印度洋海温年际异常与热带夏季季节内振荡的关系及其数值模拟研究

利用观测分析资料和SINTEX-F海气耦合长时间(70年)数值模拟结果,分析了印度洋海温年际异常与热带夏季季节内振荡(BSISO)各种传播模态之间关系及其物理过程。结果表明,印度洋海温年际异常与热带BSISO关系密切,当印度洋为正(负)偶极子情况,中东印度洋北传BSISO减弱(加强);当印度洋为正(负)海盆异常(BWA)情况,印度洋西太平洋赤道地区(40°E -180°)东传BSISO加强(减弱)。印度洋海温年际变化通过大气环流背景场和BSISO结构影响热带BSISO不同传播模态强度的年际变化。在负(正)偶极子年夏季,由于对流层大气垂直东风切变加强(减弱),对流扰动北侧的正压涡度、边界层水汽辐合加强更明显(不明显),导致形成BSISO较强(弱)的经向不对称结构,因此北传BSISO偏强(减弱)。印度洋BWA模态通过影响赤道西风背景以及海气界面热力交换,导致赤道东传BSISO强度产生变化。在正BWA年夏季,赤道地区西风较明显,当季节内振荡叠加在这种西风背景下,扰动中心的东侧(西侧)风速减弱(加强)更明显,海面蒸发及蒸发潜热减弱(加强)更明显,导致扰动中心的东侧(西侧)海温升高(降低)幅度更大,从而使边界层产生辐合(辐散)更强、水汽更多(少),因此赤道东传BSISO偏强;而在负BWA年,赤道地区西风背景减弱,以上物理过程受削弱使赤道东传BSISO偏弱。     

Variability of Summer Monsoon Rainfall in India on Inter-Annual and Decadal Time Scales

Indian Summer Monsoon Rainfall(ISMR)exhibits a prominent inter-annual variability known as troposphere biennial oscillation.A season of deficient June to September monsoon rainfall in India is followed by warm sea surface temperature(SST)anomalies over the tropical Indian Ocean and cold SST anomalies over the western Pacific Ocean.These anomalies persist until the following monsoon,which yields normal or excessive rainfall.Monsoon rainfall in India has shown decadal variability in the form of 30 year epochs of alternately occurring frequent and infrequent drought monsoons since1841,when rainfall measurements began in India.Decadal oscillations of monsoon rainfall and the well known decadal oscillations in SSTs of the Atlantic and Pacific oceans have the same period of approximately 60 years and nearly the same temporal phase.In both of these variabilities,anomalies in monsoon heat source,such as deep convection,and middle latitude westerlies of the upper troposphere over south Asia have prominent roles.     

夏季北太平洋洋中槽强度的年际变化及其与北美地面气温的关系

利用1979—2010年NCEP/NCAR再分析资料和特拉华大学地面气温资料,研究了夏季北太平洋洋中槽强度的年际变化特征、成因及其与北美地表气温的联系。结果表明,夏季北太平洋洋中槽强度存在明显的年际变化特征,并主要表现为准4 a和准5a的周期变化特征。进一步的分析表明,夏季北太平洋洋中槽强度与同期北美东南地区地表气温存在显著的负相关,当夏季北太平洋洋中槽强度异常偏强(弱)时,洋中槽下游的北美东南地区对流层中高层的位势高度异常降低(升高),对应于控制该地区的高压减弱(增强),从而使得该地区地表气温降低(升高)。此外,夏季热带印度洋海温异常对洋中槽的年际变化起了决定性的作用。当热带印度洋海温异常偏暖时,通过调节大气边界层相当位温,加热对流层大气,在热带印度洋东侧激发出东传的开尔文波,同时也使得北太平洋中东部位势高度异常升高,导致洋中槽的异常减弱。反之亦然。     

