海表温度异常影响东亚夏季风年代际变化的数值模拟

采用1950-2000年逐月观测的不同海域(全球、热带外、热带、热带印度洋-太平洋、热带印度洋及热带太平洋)海表温度分别驱动NCAR CAM3全球大气环流模式,进行了多组长时间积分试验,对比ERA-40和NCEP/NCAR再分析资料,讨论了这些海域海表温度异常对东亚夏季风年代际变化的影响。数值试验结果表明:全球、热带、热带印度洋-太平洋和热带太平洋海表温度变化对东亚夏季风的年代际变化具有重要作用,均模拟出了东亚夏季风在20世纪70年代中后期发生的年代际减弱现象,以及强、弱夏季风年代夏季大气环流异常分布的显著不同,这与观测结果较一致,表明热带太平洋是影响东亚夏季风此次年代际变化的关键海区;利用热带印度洋海表温度驱动模式模拟出的东亚夏季风在20世纪70年代中后期发生年代际增强现象,即当热带印度洋海表温度年代际偏暖(冷)时,东亚夏季风年代际增强(减弱),与热带太平洋海表温度变化对东亚夏季风年代际变化的影响相反;热带太平洋海表温度年代际背景的变化对东亚夏季风在20世纪70年代中后期的年代际减弱有重要作用。     

冬夏东亚季风环流对太平洋热状况的响应

冬夏隔季韵律关系一直是我国长期天气预报和短期气候预测的一个重要依据,然而迄今为止对它们之间的物理过程及成因机理并不十分清楚。利用NCEP/NCAR全球2.5°×2.5°网格月平均再分析资料,研究1951~2000年冬夏东亚季风环流异常变化与太平洋海面温度(SST)的关系及对关键海温区响应机理。研究指出:冬夏东亚季风环流隔季韵律关系及其年际变化与赤道东太平洋海面温度异常(SSTA)变化密切相关,冬季赤道东太平洋出现La Ni~na(El Ni~no)型的SST分布,有利冬、夏东亚季风环流加强(减弱),其影响过程通过赤道Walker环流强(弱)以及东亚地区Hadley环流强(弱)过程完成。冬季赤道东太平洋海温变化是冬、夏东亚环流季节以及年际变化的一个重要外强迫因子。     

关于东亚冬季风指数的一个讨论——东亚中、低纬冬季风的差异

利用NCEP再分析资料和NOAA海表温度等资料,分析了东亚冬季风在不同纬度上的表现。根据我们定义的东亚中纬度冬季风(The mid latitudinal East Asian winter monsoon, 简称EAWM-M)和低纬度冬季风(The low latitudinal East Asian winter monsoon, 简称EAWM-L)指数,探讨了它们对应的影响系统,并重点分析了它们与海温异常之间联系的异同。研究主要发现:(1)EAWM-L和EAWM-M指数所反映的东亚冬季大气环流形势不尽相同。在对流层低层,EAWM-L与中国南海、菲律宾附近环流异常的关系密切,EAWM-M与贝加尔湖阻塞高压的关系更为密切;在对流层中层,EAWM-M同样与贝加尔湖阻塞高压异常的联系相对更为紧密,而EAWM-L指数则与东亚大槽的关系更为紧密。在对流层高层,副热带西风急流强度变化通过调制次级环流进而与EAWM-L联系起来,而EAWM-M强弱变化主要与副热带西风急流北界的位置有关。(2)EAWM-L与冬季赤道中东太平洋和热带印度洋海温异常的联系都很紧密,而EAWM-M变化与冬季热带印度洋海温异常的联系更为密切,与赤道中东太平洋海温异常的关系相对偏弱。EAWM-L与冬季赤道中东太平洋和热带印度洋海温异常的紧密联系在年际和年代际尺度上都是存在的,而EAWM-M与冬季热带印度洋海温异常的紧密联系主要体现在年代际尺度上。     

东亚夏季风的年际到年代际变化及其与全球大气环流和海表温度的联系

The East Asian summer monsoon (EASM) underwent an interdecadal variation with interannual variations during the period from 1958 to 1997, its index tended to decline from a higher stage in the mid-1960＇s until it reached a lower stage after 1980＇s. Correlation analysis reveals that EASM is closely related with the global atmospheric circulation and sea surface temperature (SST). The differences between the weak and strong stage of EASM shows that, the summer monsoon circulation over East Asia and North Africa is sharply weakened, in the meantime, the westerlies in high latitudes and the trade--wind over the tropical ocean are also changed significantly. Over the most regions south of the northern subtropics, both air temperature in the lower troposphere and SST tended to rise compared with the strong stage of EASM. It is also revealed that the ocean-atmosphere interaction over the western Pacific and Indian Ocean plays a key role in interannual to interdecadal variation of EASM, most probably, the subtropical Indian Ocean is more important. On the other hand, the ENSO event is less related to EASM at least during the concerned period.     

