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1.
南半球环状模是南半球热带外地区环流变率的主导模态。由于南半球环状模在空间上的大尺度特征,全球多个地区的气候均与南半球环状模的变化有关。探讨南半球环状模的气候影响,是近几十年来得到广泛关注并迅速发展的新方向。围绕这个方向,分别回顾了南半球环状模对南半球和北半球气候影响的研究进展,重点阐述了南半球环状模对中国气候影响的相关工作,并从长期变化尺度上,列举了南半球环状模与气候变化方面的研究成果。纵观近几十年的研究发现,针对南半球环状模对南半球的气候影响,目前已有比较系统的认识。总体而言,在年际尺度上,南半球环状模可以通过影响垂直环流和风暴轴的位置,改变表面风速对下垫面的热力和动力驱动作用,进而对南半球的海-气-冰耦合系统产生调控。这种调控多表现出纬向对称性,同时也存在纬向非对称的局地特征。在气候变化的尺度上,南半球环状模是过去半个世纪里南半球气候变化的主要驱动力之一。关于南半球环状模对北半球尤其是中国气候的影响问题,目前也取得了许多有意义的结果。例如,南半球环状模对东亚、西非、北美的夏季风和东亚冬季风均有作用,并且可以调控中国春季华南降水等。海-气耦合过程在南半球环状模对北半球气候的影响中扮演着重要角色,与南半球环状模信号的跨季节存储和由南半球向北半球的传播均有密切关系。但是,与南半球相比,南半球环状模对北半球气候影响的研究,还有许多问题值得深入讨论和研究：一是体现在对南半球环状模信号向北传播机制上的深入认识,二是将南半球环状模的信号作为因子在季节气候预测中的实践。 相似文献
2.
This paper examines the mean annual cycle, interannual variability, and leading patterns of the tropical Atlantic Ocean simulated in a long-term integration of the climate forecast system (CFS), a state-of-the-art coupled general circulation model presently used for operational climate prediction at the National Centers for Environmental Prediction. By comparing the CFS simulation with corresponding observation-based analyses or reanalyses, it is shown that the CFS captures the seasonal mean climate, including the zonal gradients of sea surface temperature (SST) in the equatorial Atlantic Ocean, even though the CFS produces warm mean biases and underestimates the variability over the southeastern ocean. The seasonal transition from warm to cold phase along the equator is delayed 1 month in the CFS compared with the observations. This delay might be related to the failure of the model to simulate the cross-equatorial meridional wind associated with the African monsoon. The CFS also realistically simulates both the spatial structure and spectral distributions of the three major leading patterns of the SST anomalies in the tropical Atlantic Ocean: the south tropical Atlantic pattern (STA), the North tropical Atlantic pattern (NTA), and the southern subtropical Atlantic pattern (SSA). The CFS simulates the seasonal dependence of these patterns and partially reproduces their association with the El Niño-Southern Oscillation. The dynamical and thermodynamical processes associated with these patterns in the simulation and the observations are similar. The air-sea interaction processes associated with the STA pattern are well simulated in the CFS. The primary feature of the anomalous circulation in the Northern Hemisphere (NH) associated with the NTA pattern resembles that in the Southern Hemisphere (SH) linked with the SSA pattern, implying a similarity of the mechanisms in the evolution of these patterns and their connection with the tropical and extratropical anomalies in their respective hemispheres. The anomalies associated with both the SSA and NTA patterns are dominated by atmospheric fluctuations of equivalent-barotropic structure in the extratropics including zonally symmetric and asymmetric components. The zonally symmetric variability is associated with the annular modes, the Arctic Oscillation in the NH and the Antarctic Oscillation in the SH. The zonally asymmetric part of the anomalies in the Atlantic is teleconnected with the anomalies over the tropical Pacific. The misplaced teleconnection center over the southern subtropical ocean may be one of the reasons for the deformation of the SSA pattern in the CFS. 相似文献
3.
