Climatological aspects of Hadley, Walker and monsoon circulations in two phases of the Pacific Decadal Oscillation

Summary Reanalyzed 200 hPa velocity potential (χ) fields for the 1958–2005 period are used to examine the influences of the Pacific Decadal Oscillation (PDO) in the average Hadley, Walker and monsoon circulations. These circulations are quantified and separated from each other by computing their components contained in the 200 hPa χ fields for the 1958–1976 and 1977–1995 periods. The main results found here are that the Hadley and monsoon circulations show small differences between the two periods, while the Walker circulation shows weaker action centers during the second period, with its center in the western Pacific being located further west. The differences in intensity and locations of the main action centers of the Walker circulation between the first and second periods are analyzed here in terms of the PDO influences. Authors’ address: Mary T. Kayano, Samia R. Garcia, Instituto Nacional de Pesquisas Espaciais/CPTEC, Avenida dos Astronautas 1758, 12227-010 S?o José dos Campos, SP, Brazil.     

CMIP5 simulated change in the intensity of the Hadley and Walker circulations from the perspective of velocity potential

Based on the simulations of 31 global models in CMIP5, the performance of the models in simulating the Hadley and Walker circulations is evaluated. In addition, their change in intensity by the end of the 21 st century(2080–2099) under the RCP4.5 and RCP8.5 scenarios, relative to 1986–2005, is analyzed from the perspective of 200 h Pa velocity potential.Validation shows good performance of the individual CMIP5 models and the multi-model ensemble mean(MME) in reproducing the meridional(zonal) structure and magnitude of Hadley(Walker) circulation. The MME can also capture the observed strengthening tendency of the winter Hadley circulation and weakening tendency of the Walker circulation. Such secular trends can be simulated by 39% and 74% of the models, respectively. The MME projection indicates that the winter Hadley circulation and the Walker circulation will weaken under both scenarios by the end of the 21 st century. The weakening amplitude is larger under RCP8.5 than RCP4.5, due to stronger external forcing. The majority of the CMIP5 models show the same projection as the MME. However, for the summer Hadley circulation, the MME shows little change under RCP4.5 and large intermodel spread is apparent. Around half of the models project an increase, and the other half project a decrease. Under the RCP8.5 scenario, the MME and 65% of the models project a weakening of the summer southern Hadley circulation.     

Intermodel Diversity of Simulated Long-term Changes in the Austral Winter Southern Annular Mode: Role of the Southern Ocean Dipole

The Southern Annular Mode(SAM)plays an important role in regulating Southern Hemisphere extratropical circulation.State-of-the-art models exhibit intermodel spread in simulating long-term changes in the SAM.Results from Atmospheric Model Intercomparison Project(AMIP)experiments from 28 models archived in CMIP5 show that the intermodel spread in the linear trend in the austral winter(June?July?August)SAM is significant,with an intermodel standard deviation of 0.28(10 yr)?1,larger than the multimodel ensemble mean of 0.18(10 yr)?1.This study explores potential factors underlying the model difference from the aspect of extratropical sea surface temperature(SST).Extratropical SST anomalies related to the SAM exhibit a dipole-like structure between middle and high latitudes,referred to as the Southern Ocean Dipole(SOD).The role of SOD-like SST anomalies in influencing the SAM is found in the AMIP simulations.Model performance in simulating the SAM trend is linked with model skill in reflecting the SOD?SAM relationship.Models with stronger linkage between the SOD and the SAM tend to simulate a stronger SAM trend.The explained variance is about 40%in the AMIP runs.These results suggest improved simulation of the SOD?SAM relationship may help reproduce long-term changes in the SAM.     

瓦克环流指数与哈得莱环流指数

利用地面风场资料定义了瓦克环流指数与哈得莱环流指数,并研究指数的变化及其与厄尔尼诺的关系。     

南大洋淡水强迫对南半球环状模的影响分析

南半球环状模是南半球热带外大气环流变异的主导模态,对南半球海洋—大气—海冰耦合系统有重要的影响。冰川融化激发的淡水强迫是南大洋的一种重要外强迫。在历史气候记录中,南大洋淡水通量异常曾引发数次全球性气候异常事件。基于海—气完全耦合模式FOAM,在60°S以南的海洋中施加强度为1.0 Sv的理想化淡水通量异常,分析南半球环状模的响应。结果表明：南大洋淡水通量异常可使局地西风增强,且西风的增强在垂向各个层次上均有体现。西风强度的变化导致对流层中大气斜压性增强,平流层中大气斜压性减弱。此外,淡水强迫使环状模的年际变率振幅显著减弱,而年代际变率开始增强,谱能量的变化具有垂向一致性。海温和海冰等外强迫因子的变化对环状模年代际振幅的增强具有重要意义。     

