ALBERT

Description: ABSTRACT In this observational research, the seasonally stratified (October to December) Indian Ocean Dipole mode (IOD) behaviour in the tropical Indian Ocean since 1870 is investigated. Three significant climate shifts manifested themselves in the Indian Ocean during the years 1918, 1961 and 1997. Each shift is preceded by a 3-year sequence of IOD events that are unique in the entire time series. The order is such that a relatively moderate negative sea surface temperature (SST) anomaly gradient intensifies to an extreme negative IOD event which then reverses in the shift year to an extreme positive event. The last two extreme IOD events reach record-breaking magnitudes during each shift implying intensification of the shift process with time. During the year before the shift, the Mascarene High (MH) is anomalously displaced poleward and westward, while it intensifies as it anomalously moves equatorward during the shift year. Therefore the maximum contrast of south Indian Ocean pressure pattern and hence the intensity of the MH, from one year to the other within an epoch, is achieved during the shift years. The intensity of this process has been escalating during each of the three successive shifts. Despite involving to a greater extent the IOD dynamics, these IOD shifts may primarily be an expression of the south Indian Ocean basinwide dynamics rather than a direct response to internal tropical influences.

Description: ABSTRACT Monthly total precipitation and mean temperature climate surfaces, gridded to 30-arcseconds (≈1 km at the equator) and available for all global land areas, are presented. These datasets are generated with a Delta downscaling method, using the 30-arcsecond WorldClim climatologies to scale monthly anomaly grids. For monthly mean temperature, the anomalies are constructed from both the Climate Research Unit (CRU) and Willmott & Matsuura (W&M) 0.5 degree time-series datasets, whereas for monthly precipitation Global Precipitation Climatology Centre (GPCC) data are also used. The 0.5 degree anomalies are then interpolated to the 30-arcsecond resolution. Use of piecewise cubic Hermite interpolating polynomials (PCHIP) to interpolate the anomaly grids results in more physically representative Delta downscaled surfaces, compared to bilinear and cubic spline interpolation. The Delta downscaled products are compared to Global Historical Climatology Network (GHCN) station records for six test regions distributed globally. In this analysis, the Delta grids produced using the W&M time-series dataset perform better than grids produced using GPCC or CRU. Using Oregon, USA as a test region, the Delta downscaled datasets are compared to the Parameter-elevation Regressions on Independent Slopes Model (PRISM) datasets. For monthly precipitation, PRISM performs better than each of the three Delta downscaled datasets, but for mean temperature both Delta downscaled datasets outperform PRISM. Through computing the Pearson product–moment correlation coefficient between GHCN station delineated errors in the WorldClim climatologies and the Delta downscaled W&M data, it is shown that performance of the Delta grids corresponds strongly to performance of the reference climatologies. Therefore, future improvement of the 30-arcsecond Delta grids described in this article is strongly tied to advances in the high-resolution climatological data for all global land surfaces. The Delta downscaled datasets discussed herein are open-source and freely distributed at http://www.globalclimatedata.org .

Description: ABSTRACT This study demonstrates the capability of the Weather Research and Forecasting (WRF) model with four-dimensional data assimilation (WRF-FDDA) to produce a high-resolution climatography of seasonal precipitation over Israel and the surrounding areas. The system was used to dynamically downscale global Climate Forecast System (CFS) reanalysis with continuous assimilation of conventional and unconventional observations. Precipitation seasons (December-January-February) in 7 years, including two extreme dry and wet seasons observed in the past decades, were generated at 2-km spatial resolution. Verification against rain-gauge observations shows that the WRF-FDDA system effectively reproduces the spatial and inter-annual variability, as well as the timing, intensity, and length of wet and dry spells. The best agreement between model and observations was obtained at areas dominated by complex terrain, illustrating the benefit of the high-resolution lower boundary forcing in the dynamical downscaling process. In contrast, some biases were observed over coastal-flat terrain. The model was able to reproduce some of the extreme events, but exhibited limitations in the case of rare events. This specific discrepancy between the model and observations suggests that further fine tuning and different model configurations may be needed to correctly simulate extreme events. The use of an objective weather-regimes verification procedure reveals the skill of the climatography for different types of extra-tropical cyclones: while biases are larger at coastal-flat areas under shallow-cyclonic conditions, deep-cyclonic conditions lead to more significant biases in complex terrain regions. The weather-regimes dependent information may be used for further calibration of the downscaled precipitation.

Description: ABSTRACT The source region of Yangtze River in China is a part of Tibet Plateau where the hydrological processes are sensitive to climatic change. The impacts of precipitation, air temperature and evapotranspiration on annual runoff in the source region of Yangtze River during 1957–2009 are investigated in the time-period domain using wavelet analysis method and multiple regression method. Annual evapotranspiration is calculated with data of precipitation and air temperature based on Takahashi's empirical equation. This approximation of actual evapotranspiration successfully matches the mean annual water balance. Significant periods of runoff, 7–8 year, 20–21 year and 42–43 year, are revealed by using Morlet wavelet. Different significant periods are found for annual precipitation, air temperature and evapotranspiration, whereas the 7–8 year and 42–43 year periods are the same of the runoff. It is indicated by wavelet correlation coefficients that the correlations between runoff and these climatic components depends on periods. Change in the summation of runoff wavelet coefficients at different period can approximately represents the change pattern of real runoff and is correlated with the wavelet coefficients of the climatic components. The correlation can be expressed with a linear multiple regression equation which indicates that the change in annual runoff is contributed by change in annual precipitation rather than change in air temperature. This relationship between runoff and climatic components are different from that in the source region of Yellow River, in China.

Description: ABSTRACT Global solar radiation (G) and ultraviolet radiation (UV) observed at Wuhan, China from 2006 to 2012 have been used to investigate the temporal variability of both radiant fluxes and the UV/G ratios (F UV ) at different time scales for the first time in central China. Clearness index (Kt) was used to study the cloud effects on F UV in each month under different sky conditions. It turned out that F UV reached higher values in summer and lower values in winter; F UV also increased generally with cloudiness. A UV model for cloud-free condition was developed by studying the dependence of hourly UV irradiations on the relative optical air mass (m), which has been assessed through the statistical indices: mean bias error (mbe), mean-absolute bias error (mabe) and root-mean-square error (rmse) whose values were 0.32, 7.3 and 8.64%, respectively. UV clearness index, m and Kt were further used for analysing the cloud effects, and different UV models under any sky conditions have been established and validated. Meanwhile, as a modulation for different sky conditions, both cloud modification factors were also used in the construction of a new type of UV model. Finally, by comparing the statistical indices from different models, the most suitable model was chosen for validating at Wuhan University (WHU) and two other sites in different regions of China under any atmospheric conditions, which suggested that the proposed model should be modified to account for local differences to produce better estimations in larger areas.

Description: This study analyses the spatial and temporal variability and trends of rainfall, mean maximum and minimum temperatures at seasonal and annual timescales over the Upper Blue Nile River Basin, Ethiopia. Statistical and geostatistical techniques were applied to 1634 points on 10 × 10 km gridded data reconstructed from weather stations and meteorological satellite records. The data were obtained from the National Meteorological Agency of Ethiopia and cover the period between 1981 and 2010. Trends were evaluated from slopes of regression lines using the least squares method. The F -distribution test was used to determine the statistical significance of the trends. Minimum temperatures significantly increased in northern, central, southern and southeastern parts of the Basin in all seasons. At the annual scale, maximum and minimum temperatures significantly increased in over 33% of the Basin at a rate of 0.1 and 0.15 °C per decade, respectively; however, the western part (12%) of the Basin experienced declining trends on annual and seasonal timescales. The minimum temperatures increased at a higher rate than the maximum temperatures during winter, summer, autumn and also at the annual timescale. Mean annual minimum and maximum temperatures increased from 12.69 to 13.32 °C and 26.43 to 26.91 °C from 1981 to 2010, respectively. Rainfall showed statistically non-significant increasing trends of 35 mm per decade at the annual timescale. All seasons except spring season exhibited similar statistically non-significant trends. The spring season, however, showed a statistically non-significant declining trend in the north eastern (11%) part of the Basin.

Description: This study investigates whether a regional climate model (RCM) driven by a global general circulation model (GCM) in a nesting approach with observed atmospheric CO 2 concentrations shows predictability for temperature and precipitation trends during 1961–1990 in the Mediterranean area, a region strongly influenced by large-scale circulation. Resulting discrepancies between model and observations raise the question whether the model predictability increases after removing impacts of mid-latitude circulation variability. For temperature and precipitation trends we use the RCM REMO and the observational dataset E-OBS, and for atmospheric circulation the driving coupled GCM ECHAM5/MPI-OM and NCEP/NCAR reanalyses. Cross-validated multiple regression analyses between large-scale circulation and regional temperature and precipitation are performed for observed and simulated data. The impact of circulation is removed from the original temperature and precipitation data, and the trends of circulation-related and circulation-unrelated parts are compared. The circulation-related trends of models and observations show discrepancies owing to differing observed and simulated mid-latitude circulation dynamics, i.e. different temporal evolutions of North Atlantic Oscillation and East Atlantic pattern in winter and East Atlantic Jet and a blocking pattern in summer. Such differences can be related to unknown initial conditions of GCM simulations. In fact, we find strong impacts of initial conditions on mid-latitude circulation dynamics of ECHAM5/MPI-OM ensemble members over 30-year periods. The agreement between simulated and observed circulation-unrelated trends is generally higher than for original trends indicating that the predictability of this nesting approach increases by removing impacts of mid-latitude circulation variability. We conclude that initial conditions affect climate variability up to the multi-decadal timescale, at least in parts of the globe which are governed by extratropical circulation modes, and hence, hinder the comparability of simulated and observed climate trends over time periods shorter than the timescale dominated by radiative forcing. In the Mediterranean Basin the latter is definitely beyond 30 years.

Description: The impact of heating by black carbon aerosols on Indian summer monsoon has remained inconclusive. Some investigators have predicted that black carbon aerosols reduce monsoon rainfall while others have argued that it will increase monsoon rainfall. These conclusions have been based on local influence of aerosols on the radiative fluxes. The impact of aerosol-like heating in one region on the rainfall in a remote region has not been examined in detail. Here, using an atmospheric general circulation model, it has been shown that remote influence of aerosol-like heating can be as important as local influence on Indian summer monsoon. Precipitation in northern Arabian Sea and north-west Indian region increased by 16% in June to July when aerosol-like heating were present globally. The corresponding increase in precipitation due to presence of aerosol-like heating only over South Asia (local impact) and East Asia (remote impact) were 28 and 13%, respectively. This enhancement in precipitation was due to destabilization of the atmosphere in pre-monsoon season that affected subsequent convection. Moreover, pre-monsoon heating of the lower troposphere changed the circulation substantially that enabled influx of more moisture over certain regions and reduced the moist static stability of the atmosphere. It has been shown that regional aerosol heating can have large impact on the phase of upper tropospheric Rossby wave in pre-monsoon season, which acts as a primary mechanism behind teleconnection and leads to the change in precipitation during monsoon season. These results demonstrate that changes in aerosol in one region can influence the precipitation in a remote region through changes in circulation.

Description: ABSTRACT Ensembles of high-resolution regional climate model (RCM) simulations are crucial for assessing regional climate change and the associated uncertainties. This article presents an RCM ensemble generation technique which explores uncertainties arising from the positioning of synoptic systems in the large-scale atmospheric forcing by shifting the atmospheric fields from global climate model (GCM) runs horizontally. Here, we discuss how the so-called Atmospheric Forcing Shifting (AFS) affects temperature and precipitation over Europe for the period 1980–1984. We use ERA-40 reanalysis data in which the atmospheric fields are shifted to each direction by 25 and 50 km, respectively, to run RCM simulations with COSMO-CLM at 50-km resolution. The analysis of the AFS ensemble includes comparisons with E-OBS observations and COSMO-CLM runs driven by different GCMs. AFS has an evident effect on the spatiotemporal distributions of temperature and particularly precipitation, which is most pronounced during hydrological summer (May to October) when spatial weather patterns are more variable. Furthermore, AFS produces realistic changes in the likelihood and intensity of extreme precipitation. The changes induced by AFS depend strongly on orography, i.e. precipitation increases are likely to occur where moist air masses are shifted towards the windward mountain side and vice versa. Thus, increasing the RCM ensemble spread by means of AFS is a simple and useful method for sampling observed climate statistics and assessing the variability and changes in mean and extreme climate.

Description: ABSTRACT Short-term drought forecasting can be aided with an understanding of the likelihood of dry periods persisting from one season to the next. This research examines drought persistence in the Southeastern United States by identifying spatial patterns of seasonal drought frequency and persistence, using logistic regression to calculate the odds and probability of drought persisting from one season to the next, and examining the effects of El Niño-Southern Oscillation (ENSO) drought persistence in the Southeast. The 3-month climate division-scale Standardized Precipitation Index (SPI) data from 1895 to 2011 is used to examine meteorological drought. Logistic regression is well-suited to examining a binary independent variable (drought or no drought) and also circumvents many of the assumptions that limit linear regression. Results show generally weak seasonal drought persistence throughout the region. However, we do find that some areas in the Southeast United States, like North-Central Alabama are more prone to drought and drought persistence than others. Logistic regression model outcome shows that the probability of spring drought varies as a strong function of winter SPI in the central Southeast United States region. While areas in the western portion of the study region, including Texas and Oklahoma are more prone to summer-to-fall drought persistence, as the probability of fall drought is strongly related to summer SPI. Overall we conclude that seasonal drought forecasts are difficult in the Southeast United States because of infrequent drought persistence. However, the logistic regression model does provide an accurate method for probabilistic seasonal drought forecasts in the region.

Description: ABSTRACT Planners and policy makers require information about the regions for which they are responsible. However, it seems that many developing countries, including Nigeria, are not adequately prepared either for their current climates or for the impact of climate change because they lack sufficient information. We have therefore examined the variations in the thermal condition in terms of the temperature, relative humidity, effective temperature (ET), temperature–humidity index (THI) and relative strain index (RSI). We studied the spatial and temporal (1951–2009, 1951–1980, 1981–2009, decadal, seasonal and monthly averages) variations in the thermal climate of Nigeria, and we divided Nigeria into thermal climate regions for effective climate change management. Mean annual minimum, mean and maximum temperatures (with their standard deviations) were 21.4 (3.5), 27.1 (2.7) and 32.8 (3.4) °C, respectively, while the overall mean relative humidity was 62 (24.8)%. Mean ET, THI and RSI were 24.3 (0.85), 24.8 (1.83) and 0.2 (0.18) °C, respectively. The ET, THI and RSI provided contrasting expressions of thermal comfort for Nigeria, because of its varied climate. We also found that elevation; the movement of the Inter Tropical Discontinuity and urbanization affect thermal comfort in Nigeria. We conclude that thermal stress has increased in Nigeria from 2000 at most stations, especially in the south and north-western regions, and that Nigerian thermal comfort climate is heterogeneous and requires analysis of multiple thermal indices.

