VARIABILITY OF INTER-ANNUAL RELATIONSHIP BETWEEN INDIAN OCEAN DIPOLE AND HUAXI REGION’S AUTUMN PRECIPITATION

The moving-window correlation analysis was applied to investigate the relationship between autumn Indian Ocean Dipole (IOD) events and the synchronous autumn precipitation in Huaxi region, based on the daily precipitation, sea surface temperature (SST) and atmospheric circulation data from 1960 to 2012. The correlation curves of IOD and the early modulation of Huaxi region’s autumn precipitation indicated a mutational site appeared in the 1970s. During 1960 to 1979, when the IOD was in positive phase in autumn, the circulations changed from a “W” shape to an ”M” shape at 500 hPa in Asia middle-high latitude region. Cold flux got into the Sichuan province with Northwest flow, the positive anomaly of the water vapor flux transported from Western Pacific to Huaxi region strengthened, caused precipitation increase in east Huaxi region. During 1980 to 1999, when the IOD in autumn was positive phase, the atmospheric circulation presented a “W” shape at 500 hPa, the positive anomaly of the water vapor flux transported from Bay of Bengal to Huaxi region strengthened, caused precipitation ascend in west Huaxi region. In summary, the Indian Ocean changed from cold phase to warm phase since the 1970s, caused the instability of the inter-annual relationship between the IOD and the autumn rainfall in Huaxi region.     

华南秋季干旱的年代际转折及其与热带印度洋热含量的关系

采用1961—2014年逐月全球标准化降水蒸散指数(Standardized Precipitation Evapotranspiration Index,SPEI)数据集、ORA-S4海温资料及NCEP/NCAR再分析资料,对华南地区秋季干旱的年代际转折及其与热带印度洋热含量的关系进行了研究。结果表明:华南秋季SPEI主要表现为全区一致变化型,且具有明显的年代际变化特征,在1988年发生了年代际转折,转折后(前)为偏旱(涝)期。进一步分析表明,华南秋季SPEI与同期热带西印度洋海洋热含量变化呈显著的正相关关系,即当秋季热带西印度洋热含量偏低时,华南地区SPEI偏小,易发生干旱。热带西印度洋热含量异常影响华南秋季干旱的可能机制为:秋季热带印度洋热含量变化表现为""型的东西向偶极子分布,即当热带西印度洋热含量偏低时,热带东印度洋热含量将会偏高;而热带东印度洋热含量偏高将会使热带东印度洋—西太平洋海表温度偏高、外逸长波辐射偏小、降水增多,凝结潜热释放增强,产生偏强的东亚Hadley环流,使华南地区存在异常下沉运动,不利于产生降水;热带东印度洋—西太平洋海表温度偏高,还会使西北太平洋副热带高压位置偏西、面积偏大,西北太平洋存在气旋性环流异常,使华南地区受偏北气流异常控制,从而削弱了向华南地区的水汽输送。热带东印度洋—西太平洋海表温度年代际变化是热带西印度洋热含量异常影响华南秋旱年代际变化的重要环节,因此用NCAR CAM5.1全球大气环流模式进行了热带东印度洋—西太平洋海表温度年代际变化的敏感性试验,证实该区海表温度年代际升高对华南秋季年代际干旱具有重要作用。     

Decadal and interannual variability of the Indian Ocean Dipole

This study investigates the decadal and interannual variability of the Indian Ocean Dipole （IOD）. It is found that the long-term IOD index displays a decadal phase variation. Prior to 1920 negative phase dominates but after 1960 positive phase prevails. Under the warming background of the tropical ocean, a larger warming trend in the western Indian Ocean is responsible for the decadal phase variation of the IOD mode. Due to reduced latent heat loss from the local ocean, the western Indian Ocean warming may be caused by the weakened Indian Ocean westerly summer monsoon. The interannual air-sea coupled IOD mode varies on the background of its decadal variability. During the earlier period （1948-1969）, IOD events are characterized by opposing SST anomaly （SSTA） in the western and eastern Indian Ocean, with a single vertical circulation above the equatorial Indian Ocean. But in the later period （1980-2003）, with positive IOD dominating, most IOD events have a zonal gradient perturbation on a uniform positive SSTA. However, there are three exceptionally strong positive IOD events （1982, 1994, and 1997）, with opposite SSTA in the western and eastern Indian Ocean, accompanied by an El Nifio event. Consequently, two anomalous reversed Walker cells are located separately over the Indian Ocean and western-eastern Pacific; the one over the Indian Ocean is much stronger than that during other positive IOD events.     

