The impacts of the Indian summer rainfall on North China summer rainfall

Previous studies have indicated a connection between interannual variations of the Indian and North China summer rainfall. An atmospheric circulation wave pattern over the mid-latitude Asia plays an important role in the connection. The present study compares the influence of the above-normal and below-normal Indian summer rainfall on the North China summer rainfall variations. Composite analysis shows that the mid-latitude Asian atmospheric circulation and the North China rainfall anomalies during summer tend to be anti-symmetric in above-normal and below-normal Indian rainfall years. Analysis indicates that the Indian-North China summer rainfall relation tends to be stronger when larger Indian rainfall anomaly occurs during a higher mean rainfall period. The observed long-term change in the Indian-North China summer rainfall relationship cannot be explained by the impact of the El Niño-Southern Oscillation (ENSO). The present study evaluates the Indian-North China summer rainfall relationship in climate models. Analysis shows that the Indian-North China summer rainfall relationship differs largely among different climate models and among different simulations of a specific model. The relationship also displays obvious temporal variations in both individual and ensemble mean model simulations. This suggests an important role of the atmospheric internal variability in the change of the Indian-North China summer rainfall relationship.     

南亚和东亚热带夏季风强弱不同配置及其对中国夏季降水的影响北大核心

利用NOAA向外长波辐射(OLR)、NCEP/NCAR再分析资料和CN05.1降水资料,研究了南亚和东亚热带夏季风强度年际变化关系,及其强弱不同配置对中国夏季降水的影响。结果表明:南亚和东亚热带夏季风强度变化之间存在同相和反相两种配置,定义的强度同相和反相变化指数可以很好地表征该关系。同相变化模态可能与海温异常时的强El Nino(La Nina)影响有关,其反相变化模态受El Nino(La Nina)以及印度洋海盆一致模的影响,同时西太平洋副热带高压和伊朗高压位置东西偏移和强度变化也影响着不同配置的出现。两者不同配置时,对中国夏季降水的影响不同。当变化呈同相偏强时,夏季中国东部地区降水为“中间少南北多”的雨型。当变化呈反相,东亚热带夏季风偏强南亚夏季风偏弱时,夏季中国东部地区降水为“一致偏少”雨型。     

Spring Indian Ocean-western Pacific SST contrast and the East Asian summer rainfall anomaly

studying the relationship between SST in the tropical Indian Ocean （TIO）, tropical western Pacific （TWP）, and tropical eastern Pacific （TEP） and East Asian summer rainfall （EASR）, using data provided by NOAA/OAR/ESRL PSD and the National Climate Center of China for the period 1979-2008, an index, SSTDI, was defined to describe the SST difference between the TIO and TWP. In comparison with the winter ENSO, the spring SST contrast between the TIO and TWP was found to be more significantly associated with summer rainfall in East Asia, especially along the EASR band and in Northeast China. This spring SST contrast can persist into summer, resulting in a more significant meridional teleconnection pattern of lower-tropospheric circulation anomalies over the western North Pacific and East Asia. These circulation anomalies are dynamically consistent with the summer rainfall anomaly along the EASR band. When the SSTDI is higher （lower） than normal, the EASR over the Yangtze River valley, Korea, and central and southern Japan is heavier （less） than normal. The present results suggest that this spring SST contrast can be used as a new and better predictor of EASR anomalies.     

IMPACT OF ENSO ON VARIABILITY OF SUMMER MONSOON OVER ASIA AND SUMMER RAINFALL IN CHINA

In this paper, the variability of the Asian summer monsoon circulation during the ENSO period is diagnostically analyzed. Evidence suggests that every member of the Asian summer monsoon system change in varying degrees, even, oppositely, during El Nino and La Nina events. Then, the basis of the summer rainfall in east China analyzed using EOF, both region and extent of impact of the eastern tropical Pacific SSTA on the summer rainfall in East China are analyzed. This impact is closely related with development phase of the ENSO cycle, being most notable in the middle and lower reaches of the Yangtze and south of China.     