An Index Measuring the Interannual Variation of the East Asian Summer Monsoon ——The EAP Index 总被引：57，自引：9，他引：48
HUANG Gang 《大气科学进展》2004,21(1):41-52
Based on the EAP (East Asia/Pacific) teleconnection in the summer circulation anomalies over ther Northern Hemisphere, an index measuring the strength of the East Asian summer monsoon, i.e., the so-called EAP index, is defined in this paper. From the analyses of observed data, it is clearly shown that the EAP index defined in this study can well describe the interannual variability of summer rainfall and surface air temperature in East Asia, especially in the Yangtze River valley and the Huaihe River valley, Korea,and Japan. Moreover, this index can also reflect the interannual variability of the East Asian summer monsoon system including the monsoon horizontal circulation and the vertical-meridional circulation cell over East Asia. From the composite analyses of climate and monsoon circulation anomalies for high EAP index and for low EAP index, respectively, it is well demonstrated that the EAP index proposed in this study can well measure the strength of the East Asian summer monsoon. 相似文献
4.
Arctic Oscillation and Antarctic Oscillation in Internal Atmospheric Variability with an Ensemble AGCM Simulation 总被引：1，自引：0，他引：1
In this study, we investigated the features of Arctic Oscillation （AO） and Antarctic Oscillation （AAO）, that is, the annular modes in the extratropics, in the internal atmospheric variability attained through an ensemble of integrations by an atmospheric general circulation model （AGCM） forced with the global observed SSTs. We focused on the interannual variability of AO/AAO, which is dominated by internal atmospheric variability. In comparison with previous observed results, the AO/AAO in internal atmospheric variability bear some similar characteristics, but exhibit a much clearer spatial structure： significant correlation between the North Pacific and North Atlantic centers of action, much stronger and more significant associated precipitation anomalies, and the meridional displacement of upper-tropospheric westerly jet streams in the Northern/Southern Hemisphere. In addition, we examined the relationship between the North Atlantic Oscillation （NAO）/AO and East Asian winter monsoon （EAWM）. It has been shown that in the internal atmospheric variability, the EAWM variation is significantly related to the NAO through upper-tropospheric atmospheric teleconnection patterns. 相似文献
5.
June-Yi Lee Bin Wang Q. Ding K.-J. Ha J.-B. Ahn A. Kumar B. Stern O. Alves 《Climate Dynamics》2011,37(5-6):1189-1203
The retrospective forecast skill of three coupled climate models (NCEP CFS, GFDL CM2.1, and CAWCR POAMA 1.5) and their multi-model ensemble (MME) is evaluated, focusing on the Northern Hemisphere (NH) summer upper-tropospheric circulation along with surface temperature and precipitation for the 25-year period of 1981–2005. The seasonal prediction skill for the NH 200-hPa geopotential height basically comes from the coupled models’ ability in predicting the first two empirical orthogonal function (EOF) modes of interannual variability, because the models cannot replicate the residual higher modes. The first two leading EOF modes of the summer 200-hPa circulation account for about 84% (35.4%) of the total variability over the NH tropics (extratropics) and offer a hint of realizable potential predictability. The MME is able to predict both spatial and temporal characteristics of the first EOF mode (EOF1) even at a 5-month lead (January initial condition) with a pattern correlation coefficient (PCC) skill of 0.96 and a temporal correlation coefficient (TCC) skill of 0.62. This long-lead predictability of the EOF1 comes mainly from the prolonged impacts of El Niño-Southern Oscillation (ENSO) as the EOF1 tends to occur during the summer after the mature phase of ENSO. The second EOF mode (EOF2), on the other hand, is related to the developing ENSO and also the interdecadal variability of the sea surface temperature over the North Pacific and North Atlantic Ocean. The MME also captures the EOF2 at a 5-month lead with a PCC skill of 0.87 and a TCC skill of 0.67, but these skills are mainly obtained from the zonally symmetric component of the EOF2, not the prominent wavelike structure, the so-called circumglobal teleconnection (CGT) pattern. In both observation and the 1-month lead MME prediction, the first two leading modes are accompanied by significant rainfall and surface air temperature anomalies in the continental regions of the NH extratropics. The MME’s success in predicting the EOF1 (EOF2) is likely to lead to a better prediction of JJA precipitation anomalies over East Asia and the North Pacific (central and southern Europe and western North America). 相似文献
6.