1994年印度洋偶极子对两广地区后汛期降水异常的影响机理

利用1979—2017年中国地面气象台站2 400站中广东省和广西壮族自治区（简称两广地区）174站的逐日降水资料、英国Hadley中心的逐月海表温度资料、NCEP/NCAR逐月再分析资料,分析了1994年华南后汛期（7—9月）期间印度洋偶极子（Indian Ocean Dipole,IOD）对两广降水极端正异常的影响机理。结果表明,1994年的正位相IOD事件引起了两广地区后汛期偏多,影响机制主要为两方面：一方面,IOD东极子区域海温负异常时,水汽沿着印度北部-孟加拉湾北部-中南半岛-两广地区和孟加拉湾中部-中南半岛-两广地区的异常水汽输送通道向两广地区输送,使得两广地区水汽异常辐合;另一方面,IOD东极子区域海温异常偏低时,该地区对流层低层异常辐散、高层异常辐合、存在异常下沉运动,两广地区对流层低层异常辐合、高层异常辐散、受异常上升运动控制。上述物理机制共同作用,导致1994年后汛期两广地区降水极端正异常。     

热带印度洋偶极子与中国夏季年际气候异常关系的年代际变化

利用中国站点观测逐月降水和月平均气温资料以及NCEP/NCAR再分析资料,揭示了热带印度洋偶极子(IOD)与中国夏季气候异常关系的年代际变化.结果表明:IOD与中国夏季年际气候异常的关系既有稳定的一面,又存在着年代际变化.较为稳定的关系表现为:IOD与同年夏季长江黄河之间的降水变化存在显著负相关,与四川气温变化存在显著正相关;IOD与次年夏季四川降水存在显著正相关.伴随发生在20世纪70年代末的大尺度环流年代际转型,IOD与中国气候年际异常的联系亦发生变化:IOD正位相年的同年夏季降水异常型,由中国大部分地区偏少变为长江以南(北)偏多(少),气温由西南地区东部偏暖变为长江以南(北)偏冷(暖);次年夏季降水由全国大部分地区偏多变为长江以南(北)偏少(多),气温由全国大部分地区相关不显著变为黄河以南大部分地区显著偏暖.在IOD负位相年,中国夏季气候异常的特征与IOD正位相年相反.在20世纪70年代末的大尺度年代际气候转犁前后,与IOD相关的东亚大气环流异常特征明显不同.在IOD发展阶段,在70年代末以前,印度夏季风和南海季风偏强,副热带高压势力偏弱,导致中国华南大部分地区降水偏少,华北西部以及内蒙古中部等地降水偏多;70年代末以后,东亚大陆中纬度为弱的东风距平,导致新疆北部降水偏少,气温偏高,华南降水偏多.在IOD次年夏季,70年代末以前,华南、河套以及四川等地盛行偏南气流,降水偏多;70年代末以后,南亚高压和西太平洋副高偏西偏强,华南、江南降水偏少.     

塔里木河流域夏季降水年际和年代变化的环流异常

基于1961-2020年夏季（6-8月）NCAR/NCEP再分析资料和塔里木河流域（简称“塔河”）38个气象站降水资料,使用5年滑动平均分离出塔河夏季降水年代际和年际变化,选出了典型的年际变化的干年和湿年与年代际变化的干期（1963-1986年）和湿期（1989-2018年）,分析了与这两种时间尺度变化相关联的大气环流异常。结果表明,两者的变化不同,影响两者变化的大气环流不同。影响年代际变化的大气环流异常主要表现在欧亚大陆对流层从西北至东南交替出现正负位势高度异常中心,在湿期塔河东部和西部分别出现明显的东风和西南风异常,水汽主要由西北太平洋和印度洋输送至塔河,在干期则与之相反;影响年际变化的大气环流异常主要表现为在塔河东部和西部对流层分别为反气旋异常和气旋异常,在多雨年从南北方向向塔河输送的水汽增多,北风水汽通量异常向流域输送较多,南风水汽通量异常向流域输送较小且主要集中在流域西部,水汽主要由北冰洋洋和印度洋输送至塔河。在干年则与之相反。     

