The principal modes of variability of the boreal summer (JJA) Hadley circulation (HC) and their variations during 1948 - 2007 are investigated. The results show that the year-to-year variability of the JJA HC is dominated by two asymmetric modes, centered respectively in the Northern Hemisphere (referred to as the AMN) and Southern Hemisphere (referred to as the AMS) as well as a quasi-symmetric mode (QSM) about the equator. The regime change of the JJA HC is revealed by the trends of the time series of AMN and AMS. It shows that the winter hemispheric HC has changed from one regime with strong northern part and weak southern part before the 1970s to the opposite regime with weak northern part and strong southern part since the 1970s. The variability of the JJA HC mentioned above can be explained by strong warming trends in the sea surface temperature (SST) over the equatorial Indian Ocean- western Pacific warm pool and the tropical Atlantic as well as the significant decreasing and increasing trends of the large-scale meridional SST gradients respectively in the Northern Hemisphere and the Southern Hemisphere over the tropical Atlantic and the Indian Ocean. The high-frequency interannual variability of the JJA HC, however, is mainly featured by the QSM, and highly correlated with the sea surface temperature over the eastern tropical Pacific Ocean and with the Nino3.4 index, implying that ENSO influence is mainly on high-frequency interannual time scale. Further study about the regime change of the HC reveals that the weakening of the crossequatorial part of the HC is highly connected with the weakening of the meridional circulations over the tropical monsoon regions of the Eastern Hemisphere The correlation and composite analyses show that the summer monsoon over the South China Sea, the eastern part of South Asia, and West Africa are significantly influenced by the regional meridional circulations, hence, the weakening trends of all those monsoons may be affected by the weakening of the cross-equatorial circulation during the regime change of the JJA HC. Nevertheless, the summer monsoon over the western part of South Asia does not show any noticeable trend or close connection with the regional meridional circulation. Thus, the result in this paper confirms the rationality of the division of the South Asia summer monsoon region into the east and west parts by Li and Zeng (2002).%本文分析了1948～2007年北半球夏季Hadley环流的主导模态及其变率,结果表明:北半球夏季Hadley环流变率的主导模态包括两个赤道非对称模态,其主体分别位于北半球(简称为AMN)和南半球(简称ANTS)和一个赤道准对称的模态(简称QSM),AMN和AMS主要表征Hadley环流的年代际变率部分,而QSM主要表征Hadley环流的年际变率部分.AMN的时间系数呈现明显的减弱趋势,AMS的时间系数则表现为明显的增强趋势,两个模态的年代际变率表明:北半球夏季Hadley环流发生了显著的年代际转型,在1970年代以前呈现"北强南弱"型,之后转变为"南强北弱"型.印度洋-西太平洋暖池和热带大西洋赤道带海温的异常增暖以及由热带大西洋和印度洋海温非均匀增暖形成的减弱的北半球大尺度经向海温梯度和加强的南半球大尺度经向海温梯度可能是导致上述Hadley环流变率的重要影响因子.不同于两个非对称模态,QSM模态的变率主要与热带东太平洋的海温以及Nino3.4指数有明显的线性关系,说明ENSO对夏季Hadley环流的影响主要是在年际尺度上.对Hadley环流年代际转型的进一步分析发现,其越赤道部分的减弱与东半球热带季风区经向越赤道环流的减弱有密切联系.相关和合成分析的结果显示,南海季风、南亚东区季风以及西非季风的强弱与越赤道环流异常有显著相关,热带季风在这些区域的减弱趋势很可能共同受到北半球夏季Hadley环流年代际转型中越赤道环流减弱的影响.然而,南亚西区季风与经向环流没有明显相关,同时也未呈现显著的年代际趋势,这一结果从环流的角度验证了Liand Zeng(2002)将南亚季风区划分为东区和西区的合理性.
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