东亚冬夏季风关系在1970s末的年代际转变

利用NCEP/NCAR和Hadley中心的大气与海洋再分析资料,选取具有代表性的东亚冬、夏季风指数,采用滑动相关和线性回归等方法,主要讨论了受ENSO影响的东亚冬季风分量和后期夏季风之间关系的年代际变化,并分析了二者关系发生变化的原因。结果表明:在1965—1979年,受ENSO影响的冬季风与后期夏季风强度的对应关系并不明显。在1980—2004年,受ENSO影响的冬季风强,对应后期的夏季风偏弱,弱冬季风对应的后期夏季风偏强。当受ENSO影响的冬季风较强时,冬季在对流层低层西北太平洋出现了异常气旋并可以维持到次年夏季,低纬地区位势高度偏低,削弱了西太平洋副热带高压,异常气旋西部的偏北气流阻碍了西南风的北进,导致夏季风偏弱。海表温度异常在1980年前后春、夏季不同的分布型可以解释环流在不同时段内的差异。     

季节预测模式对东亚夏季环流的预测能力及其对热带海洋的响应分析

东亚夏季环流变化对中国夏季降水的年际变化有重要影响,因此需要进一步理解季节预测模式对东亚夏季环流的预测能力。利用1991~2013年美国国家环境预测中心（NCEP）、中国气象局国家气候中心（NCC）和日本东京气候中心（TCC）的三个季节预测模式（CFS V2、BCC_CSM V2和MRI-CGCM）以及NCEP/NCAR再分析资料,定量评估了模式对东亚夏季风（EASM）和夏季西太平洋副热带高压（WPSH）强度的预测能力。在此基础上,分析了模式预测的EASM和WPSH对热带海温异常的响应能力,以及ENSO事件对EASM和WPSH预测的影响,阐述了预测误差产生的原因。结果表明:整体而言,三个模式对EASM和WPSH的预测技巧较高,但TCC模式对WPSH的预测技巧相对较低。三个模式预测的850 hPa风场在西北太平洋存在一个异常气旋,使得预测的EASM偏强和WPSH偏弱。同时,二者的年际变率整体比观测小。三个模式预测的EASM和WPSH对热带海洋海温异常的响应随季节演变特征与观测比较接近,但NCEP模式和TCC模式预测的EASM对前期热带太平洋和前期、同期热带印度洋的海温异常响应要强于观测,NCC模式预测的EASM对前期和同期的热带太平洋的海温异常响应明显比观测强。此外,三个模式预测的WPSH对前期和同期的热带太平洋、热带印度洋和热带大西洋的海温异常响应明显强于观测。三个模式预测的EASM和WPSH在ENSO年的平均绝对误差（MAE）整体而言要比正常年的小很多,NCEP模式和NCC模式预测的EASM和WPSH的MAE在La Ni?a年和El Ni?o年差别不大,而TCC模式预测的EASM和WPSH的MAE在El Ni?o年比在La Ni?a年大很多,表明ENSO事件是东亚夏季环流重要的可预报源。     