Description: ABSTRACT Variations in evapotranspiration (ET) and gross primary production (GPP) due to climatic fluctuations deserve specific considerations for sustainable water and agricultural management. A Vegetation Interface Processes (VIP) model assimilated with Terra-MODIS remotely sensed leaf area index was used to simulate their spatial and temporal patterns from 2000 to 2010 in the Songhua River Basin of Northeast China at 1-km grid and 1-h time step. The predicted ET and GPP were well verified with the eddy covariance measurements, and the ET was in agreement with that derived from water balance. Over the whole basin, average annual precipitation (P), ET, GPP and water use efficiency are 458 mm, 374 mm, 1067 gC m −2 and 2.86 gC m −2  mm −1 , respectively. Both ET and GPP display significantly spatial variability, with ET presenting different and good relationships with precipitation for different kinds of land use covers (LUCs); and GPP a unified good relationship across almost all the LUCs except water body. Over the years from 2000 to 2010, Normalized Difference Vegetation Index (NDVI), especially crop NDVI, experienced a trend of increasing. GPP showed no significant trend. ET showed an increasing trend for cropland and grassland, a decreasing trend in water body and wetland, no significant trend for other vegetation types and a weakly increasing trend for the whole basin. Annually, ET roughly follows precipitation, whereas GPP was mainly regulated by radiation (Rn). The inter-annual variability for the eco-hydrological elements with the order of the coefficient of variation (CV) being P (0.13) 〉 E C (0.055) 〉 ET (0.05) 〉 Rn (0.037) 〉 GPP (0.019) 〉 NDVI (0.01) is less than their spatial variability with the order of CV being E C (0.23) 〉 P (0.22) 〉 GPP (0.19) 〉 ET (0.15) 〉 Rn (0.05). The highest spatial variability of ET and GPP in grassland implies the incurring problem of degradation in western basin. The low spatial variability in paddy relates agricultural management.

Description: Dynamic vegetation models provide the ability to simulate terrestrial carbon pools and fluxes and a useful tool to study how these are affected by climate variability and climate change. At the continental scale, the spatial distribution of climate, in particular temperature and precipitation, strongly determines surface vegetation characteristics. Model validation exercises typically consist of driving a model with observation-based climate data and then comparing simulated quantities with their observation-based counterparts. However, observation-based datasets themselves may not necessarily be consistent with each other. Here, we compare simulated terrestrial carbon pools and fluxes over North America with observation-based estimates. Simulations are performed using the dynamic vegetation model Canadian Terrestrial Ecosystem Model (CTEM) coupled to the Canadian Land Surface Scheme (CLASS) when driven with two reanalysis-based climate datasets. The driving ECMWF reanalysis data (ERA40) and NCEP/NCAR reanalysis I data (NCEP) show differences when compared to each other, as well as when compared to the observation-based climate research unit (CRU) data. Most simulated carbon pools and fluxes show important differences, particularly over eastern North America, primarily due to differences in precipitation and temperature in the two reanalysis. However, despite very different gross fluxes, the model yields fairly similar estimates of the net atmosphere-land CO 2 flux when driven with the two forcing datasets. The ERA40 driven simulation produces terrestrial pools and fluxes that compare better with observation-based estimates. These simulations do not take into account land use change or nitrogen deposition, both of which have been shown to enhance the land carbon sink over North America. The simulated sink of 0.5 Pg C year −1 during the 1980s and 1990s is therefore lower than inversion-based estimates. The analysis of spatial distribution of trends in simulated carbon pools and fluxes shows that the simulated carbon sink is driven primarily by NPP enhancements over eastern United States. © 2013 The Authors and Her Majesty the Queen in Right of Canada. International Journal of Climatology published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society.

Description: ABSTRACT Cape Verde is a semi-arid country conformed by a group of islands located off the west coast of Africa, with highly variable rainfall that appears during a single rainy season. Santiago Island, the biggest of the country, is characterized for abrupt changes of relief within small distances. The influence of geographic location and topographic parameters, such as slope gradient, exposition and elevation on the variability of rainfall in Santiago Island was studied using monthly rainfall data of 30 seasons (1981 to 2010), with daily rainfall data for 14 seasons (1997 to 2010). The number of rainfall days and the percentage of maximum daily rainfall within the monthly and seasonal totals were evaluated. Few rainy days can control the monthly and seasonal rainfall patterns of Santiago Island. Multivariate linear regressions among daily, monthly and seasonal rainfall and elevation, slope gradient, aspect, and geographic east and west coordinates as predictors were carried out. Elevation explains most of the variance in the rainfall. The coefficients of determination show an inverse relationship with the rainfall depth: moderate rainfall totals (120–150 mm monthly, 250–300 mm seasonal) produced the best correlations for seasonal and monthly rainfall, while very low (〈50 mm for monthly, 〈200 mm for seasonal) and very high amounts (〉250 mm for monthly, 〉350 mm for seasonal) resulted in poor correlations. Long-term mean rainfall was interpolated using ordinary kriging and kriging with external drift. In Santiago Island, high and more extreme rainfall events are less influenced by elevation, while low and medium rainfall events are significantly influenced by orography, with most of the rainfall appearing on high elevations. Copyright © 2013 Royal Meteorological Society

Description: For many of the low islands of the tropical Pacific, freshwater is a scarce resource. Water catchment areas are small and groundwater storage is a shallow fresh water lens. The high hydraulic conductivities of the coral and sand substrate means surface water is limited. Realization of the possible impact of climate change has highlighted the sensitivity of island communities to the availability of water. However, impact evaluation requires specialized data as well as appropriate sensitivity assessment methodologies. This is the second of a two part study. The first addressed the data problem by assembling and validating a suitable database. The second develops an island water balance model and applies a sensitivity assessment. Data are at a 2.5° × 2.5° latitude–longitude grid resolution for the Pacific bounded by coordinates 30°S to 30°N and 155°E to 120°W. Output is in the form of Climate Change Sensitivity Index maps that show the impact on the spatial redistribution of climate-determined freshwater resources under various climate scenarios. The method allows for estimation of water deficits or surpluses for low islands located in any part of the study area. Areas of high sensitivity to climatic change are those that sit between margins of very wet and very dry zones. Their extent is determined by the gradients at the margins. Steep gradients define small areas of high sensitivity, whereas gentle gradients appear as large areas of high sensitivity. Adjustments to the model for differing local surface conditions on different islands can be easily made, which allows a sensitivity assessment of individual islands, even for islands with no climate station data. The approach could be a powerful tool to gain useful information on the influence of climate change on freshwater resources of low islands. Planning decision-making is possible without knowing precisely the magnitude of climate change that might occur. Copyright © 2013 Royal Meteorological Society

Description: Forecasting tropical cyclone (TC) intensity changes over land is complicated by interactions of various surface and atmospheric features. Due to generally unfavorable conditions, many TCs weaken and decay soon after landfall. In some cases, TCs may also transition to extratropical cyclones (ETs). Despite the absence of oceanic forcing, a number of TCs have been observed to maintain or increase strength inland, termed “tropical cyclone maintenance or intensification’ (TCMIs). This study identifies the environments and characteristic features of TCMIs and explores physical processes that may help to produce an atmosphere conducive for tropical systems. The objectives are to compile an inland TC dataset over a 30-year period, quantify TC traits that may relate to maximum strength over land, and analyse surface and atmospheric conditions leading up to intensification. Of 227 inland TCs globally, 45 maintained or increased strength inland: 17 cold-core (ET), 16 warm-core (TCMI), and 12 hybrid cases. Analysis of synoptic conditions indicates that TCs persist when low-level temperature gradients are weak. Soil moisture gradients were in the vicinity of the cyclones at the time of intensification and may be forcing the TCMIs via increased surface latent heat flux (LHF). The area-averaged LHF threshold is found to be around 70 W m −2 for TCMI occurrence. In the 2 weeks leading up to each TCMI, the LHF tends to be higher than average over the intensification regions and provides further evidence of land surface forcing. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT Interannual variability of the seasonal surface air-temperature over the Indian subcontinent is investigated using observations for the period of 1900–2005. Our results demonstrate that air-temperature over India is remotely influenced by the El Niño-Southern Oscillation and locally through Indian Ocean sea surface temperature (SST) anomalies. The leading mode of variability (EOF-1, empirical orthogonal function) in the observed air-temperature displays a country-wide warming in all four seasons. The spatial pattern of EOF-1 is similar to that of composite air-temperature anomalies of warm/cold years. Above-normal air-temperature in India (country-wide warming) is positively correlated to a simultaneous El Niño conditions in the eastern Pacific during boreal summer. El Niño induced strong subsidence, weaker low-level winds, less moisture availability and enhanced incoming shortwave radiation over the north Indian Ocean and Indian subcontinent are responsible for air-temperature warming in summer. It is observed that during fall, air-temperature pattern of EOF-1 over India is highly correlated with SST over the tropical oceans. SST correlation is maximum in central Pacific and north Indian Ocean, indicating the importance of both remote and local forcing. During boreal spring and winter, air-temperature warming (EOF-1) is mainly influenced by Indian Ocean SST anomalies. Low moisture and negative sea level pressure anomalies over India indicate the existence of heat low with strong dry winds convergence, which are favourable for air-temperature warming in spring. Although El Niño peaks during winter, its impact on the air-temperature over the Indian subcontinent is weak during this season. The second EOF mode shows dipole-like air-temperature pattern with warming over the south-east and cooling in the north-western India during summer and winter, whereas spring shows opposite polarity. In case of boreal fall, EOF-2 of air-temperature displays a south-west and north-east orientation. Mechanisms responsible for these variabilities are studied in detail. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT Snow cover in the Himalayan–Tibetan region is highly variable in space and time, and influences the hydrology and climate at regional and global scales. Monthly MODIS snow cover and land surface temperature (LST) data have been analysed for the period 2000–2011. The details of the snow cover pattern during accumulation and ablation has been studied for the Indus, Ganga and Brahmaputra river basins. These river basins have shown a unique pattern of snow accumulation and ablation. The Indus and Ganga Basins have shown high snow cover percentage than the Brahmaputra during ablation period. The maximum snow cover for the three basins altogether is approximately 85% of the total geographic area which reduces to approximately 10% during ablation. Accumulation and ablation variations are clearly depicted by the LST variations of the respective basins. However, the Indus Basin has shown a unique increasing trend in snow cover, whereas the Ganga and Brahmaputra Basins have shown no significant trend in this decade. Snow cover for Tibetan region during March (accumulation) and September (ablation) months have not shown either increasing or decreasing trend from 2000 to 2011. Analysis of monthly average basin LST data has shown that Indus Basin experiences subfreezing conditions and delay in rise of peaks in comparison to Ganga and Brahmaputra which shows high LST during accumulation. This study shows that the Indus, Ganga and Brahmaputra Basins have shown different patterns in the snow cover variations during the last 12 years. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT Monthly accumulated precipitation over the tropical Panama Canal Watershed has the largest interannual variability in December, with the wettest month on record being December 2010. December accumulated precipitation over the watershed is found to vary with sea surface temperature (SST) anomalies in both the tropical North Atlantic and equatorial Pacific oceans, but a considerably stronger relationship is found with the difference between these SST anomalies. The configuration of SST anomalies in these two ocean basins during December has a strong effect on the flux of low-level moisture over the Caribbean Sea, southern Central America, and the eastern Pacific Ocean through the modification of the Caribbean Low-Level Jet (CLLJ) and the Chorro del Occidente Colombiano (CHOCO) jet. Wet Decembers in the watershed are associated with cool (warm) SST anomalies in the tropical Pacific (North Atlantic), a weakening (strengthening) of the CLLJ (CHOCO jet), and increased moisture convergence over and around Panama, with the opposite conditions associated with dry Decembers. The SST anomalies in these two ocean basins affect the distribution of daily precipitation during December differently, with SST anomalies in the Pacific (Atlantic) predominately associated with changes in the frequency of heavier (lighter) precipitation. The analysis shows that December 2010 followed the pattern of associations of wet Decembers but with the highest intensity of precipitation and CHOCO jet strength. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT In this study, fine-resolution multimodel climate projections over China are developed based on 35 climate models and two emissions scenarios (RCP4.5 and RCP8.5) from phase five of the Coupled Model Intercomparison Project (CMIP5) by means of Bias Correction and Spatial Disaggregation. The yearly-averaged temperature is projected to increase by 0.8 to 1.6 °C (0.8 to 1.7 °C), 1.5 to 2.7 °C (2 to 3.7 °C), and 1.9 to 3.3 °C (3.4 to 6 °C) under RCP4.5 (RCP8.5) in three time slices (2010–2039, 2040–2069, and 2070–2099), respectively. The most warming occurs in winter and the least in summer, and the inland areas in the northwest will warm much faster than the southeast. Under the background of surface warming, the probability of extreme low temperatures in winter defined as the monthly temperature being lower than the 9th percentile of the climatological distribution will sharply reduce to 0.1–1.7% under RCP4.5 for the period 2010–2039 and even lower for the following decades. For precipitation change, a remarkable increase is found over most areas of China except the Southwest, ranging from approximately 2 to 20%. The projected precipitation changes are highly robust in northern China, but inconsistent in southern China. In spite of widespread precipitation increases, most areas of China quantified by the Palmer Drought Severity Index are projected to become drier as a consequence of increasing evaporation driven by temperature increases. Detailed examination shows that drought that is moderate or severe according to current climate standards will become the norm in the future. Not only will incidences of severe and extreme drought increase dramatically in the future, but extreme wet events will also become more probable. Furthermore, the increasing drought risk in Southwest China and the Qinghai-Tibetan Plateau is nearly twice that for other parts of China.

Description: ABSTRACT We describe the method and performance of a bias-correction applied to high-resolution (˜10 km) simulations from a stretched-grid Regional Climate Model (RCM) over Tasmania, Australia. The bias-correction is a quantile mapping of empirical cumulative frequency distributions. Corrections are applied at a daily time step to five variables: rainfall, potential evaporation (PE), solar radiation, maximum temperature and minimum temperature. Corrections are calculated independently for each season. We show that quantile mapping of empirical distributions can be highly effective in correcting biases in RCM outputs. Cross-validation shows biases are effectively reduced across the range of cumulative frequency distributions, with few exceptions. The bias-correction is not as effective at correcting biases for values at or near zero (e.g. in rainfall simulations), although even here the bias-correction improves biases evident in the uncorrected simulations. In addition, the bias-correction improves frequency characteristics of variables such as the number of rain days. We use a detrending technique to apply the bias-correction to 140-year time series of RCM variables. We show that the bias-correction effectively preserves long-term changes (e.g. to the mean and variance) to variables projected by the uncorrected RCM simulations. Correlations between key variables are also largely preserved, thus the bias-corrected outputs reflect the dynamics of the underlying RCM. However, the bias-corrected simulations still exhibit some of the deficiencies of the RCM simulations, e.g. the tendency to underestimate the magnitude and duration of large, multi-day rain events, and the tendency to underestimate the duration of dry spells. The bias-corrected simulations for six downscaled GCMs for the A2 and B1 emissions scenarios are available to researchers from http://www.tpac.org.au .