Emerging role of Indian ocean on Indian northeast monsoon

This study examines the emerging role of Indian Ocean sea surface temperature (SST) on the inter-annual variability (IAV) of Indian north-east monsoon rainfall (NEMR). The IAV of NEMR is associated with the warm SST anomaly over east Bay-of-Bengal (BoB) (88.5^oE–98.5^oE; 8.5^oN–15.5^oN) and cool SST anomaly over east equatorial Indian Ocean (80.5^oE–103.5^oE; 6.5^oS–3.5^oN). The gradient of SST between these boxes (i.e. northern box minus southern box) shows strong and robust association with the Indian NEMR variability in the recent decades. For establishing the teleconnections, SST, mean sea level pressure, North Indian Ocean tropical storm track, and circulation data have been used. The study reveals that during the positive SST gradient years, the inter-tropical convergence zone (ITCZ) shifts northwards over the East Indian Ocean. The tropical depressions, storms and cyclones formed in the North Indian Ocean moves more zonally and strike the southern peninsular India and hence excess NEMR. While, during the negative SST gradient years, the ITCZ shifts southwards over the Indian Ocean. The tropical depressions, storms and cyclones formed in the North Indian Ocean moves more northwestward direction and after crossing 15^oN latitude re-curve to north-east direction towards head BoB and misses southern peninsular India and hence, deficient NEMR.     

赤道印度洋海温偶极子型振荡及其气候影响

对近百年观测资料的分析表明赤道印度洋海温(SST)确实存在着偶极子型振荡的变化特征,它在9～11月最强,而在1～4月最弱;年际变化(4～5年周期)和年代际变化(主要为20～25年周期)也十分清楚.这个偶极子主要有正位相型(海温西高东低)和负位相型(海温东高西低);一般正位相型的振幅强于负位相型.尽管在极个别年赤道印度洋海温偶极子似乎与太平洋ENSO无关,但总体而论,赤道印度洋海温偶极子与赤道太平洋海温偶极子(类似ENSO)有很好负相关.它们的联系主要是赤道大气纬向(Walker)环流.资料分析表明,赤道印度洋海温偶极子与亚洲南部流场、青藏高压和西太平洋副高都有明显关系,表明它对亚洲季风活动有重要影响.     

印度洋偶极子对大气环流和气候的影响

赤道印度洋SST的分析研究证实了偶极子型振荡的存在，它在9-11月较强而在1-4月较弱。若以海温西高东低为偶极子振荡正位相，以海温东高西低为负位相，则一般是正位相时振荡要强于负位相。印度洋偶极子也存在年际（主要周期为4－5年）和年代际（主要周期为25-30年）变化。分析研究表明，印度洋偶极子对亚洲季风活动有明显影响，因为亚洲地区对流层低层的风场，南亚高压和西太平洋副高强度都与印度洋偶极子有关。另外，印度洋偶极子还对北美和南印度洋（包括澳大利亚和南美）地区的大气环流和气流有影响。     

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.     

夏季印度洋海盆模与MC区域降水异常联系的进一步分析

利用英国哈德莱中心的逐月海表温度资料及NCEP/NCAR月平均再分析资料等,通过在印度洋海盆模IOBM指数(IIOB)中扣除长期趋势和两类ENSO的同期信号后,得到了修正的IOBM指数(Im IOB),并由此分析了IOBM的变化及与海洋性大陆区域降水异常的联系。结果表明:印度洋IOBM为暖位相时,不同季节的印度洋地区均呈现异常偏暖,但大气是上升还是下沉运动则在印度洋不同季节和不同区域存在很大变化。就夏季而言,印度洋大部分地区存在上升运动,这与海温异常偏暖有关。在北半球夏季,指数Im IOB存在3~5 a的周期变化。当IOBM处于正位相时,印度洋至我国东海地区大范围海温偏暖。MC(Maritime Continent,海洋性大陆)区域西部降水正异常,而MC区域东北部降水为负异常。造成这种降水分布的原因是:当指数为正时,在MC区域的西部对流层低层辐合、高层辐散,上升运动增强,且水汽辐合,而MC区域的东北部对流层低层辐散、高层辐合,上升运动不明显,水汽辐散,不易形成降水。而在对流层低层与西太平洋辐散中心对应,南北半球出现关于赤道对称的反气旋对,赤道印度洋上的异常加热激发东传的Kelvin波,加强东风异常,同时加强了KMC(海洋性大陆的核心区域)之外南北半球热带地区的这对Rossby波型。以上这些结果有利于深刻理解MC降水异常成因及热带海陆气相互作用过程。     

