台风强度模拟的海温目标观测研究

根据非线性强迫奇异向量(NFSV)型海温(SST)强迫误差识别的敏感性特征,通过观测系统模拟试验(OSSE)确定了12个热带气旋(TC)的强度模拟的海温目标观测最优布局.NFSV型SST强迫误差敏感区一般沿着台风移动路径,主要位于台风快速增强阶段.结果 表明,在NFSV型SST强迫目标观测敏感区内以90 km间隔加密海...     

Which Features of the SST Forcing Error Most Likely Disturb the Simulated Intensity of Tropical Cyclones?

Among all of the sources of tropical cyclone(TC) intensity forecast errors, the uncertainty of sea surface temperature(SST) has been shown to play a significant role. In the present study, we determine the SST forcing error that causes the largest simulation error of TC intensity during the entire simulation period by using the WRF model with time-dependent SST forcing. The SST forcing error is represented through the application of a nonlinear forcing singular vector(NFSV)structure. For the selected 12 TC cases, the NFSV-type SST forcing errors have a nearly coherent structure with positive(or negative) SST anomalies located along the track of TCs but are especially concentrated in a particular region. This particular region tends to occur during the specific period of the TCs life cycle when the TCs present relatively strong intensity, but are still intensifying just prior to the mature phase, especially within a TC state exhibiting a strong secondary circulation and very high inertial stability. The SST forcing errors located along the TC track during this time period are verified to have the strongest disturbing effect on TC intensity simulation. Physically, the strong inertial stability of TCs during this time period induces a strong response of the secondary circulation from diabatic heating errors induced by the SST forcing error. Consequently, this significantly influences the subsidence within the warm core in the eye region, which,in turn, leads to significant errors in TC intensity. This physical mechanism explains the formation of NSFV-type SST forcing errors. According to the sensitivity of the NFSV-type SST forcing errors, if one increases the density of SST observations along the TC track and assimilates them to the SST forcing field, the skill of TC intensity simulation generated by the WRF model could be greatly improved. However, this adjustment is most advantageous in improving simulation skill during the time period when TCs become strong but are still intensifying just prior to reaching full maturity. In light of this, the region along the TC track but in the time period of TC movement when the NFSV-type SST forcing errors occur may represent the sensitive area for targeting observation for SST forcing field associated with TC intensity simulation.     

与夏季北太平洋副热带中尺度海洋涡旋相联系的海气关系

采用高分辨率卫星和再分析资料,利用涡旋探测技术、滤波和合成分析等方法,对夏季北太平洋副热带地区中尺度海洋涡旋与大气的耦合关系进行了分析。结果表明:在日时间尺度上,海洋涡旋的海表温度（Sea SurfaceTemperature,简称SST）与海表风速之间不仅存在同位相的正相关关系,还存在反位相的负相关关系,即在涡旋这种中尺度上既存在海洋对大气的强迫,也存在大气对海洋的强迫。海表风速与SST同位相时,对暖（冷）涡来说,向上（下）的净热通量增强,云和降水增多（减少）;其海水温度异常和海流旋度较强,暖（冷）涡较为深厚,一定程度上表明了海洋对大气的强迫。海表风速与SST反位相时,对暖（冷）涡而言,当其处在正（负）位势高度异常、中低层相对湿度较小（大）、气温较高（低）的大气配置下,海表风速较小（大）;同时向下（上）净热通量增强,云和降水减少（增多）;涡旋海水温度异常和海流旋度较弱,这种暖（冷）涡较为浅薄;表明晴空（阴雨）条件下有利于暖（冷）涡的维持,一定程度上反映了大气对海洋的强迫作用。     

The Dynamic and Thermodynamic Effects of Relative and Absolute Sea Surface Temperature on Tropical Cyclone Intensity

Several numerical experiments were performed to investigate the dynamic and thermodynamic effects of sea surface temperature(SST) on tropical cyclone(TC) intensity.The results reveal that the relative SST within a radius of 2-3 times the radius of maximum wind contributes positively and greatly to TC intensity,while the remote SST far away from the TC center could reduce storm intensity.The change of air-sea temperature and moisture differences may be the reason why TC intensity is more sensitive to the relative rather than the absolute SST.As the inflow air moves toward the eyewall,warmer(colder) remote SST can gradually increase(decrease) the underlying surface air temperature and moisture,and thus decrease(increase) the air-sea temperature and moisture differences,which lead to less(more) energy fluxes entering the eyewall and then decrease(increase) the TC intensity and make it less sensitive to the absolute SST change.Finally,with all the related dynamic and thermodynamic processes being taken into account,a schematic diagram for the effects of relative SST and absolute SST on TC intensity is proposed.     

