ALBERT

Description:    This study examines the Indian summer monsoon hydroclimate in the National Centers for Environmental Prediction (NCEP)-Department of Energy (DOE) Reanalysis (R2), the Climate Forecast System Reanalysis (CFSR), and the Modern Era Retrospective-Analysis for Research and Applications (MERRA). The three reanalyses show significant differences in the climatology of evaporation, low-level winds, and precipitable water fields over India. For example, the continental evaporation is significantly less in CFSR compared to R2 and MERRA. Likewise the mean boreal summer 925 hPa westerly winds in the northern Indian Ocean are stronger in R2. Similarly the continental precipitable water in R2 is much less while it is higher and comparable in MERRA and CFSR. Despite these climatological differences between the reanalyses, the climatological evaporative sources for rain events over central India show some qualitative similarities. Major differences however appear when interannual variations of the Indian summer monsoon are analyzed. The anomalous oceanic sources of moisture from the adjacent Bay of Bengal and Arabian Sea play a significant role in determining the wet or dry year of the Indian monsoon in CFSR. However in R2 the local evaporative sources from the continental region play a more significant role. We also find that the interannual variability of the evaporative sources in the break spells of the intraseasonal variations of the Indian monsoon is stronger than in the wet spells. We therefore claim that instead of rainfall, evaporative sources may be a more appropriate metric to observe the relationship between the seasonal monsoon strength and intraseasonal activity. These findings are consistent across the reanalyses and provide a basis to improve the predictability of intraseasonal variability of the Indian monsoon. This study also has a bearing on improving weather prediction for tropical cyclones in that we suggest targeting enhanced observations in the Bay of Bengal (where it is drawing the most moisture from) for improved analysis during active spells of the intraseasonal variability of the Indian monsoon. The analysis suggests that the land–atmosphere interactions contribute significant uncertainty to the Indian monsoon in the reanalyses, which is consistent with the fact that most of the global reanalyses do not assimilate any land-surface data because the data are not available. Therefore, the land–atmosphere interaction in the reanalyses is highly dependent on the land-surface model and it’s coupling with the atmospheric model. Content Type Journal Article Pages 1-20 DOI 10.1007/s00382-012-1319-y Authors Vasubandhu Misra, Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL, USA P. Pantina, Science Systems and Application, Inc., 10210 Greenbelt Road, Ste. 600, Lanham, MD 20706, USA S. C. Chan, School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, UK S. DiNapoli, Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, FL, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This work uses a WRF numerical simulation from 1960 to 2005 performed at a high horizontal resolution (2 km) to analyze the surface wind variability over a complex terrain region located in northern Iberia. A shorter slice of this simulation has been used in a previous study to demonstrate the ability of the WRF model in reproducing the observed wind variability during the period 1992–2005. Learning from that validation exercise, the extended simulation is herein used to inspect the wind behavior where and when observations are not available and to determine the main synoptic mechanisms responsible for the surface wind variability. A principal component analysis was applied to the daily mean wind. Two principal modes of variation accumulate a large percentage of the wind variability (83.7%). The first mode reflects the channeling of the flow between the large mountain systems in northern Iberia modulated by the smaller topographic features of the region. The second mode further contributes to stress the differentiated wind behavior over the mountains and valleys. Both modes show significant contributions at the higher frequencies during the whole analyzed period, with different contributions at lower frequencies during the different decades. A strong relationship was found between these two modes and the zonal and meridional large scale pressure gradients over the area. This relationship is described in the context of the influence of standard circulation modes relevant in the European region like the North Atlantic Oscillation, the East Atlantic pattern, East Atlantic/Western Russia pattern, and the Scandinavian pattern. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1326-z Authors Pedro A. Jiménez, Departamento de Astrofísica y CC, de la Atmósfera, Faculatad de CC, Físicas, UCM, Avenida Complutense s/n, 28040 Madrid, Spain J. Fidel González-Rouco, Departamento de Astrofísica y CC, de la Atmósfera, Faculatad de CC, Físicas, UCM, Avenida Complutense s/n, 28040 Madrid, Spain Juan P. Montávez, Departamento de Física, Universidad de Murcia, Murcia, Spain E. García-Bustamante, Department of Geography, Justus-Liebig University of Giessen, Giessen, Germany J. Navarro, División de Energías Renovables, CIEMAT, 28040 Madrid, Spain J. Dudhia, Mesoscale and Microscale Meteorology Division, NCAR, Boulder, CO, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The water vapour feedback probably makes the largest contribution to climate sensitivity, and the second-largest contribution to its uncertainty, in the sense of disagreement between General Circulation Models (GCMs, the most physically detailed models of climate we have). Yet there has been no quantification of it which allows these differences to be attributed physically with the aim of constraining the true value. This paper develops a new breakdown of the non-cloud LW (longwave) response to climate change, which avoids the problems of the conventional breakdown, and applies it to a set of 4 GCMs. The basic physical differences are that temperature is used as the vertical coordinate, and relative humidity as the humidity variable. In this framework the different GCMs’ feedbacks look more alike, consistent with our understanding that their water vapour responses are physically very similar. Also, in the global mean all the feedback components have the same sign, allowing us to conveniently attribute the overall response fractionally (e.g. about 60% from the “partly-Simpsonian” component). The systematic cancellation between different feedback components in the conventional breakdown is lost, so now a difference in a feedback component actually contributes to a difference in climate sensitivity, and the differences between these GCMs in the non-cloud LW part of this can be traced to differences in formulation, mean climate and climate change response. Physical effects such as those due to variations in the formulation of LW radiative transfer become visible. Differences in the distribution of warming no longer dominate comparison of GCMs. The largest component depends locally only on the GCM’s mean climate, so it can in principle be calculated for the real world and validated. However, components dependent on the climate change response probably account for most of the variation between GCMs. The effect of simply changing the humidity variable in the conventional breakdown is also examined. It gives some of this improvement—the loss of the cancellations that leave the conventional breakdown of no use to understand differences between GCMs’ climate sensitivities—but not the link to mean climate. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1294-3 Authors William Ingram, Atmospheric, Oceanic and Planetary Physics, Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU UK Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The MJO modulation of sea surface chlorophyll-a (Chl) examined initially by Waliser et al. in Geophys Res Lett, ( 2005 ) is revisited with a significantly longer time-series of observations and a more systematic approach to characterizing the possible mechanisms underlying the MJO-Chl relationships. The MJO composite analysis of Chl and lead-lag correlations between Chl and other physical variables reveal regional variability of Chl and corresponding indicative temporal relationships among variables. Along the path of the MJO convection, wind speed—a proxy for oceanic vertical turbulent mixing and corresponding entrainment—is most strongly correlated with Chl when wind leads Chl by a few days. Composite Chl also displays MJO influences away from the path of the MJO convection. The role of wind speed in those regions is generally the same for Chl variability as that along the path of the MJO convection, although Ekman pumping also plays a role in generating Chl variability in limited regions. However, the wind forcing away from the MJO convection path is less coherent, rendering the temporal link relatively weak. Lastly, the potential for bio-physical feedbacks at the MJO time-scale is examined. The correlation analysis provides tantalizing evidence for local bio-feedbacks to the physical MJO system. Plausible hypothesis for Chl to amplify the MJO phase transition is presented though it cannot be affirmed in this study and will be examined and reported in a future modeling study. Content Type Journal Article Pages 1-20 DOI 10.1007/s00382-012-1321-4 Authors Daeho Jin, Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA Duane E. Waliser, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Charles Jones, Earth Research Institute, University of California, Santa Barbara, CA, USA Raghu Murtugudde, Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    A principal component decomposition of monthly sea surface temperature (SST) variability in the tropical Pacific Ocean demonstrates that nearly all of the linear trends during 1950–2010 are found in two leading patterns. The first SST pattern is strongly related to the canonical El Niño-Southern Oscillation (ENSO) pattern. The second pattern shares characteristics with the first pattern and its existence solely depends on the presence of linear trends across the tropical Pacific Ocean. The decomposition also uncovers a third pattern, often referred to as ENSO Modoki, but the linear trend is small and dataset dependent over the full 61-year record and is insignificant within each season. ENSO Modoki is also reflected in the equatorial zonal SST gradient between the Niño-4 region, located in the west-central Pacific, and the Niño-3 region in the eastern Pacific. It is only in this zonal SST gradient that a marginally significant trend arises early in the Northern Hemisphere spring (March–May) during El Niño and La Niña and also in the late summer (July–September) during El Niño. Yet these SST trends in the zonal gradient do not unequivocally represent an ENSO Modoki-like dipole because they are exclusively associated with significant positive SST trends in either the eastern or western Pacific, with no corresponding significant negative trends. Insignificant trends in the zonal SST gradient are evident during the boreal wintertime months when ENSO events typically mature. Given the presence of positive SST trends across much of the equatorial Pacific Ocean, using fixed SST anomaly thresholds to define ENSO events likely needs to be reconsidered. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1331-2 Authors Michelle L. L’Heureux, NOAA Climate Prediction Center, 5200 Auth Rd, Rm 605, Camp Springs, MD 20746, USA Dan C. Collins, NOAA Climate Prediction Center, 5200 Auth Rd, Rm 605, Camp Springs, MD 20746, USA Zeng-Zhen Hu, NOAA Climate Prediction Center, 5200 Auth Rd, Rm 605, Camp Springs, MD 20746, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The impact of climate warming on the upper layer of the Bering Sea is investigated by using a high-resolution coupled global climate model. The model is forced by increasing atmospheric CO 2 at a rate of 1% per year until CO 2 reaches double its initial value (after 70 years), after which it is held constant. In response to this forcing, the upper layer of the Bering Sea warms by about 2°C in the southeastern shelf and by a little more than 1°C in the western basin. The wintertime ventilation to the permanent thermocline weakens in the western Bering Sea. After CO 2 doubling, the southeastern shelf of the Bering Sea becomes almost ice-free in March, and the stratification of the upper layer strengthens in May and June. Changes of physical condition due to the climate warming would impact the pre-condition of spring bio-productivity in the southeastern shelf. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1301-8 Authors Hyun-Chul Lee, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Thomas L. Delworth, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Anthony Rosati, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Rong Zhang, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Whit G. Anderson, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Fanrong Zeng, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Charles A. Stock, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Anand Gnanadesikan, Johns Hopkins University, Baltimore, MD, USA Keith W. Dixon, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Stephen M. Griffies, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Surface temperatures are projected to increase 3–4°C over much of Africa by the end of the 21st century. Precipitation projections are less certain, but the most plausible scenario given by the Intergovernmental Panel on Climate Change (IPCC) is that the Sahel and East Africa will experience modest increases (~5%) in precipitation by the end of the 21st century. Evapotranspiration (E a ) is an important component of the water, energy, and biogeochemical cycles that impact several climate properties, processes, and feedbacks. The interaction of E a with climate change drivers remains relatively unexplored in Africa. In this paper, we examine the trends in E a , precipitation (P), daily maximum temperature (T max ), and daily minimum temperature (T min ) on a seasonal basis using a 31 year time series of variable infiltration capacity (VIC) land surface model (LSM) E a . The VIC model captured the magnitude, variability, and structure of observed runoff better than other LSMs and a hybrid model included in the analysis. In addition, we examine the inter-correlations of E a , P, T max , and T min to determine relationships and potential feedbacks. Unlike many IPCC climate change simulations, the historical analysis reveals substantial drying over much of the Sahel and East Africa during the primary growing season. In the western Sahel, large increases in daily maximum temperature appear linked to E a declines, despite modest rainfall recovery. The decline in E a and latent heating in this region could lead to increased sensible heating and surface temperature, thus establishing a possible positive feedback between E a and surface temperature. Content Type Journal Article Pages 1-17 DOI 10.1007/s00382-012-1299-y Authors Michael Marshall, Department of Geography, Climate Hazards Group, University of California Santa Barbara, Santa Barbara, CA, USA Christopher Funk, US Geological Survey Earth Resources Observation and Science (EROS) Center, Department of Geography, University of California Santa Barbara, Santa Barbara, CA, USA Joel Michaelsen, Department of Geography, Climate Hazards Group, University of California Santa Barbara, Santa Barbara, CA, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In this study, a new approach for extracting flow-dependent empirical singular vectors (FESVs) for seasonal prediction using ensemble perturbations obtained from an ensemble Kalman filter (EnKF) assimilation is presented. Due to the short interval between analyses, EnKF perturbations primarily contain instabilities related to fast weather variability. To isolate slower, coupled instabilities that would be more suitable for seasonal prediction, an empirical linear operator for seasonal time-scales (i.e. several months) is formulated using a causality hypothesis; then, the most unstable mode from the linear operator is extracted for seasonal time-scales. It is shown that the flow-dependent operator represents nonlinear integration results better than a conventional empirical linear operator static in time. Through 20 years of retrospective seasonal predictions, it is shown that the skill of forecasting equatorial SST anomalies using the FESV is systematically improved over that using Conventional ESV (CESV). For example, the correlation skill of the NINO3 SST index using FESV is higher, by about 0.1, than that of CESV at 8-month leads. In addition, the forecast skill improvement is significant over the locations where the correlation skill of conventional methods is relatively low, indicating that the FESV is effective where the initial uncertainty is large. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1302-7 Authors Yoo-Geun Ham, Global Modeling and Assimilation Office, NASA/GSFC Code 610.1, Greenbelt, MD, USA Michele M. Rienecker, Global Modeling and Assimilation Office, NASA/GSFC Code 610.1, Greenbelt, MD, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Atlantic meridional overturning circulation (MOC) is responsible for a climatically significant northward heat transport that is expected to decrease in response to anthropogenic global warming. Here, simulations from an ensemble of UK Met Office Hadley Centre Climate Models (HadGEM1, HadGEM2 and a 22 member perturbed physics ensemble of HadCM3-like models) are used to evaluate detection times for different MOC observing strategies. Six different detection statistics are compared, including direct observations of the MOC at two latitudes (26°N and 50°N) and several multivariate detection variables based on an optimal fingerprint of MOC change previously identified using HadCM3 (Vellinga and Wood in Geophys Res Lett 31(14):L14203, 2004 ). Using these models, and assuming perfectly observed conditions, we find no evidence to suggest that detection times would be significantly reduced by measuring the MOC at 50°N instead of (or in addition to) measurements at 26°N. Our results suggest that complementary observations of hydrographic properties in the North Atlantic may help reduce MOC detection times, but the benefits are not universal across models, nor as large as previously suggested. In addition, detection times calculated using optimal fingerprint methods are sensitive to the model-dependent estimates of covariances describing internal climate variability. This last result presents a strong case for deriving fingerprints of MOC change using dynamical/physical arguments, rather than statistical methods, in order to promote more robust results across a range of models. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1306-3 Authors Christopher D. Roberts, Met Office Hadley Centre, Fitzroy Road, Exeter, EX1 3PB UK Matthew D. Palmer, Met Office Hadley Centre, Fitzroy Road, Exeter, EX1 3PB UK Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Madden–Julian oscillation (MJO) is observed to interact with moist Kelvin waves. To understand the role of this interaction, a simple scale-interaction model is built, which describes the MJO modulation of moist Kelvin waves and the feedback from moist Kelvin waves through upscale eddy heat and momentum transfer. The backward-tilted moist Kelvin waves produce eddy momentum transfer (EMT) characterized by the lower-tropospheric westerly winds and eddy heat transfer (EHT) that warms the mid-troposphere. The EHT tends to induce the lower-tropospheric easterly winds and low pressure, which is located in front of the “westerly wind burst” induced by the EMT. Adding the eddy forcing to a neutral MJO skeleton model, we show that the EHT provides an instability source for the MJO by warming up the mid-troposphere, and the EMT offers an additional instability source by enhancing the lower-tropospheric westerly winds. The eddy forcing selects eastward propagation for the unstable mode, because it generates positive/negative eddy available potential energy for the eastward/westward modes by changing their thermal and dynamical structures. The present results show that moist Kelvin waves can provide a positive feedback to the MJO only when they are located within (or near) the convective complex (center) of the MJO. The EHT and EMT feedback works positively in the front and rear part of the MJO, respectively. These theoretical results suggest the potential importance of moist Kelvin waves in sustaining the MJO and encourage further observations to document the relationship between moist Kelvin waves and the MJO. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-011-1281-0 Authors Fei Liu, International Pacific Research Center, University of Hawaii at Manoa, Honolulu, HI, USA Bin Wang, International Pacific Research Center, University of Hawaii at Manoa, Honolulu, HI, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Using daily precipitable water ( PW ) and 850 hPa monsoon wind, which represent large-scale moisture and dynamic conditions for monsoon development, we analyze potential changes in Asian monsoon onset, retreat and duration simulated by 13 IPCC AR4 models. Most models are able to reproduce the observed temporal and spatial evolution patterns of the Asian monsoon system. Nevertheless, there are significant model biases and some models fail in reproducing the broad structure. Under a warmed climate, changes in onset and duration days are only moderate (about 3–10 days), with significant discrepancies among the models, particularly over the East Asia land area where the models are almost equally divided. In the tropical Indian Ocean, maritime continent and Indochina Peninsula, the majority of the models tend to simulate delayed onset and shortened duration while in the western North Pacific most models exhibit an early onset and longer duration. There are two reasons leading to such uncertainties: (1) the key processes determining the Asian monsoon onset/retreat are different among the models. Some are more influenced by ENSO-like processes. But in some models, monsoon onset/retreat is more significantly correlated to circulations in the tropics. (2) The model-simulated changes in these dominant processes are different. In some models, surface warming is more intense in the central and eastern Pacific Ocean with El Niño-like patterns, while others do not show such features. If the model-simulated monsoon onset/retreat is correlated to the central and eastern Pacific warming and at the same time the model simulates much larger warming of the central and eastern Pacific Ocean, then it is very likely that these models will show significant delay of south Asian monsoon onset and shortened duration. In some models, the delayed onsets are more related to the reduction of westerlies in the west of the warm pool region. The patterns of anomalous SST and wind conditions identified in this study are consistent with each other and both are likely linked to the weakening and westward shift of Walker circulation in the warm pool and maritime continent region. Increases in precipitable water associated with global warming do not change monsoon rainfall and circulation seasonality much but they can result in increased rainfall intensity once the summer monsoon is established. Content Type Journal Article Pages 1-22 DOI 10.1007/s00382-012-1289-0 Authors Huqiang Zhang, Centre for Australian Weather and Climate Research, A Partnership between the Australian Bureau of Meteorology and CSIRO, GPO Box 1289k, Melbourne, VIC 3001, Australia Ping Liang, Shanghai Regional Climate Center, China Meteorological Administration, Shanghai, China A. Moise, Centre for Australian Weather and Climate Research, A Partnership between the Australian Bureau of Meteorology and CSIRO, GPO Box 1289k, Melbourne, VIC 3001, Australia L. Hanson, Centre for Australian Weather and Climate Research, A Partnership between the Australian Bureau of Meteorology and CSIRO, GPO Box 1289k, Melbourne, VIC 3001, Australia Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Brightness temperature observations from Microwave Sounding Unit and Advanced Microwave Sounding Unit-A (AMSU-A) on board National Oceanic and Atmospheric Administration (NOAA) satellites have been widely utilized for estimating the global climate trend in the troposphere and stratosphere. A common approach for deriving the trend is linear regression, which implicitly assumes the trend being a straight line over the whole length of a time series and is often highly sensitive to the data record length. This study explores a new adaptive and temporally local data analysis method—Ensemble Empirical Mode Decomposition (EEMD)—for estimating the global trends. In EEMD, a non-stationary time series is decomposed adaptively and locally into a sequence of amplitude-frequency modulated oscillatory components and a time-varying trend. The AMSU-A data from the NOAA-15 satellite over the time period from October 26, 1998 to August 7, 2010 are employed for this study. Using data over Amazon rainforest areas, it is shown that channel 3 is least sensitive to the orbital drift among four AMSU-A surface sensitive channels. The decadal trends of AMSU-A channel 3 and other eight channels in the troposphere and stratosphere are deduced and compared using both methods. It is shown that the decadal climate trends of most AMSU-A channels are nonlinear except for channels 3–4 in Northern Hemisphere only and channels 12–13. Although the decadal trend variation of the global average brightness temperature is no more than 0.2 K, the regional decadal trend variation could be more (less) than 3 K (−3 K) in high latitudes and over high terrains. Content Type Journal Article Pages 1-17 DOI 10.1007/s00382-012-1296-1 Authors Z. Qin, Center of Data Assimilation for Research and Application, Nanjing University of Information Science and Technology, Nanjing, China X. Zou, Center of Data Assimilation for Research and Application, Nanjing University of Information Science and Technology, Nanjing, China F. Weng, Center for Satellite Applications and Research, NOAA/NESDIS, Camp Springs, MD 20746, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Climate models often use a simplified and static representation of vegetation characteristics to determine fluxes of energy, momentum and water vapour between surface and lower atmosphere. In order to analyse the impact of short term variability in vegetation phenology, we use remotely-sensed leaf area index and albedo products to examine the role of vegetation in the coupled land–atmosphere system. Perfect model experiments are carried out to determine the impact of realistic temporal variability of vegetation on potential predictability of evaporation and temperature, as well as model skill of EC-Earth simulations. The length of the simulation period is hereby limited by the availability of satellite products to 2000–2010. While a realistic representation of vegetation positively influences the simulation of evaporation and its potential predictability, a positive impact on 2 m temperature is of smaller magnitude, regionally confined and more pronounced in climatically extreme years. Content Type Journal Article Pages 2733-2746 DOI 10.1007/s00382-012-1572-0 Authors Martina Weiss, Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands Bart van den Hurk, Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands Reindert Haarsma, Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands Wilco Hazeleger, Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575 Journal Volume Volume 39 Journal Issue Volume 39, Number 11