赤道西太平洋-印度洋海温异常对亚洲夏季风的影响

本文采用了p-σ五层原始方程模式模拟并研究了赤道西太平洋-印度洋海温距平场对亚洲夏季风的影响,计算了四种不同的海温距平试验方案。试验结果表明赤道西太平洋海温正距平使对流层下层的印度低压明显加强,副高北挺,季风槽加深,同时加强了对流层上层的反气旋环流。赤道西印度洋暖海温的模拟结果与赤道西太平洋暖海温对上述系统的影响相反,而赤道西印度洋冷海温对季风环流的影响与赤道西太平洋暧海温的影响一致。试验进一步表明赤道西太平洋-印度洋海温距平的纬向梯度方向对亚洲夏季风的影响是主要的,这一结论与实际观测结果一致。本文进一步讨论了赤道海温距平对越赤道气流、印度洋赤道东-西纬向环流和非绝热加热场的影响,结果都表明赤道西太平洋海温正距平和赤道西印度洋海温负距平的模拟特征与反El Nino年亚洲夏季环流特征类似,而赤道西印度洋海员正距平的模拟特征与El Nino年亚洲夏季坏流特征类似。     

Sub-seasonal interannual variability associated with the excess and deficit Indian winter monsoon over the Western Himalayas

During the winter season (Dec., Jan., and Feb.; DJF) the western Himalaya (WH) receives one-third of its annual precipitation due to Indian winter monsoon (IWM). The IWM is characterized by eastward-moving synoptic weather systems called western disturbances. Seasonal interannual precipitation variability is positively correlated with monthly interannual variabilities. However, it was found that the monthly interannual variabilities differ. The interannual variability for Jan. is negatively correlated with that for Dec. and Feb. Because the entire seasonal interannual variability is in phase with the El Niño Southern Oscillation, it is interesting to investigate such contrasting behavior. Composite analysis based on extreme wet and dry seasons indicates that Dec. and Feb. precipitation variabilities have a high positive (low negative) correlation with eastern (western) equatorial Pacific warming (cooling), whereas Jan. precipitation variability exhibits negligible correlations. Seasonal mid/upper tropospheric cooling over the Himalayas enhances anomalous cyclonic circulation, which along with suppressed convection over the western equatorial Pacific, shifts the 200-hPa subtropical westerly jet southward over the Himalayas. Due to the upper tropospheric anomalous cyclonic circulation, mass transfer favors anticyclone formation at the mid/lower troposphere, which is enhanced in Jan. due to a warmer mid troposphere and hence decreases precipitation compared with Dec. and Feb. Additionally, a weakening of meridional moisture flux transport from the equatorial Indian Ocean to WH is observed in Jan. Further analysis reveals that mid-tropospheric and surface temperatures over WH also play dominant roles, acting as local forcing where the preceding month’s surface temperature controls the succeeding month’s precipitation.     

On the relationship between Indian summer monsoon withdrawal and Indo-Pacific SST anomalies before and after 1976/1977 climate shift

A clear shift in the withdrawal dates of the Indian Summer Monsoon is observed in the long term time series of rainfall data. Prior (posterior) to the 1976/1977 climate shift most of the withdrawal dates are associated with a late (an early) withdrawal. As a result, the length of the rainy season (LRS) over the Indian land mass has also undergone similar changes (i.e., longer (shorter) LRS prior (posterior) to the climate shift). In this study, probable reasons for this significant shift in withdrawal dates and the LRS are investigated using reanalysis/observed datasets and also with the help of an atmospheric general circulation model. Reanalysis/observational datasets indicate that prior to the climate shift the sea surface temperature (SST) anomalies in the eastern equatorial Pacific Ocean and the Arabian Sea exerted a strong influence on both the withdrawal and the LRS. After the climate shift, the influence of the eastern equatorial Pacific Ocean SST has decreased and surprisingly, the influence of the Arabian Sea SST is almost non-existent. On the other hand, the influence of the southeastern equatorial Indian Ocean has increased significantly. It is observed that the upper tropospheric temperature gradient over the dominant monsoon region has decreased and the relative influence of the Indian Ocean SST variability on the withdrawal of the Indian Summer Monsoon has increased in the post climate shift period. Sensitivity experiments with the contrasting SST patterns on withdrawal dates and the LRS in the pre- and post- climate shift scenarios, confirm the observational evidences presented above.     