有关南半球大气环流与东亚气候的关系研究的若干新进展 总被引：14，自引：15，他引：14
南半球大气环流是全球大气环流的重要组成部分,也是影响气候变化和亚洲季风系统的一个重要因素.中国气象学家很早就注意到南半球大气环流对东亚夏季风降水的影响.近年来,有关南半球气候变率的研究目前正受到世界气象学家越来越多的关注.南半球中高纬大气资料的丰富及南极涛动的确定,使得认识南半球高中纬环流的年际变动规律及其与东亚气候关系成为可能.本文主要介绍近年来有关南极涛动的年际变化与沙尘天气发生频次及东亚冬春季气候的关系,古气候资料揭示的南极涛动与华北降水的关系,以及南半球大气环流与长江中下游夏季降水的关系和南极涛动变率的可预测性等方面的研究进展.并对未来研究方向作了初步的展望. 相似文献
7.
8.
Ting Wei Jian Li Xinyao Rong Wenjie Dong Bingyi Wu Minghu Ding 《Acta Meteorologica Sinica》2018,32(6):881-895
The Chinese Academy of Meteorological Sciences Climate System Model (CAMS-CSM) is a newly developed global climate model that will participate in the Coupled Model Intercomparison Project phase 6. Based on historical simulations (1900?2013), we evaluate the model performance in simulating the observed characteristics of the Arctic climate system, which includes air temperature, precipitation, the Arctic Oscillation (AO), ocean temperature/salinity, the Atlantic meridional overturning circulation (AMOC), snow cover, and sea ice. The model?data comparisons indicate that the CAMS-CSM reproduces spatial patterns of climatological mean air temperature over the Arctic (60°?90°N) and a rapid warming trend from 1979 to 2013. However, the warming trend is overestimated south of the Arctic Circle, implying a subdued Arctic amplification. The distribution of climatological precipitation in the Arctic is broadly captured in the model, whereas it shows limited skills in depicting the overall increasing trend. The AO can be reproduced by the CAMS-CSM in terms of reasonable patterns and variability. Regarding the ocean simulation, the model underestimates the AMOC and zonally averaged ocean temperatures and salinity above a depth of 500 m, and it fails to reproduce the observed increasing trend in the upper ocean heat content in the Arctic. The large-scale distribution of the snow cover extent (SCE) in the Northern Hemisphere and the overall decreasing trend in the spring SCE are captured by the CAMS-CSM, while the biased magnitudes exist. Due to the underestimation of the AMOC and the poor quantification of air–sea interaction, the CAMS-CSM overestimates regional sea ice and underestimates the observed decreasing trend in Arctic sea–ice area in September. Overall, the CAMS-CSM reproduces a climatological distribution of the Arctic climate system and general trends from 1979 to 2013 compared with the observations, but it shows limited skills in modeling local trends and interannual variability. 相似文献
9.
Previous studies suggest that the atmospheric precursor of El Ni ?no–Southern Oscillation(ENSO) in the extratropical Southern Hemisphere(SH) might trigger a quadrapole sea surface temperature anomaly(SSTA) in the South Pacific and subsequently influence the following ENSO. Such a quadrapole SSTA is referred to as the South Pacific quadrapole(SPQ).The present study investigated the relationships between the atmospheric precursor signal of ENSO and leading modes of atmospheric variability in the extratropical SH [including the SH annular mode(SAM), the first Pacific–South America(PSA1) mode, and the second Pacific–South America(PSA2) mode]. The results showed that the atmospheric precursor signal in the extratropical SH basically exhibits a barotropic wavenumber-3 structure over the South Pacific and is significantly correlated with the SAM and the PSA2 mode during austral summer. Nevertheless, only the PSA2 mode was found to be a precursor for the following ENSO. It leads the SPQ-like SSTA by around one month, while the SAM and the PSA1 mode do not show any obvious linkage with either ENSO or the SPQ. This suggests that the PSA2 mode may provide a bridge between the preceding circulation anomalies over the extratropical SH and the following ENSO through the SPQ-like SSTA. 相似文献
10.