Description:    This study investigates future changes of Global Monsoon (GM) under anthropogenic global warming using 20 coupled models that participated in the phase five of Coupled Model Intercomparison Project (CMIP5) by comparing two runs: the historical run for 1850–2005 and the Representative Concentration Pathway (RCP) 4.5 run for 2006–2100. A metrics for evaluation of models’ performance on GM is designed to document performance for 1980–2005 and best four models are selected. The four best models’ multi-model ensemble (B4MME) projects the following changes in the twenty-first century under the RCP4.5 scenario. (1) Monsoon domain will not change appreciably but land monsoon domain over Asia tends to expand westward by 10.6 %. (2) The annual mean and range of GM precipitation and the percentage of local summer rainfall will all amplify at a significant level over most of the global region, both over land and over ocean. (3) There will be a more prominent northern-southern hemispheric asymmetry and eastern-western hemispheric asymmetry. (4) Northern Hemisphere (NH) monsoon onset will be advanced and withdrawal will be delayed. (5) Changes in monsoon precipitation exhibits huge differences between the NH and the Southern hemisphere (SH). The NH monsoon precipitation will increase significantly due to increase in temperature difference between the NH and SH, significant enhancement of the Hadley circulation, and atmospheric moistening, against stabilization of troposphere. There is a slight decrease of the Walker circulation but not significant against the inter-model spread. There are important differences between the CMIP 3 and CMIP5 results which are discussed in detail. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1564-0 Authors June-Yi Lee, Department of Meteorology and International Pacific Research Center, University of Hawaii/IPRC, POST Bldg, Room 409E, 1680 East-West Road, Honolulu, HI 96822, USA Bin Wang, Department of Meteorology and International Pacific Research Center, University of Hawaii/IPRC, POST Bldg, Room 409E, 1680 East-West Road, Honolulu, HI 96822, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This paper is dedicated to the analysis of winter cold spells over Western Europe in the simulations of the 5th phase of the Coupled Model Intercomparison Project (CMIP5). Both model biases and responses in a warming climate are discussed using historical simulations and the 8.5 W/m 2 Representative Concentration Pathway (RCP8.5) scenario, respectively on the 1979–2008 and 2070–2099 periods. A percentile-based index (10th percentile of daily minimum temperature, Q10) with duration and spatial extent criteria is used to define cold spells. Related diagnostics (intensity, duration, extent, and severity as a combination of the former three statistics) of 13 models are compared to observations and suggest that models biases on severity are mainly due to the intensity parameter rather than to duration and extent. Some hypotheses are proposed to explain these biases, that involve large-scale dynamics and/or radiative fluxes related to clouds. Evolution of cold spells characteristics by the end of the century is then discussed by comparing RCP8.5 and historical simulations. In line with the projected rise of mean temperature, “present-climate” cold spells (computed with the 1979–2008 10th percentile, Q10P) are projected to be much less frequent and, except in one model, less severe. When cold spells are defined from the future 10th percentile threshold (“future-climate” cold spells, Q10F), all models simulate a decrease of their intensity linearly related to the seasonal mean warming. Some insights are given to explain the inter-model diversity in the magnitude of the cold spells response. In particular, the snow-albedo feedback is suggested to play an important role, while for some models changes in large-scale dynamics are also not negligible. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1565-z Authors Y. Peings, CNRM-GAME, Météo-France and CNRS, Toulouse, France J. Cattiaux, CNRM-GAME, Météo-France and CNRS, Toulouse, France H. Douville, CNRM-GAME, Météo-France and CNRS, Toulouse, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In the present paper the effect of an abrupt change of the atmospheric radiative forcing is investigated by means of a global climate model that includes a mixed layer ocean. In assessing if, under such a change, the model response has a bifurcation point, the steady solution is studied for a sudden decrease of CO 2 concentration from its actual value. It is found that there is a critical threshold for CO 2 content below which the model ends up to a snowball Earth. It occurs for a few percentage changes of CO 2 concentration around the threshold because the model strongly depends on the relationship among atmospheric temperature, water vapor content and the sudden ice-albedo feedback activation, even in the subtropical regions. Moreover, results suggest that the transition to ice-covered Earth is clearly favoured when Q-flux corrections (i.e. the parameterization of ocean heat transports) are removed. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1581-z Authors Isabella Bordi, Department of Physics, Sapienza University of Rome, Rome, Italy Klaus Fraedrich, Universität Hamburg, KlimaCampus, Hamburg, Germany Alfonso Sutera, Department of Physics, Sapienza University of Rome, Rome, Italy Xiuhua Zhu, Universität Hamburg, KlimaCampus, Hamburg, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    A hindcast experiment of the Mediterranean present-day climate is performed using a fully-coupled Atmosphere–Ocean Regional Climate Model (AORCM) for the Mediterranean basin. The new model, called LMDz-NEMO-Med, is composed of LMDz4-regional as atmospheric component and of NEMOMED8 as oceanic component. This AORCM equilibrates freely, without any flux adjustment, neither in fresh water nor in heat. At its atmospheric lateral boundary conditions, it is driven by ERA-40 data from 1958 to 2001, after a spin-up of 40 years in coupled configuration. The model performance is assessed and compared with available observational datasets. The model skill in reproducing mean state and inter-annual variability of main atmospheric and oceanic surface fields is in line with that of state-of-the-art AORCMs. Considering the ocean behaviour, the inter-annual variations of the basin-scale heat content are in very good agreement with the observations. The model results concerning salt content could not be adequately validated. High inter-annual variability of deep convection in the Gulf of Lion is simulated, with 53 % of convective winters, representative of the present climate state. The role of different factors influencing the deep convection and its inter-annual variability is examined, including dynamic and hydrostatic ocean preconditioning and atmospheric surface forcing. A conceptual framework is outlined and validated in linking the occurrence of deep convection to the efficiency of the integrated surface buoyancy fluxes along the winter season to mix the initially stratified averaged water column down to the convective threshold depth. This simple framework (based only on 2 independent variables) is able to explain 60 % (resp. 69 %) of inter-annual variability of the deep water formation rate (resp. maximum mixed layer depth) for the West Mediterranean Deep Water (WMDW) formation process. Content Type Journal Article Pages 1-24 DOI 10.1007/s00382-012-1527-5 Authors Blandine L’Hévéder, Laboratoire de Météorologie Dynamique, Université Paris VI, Tour 45-55, 3e étage, Case postale 99, 4, place Jussieu, 75252 Paris Cedex 05, France Laurent Li, Laboratoire de Météorologie Dynamique, Université Paris VI, Tour 45-55, 3e étage, Case postale 99, 4, place Jussieu, 75252 Paris Cedex 05, France Florence Sevault, Centre National de Recherches Météorologiques, Météo-France, Toulouse, France Samuel Somot, Centre National de Recherches Météorologiques, Météo-France, Toulouse, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    A set of global climate model simulations for the last thousand years developed by the Max Planck Institute is compared with paleoclimate proxy data and instrumental data, focusing on surface temperatures for land areas between 30° and 75°N. The proxy data are obtained from six previously published Northern Hemispheric-scale temperature reconstructions, here re-calibrated for consistency, which are compared with the simulations utilizing a newly developed statistical framework for ranking several competing simulations by means of their statistical distance against past climate variations. The climate model simulations are driven by either “low” or “high” solar forcing amplitudes (0.1 and 0.25 % smaller total solar irradiance in the Maunder Minimum period compared to the present) in addition to several other known climate forcings of importance. Our results indicate that the high solar forcing amplitude results in a poorer match with the hemispheric-scale temperature reconstructions and lends stronger statistical support for the low-amplitude solar forcing. However, results are likely conditional upon the sensitivity of the climate model used and strongly dependent on the choice of temperature reconstruction, hence a greater consensus is needed regarding the reconstruction of past temperatures as this currently provides a great source of uncertainty. Content Type Journal Article Pages 1-11 DOI 10.1007/s00382-012-1526-6 Authors Alistair Hind, Department of Physical Geography and Quaternary Geology, Bert Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden Anders Moberg, Department of Physical Geography and Quaternary Geology, Bert Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The nocturnal precipitation in the Sichuan Basin in summer has been studied in many previous works. This paper expands the study on the diurnal cycle of precipitation in the Sichuan Basin to the whole year. Results show that the nocturnal precipitation has a specific quasi-stationary feature in the basin. It occurs not only in summer but also in other three seasons, even more remarkable in spring and autumn than in summer. There is a prominent eastward timing delay in the nocturnal precipitation, that is, the diurnal peak of precipitation occurs at early-night in the western basin whereas at late-night in the center and east of the basin. The Tibetan Plateau plays an essential role in the formation of this quasi-stationary nocturnal precipitation. The early-night peak of precipitation in the western basin is largely due to strong ascending over the plateau and its eastern lee side. In the central and eastern basin, three coexisting factors contribute to the late-night peak of precipitation. One is the lower-tropospheric southwesterly flow around the southeastern edge of the Tibetan Plateau, which creates a strong cyclonic rotation and ascendance in the basin at late-night, as well as brings abundant water vapor. The second is the descending motion downslope along the eastern lee side of the plateau, together with an air mass accumulation caused by the warmer air mass transport from the southeast of the Yunnan-Guizhou Plateau, creating a diabatic warming at low level of the troposphere in the central basin. The third is a cold advection from the plateau to the basin at late-night, which leads to a cooling in the middle troposphere over the central basin. All these factors are responsible for precipitation to occur at late-night in the central to eastern basin. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1521-y Authors Xia Jin, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing, China Tongwen Wu, National Climate Center, China Meteorological Administration, 46 Zhongguancun Nandajie, Beijing, 100081 People’s Republic of China Laurent Li, Laboratoire de Météorologie Dynamique, IPSL, CNRS/UPMC, Paris, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The summer circulation over the eastern Mediterranean and the Middle East (EMME) is dominated by persistent northerly winds (Etesians) whose ventilating effect counteracts the adiabatic warming induced by large scale subsidence. The ERA40 dataset is used to study the vertical distribution of these circulation features, which both appear to be reconciled manifestations of the South Asian monsoon influence. As predicted by past idealized modeling studies, in late spring a westward expanding upper level warm structure and subsidence areas are associated with Rossby waves excited by the monsoon convection. Steep sloping isentropes that develop over the EMME facilitate further subsidence on the western and northern periphery of the warm structure, which is exposed to the midlatitude westerlies. The northerly flow and descent over the eastern Mediterranean have maxima in July that are strikingly synchronous to the monsoon convection over northern India, where the weaker easterly jet favors a stronger Rossby wave response and consequent impact on the EMME circulation. The pronounced EMME topography modifies the monsoon induced structure, firstly, by inducing orographically locked summer anticyclones. These enhance the mid and low level northwesterly flow at their eastern flanks, leading to distinct subsidence maxima over the eastern Mediterranean and Iran. Secondly, topography amplifies the subsidence and the northerly flow over the Aegean, Red Sea, the Iraq—Gulf region and to the east of the Caspian Sea. Content Type Journal Article Pages 1-21 DOI 10.1007/s00382-012-1528-4 Authors Evangelos Tyrlis, Energy, Environment and Water Research Center, The Cyprus Institute, 20 Konstantinou Kavafi Street, 2121 Nicosia, Cyprus Jos Lelieveld, Energy, Environment and Water Research Center, The Cyprus Institute, 20 Konstantinou Kavafi Street, 2121 Nicosia, Cyprus Benedikt Steil, Max Planck Institute for Chemistry, 55020 Mainz, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Ensemble simulations with a coupled ocean-troposphere-stratosphere model for the pre-industrial era (1860 AD), late twentieth century (1990 AD) greenhouse gas (GHG) concentrations, the SRES scenarios B1, A1B, A2, as well as stabilization experiments up to the Twenty-third century with B1 and A1B scenario GHG concentrations at their values at 2100, have been analyzed with regard to the occurrence of major sudden stratospheric warmings (SSWs). An automated algorithm using 60°N and 10 hPa zonal wind and the temperature gradient between 60°N and the North Pole is used to identify this phenomenon in the large data set. With 1990 CO 2 concentrations (352 ppmv), the frequency of simulated SSWs in February and March is comparable to observation, but they are underestimated during November to January. All simulations show an increase in the number of SSWs from the pre-industrial period to the end of the twenty-first century, indicating that the increase of GHG is also reflected in the number of sudden warmings. However, a high variability partially masks the underlying trend. Multi-century averages during the stabilization periods indicate that the increase of SSWs is linear to the applied radiative forcing. A doubling of SSWs occurs when the GHG concentration reaches the level of the A2 scenario at the end of the twenty-first century (836 ppmv). The increase in SSWs in the projections is caused by a combination of increased wave flux from the troposphere and weaker middle atmospheric zonal winds. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1530-x Authors S. Schimanke, Swedish Meteorological and Hydrological Institute, Norrköping, 60176 Sweden T. Spangehl, Freie Universität Berlin, Berlin, Germany H. Huebener, Freie Universität Berlin, Berlin, Germany U. Cubasch, Freie Universität Berlin, Berlin, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This study compares the synoptic-dynamic relationship between two phases of the Pacific/North American (PNA) pattern and winter precipitation isotopes at 73 sites across the contiguous USA. We use the spatial pattern of isotope slope—the rate of changes in precipitation isotope ratios with distance—to identify features in the seasonal precipitation isotope fields related to climatic patterns, PNA positive and PNA negative. Our results show relationships between zones of high isotope slopes and the spatial position of the polar jet stream and juxtaposition of air masses associated with the PNA pattern. During a positive PNA winter, zones of high isotope slope in the eastern USA shift southward. This change is coincident with a southward displacement of the polar jet stream in this region, which leads to a greater frequency of polar air masses and 18 O-depleted isotope values of precipitation in the region. In the western USA, zones of high slope shift eastward during the positive PNA winter, associated with more frequent penetration of tropical air masses that bring 18 O-enriched precipitation to the region. Differences in δ 18 O/temperature relationships between the PNA-positive and -negative winters and contrasting δ 18 O/temperature behaviors in the eastern and western USA provide support for the role of variation in moisture source and transport as a control on the isotopic patterns. These findings highlight the importance of synoptic climate driven by PNA pattern in determining the spatial patterns of precipitation isotopes and provide constraints on paleo-water isotope interpretation and modern isotope hydrological processes. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1548-0 Authors Zhongfang Liu, Tianjin Key Laboratory of Water Resource and Environment, Tianjin Normal University, Tianjin, China Gabriel J. Bowen, Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN, USA Jeffrey M. Welker, Environment and Natural Resources Institute, University of Alaska Anchorage, Anchorage, AK, USA Kei Yoshimura, Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The effects of the northeastern Eurasian snow cover on the frequency of spring dust storms in northwestern China have been examined for the period 1979–2007. Averaged over all 43 stations in northwestern China, a statistically significant relationship has been found between dust-storm frequency (DSF) and Eurasian snow-water equivalent (SWE) during spring: mean DSF of 7.4 and 3.3 days for years of high- and low SWE, respectively. Further analyses reveal that positive SWE anomalies enhance the meridional gradients of the lower tropospheric temperatures and geopotential heights, thereby strengthening westerly jets and zonal wind shear over northwestern China and western Inner Mongolia of China, the regions of major dust sources. The anomalous atmospheric circulation corresponding to the Eurasian SWE anomalies either reinforces or weakens atmospheric baroclinicity and cyclogenesis, according to the sign of the anomaly, to affect the spring DSF. This study suggests that Eurasian SWE anomalies can be an influential factor of spring DSF in northwestern China and western Inner Mongolia of China. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1522-x Authors Yun Gon Lee, Climate Physics Laboratory, School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-747 Korea Chang-Hoi Ho, Climate Physics Laboratory, School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-747 Korea Jhoon Kim, Department of Atmospheric Sciences, Yonsei University, Seoul, Korea Jinwon Kim, Department of Atmospheric and Oceanic Sciences and Joint Institute for Regional Earth System Science and Engineering, University of California Los Angeles, Los Angeles, CA, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Regional climate models (RCMs) have been increasingly used for climate change studies at the watershed scale. However, their performance is strongly dependent upon their driving conditions, internal parameterizations and domain configurations. Also, the spatial resolution of RCMs often exceeds the scales of small watersheds. This study developed a two-step downscaling method to generate climate change projections for small watersheds through combining a weighted multi-RCM ensemble and a stochastic weather generator. The ensemble was built on a set of five model performance metrics and generated regional patterns of climate change as monthly shift terms. The stochastic weather generator then incorporated these shift terms into observed climate normals and produced synthetic future weather series at the watershed scale. This method was applied to the Assiniboia area in southern Saskatchewan, Canada. The ensemble led to reduced biases in temperature and precipitation projections through properly emphasizing models with good performance. Projection of precipitation occurrence was particularly improved through introducing a weight-based probability threshold. The ensemble-derived climate change scenario was well reproduced as local daily weather series by the stochastic weather generator. The proposed combination of dynamical downscaling and statistical downscaling can improve the reliability and resolution of future climate projection for small prairie watersheds. It is also an efficient solution to produce alternative series of daily weather conditions that are important inputs for examining watershed responses to climate change and associated uncertainties. Content Type Journal Article Pages 1-17 DOI 10.1007/s00382-012-1490-1 Authors Hua Zhang, Institute for Energy, Environment and Sustainable Communities, University of Regina, Regina, SK S4S 0A2, Canada Guo H. Huang, Institute for Energy, Environment and Sustainable Communities, University of Regina, Regina, SK S4S 0A2, Canada Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Regional climate models represent a promising tool to assess the regional dimension of future climate change and are widely used in climate impact research. While the added value of regional climate models has been highlighted with respect to a better representation of land-surface interactions and atmospheric processes, it is still unclear whether radiative heating implies predictability down to the typical scale of a regional climate model. As a quantitative assessment, we apply an optimal statistical filter to compare the coherence between observed and simulated patterns of Mediterranean climate change from a global and a regional climate model. It is found that the regional climate model has indeed an added value in the detection of regional climate change, contrary to former assumptions. The optimal filter may also serve as a weighting factor in multi-model averaging. Content Type Journal Article Pages 1-10 DOI 10.1007/s00382-012-1517-7 Authors Heiko Paeth, Institute of Geography and Geology, University of Würzburg, Am Hubland, 97074 Würzburg, Germany Birgit Mannig, Institute of Geography and Geology, University of Würzburg, Am Hubland, 97074 Würzburg, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The variability of the European climate is mostly controlled by the unstable nature of the North-Atlantic dynamics, especially in wintertime. The intra-seasonal to inter-annual fluctuations of atmospheric circulations has often been described as the alternation between a limited number of preferential weather regimes. Such discrete description can be justified by the multi-modality of the latitudinal position of the jet stream. In addition, seasonal extremes in European temperatures are generally associated with an exceptional persistence into one weather regime. Here we investigate the skill of the IPSL model to both simulate North-Atlantic weather regimes and European temperature extremes, including summer heat waves and winter cold spells. We use a set of eight IPSL experiments, with six different horizontal resolutions and the two versions used in CMIP3 and CMIP5. We find that despite a substantial deficit in the simulated poleward peak of the jet stream, the IPSL model represents weather regimes fairly well. A significant improvement is found for all horizontal resolutions higher than the one used in CMIP3, while the increase in vertical resolution included in the CMIP5 version tends to improve the wintertime dynamics. In addition to a recurrent cold bias over Europe, the IPSL model generally overestimates (underestimates) the indices of winter cold spells (summer heat waves) such as frequencies or durations. We find that the increase in horizontal resolution almost always improves these statistics, while the influence of vertical resolution is less clear. Overall, the CMIP5 version of the IPSL model appears to carry promising improvements in the simulation of the European climate variability. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1529-3 Authors Julien Cattiaux, IPSL/LSCE, Unité mixte CEA-CNRS-UVSQ, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France Benjamin Quesada, IPSL/LSCE, Unité mixte CEA-CNRS-UVSQ, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France Ara Arakélian, IPSL/LMD, Unité mixte Ecole Polytechnique-CNRS-ENS-UPMC, 75005 Paris, France Francis Codron, IPSL/LMD, Unité mixte Ecole Polytechnique-CNRS-ENS-UPMC, 75005 Paris, France Robert Vautard, IPSL/LSCE, Unité mixte CEA-CNRS-UVSQ, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France Pascal Yiou, IPSL/LSCE, Unité mixte CEA-CNRS-UVSQ, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    A dynamical wave model implemented over the North Pacific Ocean was forced with winds from three coupled global climate models (CGCMs) run under a medium-to-high scenario for greenhouse gas emissions through the twenty-first century. The results are analyzed with respect to changes in upper quantiles of significant wave height (90th and 99th percentile H S ) during boreal winter. The three CGCMs produce surprisingly similar patterns of change in winter wave climate during the century, with waves becoming 10–15 % smaller over the lower mid-latitudes of the North Pacific, particularly in the central and western ocean. These decreases are closely associated with decreasing windspeeds along the southern flank of the main core of the westerlies. At higher latitudes, 99th percentile wave heights generally increase, though the patterns of change are less uniform than at lower latitudes. The increased wave heights at high latitudes appear to be due a variety of wind-related factors including both increased windspeeds and changes in the structure of the wind field, these varying from model to model. For one of the CGCMs, a commonly used statistical approach for estimating seasonal quantiles of H S on the basis of seasonal mean sea level pressure (SLP) is used to develop a regression model from 60 years of twentieth century data as a training set, and then applied using twenty-first century SLP data. The statistical model reproduces the general pattern of decreasing twenty-first century wave heights south of ~40 N, but underestimates the magnitude of the changes by ~50–70 %, reflecting relatively weak coupling between sea level pressure and wave heights in the CGCM data and loss of variability in the statistically projected wave heights. Content Type Journal Article Pages 1-26 DOI 10.1007/s00382-012-1435-8 Authors Nicholas E. Graham, Hydrologic Research Center, San Diego, CA, USA Daniel R. Cayan, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA Peter D. Bromirski, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA Reinhard E. Flick, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This study estimates MJO change under the A1B greenhouse gas emission scenario using the ECHAM5 AGCM whose coupled version (ECHAM5/MPI-OM) has simulated best MJO variance among fourteen CGCMs. The model has a horizontal resolution at T319 (about 40 km) and is forced by the monthly evolving SST derived from the ECHAM5/MPI-OM at a lower resolution of T63 (about 200 km). Two runs are carried out covering the last 21 years of the twentieth and twenty-first centuries. The NCEP/NCAR Reanalysis products and observed precipitation are used to validate the simulated MJO during the twentieth century, based on which the twenty-first century MJO change is compared and predicted. The validation indicates that the previously reported MJO variances in the T63 coupled version are reproduced by the 40-km ECHAM5. More aspects of MJO, such as the eastward propagation, structure, and dominant frequency and zonal wavenumber in power spectrum, are simulated reasonably well. The magnitude in power, however, is still low so that the signal is marginally detectable and embedded in the over-reddened background. Under the A1B scenario, the T63 ECHAM5/MPI-OM projected an over 3 K warmer tropical sea surface that forces the 40-km ECHAM to produce wetter tropics. The enhanced precipitation variance shows more spectral enhancement in background than in most wavebands. The zonal winds associated with MJO, however, are strengthened in the lower troposphere but weakened in the upper. On the one hand, the 850-hPa zonal wind has power nearly doubled in 30–60-days bands, demonstrating relatively clearer enhancement than the precipitation in MJO with the warming. A 1-tailed Student’s t test suggests that most of the MJO changes in variance and power spectra are statically significant. Subject to a 20–100-days band-pass filtering of that wind, an EOF analysis indicates that the two leading components in the twentieth-century run have a close structure to but smaller percentage of explained-to-total variance than those in observations; the A1B warming slightly increases the explained percentage and alters the structure. An MJO index formed by the two leading principal components discloses nearly doubling in the number of prominent MJO events with a peak phase occurring in February and March. A composite MJO life cycle of these events favors the frictional moisture convergence mechanism in maintaining the MJO and the nonlinear wind-induced surface heat exchange (WISHE) mechanism also appears in the A1B warming case. On the other hand, the Slingo index based on the 300-hPa zonal wind discloses that the upper-level MJO tends to be suppressed by the A1B warming, although the loose relationship with ENSO remains unchanged. Possible cause for the different change of MJO in the lower and upper troposphere is discussed. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1532-8 Authors Ping Liu, International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, HI, USA Tim Li, International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, HI, USA Bin Wang, International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, HI, USA Minghua Zhang, School of Marine and Atmospheric Sciences, Stony Brook University, 199 Endeavour Hall, SoMAS, Stony Brook, NY 11794-5000, USA Jing-jia Luo, Research Institution for Global Change, Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan Yukio Masumoto, Research Institution for Global Change, Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan Xiaocong Wang, The State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Beijing, China Erich Roeckner, Max Planck Institute for Meteorology, Hamburg, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This paper aims at characterizing how different key cloud properties (cloud fraction, cloud vertical distribution, cloud reflectance, a surrogate of the cloud optical depth) vary as a function of the others over the tropical oceans. The correlations between the different cloud properties are built from 2 years of collocated A-train observations (CALIPSO-GOCCP and MODIS) at a scale close to cloud processes; it results in a characterization of the physical processes in tropical clouds, that can be used to better understand cloud behaviors, and constitute a powerful tool to develop and evaluate cloud parameterizations in climate models. First, we examine a case study of shallow cumulus cloud observed simultaneously by the two sensors (CALIPSO, MODIS), and develop a methodology that allows to build global scale statistics by keeping the separation between clear and cloudy areas at the pixel level (250, 330 m). Then we build statistical instantaneous relationships between the cloud cover, the cloud vertical distribution and the cloud reflectance. The vertical cloud distribution indicates that the optically thin clouds (optical thickness 〈1.5) dominate the boundary layer over the trade wind regions. Optically thick clouds (optical thickness 〉3.4) are composed of high and mid-level clouds associated with deep convection along the ITCZ and SPCZ and over the warm pool, and by stratocumulus low level clouds located along the East coast of tropical oceans. The cloud properties are analyzed as a function of the large scale circulation regime. Optically thick high clouds are dominant in convective regions (CF 〉 80 %), while low level clouds with low optical thickness (〈3.5) are present in regimes of subsidence but in convective regimes as well, associated principally to low cloud fractions (CF 〈 50 %). A focus on low-level clouds allows us to quantify how the cloud optical depth increases with cloud top altitude and with cloud fraction. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1533-7 Authors Dimitra Konsta, LMD, Ecole Polytechnique, Palaiseau, France Helene Chepfer, LMD/IPSL, Universite Pierre et Marie Curie, Paris, France Jean-Louis Dufresne, LMD/IPSL, CNRS, Universite Pierre et Marie Curie, Paris, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Greenland ice sheet is projected to be strongly affected by global warming. These projections are either issued from downscaling methods (such as Regional Climate Models) or they come directly from General Circulation Models (GCMs). In this context, it is necessary to evaluate the accuracy of the daily atmospheric circulation simulated by the GCMs, since it is used as forcing for downscaling methods. Thus, we use an automatic circulation type classification based on two indices (Euclidean distance and Spearman rank correlation using the daily 500 hPa geopotential height) to evaluate the ability of the GCMs from both CMIP3 and CMIP5 databases to simulate the main circulation types over Greenland during summer. For each circulation type, the GCMs are compared to three reanalysis datasets on the basis of their frequency and persistence differences. For the current climate (1961–1990), we show that most of the GCMs do not reproduce the expected frequency and the persistence of the circulation types and that they simulate poorly the observed daily variability of the general circulation. Only a few GCMs can be used as reliable forcings for downscaling methods over Greenland. Finally, when applying the same approach to the future projections of the GCMs, no significant change in the atmospheric circulation over Greenland is detected, besides a generalised increase of the geopotential height due to a uniform warming of the atmosphere. Content Type Journal Article Pages 1-20 DOI 10.1007/s00382-012-1538-2 Authors Alexandre Belleflamme, Laboratory of Climatology and Topoclimatology, University of Liège, Allée du 6 Aout, 2, 4000 Liège, Belgium Xavier Fettweis, Laboratory of Climatology and Topoclimatology, University of Liège, Allée du 6 Aout, 2, 4000 Liège, Belgium Charlotte Lang, Laboratory of Climatology and Topoclimatology, University of Liège, Allée du 6 Aout, 2, 4000 Liège, Belgium Michel Erpicum, Laboratory of Climatology and Topoclimatology, University of Liège, Allée du 6 Aout, 2, 4000 Liège, Belgium Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Portions of the southern and southeastern United States, primarily Mississippi, Alabama, and Georgia, have experienced century-long (1895–2007) downward air temperature trends that occur in all seasons. Superimposed on them are shifts in mean temperatures on decadal scales characterized by alternating warm (1930s–1940s, 1990s) and cold (1900s; 1960s–1970s) regimes. Regional atmospheric circulation and SST teleconnection indices, station-based cloud cover and soil moisture (Palmer drought severity index) data are used in stepwise multiple linear regression models. These models identify predictors linked to observed winter, summer, and annual Southeastern air temperature variability, the observed variance (r 2 ) they explain, and the resulting prediction and residual time series. Long-term variations and trends in tropical Pacific sea temperatures, cloud cover, soil moisture and the North Atlantic and Arctic oscillations account for much of the air temperature downtrends. Soil moisture and cloud cover are the primary predictors of 59.6 % of the observed summer temperature variance. While the teleconnections, cloud cover and moisture data account for some of the annual and summer Southeastern cooling trend, large significant downward trending residuals remain in winter and summer. Comparison is made to the northeastern United States where large twentieth century upward air temperature trends are driven by cloud cover increases and Atlantic Multidecadal Oscillation (AMO) variability. Differences between the Northeastern warming and the Southeastern cooling trends in summer are attributable in part to the differing roles of cloud cover, soil moisture, the Arctic Oscillation and the AMO on air temperatures of the 2 regions. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1437-6 Authors Jeffrey C. Rogers, Department of Geography and Atmospheric Sciences Program, The Ohio State University, Columbus, OH 43210-1361, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Northern China has been subject to increased heatwave frequency (HWF) in recent decades, which deteriorates the local droughts and desertification. More than half a billion people face drinking water shortages and worsening ecological environment. In this study, the variability in the western Tibetan Plateau snow cover (TPSC) is observed to have an intimate linkage with the first empirical orthogonal function mode of the summer HWF across China. This distinct leading mode is dominated by the decadal to inter-decadal variability and features a mono-sign pattern with the extreme value center prevailing over northern China and high pressure anomalies at mid- and upper troposphere over Mongolia and the adjacent regions. A simplified general circulation model is utilized to examine the possible physical mechanism. A reduced TPSC anomaly can induce a positive geopotential height anomaly at the mid- and upper troposphere and subsequently enhance the climatological high pressure ridge over Mongolia and the adjacent regions. The subsidence associated with the high pressure anomalies tends to suppress the local cloud formation, which increases the net radiation budget, heats the surface, and favors more heatwaves. On the other hand, the surface heating can excite high pressure anomalies at mid- and upper troposphere. The latter further strengthens the upper troposphere high pressure anomalies over Mongolia and the adjacent regions. Through such positive feedback effect, the TPSC is tied to the interdecadal variations of the northern China HWF. Content Type Journal Article Category Original Article Pages 1-10 DOI 10.1007/s00382-012-1439-4 Authors Zhiwei Wu, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China Zhihong Jiang, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China Jianping Li, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Shanshan Zhong, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China Lijuan Wang, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The regional ocean modeling system is used, at a resolution of 1/12°, to explicitly simulate the ocean circulation near the Iberian coast during two 30-year simulations forced by atmospheric fields produced by the RACMO regional climate model. The first simulation is a control run for the present climate (1961–1990) and the second is a scenario run from the IPCC A2 scenario (2071–2100). In the control run, the model reproduces some important features of the regional climate but with an overestimation of upwelling intensity, mainly attributable to inaccuracies in the coastal wind distributions when compared against reanalysis data. A comparison between the scenario and control simulations indicates a significant increase in coastal upwelling, with more frequent events with higher intensity, leading to an overall enhancement of SST variability on both the intra- and inter-annual timescales. The increase in upwelling intensity is more prominent in the northern limit of the region, near cape Finisterre, where its mean effect extends offshore for a few hundred kms, and is able to locally cancel the effect of global warming. If these results are confirmed, climate change will have a profound impact on the regional marine ecosystem. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1442-9 Authors P. M. A. Miranda, Instituto Dom Luiz, University of Lisbon, Lisbon, Portugal J. M. R. Alves, Instituto Dom Luiz, University of Lisbon, Lisbon, Portugal N. Serra, Institut für Meereskunde, University of Hamburg, Hamburg, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    A wide range of statistical tools is used to investigate the decadal variability of the Atlantic Meridional Overturning Circulation (AMOC) and associated key variables in a climate model (CHIME, Coupled Hadley-Isopycnic Model Experiment), which features a novel ocean component. CHIME is as similar as possible to the 3rd Hadley Centre Coupled Model (HadCM3) with the important exception that its ocean component is based on a hybrid vertical coordinate. Power spectral analysis reveals enhanced AMOC variability for periods in the range 15–30 years. Strong AMOC conditions are associated with: (1) a Sea Surface Temperature (SST) anomaly pattern reminiscent of the Atlantic Multi-decadal Oscillation (AMO) response, but associated with variations in a northern tropical-subtropical gradient; (2) a Surface Air Temperature anomaly pattern closely linked to SST; (3) a positive North Atlantic Oscillation (NAO)-like pattern; (4) a northward shift of the Intertropical Convergence Zone. The primary mode of AMOC variability is associated with decadal changes in the Labrador Sea and the Greenland Iceland Norwegian (GIN) Seas, in both cases linked to the tropical activity about 15 years earlier. These decadal changes are controlled by the low-frequency NAO that may be associated with a rapid atmospheric teleconnection from the tropics to the extratropics. Poleward advection of salinity anomalies in the mixed layer also leads to AMOC changes that are linked to processes in the Labrador Sea. A secondary mode of AMOC variability is associated with interannual changes in the Labrador and GIN Seas, through the impact of the NAO on local surface density. Content Type Journal Article Pages 1-22 DOI 10.1007/s00382-012-1432-y Authors A. Persechino, Ocean and Earth Science, University of Southampton, European Way, Southampton, Hampshire, SO14 3ZH UK R. Marsh, Ocean and Earth Science, University of Southampton, European Way, Southampton, Hampshire, SO14 3ZH UK B. Sinha, National Oceanography Centre, European Way, Southampton, Hampshire, SO14 3ZH UK A. P. Megann, National Oceanography Centre, European Way, Southampton, Hampshire, SO14 3ZH UK A. T. Blaker, National Oceanography Centre, European Way, Southampton, Hampshire, SO14 3ZH UK A. L. New, National Oceanography Centre, European Way, Southampton, Hampshire, SO14 3ZH UK Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This study explores how global precipitation and tropospheric water vapor content vary on the interdecadal/long-term time scale during past three decades (1988–2010 for water vapor), in particular to what extent the spatial structures of their variations relate to changes in surface temperature. EOF analyses of satellite-based products indicate that the first two modes of global precipitation and columnar water vapor content anomalies are in general related to the El Niño-Southern oscillation. The spatial patterns of their third modes resemble the corresponding linear fits/trends estimated at each grid point, which roughly represent the interdecadal/long-term changes happening during the same time period. Global mean sea surface temperature (SST) and land surface temperature have increased during the past three decades. However, the water vapor and precipitation patterns of change do not reflect the pattern of warming, in particular in the tropical Pacific basin. Therefore, other mechanisms in addition to global warming likely exist to account for the spatial structures of global precipitation changes during this time period. An EOF analysis of longer-record (1949–2010) SST anomalies within the Pacific basin (60 o N–60 o S) indicates the existence of a strong climate regime shift around 1998/1999, which might be associated with the Pacific decadal variability (PDV) as suggested in past studies. Analyses indicate that the observed linear changes/trends in both precipitation and tropospheric water vapor during 1988–2010 seem to result from a combined impact of global mean surface warming and the PDV shift. In particular, in the tropical central-eastern Pacific, a band of increases along the equator in both precipitation and water vapor sandwiched by strong decreases south and north of it are likely caused by the opposite effects from global-mean surface warming and PDV-related, La Niña-like cooling in the tropical central-eastern Pacific. This narrow band of precipitation increase could also be considered an evidence for the influence of global mean surface warming. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1443-8 Authors Guojun Gu, Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA Robert F. Adler, Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The identification of the land-atmosphere interactions as one of the key source of uncertainty in climate models calls for process-level assessment of the coupled atmosphere/land continental surface system in numerical climate models. To this end, we propose a novel approach and apply it to evaluate the standard and new parametrizations of boundary layer/convection/clouds in the Earth System Model (ESM) of Institut Pierre Simon Laplace (IPSL), which differentiate the IPSL-CM5A and IPSL-CM5B climate change simulations produced for the Coupled Model Inter-comparison Project phase 5 exercise. Two different land surface hydrology parametrizations are also considered to analyze different land-atmosphere interactions. Ten-year simulations of the coupled land surface/atmospheric ESM modules are confronted to observations collected at the SIRTA (Site Instrumental de Recherche par Télédection Atmosphérique), located near Paris (France). For sounder evaluation of the physical parametrizations, the grid of the model is stretched and refined in the vicinity of the SIRTA, and the large scale component of the modeled circulation is adjusted toward ERA-Interim reanalysis outside of the zoomed area. This allows us to detect situations where the parametrizations do not perform satisfactorily and can affect climate simulations at the regional/continental scale, including in full 3D coupled runs. In particular, we show how the biases in near surface state variables simulated by the ESM are explained by (1) the sensible/latent heat partitionning at the surface, (2) the low level cloudiness and its radiative impact at the surface, (3) the parametrization of turbulent transport in the surface layer, (4) the complex interplay between these processes. We also show how the new set of parametrizations can improve these biases. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1469-y Authors F. Cheruy, Laboratoire de Météorologie Dynamique du CNRS, 4 Place Jussieu, case courrier 99, 75252 Paris Cedex 05, France A. Campoy, Sisyphe, UPMC/CNRS, Paris, France J.-C. Dupont, IPSL, Ecole Polytechnique, 91128 Palaiseau Cedex, France A. Ducharne, Sisyphe, UPMC/CNRS, Paris, France F. Hourdin, LMD, 4 Place Jussieu, case courrier 99, 75252 Paris Cedex 05, France M. Haeffelin, IPSL, Ecole Polytechnique, 91128 Palaiseau Cedex, France M. Chiriaco, LATMOS, Paris, France A. Idelkadi, LMD, 4 Place Jussieu, case courrier 99, 75252 Paris Cedex 05, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    We assess the ability of individual models (single-model ensembles) and the multi-model ensemble (MME) in the European Union-funded ENSEMBLES project to simulate the intraseasonal oscillations (ISOs; specifically in 10–20-day and 30–50-day frequency bands) of the Indian summer monsoon rainfall (ISMR) over the Western Ghats (WG) and the Bay of Bengal (BoB), respectively. This assessment is made on the basis of the dynamical linkages identified from the analysis of observations in a companion study to this work. In general, all models show reasonable skill in simulating the active and break cycles of the 30–50-day ISOs over the Indian summer monsoon region. This skill is closely associated with the proper reproduction of both the northward propagation of the intertropical convergence zone (ITCZ) and the variations of monsoon circulation in this band. However, the models do not manage to correctly simulate the eastward propagation of the 30–50-day ISOs in the western/central tropical Pacific and the eastward extension of the ITCZ in a northwest to southeast tilt. This limitation is closely associated with a limited capacity of models to accurately reproduce the magnitudes of intraseasonal anomalies of both the ITCZ in the Asian tropical summer monsoon regions and trade winds in the tropical Pacific. Poor reproduction of the activity of the western Pacific subtropical high on intraseasonal time scales also amplify this limitation. Conversely, the models make good reproduction of the WG 10–20-day ISOs. This success is closely related to good performance of the models in the representation of the northward propagation of the ITCZ, which is partially promoted by local air–sea interactions in the Indian Ocean in this higher-frequency band. Although the feature of westward propagation is generally represented in the simulated BoB 10–20-day ISOs, the air–sea interactions in the Indian Ocean are spuriously active in the models. This leads to active WG rainfall, which is not present in the observed BoB 10–20-day ISOs. Further analysis indicates that the intraseasonal variability of the ISMR is generally underrepresented in the simulations. Skill of the MME in seasonal ISMR forecasting is strongly dependent on individual model performance. Therefore, in order to improve the model skill with respect to seasonal ISMR forecasting, we suggest it is necessary to better represent the robust dynamical links between the ISOs and the relevant circulation variations, as well as the proportion of intraseasonal variability in the individual models. Content Type Journal Article Pages 1-21 DOI 10.1007/s00382-012-1476-z Authors Shujie Ma, Institut Català de Ciències del Clima (IC3), C/Doctor Trueta 203, 08005 Barcelona, Catalonia, Spain Xavier Rodó, Institut Català de Ciències del Clima (IC3), C/Doctor Trueta 203, 08005 Barcelona, Catalonia, Spain Yongjia Song, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA Benjamin A. Cash, Center for Ocean-Land-Atmosphere Studies, Calverton, MD, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This study investigates the space–time evolution of the East Asian winter monsoon (EAWM) and its relationship with other climate subsystems. Cyclostationary Empirical Orthogonal Function (CSEOF) analysis and the multiple regression method are used to delineate the detailed evolution of various atmospheric and surface variables in connection with the EAWM. The 120 days of winter (November 17–March 16) per year over 62 years (1948–2010) are analyzed using the NCEP daily reanalysis dataset. The first CSEOF mode of 850-hPa temperatures depicts the seasonal evolution of the EAWM. The contrast in heat capacity between the continent and the northwestern Pacific results in a differential heating in the lower troposphere. Its temporal evolution drives the strengthening and weakening of the Siberian High and the Aleutian Low. The anomalous sea level pressure pattern dictates anomalous circulation, in compliance with the geostrophic relationship. Thermal advection, in addition to net surface radiation, partly contributes to temperature variations in winter. Latent and sensible heat fluxes (thermal forcing from the ocean to the atmosphere) increase with decreased thermal advection. Anomalous upper-level circulation is closely linked to the low-level temperature anomaly in terms of the thermal wind equation. The interannual variability of the seasonal cycle of the EAWM is strongly controlled by the relative strength of the Siberian High to the Aleutian Low. A stronger than normal gradient between the two pressure systems amplifies the seasonal cycle of the EAWM. The EAWM seasonal cycle in the mid-latitude region exhibits a weak negative correlation with the Arctic Oscillation and the East Atlantic/West Russia indices. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1491-0 Authors Yoojin Kim, School of Earth and Environmental Sciences, Seoul National University, 1 Gwanangno, Gwanak-gu, Seoul, 151-747 Republic of Korea Kwang-Yul Kim, School of Earth and Environmental Sciences, Seoul National University, 1 Gwanangno, Gwanak-gu, Seoul, 151-747 Republic of Korea Jong-Gap Jhun, School of Earth and Environmental Sciences, Seoul National University, 1 Gwanangno, Gwanak-gu, Seoul, 151-747 Republic of Korea Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Knowledge of cloud properties and their vertical structure is important for meteorological studies due to their impact on both the Earth’s radiation budget and adiabatic heating within the atmosphere. The objective of this study is to evaluate bulk cloud properties and vertical distribution simulated by the US National Oceanic and Atmospheric Administration National Centers for Environmental Prediction Global Forecast System (GFS) using three global satellite products. Cloud variables evaluated include the occurrence and fraction of clouds in up to three layers, cloud optical depth, liquid water path, and ice water path. Cloud vertical structure data are retrieved from both active (CloudSat/CALIPSO) and passive sensors and are subsequently compared with GFS model results. In general, the GFS model captures the spatial patterns of hydrometeors reasonably well and follows the general features seen in satellite measurements, but large discrepancies exist in low-level cloud properties. More boundary layer clouds over the interior continents were generated by the GFS model whereas satellite retrievals showed more low-level clouds over oceans. Although the frequencies of global multi-layer clouds from observations are similar to those from the model, latitudinal variations show discrepancies in terms of structure and pattern. The modeled cloud optical depth over storm track region and subtropical region is less than that from the passive sensor and is overestimated for deep convective clouds. The distributions of ice water path (IWP) agree better with satellite observations than do liquid water path (LWP) distributions. Discrepancies in LWP/IWP distributions between observations and the model are attributed to differences in cloud water mixing ratio and mean relative humidity fields, which are major control variables determining the formation of clouds. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1430-0 Authors Hyelim Yoo, Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20740, USA Zhanqing Li, Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20740, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Atlantic subpolar gyre (SPG) is one of the main drivers of decadal climate variability in the North Atlantic. Here we analyze its dynamics in pre-industrial control simulations of 19 different comprehensive coupled climate models. The analysis is based on a recently proposed description of the SPG dynamics that found the circulation to be potentially bistable due to a positive feedback mechanism including salt transport and enhanced deep convection in the SPG center. We employ a statistical method to identify multiple equilibria in time series that are subject to strong noise and analyze composite fields to assess whether the bistability results from the hypothesized feedback mechanism. Because noise dominates the time series in most models, multiple circulation modes can unambiguously be detected in only six models. Four of these six models confirm that the intensification is caused by the positive feedback mechanism. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1525-7 Authors Andreas Born, Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland Thomas F. Stocker, Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland Christoph C. Raible, Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland Anders Levermann, Potsdam Institute for Climate Impact Research, Potsdam, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Recent studies suggested that tropical cyclones (TCs) contribute significantly to the meridional oceanic heat transport by injecting heat into the subsurface through mixing. Here, we estimate the long-term oceanic impact of TCs by inserting realistic wind vortices along observed TCs tracks in a 1/2° resolution ocean general circulation model over the 1978–2007 period. Warming of TCs’ cold wakes results in a positive heat flux into the ocean (oceanic heat uptake; OHU) of ~480 TW, consistent with most recent estimates. However, ~2/5 of this OHU only compensates the heat extraction by the TCs winds during their passage. Another ~2/5 of this OHU is injected in the seasonal thermocline and hence released back to the atmosphere during the following winter. Because of zonal compensations and equatorward transport, only one-tenth of the OHU is actually exported poleward (46 TW), resulting in a marginal maximum contribution of TCs to the poleward ocean heat transport. Other usually neglected TC-related processes however impact the ocean mean state. The residual Ekman pumping associated with TCs results in a sea-level drop (rise) in the core (northern and southern flanks) of TC-basins that expand westward into the whole basin as a result of planetary wave propagation. More importantly, TC-induced mixing and air-sea fluxes cool the surface in TC-basins during summer, while the re-emergence of subsurface warm anomalies warms it during winter. This leads to a ~10 % reduction of the sea surface temperature seasonal cycle within TCs basins, which may impact the climate system. Content Type Journal Article Pages 1-20 DOI 10.1007/s00382-012-1556-0 Authors Emmanuel M. Vincent, LOCEAN-IPSL, IRD/CNRS/UPMC/MNHN, tour 45-55 4e, 4, place Jussieu, 75252 Paris Cedex 5, France Gurvan Madec, LOCEAN-IPSL, IRD/CNRS/UPMC/MNHN, tour 45-55 4e, 4, place Jussieu, 75252 Paris Cedex 5, France Matthieu Lengaigne, LOCEAN-IPSL, IRD/CNRS/UPMC/MNHN, tour 45-55 4e, 4, place Jussieu, 75252 Paris Cedex 5, France Jérôme Vialard, LOCEAN-IPSL, IRD/CNRS/UPMC/MNHN, tour 45-55 4e, 4, place Jussieu, 75252 Paris Cedex 5, France Ariane Koch-Larrouy, LEGOS, IRD/CNRS/UPS, Toulouse, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The characteristic features of Indian summer monsoon (ISM) and monsoon intraseasonal oscillations (MISO) are analyzed in the 25 year simulation by the superparameterized Community Climate System Model (SP-CCSM). The observations indicate the low frequency oscillation with a period of 30–60 day to have the highest power with a dominant northward propagation, while the faster mode of MISO with a period of 10–20 day shows a stationary pattern with no northward propagation. SP-CCSM simulates two dominant quasi-periodic oscillations with periods 15–30 day and 40–70 day indicating a systematic low frequency bias in simulating the observed modes. Further, contrary to the observation, the SP-CCSM 15–30 day mode has a significant northward propagation; while the 40–70 day mode does not show prominent northward propagation. The inability of the SP-CCSM to reproduce the observed modes correctly is shown to be linked with inability of the cloud resolving model (CRM) to reproduce the characteristic heating associated with the barotropic and baroclinic vertical structures of the high-frequency and the low-frequency modes. It appears that the superparameterization in the General Circulation Model (GCM) certainly improves seasonal mean model bias significantly. There is a need to improve the CRM through which the barotropic and baroclinic modes are simulated with proper space and time distribution. Content Type Journal Article Pages 1-11 DOI 10.1007/s00382-012-1563-1 Authors Bidyut B. Goswami, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008 India P. Mukhopadhyay, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008 India Marat Khairoutdinov, School of Marine and Atmospheric Science, New York University, Stony Brook, NY, USA B. N. Goswami, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008 India Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The boreal summer intraseasonal oscillation (BSISO) of the Asian summer monsoon (ASM) is one of the most prominent sources of short-term climate variability in the global monsoon system. Compared with the related Madden-Julian Oscillation (MJO) it is more complex in nature, with prominent northward propagation and variability extending much further from the equator. In order to facilitate detection, monitoring and prediction of the BSISO we suggest two real-time indices: BSISO1 and BSISO2, based on multivariate empirical orthogonal function (MV-EOF) analysis of daily anomalies of outgoing longwave radiation (OLR) and zonal wind at 850 hPa (U850) in the region 10°S–40°N, 40°–160°E, for the extended boreal summer (May–October) season over the 30-year period 1981–2010. BSISO1 is defined by the first two principal components (PCs) of the MV-EOF analysis, which together represent the canonical northward propagating variability that often occurs in conjunction with the eastward MJO with quasi-oscillating periods of 30–60 days. BSISO2 is defined by the third and fourth PCs, which together mainly capture the northward/northwestward propagating variability with periods of 10–30 days during primarily the pre-monsoon and monsoon-onset season. The BSISO1 circulation cells are more Rossby wave like with a northwest to southeast slope, whereas the circulation associated with BSISO2 is more elongated and front-like with a southwest to northeast slope. BSISO2 is shown to modulate the timing of the onset of Indian and South China Sea monsoons. Together, the two BSISO indices are capable of describing a large fraction of the total intraseasonal variability in the ASM region, and better represent the northward and northwestward propagation than the real-time multivariate MJO (RMM) index of Wheeler and Hendon. Content Type Journal Article Pages 1-17 DOI 10.1007/s00382-012-1544-4 Authors June-Yi Lee, International Pacific Research Center, University of Hawaii, POST Bldg, Room 409A, 1680 East–West Road, Honolulu, HI 96822, USA Bin Wang, International Pacific Research Center, University of Hawaii, POST Bldg, Room 409A, 1680 East–West Road, Honolulu, HI 96822, USA Matthew C. Wheeler, Centre for Australia Weather and Climate Research (CAWCR), Bureau of Meteorology, Melbourne, Australia Xiouhua Fu, International Pacific Research Center, University of Hawaii, POST Bldg, Room 409A, 1680 East–West Road, Honolulu, HI 96822, USA Duane E. Waliser, JIFRESSE, University of California, Los Angeles, USA In-Sik Kang, Seoul National University, Seoul, Korea Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Numerical models of climate have great difficulties with the simulation of marine low clouds in the subtropical Pacific and Atlantic Oceans. It has been especially difficult to reproduce the observed geographical distributions of the different cloud regimes in those regions. The present study discusses mechanisms proposed in previous works for changing one regime into another. One criterion is based on the theory of stratocumulus destruction through cloud top entrainment instability due to buoyancy reversal—situations in which the mixture of two air parcels becomes denser than either of the original parcels due to evaporation of cloud water. Another criterion is based on the existence of decoupling in the boundary layer. When decoupled, the stratocumulus regime changes to another in which these clouds can still exist together with cumulus. In a LES study, the authors have suggested that a combination of those two criteria can be used to diagnose whether, at a location, the cloud regime corresponds to a well-mixed stratocumulus regime, a shallow cumulus regime, or to a transitional regime where the boundary layer is decoupled. The concept is tested in the framework of an atmospheric general circulation model (GCM). It is found that several outstanding features of disagreement between simulation and observation can be interpreted as misrepresentations of the cloud regimes by the GCM. A novel criterion for switching among regimes is proposed to alleviate the effects of these misrepresentations. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1342-z Authors Heng Xiao, Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA Chien-Ming Wu, Department of Atmospheric Sciences, National Taiwan University, Taipei, 106 Taiwan C. Roberto Mechoso, Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA Hsi-Yen Ma, Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, CA, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Decadal climate predictability is examined in hindcast experiments by a multi-model ensemble using three versions of the coupled atmosphere-ocean model MIROC. In these hindcast experiments, initial conditions are obtained from an anomaly assimilation procedure using the observed oceanic temperature and salinity with prescribed natural and anthropogenic forcings on the basis of the historical data and future emission scenarios in the Intergovernmental Panel of Climate Change. Results of the multi-model ensemble in our hindcast experiments show that predictability of surface air temperature (SAT) anomalies on decadal timescales mostly originates from externally forced variability. Although the predictable component of internally generated variability has considerably smaller SAT variance than that of externally forced variability, ocean subsurface temperature variability has predictive skills over almost a decade, particularly in the North Pacific and the North Atlantic where dominant signals associated with Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are observed. Initialization enhances the predictive skills of AMO and PDO indices and slightly improves those of global mean temperature anomalies. Improvement of these predictive skills in the multi-model ensemble is higher than that in a single-model ensemble. Content Type Journal Article Pages 1-22 DOI 10.1007/s00382-012-1351-y Authors Yoshimitsu Chikamoto, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba, 277-8568 Japan Masahide Kimoto, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba, 277-8568 Japan Masayoshi Ishii, Meteorological Research Institute, Tsukuba, Japan Takashi Mochizuki, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Takashi T. Sakamoto, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Hiroaki Tatebe, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Yoshiki Komuro, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Masahiro Watanabe, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba, 277-8568 Japan Toru Nozawa, National Institute for Environmental Studies, Tsukuba, Japan Hideo Shiogama, National Institute for Environmental Studies, Tsukuba, Japan Masato Mori, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba, 277-8568 Japan Sayaka Yasunaka, National Institute for Environmental Studies, Tsukuba, Japan Yukiko Imada, Department of Civil and Environmental Engineering, Tokyo Institute of Technology, Tokyo, Japan Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Numerical experiments with different idealized land and mountain distributions are carried out to study the formation of the Asian monsoon and related coupling processes. Results demonstrate that when there is only extratropical continent located between 0 and 120°E and between 20/30°N and the North Pole, a rather weak monsoon rainband appears along the southern border of the continent, coexisting with an intense intertropical convergence zone (ITCZ). The continuous ITCZ surrounds the whole globe, prohibits the development of near-surface cross-equatorial flow, and collects water vapor from tropical oceans, resulting in very weak monsoon rainfall. When tropical lands are integrated, the ITCZ over the longitude domain where the extratropical continent exists disappears as a consequence of the development of a strong surface cross-equatorial flow from the winter hemisphere to the summer hemisphere. In addition, an intense interaction between the two hemispheres develops, tropical water vapor is transported to the subtropics by the enhanced poleward flow, and a prototype of the Asian monsoon appears. The Tibetan Plateau acts to enhance the coupling between the lower and upper tropospheric circulations and between the subtropical and tropical monsoon circulations, resulting in an intensification of the East Asian summer monsoon and a weakening of the South Asian summer monsoon. Linking the Iranian Plateau to the Tibetan Plateau substantially reduces the precipitation over Africa and increases the precipitation over the Arabian Sea and the northern Indian subcontinent, effectively contributing to the development of the South Asian summer monsoon. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1334-z Authors Guoxiong Wu, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Yimin Liu, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Buwen Dong, Department of Meteorology, National Centre for Atmospheric Science, University of Reading, Reading, UK Xiaoyun Liang, National Climate Center, China Meteorological Administration, Beijing, China Anmin Duan, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Qing Bao, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Jingjing Yu, National Meteorological Information Center, China Meteorological Administration, Beijing, China Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The spring asymmetric mode over the Tropical Indian Ocean (TIO) is characterized by contrasting patterns of rainfall and surface wind anomalies north and south of Equator. The asymmetric pattern in rainfall has evolved as a leading mode of variability in the TIO and is strongly correlated with El Niño-Southern Oscillation (ENSO) and positive Indian Ocean Dipole (IOD). The evolution of the asymmetric pattern in rainfall and surface wind during pure El Niño/IOD and co-occurrence years are examined in the twentieth century reanalysis for the period of 1871–2008 and atmospheric general circulation model (AGCM) simulations. The study revealed that spring asymmetric mode is well developed when El Niño co-occurred with IOD (positive) and is driven by the associated meridional gradients in sea surface temperature (SST) and sea level pressure (SLP). The pure El Niño composites are characterized by homogeneous (spatially) SST anomalies (positive) and weaker SLP gradients and convection, leading to weak asymmetric mode. The asymmetric mode is absent in the pure IOD (positive) composites due to the persistence of east west SST gradient for a longer duration than the co-occurrence years. The meridional gradient in SST anomalies over the TIO associated with the ENSO-IOD forcing is therefore crucial in developing/strengthening the spring asymmetric mode. The northwest Pacific anticyclonic circulation further strengthen the asymmetric mode in surface winds by inducing northeasterlies in the north Indian Ocean during pure El Niño and co-occurrence years. The simulations based on AGCM, forced by observed SSTs during the period of 1871–2000 supported the findings. The analysis of available station and ship track data further strengthens our results. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1340-1 Authors Soumi Chakravorty, Indian Institute of Tropical Meteorology, Pune, 411008 India J. S. Chowdary, Indian Institute of Tropical Meteorology, Pune, 411008 India C. Gnanaseelan, Indian Institute of Tropical Meteorology, Pune, 411008 India Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Indian summer monsoon rainfall (ISMR) over the Western Ghats (WG) and the Bay of Bengal (BoB) is marked by the intraseasonal oscillations (ISOs) with preferred 10–20-day and 30–50-day bands. On the basis of pentad Climate Prediction Center Merged Analysis Precipitation and daily sea level pressure and winds at 850 hPa derived from European Center for Medium-range Weather Forecast reanalysis, we present the structure and evolution of the ISOs linked to the ISMR variations over the WG and the BoB and the associated anomalies of the atmospheric circulation using the approaches of wavelet analysis, bandpass filtering and composite analysis. This study reveals that the activities of both the intertropical convergence zone (ITCZ) and the western Pacific subtropical high (WPSH) contribute strongly to the structure and propagation of the ISOs on intraseasonal time scales. Northward development and propagation of the ITCZ plays a critical role in the northward-propagating ISOs, but not in the westward-propagating BoB 10–20-day ISOs. The latter ISOs may be linked, instead, to the activity of synoptic-scale weather systems to the east over the western tropical Pacific. The enhanced ITCZ in the tropical Indian Ocean plays a strong role in the sudden strengthening of the WPSH during the transition from the break to active phase of the 30–50-day ISOs. We find that the strong WPSH in the Asian summer monsoon season, with generally northward advance and eastward withdrawal, promotes the formation of a northwest to southeast tilted anomalous rainfall belt over the East Asian tropical summer monsoon region and the western tropical Pacific in the 30–50-day low-frequency band. Positive (Negative) elongated rainfall anomalies with an unbroken northwest-southeast tilt, strong easterly (westerly) anomalies in the tropical Pacific, and northward advance and eastward movement of strong (weak) WPSH are favorable for maintaining the eastward propagation of the 30–50-day ISOs in the Pacific. Daily high-resolution sea surface temperature obtained from the National Oceanic and Atmospheric Administration is used to explain the propagation features of the 10–20-day ISOs in the Indian Ocean. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1352-x Authors S. Ma, Institut Català de Ciències del Clima (IC3), C/Doctor Trueta 203, 08005 Barcelona, Catalunya, Spain X. Rodó, Institut Català de Ciències del Clima (IC3), C/Doctor Trueta 203, 08005 Barcelona, Catalunya, Spain Y. Song, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA B. A. Cash, Center for Ocean-Land–Atmosphere Studies, Calverton, MD, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Under the background of a warming climate, regional climate responses may be different from place to place. How cold extreme events in China respond is still an open question. This study investigates responses of coldwave frequency (CWF) in China from observation and modeling perspectives. Observational evidences show that CWF significantly reduces across China during the warm period (1978–2009) in comparison with that during the cold period (1957–1977), concurrent with extreme value centers located in northern China during 1957–1977 and southern China during 1978–2009. The empirical orthogonal function (EOF) leading mode of CWF in the cold period is also dominant by an extreme value center prevailing over northern China, while the center exhibits a southward shift in the warm period. A seven-member multi-model ensemble (MME) from coupled model intercomparison project#3 (CMIP3) shows that southern China tends to experience more coldwaves than northern China in the twenty first century (2045–2064 and 2080–2099) under the global warming A1B forcing (with atmospheric CO 2 concentration of 720 ppm). This feature can also be seen in the leading EOF mode of MME. These results indicate that the primary response of CWF to a warming climate may be the southward shift of the maximum loading center. The enhanced western Pacific Subtropical High and weakened Siberian High during 1978–2009 may result in anomalous southerlies which bring warm and wet air to southern China. Meanwhile cold and dry air is transported from the north via a “northwest pathway” to southern China. Under the joint action of these two air masses, coldwaves may easily generate in southern China as observed in recent extreme cold events in this region. Content Type Journal Article Pages 1-10 DOI 10.1007/s00382-012-1354-8 Authors Tingting Ma, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China Zhiwei Wu, Meteorological Research Division, Environment Canada, Dorval, QC H9P 1J3, Canada Zhihong Jiang, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The interannual variation of precipitation in the southern part of Iran and its link with the large-scale climate modes are examined using monthly data from 183 meteorological stations during 1974–2005. The majority of precipitation occurs during the rainy season from October to May. The interannual variation in fall and early winter during the first part of the rainy season shows apparently a significant positive correlation with the Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO). However, a partial correlation analysis used to extract the respective influence of IOD and ENSO shows a significant positive correlation only with the IOD and not with ENSO. The southeasterly moisture flux anomaly over the Arabian Sea turns anti-cyclonically and transport more moisture to the southern part of Iran from the Arabian Sea, the Red Sea, and the Persian Gulf during the positive IOD. On the other hand, the moisture flux has northerly anomaly over Iran during the negative IOD, which results in reduced moisture supply from the south. During the latter part of the rainy season in late winter and spring, the interannual variation of precipitation is more strongly influenced by modes of variability over the Mediterranean Sea. The induced large-scale atmospheric circulation anomaly controls moisture supply from the Red Sea and the Persian Gulf. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1357-5 Authors Farnaz Pourasghar, Department of Physical Geography, Faculty of Humanities and Social Science, The University of Tabriz, Tabriz, Iran Tomoki Tozuka, Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan Saeed Jahanbakhsh, Department of Physical Geography, Faculty of Humanities and Social Science, The University of Tabriz, Tabriz, Iran Behrooz Sari Sarraf, Department of Physical Geography, Faculty of Humanities and Social Science, The University of Tabriz, Tabriz, Iran Hooshang Ghaemi, Iran Meteorological Organization, Tehran, Iran Toshio Yamagata, Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The first-branch northward-propagating intraseasonal oscillation (FNISO) over the tropical Indian Ocean (IO) often triggers the onset of the Asian summer monsoon. In this study we investigate the structures and mechanisms associated with FNISO through the diagnosis of ERA-Interim reanalysis data for the period of 1990–2009. A composite analysis is conducted to reveal the structure and evolution characteristics of the FNISO and associated background circulation changes. It is found that the FNISO convection originates from the southwestern IO and propagates eastward. After reaching the eastern IO, the major convective branch moves northward toward the northern Bay of Bengal (BoB). Two possible mechanisms may contribute to the northward propagation of the FNISO. One is the meridional asymmetry of the background convective instability. A greater background convective instability over the northern BoB may destabilize Rossby waves and cause convection to shift northward. The other is the meridional phase leading of perturbation humidity in the planetary boundary layer (PBL). Maximum PBL moisture appears to the north of the convection center, which promotes a convectively unstable stratification ahead of the convection and leads to the northward propagation of the FNISO. A PBL moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1492-z Authors Kuiping Li, Center for Ocean and Climate Research, First Institute of Oceanography, SOA, Qingdao, 266061 China Weidong Yu, Center for Ocean and Climate Research, First Institute of Oceanography, SOA, Qingdao, 266061 China Tim Li, IPRC and Department of Meteorology, University of Hawaii, Honolulu, HI, USA V. S. N. Murty, National Institute of Oceanography Regional Centre, Visakhapatnam, 530017 India Somkiat Khokiattiwong, Phuket Marine Biology Center, Phuket, Thailand T. R. Adi, Agency for Marine and Fishery Research and Development, Center for Marine and Coastal Resources Research and Development, Jakarta, Indonesia S. Budi, Agency for Marine and Fishery Research and Development, Center for Marine and Coastal Resources Research and Development, Jakarta, Indonesia Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In this study the results of the regional climate model COSMO-CLM (CCLM) covering the Greater Alpine Region (GAR, 4°–19°W and 43°–49°N) were evaluated against observational data. The simulation was carried out as a hindcast run driven by ERA-40 reanalysis data for the period 1961–2000. The spatial resolution of the model data presented is approx. 10 km per grid point. For the evaluation purposes a variety of observational datasets were used: CRU TS 2.1, E-OBS, GPCC4 and HISTALP. Simple statistics such as mean biases, correlations, trends and annual cycles of temperature and precipitation for different sub-regions were applied to verify the model performance. Furthermore, the altitude dependence of these statistical measures has been taken into account. Compared to the CRU and E-OBS datasets CCLM shows an annual mean cold bias of −0.6 and −0.7 °C, respectively. Seasonal precipitation sums are generally overestimated by +8 to +23 % depending on the observational dataset with large variations in space and season. Bias and correlation show a dependency on altitude especially in the winter and summer seasons. Temperature trends in CCLM contradict the signals from observations, showing negative trends in summer and autumn which are in contrast to CRU and E-OBS. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1452-7 Authors Klaus Haslinger, Climate Research Department, Zentralanstalt für Meteorologie und Geodynamik (ZAMG), Hohe Warte 38, 1190 Vienna, Austria Ivonne Anders, Climate Research Department, Zentralanstalt für Meteorologie und Geodynamik (ZAMG), Hohe Warte 38, 1190 Vienna, Austria Michael Hofstätter, Climate Research Department, Zentralanstalt für Meteorologie und Geodynamik (ZAMG), Hohe Warte 38, 1190 Vienna, Austria Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The current literature provides compelling evidence suggesting that an eddy-resolving (as opposed to eddy-permitting or eddy-parameterized) ocean component model will significantly impact the simulation of the large-scale climate, although this has not been fully tested to date in multi-decadal global coupled climate simulations. The purpose of this paper is to examine how resolved ocean fronts and eddies impact the simulation of large-scale climate. The model used for this study is the NCAR Community Climate System Model version 3.5 (CCSM3.5)—the forerunner to CCSM4. Two experiments are reported here. The control experiment is a 155-year present-day climate simulation using a 0.5° atmosphere component (zonal resolution 0.625 meridional resolution 0.5°; land surface component at the same resolution) coupled to ocean and sea-ice components with zonal resolution of 1.2° and meridional resolution varying from 0.27° at the equator to 0.54° in the mid-latitudes. The second simulation uses the same atmospheric and land-surface models coupled to eddy-resolving 0.1° ocean and sea-ice component models. The simulations are compared in terms of how the representation of smaller scale features in the time mean ocean circulation and ocean eddies impact the mean and variable climate. In terms of the global mean surface temperature, the enhanced ocean resolution leads to a ubiquitous surface warming with a global mean surface temperature increase of about 0.2 °C relative to the control. The warming is largest in the Arctic and regions of strong ocean fronts and ocean eddy activity (i.e., Southern Ocean, western boundary currents). The Arctic warming is associated with significant losses of sea-ice in the high-resolution simulation. The sea surface temperature gradients in the North Atlantic, in particular, are better resolved in the high-resolution model leading to significantly sharper temperature gradients and associated large-scale shifts in the rainfall. In the extra-tropics, the interannual temperature variability is increased with the resolved eddies, and a notable increases in the amplitude of the El Niño and the Southern Oscillation is also detected. Changes in global temperature anomaly teleconnections and local air-sea feedbacks are also documented and show large changes in ocean–atmosphere coupling. In particular, local air-sea feedbacks are significantly modified by the increased ocean resolution. In the high-resolution simulation in the extra-tropics there is compelling evidence of stronger forcing of the atmosphere by SST variability arising from ocean dynamics. This coupling is very weak or absent in the low-resolution model. Content Type Journal Article Pages 1-26 DOI 10.1007/s00382-012-1500-3 Authors Ben P. Kirtman, Rosenstiel School for Marine and Atmospheric Science, University of Miami, Coral Gables, FL, USA Cecilia Bitz, Department of Atmospheric Science, University of Washington, Seattle, WA, USA Frank Bryan, National Center for Atmospheric Research, Boulder, CO, USA William Collins, University of California, Berkeley, Berkeley, CA, USA John Dennis, National Center for Atmospheric Research, Boulder, CO, USA Nathan Hearn, National Center for Atmospheric Research, Boulder, CO, USA James L. Kinter III, Center for Ocean-Land–Atmosphere Studies, Calverton, MD, USA Richard Loft, National Center for Atmospheric Research, Boulder, CO, USA Clement Rousset, Rosenstiel School for Marine and Atmospheric Science, University of Miami, Coral Gables, FL, USA Leo Siqueira, Rosenstiel School for Marine and Atmospheric Science, University of Miami, Coral Gables, FL, USA Cristiana Stan, Center for Ocean-Land–Atmosphere Studies, Calverton, MD, USA Robert Tomas, National Center for Atmospheric Research, Boulder, CO, USA Mariana Vertenstein, National Center for Atmospheric Research, Boulder, CO, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Arctic climate change is analyzed in an ensemble of future projection simulations performed with the global coupled climate model EC-Earth2.3. EC-Earth simulates the twentieth century Arctic climate relatively well but the Arctic is about 2 K too cold and the sea ice thickness and extent are overestimated. In the twenty-first century, the results show a continuation and strengthening of the Arctic trends observed over the recent decades, which leads to a dramatically changed Arctic climate, especially in the high emission scenario RCP8.5. The annually averaged Arctic mean near-surface temperature increases by 12 K in RCP8.5, with largest warming in the Barents Sea region. The warming is most pronounced in winter and autumn and in the lower atmosphere. The Arctic winter temperature inversion is reduced in all scenarios and disappears in RCP8.5. The Arctic becomes ice free in September in all RCP8.5 simulations after a rapid reduction event without recovery around year 2060. Taking into account the overestimation of ice in the twentieth century, our model results indicate a likely ice-free Arctic in September around 2040. Sea ice reductions are most pronounced in the Barents Sea in all RCPs, which lead to the most dramatic changes in this region. Here, surface heat fluxes are strongly enhanced and the cloudiness is substantially decreased. The meridional heat flux into the Arctic is reduced in the atmosphere but increases in the ocean. This oceanic increase is dominated by an enhanced heat flux into the Barents Sea, which strongly contributes to the large sea ice reduction and surface-air warming in this region. Increased precipitation and river runoff lead to more freshwater input into the Arctic Ocean. However, most of the additional freshwater is stored in the Arctic Ocean while the total Arctic freshwater export only slightly increases. Content Type Journal Article Pages 1-25 DOI 10.1007/s00382-012-1505-y Authors Torben Koenigk, Rossby Centre, Swedish Meteorological and Hydrological Institute, 60176 Norrköping, Sweden Laurent Brodeau, Department of Meteorology and Bert Bolin Centre for Climate Research, Stockholm University, 106 54 Stockholm, Sweden Rune Grand Graversen, Department of Meteorology and Bert Bolin Centre for Climate Research, Stockholm University, 106 54 Stockholm, Sweden Johannes Karlsson, Department of Meteorology and Bert Bolin Centre for Climate Research, Stockholm University, 106 54 Stockholm, Sweden Gunilla Svensson, Department of Meteorology and Bert Bolin Centre for Climate Research, Stockholm University, 106 54 Stockholm, Sweden Michael Tjernström, Department of Meteorology and Bert Bolin Centre for Climate Research, Stockholm University, 106 54 Stockholm, Sweden Ulrika Willén, Rossby Centre, Swedish Meteorological and Hydrological Institute, 60176 Norrköping, Sweden Klaus Wyser, Rossby Centre, Swedish Meteorological and Hydrological Institute, 60176 Norrköping, Sweden Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The date of onset of the southwest monsoon in western India is critical for farmers as it influences the timing of crop plantation and the duration of the summer rainy season. Identifying long-term variability in the date of monsoon onset is difficult, however, as onset dates derived from the reanalysis of instrumental rainfall data are only available for the region from 1879. This study uses documentary evidence and newly uncovered instrumental data to reconstruct annual monsoon onset dates for western India for the period 1781–1878, extending the existing record by 97 years. The mean date of monsoon onset over the Mumbai (Bombay) area during the reconstruction period was 10 June with a standard deviation of 6.9 days. This is similar to the mean and standard deviation of the date of monsoon onset derived from instrumental data for the twentieth century. The earliest identified onset date was 23 May (in 1802 and 1839) and the latest 22 June (in 1825). The longer-term perspective provided by this study suggests that the climatic regime that governs monsoon advance over western India did not change substantially from 1781 to 1955. Monsoon onset over Mumbai has occurred at a generally later date since this time. Our results indicate that this change is unprecedented during the last 230 years. Following a discussion of the results, the nature of the relationship between the date of monsoon onset and the El Niño-Southern Oscillation is discussed. This relationship is shown to have been stable since 1781. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1494-x Authors George C. D. Adamson, School of Environment and Technology, University of Brighton, Lewes Road, Brighton, BN2 4GJ UK David J. Nash, School of Environment and Technology, University of Brighton, Lewes Road, Brighton, BN2 4GJ UK Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This paper analyzes seasonal and diurnal variations of MODerate resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) data at ~1.1 km for the period of 2003–2011 over a region in West-Central Texas, where four of the world’s largest wind farms are located. Seasonal anomalies are created from MODIS Terra (~10:30 a.m. and 10:30 p.m. local solar time) and Aqua (~1:30 a.m. and 1:30 p.m. local solar time) LSTs, and their spatiotemporal variability is analyzed by comparing the LST changes between wind farm pixels (WFPs) and nearby non wind farm pixels (NNWFPs) using different methods under different quality controls. Our analyses show consistently that there is a warming effect of 0.31–0.70 °C at nighttime for the nine-year period during which data was collected over WFPs relative to NNWFPs, in all seasons for both Terra and Aqua measurements, while the changes at daytime are much noisier. The nighttime warming effect is much larger in summer than winter and at ~10:30 p.m. than ~1:30 a.m. and hence the largest warming effect is observed at ~10:30 p.m. in summer. The spatial pattern and magnitude of this warming effect couple very well with the geographic distribution of wind turbines and such coupling is stronger at nighttime than daytime and in summer than winter. Together, these results suggest that the warming effect observed in MODIS over wind farms are very likely attributable to the development of wind farms. This inference is consistent with the increasing number of operational wind turbines with time during the study period, the diurnal and seasonal variations in the frequency of wind speed and direction distribution, and the changes in near-surface atmospheric boundary layer (ABL) conditions due to wind farm operations. The nocturnal ABL is typically stable and much thinner than the daytime ABL and hence the turbine enhanced vertical mixing produces a stronger nighttime effect. The stronger wind speed and the higher frequency of the wind speed within the optimal power generation range in summer than winter and at nighttime than daytime likely drives wind turbines to generate more electricity and turbulence and consequently results in the strongest warming effect at nighttime in summer. Similarly, the stronger wind speed and the higher frequency of optimal wind speed at ~10:30 p.m. than that at ~1:30 a.m. might help explain, to some extent, why the nighttime LST warming effect is slightly larger at ~10:30 p.m. than ~1:30 a.m. The nighttime warming effect seen in spring and fall are smaller than that in summer and can be explained similarly. Content Type Journal Article Pages 1-20 DOI 10.1007/s00382-012-1485-y Authors Liming Zhou, Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, 1400 Washington Avenue, Albany, NY 12222, USA Yuhong Tian, IMSG at NOAA/NESDIS/STAR, Camp Springs, MD 20746, USA Somnath Baidya Roy, Department of Atmospheric Sciences, University of Illinois, 105 South Gregory Street, Urbana, IL 61801, USA Yongjiu Dai, School of Geography, Beijing Normal University, Beijing, 100875 China Haishan Chen, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, 210044 China Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Atlantic Warm Pool (AWP) region, which is comprised of the Gulf of Mexico, Caribbean Sea and parts of the northwestern tropical Atlantic Ocean, is one of the most poorly observed parts of the global oceans. This study compares three ocean reanalyses, namely the Global Ocean Data Assimilation System of National Centers for Environmental Prediction (NCEP), the Climate Forecast System Reanalysis (CFSR) of NCEP, and the Simple Ocean Data Assimilation (SODA) for its AWP variation. The surface temperature in these ocean reanalyses is also compared with that from the Extended Range SST version 3 and Optimally Interpolated SST version 2 SST analyses. In addition we also compare three atmospheric reanalyses: NCEP-NCAR (R1), NCEP-DOE (R2), and CFSR for the associated atmospheric variability with the AWP. The comparison shows that there are important differences in the climatology of the AWP and its interannual variations. There are considerable differences in the subsurface ocean manifestation of the AWP with SODA (CFSR) showing the least (largest) modulation of the subsurface ocean temperatures. The remote teleconnections with the tropical Indian Ocean are also different across the reanalyses. However, all three oceanic reanalyses consistently show the absence of any teleconnection with the eastern equatorial Pacific Ocean. The influence of the AWP on the tropospheric temperature anomalies last for up to a one season lead and it is found to be relatively weak in R1 reanalyses. A simplified SST anomaly equation initially derived for diagnosing El Niño Southern Oscillation variability is adapted for the AWP variations in this study. The analysis of this equation reveals that the main contribution of the SST variation in the AWP region is from the variability of the net heat flux. All three reanalyses consistently show that the role of the ocean advective terms, including that associated with upwelling in the AWP region, is comparatively much smaller. The covariance of the SST tendency in the AWP with the net heat flux is large, with significant contributions from the variations of the surface shortwave and longwave fluxes. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1503-0 Authors Vasubandhu Misra, Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA Ashley Stroman, Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA Steven DiNapoli, Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, FL 32306, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This paper identifies two sources of uncertainties in model projections of temperature and precipitation: internal and inter-model variability. Eight models of WCRP-CMIP3 and WCRP-CMIP5 were compared to identify improvements in the reliability of projections from new generation models. While no significant differences are observed between both datasets, some improvements were found in the new generation models. For example, in summer CMIP5 inter-model variability of temperature was lower over northeastern Argentina, Paraguay and northern Brazil, in the last decades of the 21st century. Reliability of temperature projections from both sets of models is high, with signal to noise ratio greater than 1 over most of the study region. Although no major differences were observed in both precipitation datasets, CMIP5 inter-model variability was lower over northern and eastern Brazil in summer (especially at the end of the 21st century). Reliability of precipitation projections was low in both datasets. However, the signal to noise ratio in new generation models was close to 1, and even greater than 1 over eastern Argentina, Uruguay and southern Brazil in some seasons. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1489-7 Authors Josefina Blázquez, Centro de Investigaciones del Mar y la Atmósfera (CIMA-CONICET/FCENUBA), Ciudad Universitaria Pabellón II Piso 2, C1428EGA, Buenos Aires, Argentina Mario N. Nuñez, Centro de Investigaciones del Mar y la Atmósfera (CIMA-CONICET/FCENUBA), Ciudad Universitaria Pabellón II Piso 2, C1428EGA, Buenos Aires, Argentina Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Recent studies have shown that the Madden–Julian Oscillation (MJO) impacts the leading modes of intraseasonal variability in the northern hemisphere extratropics, providing a possible source of predictive skill over North America at intraseasonal timescales. We find that a k-means cluster analysis of mid-level geopotential height anomalies over the North American region identifies several wintertime cluster patterns whose probabilities are strongly modulated during and after MJO events, particularly during certain phases of the El Niño-Southern Oscillation (ENSO). We use a simple new optimization method for determining the number of clusters, k , and show that it results in a set of clusters which are robust to changes in the domain or time period examined. Several of the resulting cluster patterns resemble linear combinations of the Arctic Oscillation (AO) and the Pacific/North American (PNA) teleconnection pattern, but show even stronger responses to the MJO and ENSO than clusters based on the AO and PNA alone. A cluster resembling the positive (negative) PNA has elevated probabilities approximately 8–14 days following phase 6 (phase 3) of the MJO, while a negative AO-like cluster has elevated probabilities 10–20 days following phase 7 of the MJO. The observed relationships are relatively well reproduced in the 11-year daily reforecast dataset from the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2). This study statistically links MJO activity in the tropics to common intraseasonal circulation anomalies over the North American sector, establishing a framework that may be useful for improving extended range forecasts over this region. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1493-y Authors Emily E. Riddle, Climate Prediction Center, NCEP/NWS/NOAA, 5830 University Research Court, College Park, MD 20740, USA Marshall B. Stoner, Climate Prediction Center, NCEP/NWS/NOAA, 5830 University Research Court, College Park, MD 20740, USA Nathaniel C. Johnson, International Pacific Research Center, School of Ocean Earth Science and Technology, University of Hawaii, POST Bldg., Room 401, 1680 East-West Rd., Honolulu, HI 96822, USA Michelle L. L’Heureux, Climate Prediction Center, NCEP/NWS/NOAA, 5830 University Research Court, College Park, MD 20740, USA Dan C. Collins, Climate Prediction Center, NCEP/NWS/NOAA, 5830 University Research Court, College Park, MD 20740, USA Steven B. Feldstein, Department of Meteorology, The Pennsylvania State University, 503 Walker Building, University Park, PA 16802, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The mean spatiotemporal variations in tropopause parameters over the tropics (±35°, in latitude) in the Indian monsoon region are examined using the upper air data for an extended period obtained from radiosonde and Radio Occultation measurements. In general, the altitude of cold point tropopause (CPT) is a minimum near the equator and increases with latitude on either side. While CPT over the entire southern tropical latitudes and northern equatorial region is cooler (higher) during boreal winter and warmer (lower) during boreal summer, the annual pattern of CPT-temperature reverses in the northern hemispheric off-equatorial region. The temperature of lapse rate tropopause (LRT) is always negatively correlated with its altitude. While the annual variation of LRT-temperature in tropics is always positively correlated with CPT-temperature, the annual variation of LRT-altitude differs mainly in the off-equatorial regions. While the altitude of the convective tropopause is positively correlated with CPT-altitude over the latitude region 20°S–5°N, they are negatively correlated at the north of 10°N. In general, the tropical tropopause layer (TTL) is very thin (~3 km) near the equator and its thickness increases with latitude on either side of the equator to reach a peak value (of ~6 km) around ±30°. A pronounced decrease in TTL-thickness observed over the northern off-equatorial region during the ASM period can be attributed to the manifestation of very deep convection over the land near the Head Bay-of-Bengal region. The TTL-lapse-rate (γ TTL ) is large in the equatorial region and decreases with increase in latitude. While γ TTL in the northern hemispheric off-equatorial region is low during winter, it increases and becomes comparable to that over equatorial region during the ASM period. The annual variations in CPT parameters as well as the TTL- thickness are significantly modulated by quasi-biennial oscillation and the El Niño Southern Oscillation. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1496-8 Authors S. V. Sunilkumar, Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, 695022 Kerala, India Asha Babu, Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, 695022 Kerala, India K. Parameswaran, Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, 695022 Kerala, India Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In order to evaluate the future potential benefits of emission regulation on regional air quality, while taking into account the effects of climate change, off-line air quality projection simulations are driven using weather forcing taken from regional climate models. These regional models are themselves driven by simulations carried out using global climate models (GCM) and economical scenarios. Uncertainties and biases in climate models introduce an additional “climate modeling” source of uncertainty that is to be added to all other types of uncertainties in air quality modeling for policy evaluation. In this article we evaluate the changes in air quality-related weather variables induced by replacing reanalyses-forced by GCM-forced regional climate simulations. As an example we use GCM simulations carried out in the framework of the ERA-interim programme and of the CMIP5 project using the Institut Pierre-Simon Laplace climate model (IPSLcm), driving regional simulations performed in the framework of the EURO-CORDEX programme. In summer, we found compensating deficiencies acting on photochemistry: an overestimation by GCM-driven weather due to a positive bias in short-wave radiation, a negative bias in wind speed, too many stagnant episodes, and a negative temperature bias. In winter, air quality is mostly driven by dispersion, and we could not identify significant differences in either wind or planetary boundary layer height statistics between GCM-driven and reanalyses-driven regional simulations. However, precipitation appears largely overestimated in GCM-driven simulations, which could significantly affect the simulation of aerosol concentrations. The identification of these biases will help interpreting results of future air quality simulations using these data. Despite these, we conclude that the identified differences should not lead to major difficulties in using GCM-driven regional climate simulations for air quality projections. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1345-9 Authors Laurent Menut, Institut P.-S. Laplace, Laboratoire de Météorologie Dynamique, CNRS UMR 8539, Ecole Polytechnique, Palaiseau, France Om P. Tripathi, Institut P.-S. Laplace, Laboratoire de Météorologie Dynamique, CNRS UMR 8539, Ecole Polytechnique, Palaiseau, France Augustin Colette, INERIS, Institut National de l’Environnement Industriel et des Risques, Parc technologique ALATA, Verneuil en Halatte, France Robert Vautard, Laboratoire des Sciences du Climat et de l’Environnement, IPSL/CEA, Gif sur Yvette, France Emmanouil Flaounas, Institut P.-S. Laplace, Laboratoire de Météorologie Dynamique, CNRS UMR 8539, Ecole Polytechnique, Palaiseau, France Bertrand Bessagnet, INERIS, Institut National de l’Environnement Industriel et des Risques, Parc technologique ALATA, Verneuil en Halatte, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    High-resolution sedimentary paleoclimate proxy records offer the potential to expand the detection and analysis of decadal- to centennial-scale climate variability during recent millennia, particularly within regions where traditional high-resolution proxies may be short, sparse, or absent. However, time uncertainty in these records potentially limits a straightforward objective identification of broad-scale patterns of climate variability. Here, we describe a procedure for identifying common patterns of spatiotemporal variability from time uncertain sedimentary records. This approach, which we term Monte Carlo Empirical Orthogonal Function analysis, uses iterative age modeling and eigendecomposition of proxy time series to isolate common regional patterns and estimate uncertainties. As a test case, we apply this procedure to a diverse set of time-uncertain lacustrine proxy records from East Africa. We also perform a pseudoproxy experiment using climate model output to examine the ability of the method to extract shared anomalies given known signals. We discuss the advantages and disadvantages of our approach, including possible extensions of the technique. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1483-0 Authors Kevin J. Anchukaitis, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA Jessica E. Tierney, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Increasing concentrations of atmospheric CO 2 influence climate, terrestrial biosphere productivity and ecosystem carbon storage through its radiative, physiological and fertilization effects. In this paper, we quantify these effects for a doubling of CO 2 using a low resolution configuration of the coupled model NCAR CCSM4. In contrast to previous coupled climate-carbon modeling studies, we focus on the near-equilibrium response of the terrestrial carbon cycle. For a doubling of CO 2 , the radiative effect on the physical climate system causes global mean surface air temperature to increase by 2.14 K, whereas the physiological and fertilization on the land biosphere effects cause a warming of 0.22 K, suggesting that these later effects increase global warming by about 10 % as found in many recent studies. The CO 2 -fertilization leads to total ecosystem carbon gain of 371 Gt-C (28 %) while the radiative effect causes a loss of 131 Gt-C (~10 %) indicating that climate warming damps the fertilization-induced carbon uptake over land. Our model-based estimate for the maximum potential terrestrial carbon uptake resulting from a doubling of atmospheric CO 2 concentration (285–570 ppm) is only 242 Gt-C. This highlights the limited storage capacity of the terrestrial carbon reservoir. We also find that the terrestrial carbon storage sensitivity to changes in CO 2 and temperature have been estimated to be lower in previous transient simulations because of lags in the climate-carbon system. Our model simulations indicate that the time scale of terrestrial carbon cycle response is greater than 500 years for CO 2 -fertilization and about 200 years for temperature perturbations. We also find that dynamic changes in vegetation amplify the terrestrial carbon storage sensitivity relative to a static vegetation case: because of changes in tree cover, changes in total ecosystem carbon for CO 2 -direct and climate effects are amplified by 88 and 72 %, respectively, in simulations with dynamic vegetation when compared to static vegetation simulations. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1495-9 Authors G. Bala, Divecha Center for Climate Change, Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, 560012 India Sujith Krishna, Divecha Center for Climate Change, Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, 560012 India Devaraju Narayanappa, Divecha Center for Climate Change, Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, 560012 India Long Cao, Department of Earth Sciences, Zhejiang University, Hangzhou, 310027 Zhejiang Province, China Ken Caldeira, Department of Global Ecology, Carnegie Institution, 260 Panama Street, Stanford, CA 94305, USA Ramakrishna Nemani, NASA Ames Research Center, Moffett Field, CA 94035, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    We propose linear response functions to separately estimate the sea-level contributions of thermal expansion and solid ice discharge from Greenland and Antarctica. The response function formalism introduces a time-dependence which allows for future rates of sea-level rise to be influenced by past climate variations. We find that this time-dependence is of the same functional type, R ( t ) ∼ t α , for each of the three subsystems considered here. The validity of the approach is assessed by comparing the sea-level estimates obtained via the response functions to projections from comprehensive models. The pure vertical diffusion case in one dimension, corresponding to α =  −0.5, is a valid approximation for thermal expansion within the ocean up to the middle of the twenty first century for all Representative Concentration Pathways. The approximation is significantly improved for α =  − 0.7. For the solid ice discharge from Greenland we find an optimal value of α =  −0.7. Different from earlier studies we conclude that solid ice discharge from Greenland due to dynamic thinning is bounded by 0.42 m sea-level equivalent. Ice discharge induced by surface warming on Antarctica is best captured by a positive value of α = 0.1 which reflects the fact that ice loss increases with the cumulative amount of heat available for softening the ice in our model. Content Type Journal Article Pages 1-10 DOI 10.1007/s00382-012-1471-4 Authors Ricarda Winkelmann, Potsdam Institute for Climate Impact Research, Potsdam, Germany Anders Levermann, Potsdam Institute for Climate Impact Research, Potsdam, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The weakening relationship of El Nino with Indian summer monsoon reported in recent years is a major issue to be addressed. The altered relationships of Indian monsoon with various parameters excite to search for other dominant modes of variability that can influence the precipitation pattern. Since the Indian summer monsoon circulation originates in the oceanic region of the southern hemisphere, the present study investigates the association of southern extratropical influence on Indian summer monsoon using rainfall and reanalysis parameters. The effect of Southern Annular Mode (SAM) index during the month of June associated with the onset phase of Indian summer monsoon and that during July–August linked with the active phase of the monsoon were analysed separately for a period from 1951 to 2008. The extra-tropical influence over the monsoon is illustrated by using rainfall, specific humidity, vertical velocity, circulation and moisture transport. The June high SAM index enhances the lower level wind flow during the onset phase of monsoon over Indian sub-continent. The area of significant positive correlation between precipitation and SAM in June also shows enhancement in both ascending motion and specific humidity during the strong phase of June SAM. On the other hand, the June high SAM index adversely affects July–August monsoon over Indian subcontinent. The lower level wind flow weakens due to the high SAM. Enhancement of divergence and reduction in moisture transport results in the Indian monsoon region due to the activity of this high southern annular mode. The effect is more pronounced over the southwest region where the precipitation spell has high activity during the period. Significant correlation exists between SAM and ISMR, even after removing the effect of El Nino. It indicates that the signals of Indian summer monsoon characteristics can be envisaged to a certain extend using the June SAM index. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1509-7 Authors Nithin Viswambharan, Department of Atmospheric Sciences, Cochin University of Science and Technology, Lakeside Campus, Fine Arts Avenue, Cochin, 682016 India K. Mohanakumar, Department of Atmospheric Sciences, Cochin University of Science and Technology, Lakeside Campus, Fine Arts Avenue, Cochin, 682016 India Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In order to model stratocumulus clouds and coastal fog, we have coupled the University of Washington boundary layer model to the regional climate model, RegCM (RegCM-UW). By comparing fog occurrences observed at various coastal airports in the western United States, we show that RegCM-UW has success at modeling the spatial and temporal (diurnal, seasonal, and interannual) climatology of northern California coastal fog. The quality of the modeled fog estimate depends on whether coast-adjacent ocean or land grid cells are used; for the model runs shown here, the oceanic grid cells seem to be most appropriate. The interannual variability of oceanic northern California summertime fog, from a multi-decadal simulation, has a high and statistically significant correlation with the observed interannual variability ( r  = 0.72), which indicates that RegCM-UW is capable of investigating the response of fog to long-term climatological forcing. While RegCM-UW has a number of aspects that would benefit from further investigation and development, RegCM-UW is a new tool for investigating the climatology of coastal fog and the physical processes that govern it. We expect that with appropriate physical parameterizations and moderate horizontal resolution, other climate models should be capable of simulating coastal fog. The source code for RegCM-UW is publicly available, under the GNU license, through the International Centre for Theoretical Physics. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1486-x Authors Travis A. O’Brien, Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95604, USA Lisa C. Sloan, Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95604, USA Patrick Y. Chuang, Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95604, USA Ian C. Faloona, Department of Land, Air, and Water Resources, University of California, Davis, One Shields Avenue, Davis, CA 95618, USA James A. Johnstone, Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Decadal predictions have a high profile in the climate science community and beyond, yet very little is known about their skill. Nor is there any agreed protocol for estimating their skill. This paper proposes a sound and coordinated framework for verification of decadal hindcast experiments. The framework is illustrated for decadal hindcasts tailored to meet the requirements and specifications of CMIP5 (Coupled Model Intercomparison Project phase 5). The chosen metrics address key questions about the information content in initialized decadal hindcasts. These questions are: (1) Do the initial conditions in the hindcasts lead to more accurate predictions of the climate, compared to un-initialized climate change projections? and (2) Is the prediction model’s ensemble spread an appropriate representation of forecast uncertainty on average? The first question is addressed through deterministic metrics that compare the initialized and uninitialized hindcasts. The second question is addressed through a probabilistic metric applied to the initialized hindcasts and comparing different ways to ascribe forecast uncertainty. Verification is advocated at smoothed regional scales that can illuminate broad areas of predictability, as well as at the grid scale, since many users of the decadal prediction experiments who feed the climate data into applications or decision models will use the data at grid scale, or downscale it to even higher resolution. An overall statement on skill of CMIP5 decadal hindcasts is not the aim of this paper. The results presented are only illustrative of the framework, which would enable such studies. However, broad conclusions that are beginning to emerge from the CMIP5 results include (1) Most predictability at the interannual-to-decadal scale, relative to climatological averages, comes from external forcing, particularly for temperature; (2) though moderate, additional skill is added by the initial conditions over what is imparted by external forcing alone; however, the impact of initialization may result in overall worse predictions in some regions than provided by uninitialized climate change projections; (3) limited hindcast records and the dearth of climate-quality observational data impede our ability to quantify expected skill as well as model biases; and (4) as is common to seasonal-to-interannual model predictions, the spread of the ensemble members is not necessarily a good representation of forecast uncertainty. The authors recommend that this framework be adopted to serve as a starting point to compare prediction quality across prediction systems. The framework can provide a baseline against which future improvements can be quantified. The framework also provides guidance on the use of these model predictions, which differ in fundamental ways from the climate change projections that much of the community has become familiar with, including adjustment of mean and conditional biases, and consideration of how to best approach forecast uncertainty. Content Type Journal Article Pages 1-28 DOI 10.1007/s00382-012-1481-2 Authors L. Goddard, International Research Institute for Climate and Society, The Earth Institute of Columbia University, Palisades, NY, USA A. Kumar, Climate Prediction Center, National Centers for Environmental Prediction, NOAA, Silver Spring, MD, USA A. Solomon, Earth System Research Laboratory, NOAA, University of Colorado, Boulder, CO, USA D. Smith, UK Met Office, Hadley Centre, Exeter, UK G. Boer, Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, BC, Canada P. Gonzalez, International Research Institute for Climate and Society, The Earth Institute of Columbia University, Palisades, NY, USA V. Kharin, Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, BC, Canada W. Merryfield, Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, BC, Canada C. Deser, National Center for Atmospheric Research, Boulder, CO, USA S. J. Mason, International Research Institute for Climate and Society, The Earth Institute of Columbia University, Palisades, NY, USA B. P. Kirtman, Rosentiel School for Marine and Atmospheric Science, University of Miami, Miami, FL, USA R. Msadek, NOAA’s Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA R. Sutton, NCAS-Climate, Department of Meteorology, University of Reading, Reading, UK E. Hawkins, NCAS-Climate, Department of Meteorology, University of Reading, Reading, UK T. Fricker, University of Exeter, Exeter, UK G. Hegerl, University of Edinburgh, Edinburgh, UK C. A. T. Ferro, University of Exeter, Exeter, UK D. B. Stephenson, University of Exeter, Exeter, UK G. A. Meehl, National Center for Atmospheric Research, Boulder, CO, USA T. Stockdale, European Centre for Medium-Range Weather Forecasts, Reading, UK R. Burgman, Florida International University, Miami, FL, USA A. M. Greene, International Research Institute for Climate and Society, The Earth Institute of Columbia University, Palisades, NY, USA Y. Kushnir, Lamont-Doherty Earth Observatory, The Earth Institute of Columbia University, Palisades, NY, USA M. Newman, Earth System Research Laboratory, NOAA, University of Colorado, Boulder, CO, USA J. Carton, University of Maryland, College Park, MD, USA I. Fukumori, Jet Propulsion Laboratory, NASA, Pasadena, CA, USA T. Delworth, NOAA’s Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Since much of the flow of the Indus River originates in the Himalayas, Karakoram and Hindu Kush Mountains, an understanding of weather characteristics leading to precipitation over the region is essential for water resources management. This study examines the influence of upper level mid-latitude circulation on the summer precipitation over upper Indus basin (UIB). Using reanalysis data, a geopotential height index (GH) is defined at 200 hPa over central Asia, which has a significant correlation with the precipitation over UIB. GH has also shown significant correlation with the heat low (over Iran and Afghanistan and adjoining Pakistan), easterly shear of zonal winds (associated with central Asian high) and evapotranspiration (over UIB). It is argued that the geopotential height index has the potential to serve as a precursor for the precipitation over UIB. In order to assess the influence of irrigation on precipitation over UIB, a simplified irrigation scheme has been developed and applied to the regional climate model REMO. It has been shown that both versions of REMO (with and without irrigation) show significant correlations of GH with easterly wind shear and heat low. However contrary to reanalysis and the REMO version with irrigation, the REMO version without irrigation does not show any correlation between GH index and evapotranspiration as well as between geopotential height and precipitation over UIB, which is further confirmed by the quantitative analysis of extreme precipitation events over UIB. It is concluded that although atmospheric moisture over coastal Arabian sea region, triggered by wind shear and advected northward due to heat low, also contribute to the UIB precipitation. However for the availability of necessary moisture for precipitation over UIB, the major role is played by the evapotranspiration of water from irrigation. From the results it may also be inferred that the representation of irrigated water in climate models is unavoidable for studying the impact of global warming over the region. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1480-3 Authors Fahad Saeed, Climate Service Center, Helmholtz-Zentrum Geesthacht, Fischertwiete 1, 20095 Hamburg, Germany Stefan Hagemann, Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany Sajjad Saeed, Division of Geography, KU Leuven, Celestijnenlaan 200e, 3001 Heverlee, Belgium Daniela Jacob, Climate Service Center, Helmholtz-Zentrum Geesthacht, Fischertwiete 1, 20095 Hamburg, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Climate change has the potential to reduce water availability in West Africa. This study aims to quantify the expected impact of increased greenhouse gases (GHGs) on hydroclimatology of Niger River Basin (NRB). Boundary data from a general circulation model are used to force a regional climate model, to produce dynamically downscaled hydroclimatic variables of NRB under present-day (PRS) and future climate scenarios. The data were further analyzed to detect changes in atmospheric and surface water balance components and moisture recycling ratio (β). The results show that elevated GHGs (under A1B scenario) would produce a drier climate during the rainy season and a wetter climate during the dry season. A warmer climate over NRB in all months was projected. Highest temperature increase of 3 °C occurs about 14°N in May and June, and the smallest increase of 0.5 °C occurs below 8°N in wet-dry transition period. Evaporation reduces during wet season and increases during the dry periods. Humidity increases by 2 % in the dry season, but decreases by 2–4 % in the wet season. Maximum change in moisture influx of 20.7 % and outflux of 20.6 % occur in June and July, respectively. β is projected to decrease in 75 % of the months with biggest relative change of −18.4 % in June. The projected decrease in precipitation efficiency (ρ) during the wet season reaches −20.3 % in June. For PRS run, about 66 % of the available atmospheric moisture in NRB precipitates between June and September, of which around 21 % originates from local evaporation. The result suggests that under enhanced GHGs, local evaporation will contribute less to atmospheric moisture and precipitation over the basin. Projected changes in rainfall and streamflow for Upper Niger and Benue sub-basin are significantly different during the wet season. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1498-6 Authors Philip G. Oguntunde, Institute of Landscape Hydrology, Leibniz Center for Agricultural Landscape Research (ZALF), 15374 Müncheberg, Germany Babatunde J. Abiodun, Department of Environmental and Geographical Science, University of Cape Town, Cape Town, South Africa Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The boreal summer intraseasonal oscillation (BSISO) has strong convective activity centers in Indian (I), Western North Pacific (WNP), and North American (NA) summer monsoon (SM) regions. The present study attempts to reveal BSISO teleconnection patterns associated with these dominant intraseasonal variability centers. During the active phase of ISM, a zonally elongated band of enhanced convection extends from India via the Bay of Bengal and Philippine Sea to tropical central Pacific with suppressed convection over the eastern Pacific near Mexico. The corresponding extratropical circulation anomalies occur along the waveguides generated by the North African-Asian jet and North Atlantic-North European jet. When the tropical convection strengthens over the WNPSM sector, a distinct great circle-like Rossby wave train emanates from the WNP to the western coast of United States (US) with an eastward shift of enhanced meridional circulation. In the active phase of NASM, large anticyclonic anomalies anchor over the western coast of US and eastern Canada and the global teleconnection pattern is similar to that during a break phase of the ISM. Examination of the evolution of the BSISO teleconnection reveals quasi-stationary patterns with preferred centers of teleconnection located at Europe, Russia, central Asia, East Asia, western US, and eastern US and Canada, respectively. Most centers are embedded in the waveguide along the westerly jet stream, but the centers at Europe and Russia occur to the north of the jet-induced waveguide. Eastward propagation of the ISO teleconnection is evident over the Pacific-North America sector. The rainfall anomalies over the elongated band near the monsoon domain over the Indo-western Pacific sector have an opposite tendency with that over the central and southern China, Mexico and southern US, providing a source of intraseasonal predictability to extratropical regions. The BSISO teleconnection along and to the north of the subtropical jet provides a good indication of the surface sir temperature anomalies in the NH extratropics. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1394-0 Authors Ja-Yeon Moon, International Pacific Research Center, School of Ocean and Earth Science Technology, University of Hawaii at Mânoa, Honolulu, HI, USA Bin Wang, International Pacific Research Center, School of Ocean and Earth Science Technology, University of Hawaii at Mânoa, Honolulu, HI, USA Kyung-Ja Ha, Division of Earth Environmental System, Pusan National University, Busan, Korea June-Yi Lee, International Pacific Research Center, School of Ocean and Earth Science Technology, University of Hawaii at Mânoa, Honolulu, HI, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This work evaluates the skill of retrospective predictions of the second version of the NCEP Climate Forecast System (CFSv2) for the North Atlantic sea surface temperature (SST) and investigates the influence of El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) on the prediction skill over this region. It is shown that the CFSv2 prediction skill with 0–8 month lead displays a “tripole”-like pattern with areas of higher skills in the high latitude and tropical North Atlantic, surrounding the area of lower skills in the mid-latitude western North Atlantic. This “tripole”-like prediction skill pattern is mainly due to the persistency of SST anomalies (SSTAs), which is related to the influence of ENSO and NAO over the North Atlantic. The influences of ENSO and NAO, and their seasonality, result in the prediction skill in the tropical North Atlantic the highest in spring and the lowest in summer. In CFSv2, the ENSO influence over the North Atlantic is overestimated but the impact of NAO over the North Atlantic is not well simulated. However, compared with CFSv1, the overall skills of CFSv2 are slightly higher over the whole North Atlantic, particularly in the high latitudes and the northwest North Atlantic. The model prediction skill beyond the persistency initially presents in the mid-latitudes of the North Atlantic and extends to the low latitudes with time. That might suggest that the model captures the associated air-sea interaction in the North Atlantic. The CFSv2 prediction is less skillful than that of SSTA persistency in the high latitudes, implying that over this region the persistency is even better than CFSv2 predictions. Also, both persistent and CFSv2 predictions have relatively low skills along the Gulf Stream. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1431-z Authors Zeng-Zhen Hu, Climate Prediction Center, NCEP/NWS/NOAA, 5200 Auth Road (Suite 605), Camp Springs, MD 20746, USA Arun Kumar, Climate Prediction Center, NCEP/NWS/NOAA, 5200 Auth Road (Suite 605), Camp Springs, MD 20746, USA Bohua Huang, Department of Atmospheric, Oceanic, and Earth Sciences, College of Science, Gorge Mason University, 4400 University Drive, Fairfax, VA 22030, USA Wanqiu Wang, Climate Prediction Center, NCEP/NWS/NOAA, 5200 Auth Road (Suite 605), Camp Springs, MD 20746, USA Jieshun Zhu, Center for Ocean-Land-Atmosphere Studies, 4041 Powder Mill Road, #302, Calverton, MD 20705, USA Caihong Wen, Climate Prediction Center, NCEP/NWS/NOAA, 5200 Auth Road (Suite 605), Camp Springs, MD 20746, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description: Erratum to: The interannual precipitation variability in the southern part of Iran as linked to large-scale climate modes Content Type Journal Article Category Erratum Pages 1-1 DOI 10.1007/s00382-012-1404-2 Authors Farnaz Pourasghar, Department of Physical Geography, Faculty of Humanities and Social Science, The University of Tabriz, Tabriz, Iran Tomoki Tozuka, Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan Saeed Jahanbakhsh, Department of Physical Geography, Faculty of Humanities and Social Science, The University of Tabriz, Tabriz, Iran Behrooz Sari Sarraf, Department of Physical Geography, Faculty of Humanities and Social Science, The University of Tabriz, Tabriz, Iran Hooshang Ghaemi, Iran Meteorological Organization, Tehran, Iran Toshio Yamagata, Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    We evaluate three categories of variables for explaining the spatial pattern of warming and cooling trends over land: predictions of general circulation models (GCMs) in response to observed forcings; geographical factors like latitude and pressure; and socioeconomic influences on the land surface and data quality. Spatial autocorrelation (SAC) in the observed trend pattern is removed from the residuals by a well-specified explanatory model. Encompassing tests show that none of the three classes of variables account for the contributions of the other two, though 20 of 22 GCMs individually contribute either no significant explanatory power or yield a trend pattern negatively correlated with observations. Non-nested testing rejects the null hypothesis that socioeconomic variables have no explanatory power. We apply a Bayesian Model Averaging (BMA) method to search over all possible linear combinations of explanatory variables and generate posterior coefficient distributions robust to model selection. These results, confirmed by classical encompassing tests, indicate that the geographical variables plus three of the 22 GCMs and three socioeconomic variables provide all the explanatory power in the data set. We conclude that the most valid model of the spatial pattern of trends in land surface temperature records over 1979–2002 requires a combination of the processes represented in some GCMs and certain socioeconomic measures that capture data quality variations and changes to the land surface. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1418-9 Authors Ross McKitrick, Department of Economics, University of Guelph, Guelph, ON, Canada Lise Tole, Department of Economics, Strathclyde University, Glasgow, UK Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Recent winter seasons have evidenced that global warming does not exclude the occurrence of exceptionally cold and/or snowy episodes in the Northern mid-latitudes. The expected rarefaction of such events is likely to exacerbate both their societal and environmental impacts. This paper therefore aims to evaluate model uncertainties underlying the fate of wintertime cold extremes over Europe. Understanding why climate models (1) still show deficiencies in simulating present-day features and (2) differ in their responses under future scenarios for the twentyfirst century indeed constitutes a crucial challenge. Here we propose a weather-regime approach in order to separate the contributions of large-scale circulation and non-dynamical processes to biases or changes in the simulated mean and extreme temperatures. We illustrate our methodology from the wintertime occurrence of extremely cold days in idealized atmosphere-only experiments performed with two of the CMIP5 climate models (CNRM-CM5 and IPSL-CM5A-LR). First we find that most of the present-day temperature biases are due to systematic errors in non-dynamical processes, while the main features of the large-scale dynamics are well captured in such experiments driven by observed sea-surface temperatures, with the exception of a generalized underestimation of blocking episodes. Then we show that uncertainties associated with changes in large-scale circulation modulate the depletion in cold extremes under an idealized scenario for the late twentyfirst century. These preliminary results suggest that the original methodology proposed in this paper can be helpful for understanding spreads of larger model-ensembles when simulating the response of temperature extremes to climate change. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1436-7 Authors Julien Cattiaux, Météo-France/CNRM-GAME, 42 avenue Gaspard Coriolis, 31057 Toulouse, France Hervé Douville, Météo-France/CNRM-GAME, 42 avenue Gaspard Coriolis, 31057 Toulouse, France Aurélien Ribes, Météo-France/CNRM-GAME, 42 avenue Gaspard Coriolis, 31057 Toulouse, France Fabrice Chauvin, Météo-France/CNRM-GAME, 42 avenue Gaspard Coriolis, 31057 Toulouse, France Chloé Plante, Météo-France/CNRM-GAME, 42 avenue Gaspard Coriolis, 31057 Toulouse, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Canonical El Niño has a warming center in the eastern Pacific (EP), but in recent decades, El Niño warming center tends to occur more frequently in the central Pacific (CP). The definitions and names of this new type of El Niño, however, have been notoriously diverse, which makes it difficult to understand why the warming center shifts. Here, we show that the new type of El Niño events is characterized by: 1) the maximum warming standing and persisting in the CP and 2) the warming extending to the EP only briefly during its peak phase. For this reason, we refer to it as standing CP warming (CPW). Global warming has been blamed for the westward shift of maximum warming as well as more frequent occurrence of CPW. However, we find that since the late 1990s the standing CPW becomes a dominant mode in the Pacific; meanwhile, the epochal mean trade winds have strengthened and the equatorial thermocline slope has increased, contrary to the global warming-induced weakening trades and flattening thermocline. We propose that the recent predominance of standing CPW arises from a dramatic decadal change characterized by a grand La Niña-like background pattern and strong divergence in the CP atmospheric boundary layer. After the late 1990s, the anomalous mean CP wind divergence tends to weaken the anomalous convection and shift it westward from the underlying SST warming due to the suppressed low - level convergence feedback . This leads to a westward shift of anomalous westerly response and thus a zonally in-phase SST tendency, preventing eastward propagation of the SST anomaly. We anticipate more CPW events will occur in the coming decade provided the grand La Niña-like background state persists. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1427-8 Authors Baoqiang Xiang, International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, 1680 East–West Rd., Honolulu, HI 96822, USA Bin Wang, International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, 1680 East–West Rd., Honolulu, HI 96822, USA Tim Li, International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, 1680 East–West Rd., Honolulu, HI 96822, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The impact of the simulated large-scale atmospheric circulation on the regional climate is examined using the Weather Research and Forecasting (WRF) model as a regional climate model. The purpose is to understand the potential need for interior grid nudging for dynamical downscaling of global climate model (GCM) output for air quality applications under a changing climate. In this study we downscale the NCEP-Department of Energy Atmospheric Model Intercomparison Project (AMIP-II) Reanalysis using three continuous 20-year WRF simulations: one simulation without interior grid nudging and two using different interior grid nudging methods. The biases in 2-m temperature and precipitation for the simulation without interior grid nudging are unreasonably large with respect to the North American Regional Reanalysis (NARR) over the eastern half of the contiguous United States (CONUS) during the summer when air quality concerns are most relevant. This study examines how these differences arise from errors in predicting the large-scale atmospheric circulation. It is demonstrated that the Bermuda high, which strongly influences the regional climate for much of the eastern half of the CONUS during the summer, is poorly simulated without interior grid nudging. In particular, two summers when the Bermuda high was west (1993) and east (2003) of its climatological position are chosen to illustrate problems in the large-scale atmospheric circulation anomalies. For both summers, WRF without interior grid nudging fails to simulate the placement of the upper-level anticyclonic (1993) and cyclonic (2003) circulation anomalies. The displacement of the large-scale circulation impacts the lower atmosphere moisture transport and precipitable water, affecting the convective environment and precipitation. Using interior grid nudging improves the large-scale circulation aloft and moisture transport/precipitable water anomalies, thereby improving the simulated 2-m temperature and precipitation. The results demonstrate that constraining the RCM to the large-scale features in the driving fields improves the overall accuracy of the simulated regional climate, and suggest that in the absence of such a constraint, the RCM will likely misrepresent important large-scale shifts in the atmospheric circulation under a future climate. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1440-y Authors Jared H. Bowden, U.S. EPA National Exposure Research Laboratory, Research Triangle Park, NC, USA Christopher G. Nolte, U.S. EPA National Exposure Research Laboratory, Research Triangle Park, NC, USA Tanya L. Otte, U.S. EPA National Exposure Research Laboratory, Research Triangle Park, NC, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Vertical stratification changes at low frequency over the last decades are the largest in the western-central Pacific and have the potential to modify the balance between ENSO feedback processes. Here we show evidence of an increase in thermocline feedback in the western-central equatorial Pacific over the last 50 years, and in particular after the climate shift of 1976. It is demonstrated that the thermocline feedback becomes more effective due to the increased stratification in the vicinity of the mean thermocline. This leads to an increase in vertical advection variability twice as large as the increase resulting from the stronger ENSO amplitude (positive asymmetry) in the eastern Pacific that connects to the thermocline in the western-central Pacific through the basin-scale ‘tilt’ mode. Although the zonal advective feedback is dominant over the western-central equatorial Pacific, the more effective thermocline feedback allows for counteracting its warming (cooling) effect during warm (cold) events, leading to the reduced covariability between SST and thermocline depth anomalies in the NINO4 (160°E–150°W; 5°S–5°N) region after the 1976 climate shift. This counter-intuitive relationship between thermocline feedback strength as derived from the linear relationship between SST and thermocline fluctuations and stratification changes is also investigated in a long-term general circulation coupled model simulation. It is suggested that an increase in ENSO amplitude may lead to the decoupling between eastern and central equatorial Pacific sea surface temperature anomalies through its effect on stratification and thermocline feedback in the central-western Pacific. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1504-z Authors Boris Dewitte, Laboratoire d’Etudes en Géophysique et Océanographie Spatiales, Toulouse, France Sang-Wook Yeh, Department of Environmental Marine Science, Hanyang University, Ansan, Korea Sulian Thual, Laboratoire d’Etudes en Géophysique et Océanographie Spatiales, Toulouse, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This paper analyses the behavior of extreme events of surface precipitation and temperature inside the Pacific and Caribbean Catchment Basins in Colombia using several datasets such as observations, reconstructed data, NCEP-NCAR and ERA-40 reanalyses and data from the regional model REMO. We use an extreme value method that selects the time series excesses over a nonstationary threshold and adjusts them to a generalized Pareto distribution. The goodness of fit is evaluated through a test that includes the Cramer–von Mises, Kolmogorov–Smirnov and Anderson–Darling statistics and the p values generated by parametric bootstrap resampling. The test not only evaluates the goodness of fit but also the threshold choice. The parameters are presented in maps that allow recognition of the features of the extreme behaviour inside the catchment basins, and differences and similarities between them. Maps of return periods for the maximum extreme events are also presented. A strong influence of the El Niño–Southern oscillation on the extreme events of both temperature and precipitation is found in the two catchment basins. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1487-9 Authors Isabel Hoyos, Grupo de Física y Astrofísica Computacional, FACom, Instituto de Física, Universidad de Antioquia, Medellín, Colombia Astrid Baquero-Bernal, Grupo de Simulación del Sistema Climático Terrestre, Departamento de Física, Universidad Nacional de Colombia, Cra. 30 No. 45-03, Bogotá, Colombia Daniela Jacob, Climate Service Center, Fischertwiete 1, 20095 Hamburg, Germany Boris A. Rodríguez, Grupo de Física y Astrofísica Computacional, FACom, Instituto de Física, Universidad de Antioquia, Medellín, Colombia Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In this paper, we apply finite-mixture-model-based clustering algorithms to cluster post-landfall tracks of tropical cyclones (TCs) making landfall over China. Because existing studies find that landfall surfaces or elevations affect post-landfall TC movements, we also take account of elevations in addition to time orders in this model. Our study reveals three clusters, with cluster 1 making landfall in Hainan province and moving across the western coast of Guangdong province. Most of the TC tracks in cluster 1 move over the ocean and make secondary landfalls over Yunnan province of China and Vietnam. Cluster 1 finally dissipates inland and moves westward as a result of the westward-shift subtropical high, westward steering flow, easterly vertical wind shear and strong mountainous blocking. Cluster 2 makes landfall over Guangdong and Fujian provinces. TCs in cluster 2 subsequently move inland and disappear due largely to westward-shift subtropical high, easterly steering flow, easterly vertical wind shear and relatively strong mountainous blocking. Cluster 3 makes landfall along the Fujian and Zhejiang coast and sustains a long period of time, recurving mostly to the mid-latitude region owing to the surrounding eastward-shift subtropical high, westerly vertical wind shear, weak mountainous blocking and westerly steering flow. Because cluster 2 is significantly associated with La Niña events, TCs more likely make landfall over southeastern China coast and move westward or northwestward without recurving. Cluster 3 sustains a longer time than clusters 1 and 2 in spite of its weak horizontal and vertical water vapor supply. TCs in cluster 3 interact actively with westerlies during the post-landfall period. However, we cannot observe any analogous interactions with the mid-latitude westerlies in clusters 1 and 2. TCs of clusters 1 and 2 are influenced by summer monsoon flows. Moreover, summer monsoon exerts a greater influence on cluster 1 than cluster 2. The composite 200 hPa divergence of cluster 3 is stronger than that of clusters 1 and 2. This explains to some degree why cluster 3 sustains longer than clusters 1 and 2 after making landfall. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1519-5 Authors Wei Zhang, Department of Geography and Resource Management, The Chinese University of Hong Kong, Shatin, Hong Kong, China Yee Leung, Department of Geography and Resource Management, The Chinese University of Hong Kong, Shatin, Hong Kong, China Yuanfei Wang, Key Laboratory of Geo-information Science, Ministry of Education, East China Normal University, Shanghai, China Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The extra-tropical response to El Niño in configurations of a coupled model with increased horizontal resolution in the oceanic component is shown to be more realistic than in configurations with a low resolution oceanic component. This general conclusion is independent of the atmospheric resolution. Resolving small-scale processes in the ocean produces a more realistic oceanic mean state, with a reduced cold tongue bias, which in turn allows the atmospheric model component to be forced more realistically. A realistic atmospheric basic state is critical in order to represent Rossby wave propagation in response to El Niño, and hence the extra-tropical response to El Niño. Through the use of high and low resolution configurations of the forced atmospheric-only model component we show that, in isolation, atmospheric resolution does not significantly affect the simulation of the extra-tropical response to El Niño. It is demonstrated, through perturbations to the SST forcing of the atmospheric model component, that biases in the climatological SST field typical of coupled model configurations with low oceanic resolution can account for the erroneous atmospheric basic state seen in these coupled model configurations. These results highlight the importance of resolving small-scale oceanic processes in producing a realistic large-scale mean climate in coupled models, and suggest that it might may be possible to “squeeze out” valuable extra performance from coupled models through increases to oceanic resolution alone. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1518-6 Authors Andrew Dawson, School of Mathematics, University of East Anglia, Norwich, UK Adrian J. Matthews, School of Mathematics, University of East Anglia, Norwich, UK David P. Stevens, School of Mathematics, University of East Anglia, Norwich, UK Malcolm J. Roberts, Met Office Hadley Centre, Exeter, UK Pier Luigi Vidale, Department of Meteorology, National Centre for Atmospheric Science, University of Reading, Reading, UK Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Level 3 (3A25) TRMM Precipitation Radar (PR) data are used for 13 years period (1998–2010) to prepare climatology of TRMM PR derived near surface rain (Total rain) and rain fractions for the 4-months duration of Indian Summer Monsoon season (June–September) as well as for individual months. It is found that the total rain is contributed mostly (99 %) by two rain fractions i.e. stratiform and convective rain fractions for the season as well as on the monthly basis. It is also found that total rain estimates by PR are about 65 % of the gauge measured rain over continental India as well as on sub-regional basis. Inter-annual variability of TRMM-PR rain estimates for India mainland and its sub-regions as well as over the neighboring oceanic regions, in terms of coefficient of variability (CV) is discussed. The heaviest rain region over north Bay of Bengal (BoB) is found to have the lowest CV. Another sub-region of low CV lies over the eastern equatorial Indian ocean (EEIO). The CVs of total rain as well as its two major constituents are found to be higher on monthly basis compared to seasonal basis. Existence of a well known dipole between the EEIO and the north BoB is well recognized in PR data also. Significant variation in PR rainfall is found over continental India between excess and deficit monsoon seasons as well as between excess and deficit rainfall months of July and August. Examination of rainfall fractions between the BoB and Central India on year to year basis shows that compensation in rainfall fractions exists on monthly scale on both the regions. Also on the seasonal and monthly scales, compensation is observed in extreme monsoon seasons between the two regions. However, much less compensation is observed between the north BoB and EEIO belts in extreme rain months. This leads to speculation that the deficit and excess seasons over India may result from slight shift of the rainfall from Central India to the neighboring oceanic regions of north BoB. Contribution of stratiform and convective rain fractions have been also examined and the two fractions are found to contribute almost equally to the total rain. Results are further discussed in terms of the possible impact of the two rain fractions on circulation based on possible difference is vertical profiles of latent heat of two types of rain. Substantial differences in the lower and upper tropospheric circulation regimes are noticed in both deficit and excess monsoon months/seasons, emphasizing the interaction between rainfall (latent heat) and circulation. Content Type Journal Article Pages 1-24 DOI 10.1007/s00382-012-1502-1 Authors Samir Pokhrel, Indian Institute of Tropical Meteorology, Pune, 411008 India D. R. Sikka, Indian Institute of Tropical Meteorology, Pune, 411008 India Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    We investigate the simulated temperature and precipitation of the HIRHAM regional climate model using systematic variations in domain size, resolution and detailed location in a total of eight simulations. HIRHAM was forced by ERA-Interim boundary data and the simulations focused on higher resolutions in the range of 5.5–12 km. HIRHAM outputs of seasonal precipitation and temperature were assessed by calculating distributed model errors against a higher resolution data set covering Denmark and a 0.25° resolution data set covering Europe. Furthermore the simulations were statistically tested against the Danish data set using bootstrap statistics. The results from the distributed validation of precipitation showed lower errors for the winter (DJF) season compared to the spring (MAM), fall (SON) and, in particular, summer (JJA) seasons for both validation data sets. For temperature, the pattern was in the opposite direction, with the lowest errors occurring for the JJA season. These seasonal patterns between precipitation and temperature are seen in the bootstrap analysis. It also showed that using a 4,000 × 2,800 km simulation with an 11 km resolution produced the highest significance levels. Also, the temperature errors were more highly significant than precipitation. In similarly sized domains, 12 of 16 combinations of variables, observation validation data and seasons showed better results for the highest resolution domain, but generally the most significant improvements were seen when varying the domain size. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1513-y Authors Morten A. D. Larsen, Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark Peter Thejll, Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark Jens H. Christensen, Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark Jens C. Refsgaard, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark Karsten H. Jensen, Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The projected climate change signals of a five-member high resolution ensemble, based on two global climate models (GCMs: ECHAM5 and CCCma3) and two regional climate models (RCMs: CLM and WRF) are analysed in this paper (Part II of a two part paper). In Part I the performance of the models for the control period are presented. The RCMs use a two nest procedure over Europe and Germany with a final spatial resolution of 7 km to downscale the GCM simulations for the present (1971–2000) and future A1B scenario (2021–2050) time periods. The ensemble was extended by earlier simulations with the RCM REMO (driven by ECHAM5, two realisations) at a slightly coarser resolution. The climate change signals are evaluated and tested for significance for mean values and the seasonal cycles of temperature and precipitation, as well as for the intensity distribution of precipitation and the numbers of dry days and dry periods. All GCMs project a significant warming over Europe on seasonal and annual scales and the projected warming of the GCMs is retained in both nests of the RCMs, however, with added small variations. The mean warming over Germany of all ensemble members for the fine nest is in the range of 0.8 and 1.3 K with an average of 1.1 K. For mean annual precipitation the climate change signal varies in the range of −2 to 9 % over Germany within the ensemble. Changes in the number of wet days are projected in the range of ±4 % on the annual scale for the future time period. For the probability distribution of precipitation intensity, a decrease of lower intensities and an increase of moderate and higher intensities is projected by most ensemble members. For the mean values, the results indicate that the projected temperature change signal is caused mainly by the GCM and its initial condition (realisation), with little impact from the RCM. For precipitation, in addition, the RCM affects the climate change signal significantly. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1510-1 Authors Sven Wagner, Institute for Meteorology and Climate Research (IMK-IFU), KIT, Kreuzeckbahnstraße 19, 82467 Garmisch-Partenkirchen, Germany Peter Berg, Institute for Meteorology and Climate Research (IMK-TRO), KIT, Hermann-von-Helmholtz Platz 1, Bldg. 435, 76344 Eggenstein-Leopoldshafen, Germany Gerd Schädler, Institute for Meteorology and Climate Research (IMK-TRO), KIT, Hermann-von-Helmholtz Platz 1, Bldg. 435, 76344 Eggenstein-Leopoldshafen, Germany Harald Kunstmann, Institute for Meteorology and Climate Research (IMK-IFU), KIT, Kreuzeckbahnstraße 19, 82467 Garmisch-Partenkirchen, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Because of model biases, projections of future climate need to combine model simulations of recent and future climate with information on observed climate. Here, 10 methods for projecting the distribution of daily mean temperatures are compared, using six regional climate change simulations for Europe. Cross validation between the models is used to assess the potential performance of the methods in projecting future climate. Delta change and bias correction type methods show similar cross-validation performance, with methods based on the quantile mapping approach doing best in both groups due to their apparent ability to reduce the errors in the projected time mean temperature change. However, as no single method performs best under all circumstances, the optimal approach might be to use several well-behaving methods in parallel. When applying the various methods to real-world temperature projection for the late 21st century, the largest intermethod differences are found in the tails of the temperature distribution. Although the intermethod variation of the projections is generally smaller than their intermodel variation, it is not negligible. Therefore, it should be preferably included in uncertainty analysis of temperature projections, particularly in applications where the extremes of the distribution are important. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1515-9 Authors Jouni Räisänen, Department of Physics, University of Helsinki, P.O. Box 48 (Erik Palménin aukio 1), 00014 Helsinki, Finland Olle Räty, Department of Physics, University of Helsinki, P.O. Box 48 (Erik Palménin aukio 1), 00014 Helsinki, Finland Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    It is well known from previous research that significant differences exist amongst reanalysis products from different institutions. Here, we compare the skill of NCEP-R (reanalyses by the National Centers for Environmental Prediction, NCEP), ERA-int (the European Centre of Medium-range Weather Forecasts Interim), JCDAS (the Japanese Meteorological Agency Climate Data Assimilation System reanalyses), MERRA (the Modern Era Retrospective-Analysis for Research and Applications by the National Aeronautics and Space Administration), CFSR (the Climate Forecast System Reanalysis by the NCEP), and ensembles thereof as predictors for daily air temperature on a high-altitude glaciated mountain site in Peru. We employ a skill estimation method especially suited for short-term, high-resolution time series. First, the predictors are preprocessed using simple linear regression models calibrated individually for each calendar month. Then, cross-validation under consideration of persistence in the time series is performed. This way, the skill of the reanalyses with focus on intra-seasonal and inter-annual variability is quantified. The most important findings are: (1) ERA-int, CFSR, and MERRA show considerably higher skill than NCEP-R and JCDAS; (2) differences in skill appear especially during dry and intermediate seasons in the Cordillera Blanca; (3) the optimum horizontal scales largely vary between the different reanalyses, and horizontal grid resolutions of the reanalyses are poor indicators of this optimum scale; and (4) using reanalysis ensembles efficiently improves the performance of individual reanalyses. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1501-2 Authors Marlis Hofer, Innrain 52f, Institute of Meteorology and Geophysics, University of Innsbruck, 6020 Innsbruck, Austria Ben Marzeion, Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria Thomas Mölg, Chair of Climatology, Technische Universität Berlin, Berlin, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Global warming exerts a lengthening effect on the growing season, with observational evidences emerging from different regions over the world. However, the difficulty for a global overview of this effect for the last century arises from limited availability of the long-term daily observations. In this study, we find a good linear relationship between the start (end) date of local growing season (LGS) and the monthly mean temperature in April (October) using the global gridded daily temperature dataset for 1960–1999. Using homogenized daily temperature records from nine stations where the time series go back to the beginning of the twentieth century, we find that the rate of change in the start (end) date of the LGS for per degree warming in April (October) mean temperature keeps nearly constant throughout the time. This enables us to study LGS changes during the last century using global gridded monthly mean temperature data. The results show that during the period 1901–2009, averaged over the observation areas, the LGS length has increased by a rate of 0.89 days decade −1 , mainly due to an earlier start (−0.58 days decade −1 ). This is smaller than those estimates for the late half of the twentieth century, because of multidecadal climate variability (MDV). A MDV component of the LGS index series is extracted by using Ensemble Empirical Mode Decomposition method. The MDV exhibits significant positive correlation with the Atlantic Multi–decadal Oscillation (AMO) over most of the Northern Hemisphere lands, but negative in parts of North America and Western Asia for start date of LGS. These are explained by analyzing differences in atmospheric circulation expressed by sea level pressure departures between the warm and cool phases of AMO. It is suggested that MDV in association with AMO accelerates the lengthening of LGS in Northern Hemisphere by 53 % for the period 1980–2009. Content Type Journal Article Pages 1-11 DOI 10.1007/s00382-012-1438-5 Authors Jiangjiang Xia, Key Laboratory of Regional Climate-Environment for East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029 China Zhongwei Yan, Key Laboratory of Regional Climate-Environment for East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029 China Peili Wu, Hadley Centre, Met Office, Exeter, EX1 3PB UK Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Tropical upper tropospheric humidity, clouds, and ice water content, as well as outgoing longwave radiation (OLR), are evaluated in the climate model EC Earth with the aid of satellite retrievals. The Atmospheric Infrared Sounder and Microwave Limb Sounder together provide good coverage of relative humidity. EC Earth’s relative humidity is in fair agreement with these observations. CloudSat and CALIPSO data are combined to provide cloud fractions estimates throughout the altitude region considered (500–100 hPa). EC Earth is found to overestimate the degree of cloud cover above 200 hPa and underestimate it below. Precipitating and non-precipitating EC Earth ice definitions are combined to form a complete ice water content. EC Earth’s ice water content is below the uncertainty range of CloudSat above 250 hPa, but can be twice as high as CloudSat’s estimate in the melting layer. CERES data show that the model underestimates the impact of clouds on OLR, on average with about 9 W m −2 . Regionally, EC Earth’s outgoing longwave radiation can be ∼20 W m −2 higher than the observation. A comparison to ERA-Interim provides further perspectives on the model’s performance. Limitations of the satellite observations are emphasised and their uncertainties are, throughout, considered in the analysis. Evaluating multiple model variables in parallel is a more ambitious approach than is customary. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1511-0 Authors M. S. Johnston, Department of Earth and Space Sciences, Chalmers University of Technology, 412 96 Göteburg, Sweden P. Eriksson, Department of Earth and Space Sciences, Chalmers University of Technology, 412 96 Göteburg, Sweden S. Eliasson, Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology, Space Campus 1, 98128 Kiruna, Sweden C. G. Jones, Swedish Meteorological and Hydrological Institute (SMHI), Folkborgsvägen 1, 601 76 Norrköping, Sweden R. M. Forbes, ECMWF, Shinfield Park, Reading, Berkshire, RG2 9AX UK D. P. Murtagh, Department of Earth and Space Sciences, Chalmers University of Technology, 412 96 Göteburg, Sweden Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Recently, a new conceptual framework for deep convection scheme triggering and closure has been developed and implemented in the LMDZ5B general circulation model, based on the idea that deep convection is controlled by sub-cloud lifting processes. Such processes include boundary-layer thermals and evaporatively-driven cold pools (wakes), which provide an available lifting energy that is compared to the convective inhibition to trigger deep convection, and an available lifting power (ALP) at cloud base, which is used to compute the convective mass flux assuming the updraft vertical velocity at the level of free convection. While the ALP closure was shown to delay the local hour of maximum precipitation over land in better agreement with observations, it results in an underestimation of the convection intensity over the tropical ocean both in the 1D and 3D configurations of the model. The specification of the updraft vertical velocity at the level of free convection appears to be a key aspect of the closure formulation, as it is weaker over tropical ocean than over land and weaker in moist mid-latitudes than semi-arid regions. We propose a formulation making this velocity increase with the level of free convection, so that the ALP closure is adapted to various environments. Cloud-resolving model simulations of observed oceanic and continental case studies are used to evaluate the representation of lifting processes and test the assumptions at the basis of the ALP closure formulation. Results favor closures based on the lifting power of sub-grid sub-cloud processes rather than those involving quasi-equilibrium with the large-scale environment. The new version of the model including boundary-layer thermals and cold pools coupled together with the deep convection scheme via the ALP closure significantly improves the representation of various observed case studies in 1D mode. It also substantially modifies precipitation patterns in the full 3D version of the model, including seasonal means, diurnal cycle and intraseasonal variability. Content Type Journal Article Pages 1-22 DOI 10.1007/s00382-012-1506-x Authors Catherine Rio, Laboratoire de Météorologie Dynamique, CNRS/IPSL, UPMC, Tr 45-55, 3e et, B99 Jussieu, 75005 Paris, France Jean-Yves Grandpeix, LMD, Paris, France Frédéric Hourdin, LMD, Paris, France Francoise Guichard, Centre National de la Recherche Météorologique (CNRM/GAME), Toulouse, France Fleur Couvreux, Centre National de la Recherche Météorologique (CNRM/GAME), Toulouse, France Jean-Philippe Lafore, Centre National de la Recherche Météorologique (CNRM/GAME), Toulouse, France Ann Fridlind, Goddard Institute for Space Studies, NASA/GISS, New York, USA Agnieszka Mrowiec, Goddard Institute for Space Studies, NASA/GISS, New York, USA Romain Roehrig, LMD, Paris, France Nicolas Rochetin, LMD, Paris, France Marie-Pierre Lefebvre, LMD/CNRM, Paris, France Abderrahmane Idelkadi, LMD, Paris, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Several studies have analysed the atmospheric response to sea-ice changes in the Arctic region, but only few have considered the Antarctic. Here, the atmospheric response to sea-ice variability in the Southern Hemisphere is investigated with the atmospheric general circulation model ECHAM5. The model is forced by the present and a projected future seasonal cycle of Antarctic sea ice. In September, the mean atmospheric response exhibits distinct similarities to the structure of the negative phase of the Southern Annular Mode, the leading mode of Southern Hemisphere variability. In the reduced Antarctic sea-ice integration, there is an equatorward shift of the Southern Hemisphere mid-latitude jet and the storm tracks. In contrast to a recent previous study, our findings indicate that a substantial impact of Southern Hemispheric future sea-ice reduction on the mid-latitude circulation cannot be ruled out. Content