Secular spring rainfall variability at local scale over Ethiopia: trend and associated dynamics

Spring rainfall secular variability is studied using observations, reanalysis, and model simulations. The joint coherent spatio-temporal secular variability of gridded monthly gauge rainfall over Ethiopia, ERA-Interim atmospheric variables and sea surface temperature (SST) from Hadley Centre Sea Ice and SST (HadISST) data set is extracted using multi-taper method singular value decomposition (MTM-SVD). The contemporaneous associations are further examined using partial Granger causality to determine presence of causal linkage between any of the climate variables. This analysis reveals that only the northwestern Indian Ocean secular SST anomaly has direct causal links with spring rainfall over Ethiopia and mean sea level pressure (MSLP) over Africa inspite of the strong secular covariance of spring rainfall, SST in parts of subtropical Pacific, Atlantic, Indian Ocean and MSLP. High secular rainfall variance and statistically significant linear trend show consistently that there is a massive decline in spring rain over southern Ethiopia. This happened concurrently with significant buildup of MSLP over East Africa, northeastern Africa including parts of the Arabian Peninsula, some parts of central Africa and SST warming over all ocean basins with the exception of the ENSO regions. The east-west pressure gradient in response to the Indian Ocean warming led to secular southeasterly winds over the Arabian Sea, easterly over central Africa and equatorial Atlantic. These flows weakened climatological northeasterly flow over the Arabian Sea and southwesterly flow over equatorial Atlantic and Congo basins which supply moisture into the eastern Africa regions in spring. The secular divergent flow at low level is concurrent with upper level convergence due to the easterly secular anomalous flow. The mechanisms through which the northwestern Indian Ocean secular SST anomaly modulates rainfall are further explored in the context of East Africa using a simplified atmospheric general circulation model (AGCM) coupled to mixed-layer oceanic model. The rainfall anomaly (with respect to control simulation), forced by the northwestern Indian Ocean secular SST anomaly and averaged over the 30-year period, exhibits prevalence of dry conditions over East and equatorial Africa in agreement with observation. The atmospheric response to secular SST warming anomaly led to divergent flow at low levels and subsidence at the upper troposphere over regions north of 5^° S on the continent and vice versa over the Indian Ocean. This surface difluence over East Africa, in addition to its role in suppressing convective activity, deprives the region of moisture supply from the Indian Ocean as well as the Atlantic and Congo basins.     

前冬印度洋海盆一致模对ENSO衰减期华南春季降水的影响

利用逐月台站观测降水、HadISST1.1海温和ERA5大气再分析资料,研究了前冬印度洋海盆一致模（Indian Ocean Basin,IOB）对华南春季降水（SCSR）与ENSO关系的影响,并分析了IOB通过调控ENSO环流异常进而影响SCSR的可能机制。结果表明:当前冬El Ni?o（La Ni?a）与IOB暖（冷）位相同时发生时,SCSR显著增多（减少）;而当El Ni?o或La Ni?a单独发生而IOB处于中性时,SCSR并无明显多寡倾向。其原因在于,当El Ni?o与IOB暖相位并存时,前冬热带印度洋和赤道中东太平洋均为正海温异常（Sea-Surface Temperature Anomaly,SSTA）,且印度洋SSTA强度可一直维持至春季。在对流层低层,春季赤道中东太平洋的正SSTA激发出异常西北太平洋反气旋（Western North Pacific Anticyclone,WNPAC）。而热带印度洋的正SSTA在副热带印度洋激发出赤道南北反对称环流,赤道以北的东风异常有利于异常WNPAC西伸;赤道以南的西风异常与来自赤道西太平洋的东风异常在东印度洋辐合上升,气流至西北太平洋下沉,形成经向垂直环流,有利于春季WNPAC维持。在对流层高层,印度洋的正SSTA在热带印度洋上空激发出位势高度正异常,随之形成的气压经向梯度加强了东亚高空副热带西风急流,进而在华南上空形成异常辐散环流。WNPAC的西伸和加强可为华南提供充足的水汽,同时高空辐散在华南引发水汽上升运动,共同导致SCSR正异常。而若El Ni?o发生时IOB处于中性状态,El Ni?o相关的SSTA衰减较快,春季WNPAC不显著,SCSR无明显多寡趋势。      