利用NCAR大气模式CAM3.0探讨了包括山脉和海陆分布等下边界条件对北极涛动(AO)或北半球环状模(NAM)形成的影响.主要进行了控制试验和两个地形敏感性数值试验,对比了控制试验结果与资料分析结果.控制试验和NCEP/NCAR资料分析结果对比指出,CAM3.0模式能够较好地模拟出AO的水平环状结构和垂直相当正压结构.在去除山脉的敏感性试验中发现,AO中纬度北太平洋和北大西洋两个活动中心的强度、范围和位置发生变化.在水球敏感性试验中发现,AO两个大洋上的活动中心被环绕极地主活动中心的环状结构代替.两个敏感性试验共同表明,AO是大气内部某种过程作用的结果,它的存在本身不取决于下边界条件的改变.山脉和海陆分布主要影响的是AO的具体形态.通过控制试验和两个敏感性试验对比,又分析了准定常波和瞬变波对AO的影响.去除山脉和海陆分布热力差异共同强迫的准定常波,AO纬向平均纬向风高纬地区平流层异常最大值活动中心消失,这表明在平流层准定常波和纬向平均流的相互作用与AO活动关系密切.在只有对流层瞬变波作用时,AO仍然存在,表明准定常波不是AO存在的必要条件.平流层准定常波与平均流的相互作用和对流层瞬变波与平均流的相互作用与AO形成之间的具体关系有待通过E-P通量诊断进一步确定. 相似文献
11.
NUMERICAL EXPERIMENTS FOR CLIMATE VARIABILITY OF MONSOON UNDER THE UNITED EFFECTS OF ANTARCTIC ICE COVER AND SEA SURFACE TEMPERATURE* 
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下载免费PDF全文 In order to study the mechanisms of climate natural variability under the united effects of antarctic ice cover(AIC) and sea surface temperature(SST),we have done a series of numerical experiments for the climate variability of the period from January 1981 to December 1983 by using a threelevel atmospheric general circulation model(AGCM).Firstly we conduct climate integration for sixyears,then 3 years' control integration from January 16 of the seventh year.Moreover,we do threesensitivity experiments,which are the sensitivity experiments forced by observed SST and AIC.observed SST and climatic AIC,observed AIC and climatic SST respectively,to study the climate variability and its mechanisms affected by SST and AIC.We put emphasis on the variability of East Asiamonsoon and the Southern Hemisphere(SH) circulation.In this paper,introduction is made to theresults of control test and the sensitivity experiment forced by observed SST and AIC,and the predictability of the monsoon climate variability is discussed. 相似文献
12.
In this paper, the relation between Asian summer monsoon circulation and sea surface temperature anomalies over equatorial central-eastern Pacific is investigated by using a global spectral model. This model has nine layers in the vertical and the model variables are represented in the horizontal as truncated expansions of the surface spherical harmonics with rhomboidal truncation at wave number 15. The model involves comparatively complete physical pro-cesses and parameterizations with mountains.Using the above model, two experimental schemes are designed, namely control case and anomalous sea surface temperature case. The above two schemes are respectively integrated for forty days and the simulated results are ob-tained from the last 30-day averaged simulations.The simulations show that positive SST anomalies over equatorial central-eastern Pacific weakens Indian mon-soon circulation, decreases precipitation in Indian sub-continent whereas it intensifies East Asian monsoon circula-tion and increases precipitation in East Asian area. All these results reflect the characteristics of Asian summer mon-soon during the El Nino period. In this paper, SST anomalies over equatorial central-eastern Pacific have a direct in-fluence on the intensity and position of subtropical high via the wave train over Northern Hemisphere, which is simi-lar to that suggested by Nitta (1987) and the wave train over Southern Hemisphere has an influence on the intensity of Mascarene high and Australia high resulting in affecting cross equatorial flow. As a result, atmospheric interior heat sources and sinks are redistributed because of the change of cross equatorial flow. And the response of atmos-phere to the new heat source and sink has a significant influence on Asian summer monsoon. 相似文献
13.