Description: ABSTRACT In-situ observations of surface air temperature at 55 weather stations in Iran are analysed for homogeneity and trends over the period 1960–2010. Among them 32 stations have data available for the whole period. The other 23 stations with shorter records are used only to confirm variability during overlapping periods. Discontinuities in the temperature series relate mostly to relocation and changes of environmental conditions at individual stations. These changes alter the statistical characteristics of temperature, including the mean, variance, and frequency distribution and introduce uncertainties in spatially averaged trends. This article determines new estimates of temperature trends over Iran after the detection of artificial change points and application of homogenization. The regional trend of temperature is estimated using seasonal and annual minimum and maximum temperature from stations that have identical variability across the country. The country may be segmented to 10 such regions in terms of trends and variability of temperature. There is little doubt that temperatures have increased in all regions at nearly equal rates of 0.4–0.5 and 0.2–0.3 (°C/decade) for minimum and maximum temperature, respectively in Iran. The finding in earlier work of a few individual stations with negative trends is found to be due to artificial effects such as relocation.

Description: ABSTRACT Ethiopia has wide eco-environmental diversity ranging from extreme heat at one of the lowest places in the world to one of the coolest summits in Africa. Associated with this environmental diversity and climate change, climatic extremes are expected to change over time and also vary across eco-environments in the country. This study was conducted to examine the trends of past precipitation and temperature extremes over three eco-environments in Ethiopia. The study involved analysis of 20 extreme indices computed from daily temperature and precipitation data spanning over 42 years (1967–2008). The climate data were obtained from 11 stations selected from three major eco-environments (pastoral, agropastoral and highland). The results indicated positive trends for maximum value of the maximum temperature (TXx), warm days (TX90p), warm nights (TN90p) and warm spell duration indicators (WSDI) and negative trends for cool days (TX10p), cool nights (TN10p) and cold spell duration indicators (CSDI) in more than 8 of the 11 stations studied. However, most of the trends were not significant at many of the stations and the significant trends were not uniquely differentiated by eco-environments. Unlike temperature extremes, precipitation extreme trends showed high variability among nearby stations within eco-environments and were not significant at many of the stations studied. It is concluded that trends of temperature and precipitation extremes vary considerably among stations located within a given eco-environment indicating that the response of local climate to global warming could be different in physiographically diverse regions.

Description: Historical datasets of instrumental temperature, rainfall and atmospheric pressure observations have recently been developed for southeastern Australia (SEA), extending the regional climate record back to 1860. In this study we use the newly extended datasets to conduct the first multivariate examination of SEA climatic changes from 1860 to 2009. The climate in SEA is highly variable in response to fluctuations in large-scale circulation features including El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). To examine how teleconnection patterns in the SEA region have changed over time, we then applied a path analysis over the 1871–2009 period to isolate the independent relationships between SEA climate variables, ENSO and the IOD. The extended data revealed several relatively unknown periods of 19th century SEA climate variations. Cool and wet conditions were identified in the early parts of the 1860s, 1870s and 1890s, while dry conditions were found in the late 1870s, 1880–1885, and during the well-known Federation Drought (1895–1902). Path analysis identified a decrease in the influence of ENSO on SEA rainfall during 1920–1959, particularly in the austral winter. Increasing correlations between the IOD and annual SEA rainfall and pressure were found in the recent 1970–2009 period, but appear to be within the range of natural variability in the context of the last 140 years. Despite large changes in the correlations between SEA rainfall, ENSO and the IOD, correlations between SEA rainfall and temperature remained stable over 1871–2009. Similar results were obtained using 20th Century Reanalysis data for 1871–2009, supporting the quality of the extended historical datasets and providing verification for the reanalysis data in SEA from the late 19th century.

Description: Only few reliable records are available covering more than 5 years of meteorological conditions on Arctic glaciers. Here, we report on the operation of an automatic weather station at the Austfonna ice cap, Svalbard, over an 8-year period from 2004 to 2012. Time series of measured and derived quantities are analysed to characterize meteorological conditions close to the equilibrium line altitude at ∼400 m.a.s.l. The mean annual temperature is −8.3 °C but exhibits large variability such that excursions above 0 °C occur even during winter. In general, relative air humidity is high and evaluating the wind pattern, we find that moisture is primarily advected from south-easterly directions. Net radiation is dominated by shortwave radiation and, hence, surface albedo plays an important role in the radiation budget. Frequent summer snowfalls, as observed in 2008, have the ability to maintain a high albedo over much of the ablation season, thereby having large impact on the energy balance as well as on glacier mass balance. Cloudiness is assessed using records of incoming longwave radiation. Analyzing the radiation data, we find evidence for the radiation paradox, i.e. an increase of average net radiation (2004–2012) from −15.7 W m −2 for clear-sky conditions to 7.3 W m −2 during overcast skies.

Description: ABSTRACT The Arctic Oscillation (AO) and ENSO have dominant influences on the extratropical and tropical climate variabilities in the Northern Hemisphere, respectively. In this paper, we document a strengthening of the AO–ENSO relationship in January after the mid-1990s, when the interannual variability of the previous September Arctic sea ice cover (ASIC) had consistently increased, by analyzing the AO, ENSO, and associated atmosphere–ocean variability. Our analysis further suggests that the larger ASIC interannual variability may account for the larger scale wave train across the Pacific basin, and the AO is strongly coupled to the circulation in the North Pacific. The strengthened connection between the −AO and Aleutian Low (AL) may contribute to the identified closer relationship between the −AO and ENSO.

Description: ABSTRACT A 175 years long homogenized composite record of monthly mean temperatures is presented for Oslo, the capital of Norway. The early raw data have been digitised and quality controlled, and monthly means have been calculated. Some early original observations carried out in a Wild screen (1877–1936) were found to be spuriously high because of inappropriate sheltering from sunlight. These spurious temperatures were not used in the composite record, but alternative temperatures measured (1837–1933) by thermometers placed outside windows at the Astronomical Observatory were used instead. No inhomogeneity was detected in the latter series after adding an instrument correction of +0.3 °C, but the start year of the correction remains uncertain. The more recent part of the composite record used the long-term series (1937 to present) from Blindern in Oslo, the premises of The Norwegian Meteorological Institute. Two small inhomogeneities were detected in the Blindern series, possibly caused by a weak urban heat island effect or growing/cutting of trees. The study revealed that the annual mean temperature has increased by 1.5 °C in the period 1838–2012. The most pronounced increase in annual temperature occurred during the last 50 years, and in the early 20th century that ended with a local maximum in the 1930s. The temperature has increased significantly in all seasons; however, the temperature increase in summer was less than a half of that in winter and spring, which were the seasons with largest increase. In addition the monthly mean temperature of the coldest month in each year has increased two times faster than the warmest one. The most significant temperature variations were associated to ∼ 5-year time scales in its early part, but since 1930 and up to present, the dominant time scales were 10–20 years.

Description: ABSTRACT The onset, retreat and the length of growing season in the north-eastern region of Sri Lanka were investigated using daily rainfall data for the period 1961 to 2000. Data from three weather stations situated in the coastal belt in the northern and eastern parts of Sri Lanka (Jaffna, Trincomalee and Batticaloa) that receive rainfall predominantly from the northeast monsoon were selected for this study. A method based on cumulative rainy days was utilized in the determination of the onset and retreat dates. It is shown that there is substantial interannual variability in onset and retreat dates. The mean onset and retreat dates fall on the standard week 38.3 ± 2.7 and 53.0 ± 2.9, respectively. The mean duration of the growing season is 14.7 ± 3.4 weeks. The retreat date and thus the length of growing season could be extended by 2 weeks if the probability of occurrence of rain during the onset is favourable for the retreat. The results indicate that there has been no significant trend in the onset and retreat dates during the last 40 years in the dry zone of Sri Lanka. The onset date and the length of growing season are weakly correlated with early onset dates leading to longer growing seasons. The study concludes that rainy days could be used successfully to determine the mean rainfall onset and retreat dates in the dry zone of Sri Lanka.

Description: ABSTRACT A 256-year composite monthly rainfall record representative of the lowlands around Carlisle, northwest England is presented, providing the third longest instrumental record of rainfall available in the UK and expanding the sparse network of long homogenous rainfall series. This article describes the construction of the rainfall record for the period 1757–2012 and presents analyses of long-term annual and seasonal variability, with a particular focus on wet/dry extremes. Three primary rainfall records from Carlisle underpin the reconstruction, with others selected based on length of record and proximity to the primary recording stations. Linear regression and adjustment factors were applied to create a homogenous continuous rainfall record, assessed by cross-comparison with other long-term UK rainfall records. Double-mass curve and standard normal homogeneity tests using long records representative of Manchester and Appleby confirmed that the Carlisle record is homogenous, but includes a period of uncertainty during the period 1886–1911, which is within the recognized instrumental phase of recording. Analysis of the series shows long-term temporal rainfall variability, with seasonal rainfall totals showing a significant increase in winter rainfall. An increasing trend in annual rainfall was also identified but is not significant. Several previously documented notable extreme wet (e.g. winter 1834) and dry (e.g. summer 1995) seasons were identified, along with several additional seasons (e.g. winter 1764 and summer 1891) that can be considered as extreme and occurred during the more poorly understood 18th and early 19th centuries. These results allow recent extremes to be placed within the context of long-term natural variability. At the decadal timescale seasonal rainfall totals are highly variable. The wettest season fluctuates between autumn and summer until the late 20th century (1990s), when winter became wetter than any other season for the first time on record.

Description: ABSTRACT Policymakers, governments and aid agencies require operational environmental monitoring in support of evidence-based policy-making and resource deployment in crisis situations. For Africa, this is only feasible at sub-continental scale with a large network of automated meteorological stations, a large number of highly coordinated field observers or with satellite remote sensing. The challenge with satellite data lies in the derivation of meaningful environmental indicators. This article describes a conceptual framework for understanding satellite-derived indicators of rainfall and vegetation greenness trends over Africa. It attributes observed vegetation changes to climatic (i.e. rainfall linked) and non-climatic drivers. A decade of annual rainfall and vegetation data over sub-Saharan Africa was analysed using satellite-based rainfall estimates [Famine Early Warning System Rainfall Estimation 2.0 (FEWSNET RFE 2.0)] from National Oceanic and Atmospheric Administration's (NOAA's) Climate Prediction Centre and the Normalized Difference Vegetation Index (NDVI) obtained from the Satellite Pour l'Observation de la Terre Vegetation (SPOT-VGT) sensor. Rainfall and vegetation greenness trends were analysed for 759 administrative regions of sub-Saharan Africa to identify those regions that have experienced a negative, positive or stable rainfall/vegetation trend over the period 2001–2010. The character of the relationship between the annual rainfall and max NDVI trends were examined to identify areas where the changes in greenness could be attributed to climatic (rainfall) and non-climatic (human land use or ecological disturbance) changes. Regions where increasing rainfall was associated with vegetation greening were found in West Africa, Central African Republic, West Cameroon and northeastern part of South Africa, whereas areas with evidence of ‘climatic vegetation degradation’ were located in Southern Madagascar, Nigeria, Kenya and the Garden Route region of South Africa.

Description: ABSTRACT The North African climate is analysed for August during a 32-year period using the European Centre for Medium Range Weather Forecast (ECMWF) global data set to investigate the intensity variability at 600 mb of the subtropical highs, Africa easterly jet (AEJ) with two embedded local wind maxima, and African easterly waves over North Africa and the Arabian Peninsula. The variability of these synoptic weather systems is higher in East Africa. The most noticeable variability of intensity occurred with easterly waves. Maintenance of easterly waves from the Arabian Peninsula into East Africa is dependent on strong zonal gradients from the AEJ through shear vorticity. These zonal gradients were induced by the strengthening of the subtropical highs and the presence of a westerly jet in Central Africa and south of the Arabian Peninsula. During positive ENSO periods, these systems are generally weaker while in negative periods are stronger. The focus of this research is to investigate the role of the Arabian High and eastern local wind maximum (LWM E ) on complementing the Saharan High and western local wind maximum (LWM W ). It is found that an intense local wind maximum in East Africa helps maintain the easterly waves and their westward propagation from the Arabian Peninsula.

Description: ABSTRACT The purpose of this paper is to describe the variability of sunshine duration and air temperature and to assess the relationship between these variables. The paper is based on daily means of air temperature and daily sunshine duration totals from the 1884–2012 period. The variability analyses of these variables for the whole period is supplemented by additional trend analyses for two separate time periods described in the literature as dimming (1958–1985) and brightening (1985–2012). All the data come from the Jagiellonian University Research Station in Krakow. In Krakow, there is a decrease in the number of sunshine hours in the 1950s to 1980s period followed by an increase in the last two decades of the 20th century. The long-term course of air temperature in Krakow confirms the results obtained in other places in the world and exhibits increasing warming of the climate. The temperature increase is visible in each month but is highest in winter (exceeding 1.5 °C per 100 years). Therefore, it may be assumed that the increase in air temperature, which has been particularly strong since the 1980s, showed the full effect of global warming in the brightening period that was previously masked by global dimming as shown in the sunshine duration records. The time course of air temperature for the whole study period (1884–2012) shows a statistically significant positive linear trend. The present warming is influenced probably by the air-quality improvement (brightening period) and an increase in sunshine duration in the years 1980–2012.

Description: The relationship between long-term variations in summer temperature in Japan since the early 20th century and the large-scale atmospheric circulation field was analysed. The combined influence of various circulation predictors on temperature variations was analysed via a multicomponent canonical correlation analysis (CCA). The positive phase of the first CCA mode is related to a positive temperature anomaly across Japan, characterized by a weak blocking high over the Okhotsk Sea, and a strong North Pacific subtropical high (NPSH) over Japan. The positive phase of the second CCA mode corresponds to a positive temperature anomaly in southwestern Japan, and a negative anomaly in northern Japan, characterized by an anticyclonic circulation anomaly over eastern China and a cyclonic anomaly over the northwestern Pacific. By investigating the temporal changes in CCA scores, we detected an abrupt increase in the first CCA score in the early 1910s and a long-term increasing trend in the second CCA score since the early 20th century. The abrupt increase in the first CCA score indicates an abrupt increase in temperature throughout Japan. By investigating changes in the circulation field associated with this abrupt warming, we determined that intensification of zonal flow over the Okhotsk Sea was responsible. The increasing trend of the second CCA score indicates an increase in the regional difference in summer temperatures between northern and southwestern Japan. After investigating changes in the circulation field before and after the mid-20th century, we suggest that the southwestward shift of the NPSH and the weakening of anticyclonic circulation anomalies over the northwestern Pacific were responsible for this increase in the regional temperature difference.