2015/2016超强El Nio对中国南方冬春季降水的影响分析

利用1981—2016年的中国160站降水资料、OISST海温资料和NCEP/NCAR大气环流资料,对比分析了中等强度El Nio和2015/2016超强El Nio对中国东南部、江淮流域和西南地区冬春季降水影响的异同。结果表明:在中等强度El Nio的冬季,偏暖的赤道中东太平洋海表面温度(Sea Surface Temperature,SST)所激发的西北太平洋和日本附近的异常反气旋环流,其异常的西南风会加强南海—西北太平洋的水汽向中国东部输送,造成中国东南部和江淮流域的降水一致偏多。2015/2016超强El Nio的冬季,赤道中东太平洋SST的强度异常偏强,中国东部异常偏冷的表面气温和对流层低层温度加强大陆冷高压,长江流域及其以北地区受异常强的北风控制,从而造成中国东南部降水增多、江淮流域降水减少。在2015/2016超强El Nio事件衰减位相的春季,中国东南部和西南部降水的增加主要归因于异常偏暖的西北印度洋和东南印度洋SST的作用。经CAM5模式试验证明,西北印度洋异常偏暖的SST引起了北印度洋的异常西南风,激发了孟加拉湾—西北太平洋的异常反气旋,加强了印度洋和南海—西北太平洋的水汽向中国西南和东南部输送。此外,东南印度洋异常偏暖的SST还会激发局地异常上升运动,通过经向垂直环流加强南海—西北太平洋异常下沉运动,诱使中国东南部的上升运动加强,导致降水增多。     

Some aspects of an early summer monsoon onset over Andaman Sea and further progress towards the Indian mainland

This work addresses the mechanisms that leads to an early onset of monsoon over Andaman Sea but advances further rapidly (slowly) to the Indian mainland resulting in the early (delayed) onset over Kerala. The upper tropospheric temperature, production of kinetic energy (KE) and outgoing long wave radiation (OLR) from the month of May till onset over Kerala are analysed for two delayed onset years (1997, 1995) and two early onset years (2004, 1990). It is observed that the maximum temperature over Tibetan plateau (TP), an increase in the production of KE and strong equatorial convection in early May, is associated with early onset over Andaman Sea. However, when there is a lull in advance of monsoon after the early onset over Andaman Sea, shifting of the warm region south of TP, weak production of KE in the lower troposphere and convective region shifting to Western Pacific resulted in the delayed onset over Kerala in 1997 and 1995. During the early onset years viz. 2004 and 1990, the warm region moving westwards, high production of KE extending to mid troposphere and deep convection moving westwards in the north Indian Ocean (10–15°N) is noticed.     

TROPICAL SEA SURFACE TEMPERATURE ANOMALY AND INDIAN SUMMER MONSOON*

The time series of the sea surface temperature(SST) anomaly,covering the eastern (western) equatorial Pacific,central Indian Ocean,Arabian Sea.Bay of Bengal and South China Sea(SCS),have been analyzed by using wavelet transform.Results show that there exists same interdeeadal variability of SST in the tropical Pacific and tropical Indian Ocean,and also show that the last decadal abrupt change occurred in the 1970s.On the interannual time scale,there is a similar interannual variability among the equatorial central Indian Ocean and the adjacent three sea basins(Arabian Sea.Bay of Bengal and South China Sea).but the SST interannual changes of the Indian Ocean lagged 4-5 months behind that of the equatorial central-east Pacific.Meanwhile,the interannual variability and long-range change between SST anomaly and Indian summer monsoon rainfall in recent decades have been explained and analyzed.It indicates that there existed a wet(dry) period in India when the tropical SST was lower(higher)than normal,but there was a lag of phase between them.     