Distinct patterns of monthly Southern Annular Mode events

The Southern Annular Mode (SAM) is the leading mode of atmospheric variability in the mid–high latitudes of the Southern Hemisphere, representing large-scale variations in pressure and the polar front jet (PFJ). In SAM events, the combination of the SAM and other modes may result in different atmospheric patterns. In this study, a neural-network-based cluster technique, the self-organizing map, was applied to extract the distinct patterns of SAM events on the monthly time scale based on geopotential height anomalies at 500 hPa. Four pairs of distinguishable patterns of positive and negative SAM events were identified, representing the diversity in spatial distribution, especially the zonal symmetry of the center of action at high latitudes—that is, symmetric patterns, split-center patterns, West Antarctica patterns, and a tripole pattern. Although the SAM is well known to be belt-shaped, within the selected SAM events, the occurrence frequency of symmetric patterns is only 23.8%—less than that of West Antarctica patterns. Diverse PFJ variations were found in the symmetric and asymmetric patterns of SAM events. The more asymmetric the spatial distribution of the pressure anomaly, the more localized the adjusted zonal wind anomaly. The adjusted PFJ varied in meridional displacement and strength in different patterns of SAM events. In addition, the entrance and exit of the jet changed in most of the patterns, especially in the asymmetric patterns, which might result in different climate impacts of the SAM.摘要南半球环状模 (SAM) 是南半球中–高纬度地区大气变化的主导模态, 表现为气压和极锋急流 (PFJ) 的大尺度变动, 形成强烈的气候影响. 当SAM事件发生时, 气压场异常可呈现出不同的空间结构. 本文利用自组织映射网络方法对月尺度的SAM事件进行分类, 可识别出四对具有显著差异的正, 负SAM事件类型, 包括对称型, 中心分裂型, 西南极洲型和一种三极型分布. 气压异常的空间分布越不对称, 调整后的纬向风异常越局地化. PFJ的经向位移和强度变化入口和出口的变化, 可能导致了SAM的不同气候影响.     

The Springtime North Asia Cyclone Activity Index and the Southern Annular Mode

The relationship between the North Asia cyclone （NAC） activity and the Southern Annular Mode （SAM） is documented in this research. The definition of the NAC index （NACI） is based on the atmospheric relative vorticity in North Asia. The analysis yields a significant positive correlation between previous winter Southern Annular Mode index （SAMI） and spring NACI in the interannual variability, with a correlation coefficient of 0.51 during 1948-2000. Analysis of the NAC-related and SAM-related atmospheric general circulation variability demonstrates such a relationship. The study further reveals that when the winter SAM becomes strong, the springtime atmospheric convection in tropical western Pacific will intensify and the local Hadley circulation will be strengthened. As a result, the abnormal subsiding motion over South China makes the temperature gradient intensified in the low level and strengthens the jet in the high level, both of which are beneficial to the development of NAC activity.     

南半球环状模事件的准地转调整过程分析

本文利用NCEP/NCAR逐日再分析资料,分析了南半球环状模（SAM）事件生命过程中的准地转调整过程.由于SAM沿纬圈的水平尺度远远大于临界尺度罗斯贝变形半径,因此要求纬向风场在地转调整过程向气压场适应以达到地转平衡.研究结果表明,在纬向平均环流中,异常Ferrel环流强度的变化超前于SAM强度变化约2/16位相,异常Ferrel环流能够通过超前的整层大气质量的经向输运,改变中高纬度的质量分布装状况,导致中、高纬度地区间的位势高度梯度异常变化,而中、高纬度地区间的位势高度梯度异常发生变化就意味着SAM强度和位相发生变化;而当SAM强度和位相发生改变后,即中、高纬度地区南北方向上的位势高度梯度发生变化后,可破坏中纬度地区纬向风场与位势高度场之间的地转平衡,产生地转偏差;地转偏差产生后,又可驱动经向散度风场,造成Ferrel环流异常的变化,由此形成一个自我内部循环调整过程.     