An investigation into the mechanisms of changes in mid-latitude mean sea level pressure as greenhouse gases are increased

When greenhouse gases are increased in coupled GCM experiments there is both a direct effect and an indirect effect due to changes in the surface conditions. In this study we carry out experiments with a perpetual winter atmosphere only model in order to investigate the influence of changes to the surface conditions (sea surface temperatures, sea-ice and snow amount) on the Northern Hemisphere winter mid-latitude mean sea level pressure response. The surface conditions for the perpetual winter model experiments are prescribed from time averages of the HadCM2 control and greenhouse gas experiments. Forcing the perpetual winter model with the HadCM2 greenhouse gas surface conditions produces a negative mean sea level pressure (MSLP) response across both Northern Hemisphere ocean basins, as was found in the coupled model HadCM2 experiment. Additional PW model experiments show that the sea surface temperature forcing from the HadCM2 greenhouse gas experiment dominates the snow and soil moisture content forcings. The sea-ice forcing from the HadCM2 greenhouse gas experiment reduces MSLP at high latitudes. In the north Pacific region MSLP decreases when the global mean warming is applied to the sea surface temperature forcing field at all open sea points. In the north Atlantic region the increased tropics to mid-latitude meridional sea surface temperature gradient is required for MSLP to decrease. These experiments show that the MSLP response in the Northern Hemisphere mid-latitude storm track regions is sensitive to the non-local sea surface temperature anomaly pattern.     

Predictability of SST forced climate signals in two atmospheric general circulation models

 The predictability of atmospheric responses to global sea surface temperature (SST) anomalies is evaluated using ensemble simulations of two general circulation models (GCMs): the GENESIS version 1.5 (GEN) and the ECMWF cycle 36 (ECM). The integrations incorporate observed SST variations but start from different initial land and atmospheric states. Five GEN 1980–1992 and six ECM 1980–1988 realizations are compared with observations to distinguish predictable SST forced climate signals from internal variability. To facilitate the study, correlation analysis and significance evaluation techniques are developed on the basis of time series permutations. It is found that the annual mean global area with realistic signals is variable dependent and ranges from 3 to 20% in GEN and 6 to 28% in ECM. More than 95% of these signal areas occur between 35 °S–35 °N. Due to the existence of model biases, robust responses, which are independent of initial condition, are identified over broader areas. Both GCMs demonstrate that the sensitivity to initial conditions decreases and the predictability of SST forced responses increases, in order, from 850 hPa zonal wind, outgoing longwave radiation, 200 hPa zonal wind, sea-level pressure to 500 hPa height. The predictable signals are concentrated in the tropical and subtropical Pacific Ocean and are identified with typical El Ni?o/ Southern Oscillation phenomena that occur in response to SST and diabatic heating anomalies over the equatorial central Pacific. ECM is less sensitive to initial conditions and better predicts SST forced climate changes. This results from (1) a more realistic basic climatology, especially of the upper-level wind circulation, that produces more realistic interactions between the mean flow, stationary waves and tropical forcing; (2) a more vigorous hydrologic cycle that amplifies the tropical forcing signals, which can exceed internal variability and be more efficiently transported from the forcing region. Differences between the models and observations are identified. For GEN during El Ni?o, the convection does not carry energy to a sufficiently high altitude, while the spread of the tropospheric warming along the equator is slower and the anomaly magnitude smaller than observed. This impacts model ability to simulate realistic responses over Eurasia and the Indian Ocean. Similar biases exist in the ECM responses. In addition, the relationships between upper and lower tropospheric wind responses to SST forcing are not well reproduced by either model. The identification of these model biases leads to the conclusion that improvements in convective heat and momentum transport parametrizations and basic climate simulations could substantially increase predictive skill. Received: 25 April 1996 / Accepted: 9 December 1996     

NUMERICAL STUDY ON THE EFFECTS OF SEA SURFACE TEMPERATURE ON TROPICAL CYCLONE INTENSITY-PART I:NUMERICAL EXPERIMENT OF THE TROPICAL CYCLONE INTENSITY RELATED TO SST *

In the context of a model of tropical cyclone intensity based on an improved meso-scale atmospheric model, numerical simulation is performed of the track and intensity variation of tropical cyclones(TC) arising from sea surface temperature(SST) variation over a specified sea region. Evidence suggests that the model is capable of modeling quite welt the track and intensity of TC:SST variation leads to an abrupt change in the cyclone intensity:the response of the cyclone to the abrupt SST change lasts 8-12 h.     