Contrasting Impacts of South and North Tropical Indian Ocean Sea Surface Temperature Anomalies on East Asian Summer Climate

Using observational data and model simulations,the author find that the North Indian Ocean(NIO)sea surface temperature(SST)anomalies can trigger an eastward atmospheric Kelvin wave propagating into the equatorial western Pacific,inducing an anomalous anticyclone over the subtropical Northwest Pacific(NWP)and resulting in prominent summer climate anomalies in the East Asia-Northwest Pacific(EANWP)region.However,the response of tropospheric temperatures and atmospheric Kelvin waves to the South Indian Ocean(SIO)SST anomalies is weak;as a result,the impact of the SIO SST anomalies on the EANWP summer climate is weak.The contrasting impacts of NIO and SIO SST anomalies on the EANWP summer climate is possibly due to the different mean state of SSTs in the two regions.In summer,the climatological SSTs in the NIO are higher than in the SIO,leading to a stronger response of atmospheric convection to the NIO SST anomalies than to the SIO SST anomalies.Thus,compared with the SIO SST anomalies,the NIO SST anomalies can lead to stronger tropospheric air temperature anomalies and atmospheric Kelvin waves to affect the EANWP summer climate.     

气候系统模式FGOALS模拟的南亚夏季风：偏差和原因分析

本文通过与观测和再分析资料的对比,评估了LASG/IAP发展的气候系统模式FGOALS的两个版本FGOALS-g2和FGOALS-s2对南亚夏季风的气候态和年际变率的模拟能力,并使用水汽收支方程诊断,研究了造成降水模拟偏差的原因。结果表明,两个模式夏季气候态降水均在陆地季风槽内偏少,印度半岛附近海域偏多,在降水年循环中表现为夏季北侧辐合带北推范围不足。FGOALS-g2中赤道印度洋"东西型"海温偏差导致模拟的东赤道印度洋海上辐合带偏弱,而FGOALS-s2中印度洋"南北型"海温偏差导致模拟的海上辐合带偏向西南。水汽收支分析表明,两个模式中气候态夏季风降水的模拟偏差主要来自于整层积分的水汽通量,尤其是垂直动力平流项的模拟偏差。一方面,夏季阿拉伯海和孟加拉湾的海温偏冷而赤道西印度洋海温偏暖,造成向印度半岛的水汽输送偏少;另一方面,对流层温度偏冷,冷中心位于印度半岛北部对流层上层,同时季风槽内总云量偏少,云长波辐射效应偏弱,对流层经向温度梯度偏弱以及大气湿静力稳定度偏强引起的下沉异常造成陆地季风槽内降水偏少。在年际变率上,观测中南亚夏季风环流和降水指数与Ni?o3.4指数存在负相关关系,但FGOALS两个版本模式均存在较大偏差。两个模式中与ENSO暖事件相关的沃克环流异常下沉支和对应的负降水异常西移至赤道以南的热带中西印度洋,沿赤道非对称的加热异常令两个模式中越赤道环流季风增强,导致印度半岛南部产生正降水异常。ENSO相关的沃克环流异常下沉支及其对应的负降水异常偏西与两个模式对热带南印度洋气候态降水的模拟偏差有关。研究结果表明,若要提高FGOALS两个版本模式对南亚夏季风气候态模拟技巧,需减小耦合模式对印度洋海温、对流层温度及云的模拟偏差;若要提高南亚夏季风和ENSO相关性模拟技巧需要提高模式对热带印度洋气候态降水以及与ENSO相关的环流异常的模拟能力。     