南印度洋大气垂直环流与江西6月降水的关系

使用江西省82站1959—2016年6月降水资料和NCEP/NCAR逐月再分析资料,研究了南印度洋大气垂直环流与江西6月降水的关系,并运用大尺度局地涡度倾向变化方程诊断了年际、年代际变化引起的局地涡度倾向异常对江西6月降水的贡献,解释了南印度洋大气垂直环流与江西6月降水年际关系发生年代际改变的原因。结果表明南印度洋大气垂直环流与江西6月降水有密切的关系,且两者的年际关系存在年代际变化:（1）二者关系在1960年代末和1990年代初发生了两次转变,1969年前为显著正相关,1969—1989年相关性不明显,1990年后又转变为显著正相关。（2）江西6月降水偏多年,500 hPa上东亚地区从中高纬到低纬为“+ - +”距平符号分布,江西区域异常正涡度,低层南北风距平在江西上空交汇;降水偏少年环流异常则相反。（3）南印度洋大气垂直环流可引起东亚环流异常,使江西区域涡度正异常;但其影响与背景场的变化有关。动力诊断表明,1969—1989年南印度洋大气垂直环流年际异常对江西局地涡度为正贡献,但年代际异常为负贡献,削弱了年际异常的作用;1990—2016年阶段年际异常为正贡献,同时年代际异常也为正贡献,加强了年际异常的作用,使得其与江西6月降水的正相关显著。      

有无El Ni?o情况下印度洋偶极子演变特征及机理研究

基于NCEP、SODA等再分析资料,采用合成分析和2.5层简化海洋模型数值模拟等方法,分析了El Ni?o和正印度洋偶极子(IOD)事件不同配置情形下印度洋海温异常的演变特征,并重点探讨了联合IOD和独立IOD事件中,关键海区海温异常的发展演变及其可能机制。对于联合IOD事件,初期马里沿岸的增暖可能对其发生起主要的激发作用;而对于独立IOD事件的发生,则可能是赤道东南印度洋的降温起主导作用。不同类型IOD事件中,热带印度洋海表温度异常(SSTA)和海面高度异常(SSHA)的演变特征有明显差别,孟加拉湾上空降水异常所起的作用也不一样,印度洋不同海区混合层温度异常的演变机制也有显著不同。基于2.5层简化海洋模式结果的分析表明,各个海区的热力、动力过程在不同IOD事件有着不同的作用。例如在索马里沿岸海区:对于联合IOD事件,西印度洋赤道东风异常和索马里沿岸东北风异常,有利于该海区出现纬向平流热输送和海表热通量正异常,从而增暖。而对于独立IOD事件,阿拉伯海上空的强西南风异常,加强了索马里沿岸底层冷水的上翻和海表的热通量损失,导致前期纬向平流和夹卷混合的负异常以及后期海表热通量的负异常,使得该海区变冷。      

Possible Linkage Between Tropical Indian Ocean SST Anomalies and the Date ofFirst and Last Tropical Cyclones Landfalling in the Chinese Mainland