Description: The meridional displacement of East Asia jet (EAJ) is characterized by the leading mode of upper tropospheric zonal wind variability over East Asia in boreal summer, and is closely related to the East Asia summer monsoon and downstream climate. Present study reveals that the meridional displacement of EAJ is associated with tropical Indian Ocean (TIO) SST anomalies. When the TIO SST is higher than normal, the overlying tropospheric air warms up through the modulation of the TIO SST on tropical convection. The anomalous convection forces a Kelvin wave wedge penetrating into the equatorial western Pacific, leading to a decrease in precipitation near the Philippines. Combined with the climatological easterly shear over the subtropical western North Pacific, the Pacific-Japan/East Asia-Pacific (PJ/EAP) teleconnection is induced along the East Asia coast. The PJ/EAP-related upper-level anomalous cyclone accelerates westerly in the south flank of EAJ and decelerates westerly in the north flank. Thus, EAJ shifts southward. In contrast, the EAJ shifts northward when the TIO SST is lower than normal. Copyright © 2011 Royal Meteorological Society

Description: Early detection of extreme drought and flood events either over the whole globe or a broad geographical region, and timely dissemination of this information, is indispensable for mitigation and disaster preparedness. Recently, the APEC Climate Center (APCC) has launched a global precipitation variation monitoring product based on the Climate Anomaly Monitoring System-Outgoing Longwave Radiation Precipitation Index (CAMS-OPI) data. Here we quantify the reliability of CAMS-OPI, as well as other gauge-satellite-merged and reanalysis precipitation datasets, for the purpose of monitoring large-scale precipitation variability in East Asia. The ground truth is the newly available gauge-based data from the project titled ‘Asian Precipitation—Highly-Resolved Observational Data Integration Towards Evaluation (APHRODITE) of the Water Resources’. It is found that the seasonal-to-interannual rainfall deficit and surplus given by various reanalysis systems sometimes do not match the spatial patterns seen in the APHRODITE data. Moreover, maps showing the Standardized Precipitation Index (SPI) become less and less reliable as the time scale based on which values are calculated increases. In contrast, the performance of gauge-satellite-based rainfall datasets is satisfactory and the quality of SPI maps does not decay as the time scale increases. Overall, CAMS-OPI is found to be reliable for monitoring large-scale precipitation variations over the East Asian sector. Copyright © 2011 Royal Meteorological Society

Description: The connection between Tropical Pacific and North Pacific variability is investigated in a state-of-the-art coupled ocean-atmosphere model, comparing two 20th century simulations at T30 and T106 atmospheric horizontal resolutions. Despite a better simulation of the frequency and the spatial distribution of the Tropical Pacific anomalies associated with the El Niño Southern Oscillation (ENSO) in the high-resolution experiment, the response in the North Pacific is scarcely different from the low-resolution experiment where the ENSO variability is weaker and at higher than observed frequency. In the North Pacific, the response of surface atmospheric fields to the variability in the Tropical Pacific appears to be affected by local coupling processes significantly different in the two experiments. The coupling between sea level pressure (SLP) and sea surface temperature (SST) in the North Pacific as well as the influence of the Tropical Pacific SST has been measured here by means of the ‘coupled manifold’ technique. In the low-resolution case the SLP variances linked to the fraction of North Pacific SST not influenced by the Tropical Pacific are weak suggesting that the remote influence is strong, consistently with the observations. On the contrary, in the high-resolution experiment the fractions and the patterns of the SLP variances due to the Tropical Pacific SST and those linked to the North Pacific SST are comparable. In the latter case, model systematic errors in the northwestern Pacific influences the local coupling processes thus triggering the remote response. We conclude that an increased atmospheric horizontal resolution does not reduce the coupled model systematic errors in the representation of the teleconnection between the North and the Tropical Pacific and that the validation of coupled models has to consider both remote and local processes. Copyright © 2011 Royal Meteorological Society

Description: In this work we present a homogenized high-resolution data set composed of 200 daily precipitation series spanning the last 90 years, located over an area centred on the Trentino—South Tyrol region (central part of the European Alps), in a transition zone between the climates of the southern and northern slopes of the Alps. We analysed the trends of total precipitation (TP), wet days (WD) and average intensity (PI), as well as trends of the number of events and precipitation amounts belonging to 12 different daily intensity categories. For an easier understanding of geographical patterns, we set up a gridded data set in terms of anomalies, with a spatial resolution of 0.1°. All the statistics were analysed for trend over the entire period spanned by the data and on subperiods of variable length. On regional average, we found a weak decrease in TP (about 1%/decade with respect to the 1971–2000 mean) over the entire studied period (1922–2009), which was statistically significant only in spring. Gridded data show that the decrease is related to a reduction in the number of WD in the eastern part of the study area, and a decrement in PI in the western part, with orography playing a clear role in this differentiation. On a daily scale, trends of the strongest events present scarce spatial coherence and are only locally significant, however the results are highly dependent on the period analysed. Comparisons with previous low-resolution studies on the same area underline the importance of a high-resolution data set in characterizing spatial variability of climatic trends in precipitation. Copyright © 2011 Royal Meteorological Society

Description: The relationship between temperature variability inside Scarisoara Ice Cave (NW Romania) and large-scale atmospheric circulation and sea surface temperature (SST) anomalies is investigated. A composite analysis reveals that high (low) temperatures inside the cave are associated with high (low) air temperature over a large area that covers central and Eastern Europe. Coherent large-scale patterns are identified in the field of SST anomalies. The corresponding atmospheric circulation patterns favour relatively warm (cold) air advection towards the cave region during high (low) temperatures inside the cave. A correlation analysis reveals that the atmospheric circulation patterns associated with cave temperature variability is mainly the result of the East Atlantic-Western Russia ( r = − 0.38) teleconnection pattern. Our results have strong implications for the interpretation of climate variability in the region as recorded by the ice block within the Scarisoara Ice Cave, which is more than 3000 years old. Copyright © 2011 Royal Meteorological Society

Description: An examination of synoptic environments conducive to urban thunderstorm development surrounding Atlanta, Georgia, was conducted. Synoptic environmental characteristics were evaluated utilising regional analyses of heights, winds, temperature, and moisture characteristics as well as several thermodynamic variables. A flexible beta cluster analysis was used in order to identify disparate groupings of common synoptic and thermodynamic environments. These groupings were then further analysed through the production of synoptic composites, radar, and lightning distributions. Although the analysis stratified the data by days with weak synoptic flow and maritime tropical airmass characteristics, enough variation existed within the synoptic and thermodynamically grouped parameters to identify four distinct environments present during urban thunderstorms. Environments exhibiting moderate thermodynamic instability were found to produce the highest flash rates and most frequent composite reflectivity days. Copyright © 2011 Royal Meteorological Society

Description: A fully coupled atmospheric-biospheric regional climate model, GEMRAMS, was used to evaluate potential effects of land-use/land-cover changes (LULCC) on near-surface atmosphere over a southern South American domain at seasonal time scales. In GEMRAMS, leaf area index and canopy conductance are computed based on modelled temperature, solar radiation, and the water status of the soil and air, allowing a two-way interaction between canopy and atmosphere. Several austral spring-early summer simulations were conducted using land cover representing current (i.e. agricultural landscape), natural (i.e. before European settlement), and afforestation scenarios for three periods associated with El Niño-Southern Oscillation (ENSO) conditions. The shift to agriculture resulted in a generalized decrease in albedo, reducing the available energy at the near-surface. The energy partitioning between latent and sensible heat fluxes changed, leading to distinct temperature responses. A shift from grass to agriculture led to cooler and wetter near-surface atmospheric conditions. Warmer temperatures resulted from the conversion of wooded grasslands or forest to agriculture. The LULCC-induced signal was spatially heterogeneous and with a seasonal component associated with vegetation phenology. A significant decrease in maximum temperatures in the southern and central Pampas led to a decrease in the diurnal temperature range. Basing on some observational studies in this region our results suggest a potential strong influence of LULCC on the maximum temperatures in central Argentina in summer. Afforestation resulted overall in cooler temperatures. For both LULCC scenarios the direction of the energy fluxes and temperature changes remained in general the same in two extreme ENSO years, although for some vegetation conversions the signal reversed direction. Overall, the impacts were enhanced during a dry year, but the response also depended on the vegetation types involved in the conversion. The effects on precipitation were insignificant in the agriculture-conversion scenario and a general increase was found in the afforested scenario. Copyright © 2011 Royal Meteorological Society

Description: The variability of seasonal precipitation anomalies over Europe displays significant spectral peaks at quasi-quadrennial periods, appearing in power spectra of the two leading principal components of the field. Band-pass filtering of these time series reveals the presence of two different signals within the quadrennial band, each associated to distinct precipitation, sea level pressure (SLP) and sea surface temperature (SST) patterns, obtained here by means of linear regression and composite analysis. One of the quadrennial components seems to be connected with the impact of El Niño-Southern Oscillation (ENSO) over Europe, that leads to anomalous SLP over the North Sea, southwestern Scandinavia and central Europe, along with precipitation anomalies of the same sign in a wide band extending from the British Isles into eastern Europe, fringed by anomalies of the opposite sign along the Norwegian coast and in the western Mediterranean area. The other quadrennial component bears no correlation with the Tropical Pacific SST anomalies, and seems connected with SST anomalies in the equatorial Atlantic, and in the Mediterranean, North and Norwegian Seas. The precipitation anomalies feature a contrast between the northern and southern regions of Europe akin to that associated with the North Atlantic Oscillation (NAO). The SLP pattern resembles the NAO structure, but with a marked eastward shift of its two centres of action towards central Europe and the Barents Sea, respectively. Copyright © 2011 Royal Meteorological Society

Description: The relationships between large-scale atmospheric circulation types and seasonal regimes of daily precipitation over Iran are assessed using daily precipitation from a high-resolution gridded dataset provided by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) Project. Regional spatial modes of daily precipitation variability were identified by S-mode Principal Component Analysis (PCA) with Varimax rotation, applied to the subset of days when at least 10% of all grid-points over Iran received precipitation ≥ 5 mm. The study refers to the period 1961–2004 and is carried out for each season (excluding summer) separately. To characterize the dynamical features associated with each regional precipitation regime (PR), composites of daily atmospheric fields are computed by only averaging days with rotated PCA scores ≥ 1.5 (strong positive phase). In autumn and winter, Iran is divided into five PRs, while four PRs are identified in spring. Results suggest that the spatial distribution of precipitation over Iran is largely governed by the geographical position of both the mid-tropospheric trough over the Middle East and the Arabian anticyclone. In fact, in almost all PRs, the trough, as a pre-conditioning factor, leads to regional-scale ascending motions, whereas the Arabian anticyclone induces low-tropospheric moisture transports from southern water bodies into the cyclonic systems near Iran, triggering rain-generating conditions. Copyright © 2011 Royal Meteorological Society

Description: Synoptic weather typing depends on the method and its implementation. This paper demonstrates the utility of a 2-phase batch self-organising map procedure (CP2) in comparison with a procedure (CP1) consisting of T-mode principal component analysis followed by convergent K-means clustering, for classifying the synoptic weather types over east Australia. Four classifications were obtained for the 52-year NCEP/NCAR dataset, one from CP1 and three from CP2. These classifications were examined in terms of grouping quality, mean type maps, type frequencies, lifetime and transitions, and in relation to the Southern Oscillation Index (SOI). The results showed that the classifications derived from different procedures are inter-confirmative and capture a similar set of major synoptic situations influencing east Australia, each having counterparts in previous studies and conforming well to local synoptic experience. In particular, this study demonstrates that CP2 is a promising tool for the purpose of weather typing with a two-fold utility: CP2 can perform data projection (neighbourhood width 〉 = 1) and provide a flexible means for visualising the broad distribution of the daily weather patterns in the dataset; and the procedure can also conduct cluster analysis (neighbourhood width = 0) and produce results equivalent to those from CP1. It was found that performing data clustering or data projection may affect the inference associated with type frequencies. While focusing on methodology, the analysis has also shown that the frequency of synoptic weather types fluctuates on the seasonal, interannual, and decadal scales. From the 1970s to 1995 there was a decline in the occurrence of a few weather types associated with an east-coast trough extending over New South Wales from the north, and a slight increase in the activity of some anticyclonic types. Significant correlations were also identified between SOI and frequencies of synoptic weather types on the annual and seasonal timescales. Copyright © 2011 Royal Meteorological Society

Description: ABSTRACT The vegetation green-up date (GUD) is widely expected to advance to an earlier date in the year in response to increasing spring temperature. However, although it is well known that climatic warming has been more intensive at higher latitudes than at lower latitudes in the Northern Hemisphere, it is not known whether this greater climatic warming at higher latitudes has led to a greater advance of vegetation GUD during the past few decades. We have first determined GUD from the satellite-derived greenness vegetation index for all the vegetated land between 30 and 75°N from 1982 to 2008, and then investigated the latitudinal pattern of the inter-annual trend of GUD and its relation to pre-season temperature. The results show a generally greater GUD advance in lower latitudes, in spite of the faster increase in pre-season temperature in higher latitudes. We find that 88% of the latitudinal variability in the GUD trend can be explained by the sensitivity of the GUD response to pre-season temperature. The GUD change has also resulted in a northward shift of GUD isolines by 1°–4° between 50 and 65°N during the period. These findings highlight the sensitivity of vegetation response to temperature in shaping the spatial pattern of spring phenological change and suggest that temperate ecosystems may experience greater phenological change if the current latitudinal pattern of climatic warming continues.

Description: ABSTRACT In this study, we investigated the impact of the spring North Pacific Oscillation (NPO) on typhoon frequency over the Western North Pacific (WNP, north of the equator and west of dateline). To reduce the statistical uncertainty, all the data we used have been excluded the El Nino/Southern Oscillation signals. We found that the spring NPO positively correlated with the annual typhoon number over the WNP (hereafter TNWNP) during the period 1968–2010, with a correlation coefficient of 0.62 (above the 99% significance level). When the Northern Low and Southern High pattern over the north Pacific weakens, the TNWNP tends to increase. A positive phase of spring NPO is associated with the tropical circulation and sea surface temperature (SST) anomalies in the North Pacific, which may lead to unfavorable dynamical and thermal conditions for typhoon genesis over the WNP during JJAS. The negative anomaly of SST over the WNP, associated with the positive spring NPO anomaly, is connected to the tropical atmospheric circulations from spring to summer via good oceanic seasonal persistence and air–sea interaction. Thus, the spring NPO-related variability of the tropical atmospheric circulation as well as the SST can affect typhoon activity over the WNP. In addition, the spring NPO can also be adopted as a predictor for the summer rainfall in South China, since the spring NPO can modulate the WNP SST and western Pacific subtropical high from spring to summer.