登陆台湾岛热带气旋强度和结构变化的统计分析

利用1949—2008年共60年的《台风年鉴》、《热带气旋年鉴》资料及CMA-STI热带气旋最佳路径数据集,2001—2008年美国联合台风警报中心(JTWC)热带气旋尺度相关资料及日本气象厅(JMA)的TBB资料,统计分析西北太平洋(包括南海)热带气旋(TC)在登陆台湾过程中强度和结构变化的基本特征,主要结论有:(1)TC登陆台湾时强度为台风及以上级别的样本数占总样本数约60%,主要出现在6—9月,东部登陆TC的强度一般比在西部登陆的强;(2)大部分TC在岛上维持6 h左右,登陆时最大风速≤5级和强度为超强台风的TC穿越台湾岛时移动比较缓慢;(3)126个登陆台湾的TC样本过岛后近中心海平面气压平均增加5.61 hPa,近中心最大风速平均减小3.58 m/s,在台湾东部地区登陆TC的衰减率比在西部登陆的大3倍左右;(4)TC在登陆台湾前6 h至离岛后6 h期间其8级和10级风圈半径均明显减小,TC形状略呈长轴为NE-SW向的椭圆状,而其最大风速的半径却逐渐增大;(5)TBB分析结果显示,TC登陆台湾前,其外围对流主要出现在南侧和西侧,结构不对称,登陆以后,TC北部及东部的对流显著发展,外围结构区域对称;但中心附近的强对流则从登陆前6 h开始逐渐减弱消失。表明TC穿越台湾过程中内核结构松散、强度减弱。     

西北太平洋热带气旋强度变化的若干特征

使用NOAA海表温度资料、ECMWF再分析资料和JTWC台风最佳路径数据,对1984—2013年30年西北太平洋热带区域（100 °E~180 °,0~60 °N）内热带气旋（TC）的强度变化特征及其与环境风垂直切变（VWS）、海表温度（SST）、最大风速半径（RMW）的关系作了统计分析,尤其关注TC强度突变。结果表明:（1）在研究区域内,TC样本中35.2%强度稳定,52.8%强度变化缓慢,仅12.0%强度突变,约92.7%的迅速加强TC样本发生在其台风及以上强度等级;（2）2000年以来,TC强度稳定样本减少,强度迅速变化样本增多。5月和9—10月是TC强度突变的高频期;（3）超过12 m/s的环境VWS下TC迅速加强较少,且只有台风及以上强度TC才能在大于12 m/s的VWS下迅速加强;（4）TC加强和迅速加强主要在28.5~30.0 ℃的SST洋面上发生,在较低SST下仍迅速加强的TC强度等级较高;（5）TC样本的RMW多小于100 km,其中强度突变TC RMW峰值区在20~40 km;（6）加强TC的RMW的24 h变化一般减小,减弱TC的RMW则增大;其中强度突变TC尤其明显,超强台风发生迅速加强时,RMW减小的比率达84.6%,但仍有15.4%比率的RMW增大。      

A mechanism for long-term changes of Atlantic tropical cyclone intensity

Although previous studies reported upward trends in the basin-wide average lifetime, annual frequency, proportion of intense hurricanes and annual accumulated power dissipation index of Atlantic tropical cyclones (TCs) over the past 30?years, the basin-wide intensity did not increase significantly with the rising sea surface temperature (SST). Observational analysis and numerical simulation conducted in this study suggest that Sahel rainfall is the key to understanding of the long-term change of Atlantic TC intensity. The long-term changes of the basin-wide TC intensity are generally associated with variations in Sahara air layer (SAL) activity and vertical wind shear in the main development region (MDR), both of which are highly correlated with Sahel rainfall. The drying Sahel corresponds to an equatorward shift in the African easterly jet and African easterly wave activity, introducing the SAL to lower latitudes and increasing the MDR vertical wind shear. As a result, Atlantic TCs are more vulnerable to the suppressing effects of the SAL and vertical wind shear. Since the SST warming, especially in the tropical Indian Ocean, is a dominant factor for the Sahel drying that occurred over the past 30?years, it is suggested that the remote effect of SST warming is important for the long-term change of Atlantic TC intensity. Although influence of the AMO warm phase that started in the early 1990s alone can provide a favorable condition for TC intensification, its influence may have been offset by the influence of the ongoing SST warming, particularly in the Indian Ocean. As a result, there was no significant trend observed in the basin-wide average and peak intensity of Atlantic TCs.     