2009/2010年北半球冬季异常低温分析 总被引：3，自引：0，他引：3
分析了2009/2010年冬季(2009年12月1日至2010年2月28日,简称09/10年冬季)北半球地面气温异常特征及同期的水平与垂直环流场的异常结构。结果表明地面气温的异常呈现出带状的分布,表现为在低纬度为正异常、中纬度负异常及高纬度正异常的"正负正"的分布特征,最大的降温区在欧亚大陆和美国东部,其中局部的降温超过了-4℃。09/10年冬季北半球中纬度的地面气温相比过去15年冬季的平均值下降了近1℃,而在欧亚大陆的局部地区降温超过了-8℃。水平环流场的异常特征为:海平面气压和位势高度均表现为高纬度正异常而中纬度负异常的"北高南低"的分布特征,与此同时,中纬度出现气旋式的异常环流而高纬出现反气旋式的异常环流,这种分布形势在高低层表现得较为一致。经圈环流异常特征为:费雷尔环流减弱,中纬度出现异常的上升运动而高纬度出现异常的下沉运动,与此同时,中纬度对流层气温降低,而低纬度和高纬度的对流层气温升高,副热带急流增强,而极地急流减弱。09/10年冬季北半球环流的异常特征与北半球环状模(NAM)负位相时的极为相似。对多年冬季北半球地面气温和NAM指数进行合成和相关分析,结果表明当NAM处于正(负)位相时,北半球中纬度地面气温出现正(负)异常带,并且在欧亚大陆和美国东部最为显著,局部升温(降温)的幅度达到2℃。在热带外地区,经向温度平流是控制温度局地变化的关键因子。NAM影响北半球地面气温的物理机制分析表明,NAM主要是通过影响经向温度平流来影响北半球中纬度气温的。当NAM为正位相时,北半球费雷尔环流加强,中纬度带和高纬度带发生大气质量的交换,海平面气压场表现为中纬度异常高压而高纬度异常低压的"南高北低"的分布特征,中纬度地表出现异常的南风,进而经向暖平流加强,最终导致中纬度地面气温升高,NAM负位相年时与之相反。这个结果揭示了NAM作为自然变率对中纬度地面气温的调控作用。 相似文献
14.
Atmospheric water vapor content(WVC) is a critical factor for East Asian winter precipitation. This study investigates the dominant modes of interannual variability in WVC over East Asia during winter and their underlying mechanisms.Based on the empirical orthogonal function(EOF) method, the leading mode(EOF1, R~2 = 28.9%) of the interannual variability in the East Asian winter WVC exhibits a meridional dipole pattern characterized by opposite WVC anomalies over northeastern China and eastern China; the second mode(EOF2, R~2 = 24.3%) of the interannual variability in the East Asian winter WVC exhibits a monopole pattern characterized by consistent WVC anomalies over eastern China. EOF1 is mainly modulated by two anomalous zonal water vapor transport(WVT) branches over northeastern China and eastern China, which are associated with an anomalous atmospheric wave train over Eurasia affected by sea ice cover in the Kara Sea-Barents Sea(SIC-KSBS) area in the preceding October-November(ON). EOF2 is mainly modulated by an anomalous westerly WVT branch over eastern China, which is associated with a circumglobal atmospheric zonal wave train in the Northern Hemisphere. This circumglobal zonal wave train is modulated by concurrent central and eastern tropical Pacific sea surface temperature anomalies. The SIC-KSBS anomalies in ON and the concurrent SST anomalies over tropical Pacific may partially account for the interannual variability of EOF1 and EOF2 winter WVC, and thus may provide a theoretical basis for improving the prediction of winter climate over East Asia. 相似文献
15.