Bases on the NCEP / NCAR reanalysis products, HadISST dataset, and data of tropical cyclone (TC)landfalling in the Chinese mainland during 1960-2019, the possible impacts of Indian Ocean Dipole (IOD) mode andIndian Ocean basin (IOB) mode on the last-TC-landfall date (LLD) and first-TC-landfall date (FLD), respectively, areinvestigated in this study. The LLD is in significantly negative correlation with autumn IOD on the interannual time-scale and their association is independent of El Ni?o-Southern Oscillation (ENSO). The LLD tends to be earlier when theIOD is positive while becomes later when the IOD is negative. An anomalous lower-level anticyclone is located aroundthe Philippines during October-November, resulting from the change of Walker circulation over the tropical Indo-westPacific Ocean forced by sea surface temperature (SST) anomalies related to a positive IOD event. The Philippinesanticyclone anomaly suppresses TCs formation there and prevents TCs from landfalling in the Chinese mainland due tothe anomalous westerly steering flows over southeast China during October-November, agreeing well with the earlierLLD. However, the robust connection between spring IOB and FLD depends on ENSO episodes in preceding winter.There is an anticyclonic anomaly around the Philippines caused by the tropical SST anomalies through modulating theWalker circulation during May-June when the IOB is warming in the El Ni?o decaying phase. Correspondingly, the TCsgenesis is less frequent near the Philippines and the mid-level steering flows associated with the expanded westernPacific subtropical high are disadvantageous for TCs moving towards southeast China and making landfall during May-June, in accordance with the later FLD. By contrast, cooling IOB condition in spring of a La Ni?a decaying year andnegative IOD cases during autumn could produce a completely reversed atmospheric circulation response, leading to anearlier FLD and a later LLD over the Chinese mainland, respectively.     

秋季热带印度洋偶极子年际振荡对长江上游径流量多寡的影响分析

利用年际增量、小波分析和回归分析等方法深入分析了秋季热带印度洋偶极子(Tropical Indian Ocean Dipole,TIOD)年际振荡对次年长江上游年径流量的影响特征及其物理机理。结果表明,长江上游年径流量和秋季TIOD均具有显著的年际振荡特征,在20世纪80年代以前和90年代之后尤为明显。两者的滞后相关系数为0.42,通过了99.9%的显著性检验,即秋季TIOD较前一年增强(减弱),有利于次年长江上游径流量较前一年偏多(少)。秋季TIOD对次年长江上游年径流量多寡的影响,是通过调制降水,尤其是夏季降水来实现的。当秋季TIOD增强时,赤道印度洋海温呈东西"-+"分布,其中偏暖区延伸至南北纬20°,偏冷区与西太平洋的偏冷区相通。赤道印度洋至西太平洋上空激发出增强的Walker型环流,中心位于印度洋正上方。随着时间的发展,暖性Kelvin波产生并向东传播,印度洋偏暖区以及冷暖海温差异中心东移。至次年夏季,西印度洋暖海温中心移动至东印度洋边缘至南海区域,偏冷海区东退至日界线附近。印度洋上空增强的Walker型环流消失,高层转为偏东气流与105°E附近加强爬升的气流相连。与此同时,105°E以东的Walker环流加强,高层为西风,400 hPa以下为深厚的东风区。高低空环流相互耦合并配合科氏力的影响,赤道以北副热带地区负涡度增强,西太平洋副热带高压偏大偏强,异常反气旋北扩,系统外围的西南气流加强南海和孟湾水汽的输送,使得次年夏季长江上游全流域处于水汽辐合上升区,降水显著偏多,从而影响了长江上游年径流量的多寡。     

A model study on oceanic processes during the Indian Ocean Dipole termination

In this study, the processes affecting the temperature variability over the Southeastern Tropical Indian Ocean (STIO) during 1958–2000, accomplishing the positive and negative Indian Ocean Dipole (IOD) events are analyzed. The upper ocean heat budget analysis of the STIO has been carried out to understand the oceanic process during the termination of the recent strongest IOD events. The three recent strongest positive IOD events (1961, 1994 and 1997) and a strong negative IOD event (1996) are considered for detailed analysis. The heat budget analysis revealed that the positive net-surface heat flux and vertical advection played dominant roles in the termination of 1997 IOD event, whereas horizontal and vertical advections are responsible for the termination of IOD events during 1961 and 1994. The anomalous negative surface heat flux and horizontal advection caused the dipole termination during the negative dipole year 1996. The findings are well supported by the analysis of anomaly correlation between model upper ocean heat content tendency and heat budget components. Significant intra-seasonal oscillations (ISOs) in sea surface temperature (SST) anomaly are seen during the initial phase of termination in the eastern equatorial Indian Ocean during 1961 and 1994 IOD events. The influence of ISOs in SST is not so evident during the IOD termination in 1997. It is found that the termination processes have started more than a month prior to the actual IOD termination.     