Description: The NCEP/NCAR reanalysis and precipitation data from the Australian Bureau of Meteorology are used to analyse variability in rainfall during the austral summer (December–March, DJFM) in northeast Australia (NEA). NEA rainfall shows a marked decrease over the past 50 years, mainly in the austral summer. Our analysis reveals that the summer rainfall decrease in NEA is generally an interdecadal phenomenon. The declining trend has an imprint in the tropical Australian summer monsoon (TASM). Not only does TASM have a phase-to-phase influence on NEA summer rainfall at the interannual scale, it is also closely linked with interdecadal variation in NEA summer rainfall. Thus, the decrease in NEA at the interdecadal scale could be attributed to corresponding variation in TASM. Moreover, the coupled linkage between TASM and NEA summer rainfall appears to be largely independent of El Niño–Southern Oscillation. One possible reason for the interdecadal weakening trend in TASM is a sustained interdecadal warming trend in sea surface temperature (SST) over the Wharton Basin (100°–130°E, 20°–5°S). When the Wharton Basin is in a cold state, anomalous westerlies occur in the lower troposphere in the TASM region, and cyclonic circulation anomalies and rising flows occur in the low and middle troposphere over NEA, which are associated with a strong TASM situation, consequently favouring enhanced rainfall over NEA; the opposite occurs in the case of a warm Wharton Basin. SST over the Wharton Basin has shown a continuous warming trend over the past 60 years, contributing to the weakening of TASM and, consequently, a decrease in NEA summer rainfall. Copyright © 2011 Royal Meteorological Society

Description: We estimate the potential predictability of European winter temperature using factors based on physical studies of their influences on European winter climate. These influences include sea surface temperature patterns in different oceans, major tropical volcanoes, the quasi-biennial oscillation in the tropical stratosphere, and anthropogenic climate change. We first assess the predictive skill for winter mean temperature in northern Europe by evaluating statistical hindcasts made using multiple regression models of temperature for Europe for winter and the January–February season. We follow this up by extending the methodology to all of Europe on a 5° × 5° grid and include rainfall for completeness. These results can form the basis of practical prediction methods. However, our main aim is to develop ideas to act as a benchmark for improving the performance of dynamical climate models. Because we consider only potential predictability, many of the predictors have estimated values coincident with the winter season being forecast. However, in each case, these values are predictable on average with considerable skill in advance of the winter season. A key conclusion is that to reproduce the results of this paper, dynamical forecasting models will require a fully resolved stratosphere. Copyright © 2011 Royal Meteorological Society and British Crown copyright, the Met Office

Description: Extreme (maximum and minimum) temperature records are analysed for search of spatio-temporal variation patterns in regional climate change throughout Turkey based on 165 stations, as many stations as possible, and as far back as possible, in the period 1961–2008. The main purpose is to better understand the possibility of annual trends during the warm and cold periods. Homogeneity and Mann-Kendall trend test methodologies are applied in order to identify statistically significant trends at each station. It is observed that there is a significant warming in both annual maximum and minimum temperature records. Increasing trends in warm period are comparatively stronger than cold periods so far as the maximum temperatures are concerned. There are also increasing trends in annual minimum temperature during warm periods with almost no change in minimum temperatures of cold periods. The observations indicate that the temperature began to increase in the beginning of the 1980s. Nine stations are chosen from different geographical regions in order to compare monthly average temperature values of 1961–1984 with 1985–2008 and a marked increase was observed on extreme temperatures in these stations. Copyright © 2011 Royal Meteorological Society

Description: Aiming at better understanding the causes and processes leading to meteorological dry and wet episodes in western Iran, the present study investigates the space and time variability of winter dry/wet events and their associated large-scale atmospheric driving circulations. Using monthly precipitation data of 140 weather stations for 1965–2000, meteorological dry/wet spells are assessed through the 1-month Standardized Precipitation Index (SPI). By applying the Principal Component Analysis (PCA) with Varimax rotation to the SPI field, three sub-regions of independent climatic variability are identified. Large-scale daily atmospheric circulation patterns are then classified into twelve circulation types (CTs) by applying PCA to the 500 hPa geopotential height fields coupled with the non-iterative K-means clustering technique, in which extreme rotated principal component scores, in their positive and negative phases, are considered as potential groups and their centroids are computed by averaging all days that fulfil the extreme score criterion. The linkage between daily CTs and winter dry/wet spells in the region is investigated by correlating the frequencies of occurrence of each CT with the time series of dry/wet events, derived from both the rotated principal component scores of SPI and the SPI series at three representative stations of the identified sub-regions. Severe/extreme events have also been considered. A performance index is applied to test the relationship between CTs and dry/wet events. Results show statistically significant correlations between the frequencies of occurrence of the identified CTs and of dry/wet spells at the three representative stations, suggesting that particular CTs are potential predictors for the winter dry/wet events in western Iran. Copyright © 2011 Royal Meteorological Society

Description: The application of spatial-temporal stochastic rainfall models to semi-arid or arid areas is expected to be particularly challenging because of the high variability of rainfall, sparse rain gauge networks with significant periods of missing rainfall and potential data quality issues. In this article, a generalized linear model (GLM) has been fitted to daily rainfall data from the period 1975–1999 for 13 gauges in a 7660 km 2 sub-basin of the Limpopo basin in Botswana, with the objective of exploring applicability of the GLM for infilling historic records and for climate change analysis. Several relevant statistics of rainfall space-time variability were used to analyse model performance, including use of an independent validation period and sites that were not used in the fitting. The GLM was considered to simulate rainfall adequately for the purpose of sub-basin-scale water resource studies, although the model uncertainty is high. The main factors affecting rainfall space-time variability were found to be seasonality, autocorrelation of daily rainfall, altitude, latitude and longitude. Addition of large-scale drivers of rainfall (pressure, humidity and temperature) further improved representation of inter-annual variability, and this link to large-scale climate potentially facilitates downscaling of global climate model outputs. Although the model was locally sensitive to data quality issues, there was no evidence that these issues affected sub-basin scale analysis. Copyright © 2011 Royal Meteorological Society

Description: Although the Sahel precipitation decreased during the 1970s, it has increased since the mid-1980s. This trend shift also occurred throughout the world with weaker signatures than in Sahel. The Sahel trend shift was remarkably similar to the trend shift of the difference in SSTs between Northern Hemisphere and Southern Hemisphere; that is, until the 1980s the NH SST had decreased relative to the SH SST, whereas after the 1980s this trend reversed. Concurrent with this shift were trend shifts in the NH-SH contrast in atmospheric temperature and humidity and in inter-hemispheric meridional winds around Sahel. It appears that the NH-SH SST contrast determines the long-term variation of precipitation over Sahel through the shift of inter-hemispheric atmospheric circulations. Copyright © 2011 Royal Meteorological Society

Description: Surface temperature (Ts), sea-level pressure (SLP), and zonal wind (U) fields from NCEP/NCAR reanalysis spanning the period 1949–2006 have been jointly analysed by means of principal component analysis to assess the dominant modes of climate variability in the inter-American region, including the eastern Pacific and Atlantic Oceans. Two aspects of these time-series were analysed: the annual cycle and its residual. Seasonal modes 1 and 2 take the form of north-south dipoles of Ts over oceanic and terrestrial environments. Seasonal mode 3 derives from SLP changes over the Amazon with a Caribbean wind response. Temporal fluctuations in the residual (ANOM) fields are resolved by six modes. ANOM1–3 is dominated by SLP and Ts changes across the equator in the Atlantic and east Pacific. The ANOM4 pattern is an isolated wind mode in the NE Atlantic. ANOM5 and 6 are dominated by sea temperature patterns in the South Atlantic and in the path of African easterly waves crossing the tropical Atlantic. The residual time scores are marked by significant warming (Ts) trends and co-located lower pressure. Cycles in the anomaly time scores tend to occur at 2- to 5-year periods. Modes dominated by atmospheric variables exhibit greater high-frequency ‘noise’ than oceanic modes. Predictability is assessed through development of multivariate regression models trained on Caribbean rainfall and related target time-series. The seasonal dipole between North and South America is found to modulate Pacific El Niño-Southern Oscillation and tropical Atlantic influence on Caribbean rainfall. Copyright © 2011 Royal Meteorological Society

Description: Current climate and future climate-warming runs with the RegCM Regional Climate Model (RCM) at 50 and 11 km-resolutions forced by the ECHAM GCM are used to examine whether the increased resolution of the RCM introduces novel information in the precipitation field when the models are run for the mountainous region of the Hellenic peninsula. The model results are inter-compared with the resolution of the RCM output degraded to match that of the GCM, and it is found that in both the present and future climate runs the regional models produce more precipitation than the forcing GCM. At the same time, the RCM runs produce increases in precipitation with climate warming even though they are forced with a GCM that shows no precipitation change in the region. The additional precipitation is mostly concentrated over the mountain ranges, where orographic precipitation formation is expected to be a dominant mechanism. It is found that, when examined at the same resolution, the elevation heights of the GCM are lower than those of the averaged RCM in the areas of the main mountain ranges. It is also found that the majority of the difference in precipitation between the RCM and the GCM can be explained by their difference in topographic height. The study results indicate that, in complex topography regions, GCM predictions of precipitation change with climate warming may be dry biased due to the GCM smoothing of the regional topography. Copyright © 2011 Royal Meteorological Society

Description: It has been known for some time that the potential exists to construct a long daily series of surface pressure for the city of Paris because of the existence of various instrumental series that might be pieced together. The remarkable weather diary kept by the physician, Louis Morin, sets the start of the potential series to 1670, and various other series have now been digitized and corrected to allow the formation of a daily series of Mean Sea-Level Pressure (MSLP) that spans the greater part of 300 years. Unfortunately, a gap in the series still exists for the period 1726–1747, where it appears that no barometer observations have survived. This paper discusses the sources of the various pressure data used in the Paris daily MSLP series, and describes how these observations have been corrected and amalgamated to form a near-continuous, homogeneous series. The data are compared with a similar long daily series that has been independently constructed for the city of London (1692–2007). This comparison indicates that the two MSLP series are of a sufficient quality to provide useful information about the atmospheric circulation across western Europe over the last 300 years. Copyright © 2011 Royal Meteorological Society

Description: The availability of water, and its quality, are among the chief growing concerns that are expected to cause serious questions over the sustainability of the Mediterranean region especially under the present climate change impacts, in general, and over the water resources of Turkey, in particular, due to precipitation pattern variability and sensitivity at arid and semi-arid regions within the country. In order to assess climate impacts concerning precipitation over Turkey, expected spatio-temporal changes are examined by considering precipitation time series at 165 stations across the country covering the period from 1961 to 2008 for almost 5 decades. Methodologically, homogeneity and Mann-Kendall rank correlation trend tests are applied at each station for the identification of statistically significant trends. Long-term annual precipitation averages indicate a decreasing trend with some oscillations during the last 5 decades. However, in more refined periods, average precipitation decreased during 1968–1973 and 1998–2008, whereas it increased during 1973–1981 and 1989–1998. As a result, there is a significant decrease of about 12, 13, and 35% at all stations in annual, rainy period, and winter season total precipitations, respectively. On the other hand, increases occurred at 4, 3 and 19% of all stations in annual, rainy, and autumn season, respectively. Copyright © 2011 Royal Meteorological Society

Description: Multivariate regression trees (MRTs) have been used in synoptic climatology to construct “circulation-to-environment” synoptic classifications. Because the goal of an MRT is to maximize discrimination of the environmental predictand variables, performance in terms of the synoptic-scale circulation predictors is typically sacrificed. This paper introduces a semi-supervised approach in which a weighted combination of synoptic-scale predictors and environmental variables serve as predictands in a MRT. Results for southern British Columbia, Canada, indicate that (1) a semi-supervised MRT can outperform a fully supervised MRT in terms of discrimination of the surface environment; (2) weighting allows the synoptic classifier to behave as a fully unsupervised model, a fully supervised model, or intermediate between the two ends of the spectrum; and (3) the optimum trade-off between circulation and environment must be chosen by the user depending on specific needs. © 2011 Crown in the right of Canada. Published by John Wiley & Sons Ltd.

Description: The climatology and hydrology of western North America display strong periodic cycles which are correlated with the low-frequency Pacific Decadal Oscillation (PDO). The PDO's signature is seen throughout the entire North Pacific region, with related significant associations to hydrology and ecology in western North America and northeastern Asia. Therefore, the status of the PDO in a warmer world caused by anthropogenic climate change is of great interest. We developed early 21st-century projections of the PDO, using data from archived runs of the most recent high-resolution global climate models from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (Phase 3 of the Coupled Model Intercomparison Project). Because of the geographical adjacency and hypothesized interactions between the PDO and the El Niño-Southern Oscillation (ENSO), and between the PDO and the North Atlantic Oscillation/Arctic Oscillation (NAO/AO), we also developed concurrent projections of ENSO and the NAO and examined their relationships with the projected PDO. For the B1, A1B and A2 Special Report on Emissions Scenarios (SRES) emission scenarios, the PDO projections for 2000–2050 showed a weak multi-model mean shift towards more occurrences of the negative phase PDO, which becomes statistically significant for the time period 2000–2099. However, not all the models showed a consistent shift to negative PDO conditions. Copyright © 2011 Royal Meteorological Society

Description: On the basis of detrended fluctuation analysis (DFA), a new method, moving cut data-DFA (MC-DFA), was presented to detect abrupt dynamic change in correlated time series. The numerical tests show the capability of the presented method to detect abrupt change time-instants in model time series generated by Logistic map. Moving DFA (MDFA) and approximate entropy (ApEn) can provide some information such as a single time-instant of abrupt dynamic change, but both of them cannot exactly detect all of the abrupt change regions. Some traditional methods, such as moving t -test, Cramer method, Mann–Kendall test and Yamamoto method, even cannot provide any information of abrupt dynamic change in these model time series. Meanwhile, results showed that windows sizes and strong noise have some less effect on the MC-DFA results. In summary, MC-DFA provides a reliable measure to detect the abrupt dynamic change in correlated time series, and perfectively make up the deficiencies of MDFA and ApEn. The applications in daily surface air pressure records further verify the validity of the present method. Copyright © 2011 Royal Meteorological Society

Description: In southern Spain a longitudinal pluviometric gradient occurs under Mediterranean climatic conditions in an area that has at least two months of drought each year and very irregular rainfall. From the Strait of Gibraltar to the Cabo de Gata in Almería there is a reduction of more than 1,000 mm year −1 in the mean annual rainfall. Analysis of the frequency and intensity of drought periods is necessary in regions where water resources are scarce and the ecosystem is highly dependent on water availability. Drought periods in the region described above were analysed using the DDSLR (Dry Days Since Last Rain) index to i) characterize trends in number of dry days and the amount of rainfall along the pluviometric gradient, ii) assess how the DDSLR index changes along the gradient from semiarid to humid Mediterranean regions, and iii) contribute to studies addressing the importance of climatic gradients in relation to the dynamics of various ecosystems. The DDSLR method enables calculation of the probability of the occurrence of dry days (days without rain) at any time of the year, assessment of the risk of water deficits for vegetation during plant growth periods, and evaluation of the temporal variability of hydrological/geomorphological processes that depend on soil moisture throughout the year. The results showed that there is a critical period of rainfall scarcity during September and October, which has implications for the availability of soil water resources for vegetation. In short-term, the major problems are predicted for the Spanish Southern Mediterranean due to the increasing irregularity in precipitations: the number of days with no rains and drought periods are increased, but also a significant increment in the probabilities of extreme rainfalls exceeding 30 mm day −1 . Copyright © 2011 Royal Meteorological Society