月内尺度南半球环状模对应的大气环流异常传播特征 总被引：5，自引：3，他引：2
本文利用NCEP/NCAR逐日再分析资料, 分析了南半球环状模 (Southern Hemisphere Annular Mode, 简称SAM) 在月内时间尺度 (sub-monthly timescales, 5～30天)上相关环流的垂直和水平传播特征。结果表明, 月内SAM对应的纬向风异常和温度异常具有明显自南极地区向南半球中纬度地区水平传播特征; 在垂直方向上, 纬向风异常为明显上传特征, 温度异常则具有在极地和高纬度地区明显上升、低纬度地区下沉的特点。月内SAM对应的南半球中高纬度地区上传信号表明, 在较短的月内时间尺度上, 对流层信号可以突破对流层顶, 上传达到平流层; 而月内SAM对应的整层南传信号表明, 南极地区环流变化超前于中高纬度地区, 因此在1～3周的月内时间尺度上, 极区信号可能对中高纬地区信号具有指示意义。 相似文献
16.
NUMERICAL EXPERIMENTS FOR CLIMATE VARIABILITY OF MONSOON UNDER THE UNITED EFFECTS OF ANTARCTIC ICE COVER AND SEA SURFACE TEMPERATURE 总被引：1，自引：0，他引：1
In order to study the mechanisms of climate natural variability under the united effects of antarc-tic ice cover(AIC)and sea surface temperature(SST),we have done a series of numerical experi-ments for the climate variability of the period from January 1981 to December 1983 by using a three-level atmospheric general circulation model(AGCM).Firstly we conduct climate integration for sixyears,then 3 years'control integration from January 16 of the seventh year.Moreover,we do threesensitivity experiments,which are the sensitivity experiments forced by observed SST and AIC.ob-served SST and climatic AIC,observed AIC and climatic SST respectively,to study the climate vari-ability and its mechanisms affected by SST and AIC.We put emphasis on the variability of East Asiamonsoon and the Southern Hemisphere(SH)circulation.In this paper,introduction is made to theresults of control test and the sensitivity experiment forced by observed SST and AIC,and the pre-dictability of the monsoon climate variability is discussed. 相似文献
17.
春季南半球环状模与长江流域夏季降水的关系：Ⅱ 印度洋和南海海温的“海洋桥”作用 总被引：6，自引：3，他引：3
用回归、合成、相关、ESVD等方法分析了春夏季印度洋、南海海温异常在春季南半球环状模(SAM)与夏季长江中下游降水关系中的作用.研究发现春季南半球环状模指数(SAMI)正(负)异常时,同期南印度洋中高纬、北印度洋海域海温出现了明显正(负)异常,这种海温的正(负)异常在夏季依然存在,并且北印度洋的海温异常得到加强.对印度洋和南海海域详细划分区域后的进一步分析表明春季南半球热带外大气环流(SAM)异常可以强迫南印度洋中高纬海域海温发生明显异常.这种异常可以持续到夏季,而且表现出传播特性,即南印度洋中高纬海温异常可以传播到北印度洋(包括阿拉伯海和孟加拉湾)和南海海域,加强这些海域的海温异常.对东亚夏季风与夏季海温关系的分析表明东亚夏季风异常对应的夏季北印度洋、南海海温异常与春季SAM异常对应的夏季北印度洋、南海海温异常的形势相似,符号相反.说明印度洋、南海海温是春季SAM影响夏季长江中下游降水的一个"桥梁".基本思路为强(弱)春季SAM可以引起南印度洋中高纬海域海温的偏高(偏低);南印度洋中高纬海域偏高(偏低)的海温从春季持续到夏季并且传播到阿拉伯海、孟加拉湾、南海海域;这些海区偏高(偏低)的海温可以导致东亚夏季风减弱(加强),而东亚夏季风减弱(加强)是长江中下游降水偏多(偏少)的一种有利条件. 相似文献
18.
