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.     

Impacts of Previous Winter Kuroshio SSTA on Summer Rainfall in China

1. IntroductionAs well known, Kuroshio is a famous and strongwest boundary current in the North Pacific. It trans-fers enormous energy from the low latitudes to themid-high latitudes and releases huge heat flux to theatmosphere above (Hsiung, 1985). The variation ofKuroshio exerts great influence on weather and cli-mate in East Asian.During 1950-60s, Lü (1950, 1964) found that thewestern North Pacific SSTA had a close relation withsummer rainfall in China. In the 1970s, evidencesshowed…     

Phase transition of the Pacific decadal oscillation and decadal variation of the East Asian summer monsoon in the 20th century

This paper focuses on the relationship between the phase transition of the Pacific decadal oscillation (PDO) and decadal variation of the East Asian summer monsoon (EASM) in the twentieth century. The first transition occurred in the 1940s, with an enhanced SST in the North Pacific and reduced SST in the tropical eastern Pacific and South Indian Ocean. In agreement with these SST changes, a higher SLP was found in most parts of the Pacific, while a lower SLP was found in the North Pacific and most parts of the Indian Ocean. In this case, the EASM was largely enhanced with a southerly anomaly in the lower troposphere along the east coast of China. Correspondingly, there was less rainfall in the Yangtze River valley and more rainfall in northern and southern China. An opposite change was found when the PDO reversed its phase in the late 1970s. In the tropical Indian Ocean and western Pacific, however, the SST was enhanced in both the 1940s and 1970s. As a result, the western Pacific subtropical high (WPSH) tended to extend westward with a larger magnitude in the 1970s. The major features were reasonably reproduced by an atmospheric general circulation model (IAP AGCM4.0) prescribed with observed SST and sea ice. On the other hand, the westward extension of the WPSH was exaggerated in the 1970s, while it was underestimated in the 1940s. Besides, the spatial pattern of the simulated summer rainfall in eastern China tended to shift southward compared with the observation.     

Anomalous Western Pacific Subtropical High during Late Summer in Weak La Nia Years: Contrast between 1981 and 2013

Both 1981 and 2013 were weak La Niña years with a similar sea surface temperature (SST) anomaly in the tropical Pacific, yet the western Pacific subtropical high (WPSH) during August exhibited an opposite anomaly in the two years. A comparison indicates that, in the absence of a strong SST anomaly in the tropics, the cold advection from Eurasian high latitudes and the convection of the western Pacific warm pool play important roles in influencing the strength and position of the WPSH in August. In August 1981, the spatial pattern of 500 hPa geopotential height was characterized by a meridional circulation with a strong ridge in the Ural Mountains and a deep trough in Siberia, which provided favorable conditions for cold air invading into the lower latitudes. Accordingly, the geopotential height to the north of the WPSH was reduced by the cold advection anomaly from high latitudes, resulting in an eastward retreat of the WPSH. Moreover, an anomalous cyclonic circulation in the subtropical western Pacific, excited by enhanced warm pool convection, also contributed to the eastward retreat of the WPSH. By contrast, the influence from high latitudes was relatively weak in August 2013 due to a zonal circulation pattern over Eurasia, and the anomalous anticyclonic circulation induced by suppressed warm pool convection also facilitated the westward extension of the WPSH. Therefore, the combined effects of the high latitude and tropical circulations may contribute a persistent anomaly of the WPSH in late summer, despite the tropical SST anomaly being weak.     