Description: This study investigates the anomalous patterns of the oceanic and atmospheric heat transport, transient waves and the interaction between transient waves and the mean zonal flow. The evaluation is based on simulations performed with a coupled model forced by an increase of 1 Sverdrup of freshwater flux into the North Atlantic Ocean. It is found that an increase of freshwater flux in the North Atlantic leads to a weakening in the Northern Hemisphere (NH) oceanic heat transport by up to 1 Petawatt (10 15 ) PW but results in an intensification of the Southern Hemisphere (SH) total heat transport. This inter-hemispheric seesaw leads to substantial changes in transient wave activity which is associated with anomalous meridional temperature flux and eddy kinetic energy (EKE). During winter in the NH, weakening of the thermohaline circulation induces an increase in the storm track activity. However, a reduction in storm tracks is found over the extratropical regions of the SH. The correspondence between the anomalies of storm track intensity and Eady growth rate lead to the conclusion that changes in the transient eddy activity are mostly generated by changes in baroclinic conditions. Furthermore, calculations of the E vector show that interaction between transient and mean flow is most pronounced over the North Atlantic where stronger storm tracks enhance the mean westerlies, thus minimising the effect of changes in the oceanic heat transport. Copyright © 2011 Royal Meteorological Society

Description: Evapotranspiration is one of the most important factors in agriculture and the hydrological cycle that can be influenced by global warming and climatic changes. In this study, the trend of reference crop evapotranspiration (ET 0 ) computed by penman Penman-Monteith equation and surveyed in 42 synoptic weather stations during last 3 decades (1975–2005). Nonparametric statistical test, Kendall's rank correlation (or τ test) was used to determine ET 0 trends. Although, downward and upward trends were observed, increasing trends had more frequency. Spatial analysis of results indicated upward trends especially in the boundary parts of the country while, no significant trends were distributed in central parts. In addition, correlation analysis between ET 0 trend and other climatic parameters trend (the trend of mean temperature, mean of minimum temperature, and mean of maximum temperature, mean of relative humidity, mean of wind speed and mean of sunshine duration) showed that wind is the most effective parameter on ET 0 . Iran losses more than 70% of annual precipitation by ET 0 . It is obvious that in this country where there are many limitations for water resources management, increase in ET 0 could lead to more problems. Copyright © 2011 Royal Meteorological Society

Description: Two statistical indices are developed to represent the northward progression of rainfall between late January and late July over eastern Africa. The first, a Seasonal Location Index, extracts the seasonal phase associated with a daily rainfall map using a principal component analysis. The second, a Rainfall Cluster Index, describes the seasonal cycle as a progression through a series of rainfall patterns that have been defined using a cluster analysis. The indices are used to identify large seasonal shifts in the latitude of rainfall, to examine when these shifts occur, and to investigate circulation features associated with perturbations ahead of and behind the mean seasonal cycle. These indices are potentially useful for exploring the predictability of rainfall transitions at both short-range and seasonal timescales. Copyright © 2012 Royal Meteorological Society

Description: Climate change data for Austria have been produced for the period from 2008 to 2040, with a temporal/spatial resolution of 1 d and 1 km 2 . The climate change data are based on historical daily weather station data from 1975 to 2007, and linear regression modelling with repeated bootstrapping. The spatial resolution is based on 60 climate clusters which represent homogenous climates with respect to mean annual precipitation sums and mean annual temperatures from the period 1961 to 1990. For each climate cluster, a regression model fit has been performed and extrapolated for the period 2008–2040. The integral parts of our regression model are: (1) the extrapolation of the observed linear temperature trend from 1975 to 2007, by using an average national trend of approximately 0.05 °C per year derived from a homogenized dataset, and (2) the repeated bootstrapping of historical temperature residuals, and of the observations for some other weather parameters, such as solar radiation, precipitation, relative humidity and wind speed. Thus, we ensure consistent physical, spatial and temporal correlations. Precipitation scenarios have been developed to account for any possible wider range of precipitation patterns. These scenarios include increased/decreased annual precipitation sums, as well as unchanged annual precipitation sums, but with different seasonal distributions. These climate change data are available at: http://www.landnutzung.at/Klima_Daten.html Copyright © 2012 Royal Meteorological Society

Description: A complex interaction of local meteorology and source characteristics regularly leads to nocturnal smog events during winter in Christchurch, New Zealand. The main focus of this article is on improving understanding of the relationship between atmospheric processes operating at a range of scales that leads to poor air quality in such urban environments. This research therefore aims to provide a quantitative analysis of atmospheric influences on particulate matter pollution in Christchurch across a wide range of spatial and temporal scales, from local to hemispheric and daily to interannual. The probability of exceeding the National Environmental Standard for PM 10 for a range of local atmospheric conditions is calculated using the classification and regression trees technique, and links between these probabilities, local meteorology and synoptic weather situations are established. The effect of the transition between synoptic types on local air quality is also examined, and the progression of anticyclones across the country is identified to be the dominant synoptic control mechanism. It is shown that variation in latitudinal location of the path of anticyclones over New Zealand influences the predicted exceedance probability. On interdecadal and hemispheric scales, it is found that the particular combination of local and synoptic atmospheric conditions that favours air quality degradation shows a reoccurring pattern of frequency maxima (and minima) with a periodicity of approximately 14-16 years. In relation to the identified interdecadal variability of synoptic circulation, a close relationship to Southern Hemisphere pressure anomalies at high latitudes is revealed. The results of this research show that, in addition to daily weather variation, air quality in Christchurch is influenced by longer-term climatic processes that operate on interannual hemispheric scales with the implication that, in general, air pollution potential can also be expected to vary on a periodic interdecadal time scale. Copyright © 2012 Royal Meteorological Society

Description: In this paper, ring-width chronologies of pine trees ( Pinus sylvestris var. mongolica ) from one sampling site in the northern Greater Higgnan Mountains, China, were constructed. The results of growth-climate responses show that mean temperature is the limiting factor affecting radial growth of pine trees in the study area. Consequently, mean temperature from May to October from 1717 to 2008 has been reconstructed using the standard chronology. For the calibrated period (1957-2008), the explained variance of the reconstruction is 57%. The characteristics of the reconstruction expose that mean temperature has increased since the 1970s, and the decade 2000s and 1990s are also ranked as the warm decades on record. However, this period from 1970s to now is not exceptional within the past 300 years. By applying an 11-year moving average to the reconstruction, three warm periods and three cold periods are evident. The warm and cold periods of the reconstructed mean temperature correspond well with other reconstructions. Power spectral and wavelet analysis demonstrated the existence of significant ∼70- and ∼100-year cycles of variability. Furthermore, the reconstruction and North Atlantic Oscillation Index showed a significant positive correlation ( r = 0.34, n = 136, p 〈 0.0001). Copyright © 2012 Royal Meteorological Society

Description: A comparative performance analysis was studied on well-known drought indices [Standardized Precipitation Index (SPI), Palmer Drought Severity Index (PDSI) and its moisture anomaly index (Orig-Z), self-calibrated Palmer Drought Severity Index (SC-PDSI) and its moisture anomaly index (SC-Z)] to determine the most appropriate index for assessing corn ( Zea mays L.) yield in four crop regions (Aydın, Denizli, Afyon, Uşak) in western Turkey and to evaluate the vulnerability of corn production to climate change with future projections provided by the Hadley Centre for Climate Prediction and Research ENSEMBLES project (HadCM3Q0). A series of curvilinear regression-based crop-yield models were developed for each of the crop region based on the drought indices. The crop-yield model that performed best at high-drought risk years was the SC-PDSI in Aydın region and the PDSI in Denizli, Afyon and Uşak regions. The SC-PDSI index in Aydın region described 75.1% of the measured variability. The PDSI index in Denizli Ayfon and Uşak regions explained 69.8, 71.3 and 66.4% of the measured yield variability, respectively. The vulnerability of the corn yield to HadCM3Q0 projections was evaluated for Aydın and Afyon regions due to the resolution of the regional climate model. For the high-drought risk years, the expected decrease in corn yields was 2.1 ton ha −1 in Aydın region and 0.014 ton ha −1 for Afyon region. For the low drought risk years, the crop yield models predicted the expected decrease in corn yield as 0.104 ton ha −1 in Afyon region. However, there was a positive yield response by 0.022 ton ha −1 in Aydın region. Copyright © 2012 Royal Meteorological Society

Description: An experimental, district-level system was developed to forecast droughts and floods over South Korea to properly represent local precipitation extremes. The system is based on the Asia-Pacific Economic Cooperation (APEC) Climate Center (APCC) multimodel ensemble (MME) seasonal prediction products. Three-month lead precipitation forecasts for 60 stations in South Korea for the season of March to May are first obtained from the coarse-scale MME prediction using statistical downscaling. Owing to the relatively small variance of the MME and regression-based downscaling outputs, the downscaled MME (DMME) products need to be subsequently inflated. The final station-scale precipitation predictions are then used to produce drought and flood forecasts on the basis of the Standardized Precipitation Index (SPI). The performance of three different inflation schemes was also assessed. Of these three schemes, the method that simply rescales the variance of predicted rainfall to that based on climate records, irrespective of the prediction skill or the DMME variance itself at a particular station, gives the best overall improvement in the SPI predictions. However, systematic biases in the prediction system cannot be removed by variance inflation. This implies that DMME techniques must be further improved to correct the bias in extreme drought/flood predictions. Overall, it is seen that DMME, in conjunction with variance inflation, can predict hydrological extremes with reasonable skill. Our results could inform the development of a reliable early warning system for droughts and floods, which is invaluable to policy makers and stakeholders in agricultural and water management sectors, and so forth and is important for mitigation and adaptation measures. Copyright © 2012 Royal Meteorological Society

Description: The impact of Amazon Basin convection and cold fronts on the formation and maintenance of the South Atlantic convergence zone (SACZ) is studied using aqua-planet simulations with a general circulation model. In the model, a circular patch of warm sea-surface temperature (SST) is used to mimic the effect of the Amazon Basin on South American monsoon convection. The aqua-planet simulations were designed to study the effect of the strength and latitude of Amazon Basin convection on the formation of the SACZ. The simulations indicate that the strength of the SACZ increases as the Amazon convection intensifies and is moved away from the equator. Of the two controls studied here, the latitude of the Amazon convection exerts the strongest effect on the strength of the SACZ. An analysis of the synoptic-scale variability in the simulations shows the importance of frontal systems in the formation of the aqua-planet SACZ. Composite time series of frontal systems that occurred in the simulations show that a robust SACZ occurs when fronts penetrate into the subtropics and become stationary there as they cross eastward of the longitude of the Amazon Basin. Moisture convergence associated with these frontal systems produces rainfall not along the model SACZ region and along a large portion of the northern model Amazon Basin. Simulations in which the warm SST patch was too weak or too close to the equator did not produce frontal systems that extended into the tropics and became stationary, and did not form a SACZ. In the model, the SACZ forms as Amazon Basin convection strengthens and migrates far enough southward to allow frontal systems to penetrate into the tropics and stall over South America. This result is in agreement with observations that the SACZ tends to form after the onset of the monsoon season in the Amazon Basin. Copyright © 2012 Royal Meteorological Society

Description: The diurnal cycle of precipitation over South Africa during summer is analysed for the first time using hourly precipitation data from 103 stations for the period 1998–2007. Using harmonic analysis, we found the presence of a distinct diurnal cycle over most of South Africa regarding both the frequency and amount of precipitation events. The standardized amplitudes, indicative of the strength of the diurnal cycle across a region, are strongest over the interior and along the east coast of South Africa with up to 70% explained variance associated with the diurnal cycle. The time of maximum precipitation is late afternoon to early evening in the interior, and midnight to early morning along the Agulhas Current as well as inland in the northeast of the county. The proximity of the warm Agulhas Current plays a role in the diurnal cycle of rainfall at coastal stations. There is an early morning maximum in precipitation in the South West of the country with small amplitude in the diurnal cycle there. On average, the peak in precipitation amount leads the peak in frequencies of precipitation by 30 min to 1 h. Together with a high resolution climatological summer rain rate presented here and a detailed table for all stations, this study is a benchmark upon which model output or satellite estimate of the diurnal cycle can be compared within the region. Copyright © 2012 Royal Meteorological Society

Description: The statistical relationships among the various 10°–70°E mid-latitude blocking anticyclone parameters and the weather of the Arabian Peninsula (AP) (35°–60°E, 12°–32°N) over a 40-year period (1968-2007), on seasonal, interannual, decadal and long-term scales, are studied. The studied parameters include the number of blocking anticyclone events, the duration, the intensity, and the longitude at the blocking anticyclone onset. It is found that 31% of the Northern Hemisphere mid-latitude blocking anticyclone events occurred over the 10°–70°E longitudes, and out of these, the maximum number of mid-latitude blocking anticyclone event onsets are at 30°E (24%). On the seasonal basis, the annual and decadal relationships of the 10°–70° blocking anticyclones with the El-Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) indices are presented. The results show that the number of days the blocking anticyclones persists is sensitive to the ENSO phase. The mid-latitude blocking anticyclone occurrence over the 10°–70°E longitudes is indicative of the reduced surface temperature variance, both upstream and downstream, during the blocking anticyclone period, over the AP. A shift in the mean surface temperature distribution occurs, in all seasons, during the blocking anticyclone period. The blocking anticyclones initiate a surface temperature change (both positive and negative) that persists even after the blocking anticyclone's decay. The AP surface weather during the months of October, November, and December is affected more by the occurrence of mid-latitude blocking anticyclones over the 10°–70°E longitudes in the presence the of El-Niño phase. Copyright © 2012 Royal Meteorological Society

Description: Owing to the large-scale transport of pollution-derived aerosols from the mid-latitudes to the Arctic, most of the aerosols are coated with acidic sulfate during winter in the Arctic. Recent laboratory experiments have shown that acid coating on dust particles substantially reduces the ability of these particles to nucleate ice crystals. Simulations performed using the Limited Area version of the Global Multiscale Environmental Model (GEM-LAM) are used to assess the potential effect of acid-coated ice nuclei on the Arctic cloud and radiation processes during January and February 2007. Ice nucleation is treated using a new parameterization based on laboratory experiments of ice nucleation on sulphuric acid-coated and uncoated kaolinite particles. Results show that acid coating on dust particles has an important effect on cloud microstructure, atmospheric dehydration, radiation and temperature over the Central Arctic, which is the coldest part of the Arctic. Mid and upper ice clouds are optically thinner while low-level mixed-phase clouds are more frequent and persistent. These changes in the cloud microstructures affect the radiation at the top of the atmosphere with longwave negative cloud forcing values ranging between 0 and − 6 W m −2 over the region covered by the Arctic air mass. Copyright © 2012 Royal Meteorological Society