Variability and predictability of Northeast China climate during 1948–2012

In this work, authors examine the variabilities of precipitation and surface air temperature (T2m) in Northeast China during 1948–2012, and their global connection, as well as the predictability. It is noted that both the precipitation and T2m variations in Northeast China are dominated by interannual and higher frequency variations. However, on interdecadal time scales, T2m is shifted significantly from below normal to above normal around 1987/1988. Statistically, the seasonal mean precipitation and T2m are largely driven by local internal atmospheric variability rather than remote forcing. For the precipitation variation, circulation anomalies in the low latitudes play a more important role in spring and summer than in autumn and winter. For T2m variations, the associated sea surface pressure (SLP) and 850-hPa wind (uv850) anomalies are similar for all seasons in high latitudes with significantly negative correlations for SLP and westerly wind anomaly for uv850, suggesting that a strong zonal circulation in the high latitudes favors warming in Northeast China. The predictability of precipitation and T2m in Northeast China is assessed by using the Atmospheric Model Inter-comparison Project type experiments which are forced by observed sea surface temperature (SST) and time-evolving greenhouse gas (GHG) concentrations. Results suggest that T2m has slightly higher predictability than precipitation in Northeast China. To some extent, the model simulates the interdecadal shift of T2m around 1987/1988, implying a possible connection between SST (and/or GHG forcing) and surface air temperature variation in Northeast China on interdecadal time scales. Nevertheless, the precipitation and T2m variations are mainly determined by the unpredictable components which are caused by the atmospheric internal dynamic processes, suggesting low predictability for the climate variation in Northeast China.     

NUMERICAL SIMULATIONS OF THE PACIFIC MERIDIONAL MODE IMPACTS ON TROPICAL CYCLONES ACTIVITY OVER THE WESTERN NORTH PACIFIC

Based on analyses of the relationship between Pacific Meridional Mode (PMM) and number of tropical cyclones (TCs) activity over the western North Pacific, the impacts of the PMM on Tc activity over the western North Pacific are studied using numerical simulations with an Atmospheric General Circulation Model (CAM3) of National Center for Atmospheric Research (of USA). The result shows that the PMM has impacts on the large-scale generating environment of TCs, thus affecting their number and strength. The numerical simulations using the NCAR CAM3 indicate that with the inclusion of the forcing from sea surface temperature (SST) of the PMM, there appears a decreased magnitude of the vertical zonal wind shear, large proportion of relative humidity, anomalous westerly wind at low levels and anomalous easterly wind at high levels, in association with anomalous cyclonic circulation at low levels and anomalous anti-cyclonic circulation at high levels over the tropical western Pacific. Thus, the PMM provides favorable environment for the typhoon genesis. In the sensitivity experiment, TCs have larger strength, lower SST at the center, stronger tangential wind at 850 hPa and intensified warm cores at high levels. In this paper, the simulation results are similar to that in the data analyses, which reveals the important impact of the PMM on TC activity over the western North Pacific.     

夏季赤道西印度洋对大气的强迫作用及其机制研究

基于1982—2015年高分辨率海气资料,从海表面温度(Sea Surface Temperature, SST)和海表面风速相关关系的角度研究了年际尺度上赤道印度洋的海气关系。结果表明,印度洋的海气关系具有明显区域性和季节性特征,即整个印度洋除赤道东南印度洋和赤道西印度洋SST与海表风速在夏季(7—9月)为显著正相关关系,主要表现为海洋影响大气;其他地区和月份均为负相关关系,主要表现为大气对海洋的强迫作用。回归分析发现,夏季赤道西印度洋SST异常可能通过海平面气压调整机制影响海表面风场,即海温增温使边界层空气增暖,海表面风场辐合增强;反之则相反。此外,还利用AM2.1模式进行模拟试验,试验结果成功地再现了夏季赤道西印度洋海表面温度与海表风速之间的正相关关系。     