A timescale decomposed threshold regression downscaling approach to forecasting South China early summer rainfall

A timescale decomposed threshold regression(TSDTR) downscaling approach to forecasting South China early summer rainfall(SCESR) is described by using long-term observed station rainfall data and NOAA ERSST data. It makes use of two distinct regression downscaling models corresponding to the interannual and interdecadal rainfall variability of SCESR.The two models are developed based on the partial least squares(PLS) regression technique, linking SCESR to SST modes in preceding months on both interannual and interdecadal timescales. Specifically, using the datasets in the calibration period 1915–84, the variability of SCESR and SST are decomposed into interannual and interdecadal components. On the interannual timescale, a threshold PLS regression model is fitted to interannual components of SCESR and March SST patterns by taking account of the modulation of negative and positive phases of the Pacific Decadal Oscillation(PDO). On the interdecadal timescale, a standard PLS regression model is fitted to the relationship between SCESR and preceding November SST patterns. The total rainfall prediction is obtained by the sum of the outputs from both the interannual and interdecadal models. Results show that the TSDTR downscaling approach achieves reasonable skill in predicting the observed rainfall in the validation period 1985–2006, compared to other simpler approaches. This study suggests that the TSDTR approach,considering different interannual SCESR-SST relationships under the modulation of PDO phases, as well as the interdecadal variability of SCESR associated with SST patterns, may provide a new perspective to improve climate predictions.     

淮河流域夏季降水异常与若干气候因子的关系

基于旋转经验正交函数分解 (REOF) 方法探讨淮河流域1961—2010年夏季降水与厄尔尼诺/南方涛动 (ENSO)、北大西洋涛动 (NAO)、印度洋偶极子 (IOD)、太平洋年代际振荡 (PDO) 之间的关系,并进一步分析各气候因子不同位相单独以及联合对淮河流域夏季降水的影响。结果表明:淮河流域夏季降水与ENSO,PDO,NAO,IOD等气候因子具有较稳定的相关性,其中,PDO和IOD是影响淮河流域夏季降水的关键因子,且PDO与夏季降水呈显著负相关关系;各气候因子的冷暖位相单独及联合对淮河流域夏季降水的影响不同,PDO的冷期以及NAO,IOD冷位相使流域北部的夏季降水量呈显著增加趋势,PDO分别联合ENSO,NAO和IOD的冷、暖位相对流域北部地区和淮河上游地区的夏季降水影响显著。     

西藏夏季降水的季节内变化特征及其影响系统

利用西藏地区1980-2013年夏季降水量资料、NCEP再分析资料等,分析了西藏地区夏季降水主模态季节内变化特征,尤其是盛夏7和8月降水异常对应的大尺度环流特征和影响系统。结果表明:西藏夏季降水存在明显的季节内变化,6和7月降水主模态的时间系数变化具有较好的持续性,而7和8月降水主模态的时间系数的相关关系明显减弱。西藏地区7和8月降水偏多年,西藏地区上游低层纬向风场均呈西风异常,但是水汽来源有差异;同时欧亚中高纬地区对流层中高层环流存在显著差异。西藏7月降水与南亚高压强度存在显著负相关关系,南亚高压偏强/弱时,降水偏少/多。西藏8月降水与南亚高压的位置关系更密切,南亚高压偏南/北,降水偏多/少。     

热带印度洋海温海盆一致模的变化规律及其对东亚夏季气候影响的回顾

近十几年,热带印度洋对全球气候的作用越来越受到关注.本文从热带印度洋气候态特征、海温海盆一致模的变化规律以及对东亚夏季气候的影响方面回顾了这些研究工作,并且对这些研究以及存在的不足做了系统的总结.     

青藏高原和亚洲夏季风动力学研究的新进展

亚洲夏季风环流受海陆和伊朗高原—青藏高原大地形的热力作用调控.亚洲季风所释放的巨大潜热又对大气环流形成反馈.这种相互反馈过程十分复杂,揭示其物理过程对理解气候变化格局的形成和变化以及提高天气预报及气候预测的准确率十分重要.夏季北半球副热带对流层上层环流的主要特征是存在庞大的南亚高压(SAH)以及强大的对流层上层温度暖中心(UTTM).本文介绍了温度—加热垂直梯度(T-Q_Z)理论的发展,并用以揭示SAH和UTTM的形成机制.指出沿副热带欧亚大陆东部的季风对流潜热加热及其中西部的表面感热加热和高层长波辐射冷却是导致SAH和UTTM在南亚上空发展的原因.文中还介绍了Gill模型用于上部对流层研究的局限性及解决的办法.