Description: This article discusses patterns in the long-term and seasonal occurrence of deep cyclones over Krakow. This study analysed the frequency of occurrence of air pressure values equal to or lower than the 1st percentile (equivalent to ≤995.3 hPa) of all air pressure values recorded at 12:00 UTC over a period of 110 years (1900/1901–2009/2010). Special attention was devoted to the tracks of deep cyclones. No distinct changes were found in the frequency of occurrence of deep cyclones during the study period. Overall the frequency peaked in December, but in recent years there has been an increase in frequency towards the end of winter and beginning of spring. A similar general lack of noticeable change in the number of days with deep cyclones can also be found in specific tracks. There were minor increases in the frequency of occurrence of cyclones from the Norwegian Sea (T1), the Atlantic (T3), Bay of Biscay (T6) and the Mediterranean (T7) after 1950. The study also found confirmation of the theory that cyclone tracks had shortened at their northeastern extremities. Copyright © 2012 Royal Meteorological Society

Description: Trends in daily maximum and minimum extreme temperature indices were investigated for 28 weather stations in South Africa, not only for the common period of 1962–2009, but also for longer periods which the individual record lengths of the stations would allow. The utilized weather stations had limited gaps in their time series, did not undergo major moves, or had their exposure compromised during the study period, as to influence the homogeneity of their time series. The indices calculated were forthcoming from those developed by the WMO/CLIVAR Expert Team on Climate Change Detection and Indices (ETCCDI), but only those applicable to the South African climate were selected. The general result is that warm extremes increased and cold extremes decreased for all of the weather stations. The trends however vary on a regional basis, both in magnitude and statistical significance, broadly indicating that the western half, as well as parts of the northeast and east of South Africa, show relatively stronger increases in warm extremes and decreases in cold extremes than elsewhere in the country. These regions coincide to a large degree with the thermal regimes in South Africa which are susceptible to extreme temperatures. The annual absolute maximum and minimum temperatures do not reflect the general trends displayed by the other indices, showing that individual extreme events cannot always be associated with observed long-term climatic trends. The analyses of longer time series than the common period indicate that it is highly likely that warming accelerated since the mid-1960s in South Africa. Copyright © 2012 Royal Meteorological Society

Description: This article examines whether the temporal clustering of flood events can be explained in terms of climate variability or time-varying land-surface state variables. The point process modelling framework for flood occurrence is based on Cox processes, which can be represented as Poisson processes with randomly varying rate of occurrence. In the special case that the rate of occurrence is deterministic, the Cox process simplifies to a Poisson process. Poisson processes represent flood occurrences which are not clustered. The Cox regression model is used to examine the dependence of the rate of occurrence on covariate processes. We focus on 41 stream gauge stations in Iowa, with discharge records covering the period 1950–2009. The climate covariates used in this study are the North Atlantic Oscillation (NAO) and the Pacific/North American Teleconnection (PNA). To examine the influence of land-surface forcing on flood occurrence, the antecedent 30 d rainfall accumulation is considered. In 27 out of 41 stations, either PNA or NAO, or both are selected as significant predictors, suggesting that flood occurrence in Iowa is influenced by large-scale climate indices. Antecedent rainfall, used as a proxy for soil moisture, plays an important role in driving the occurrence of flooding in Iowa. These results point to clustering as an important element of the flood occurrence process. Copyright © 2012 Royal Meteorological Society

Description: Climatology is one of the areas that rely on collecting huge quantities of data. The longer a time period is observed, the better; the more parameters are included, the better. But the human mind cannot easily extract useful information from the abundance of data; thus, many valuable facts may be overlooked. Having that in mind, the authors of this paper have focused on the data condensation with the goal of gathering more information about the underlying trends of the main climatologic parameters change to show climatic variability. The data from the Belgrade Meteorological Observatory are analysed using a number of different methods of multivariate statistical analysis. Separation of the years and time periods with similar weather pattern characteristics was successful and indicates that there is a trend of temperature increase, as well as a trend of the temperature range decrease. Copyright © 2012 Royal Meteorological Society

Description: This study examines synoptic situations of severe local convective storms (mesoscale severe weather associated with deep convections such as tornado and hail) during the pre-monsoon season (from March to May) in Bangladesh. We compared composite meteorological fields on severe local convective storm days (SLCSD) with those on non-severe local convective storm days (NSLCSD). Moisture inflow from the Bay of Bengal is enhanced with intensification of southwesterly wind at 950 hPa on SLCSD compared with NSLCSD. The temperature is higher at 800 hPa over the inland area of the Indian subcontinent including Bangladesh on SLCSD than NSLCSD. At 550 hPa, a trough over Bangladesh develops on SLCSD compared with NSLCSD. This leads to the development of a thermal trough over the inland area of the Indian subcontinent and enhancement of cold advection from the northwest into Bangladesh on SLCSD at this level. This synoptic situation produces great potential instability of the atmosphere in Bangladesh on SLCSD during the pre-monsoon season. Composite distributions of lifted index, precipitable water and convective available potential energy on SLCSD and NSLCSD over south Asia show distinct differences of these parameters between these two categories with statistical significance especially in and around Bangladesh. These differences indicate that the atmospheric environment has great potential instability especially in and around Bangladesh on SCLSD under the synoptic situations shown in this study. Copyright © 2012 Royal Meteorological Society

Description: The near-surface air temperature lapse rate is an important tool for spatially distributing temperatures in snow- and ice-melt models, but is difficult to parameterize, as it is not simply correlated with boundary-layer meteorological variables, such as temperature itself. This contribution quantifies spring-autumn lapse rate variability over 5 years at Vestari-Hagafellsjökull, a southerly outlet of Langjökull in Iceland. It is observed that summer lapse rates (0.57 °C 100 m −1 ) are significantly lower than non-summer rates, and are also lower than the Saturated Adiabatic Lapse Rate (SALR), which is often adopted in melt models. This is consistent with reduced near-surface temperature sensitivity to free-atmosphere temperature change during the occurrence of melting. A Variable Lapse Rate (VLR) regression model is calibrated with standardized, 750 hPa temperature anomalies derived from ERA-Interim climatology, which is shown to be highly significantly correlated with near-surface temperatures. The modelled VLR overestimates cumulative June–September Positive Degree Days (PDDs) by 3% when used to extrapolate temperatures from 1100 to 500 m a.s.l. on the glacier, whereas the SALR overestimates cumulative PDDs by 14%. ERA-Interim data therefore appear to offer a good representation of free-atmosphere temperature variability over Vestari-Hagafellsjökull, and the modelling approach offers a simple means of improving lapse rate parameterizations in melt models. Copyright © 2012 Royal Meteorological Society

Description: The knowledge of aridity is needed to explain landscape characteristics and the rational utilization of water resources. With global warming, an increase in aridity is expected for many parts of the world. This study was undertaken to analyse monthly and annual aridity index ( I A ), utilizing ratio of precipitation ( P ) over reference evapotranspiration (ET o ), at ten stations located in critical agricultural regions in Iran. The Kendall and Spearman tests with considering serial correlation effect were used for analysis. An increase in aridity, as the P /ET o index decreased, was found during the study period, and the increase was more obvious in the semi-arid region of Iran than in the humid region. The increase of aridity was caused by the concurrent occurrences of negative P trends and positive ET o trends. The results indicated the lowest numbers of the negative I A trends in the July series and the highest numbers in the January, February and March series. According to the Theil–Sen's estimator, the annual P of the semi-arid and humid regions decreased at the average rates of (−)22.87 and (−)6.24 mm year −1 per decade respectively, whereas the annual ET o values in the semi-arid and humid regions increased by (+)11.42 and (+)6.82 mm year −1 per decade, respectively. Copyright © 2012 Royal Meteorological Society

Description: ABSTRACT Daily measurements of snow depth, cumulated precipitation and mean temperature from 105 meteorological stations with continuous record over the 1961–2010 period were analysed for trends with the Mann–Kendall nonparametric test. Trends in the number of days with snow cover, and in the mean snow depth are decreasing at 29 and 18% of the stations, respectively. The decrease in snow depth affects the intra-Carpathian region and Northeastern Romania. The most dramatic change concerns the number of snowfall days, which is decreasing at 82% of the locations. There is a slight decrease in precipitation amount, significant at only 8% of the stations. The mean temperature is increasing at 47% of the stations, while the number of days with temperature over 0 °C shows upward trends at 63% of the stations. Overall, the winter season in Romania has changed substantially. All snow-related parameters show significant negative correlations with the North Atlantic Oscillation (NAO) index for winter. The NAO has a strong impact throughout the country suggesting that the winter variability in Romania is driven by the large-scale circulation over the North Atlantic. Copyright © 2013 Royal Meteorological Society

Description: The article presents characteristics of long-term variability in sunshine duration and total solar radiation in Krakow, reconstructed based on the relative intensity of total radiation (global transmission). The analysis is based on hourly sunshine duration data from the period 1884–2010 and total radiation intensity data from the period 2003–2007. Results show that the variability of incident solar radiation depends on global factors – circulation-type and anthropogenic-type factors, in particular by industralization between the 1950s and 1980s – which are affected by local conditions. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT The ability of circulation type classifications (CTCs) to resolve surface climatic and environmental variables is essential with respect to their application in synoptic climatological applications. This ‘synoptic skill’ depends on several factors including inherent properties of classification methods but as well varying boundary conditions. In this contribution the relevance of the size of the spatial domain for which CTCs are derived is investigated. To this end varying automated CTCs are applied to daily gridded sea level pressure data for 1950–2010 and in each case eight spatial domains of varying size centred around 44 locations spread over the greater north Atlantic European region. For the resulting more than 7000 CTCs the synoptic skill for daily temperature and precipitation taken from the E-OBS v4.0 data set has been estimated using varying evaluation metrics. Resulting values of evaluation metrics aggregated according to varying domain sizes reveal a distinct influence of the size of the domain on the synoptic skill of CTCs. In general highest skill appears to be achieved for domain sizes with a horizontal dimension of roughly 1300–1800 km (in west–east direction) thus covering most frequent size ranges of synoptic scale systems. However, optimal domain sizes tend to be smaller for precipitation (compared to temperature) in summer (compared to winter) and in more continental regions (compared to more oceanic regions). Distinct deviations from the overall finding of relatively small optimal domains emerge for temperatures above/below certain thresholds for which in certain locations and seasons continental scale domains yield highest synoptic skill. Finally the comparison of varying CTCs concerning the effect of the domain size for synoptic skill shows marked differences between methods and moreover clearly elucidates that differences in synoptic skill that can be attributed to varying domain sizes reach comparable magnitude than those related to differing methods. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT This study investigates a deterministic index ( I D ) for shaping regular coherent structures in meteorological and climatic variables. The developed I D was applied to the large-scale daily precipitation series derived from the data sets of the reanalysis available from National Centres for Environmental Prediction/National Centre for Atmospheric Research for the period of 1950–2011 over the Mediterranean Basin. In order to show advantages and disadvantages of I D , its results were compared with two other persistence indexes known in the literature. One of persistence index is estimated by using autocorrelation function ( I ACF ), and the other is based on conditional probability ( I CP ). Following the patterns of I D , the low values are obtained over the countries facing the sea including Greece, Italy, Turkey, Syria, Lebanon, Cyprus, Macedonia, Albania, Serbia, Slovenia, France, Portugal, Spain, Morocco and Tunisia, almost all of which are characterized with dry summer subtropical Mediterranean climate. On the other hand, the meaningful climatic patterns of I ACF are observed only over the Mediterranean Sea. At the same time as employing I CP to wet state of the precipitation series, the same patterns are obtained as I ACF . However, for dry state, I CP produces noticeable similar patterns to I D , generally in arid and semi-arid areas. The major results show that the patterns estimated by I D could be better understandable in point of view of its representing traditional spatio-temporal meteorological features. That is, the patterns of I D correspond to well-known climatic and weather features for large-scale daily precipitation values associated with influences of the atmospheric disturbances and physical geographical features such as topography and land–sea distribution over the Mediterranean Basin. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT Researchers have come to differing conclusions about multi-decadal changes in the position of the western ridge of the Bermuda High during the summer. This communication examines trends for 72 periods within the entire period of record (1948–2012), while also comparing variances in the Western Bermuda High Index (WBHI) and the latitudinal position of the western ridge. The western ridge has not moved significantly eastward or westward over either 1948–2012 or the past 60 years. The western ridge underwent a major westward shift from 1976 to 1977, but it did not stay in that western position during subsequent decades. Over the past 30–40 years, the western ridge has moved significantly eastwards and southwards. Finally, the WBHI, rather than the latitude of the western ridge, has had a definitive increase in variance over time and is the likely cause of increased summer rainfall variability in the southeastern United States.

Description: ABSTRACT In a recent article, Diem (2012) examined the impact of the North Atlantic Subtropical High (NASH) and atmospheric moistening on summer rainfall variability in the Atlanta, Georgia region. In his paper, he indicated that the results concerning the variability of the NASH western ridge discussed in Li et al . (2011) are incorrect. We present new evidence here to show that the comments by Diem (2012) are unjustified; the main conclusions in Li et al . (2011) were drawn according to the data over a 60-year (1948–2007) and 45-year (1958–2002) periods of NCEP/NCAR and ERA-40 reanalysis, whereas conclusions of Diem (2011) were based upon the trend analysis of a similar but different index of the NASH using sub-periods of 1948–2009 (since late 1970s). The comment emphases the importance of climate dynamics to study precipitation variability over the Southeastern US and further strengthens the conclusions originally put forth in Li et al . (2011): over the last 60 years (1948–2007), the NASH has shown a significant trend of westward movement, and the meridional movement of the NASH western-ridge (i.e. its latitudinal change) has enhanced in the recent three decades. Copyright © 2013 Royal Meteorological Society

Description: ABSTRACT The variability of shortwave downward solar irradiance (SDR) received at Earth's surface over Thessaloniki, Greece for the period 1993–2011 is investigated, focusing on the effects from the aerosols variability on the irradiance trends derived for different solar zenith angles (SZA). Linear trends have been calculated for the entire dataset, for cloud-free cases, and for different SZAs, separately for each season. The global upward trend in SDR after 1990 (0.33% year −1 ) is reconfirmed and is found to depend strongly on SZA, ranging from ∼0.1 to +0.6% year −1 . The long term changes in aerosols in conjunction with the local aerosol patterns result in differences of up to 0.1% year −1 in the derived trends in SDR between morning and afternoon hours. Finally, based on the analysis of the cumulative sums of the differences in monthly averages of SDR from the long term mean we report signs of a slowdown in the upward trend in SDR during the beginning of the 2000s. Copyright © 2012 Royal Meteorological Society

Description: ABSTRACT Tropical cyclone (TC) is the most hazardous high-impact weather system in Taiwan. TC seasonal prediction affecting the Taiwan area is extremely challenging because of the relatively small target area and highly variable TC genesis locations and tracks. This paper presents an empirical seasonal forecast model for predicting TC activity around Taiwan during the peak season (June through September). The predictand is the accumulative cyclone kinetic energy (ACE) of the invading TCs within the ‘influence domain’ defined as an area extending 300 km away from the coast. The predictors consist of the sea surface temperatures (SSTs) and their tendency over the tropical Indo-Pacific Ocean and the sea level pressure (SLP) over extratropical East Asia during the spring season. The source of the predictability is rooted in the spring to summer evolution of the monsoon subtropical high-ENSO system in association with the evolution of the Indo-Pacific SST anomalies. When the spring SST anomaly is warm over equatorial western Pacific, while it is cold but with a warming tendency over tropical South Indian Ocean, the coupled atmospheric and oceanic anomalies evolve into a favourable large scale condition conducive for active typhoon occurrence in the Taiwan area during the ensuing summer. The empirical prediction model presented in this paper has important implications for predicting TCs affecting a much larger area covering southeast China and the East China Sea ( r  = 0.97).