2013年汛期气候预测的先兆信号及其应用

本文系统回顾了2013年汛期气候预测的主要先兆信号。其信号特征是：2013年前期赤道中东太平洋呈正常略偏冷的状态、冬季北极海冰异常偏少、青藏高原积雪偏少,这些特征对后期东亚夏季风有明显影响。通过对前期先兆信号的分析,国家气候中心比较准确地预测了东亚夏季风偏强、我国夏季主要多雨带偏北的特征,以及南海夏季风爆发偏早、长江中下游入梅偏晚且雨量少雨期短、华北雨季提前雨量偏多的季节内过程演变趋势。最后对汛期气候预测存在的不足进行了分析和讨论。     

东亚夏季风的研究进展及其需进一步研究的问题

回顾了近年来关于东亚夏季风的结构特征以及年际、季内的变化特征及其成因的研究进展;并且回顾了关于东亚夏季风的数值模拟和可预测性的最新研究进展,特别是系统地回顾了东亚夏季风与印度季风特征的异同以及ENSO循环、西太平洋暖池和青藏高原在东亚夏季风的年际、季内变化的作用。还提出在关于东亚夏季风变化及其模拟和预测等方面需进一步研究的问题。     

MPI-ESM-LR模式中RCP8.5情景下春季北极海冰突变对东亚夏季降水的影响

利用MPI-ESM-LR模式RCP8.5情景下海冰浓度、降水、海表面温度、500 hPa位势高度和850 hPa风场等数据,对比分析了一次北极海冰突变前后春季海冰与东亚夏季降水关系的差异,并探究其可能成因。结果表明:1)北极海冰突变导致北极海冰浓度(Sea Ice Concentration,SIC)和ENSO对东亚夏季降水的影响均发生变化。突变前SIC和ENSO共同影响降水年际变化;突变后ENSO主导降水EOF的第一模态,SIC主导降水EOF的第二模态;2)北极海冰突变前,ENSO和SIC通过500 hPa经向波列,影响整个东亚地区的850 hPa风场,最终导致三极子型降水模态。突变后,ENSO通过500 hPa经向波列,影响华南地区的850 hPa风场,导致降水的偶极子空间模态,从而主导降水EOF的第一模态;同时SIC通过东亚地区500 hPa纬向波列,影响北方850 hPa风场,最终主导降水EOF的第二模态。3)北极海冰突变后,ENSO和SIC对东亚夏季降水的影响存在区域差异。北极海冰突变前,ENSO和SIC共同影响南北方降水;北极海冰突变后,SIC主要影响北方降水,ENSO主要影响南方降水。     

Possible Impacts of the Arctic Oscillation on the Interdecadal Variation of Summer Monsoon Rainfall in East Asia

The influences of the wintertime AO (Arctic Oscillation) on the interdecadal variation of summer monsoon rainfall in East Asia were examined. An interdecadal abrupt change was found by the end of the 1970s in the variation of the AO index and the leading principal component time series of the summer rainfall in East Asia, The rainfall anomaly changed from below normal to above normal in central China, the southern part of northeastern China and the Korean peninsula around 1978. However,the opposite interdecadal variation was found in the rainfall anomaly in North China and South China.The interdecadal variation of summer rainfall is associated with the weakening of the East Asia summer monsoon circulation. It is indicated that the interdecadal variation of the AO exerts an influence on the weakening of the monsoon circulation. The recent trend in the AO toward its high-index polarity during the past two decades plays important roles in the land-sea contrast anomalies and wintertime precipitation anomaly. The mid- and high-latitude regions of the Asian continent are warming, while the low-latitude regions are cooling in winter and spring along with the AO entering its high-index polarity after the late 1970s. In the meantime, the precipitation over the Tibetan Plateau and South China is excessive, implying an increase of soil moisture. The cooling tendency of the land in the southern part of Asia will persist until summer because of the memory of soil moisture. So the warming of the Asian continent is relatively slow in summer. Moreover, the Indian Ocean and Pacific Ocean which are located southward and eastward of the Asian land, are warming from winter to summer. This suggests that the contrast between the land and sea is decreased in summer. The interdecadal decrease of the land-sea heat contrast finally leads to the weakening of the East Asia summer monsoon circulation.     

Diversity of the Coupling Wheels in the East Asian Summer Monsoon on the Interannual Time Scale: Challenge of Summer Rainfall Forecasting in China

Two types of three-dimensional circulation of the East Asian summer monsoon(EASM) act as the coupling wheels determining the seasonal rainfall anomalies in China during 1979–2015. The first coupling mode features the interaction between the Mongolian cyclone over North Asia and the South Asian high(SAH) anomalies over the Tibetan Plateau at 200 hPa. The second mode presents the coupling between the anomalous low-level western Pacific anticyclone and upperlevel SAH via the meridional flow over Southeast Asia. These two modes are responsible for the summer rainfall anomalies over China in 24 and 7 out of 37 years, respectively. However, the dominant SST anomalies in the tropical Pacific, the Indian Ocean, and the North Atlantic Ocean fail to account for the first coupling wheel's interannual variability, illustrating the challenges in forecasting summer rainfall over China.     