Sensitivity Experiments on the Poleward Shift of Tropical Cyclones over the Western North Pacific under Warming Ocean Conditions

Recent studies found that in the context of global warming, the observed tropical cyclones (TCs) exhibit significant poleward migration trend in terms of the mean latitude where TCs reach their lifetime-maximum intensity in the western North Pacific (WNP). This poleward migration of TC tracks can be attributed to not only anthropogenic forcing (e.g., continuous increase of sea surface temperature (SST)), but also impacts of other factors (e.g., natural variability). In the present study, to eliminate the impacts of other factors and thus focus on the impact of unvaried SST on climatological WNP TC tracks, the mesoscale Weather Research and Forecasting (WRF) model is used to conduct a suite of idealized sensitivity experiments with increased SST. Comparisons among the results of these experiments show the possible changes in climatological TC track, TC track density, and types of TC track in the context of SST increase. The results demonstrate that under the warmer SST conditions, the climatological mean TC track systematically shifts poleward significantly in the WNP, which is consistent with the previous studies. Meanwhile, the ocean warming also leads to the decreased (increased) destructive potential of TCs in low (middle) latitudes, and thus northward migration of the region where TCs have the largest impact. Further results imply the possibility that under the ocean warming, the percentage of TCs with westward/northwestward tracks decreases/increases distinctly.     

冬季东太平洋峡谷风的季节内变化及相联系的海气特征

基于高分辨的卫星资料和再分析资料,本文采用合成分析、相关分析和带通滤波等方法研究了季节内时间尺度上东太平洋峡谷风的变化,并首先发现冬季东太平洋峡谷风存在4~16 d的季节内变化周期。进一步分析表明在该时间尺度上峡谷风异常与局地海温异常之间的关系存在由负相关到正相关的明显转变,在峡谷风强度达到最大之前及最大时,峡谷风异常与局地海温异常之间的关系主要表现为大气对海洋的强迫作用,北风分量的加强使中高纬度干冷空气进入峡谷风地区,海表面的净热通量损失使得海温降低。在峡谷风强度达到最大之后其与局地海温异常的关系则转变为海洋对大气的强迫作用,冷海温异常可一直持续到峡谷风强度达到最大后的第六天。冷海温异常的维持使得湍流混合受到抑制,导致其上的海表面风速减小。此外,峡谷风的季节内变化可能与东太平洋至北美上空的大气环流异常及其演变有关。在湾区峡谷风达到最大之前,北太平洋海平面气压正异常逐渐东移南下并在其最大时到达墨西哥湾上空,使得北美高压增强,湾区两侧气压差增大,对应湾区峡谷风达到最大。     

春季我国东部海洋温度锋区对大气的强迫作用及其机制研究

首先, 采用高分辨率的卫星资料研究了春季我国东部海区海洋锋区附近的海温与风场之间的关系, 资料分析表明海温与海表面风速之间存在明显的正相关关系, 特别是在海洋锋强的年份, 这种正相关关系更明显。资料分析还表明春季是黄海、 东海海洋锋最强的季节, 海温与海表面风速的对应关系在春季尤为明显。然后, 采用一个高分辨率和先进物理方案的中尺度模式探讨了海洋影响大气的机制。控制试验再现了海洋锋区附近海温与海表面风速之间的正相关关系。模拟的边界层垂直结构说明海温能够明显改变锋区两侧边界层大气的稳定度和垂直混合的强弱, 证明了垂直混合机制的存在。而另一方面, 对控制试验和平滑海温试验的水平动量方程中各收支项的比较分析发现, 由于海洋锋的存在而产生的气压梯度力对穿越锋区的空气的加速也有相当重要的贡献。综合观测和模拟结果说明春季我国东部海区海洋温度锋区的海洋—大气相互作用过程中海洋对大气的影响非常明显, 在海洋影响大气的机理方面, 海平面气压调整机制和垂直混合机制都在起作用。     

Impact of Surface forcing on simulating Sea Surface Temperature in the Indian Ocean – A study using Regional Ocean Modeling System (ROMS)

The sensitivity of different atmospheric forcing on the simulation of Sea Surface Temperature (SST) in the Indian Ocean is examined using Regional Ocean Modeling System (ROMS). Model simulations using three different atmospheric forcings from the National Centers for Environmental Prediction (NCEP; 2.5 deg), National Centre for Medium Range Weather Forecasting (NCMRWF; 0.25 deg) and TropFlux (0.5 deg) are analyzed here. Model sensitivity to the atmospheric forcing is studied by analyzing the response of SST and mixed layer depth (MLD) using statistical methods. Results show that the response of NCMRWF and TropFlux forcing was almost similar in capturing the variability of SST in comparison with the corresponding observations. But NCEP was unable to capture SST variability, especially over the central part of the Arabian Sea (AS). It is shown that deeper MLD simulations by NCEP forcing due to the high magnitude of wind resulted in an unrealistic simulation of SST.     