Decadal variation of the Northern Hemisphere Annular Mode and its influence on the East Asian trough 总被引：1，自引：0，他引：1 下载免费PDF全文
下载免费PDF全文 We analyze the decadal variation of the stratosphere troposphere coupled system around the year 2000 by using the NCEP reanalysis-2 data.Specifically,the relationship between the Northern Hemisphere Annular Mode(NAM) and the tropospheric East Asian trough is investigated in order to find the effective stratospheric signals during cold air outbreaks in China.Statistical analyses and dynamic diagnoses both indicate that after 2000,increased stratospheric polar vortex disturbances occur and the NAM is mainly in negative phase.The tropospheric polar areas are directly affected by the polar vortex,and in the midlatitudes,the Ural blocking high and East Asian trough are more active,which lead to enhanced cold air activities in eastern and northern China.Further investigation reveals that under this circulation pattern,downward propagations of negative NAM index are closely related to the intensity variation of the East Asian trough.When negative NAM anomalies propagate down to the upper troposphere and reach a certain intensity(standardized NAM index less than 1),they result in apparent reinforcement of the East Asian trough,which reaches its maximum intensity about one week later.The northerly wind behind the trough transports cold air southward and eastward,and the range of influence and the intensity are closely associated with the trough location.Therefore,the NAM index can be used as a measure of the signals from the disturbed stratosphere to give some indication of cold air activities in China. 相似文献
19.
Precipitation interannual variability in South America from the WCRP-CMIP3 multi-model dataset 总被引：1，自引：1，他引：0
The ability of coupled climate models from the WCRP-CMIP3 multi-model dataset to reproduce the interannual seasonal variability of precipitation in South America and the influence of the Southern Annular Mode (SAM) and El Niño-Southern Oscillation (ENSO) on such variability is examined. Models are able to reproduce the northward migration of the precipitation variability maximum during autumn and winter and its later return towards the south during spring and summer as well as the high variability throughout the year in southern Chile. Nevertheless, most of them have problems in representing accurately the variability associated with the South Atlantic convergence zone during summer and the typical maximum of variability in the subtropical continent during autumn and winter. The annular-like structure characteristic of the SAM influence on the Southern Hemisphere circulation is basically simulated by all models, but they have serious deficiencies in representing the observed relationship between SAM and both precipitation and circulation anomalies in South America. In addition, most of the models are not able to reproduce the typical wavetrains observed in the circulation anomalies in the Southern Hemisphere associated to ENSO. Only few models, previously identified as those with reasonable ENSO representation at the equatorial Pacific, have evidences of such wavetrains. Coherently, they exhibit the best representation of the ENSO signal in the South American precipitation. Results show that considerable improvement in the model representation of the climate variability in South America and in the associated large-scale teleconnections is still needed. 相似文献
20.
In order to study the physical nature of the Arctic Oscillation (AO) or the Northern Hemisphere annular mode (NAM), the linear stochastic model, constructed empirically from the output of a GCM run, is used to investigate the modal aspects of this climate variability mode. Theoretical analysis on the dominant modal response to the stochastic forcing is performed in terms of the maximum variance contribution. As the modal aspect of AO/NAM, two dominant normal modes are found to resemble the spatial structure of the AO/NAM represented by the leading EOF. These stochastically forced normal modes are regarded to be physical modes and thus can explain many fundamental features of the AO/NAM such as the barotropic annular dipole, tropospher–stratosphere coupling, and its dominance over the wintertime Northern Hemisphere. It then follows that the origin or physical nature of the AO/NAM can be attributed to the behaviors of the dominant annular normal modes. A key distinction of this study from previous eddy driving theories is that, to drive the variability, eddy forcing needs not to have a spatial structure completely coinciding with the pattern of the NAM, since the latter is mainly decided by the linear normal modes. 相似文献