基于BCC_CSM模式的中国东部夏季降水预测检验及订正

基于国家气候中心第二代季节预测模式的历史回报试验数据,检验了模式对我国东部夏季降水的预测能力,探讨了预测误差形成的可能原因,并应用降尺度方法提高了模式的降水预测技巧。分析表明:（1）模式能在一定程度上把握我国东部夏季降水时空变率的两个主要模态（偶极子型模态和全区一致型模态）,但是不同超前时间的预测在刻画模态方差贡献、异常空间分布特征、时间系数的年际变化等方面存在明显误差;（2）模式能够合理预测大尺度环流和海表温度（SST）的变化特征,但是对中国东部夏季降水的总体预测技巧有限,这与模式不能准确刻画西太平洋副热带高压、大陆高压、中高纬阻塞高压等环流系统以及热带太平洋、印度洋SST变率对中国东部降水模态的影响有关;（3）针对1991~2003年回报试验数据中的500 hPa位势高度、850 hPa纬向风和经向风、SST变量,在全球范围内寻找并定位与中国东部站点降水关系最密切的预报因子,进而建立针对降水预测的单因子线性回归、多因子逐步和多元回归模型。采用2004~2013年回报试验对所建立的降水预测模型进行了独立检验,结果表明:所建立的降尺度预测模型能显著提高中国东部地区夏季降水的预报技巧。以6月1日起报试验为例,预测的第一模态（第二模态）与观测的空间相关系数由原始的0.12（0.48）提高到了0.58（0.80）,时间相关系数则从0.47（0.15）提高到0.80（0.67）;其它超前时间的预测试验中,降尺度预测模型的降水预测技巧相比模式原始预测技巧也同样明显提高。     

MRI-CGCM模式气候预测回报试验在东亚夏季的检验和降尺度订正

应用1979—2010年MRI-CGCM模式回报、NCEP/NCAR再分析数据和中国东部降水观测资料检验了模式对东亚夏季风的模拟能力,并利用模式500 hPa高度场回报资料建立了中国东部夏季降水的奇异值分解（SVD）降尺度模型。模式较好地模拟了亚洲季风区夏季降水的气候态,但模拟的季风环流偏弱、偏南,导致降水偏弱。模拟降水的方差明显偏小,且模拟降水的外部、内部方差比值低,模拟降水受模式初值影响较大。模式对长江雨型的模拟能力最高,华南雨型次之,华北雨型最低。模式对东亚夏季风第1模态的模拟能力明显高于第2模态。对于东亚夏季风第1模态,模式模拟出了西太平洋异常反气旋,但强度偏弱,且未模拟出中高纬度的日本海气旋、鄂霍次克海反气旋,导致长江中下游至日本南部降水偏弱。各时次模拟环流均能反映但低估了ENSO衰减、印度洋偏暖对西太平洋反气旋的增强作用。对于东亚夏季风第2模态,模式对西太平洋的“气旋-反气旋”结构有一定的模拟能力,但未模拟出贝加尔湖异常反气旋和东亚沿海异常气旋,导致中国东部“北少南多”雨型在模拟中完全遗漏。仅超前时间小于4个月的模拟降水能够反映ENSO发展对降水分布的作用。通过交叉检验选取左场时间系数可以提高降尺度模型的预测技巧,SVD降尺度模型在华南、江南、淮河、华北4个区域平均距平相关系数分别为0.20、0.23、0.18、0.02,明显高于模式直接输出。      

EFFECTS OF ENSO ON THE RELATIONSHIP BETWEEN IOD AND SUMMER RAINFALL IN CHINA

Based on the data of 1950 – 1999 monthly global SST from Hadley Center, NCAR/NCEP reanalysis data and rainfall over 160 weather stations in China, investigation is conducted into the difference of summer rainfall in China (hereafter referred to as the “CS rainfall”) between the years with the Indian Ocean Dipole (IOD) occurring independently and those with IOD occurring along with ENSO so as to study the effects of El Ni?o - Southern Oscillation (ENSO) on the relationship between IOD and the CS rainfall. It is shown that CS rainfall will be more than normal in South China (centered in Hunan province) in the years of positive IOD occurring independently; the CS rainfall will be less (more) than normal in North China (Southeast China) in the years of positive IOD occurring together with ENSO. The effect of ENSO is offsetting (enhancing) the relationship between IOD and summer rainfall in Southwest China, the region joining the Yangtze River basin with the Huaihe River basin (hereafter referred to as the “Yangtze-Huaihe basin”) and North China (Southeast China). The circulation field is also examined for preliminary causes of such an influence.     