WIND STRESS ANOMALY MODEL ON ENSO TIME SCALES IN COMPLEX MODELS

The purpose of this paper is to evaluate the tropical Pacific wind stress anomalies produccd on monthly to interannual time scales by the complex general circulation model (GCM) of the center for Ocean Land Atmosphere Interactions (C.O.L.A.) at low (R15) resolutions. The model is integraed using observed sea surface temperature (SST) for ten years 1979 through 1988. The model simulates generally realistic wind stress anomaly (WSA). The model-generated data set of WSA was used to force the Zebiax Cane ocean model (ZCOM) for ten years. The modeled (SST) anomalies were compared to the observed SST anomalies. The ZCOM simulation shows realistic 1982/83 and 1986/87 warm episodes along the equator, but could produce less realistic 1984/85 and 1988/89 cold episodes along the equator due to lack of wind stress forcing in the mean model. Time series of the NINO3 index (measuring the SST anomaly in the mid-eastern Pacific) is realistic for the ZCOM simulation.     

超强台风威马逊快速增强及大尺度环流特征

超强台风威马逊（1409）登陆前发生快速增强现象,并成为我国有气象记录以来的最强登陆台风。该文利用中国气象局台风最佳路径资料、NCEP FNL分析资料、NOAA高分辨率逐日最优插值海表温度融合分析资料和天气学、动力学诊断分析方法,分析这次罕见的台风快速增强过程。研究结果表明:威马逊（1409）快速增强与持续有利背景场有关,如海温异常偏暖、低空急流和越赤道气流的增强、环境风垂直切变维持较小、高层维持较强流出气流等。尤其是台风下游大气处于热力不稳定,在其他有利因子的共同作用下,台风移入热力不稳定环境场中,有利于台风环流内部对流活动的增强和对流凝结潜热效率的增加,从而有利于台风强度增加。动能诊断方程表明:威马逊（1409）快速增强期间低层动能主要来源于风穿越等压线所作的功,这与台风环流内强降雨释放的对流凝结潜热驱动台风中心附近上升、外围下沉的垂直环流圈的加强紧密联系。     

Intra-annual link of spring and autumn precipitation over France

In a previous study, an intra-annual relationship of observed precipitation, manifested by negative correlations between domain-averaged spring and autumn precipitation of the same year, was found in two domains covering France and Central Europe for the period 1972–1990 (Hirschi et al., J Geophys Res 112(D22109), 2007). Here, this link and its temporal evolution over France during the twentieth century is further investigated and related to the atmospheric circulation and North Atlantic/Mediterranean sea surface temperature (SST) patterns. Observational datasets of precipitation, mean sea level pressure (MSLP), atmospheric teleconnection patterns, and SST, as well as various global and regional climate model simulations are analyzed. The investigation of observed precipitation by means of a running correlation with a 30-year time window for the period 1901–present reveals a decreasing trend in the spring-to-autumn correlations, which become significantly negative in the second half of the twentieth century. These negative correlations are connected with similar spring-to-autumn correlations in observed MSLP, and with negatively correlated spring East Atlantic (EA) and autumn Scandinavian (SCA) teleconnection pattern indices. Maximum covariance analyses of SST with these atmospheric variables indicate that at least part of the identified spring-to-autumn link is mediated through SST, as spring precipitation and MSLP are connected with the same autumn SST pattern as are autumn precipitation, MSLP and the SCA pattern index. Except for ERA-40 driven regional climate models from the EU-FP6 project ENSEMBLES, the analyzed regional and global climate models, including Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) simulations, do not capture this observed variability in precipitation. This is associated with the failure of most models in simulating the observed correlations between spring and autumn MSLP. While the causes for the identified relationship cannot be fully established its timing suggests a possible link with increased aerosol loading in the global dimming period.