东亚夏季风强度的多尺度统计预测模型

东亚夏季风强度的变化与中国雨带和旱涝分布密切相关。为了做好东亚夏季风强度的短期气候预测,采用小波分析、Lanczos滤波器、交叉检验等方法,研究了东亚夏季风强度的多尺度变化特征,在年际与年代际尺度上分别寻找了它在前冬海温场、200 hPa纬向风场上的前兆信号,并利用最优子集回归建立了东亚夏季风强度的多尺度统计物理预测模型。结果表明:东亚夏季风强度存在准4年、准13年和准43年的周期振荡。年际尺度上,前冬赤道东太平洋(10°N~10°S,160°W~80°W)海温与东亚夏季风强度有最强的显著负相关,且它与东亚夏季风强度在200 hPa纬向风场上的前兆信号有较强的负相关;年代际尺度上,南半球60°S与35°S附近200 hPa纬向风之差与东亚夏季风强度有最强的显著正相关,且它与东亚夏季风强度在热带印度洋、低纬度东南太平洋、低纬度南大西洋的海温及亚洲副热带200 hPa纬向风等前兆信号有强的正相关。通过探讨这两个前兆因子对东亚夏季风强度的预测意义,揭示了他们影响东亚夏季风强度年际和年代际变化的可能物理过程。所建立的东亚夏季风强度多尺度最优子集回归预测模型,不仅对东亚夏季风强度的年际变化具有较好的预测能力,而且对异常极值年份也具有一定的预测能力。     

Interdecadal variability of the East Asian summer monsoon in an AGCM

It is well known that significant interdecadal variation of the East Asian summer monsoon （EASM） occurred around the end of the 1970s. Whether these variations can be attributed to the evolution of global sea surface temperature （SST） and sea ice concentration distribution is investigated with an atmospheric general circulation model （AGCM）. The model is forced with observed monthly global SST and sea ice evolution through 1958-1999. A total of four integrations starting from different initial conditions are carried out. It is found that only one of these reproduces the observed interdecadal changes of the EASM after the 1970s, including weakened low-level meridional wind, decreased surface air temperature and increased sea level pressure in central China, as well as the southwestward shift of the western Pacific subtropical high ridge and the strengthened 200-hPa westerlies. This discrepancy among these simulated results suggests that the interdecadal variation of the EASM cannot be accounted for by historical global SST and sea ice evolution. Thus, the possibility that the interdecadal timescale change of monsoon is a natural variability of the coupled climate system evolution cannot be excluded.     

NUMERICAL SIMULATION OF SSTA IMPACTS UPON THE INTERDECADAL VARIATION OF THE CROSS-EQUATORIAL FLOWS IN EASTERN HEMISPHERE

Impacts of regional sea surface temperature(SST)anomalies on the interdecadal variation of the cross-equatorial flows(CEFs)in Eastern Hemisphere are studied using numerical simulations with a global atmospheric circulation model(NCAR CAM3)driven with 1950-2000 monthly SSTs in different marine areas(the globe,extratropics,tropics,tropical Indian Ocean-Pacific,and tropical Pacific)and ERA-40reanalysis data.Results show that all simulations,except the one driven with extratropical SSTs,can simulate the interdecadal strengthening of CEFs around Somali,120oE,and 150oE that occurred in the midand late-1970s.Among those simulated CEFs,the interdecadal variability in Somali and its interdecadal relationship with the East Asian summer monsoon are in better agreement with the observations,suggesting that changes in the SSTs of tropical oceans,especially the tropical Pacific,play a crucial role in the interdecadal variability of CEFs in Somali.The interdecadal change of CEFs in Somali is highly associated with the interdecadal variation of tropical Pacific SST.As the interdecadal warmer(colder)SST happens in the tropical Pacific,a"sandwich"pattern of SST anomalies,i.e."+,-,+"("-,+,-"),will occur in the eastern tropical Pacific from north to south with a pair of anomalous anticyclone(cyclone)at the lower troposphere;the pair links to another pair of anomalous cyclone(anticyclone)in the tropical Indian Ocean through an atmospheric bridge,and thus strengthens(weakens)the CEFs in Somali.     

Interdecadal Enhancement of the Walker Circulation over the Tropical Pacific in the Late 1990s

The Walker circulation is one of the major components of the large-scale tropical atmospheric circulation and variations in its strength are critical to equatorial Pacific Ocean circulation.It has been argued in the literature that during the 20th century the Walker circulation weakened,and that this weakening was attributable to anthropogenic climate change.By using updated observations,we show that there has been a rapid interdecadal enhancement of the Walker circulation since the late 1990s.Associated with this enhancement is enhanced precipitation in the tropical western Pacific,anomalous westerlies in the upper troposphere,descent in the central and eastern tropical Pacific,and anomalous surface easterlies in the western and central tropical Pacific.The characteristics of associated oceanic changes are a strengthened thermocline slope and an enhanced zonal SST gradient across the tropical Pacific.Many characteristics of these changes are similar to those associated with the mid-1970s climate shift with an opposite sign.We also show that the interdecadal variability of the Walker circulation in the tropical Pacific is inversely correlated to the interdecadal variability of the zonal circulation in the tropical Atlantic.An enhancement of the Walker circulation in the tropical Pacific is associated with a weakening zonal circulation in the tropical Atlantic and vise versa,implying an inter-Atlantic-Pacific connection of the zonal overturning circulation variation.Whether these recent changes will be sustained is not yet clear,but our research highlights the importance of understanding the interdecadal variability,as well as the long-term trends,that influence tropical circulation.     

