ALBERT

Description: By using the monthly ERA-40 reanalysis data and observed rainfall data, we investigated the effect of the Indian summer monsoon (ISM) on the South Asian High (SAH) at 200 hPa, and the role played by the SAH in summer rainfall variation over China. It is found that in the interannual timescale the east–west shift is a prominent feature of the SAH, with its center either over the Iranian Plateau or over the Tibetan Plateau. When the ISM is stronger (weaker) than normal, the SAH shifts westward (eastward) to the Iranian Plateau (Tibetan Plateau). The east–west position of SAH has close relation to the summer rainfall over China. A westward (eastward) location of SAH corresponds to less (more) rainfall in the Yangtze-Huai River Valley and more (less) rainfall in North China and South China. A possible physical process that the ISM affects the summer rainfall over China via the SAH is proposed. A stronger (weaker) ISM associated with more (less) rainfall over India corresponds to more (less) condensation heat release and anomalous heating (cooling) in the upper troposphere over the northern Indian peninsula. The anomalous heating (cooling) stimulates positive (negative) height anomalies to its northwest and negative (positive) height anomalies to its northeast in the upper troposphere, causing a westward (eastward) shift of the SAH with its center over the Iranian Plateau (Tibetan Plateau). As a result, an anomalous cyclone (anticyclone) is formed over the eastern Tibetan Plateau and eastern China in the upper troposphere. The anomalous vertical motions in association with the circulation anomalies are responsible for the rainfall anomalies over China. Our present study reveals that the SAH may play an important role in the effect of ISM on the East Asian summer monsoon.

Description: National Centers for Environmental Prediction recently upgraded its operational seasonal forecast system to the fully coupled climate modeling system referred to as CFSv2. CFSv2 has been used to make seasonal climate forecast retrospectively between 1982 and 2009 before it became operational. In this study, we evaluate the model’s ability to predict the summer temperature and precipitation over China using the 120 9-month reforecast runs initialized between January 1 and May 26 during each year of the reforecast period. These 120 reforecast runs are evaluated as an ensemble forecast using both deterministic and probabilistic metrics. The overall forecast skill for summer temperature is high while that for summer precipitation is much lower. The ensemble mean reforecasts have reduced spatial variability of the climatology. For temperature, the reforecast bias is lead time-dependent, i.e., reforecast JJA temperature become warmer when lead time is shorter. The lead time dependent bias suggests that the initial condition of temperature is somehow biased towards a warmer condition. CFSv2 is able to predict the summer temperature anomaly in China, although there is an obvious upward trend in both the observation and the reforecast. Forecasts of summer precipitation with dynamical models like CFSv2 at the seasonal time scale and a catchment scale still remain challenge, so it is necessary to improve the model physics and parameterizations for better prediction of Asian monsoon rainfall. The probabilistic skills of temperature and precipitation are quite limited. Only the spatially averaged quantities such as averaged summer temperature over the Northeast China of CFSv2 show higher forecast skill, of which is able to discriminate between event and non-event for three categorical forecasts. The potential forecast skill shows that the above and below normal events can be better forecasted than normal events. Although the shorter the forecast lead time is, the higher deterministic prediction skill appears, the probabilistic prediction skill does not increase with decreased lead time. The ensemble size does not play a significant role in affecting the overall probabilistic forecast skill although adding more members improves the probabilistic forecast skill slightly.

Description: The south peninsular part of India gets maximum amount of rainfall during the northeast monsoon (NEM) season [October to November (OND)] which is the primary source of water for the agricultural activities in this region. A nonlinear method viz., Extreme learning machine (ELM) has been employed on general circulation model (GCM) products to make the multi-model ensemble (MME) based estimation of NEM rainfall (NEMR). The ELM is basically is an improved learning algorithm for the single feed-forward neural network (SLFN) architecture. The 27 year (1982–2008) lead-1 (using initial conditions of September for forecasting the mean rainfall of OND) hindcast runs (1982–2008) from seven GCM has been used to make MME. The improvement of the proposed method with respect to other regular MME (simple arithmetic mean of GCMs (EM) and singular value decomposition based multiple linear regressions based MME) has been assessed through several skill metrics like Spread distribution, multiplicative bias, prediction errors, the yield of prediction, Pearson’s and Kendal’s correlation coefficient and Wilmort’s index of agreement. The efficiency of ELM estimated rainfall is established by all the stated skill scores. The performance of ELM in extreme NEMR years, out of which 4 years are characterized by deficit rainfall and 5 years are identified as excess, is also examined. It is found that the ELM could expeditiously capture these extremes reasonably well as compared to the other MME approaches.