Description: ABSTRACT The implications of geographical factors (i.e. elevation, freshwater area, urbanization, and proximity to coast) and climatic factors (i.e. air temperature, wind speed, relative humidity, and solar radiation) with regard to drought trend were investigated by analyzing the monthly averaged daily climate data (i.e. precipitation, temperature, wind speed, relative humidity, and solar radiation) recorded from 1973 to 2006 at 53 climatological stations in South Korea. In addition, geographical characteristics were identified by GIS analysis and land cover data were analyzed for these 53 stations. Further, multivariate regression analyses were performed by setting the Standardized Precipitation Index (SPI) trend as the dependent variable and the geographical and climatic factors as the independent variables. The results indicated that, overall, the droughts in South Korea are becoming more severe in spring for the short- and seasonal term; however, the droughts in all seasons are becoming less severe for the long term. The effects of geographical and climatic factors on drought trends are also closely related to seasonality and drought duration for short-, seasonal-, and medium-term droughts, but not for long-term droughts. Therefore, for efficient water resources management for drought preparedness, drought duration and seasonality should also be considered along with geographical and climatic characteristics of a region.Copyright © 2012 Royal Meteorological Society

Description: ABSTRACT 5-year 3-hourly wind speed and direction, air temperature over land and sea surface temperature data are analysed in order to characterize sea breezes at Cotonou (Republic of Benin, West Africa). A set of criteria based on the diurnal reversal of wind direction, the thermal gradient and the number of sunshine hours is used to identify sea breeze days. Statistics are presented that describe the occurrence, duration and strength of the sea breezes. It is found that the sea breeze occurs during all seasons in this region. Sea breeze occurrence is found to be strongly influenced by the West African monsoon. The frequency of occurrence shows clearly two regimes characterized by two maxima (December and May). Sea breezes occur about half of the days during non-monsoon season (winter). Land/sea breezes onset and cessation times show a pronounced seasonal variation. Sea breeze strength is greater during the West African monsoon season. A hodograph analysis reveals the existence of both clockwise and anticlockwise rotation. Average hodographs show clearly the onshore–offshore nature of the sea breeze circulation, but have indeterminate rotation. There is a significant seasonal effect of large-scale flows on sea breeze hodograph patterns. Copyright © 2012 Royal Meteorological Society

Description: ABSTRACT The asynchronous regional regression model (ARRM) is a flexible and computationally efficient statistical model that can downscale station-based or gridded daily values of any variable that can be transformed into an approximately symmetric distribution and for which a large-scale predictor exists. This technique was developed to bridge the gap between large-scale outputs from atmosphere–ocean general circulation models (AOGCMs) and the fine-scale output required for local and regional climate impact assessments. ARRM uses piecewise regression to quantify the relationship between observed and modelled quantiles and then downscale future projections. Here, we evaluate the performance of three successive versions of the model in downscaling daily minimum and maximum temperature and precipitation for 20 stations in North America from diverse climate zones. Using cross-validation to maximize the independent comparison period, historical downscaled simulations are evaluated relative to observations in terms of three different quantities: the probability distributions, giving a visual image of the skill of each model; root-mean-square errors; and bias in nine quantiles that represent both means and extremes. Successive versions of the model show improved accuracy in simulating extremes, where AOGCMs are often most biased and which are frequently the focus of impact studies. Overall, the quantile regression-based technique is shown to be efficient, robust, and highly generalizable across multiple variables, regions, and climate model inputs. Copyright © 2012 Royal Meteorological Society

Description: We compare the characteristics of synthetic European droughts generated by the HiGEM 1 coupled climate model run with present day atmospheric composition with observed drought events extracted from the CRU TS3 data set. The results demonstrate consistency in both the rate of drought occurrence and the spatiotemporal structure of the events. Estimates of the probability density functions for event area, duration and severity are shown to be similar with confidence 〉 90%. Encouragingly, HiGEM is shown to replicate the extreme tails of the observed distributions and thus the most damaging European drought events. The soil moisture state is shown to play an important role in drought development. Once a large-scale drought has been initiated it is found to be 50% more likely to continue if the local soil moisture is below the 40 th percentile . In response to increased concentrations of atmospheric CO 2 , the modelled droughts are found to increase in duration, area and severity. The drought response can be largely attributed to temperature driven changes in relative humidity. Copyright © 2012 Royal Meteorological Society

Description: Temperature estimation methods usually involve regression followed by kriging of residuals (residual kriging). Despite the performance of such models, there is invariably a residual which is not necessarily unpredictable because it may still be correlated in time. We set out to analyse such residuals through resort to autoregressive processes. It is shown that the optimal period varies depending on whether it is identified by functions of the form res d  =  f (res d−1 , res d−2 , …, res d − p ) or by partial correlations. Autoregressive processes significantly improve estimates, which are evaluated by cross-validations. Finally, the two following points are discussed: (1) the assumptions of the autoregressive model on the residuals (the assumptions of linearity, stationarity of space and time are verified empirically) and (2) the identification of the days for which the introduction of this model is really interesting.

Description: A data set of observed fire weather in Australia from 1973–2010 is analysed for trends using the McArthur Forest Fire Danger Index (FFDI). Annual cumulative FFDI, which integrates daily fire weather across the year, increased significantly at 16 of 38 stations. Annual 90th percentile FFDI increased significantly at 24 stations over the same period. None of the stations examined recorded a significant decrease in FFDI. There is an overall bias in the number of significant increases towards the southeast of the continent, while the largest trends occur in the interior of the continent and the smallest occur near the coast. The largest increases in seasonal FFDI occurred during spring and autumn, although with different spatial patterns, while summer recorded the fewest significant trends. These trends suggest increased fire weather conditions at many locations across Australia, due to both increased magnitude of FFDI and a lengthened fire season. Although these trends are consistent with projected impacts of climate change on FFDI, this study cannot separate the influence of climate change, if any, with that of natural variability. Copyright © 2012 Royal Meteorological Society

Description: This study examines the performance of National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) over the Indian monsoon region in 100 years long coupled run, in terms of biases of sea surface temperature (SST), rainfall and circulation. The study further explores the role of the feedback processes in maintaining these biases. The model simulates reasonable monsoon climatology during JJAS (June–September). It shows dry (wet) rainfall bias concomitant with cold (warm) SST bias over east (west) equatorial Indian Ocean. These biases of SST and rainfall affect both lower- and upper-level circulations in a feedback process, which in turn regulates the SST and rainfall biases by maintaining a coupled feedback process. A dry (wet) rainfall bias over east (west) Indian Ocean induces anomalous low level easterlies over tropical Indian Ocean and causes cold SST bias over east Indian Ocean by triggering evaporation and warm SST bias over west Indian Ocean through advection of warm waters. The persistent SST bias retains the zonal asymmetric heating and meridional temperature gradient resulting in a circum-global subtropical westerly jet core, which in turn magnifies the mid-latitude disturbances and decreases the Mascarene high. The decreased Mascarene high diminishes the strength of monsoon cross-equatorial flow and results in less upwelling as compared to that in the observation. It further increases the SST bias over the West Indian Ocean. The coupled interaction among SST, rainfall and circulation works in tandem through a closed feedback loop to maintain the model biases over tropical Indian Ocean. Copyright © 2012 Royal Meteorological Society

Description: The present study aims to determine synoptic regimes associated with rain and no-rain days in south-eastern Queensland. A k-means cluster analysis is used on upper-air data from Brisbane Airport to identify dominant weather regimes for the region. Eight weather regimes appear to succinctly describe the main types of conditions experienced in south-eastern Queensland. Using rainfall data from 307 sites across the region, the rainfall associated with each weather regime is then characterized. Four of the regimes are associated with moist conditions; each accounts for about 15–20% of the mean total annual rainfall. These regimes preferentially occur during the summer and are characterized by high onshore moisture flux. The four dry regimes are characterized by southerly moisture flux and generally occur throughout the year. These regimes combined account for less than 25% of the mean total annual rainfall but more than 60% of the days in a year. Back trajectories at five levels were computed for each of the regimes using the HYSPLIT model. Case studies were selected for the four wet regimes and examined in detail. The wet regimes exhibit geographically shorter back trajectories, particularly at lower levels, than the dry regimes. In addition, there is usually a northerly component to the 500 m level trajectories close to Brisbane for the wet regimes. Copyright © 2012 Royal Meteorological Society

Description: This study examines the daily observed temperature at the Faraday/Vernadsky station in the Antarctic Peninsula for the period February 1947 through January 2011. Faraday/Vernadsky is experiencing a significant warming trend of about 0.6 °C/decade over the last few decades. Concurrently, the magnitude of extremely cold temperatures has reduced while there is no evidence for an increase of the annual maximum temperature. An empirical mode decomposition reveals that most of the temperature variability occurs on intraannual time scales and that changes in the magnitude of the annual cycle can be explained by a simple periodic stochastic process. Extremely cold temperatures below a threshold follow a generalised Pareto distribution (GPD) with a negative shape parameter and thus are bounded. We find that the extremely cold behaviour in the first half of the record is significantly different from the second half. At the same time there is no evident increase of warm temperatures or in the location of the maximum of the temperature probability distribution. These findings provide evidence that at Faraday/Vernadsky, it is the change in the shape of the temperature distribution that has substantially contributed to the observed warming over the last few decades. Furthermore, we find evidence for clustering of extreme cold events and show that they are predictable a few days in advance using a precursor-based prediction scheme. Copyright © 2012 Royal Meteorological Society

Description: A number of recent studies have examined trends in sea ice cover using ordinary least squares regression. In this study, quantile regression is applied to analyse other aspects of the distribution of sea ice extent. More specifically, trends in the mean, maximum, and minimum sea ice extent in the Arctic and Antarctic are investigated. While there is a significant decreasing trend in mean Arctic sea ice extent of − 4.5% per decade from 1979 through 2010, the Antarctic results show a small positive trend of 2.3% per decade. In some cases such as the Antarctic minimum ice cover, selected quantile regressions yield slope estimates that differ from trends in the mean. It was also found that the variability in Antarctic sea ice extent is higher than that in the Arctic. Copyright © 2012 Royal Meteorological Society

Description: The linkages between the El Niño Southern Oscillation (ENSO) and the sea ice extent (SIE) in the Weddell Sea (South Atlantic) and the Bellingshausen–Amundsen Sea (South-Eastern Pacific) sectors of the Southern Ocean have been studied for the period 1979–2005 using crosswavelet analysis. The analysis showed that the relationship between the tropical expression of ENSO and the SIE in these two areas are different before and after 1992. Further, we investigated the structure and strength of the regional Ferrel cell (RFC) during El Niño and La Niña episodes using composite latitude–pressure cross-sections of wind anomalies for these two periods. Contrasting features were observed in the structure and strength of the RFC before and after 1992 in both, the South Atlantic and the South-Eastern Pacific. These modulations in the RFC control the heat transport from the tropics to high latitudes and hence the extent of sea ice in both the regions. We propose that the modulation of the RFC is responsible for the phase shift in the tropical–polar teleconnection. Copyright © 2012 Royal Meteorological Society

Description: During the 20th century and early 21st century, the subtropical ridge (STR) in the vicinity of eastern Australia has intensified significantly. While the position of the STR has often been the focus of past studies, its influence on rainfall across southern Australia is less pronounced than the influence of the intensity of the ridge. Most of the Australian continent has experienced above average rainfall from 1997 to 2009, but areas of below average rainfall, including southeastern Australia (SEA), exist and tend to match the pattern of the influence of the STR on Australian rainfall. The spatial extent makes the 1997–2009 rainfall deficit in SEA different from previous dry decades such as 1935–1945. It is also noted that the annual cycle of the rainfall deficiency centred on a continuum from March to October overlaps well with the time of the year when the STR intensity has a strong influence on rainfall. Using simple linear statistics, a rainfall decline for the period 1997–2009 equivalent to nearly two thirds of that observed can be inferred from the intensification of the ridge. The apparent southward shift of the ridge in certain seasons does not appear to have had an additional effect on SEA rainfall. During the 1935–1945 drought, almost a third of the rainfall decline can be attributed to the strengthening of the ridge. Finally, it was observed that the intensification of the STR was not monotonic during the 20th century but happened mostly during two extended periods: from 1900 to the 1940s, culminating at the time of the 1935–1945 dry decade, and from 1970 to 2010 culminating with the 1997–2009 rainfall deficit in SEA. That multidecadal behaviour is reminiscent of the global warming of the planet. Copyright © 2012 Royal Meteorological Society

Description: This study examines the daily observed temperature at the Faraday/Vernadsky station in the Antarctic Peninsula for the period February 1947 through January 2011. Faraday/Vernadsky is experiencing a significant warming trend of about 0.6 °C/decade over the last few decades. Concurrently, the magnitude of extremely cold temperatures has reduced while there is no evidence for an increase of the annual maximum temperature. An empirical mode decomposition reveals that most of the temperature variability occurs on intraannual time scales and that changes in the magnitude of the annual cycle can be explained by a simple periodic stochastic process. Extremely cold temperatures below a threshold follow a generalised Pareto distribution (GPD) with a negative shape parameter and thus are bounded. We find that the extremely cold behaviour in the first half of the record is significantly different from the second half. At the same time there is no evident increase of warm temperatures or in the location of the maximum of the temperature probability distribution. These findings provide evidence that at Faraday/Vernadsky, it is the change in the shape of the temperature distribution that has substantially contributed to the observed warming over the last few decades. Furthermore, we find evidence for clustering of extreme cold events and show that they are predictable a few days in advance using a precursor-based prediction scheme. Copyright © 2012 Royal Meteorological Society

Description: A number of recent studies have examined trends in sea ice cover using ordinary least squares regression. In this study, quantile regression is applied to analyse other aspects of the distribution of sea ice extent. More specifically, trends in the mean, maximum, and minimum sea ice extent in the Arctic and Antarctic are investigated. While there is a significant decreasing trend in mean Arctic sea ice extent of − 4.5% per decade from 1979 through 2010, the Antarctic results show a small positive trend of 2.3% per decade. In some cases such as the Antarctic minimum ice cover, selected quantile regressions yield slope estimates that differ from trends in the mean. It was also found that the variability in Antarctic sea ice extent is higher than that in the Arctic. Copyright © 2012 Royal Meteorological Society