Interdecadal Change of the Relationship Between the Tropical Indian Ocean Dipole Mode and the Summer Climate Anomaly in China

The interdecadal change of the relationship between the tropical Indian Ocean dipole(IOD) mode and the summer climate anomaly in China is investigated by using monthly precipitation and temperature records at 210 stations in China and the NCEP/NCAR reanalysis data for 1957-2005.The results indicate that along with the interdecadal shift in the large-scale general circulation around the late 1970s,the relationship between the IOD mode and the summer climate anomaly in some regions of China has significantly changed.Before the late 1970s,a developing IOD event is associated with an enhanced East Asian summer monsoon,which tends to decrease summer precipitation and increase summer temperature in South China;while after the late 1970s,it is associated with a weakened East Asian summer monsoon,which tends to increase(decrease) precipitation and decrease(increase) temperature in the south(north) of the Yangtze River.During the next summer,following a positive IOD event,precipitation is increased in most of China before the late 1970s,while it is decreased(increased) south(north) of the Yangtze River after the late 1970s.There is no significant correlation between the IOD and surface air temperature anomaly in most of China in the next summer before the late 1970s;however,the IOD tends to increase the next summer temperature south of the Yellow River after the late 1970s.     

印度洋海温异常对西太平洋副热带高压的影响——大气中的两级热力适应

利用热力适应理论讨论了印度洋海温异常影响中国天气和西太平洋副热带高压异常的物理机制。结果表明 :通过第一级热力适应 ,印度洋上的海温异常形成低层气旋式环流 ,并在其东侧的偏南气流中产生对流性降水 ;然后通过第二级热力适应 ,在 50 0 h Pa上导致西太平洋副热带高压加强西伸、2 0 0 h Pa上南亚高压异常增强。从而证明两级热力适应是导致印度洋海温异常影响中国天气气候异常的重要物理机制。     

IOD influence on the early winter tibetan plateau snow cover: diagnostic analyses and an AGCM simulation

Using diagnostic analyses and an AGCM simulation, the detailed mechanism of Indian Ocean Dipole (IOD) influence on the early winter Tibetan Plateau snow cover (EWTPSC) is clarified. In early winter of pure positive IOD years with no co-occurrence of El Ni?o, the anomalous dipole diabatic heating over the tropical Indian Ocean excites the baroclinic response in the tropics. Since both baroclinic and barotropic components of the basic zonal wind over the Arabian Peninsula increase dramatically in early winter due to the equatorward retreat of the westerly jet, the baroclinic mode excites the barotropic Rossby wave that propagates northeastward and induces a barotropic cyclonic anomaly north of India. This enables the moisture transport cyclonically from the northern Indian Ocean toward the Tibetan Plateau. The convergence of moisture over the plateau explains the positive influence of IOD on the EWTPSC. In contrast, the basic zonal wind over the Arabian Peninsula is weak in autumn. This is not favorable for excitation of the barotropic Rossby wave and teleconnection, even though the IOD-related diabatic heating anomaly in autumn similar to that in early winter exists. This result explains the insignificant (significant positive) partial correlation between IOD and the autumn (early winter) Tibetan Plateau snow cover after excluding the influence of ENSO. The sensitivity experiment forced by the IOD-related SST anomaly within the tropical Indian Ocean well reproduces the baroclinic response in the tropics, the teleconnection from the Arabian Peninsula, and the increased moisture supply to the Tibetan Plateau. Also, the seasonality of the atmospheric response to the IOD is simulated.     

EFFECTS OF INDIAN OCEAN SSTA WITH ENSO ON WINTER RAINFALL IN CHINA

Based on Hadley Center monthly global SST, 1960-2009 NCEP/NCAR reanalysis data and observation rainfall data over 160 stations across China, the combined effect of Indian Ocean Dipole （IOD） and Pacific SSTA （ENSO） on winter rainfall in China and their different roles are investigated in the work. The study focuses on the differences among the winter precipitation pattern during the years with Indian Ocean Dipole （IOD） only, ENSO only, and IOD and ENSO concurrence. It is shown that although the occurrences of the sea surface temperature anomalies of IOD and ENSO are of a high degree of synergy, their impacts on the winter precipitation are not the same. In the year with positive phase of IOD, the winter rainfall will be more than normal in Southwest China （except western Yunnan）, North China and Northeast China while it will be less in Yangtze River and Huaihe River Basins. The result is contrary during the year with negative phase of IOD. However, the impact of IOD positive phase on winter precipitation is more significant than that of the negative phase. When the IOD appears along with ENSO, the ENSO signal will enhance the influence of IOD on winter precipitation of Southwest China （except western Yunnan）, Inner Mongolia and Northeast China. In addition, this paper makes a preliminary analysis of the circulation causes of the relationship between IOD and the winter rainfall in China.     