Type Journal Article Pages 1-12 DOI 10.1007/s00382-012-1507-9 Authors Jürgen Bader, Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, Germany Martin Flügge, Geophysical Institute, University of Bergen, Bergen, Norway Nils Gunnar Kvamstø, Geophysical Institute, University of Bergen, Bergen, Norway Michel D. S. Mesquita, Bjerknes Centre for Climate Research, Uni Research, Bergen, Norway Aiko Voigt, Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, Germany Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Soil physical characteristics can influence terrestrial hydrology and the energy balance and may thus affect land–atmosphere exchanges. However, only few studies have investigated the importance of soil textures for climate. In this study, we examine the impact of soil texture specification in a regional climate model. We perform climate simulations over Europe using soil maps derived from two different sources: the soil map of the world from the Food and Agricultural Organization and the European Soil Database from the European Commission Joint Research Center. These simulations highlight the importance of the specified soil texture in summer, with differences of up to 2 °C in mean 2-m temperature and 20 % in precipitation resulting from changes in the partitioning of energy at the land surface into sensible and latent heat flux. Furthermore, we perform additional simulations where individual soil parameters are perturbed in order to understand their role for summer climate. These simulations highlight the importance of the vertical profile of soil moisture for evapotranspiration. Parameters affecting the latter are hydraulic diffusivity parameters, field capacity and plant wilting point. Our study highlights the importance of soil properties for climate simulations. Given the uncertainty associated with the geographical distribution of soil texture and the resulting differences between maps from different sources, efforts to improve existing databases are needed. In addition, climate models would benefit from tackling unresolved issues in land-surface modeling related to the high spatial variability in soil parameters, both horizontally and vertically, and to limitations of the concept of soil textural class. Content Type Journal Article Pages 1-19 DOI 10.1007/s00382-012-1395-z Authors Benoit P. Guillod, Institute for Atmospheric and Climate Science, ETH Zurich, Zürich, Switzerland Edouard L. Davin, Institute for Atmospheric and Climate Science, ETH Zurich, Zürich, Switzerland Christine Kündig, Institute for Atmospheric and Climate Science, ETH Zurich, Zürich, Switzerland Gerhard Smiatek, Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Garmisch-Partenkirschen, Germany Sonia I. Seneviratne, Institute for Atmospheric and Climate Science, ETH Zurich, Zürich, Switzerland Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The Geophysical Fluid Dynamics Laboratory has developed an ensemble coupled data assimilation (ECDA) system based on the fully coupled climate model, CM2.1, in order to provide reanalyzed coupled initial conditions that are balanced with the climate prediction model. Here, we conduct a comprehensive assessment for the oceanic variability from the latest version of the ECDA analyzed for 51 years, 1960–2010. Meridional oceanic heat transport, net ocean surface heat flux, wind stress, sea surface height, top 300 m heat content, tropical temperature, salinity and currents are compared with various in situ observations and reanalyses by employing similar configurations with the assessment of the NCEP’s climate forecast system reanalysis (Xue et al. in Clim Dyn 37(11):2511–2539, 2011 ). Results show that the ECDA agrees well with observations in both climatology and variability for 51 years. For the simulation of the Tropical Atlantic Ocean and global salinity variability, the ECDA shows a good performance compared to existing reanalyses. The ECDA also shows no significant drift in the deep ocean temperature and salinity. While systematic model biases are mostly corrected with the coupled data assimilation, some biases (e.g., strong trade winds, weak westerly winds and warm SST in the southern oceans, subsurface temperature and salinity biases along the equatorial western Pacific boundary, overestimating the mixed layer depth around the subpolar Atlantic and high-latitude southern oceans in the winter seasons) are not completely eliminated. Mean biases such as strong South Equatorial Current, weak Equatorial Under Current, and weak Atlantic overturning transport are generated during the assimilation procedure, but their variabilities are well simulated. In terms of climate variability, the ECDA provides good simulations of the dominant oceanic signals associated with El Nino and Southern Oscillation, Indian Ocean Dipole, Pacific Decadal Oscillation, and Atlantic Meridional Overturning Circulation during the whole analyzed period, 1960–2010. Content Type Journal Article Pages 1-29 DOI 10.1007/s00382-012-1412-2 Authors You-Soon Chang, Geophysical Fluid Dynamics Laboratory, Princeton University Forrestal Campus, 201 Forrestal Road, Princeton, NJ 08540, USA Shaoqing Zhang, Geophysical Fluid Dynamics Laboratory, Princeton University Forrestal Campus, 201 Forrestal Road, Princeton, NJ 08540, USA Anthony Rosati, Geophysical Fluid Dynamics Laboratory, Princeton University Forrestal Campus, 201 Forrestal Road, Princeton, NJ 08540, USA Thomas L. Delworth, Geophysical Fluid Dynamics Laboratory, Princeton University Forrestal Campus, 201 Forrestal Road, Princeton, NJ 08540, USA William F. Stern, Geophysical Fluid Dynamics Laboratory, Princeton University Forrestal Campus, 201 Forrestal Road, Princeton, NJ 08540, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This paper assesses potential predictability of decadal variations in the El Niño/Southern Oscillation (ENSO) characteristics by constructing and performing simulations using an empirical nonlinear stochastic model of an ENSO index. The model employs decomposition of global sea-surface temperature (SST) anomalies into the modes that maximize the ratio of interdecadal-to-subdecadal SST variance to define low-frequency predictors called the canonical variates (CVs). When the whole available SST time series is so processed, the leading canonical variate (CV-1) is found to be well correlated with the area-averaged SST time series which exhibits a non-uniform warming trend, while the next two (CV-2 and CV-3) describe secular variability arguably associated with a combination of Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) signals. The corresponding ENSO model that uses either all three (CVs 1–3) or only AMO/PDO-related (CVs 2 and 3) predictors captures well the observed autocorrelation function, probability density function, seasonal dependence of ENSO, and, most importantly, the observed interdecadal modulation of ENSO variance. The latter modulation, and its dependence on CVs, is shown to be inconsistent with the null hypothesis of random decadal ENSO variations simulated by multivariate linear inverse models. Cross-validated hindcasts of ENSO variance suggest a potential useful skill at decadal lead times. These findings thus argue that decadal modulations of ENSO variability may be predictable subject to our ability to forecast AMO/PDO-type climate modes; the latter forecasts may need to be based on simulations of dynamical models, rather than on a purely statistical scheme as in the present paper. Content Type Journal Article Pages 1-15 DOI 10.1007/s00382-012-1424-y Authors S. Kravtsov, Department of Mathematical Sciences, Atmospheric Sciences Group, University of Wisconsin-Milwaukee, P. O. Box 413, Milwaukee, WI 53201, USA Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    Boreal summer intraseasonal oscillations (BSISOs) manifest in the active and break spells and act as the primary building block of the Indian summer monsoon. Although recent research has evolved a basic framework for understanding the scale selection and northward propagation of the BSISO, the role of different hydrometeors in modulating these processes remains poorly explored. In this study, TRMM-2A12 retrievals and Modern Era Retrospective-analysis for Research and Applications reanalysis data are examined to establish relationship between cloud hydrometeors and other atmospheric dynamical parameters with the northward propagation of the BSISOs. The study reveals that the cloud liquid water leads the deep convection during the northward propagation of BSISOs in the lower troposphere, while the cloud ice slightly lags the convection. This distribution indicates the occurrence of a possible mechanism of the lower level moistening through the large scale moisture advection in lower atmosphere and boundary layer (PBL) convergence, followed by triggering of the deep convection. The analyses of moisture advection and the dynamical fields with respect to the convection center show that low level moistening is a manifestation of the barotropic vorticity and PBL convergence of moisture anomaly north of the convection center. A new internal dynamical-thermodynamical mechanism is unraveled to understand the reason behind the middle tropospheric heating maximum and its role on the northward propagation. It is shown that the enhanced moisture perturbation in lower levels together with the heat transport by the sub-grid scale eddies within the PBL induces lower level instability required to precondition the lower atmosphere for triggering the deep convection. Vigorous upward motion inside the deep convection uplifts the liquid hydrometeors to upper levels and the formation of precipitable ice leads to the heating maxima in the middle troposphere. To check the robustness of the proposed hypothesis, similar analysis is performed for the weak northward propagating BSISO cases. Content Type Journal Article Pages 1-14 DOI 10.1007/s00382-012-1425-x Authors S. Abhik, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008 India M. Halder, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008 India P. Mukhopadhyay, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008 India X. Jiang, Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, CA, USA B. N. Goswami, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411008 India Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    During the last glacial, major abrupt climate events known as Heinrich events left distinct fingerprints of ice rafted detritus, and are thus associated with iceberg armadas; the release of many icebergs into the North Atlantic Ocean. We simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere–ocean model. In our model, dynamic-thermodynamic icebergs influence the climate through two direct effects. First, melting of the icebergs causes freshening of the upper ocean, and second, the latent heat used in the phase-transition of ice to water results in cooling of the iceberg surroundings. This cooling effect of icebergs is generally neglected in models. We investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off. At the peak of the simulated Heinrich event, icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects. The cause of this intriguing result must be sought in the involvement of a secondary mechanism: facilitation of sea-ice formation, which can disturb deep water production at key convection sites, with consequences for the thermohaline circulation. We performed additional sensitivity experiments, designed to explore the effect of the more plausible distribution of the dynamic icebergs’ melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid-latitude North Atlantic (NA) Ocean. The early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments, which must be a distribution-effect: the dynamically distributed icebergs quickly affect western NADW formation, which synergizes with direct sea-ice facilitation, causing an earlier sea-ice expansion and climatic response. Furthermore, compared to dynamic-thermodynamic icebergs, a homogeneous hosing overestimates the fresh water flux in the Eastern Ruddiman belt, causing a fresh anomaly in the Eastern North Atlantic, leading to a delayed recovery of the circulation after the event. Content Type Journal Article Pages 1-13 DOI 10.1007/s00382-012-1421-1 Authors J. I. Jongma, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands H. Renssen, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands D. M. Roche, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In order to understand potential predictability of the ocean and climate at the decadal time scales, it is crucial to improve our understanding of internal variability at this time scale. Here, we describe a 20-year mode of variability found in the North Atlantic in a 1,000-year pre-industrial simulation of the IPSL-CM5A-LR climate model. This mode involves the propagation of near-surface temperature and salinity anomalies along the southern branch of the subpolar gyre, leading to anomalous sea-ice melting in the Nordic Seas, which then forces sea-level pressure anomalies through anomalous surface atmospheric temperatures. The wind stress associated to this atmospheric structure influences the strength of the East Greenland Current across the Denmark Strait, which, in turn, induces near-surface temperature and salinity anomalies of opposite sign at the entrance of the Labrador Sea. This starts the second half of the cycle after approximatively 10 years. The time scale of the cycle is thus essentially set by advection of tracers along the southern branch of the subpolar gyre, and by the time needed for anomalous East Greenland Current to accumulate heat and freshwater anomalies at the entrance of the Labrador Sea. The Atlantic meridional overturning circulation (AMOC) does not play a dominant role in the mode that is confined in the subpolar North Atlantic, but it also has a 20-year preferred timescale. This is due to the influence of the propagating salinity anomalies on the oceanic deep convection. The existence of this preferred timescale has important implications in terms of potential predictability of the North Atlantic climate in the model, although its realism remains questionable and is discussed. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1402-4 Authors R. Escudier, Institut Mediterrani d’Estudis Avancats, IMEDEA (CSIC-UIB), Mallorca, Spain J. Mignot, Laboratoire d’ocanographie et du climat: experimentation et approches numeriques, IPSL/UPMC/CNRS/IRD/MNHN, case 100, 4 place Jussieu, 75005 Paris, France D. Swingedouw, IPSL/LSCE, Gif-sur-Yvette, CEA Saclay, Orme des Merisiers, France Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    The ENSEMBLES multi-model and perturbed-parameter decadal re-forecasts are used to assess multi-year forecast quality for global-mean surface air temperature (SAT) and North Atlantic multi-decadal sea surface temperature variability (AMV). Two issues for near-term climate prediction, not discussed so far, are addressed with these two examples: the impact of the choice of the observational reference period, and of the number of years included in the forecast average. Taking into account only years when both observational and model data are available, instead of using the full record, to estimate observed climatologies produces systematically (although not statistically significantly different) higher ensemble-mean correlations and lower root mean square errors in all forecast systems. These differences are more apparent in the second half of the decadal prediction, which suggests an influence of non-stationary long-term trends. Also, as the forecast period averaged increases, the correlation for both global-mean SAT and AMV is generally higher. This also suggests an increasing role for the variable external forcing as when forecast period averaged increases, unpredictable internal variability is smoothed out. The results show that predicting El Niño-Southern Oscillation beyond one year is a hurdle for current global forecast systems, which explains the positive impact of the forecast period averaging. By comparing initialized and uninitialized re-forecasts, the skill assessment confirms that variations of the global-mean SAT are largely controlled by the prescribed variable external forcing. By contrast, the initialization improves the skill of the AMV during the first half of the forecast period. In an operational context, this would lead to improved predictions of the AMV from initializing internal climate fluctuations. The coherence between the multi-model and perturbed-parameter ensemble supports that conclusion for boreal summer and annual means, while the results show less consistency for boreal winter. Content Type Journal Article Pages 1-16 DOI 10.1007/s00382-012-1413-1 Authors J. García-Serrano, Institut Català de Ciències del Clima (IC3), Barcelona, Spain F. J. Doblas-Reyes, Institut Català de Ciències del Clima (IC3), Barcelona, Spain Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In this study, the phase-locking of El Nino Southern Oscillation (ENSO) in a coupled model with different physical parameter values is investigated. It is found that there is a dramatic change in ENSO phase-locking in response to a slight change in the Tokioka parameter, which is a minimum entrainment rate threshold in the cumulus parameterization. With a smaller Tokioka parameter, the model simulates ENSO peak in the boreal summer season rather than in the winter season as observed. It is revealed that the differences in climatological zonal sea surface temperature (SST) gradient and its associated mean state changes are crucial to determine the phase-locking of ENSO. In the simulations with smaller Tokioka parameter values, climatological zonal SST gradient during the boreal summer is excessively large, because the zonally-asymmetric SST change (i.e., SST increase is relatively smaller over the eastern Pacific) is maximum during the boreal summer when the eastern Pacific SST is the coolest of the year. The enhanced climatological zonal SST gradient in boreal summer reduces the convection over the eastern Pacific, which leads to a weakening of air–sea coupling strength. The minimum coupling strength during summer prevents SST anomalies from further development in the following season, which favors SST maximum during summer. In addition, enhanced zonal SST gradient and resultant thermocline shoaling over the eastern Pacific lead to excessive zonal advective feedback and thermocline feedback. Atmospheric damping is also weakened during boreal summer season. These changes due to feedback processes allow an excessive development of SST anomalies during the summer time, and lead to a summer peak of ENSO. The importance of basic state change for the ENSO phase-locking is also validated in a multi-model framework using the Coupled Model Intercomparison Project phase-3 archive. It is found that several of the climate models have the same problem in producing a summer peak of ENSO. Consistent with the simulations with different physical parameter values, these models have minimum air–sea coupling strength during the boreal summer season. Also, they have stronger climatological zonal SST gradient and shallower climatological thermocline depth over the eastern Pacific during the boreal summer season. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1420-2 Authors Yoo-Geun Ham, Global Modeling and Assimilation Office, GSFC/NASA, Greenbelt, MD, USA Jong-Seong Kug, Korea Ocean Research and Development Institute, Ansan, Korea Daehyun Kim, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, NY, USA Young-Ho Kim, Korea Ocean Research and Development Institute, Ansan, Korea Dong-Hoon Kim, Department of Atmospheric Science, Yonsei University, Seoul, Korea Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    A continental scale evaluation of Antarctic surface winds is presented from global ERA-40 and ERA-Interim reanalyses and RACMO2/ANT regional climate model at 55 and 27 km horizontal resolution, based on a comparison with observational data from 115 automatic weather stations (AWS). The Antarctic surface wind climate can be classified based on the Weibull shape factor k w . Very high values ( k w  〉 3) are found in the interior plateaus, typical of very uniform katabatic-dominated winds with high directional constancy. In the coast and all over the Antarctic Peninsula the shape factors are similar to the ones found in mid-latitudes ( k w  〈 3) typical of synoptically dominated wind climates. The Weibull shape parameter is systematically overpredicted by ERA reanalyses. This is partly corrected by RACMO2/ANT simulations which introduce more wind speed variability in complex terrain areas. A significant improvement is observed in the performance of ERA-Interim over ERA-40, with an overall decrease of 14 % in normalized mean absolute error. In escarpment and coastal areas, where the terrain gets rugged and katabatic winds are further intensified in confluence zones, ERA-Interim bias can be as high as 10 m s −1 . These large deviations are partly corrected by the regional climate model. Given that RACMO2/ANT is an independent simulation of the near-surface wind speed climate, as it is not driven by observations, it compares very well to the ERA-Interim and AWS-115 datasets. Content Type Journal Article Pages 1-24 DOI 10.1007/s00382-012-1396-y Authors Javier Sanz Rodrigo, von Karman Institute for Fluid Dynamics (VKI), Sint-Genesius-Rode, Belgium Jean-Marie Buchlin, von Karman Institute for Fluid Dynamics (VKI), Sint-Genesius-Rode, Belgium Jeroen van Beeck, von Karman Institute for Fluid Dynamics (VKI), Sint-Genesius-Rode, Belgium Jan T. M. Lenaerts, Institute for Marine and Atmospheric Research (IMAU), Utrecht University, Utrecht, The Netherlands Michiel R. van den Broeke, Institute for Marine and Atmospheric Research (IMAU), Utrecht University, Utrecht, The Netherlands Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    In recent decades, the need of future climate information at local scales have pushed the climate modelling community to perform increasingly higher resolution simulations and to develop alternative approaches to obtain fine-scale climatic information. In this article, various nested regional climate model (RCM) simulations have been used to try to identify regions across North America where high-resolution downscaling generates fine-scale details in the climate projection derived using the “delta method”. Two necessary conditions were identified for an RCM to produce added value (AV) over lower resolution atmosphere-ocean general circulation models in the fine-scale component of the climate change (CC) signal. First, the RCM-derived CC signal must contain some non-negligible fine-scale information—independently of the RCM ability to produce AV in the present climate. Second, the uncertainty related with the estimation of this fine-scale information should be relatively small compared with the information itself in order to suggest that RCMs are able to simulate robust fine-scale features in the CC signal. Clearly, considering necessary (but not sufficient) conditions means that we are studying the “potential” of RCMs to add value instead of the AV, which preempts and avoids any discussion of the actual skill and hence the need for hindcast comparisons. The analysis concentrates on the CC signal obtained from the seasonal-averaged temperature and precipitation fields and shows that the fine-scale variability of the CC signal is generally small compared to its large-scale component, suggesting that little AV can be expected for the time-averaged fields. For the temperature variable, the largest potential for fine-scale added value appears in coastal regions mainly related with differential warming in land and oceanic surfaces. Fine-scale features can account for nearly 60 % of the total CC signal in some coastal regions although for most regions the fine scale contributions to the total CC signal are of around ∼5 %. For the precipitation variable, fine scales contribute to a change of generally less than 15 % of the seasonal-averaged precipitation in present climate with a continental North American average of ∼5 % in both summer and winter seasons. In the case of precipitation, uncertainty due to sampling issues may further dilute the information present in the downscaled fine scales. These results suggest that users of RCM simulations for climate change studies in a delta method framework have little high-resolution information to gain from RCMs at least if they limit themselves to the study of first-order statistical moments. Other possible benefits arising from the use of RCMs—such as in the large scale of the downscaled fields– were not explored in this research. Content Type Journal Article Pages 1-18 DOI 10.1007/s00382-012-1415-z Authors Alejandro Di Luca, Centre ESCER (Étude et Simulation du Climat à l’Échelle Régionale), Département des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal (UQAM), PK-6530 B.P. 8888, Succ. Centre-ville, Montréal, QC H3C 3P8, Canada Ramón de Elía, Centre ESCER (Étude et Simulation du Climat à l’Échelle Régionale), Consortium Ouranos, 550 Sherbrooke West, 19th floor, West Tower, Montréal, QC H3A 1B9, Canada René Laprise, Centre ESCER (Étude et Simulation du Climat à l’Échelle Régionale), Département des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal (UQAM), PK-6530 B.P. 8888, Succ. Centre-ville, Montréal, QC H3C 3P8, Canada Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575

Description:    This study examines wave disturbances on submonthly (6–30-day) timescales over the tropical Indian Ocean during Southern Hemisphere summer using Japanese Reanalysis (JRA25-JCDAS) products and National Oceanic and Atmospheric Administration outgoing longwave radiation data. The analysis period is December–February for the 29 years from 1979/1980 through 2007/2008. An extended empirical orthogonal function (EEOF) analysis of daily 850-hPa meridional wind anomalies reveals a well-organized wave-train pattern as a dominant mode of variability over the tropical Indian Ocean. Daily lagged composite analyses for various atmospheric variables based on the EEOF result show the structure and evolution of a wave train consisting of meridionally elongated troughs and ridges along the Indian Ocean Intertropical Convergence Zone (ITCZ). The wave train is oriented in a northeast–southwest direction from Sumatra toward Madagascar. The waves have zonal wavelengths of about 3,000–5,000 km and exhibit westward and southwestward phase propagation. Individual troughs and ridges as part of the wave train sequentially travel westward and southwestward from the west of Sumatra into Madagascar. Meanwhile, eastward and northeastward amplification of the wave train occurs associated with the successive growth of new troughs and ridges over the equatorial eastern Indian Ocean. This could be induced by eastward and northeastward wave energy dispersion from the southwestern to eastern Indian Ocean along the mean monsoon westerly flow. In addition, the waves modulate the ITCZ convection. Correlation statistics show the average behavior of the wave disturbances over the tropical Indian Ocean. These statistics and other diagnostic measures are used to characterize the waves obtained from the composite analysis. The waves appear to be connected to the monsoon westerly flow. The waves tend to propagate through a band of the large meridional gradient of absolute vorticity produced by the mean monsoon westerly flow. This suggests that the monsoon westerly flow provides favorable background conditions for the propagation and maintenance of the waves and acts as a waveguide over the tropical Indian Ocean. The horizontal structure of the wave train may be interpreted as that of a mixture of equatorial Rossby waves and mixed Rossby-gravity wavelike gyres. Content Type Journal Article Pages 1-21 DOI 10.1007/s00382-012-1417-x Authors Yoshiki Fukutomi, Research Institute for Global Change, JAMSTEC, 3173-25 Showamachi, Kanazawaku, Yokohama, Kanagawa, 236-001 Japan Tetsuzo Yasunari, Hydrospheric Atmospheric Research Center, Nagoya University, Furocho, Chikusaku, Nagoya, Aichi, 464-8601 Japan Journal Climate Dynamics Online ISSN 1432-0894 Print ISSN 0930-7575