夏季厄尔尼诺-Modoki和东部型ENSO海表温度异常分布型特征及其与海洋性大陆区域气候异常的联系

2007年,Ashok等揭示了赤道太平洋区域存在一种三极型分布海表温度异常并称之为厄尔尼诺-Modoki,同时定义了相应的海表温度异常指数EMI（记为I_EM）。在此基础上,利用英国哈得来中心逐月海表温度资料、美国NCEP/NCAR月平均再分析数据集、美国国家海洋和大气管理局（NOAA）逐月降水资料（CMAP）,通过在太平洋海表温度异常中扣除厄尔尼诺-Modoki信号后,在Nino1+2区域上定义了东太平洋型海表温度异常指数EPNI（I_EPN）。据此,由I_EPN和I_EM可构成描述热带太平洋海表温度异常变化的一对指数。分析了两个指数相应的海气状态及对海洋性大陆区域气候异常的影响。结果表明,厄尔尼诺-Modoki和东太平洋型海表温度异常及其影响存在显著差异。在北半球夏季,当I_EM处于正位相时,热带太平洋海表温度异常呈现“负-正-负”的结构,海洋性大陆大部分区域海表温度异常为负,此时对流层低层太平洋地区辐合,海洋性大陆地区辐散,对流层高层太平洋地区辐散,海洋性大陆地区辐合。对应于辐合辐散中心,存在着自赤道中太平洋分别向赤道东太平洋和海洋性大陆中东部地区的异常垂直环流圈,同时也存在自海洋性大陆西部向印度洋西部的垂直环流。大气在海洋性大陆区域北部加热,南部冷却;在太平洋地区西部加热而东部冷却;在海洋性大陆区域10°N以南降水偏少,而10°N以北降水偏多。当I_EPN处于正位相时,热带太平洋海表温度异常呈现“西负东正”分布型,海洋性大陆区域海表温度异常呈现“西正东负”分布,对流层低层海洋性大陆地区辐散中心范围偏大、位置偏东、强度偏强,太平洋地区辐合中心范围偏小、位置偏东,热带环流异常在垂直方向上呈斜压结构,海洋性大陆区域北部大气加热而南部冷却,太平洋地区大气均呈加热正异常,海洋性大陆大部分区域降水均偏少,赤道太平洋降水偏多。以上这些结果有利于深刻理解热带太平洋海表温度异常的特征及其对海洋性大陆区域气候的影响。      

Extreme monsoons over East Asia: Possible role of Indian Ocean Zonal Mode

Summary The influence of the Indian Ocean Zonal Mode on the extreme summer monsoon rainfall over East Asia (China, Korea, Japan) has been investigated applying simple statistical techniques of correlation and composite analysis. While the observed rainfall data are used as a measure of rainfall activity, the NCEP-NCAR Reanalysis data are used to examine the circulation features associated with the extreme monsoon phases and the dynamics of the zonal mode – monsoon variability connections. The data used covers the period 1960 to 2000.The equatorial Indian Ocean is dominated by westerly winds blowing towards Indonesia. However, during the positive phase of the zonal mode, an anomalous, intensified easterly flow prevails, consistent with the positive (negative) sea surface temperature anomalies over the western (southeastern) equatorial Indian Ocean. This positive phase of the zonal mode enhances summer monsoon activity over China, but suppresses the monsoon activity over the Korea-Japan sector, 3 to 4 seasons later. The relationship is more consistent and stronger over the Korea-Japan region than over China.The Indian Ocean influences the monsoon variability over East Asia via the northern hemisphere mid-latitudes or via the eastern Indian Ocean/west Pacific route. The monsoon-desert mechanism induces strong subsidence northwest of India due to the anomalous convection over the Indian Ocean region associated with the positive phase of the zonal mode. This induces a zonal wave pattern over the mid-latitudes of Asia propagating eastwards and displacing the north Pacific subtropical high over East Asia. The warming over the eastern Indian Ocean/west Pacific inhibits the westward extension of the north Pacific sub-tropical high. The location and shape of this high plays a dominant role in the monsoon variability over East Asia. The memory for delayed impact, three to four seasons later, could be carried by the surface boundary conditions of Eurasian snow cover via the northern channel or the equatorial SSTs near the Indonesian Through Flow via the southern channel.