Interannual Variability of Autumn Precipitation over South China and its Relation to Atmospheric Circulation and SST Anomalies

The interannual variability of autumn precipitation over South China and its relationship with atmospheric circulation and SST anomalies are examined using the autumn precipitation data of 160 stations in China and the NCEP-NCAR reanalysis dataset from 1951 to 2004. Results indicate a strong interannual variability of autumn precipitation over South China and its positive correlation with the autumn western Pacific subtropical high （WPSH）. In the flood years, the WPSH ridge line lies over the south of South China and the strengthened ridge over North Asia triggers cold air to move southward. Furthermore, there exists a significantly anomalous updraft and cyclone with the northward stream strengthened at 850 hPa and a positive anomaly center of meridional moisture transport strengthening the northward warm and humid water transport over South China. These display the reverse feature in drought years. The autumn precipitation interannual variability over South China correlates positively with SST in the western Pacific and North Pacific, whereas a negative correlation occurs in the South Indian Ocean in July. The time of the strongest lag-correlation coefficients between SST and autumn precipitation over South China is about two months, implying that the SST of the three ocean areas in July might be one of the predictors for autumn precipitation interannual variability over South China. Discussion about the linkage among July SSTs in the western Pacific, the autumn WPSH and autumn precipitation over South China suggests that SST anomalies might contribute to autumn precipitation through its close relation to the autumn WPSH.     

西南地区秋季干旱的年代际转折及其可能原因分析

采用1961~2012年中国气象局753站降水和温度资料、NCEP/NCAR全球大气再分析资料、NOAA海表温度资料等,应用观测统计分析和全球大气环流模式NCAR CAM5.1数值模拟,基于标准化降水蒸散指数(SPEI),对我国西南秋季干旱的年代际转折及其可能原因进行了分析。观测分析结果表明:(1)西南秋季干旱的主要分布型为全区一致型;西南秋季SPEI在1994年发生年代际突变,突变后(前)为偏旱(涝)期。(2)西南秋季偏旱期的主要环流特征是,西太平洋副热带高压位置偏西、面积偏大、强度偏强,南支槽偏弱,西南地区存在下沉运动。(3)热带东印度洋-西太平洋的海表温度年代际升高对西南秋季SPEI在1994年发生年代际突变有重要作用,该关键海区海表温度异常升高,一是会使秋季西南地区500 hPa高度场偏高,南支槽减弱;二是产生偏强的Hadley环流,使得我国西南地区存在下沉运动;三是会在西太平洋激发气旋性环流,使我国西南地区被偏北气流控制,削弱了向我国西南地区的水汽输送,容易造成该地区的秋季干旱。应用NCAR CAM5.1全球大气环流模式进行了关键海区海表温度年代际变化的敏感性试验,验证了观测分析结果,即秋季关键海区海表温度年代际升高对西南秋季年代际变旱有重要作用。     