ENSO与中国东部夏季降水异常关系的年代际变化

根据1960—2011年Had ISST资料集中的月平均海表温度资料和中国753站逐日降水资料,基于转经验正交函数分解等分析方法,发现中国夏季降水的变化具有明显的独立性特征,可以分为相对独立的11个雨区,并在此基础上讨论了11个雨区夏季降水与ENSO的相关关系及其年代际变化,发现不同雨区的夏季降水与ENSO相关关系的年代际变化特征不尽相同,据此可分为3种类型:第1类为稳定不相关型,代表区为东北地区、长江中下游地区、江南地区、闽赣地区、环琼州海峡地区;第2类为稳定相关型,代表区为河套地区、黄河中下游地区;第3类为相关关系变化型,代表区为辽吉地区、黄淮地区、淮河流域以及两广地区。而在第3类相关关系变化型中,4个雨区夏季降水与ENSO相关关系的年代际突变时间也存在差异,两广地区的突变年份在1975年左右,辽吉地区和黄淮地区的突变年份在1980年左右,淮河流域的突变年份在1985年左右。     

太平洋—印度洋海温与我国东部旱涝型年代际变化的关系

对我国东部各区域的夏季降水的正交小波分解表明,其大于28年的分量可以很好地表示华北和长江中下游地区在20世纪70年代前后旱涝相反的年代际变化特征。合成分析表明,北太平洋、热带印度洋海温和东亚高空急流与我国东部夏季旱涝型的年代际变化密切相关。东亚高空急流和西太平洋副热带高压在70年代前后的年代际差异对旱涝型发生年代际变化起到重要作用;北太平洋和热带印度洋海温的年代际变化近百年来是协同一致的,二者有可能共同对旱涝型的变化产生影响。进一步分析指出,北太平洋—热带印度洋海温的变化与急流和副高的南北位置在年际和年代际尺度上都密切相关。可见,北太平洋-热带印度洋海表温度异常(SSTA)对于我国东部旱型涝的年代际变化确实具有重要的预示作用。     

青藏高原西侧绕流风系的变化及其与东亚夏季风和我国华北地区夏季降水的关系

从观测资料分析了青藏高原西侧绕流偏北风系的年际和年代际变化及其与东亚夏季风和华北地区夏季降水的关系。研究表明 ,前者对后者有很大影响 ,若夏季青藏高原西侧绕流的偏北风系强 ,则东亚夏季风偏南风分量强 ,且华北地区夏季降水可能偏多 ;相反 ,若夏季青藏高原西侧绕流的偏北风系弱 ,则东亚夏季风的偏南风分量弱 ,且华北地区夏季降水可能偏少。分析结果还表明 ,由于从 1 965年之后 (特别从 1 977年之后 ) ,高原西侧绕流的偏北风系减弱 ,可能导致了东亚夏季风的偏南风分量减弱 ,使得输向华北的水汽大大减弱 ,且引起华北地区降水减少 ,发生了持续严重干旱。     

Interdecadal shift in the relationship between the East Asian summer monsoon and the tropical Indian Ocean

In this work, the authors investigate changes in the interannual relationship between the East Asian summer monsoon (EASM) and the tropical Indian Ocean (IO) in the late 1970s. By contrasting the correlations of the EASM index (EASMI) with the summer IO sea surface temperature anomaly (SSTA) between 1953–1975 and 1978–2000, a pronounced different correlation pattern is found in the tropical IO. The SSTA pattern similar to the positive Indian Ocean Dipole (IOD) shows a strongly positive correlation with the EASMI in 1953–1975. But in 1978–2000, significant negative correlation appears in the northern IO and the IOD-like correlation pattern disappears. It is indicated that the summer strong IOD events in 1953–1975 can cause a weaker-than-normal western North Pacific (WNP) subtropical high, which tends to favor a strong EASM. In 1978–2000, the connection between the summer IOD and the WNP circulation is disrupted by the climate shift. Instead, the northern IO shows a close connection with the WNP circulation in 1978–2000. The warming over the northern IO is associated with the significant enhanced 500 hPa geopotential height and an anomalous anticyclone over the WNP. The change in the IO–EASM relationship is attributed to the interdecadal change of the background state of the ocean–atmosphere system and the interaction between the ENSO and IO. In recent decades, the tropical IO and tropical Pacific have a warmer mean SST, which has likely strengthened (weakened) the influence of the northern IO (IOD) on the EASM. In addition, due to the increase in the ENSO variability along with the higher mean equatorial eastern Pacific SST in 1978–2000, the influence of ENSO on the East Asian summer circulation experiences a significant strengthening after the late 1970s. Because the warming over the northern IO is associated with the significant warming in the equatorial eastern Pacific, the strengthened ENSO–EASM relationship has likely also contributed to the strengthened relationship between the northern IO and the EASM in 1978–2000.     

