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。     

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。     

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     

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。     

Changes in the Walker and Hadley circulations associated with the Southern Annular Mode

Extratropical impacts on tropical climates are one of the most exciting areas of meteorological investigation in recent times. The present study elucidates the seasonal impact of the Southern Annular Mode (SAM) on predominant tropical circulations such as Hadley and Walker. The velocity potential at 200 hPa is used to understand the spatio-temporal variability in tropical circulations in the boreal summer and winter seasons. The results show an intensification of seasonal velocity potential in the composite of the low phase of the SAM. The seasonal climatological values of velocity potential observed for the period from 1979 to 2012 are of lesser magnitudes than earlier findings. The convergence/divergence locations of tropical circulation have shifted from their mean positions in the alternative phase of the SAM. The low-level convergence in the southern hemispheric Hadley circulation (HC) is enhanced in the composite of the positive phase of the SAM; however, the SAM’s effect on the HC is no stronger in the summer. Another interesting feature noted in the present study is the weakening of the Walker circulation associated with the positive phase of the SAM, which can influence the basic state of the tropical Pacific Ocean. The SAM’s interannual variability exhibits a significant positive trend in winter. The study reveals that the positive phase of the SAM could be a possible explanation for the recent changes in the tropical circulation patterns; however, the variability in tropical circulation anomalies associated with the SAM should be noted on seasonal and monthly scales to understand the dynamical mechanism behind the relationship. The impact of the SAM on tropical circulation may continue in future decades, as this southern extratropical vacillation is predicted to remain in a positive phase due to the increase in greenhouse gases and the variability in ozone.     

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.     

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.     

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.     

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.     

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.     

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.     

The bi-decadal rainfall cycle,Southern Annular Mode and tropical cyclones over the Limpopo River Basin,southern Africa

The association between bi-decadal rainfall variability over southern Africa and the rainfall contributed by tropical cyclonic systems from the Southwest Indian Ocean (SWIO) provides a potential means towards understanding decadal-scale variability over parts of the region. A multi-decadal period is considered, focusing on the anomalous tropospheric patterns that induced a particularly wet 8-year long sub-period over the Limpopo River Basin. The wet sub-period was also characterized by a larger contribution to rainfall by tropical cyclones and depressions. The findings suggest that a broadening of the Hadley circulation underpinned by an anomalous anticyclonic pattern to the east of southern Africa altered tropospheric steering flow, relative vorticity and moisture contents spatially during the sub-period of 8 years. These circulation modulations induced enhanced potential for tropical systems from the SWIO to cause precipitation over the Limpopo River Basin. The same patterns are also conducive to increasing rainfall over the larger subcontinent, therefore explaining the positive association in the bi-decadal rainfall cycle and rainfall contributed by tropical cyclonic systems from the SWIO. An overview of regional circulation anomlies during alternating near-decadal wet and dry epochs is given. The regional circulation anomalies are also explained in hemispheric context, specifically in relation to the Southern Annular Mode, towards understanding variation over other parts of the Southern Hemisphere at this time scale.     

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...     

Relationships among the monsoon-like southwest Australian circulation,the Southern Annular Mode,and winter rainfall over southwest Western Australia

This study examines the relationships among the monsoon-like southwest Australian circulation (SWAC), the Southern Annular Mode (SAM), and southwest Western Australia winter rainfall (SWR), based on observed rainfall, reanalysis datasets, and the results of numerical modeling. By decomposing the SWAC into two components using a linear model, i.e. the component related to SAM (RSAM) and the component unrelated to SAM (SWACI*), we find it is the SWACI* that shows a significant influence on SWR. Similarly, it is the component of SAM associated with SWAC that exhibits an impact on SWR, whereas the component unrelated to SAM. A similar result is obtained in terms of the circulation associated with SWAC and the SAM. These facts suggest the SAM plays an indirect role in influencing SWR, and raise the possibility that SWAC acts as a bridge between the SAM and SWR, by which the SAM passes its influences onto SWR. This is due to the fact that the variations of SWAC are closely linked to the thermal contrast between land and sea across the southern Indian Ocean and southwest Australia. By contrast, the SAM does not significantly relate to this thermal structure, particularly for the component unrelated to SWAC. The variations of surface sea temperature over the southern Indian Ocean contribute to the favored rainfall circulation patterns. This finding is supported by the numerical modeling results. The strong coupling between SWAC and SWR may be instrumental for understanding the interactions between SWR and the southern Indian Ocean, and provides another perspective in examining the variations in SWR.     

Simulation of the Northern and Southern Hemisphere Annular Modes by CAMS-CSM

As leading modes of the planetary-scale atmospheric circulation in the extratropics, the Northern Hemisphere(NH)annular mode(NAM) and Southern Hemisphere(SH) annular mode(SAM) are important components of global circulation, and their variabilities substantially impact the climate in mid-high latitudes. A 35-yr(1979-2013) simulation by the climate system model developed at the Chinese Academy of Meteorological Sciences(CAMS-CSM) was carried out based on observed sea surface temperature and sea ice data. The ability of CAMS-CSM in simulating horizontal and vertical structures of the NAM and SAM, relation of the NAM to the East Asian climate, and temporal variability of the SAM is examined and validated against the observational data. The results show that CAMS-CSM captures the zonally symmetric and out-of-phase variations of sea level pressure anomaly between the midlatitudes and polar zones in the extratropics of the NH and SH. The model has also captured the equivalent barotropic structure in tropospheric geopotential height and the meridional shifts of the NH and SH jet systems associated with the NAM and SAM anomalies. Furthermore, the model is able to reflect the variability of northern and southern Ferrel cells corresponding to the NAM and SAM anomalies. The model reproduces the observed relationship of the boreal winter NAM with the East Asian trough and air temperature over East Asia. It also captures the upward trend of the austral summer SAM index during recent decades. However, compared with the observation, the model shows biases in both the intensity and center locations of the NAM's and SAM's horizontal and vertical structures. Specifically, it overestimates their intensities.