热带印度洋偶极子模态的不对称性及其成因的数值研究

热带印度洋偶极子 (Indian Ocean Dipole) 是印度洋海域内海洋和大气环流年际变化的主要特征模态之一, 在热带海气耦合系统中起到非常重要的作用。同热带太平洋的ENSO现象类似, 热带印度洋偶极子也呈现出显著的不对称性。本文利用中国科学院大气物理研究所发展的全球海洋环流模式, 在观测风应力距平的强迫下, 评估了模式对热带印度洋季节变化、 热带印度洋偶极子 (IOD) 模态及其不对称性的模拟能力, 并且通过数值试验分析了IOD模态不对称性特征及其对气候平均态的影响。对照观测资料, 模式较好地再现了热带印度洋SST在季风驱动下的季节变化特征。在年际时间尺度上, 模式不仅能够再现IOD指数的变化趋势, 而且可以成功模拟出IOD模态的空间分布特征, 即表层和次表层海温在西印度洋表现为正异常, 在东印度洋表现为负异常。可见, 对于热带印度洋而言, IOD模态主要是对风应力异常的响应。热带印度洋海温与Niño3.4指数的相关性分析表明, 模式能够模拟出超前热带太平洋ENSO现象2～4个月时海温的偶极子型分布, 但是不能模拟出滞后ENSO现象2个月左右的全海盆增暖模态, 可能是因为模式试验中没有考虑热通量年际异常的强迫。同时, 模式模拟的IOD模态具有同观测结果相类似的不对称性, 进一步的敏感性试验表明风应力的不对称性对偶极子指数的不对称性贡献较小, 次表层及以下海温的不对称性可能主要受到海洋内部非线性动力过程的影响。通过数值试验, 本文还发现热带印度洋海温的不对称性对气候平均态会有影响, 而这种不对称性长期积累后, 会导致上层热带印度洋温度层结趋于稳定状态。     

近期气候预测系统IAP-DecPreS对印度洋偶极子的回报技巧:全场同化和异常场同化的比较

印度洋偶极子（IOD）是热带印度洋年际变率主导模态之一,对于区域乃至全球气候有重要影响。准确预报IOD对于短期气候预测具有重要意义。中国科学院大气物理研究所最近建立了近期气候预测系统IAP-DecPreS,其初始化方案采用“集合最优插值—分析增量更新”（EnOI-IAU）方案,能够同化观测的海洋次表层温度廓线资料。本文分析了IAP-DecPreS季节回报试验对IOD的回报技巧,重点比较了全场同化和异常场同化两种初始化策略下预测系统对IOD的回报技巧。分析表明,8月起报秋季IOD,无论从确定性预报还是概率性预报的角度,基于全场同化的回报试验技巧均高于异常场同化的回报试验。对于5月起报的秋季IOD,基于两种初始化策略的回报试验技巧相当。研究发现,全场同化策略相对于异常场的优势主要源于它提高了对伴随ENSO发生的IOD的预报技巧。ENSO遥强迫触发的热带东印度洋“风—蒸发—SST”正反馈过程是IOD发展和维持的关键。采用全场同化策略的回报结果能够更好地模拟出IOD发展过程中ENSO遥强迫产生的异常降水场和异常风场的空间分布特征;而采用异常场同化策略,模拟的异常降水场和风场偏差较大。导致两种初始化策略预测结果技巧差异的主要原因是,全场同化能够减小模式对热带印度洋气候平均态降水固有的模拟偏差,从而提升了热带印度洋对ENSO遥强迫响应的模拟能力。而异常场同化由于在同化过程中保持了模式固有的气候平均态,因此模拟的热带印度洋对ENSO遥强迫的响应存在与模式自由积分类似的模拟偏差。     

The interannual precipitation variability in the southern part of Iran as linked to large-scale climate modes

The interannual variation of precipitation in the southern part of Iran and its link with the large-scale climate modes are examined using monthly data from 183 meteorological stations during 1974–2005. The majority of precipitation occurs during the rainy season from October to May. The interannual variation in fall and early winter during the first part of the rainy season shows apparently a significant positive correlation with the Indian Ocean Dipole (IOD) and El Ni?o-Southern Oscillation (ENSO). However, a partial correlation analysis used to extract the respective influence of IOD and ENSO shows a significant positive correlation only with the IOD and not with ENSO. The southeasterly moisture flux anomaly over the Arabian Sea turns anti-cyclonically and transport more moisture to the southern part of Iran from the Arabian Sea, the Red Sea, and the Persian Gulf during the positive IOD. On the other hand, the moisture flux has northerly anomaly over Iran during the negative IOD, which results in reduced moisture supply from the south. During the latter part of the rainy season in late winter and spring, the interannual variation of precipitation is more strongly influenced by modes of variability over the Mediterranean Sea. The induced large-scale atmospheric circulation anomaly controls moisture supply from the Red Sea and the Persian Gulf.