INTERANNUAL VARIATION OF INDEX OF EAST ASIAN LAND-SEA THERMAL DIFFERENCE AND ITS RELATION TO MONSOON CIRCULATION AND RAINFALL OVER CHINA*

This paper proposes an index of land-sea thermal difference(ILSTD)that describes its zonal and meridional strength responsible for East Asian monsoon circulation to study its relation to the East Asian monsoon circulation and the summer rainfall over China on an interannual basis.Results are as follows:(1)ILSTD can be used to measure the strength of East Asian summer monsoon in such a way that the strong(weak)ILSTD years are associated with strong(weak)summer monsoon circulation.(2)The index also reflects well summer rainfall anomaly over the eastern part of China.In the strong index years,rain belt is mainly located over the northern China,and serious drought emerges in the Jianghuai valleys and mid-lower reaches of the Changjiang River,along with increase of rainfall in North and South China,but in the weak years it is contrary.(3)Besides,the index has obvious QBO and quasi 4-year oscillations,but the periods and amplitudes have significant changes on an interdecadal basis.     

INTERANNUAL VARIATION OF INDEX OF EAST ASIAN LAND-SEA THERMAL DIFFERENCE AND ITS RELATION TO MONSOON CIRCULATION AND RAINFALL OVER CHINA

This paper proposes an index of land-sea thermal difference(ILSTD)that describes its zonal andmeridional strength responsible for East Asian monsoon circulation to study its relation to the EastAsian monsoon circulation and the summer rainfall over China on an interannual basis.Results are asfollows:(1)ILSTD can be used to measure the strength of East Asian summer monsoon in such away that the strong(weak)ILSTD years are associated with strong(weak)summer monsooncirculation.(2)The index also reflects well summer rainfall anomaly over the eastern part of China.In the strong index years,rain belt is mainly located over the northern China,and serious droughtemerges in the Jianghuai valleys and mid-lower reaches of the Changjiang River,along with increaseof rainfall in North and South China,but in the weak years it is contrary.(3)Besides,the index hasobvious QBO and quasi 4-year oscillations,but the periods and amplitudes have significant changes onan interdecadal basis.     

我国夏季降水与全球气温场的关系

用蒙特卡罗的相关方法研究我国夏季大尺度降水与全球气温场关系，发现我国夏季大尺度降水与全球气温场有密切相关，以同期相关为最密切；除与热带西太平洋、东太平洋等海域有密切相关外，还与欧洲东部、北美洲东北部以及东亚等内陆地区的不同季节气温场有关系。研究还表明，印度洋地区春季气温和北美洲北部及北太平洋西南部夏季气温偏高时，长江中下游夏季易涝。且它们与长江中下游夏季降水的相关均有阶段性，相关密切程度随时间有所增强。     

El Nino事件对其衰减阶段夏季中国降水季节内演变的影响及其机理

利用1979—2007年NOAA重建海温逐月资料和中国160站夏季降水资料,使用扩展奇异值分解（extended singular value decomposition,ESVD）方法,研究了冬季热带太平洋海温异常与次年夏季中国降水异常季节内演变型之间的关系,指出前冬El Nino事件是与次年夏季中国降水季节内变化相联系的最重要的热带太平洋海温异常模态。相应的降水异常季节内变化情况为：6月在长江以南为正异常,江淮流域有负异常;7月在华南沿海有负降水异常,而正异常北进到长江流域,华北地区也出现正降水异常;8月在长江南北分别为少雨和多雨。进一步研究前冬El Nino事件与次年春夏印度洋、太平洋海温异常、对流层低层风场异常以及副热带高压等的联系,结果表明：El Nio事件发生的次年春夏,热带西太平洋周边存在东负西正的海温异常分布;西太平洋反气旋异常较强;副高在6月、7月偏西偏北,但在8月迅速南退。虽然与El Nino事件相联系的6月与7月、8月的降水型不同,但是西太平洋反气旋异常带来的充沛水汽造成7月长江流域雨季多雨,8月副高迅速南退带来的又一次长江流域降水,造成了El Nino事件发生次年夏季长江流域涝而华南沿海旱的夏季平均降水异常型。