Description: The sea surface temperature anomaly pattern differs between the central Pacific (CP) and eastern Pacific (EP) El Niños during boreal summer. It is expected that the respective atmospheric response will be different. In order to identify differences in the responses to these two phenomena, we examine the Community Atmosphere Model Version 4 simulations forced with observed monthly sea surface temperature during 1979–2010 and compare with the corresponding observations. For CP El Niño, a triple precipitation anomaly pattern appears over East Asia. During EP El Niño, the triple pattern is not as significant as and shifts eastward and southward compared to CP El Niño. We also examine the influence of CP La Niña and EP La Niña on East Asia. In general, the impact of CP (EP) La Niña on tropics and East Asia seems to be opposite to that of CP (EP) El Niño. However, the impacts between the two types of La Niña are less independent compared to the two types of warm events. Both types of El Niño (La Niña) correspond to a stronger (weaker) western North Pacific summer monsoon. The sensitivity experiments support this result. But the CP El Niño (La Niña) may have more significant influence on East Asia summer climate than EP El Niño (La Niña), as the associated low-level anomalous wind pattern is more distinct and closer to the Asian continent compared to EP El Niño (La Niña).

Description: Since the Mediterranean Sea is halfway between subtropical and middle latitudes, and it represents a marginal oceanic region, research has tended to focus on how large-scale modes of atmospheric variability modulate its surface temperature. Conversely, the present study examines the potential influence of the Mediterranean Sea surface temperature (SST) anomalies on the Northern Hemisphere atmospheric circulation. In particular, this work explores the large-scale changes in the global circulation forced/influenced by the eastern Mediterranean summer-autumn SST pattern. To isolate the atmospheric response, AGCM sensitivity experiments with prescribed SST over the Mediterranean Sea and climatology elsewhere are analysed. Observational diagnostics upon the period used to define the boundary conditions (1979–2002) are also interpreted. Our results support the hypothesis of an atmospheric pattern initiated in the Mediterranean basin, pointing out both a local baroclinic response and a barotropic circumglobal anomaly. This atmospheric teleconnection pattern projects onto a hemispheric wave-like structure, reflecting the waveguide effect of the westerly jets. Results suggest, thereby, that the recurrent summer-autumn circumglobal teleconnection pattern can be excited locally by changes in the atmosphere over the Mediterranean region. A linear behaviour is found upon a regional impact over northeastern Africa. The remote impacts present however a nonlinear signature: anomalous warm conditions influencing on northern Europe and Euro–Asia, whereas anomalous cold conditions impacting more on the North Pacific basin. Limitations in our model setup are also discussed.

Description: Atlantic Multidecadal Variability (AMV) is investigated in a millennial control simulation with the Kiel Climate Model (KCM), a coupled atmosphere–ocean–sea ice model. An oscillatory mode with approximately 60 years period and characteristics similar to observations is identified with the aid of three-dimensional temperature and salinity joint empirical orthogonal function analysis. The mode explains 30 % of variability on centennial and shorter timescales in the upper 2,000 m of the North Atlantic. It is associated with changes in the Atlantic Meridional Overturning Circulation (AMOC) of ±1–2 Sv and Atlantic Sea Surface Temperature (SST) of ±0.2 °C. AMV in KCM results from an out-of-phase interaction between horizontal and vertical ocean circulation, coupled through Irminger Sea convection. Wintertime convection in this region is mainly controlled by salinity anomalies transported by the Subpolar Gyre (SPG). Increased (decreased) dense water formation in this region leads to a stronger (weaker) AMOC after 15 years, and this in turn leads to a weaker (stronger) SPG after another 15 years. The key role of salinity variations in the subpolar North Atlantic for AMV is confirmed in a 1,000 year long simulation with salinity restored to model climatology: No low frequency variations in convection are simulated, and the 60 year mode of variability is absent.