东亚夏季风强弱年大气环流和热源异常对比分析

根据黄刚等定义的东亚夏季风指数, 对强、弱东亚夏季风年大气环流、大气热源和外强迫源SST的差异进行分析, 结果表明:强 (弱) 东亚夏季风年前期冬季到夏季, 太平洋SSTA为La Ni?a (El Ni?o) 型分布, 西太平洋暖池SST暖 (冷), 使得暖池附近对流活动较强 (较弱)。与此同时, 南亚大陆从印度半岛、青藏高原南部、中南半岛至华南大气异常加热 (变冷), 并且海陆热力对比加强 (减弱), 有利于出现强 (弱) 的东亚夏季风。此外, 由于暖池附近对流活动强 (弱), 该地区上升气流较强 (弱), Walker环流增强 (减弱), 当强 (弱) 的东亚夏季风向北推进时, 副热带西风急流北撤位置偏北 (南), 副热带高压位置也偏北 (南), 7月至8月华北 (江淮流域) 位于副热带西风急流南侧, 降水偏多, 江淮流域 (华北) 降水偏少。并给出与东亚夏季风年际变异有关的大气环流和SST异常的物理图像。     

Atmospheric Circulation Cells Associated with Anomalous East Asian Winter Monsoon

Atmospheric circulation cells associated with anomalous East Asian Winter Monsoon (EAWM) were studied using the 1948/49 to 2002/03 NCEP/NCAR reanalysis and NCAR CAM3 AGCM simulations with monthly global sea surface temperatures from 1950 to 2000. Several atmospheric cells in the Pacific [i.e., the zonal Walker cell (ZWC) in the tropic, the Hadley cell in the western Pacific (WPHC), the midlatitude zonal cell (MZC) over the central North Pacific, and the Hadley cell in the eastern Pacific (EPHC)] are associated with anomalous EAWM. When the EAWM is strong, ZWC, WPHC, and MZC are enhanced, as opposed to EPHC. The anomalous enhanced ZWC is characterized by air parcels rising in the western tropical Pacific, flowing eastward in the upper troposphere, and descending in the tropical central Pacific before returning to the tropical western Pacific. The enhanced MZC has characteristics opposite those of the enhanced ZWC in the central North Pacific. The anomalous WPHC shows air parcels rising in the western Pacific, as in the case of ZWC, followed by flowing northward in the upper troposphere and descending in the west North Pacific, as in the case of the enhanced MZC before returning to the western tropical Pacific. The anomalous EPHC is opposite in properties to the anomalous WPHC. Opposite characteristics are found during the weak EAWM period. The model simulations and the observations show similar characteristics and indicate the important role of sea surface temperature. A possible mechanism is proposed to link interannual variation of EAWM with the central-eastern tropical Pacific sea surface temperature anomaly (SSTA).     

THE INTERDECADAL VARIATION OF THE SOUTH ASIAN HIGH AND ITS ASSOCIATION WITH THE SEA SURFACE TEMPERATURE OF TROPICAL AND SUBTROPICAL REGIONS