Potential changes in extremes and links with the Southern Annular Mode as simulated by a multi-model ensemble

We assess the likely changes in climate extremes under enhanced greenhouse gases over the southern extratropics, with emphasis in southern South America and sub-Antarctic seas, through the analysis of extreme indices measured from models participating in the IPCC 4th Assessment Report. We discuss how the anthropogenic climate change under A1B scenario influences both the patterns of mean change of extreme indices and the likelihood of occurrence of severe extreme indices. The likelihood of occurrence of a year with a large number of days with “warm” minimum temperatures is estimated to increase by a factor of 4 by the end of this century over most of the southern extratropics. By that time, the risk of “severe” precipitation intensity is projected to rise in most areas with the exception of the subtropical anticyclones, which experience particularly strong drying. Over the Southern Ocean this likelihood has increased to over 60%. Corresponding estimates of the changing likelihood for very long dry spells show a banded structure with positive ratios to the north of about 50° S and negative ratios in the sub Antarctic seas. In southern South America this risk about doubled between present and future climates. Then, we explore if the Southern Annular Mode influences the occurrence of severe extreme indices during the period 2070–2099. Its positive phase inhibits the extremely warm minimum temperatures in the Southern Ocean, with the exception of the eastern Bellingshausen Sea, and favors severe frost days to the north of the Ross Sea. Temperature indices show very little change induced by the SAM to the north of 50° S. Severe dry spells are inhibited during the positive phase along the sub Antarctic seas, while the mid-latitudes, including most of Patagonia, show the opposite behaviour. The Southern Ocean reveals a non-uniform distribution with both increases and decreases in the occurrence of heavier precipitation during positive SAM.     

南半球环状模气候影响的若干研究进展

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。     

Impact of the Preceding Boreal Winter Southern Annular Mode on the Summertime Somali Jet

One of the major high-latitude circulation systems in the Southern Hemisphere is the Southern Annular Mode（SAM）. Its effect on the Somali Jet（SMJ）, which connects the Southern and Northern hemispheres, cannot be ignored. The present reported results show that time series of both the Southern Annular Mode Index（SAMI） during the preceding winter and the summertime Somali Jet intensity Index（SMJI） display a significant increasing trend and have similar interdecadal variation. The latter was rather strong around 1960, then became weaker up to the mid-1980 s, before starting to strengthen again. The lead-lag correlations of monthly mean SAMI with the following summertime SMJI showed significant positive correlations in November, December, and January. There are thus connections across two seasons between the SAM and the SMJ. The influence of the winter SAM on the summer SMJ was explored via analyses of SST anomalies in the Southern Indian Ocean. During strong（weak） SAM/SMJ years, the SST east of Madagascar is colder（warmer） while the SST west of Australia is warmer（colder）, corresponding to the positive（negative） Southern Indian Ocean Dipole-like（SIODL） event. Subsequently, the SIODL excites an anticyclone located over the Arabian Sea in summer through air-sea coupling from winter to summer, which causes an increase in the summer SMJ intensity. The anticyclone/high branch of the SAM over the Southern Hemisphere subtropics and the cyclone/low over the east coast of Madagascar play an important role in the formation of Southern Indian Ocean ＂bridge＂ from winter to summer.     

月内尺度南半球环状模对应的大气环流异常传播特征

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。     

1991 年夏暴雨期间Walker 环流和Hadley 环流的特征

该文分析了1991年7月中国江淮暴雨洪水期间赤道太平洋沃克环流及沿115°E哈得莱环流的逐日演变特征。结果发现在暴雨时期赤道东风减弱,西风加强,反哈得莱环流（即季风环流）在中国东部上空加强。在暴雨期间,高低空散度风场的分析指出,在低空出现明显的散度风越赤道气流,而在对流层上部出现散度风的流出。     