Description: Based on a generated time series for the central pressure of the Siberian High, and on defining a robust Siberian High Index (SHI), the behavior of this atmospheric center of action is examined from 1949 to 2010 with regard to inter-annual variations, persistence, trends, abrupt changes, spectral analysis and interactions. The interannual variability in the central pressure of the Siberian High is considerable. The mean downward linear and non-linear trend over the entire period (1949–2010) is estimated and is found to be statistically significant at the 95 % confidence level. Low frequency variation and linearity within the SHI time series are found from the persistence analysis. Using spectral analysis, the center of action of the Siberian High is characterized by non-periodic behavior; the peaks occur only at the lowest frequency and may be related to the Sea Surface Temperature (SST) over the El Niño region. The Siberian High is affected by the Hadley circulation cell; there is no detectable connection between the Walker circulation cell and the Siberian High. SSTs over the El Niño region may affect the Siberian High. Interactions between the Siberian High and the SSTs over the tropical Atlantic Ocean are absent. The SHI is positively correlated to surface air temperatures over Saudi Arabia, and this is statistically significant in the western and north-western regions.

Description: This study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the Andes from subtropical to tropical South America analysing wintertime in situ daily minimum temperature observations from Argentina, Bolivia and Peru and ERA-40 reanalysis over the 1975–2001 period. Cold surges usually last 2 or 3 days but are generally less persistent in the southern La Plata basin compared to tropical regions. On average, three to four cold surges are reported each year. Our analysis reveals that 52 % of cold episodes registered in the south of La Plata basin propagate northward to the northern Peruvian Amazon at a speed of around 20 m s −1 . In comparison to cold surges that do not reach the tropical region, we demonstrate that these cold surges are characterized, before they reach the tropical region, by a higher occurrence of a specific circulation pattern associated to southern low-level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect. Finally, the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the Peruvian and Bolivian Amazon in the last 20 years.

Description: The Greenland ice sheet is projected to be strongly affected by global warming. These projections are either issued from downscaling methods (such as Regional Climate Models) or they come directly from General Circulation Models (GCMs). In this context, it is necessary to evaluate the accuracy of the daily atmospheric circulation simulated by the GCMs, since it is used as forcing for downscaling methods. Thus, we use an automatic circulation type classification based on two indices (Euclidean distance and Spearman rank correlation using the daily 500 hPa geopotential height) to evaluate the ability of the GCMs from both CMIP3 and CMIP5 databases to simulate the main circulation types over Greenland during summer. For each circulation type, the GCMs are compared to three reanalysis datasets on the basis of their frequency and persistence differences. For the current climate (1961–1990), we show that most of the GCMs do not reproduce the expected frequency and the persistence of the circulation types and that they simulate poorly the observed daily variability of the general circulation. Only a few GCMs can be used as reliable forcings for downscaling methods over Greenland. Finally, when applying the same approach to the future projections of the GCMs, no significant change in the atmospheric circulation over Greenland is detected, besides a generalised increase of the geopotential height due to a uniform warming of the atmosphere.

Description: The East Asian summer monsoon (EASM) features strong humid low-level southerly flows and abundant rainfall over the subtropical East Asia. This study identified how condensational heating generated by the EASM rainfall can affect the EASM circulation by contrasting two 10-member ensembles of atmospheric General Circulation Model experiments with Community Climate Model version 3/National Center for Atmospheric Research respectively with and without feedback of condensational heating over the East Asian domain. Major results inferred from the experiments are as follows. Condensational heating is found to absolutely dominate diabatic heating over East Asia. Exclusion of the feedback of condensational heating leads to a significant weakening of summertime tropospheric warming over land and thus a large reduction of the land-sea thermal contrast between entire Asian continent and surrounding oceans. Associated with this, the lower-level EASM flows are weakened, South Asian High at 200 hPa migrates southward with reduced intensity and breaks over East Asia with southerly flows prevailing in the upper troposphere, in contrast to northerly flows in reality. Consequently, local EASM meridional cell disappears and the baroclinic structure featured by the EASM circulation that is dynamically determined by convective condensational heating over East Asia is altered to a barotropic structure. Therefore, it is concluded that the feedback of condensational heating acts to largely enhance lower-level flows of the EASM and essentially determine its baroclinic structure and meridional cell, once the solar radiation and inhomogeneity of the Earth’s surface form low-level monsoon flows in East Asia by enhancing land-sea thermal contrast.