This study aims to explore the interdecadal variation of South Asian High (SAH) and its relationship with SST (Sea surface temperature) of the tropical and subtropical regions by using the NCEP/NCAR monthly reanalysis data from 1948 to 2012, based on the NCAR CAM 3.0 general circulation model. The results show that: 1) the intensity of SAH represents a remarkable interdecadal variation characteristic, the intensity of SAH experienced from weak to strong at the late 1970s, and after the late 1970s , its strength is enhanced and the area is expanded in the east-west direction. The expansion degree is greater westward than eastward, while it is opposite in summer. 2) Corresponding to the interdecadal variation of SAH intensity, after the late 1970s, the divergent component of wind field has two ascending and three descending areas. Of the two ascending areas, one is located in the East Pacific, the other location varies with the season from the Indian Ocean in winter to the South China Sea and West Pacific in summer. Three descending areas are located in the north-central Africa, the East Asia and the Middle Pacific region respectively. 3) Corresponding to the interdecadal variation of SAH intensity, the rotational component of wind field at the lower level is an anomalous cyclone over the South China Sea and West Pacific in summer, while in winter, it is an anomalous cyclone over the Indian Ocean, and an anomalous anticyclone over the equatorial Middle Pacific. 4) Numerical simulations show that the interdecadal variation of SAH is closely related to the SST of the tropical and subtropical regions. The SST of Indian Ocean plays an important role in winter, while in summer, the SST of the South China Sea and West Pacific plays an important role, and the SST of the East Pacific also plays a certain role.     

东亚-北太平洋偶极型气压场及其与东亚季风年际变化的关系

利用美国NCEP/NCAR的月平均再分析资料,研究东亚-太平洋地区地面气压的耦合模态与东亚副热带季风异常的关系,结果表明:在亚洲大陆和北半球太平洋之间气压场的偶极子模态主要反映了东亚地区东西向气压梯度的异常.从20世纪60年代到70年代中期,东亚-太平洋的这种偶极子表现为蒙古地区气压偏低和太平洋地区气压偏高的特征,而从20世纪70年代后期到90年代,则表现为蒙古地区气压偏高和太平洋地区气压偏低的特征.在偶极子指数值较高的年份,冬季(或夏季)蒙古高压(或蒙古低压)和太平洋阿留申低压(或太平洋副热带高压)较强     

过去1500年典型暖期东亚夏季风年代际变化特征对比及其可能成因

利用通用地球系统模式开展的过去1500年气候模拟全强迫试验和对照试验结果，在验证模式模拟性能的基础上，采用多变量经验正交函数分解等方法，对比分析了典型暖期东亚夏季风年代际变化特征及其成因机制。结果表明，两个典型暖期东亚夏季风变化的主周期均为准10 a和准20 a。中世纪暖期黄河流域至日本南部一带降水偏多，长江流域以南和西北太平洋一带降水偏少；现代暖期东亚夏季风降水表现为“南涝北旱”型分布特征。内部变率是影响典型暖期东亚夏季风变化的主控因子之一，其中太平洋年代际振荡起决定性作用。当太平洋年代际振荡处于正位相时，热带西太平洋（东亚大陆）变暖（变冷），东亚地区海、陆热力差减小，对应弱的东亚夏季风。另外，中世纪暖期海平面气压的动态变化对应850 hPa风场在西北太平洋（日本海）一带均出现了经向排列的异常反气旋（气旋），从而导致中国南部（北部）降水偏多（偏少）。      

中国东部地区夏季降水和环流的年代际转型及其与PDO的联系

利用1951—2013年全国160个测站逐月降水资料、NCEP/NCAR月平均再分析资料和NOAA全球月平均海表温度等资料,分析了中国东部地区夏季降水的年代际转型及相关大气环流变化。研究结果表明,1970s中后期和1990s PDO两次位相转换给中国东部地区夏季降水带来显著的年代际变化,前者使得东亚夏季风进一步减弱,夏季雨带南退至长江中下游地区,后者使得东亚夏季风恢复增强,雨带北移至淮河流域。进一步研究发现,1990s PDO年代际突变导致东亚夏季大气环流场发生显著变化,贝加尔湖地区增暖导致向北的经向温度梯度增大以及副热带高压的东退北抬是导致1990s东部地区夏季降水年代际变化的可能原因。     

东亚夏季风次季节变化研究进展

东亚夏季风次季节（10~90 d）变化是中国夏季持续性强降水、高温热浪等高影响天气事件的重要环流载体,处于天气预报上限和气候季节预测下限之间的预报过渡区。研究表明:东亚夏季风次季节变化是东亚夏季风的固有物理特征,它和季节进程之间的时间锁相关系是东亚夏季风次季节变化潜在可预报性的重要来源。东亚夏季风次季节变化与Madden-Julian振荡（MJO）存在显著差异,试图通过MJO来预测东亚夏季风次季节变化的不确定性较大。东亚夏季风次季节预测的另一重要来源是下垫面外强迫,包括欧亚大陆春季积雪、中国东部春季土壤湿度和厄尔尼诺-南方涛动（ENSO）事件。此外,去趋势偏-交叉相关分析统计方法能够分析东亚夏季风多因子和多时间尺度问题。目前,亟需解决的科学问题包括:东亚夏季风次季节模态的客观定量描述、造成东亚夏季风次季节模态年际变化的关键物理过程、不同外强迫因子对东亚夏季风次季节模态的共同影响。     