The Southern Annular Mode (SAM) in PMIP2 simulations of the last glacial maximum

The increasing trend of the Southern Annular Mode （SAM） in recent decades has influenced climate change in the Southem Hemisphere （SH）.How the SAM will respond increased greenhouse gas concentrations in the future remains uncertain.Understanding the variability of the SAM in the past under a colder climate such as during the Last Glacial Maximum （LGM） might provide some understanding of the response of the SAM under a future warmer climate.We analyzed the changes in the SAM during the LGM in comparison to pre-industrial （PI） simulations using five coupled ocean-atmosphere models （CCSM,FGOALS,IPSL,MIROC,HadCM） from the second phase of the Paleoclimate Modelling Intercomparison Project （PMIP2）.In CCSM,MIROC,IPSL,and FGOALS,the variability of the simulated SAM appears to be reduced in the LGM compared to the PI simulations,with a decrease in the standard deviation of the SAM index.Overall,four out of the five models suggest a weaker SAM amplitude in the LGM consistent with a weaker SH polar vortex and westerly winds found in some proxy records and model analyses.The weakening of the SAM in the LGM was associated with an increase in the vertical propagation of Rossby waves in southern high latitudes.     

Influence of the Preceding Austral Summer Southern Hemisphere Annular Mode on the Amplitude of ENSO Decay

There is increasing evidence of the possible role of extratropical forcing in the evolution of ENSO.The Southern Hemisphere Annular Mode(SAM) is the dominant mode of atmospheric circulation in the Southern Hemisphere extratropics.This study shows that the austral summer(December–January–February; DJF) SAM may also influence the amplitude of ENSO decay during austral autumn(March–April–May;MAM).The mechanisms associated with this SAM–ENSO relationship can be briefly summarized as follows:The SAM is positively(negatively) correlated with SST in the Southern Hemisphere middle(high) latitudes.This dipole-like SST anomaly pattern is referred to as the Southern Ocean Dipole(SOD).The DJF SOD,caused by the DJF SAM,could persist until MAM and then influence atmospheric circulation,including trade winds,over the Nio3.4 area.Anomalous trade winds and SST anomalies over the Nio3.4 area related to the DJF SAM are further developed through the Bjerkness feedback,which eventually results in a cooling(warming) over the Nio3.4 area followed by the positive(negative) DJF SAM.     

Slowing down of the summer Southern Hemisphere Annular Mode trend against the background of ozone recovery

20世纪末期,南半球热带外地区经历了显著的气候变化,包括夏季南半球环状模(SAM)的显著上升趋势,伴随着南极半岛的增暖和别林斯高晋海的海冰融化.这些趋势主要是由20世纪末期南极平流层臭氧消耗所驱动的.本文发现,自2001年左右以来,在南极平流层臭氧恢复的背景下,观测到的夏季SAM的上升趋势已经趋于平缓,验证了前人利用数值模拟预测的夏季SAM上升趋势减缓现象.与SAM在臭氧恢复后趋势只减缓但没有逆转不同,南极地表气温和海冰的趋势发生了逆转.南极半岛由变暖趋势转为降温趋势,别林斯高晋海域的海冰由融化趋势转为增多趋势.     

Climate links and recent extremes in antarctic sea ice, high-latitude cyclones, Southern Annular Mode and ENSO

In this article, we study the climate link between the Southern Annular Mode (SAM) and the southern sea-ice extent (SIE), and discuss the possible role of stationary waves and synoptic eddies in establishing this link. In doing so, we have used a combination of techniques involving spatial correlations of SIE, eddy streamfunction and wind anomalies, and statistics of high-latitude cyclone strength. It is suggested that stationary waves may be amplified by eddy anomalies associated with high latitude cyclones, resulting in more sea ice when the SAM is in its positive phase for most, but not all, longitudes. A similar association is observed during ENSO (La Ni?a years). Although this synergy in the SAM/ENSO response may partially reflect preferential areas for wave amplification around Antarctica, the short extent of the climate records does not allow for a definite causality connection to be established with SIE. Stronger polar cyclones are observed over the areas where the stationary waves are amplified. These deeper cyclones will break up and export ice equatorward more efficiently, but the near-coastal regions are cold enough to allow for a rapid re-freeze of the resulting ice break-up. We speculate that if global warming continues this same effect could help reverse the current (positive) Antarctic SIE trends once the ice gets thinner, similarly to what has been observed in the Northern Hemisphere.     

An examination of the relationship between the Southern Annular Mode and Antarctic surface air temperatures in the CMIP5 historical runs

Climate Dynamics - Strong relationships exist between the Southern Annular Mode (SAM) and surface air temperature (SAT) across much of Antarctica. Changes in the SAM will have a profound influence...