The Intraseasonal Variation of Summer Monsoon Circulation and Rainfall in East Asia
The Intraseasonal Variation of Summer Monsoon Circulation and Rainfall in East Asia
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发表时间：
2010
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通讯作者：
Sun Tonghua
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作者：
Sun Tonghua
【摘要】：基于常规气象资料和归一化有限时间变分(NFTV)方法,分析了东亚夏季风(EASM)环流的演变特征,发现夏季东亚地区存在两次明显的次季节突变,主要表现为西太平洋副热带高压(WPSH)的两次东缩和北跳,一次发生在6月中旬,另一次发生在7月下旬。与东亚降水分布相比，东亚雨带在季节演变上也出现了两次明显的北跳，分别对应江淮至日本的梅雨期初期以及华北和东北地区的雨季。与第一次跳变相比，副高的第二次跳变更为明显。第一次跳变主要是南海对流活动的增强造成的，第二次跳变主要是由于南海对流活动的增强造成的。通过西太平洋暖池上空对流活动和高纬度环流系统的共同影响，通过西太平洋暖池上空罗斯贝波列向东北传播和高纬度罗斯贝波下游传播的锁相，西太平洋暖池上空对流活动和高纬度环流系统对副高第二次北跳起到了关键作用。西缘潜热的影响导致了副高的季节内低频振荡。NFTV分析表明，高纬度地区低层环流的调整随着季节的演变而逐渐减弱，这与中高纬度的温差有关。而夏季后半段，高层环流的调整则随着海拔的升高而趋于加强。此外，这种演变的相似性也表明东亚地区大气环流呈现出明显的变化趋势。副高第二次跳跃后的状态明显不同。南半球环流对南海和副副高对流活动的增强起着重要作用。6月中旬，这两个地区对流活动的增强是由于南海西缘西风的加强和东伸，与马斯克林高压(MH)有深入的牵连。7月中旬，澳大利亚高压(AH)的增强导致南海和副高的对流活动增强。东北边缘越赤道气流增强，大量来自南半球的冷空气侵入暖池区，增加了该区大气的不稳定和低层辐合，导致暖池区对流活动增强。但夏季上半段，夏季前半段，AH强度与其东北边缘越赤道气流的关系可以通过暖池区的对流活动来调节。 WPWP,导致AH与伴生的越赤道气流趋势相反。南半球冬季,AH趋于减弱并伴有低频振荡,这既受到澳大利亚地表温度的影响,又受到上游MH能量分散的影响。AH的减弱趋势受前者影响,而MH在AH的低频振荡中起主导作用。
Based on routine meteorological data and the method of the normalized finite temporal variation(NFTV),the evolution features of East Asian summer monsoon(EASM) circulation are analyzed.It is found that there exist two distinct subseasonal abrupt changes in East Asia during the summertime,characterized primarily by two eastward contractions and northward jumps of the western Pacific subtropical high(WPSH),one of which is in middle June and the other is in late July.Because of the close relationship between the WPSH and rainfall distributions in East Asia,the rain belt in East Asia exhibits two obvious northward jumps in the seasonal evolution as well,corresponding to the beginning of the Meiyu period from the Yangtze-Huaihe valley to Japan and the rainy season in North China and Northeast China,respectively.Compared with the first jump,the second jump of the WPSH is much more evident.The first jump is mainly caused by the enhancement of the convective activities in the South China Sea(SCS) while the second jump is influenced by both the convective activities over the western Pacific warm pool(WPWP) and the circulation systems in high latitudes.Through the phase-locking of the northeastward propagation of the Rossby wave trains from the WPWP and the downstream propagation of the Rossby waves in high latitudes,both the convective activities over the WPWP and the circulation systems in high latitudes play a key role in the second northward jump of the WPSH.In addition,the interactions between the WPSH and the release of the latent heat on its western edge lead to the intraseasonal low-frequency oscillation of the WPSH.The analyses of NFTV indicate that the adjustment of the low-level circulation in high latitudes tends to weaken gradually with the seasonal evolution,which is related to the temperature difference between middle and high latitudes.By contrast,the adjustment of the high-level circulation tends to intensify with the altitude during the latter half of the summertime.Besides,the evolution of the similarity also shows that the atmospheric circulation in East Asia exhibits a distinctly different state after the second jump of the WPSH.The Southern Hemisphere circulation plays an important role in the enhancement of the convective activities in the SCS and the WPWP.In middle June,the enhancement of the convective activities in these two regions is due to the intensification and eastward extension of the westerly on the western edge of the SCS,which is deeply involved with the Mascarene high(MH).In middle July,the enhancement of the Australian high(AH) leads to the intensification of the cross-equatorial flow on its northeastern edge,and a large amount of cold air from the Southern Hemisphere invades into the warm pool region,which increases the atmospheric instability and the low-level convergence over this region.As a result,the convective activities in the warm pool are enhanced.In the first half of the summertime,however,the relationship between the intensity of the AH and the cross-equatorial flow on its northeastern edge can be modulated by the convective activities in the WPWP,resulting in an opposite trend between the AH and the associated cross-equatorial flow.During the austral wintertime,the AH tends to weaken with the low-frequency oscillation,which is affected by both the surface temperature in Australia and the energy dispersion of the upstream MH.The weakening trend of the AH is influenced by the former while the MH plays a dominant role in the low-frequency oscillation of the AH.