前期热带太平洋次表层海温异常与东亚夏季风的可能联系

利用1979—2012年日本气象厅次表层海温资料和NCEP/NCAR再分析资料,分析了前期冬季热带太平洋次表层海温与东亚夏季风的关系,并讨论了其可能机制。结果表明,前期冬季热带太平洋次表层海温与后期东亚夏季风强弱有显著的相关关系。冬季次表层海温呈现东正西负的类El Nio分布型时,夏季副热带高压偏强,西北太平洋地区受反气旋型环流控制,能将大量的水汽输送到长江和淮河流域,有利于水汽在该区域辐合,为夏季降水偏多创造了条件,此时东亚夏季风活动整体偏弱,反之亦然。但类El Nio分布型对东亚夏季气候变化的影响较类La Nia分布型更显著。此外,冬季热带太平洋次表层海温可能通过其自身能够持续性地影响东亚—太平洋地区的大气环流异常,次表层海温随季节变化有明显的发展和移动趋势:冬季西太平洋暖池次表层冷(暖)海温不断堆积,沿温跃层向东传播使得中东太平洋次表层海温逐渐变冷(暖),冷(暖)海温上翻加强使得海表温度异常,进一步影响到西太平洋副热带高压的位置和强度,并在东亚地区形成经向遥相关波列,通过西北太平洋地区异常反气旋(气旋)环流的作用,影响东亚地区大气环流以及气候变化。     

东亚夏季风的自然变率——NCARCam3模拟结果分析

利用季节循环的全球观测海表温度及海冰驱动NCARCam3全球大气环流模式的100a模拟结果,通过定义东亚夏季风指数,分析了模拟的大气内部变化中东亚夏季风的变化特征。结果表明：模拟的东亚夏季风自然变率主要表现为3—7a较显著的年际周期,并具有较明显的年代际变化特征。在弱夏季风年代,亚洲大陆海平面气压增强,日本附近及东亚沿海地区海平面气压降低;500hPa位势高度上,欧洲地区为负高度距平,里海附近地区为正高度距平,日本及其以东太平洋为负高度距平,易形成类似欧亚（EU）型的遥相关波列。在强夏季风年代,其环流异常分布基本与弱夏季风年代相反。模拟的东亚夏季风变化与夏季大气内部500hPa高度场上EU型遥相关波列的关系密切。     

华北雨季开始早晚与大气环流和海表温度异常的关系

本文利用国家气候中心的1961~2016年华北雨季监测资料、美国国家环境预报中心/大气研究中心（NCEP/NCAR）的大气再分析资料、NOAA海表温度资料,分析了华北雨季开始早晚的气候特征,然后利用合成分析、回归分析等方法,研究了华北雨季开始早晚与大气环流系统和关键区域海表温度的关系。结果表明,56 a来华北雨季开始最早在7月6日,最晚在8月10日,1961~2016年华北雨季开始平均日期是7月18日。华北雨季开始时间具有显著的年际变化,但雨季发生早晚的长期变化趋势不太明显。华北雨季开始早晚与西太平洋副热带高压（简称副高）、东亚副热带西风急流、东亚夏季风等环流系统的活动关系密切,当对流层高层副热带西风急流建立偏早偏强,中层西太平洋副高第二次北跳偏早,低层东亚夏季风北进提前时,华北雨季开始偏早,反之华北雨季开始偏晚。华北雨季开始早晚与春、夏季热带印度洋、赤道中东太平洋海表温度关系显著且稳定,当Ni?o3.4指数和热带印度洋全区海表温度一致模态（IOBW）为正值时,贝加尔湖大陆高压偏强,副高偏强偏南,东亚夏季风偏弱,导致华北雨季开始偏晚;当海表温度指数为负值时,则华北雨季开始偏早。