The Principal Modes of Variability of the Boreal Summer Hadley Circulation and Their Variations
The Principal Modes of Variability of the Boreal Summer Hadley Circulation and Their Variations
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发表时间：
2011
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通讯作者：
L. Jianping
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作者：
L. Jianping
研究了1948-2007年夏季Hadley环流的主要变率模态及其变化特征。结果表明，夏季Hadley环流的年际变率主要由两个非对称模态控制，分别以北方半球为中心（简称AMN）和南半球（称为AMS）以及准对称模式（QSM）AMN和AMS时间序列的变化趋势揭示了JJA HC的体制变化，表明冬季半球HC已由北方强、南方弱的体制变化70年代以来，赤道印度洋-西太平洋暖池和热带大西洋的海表温度（SST）有明显的增暖趋势，而热带大西洋的大暖池有明显的减少和增加趋势，这可以解释JJA HC的上述变化。热带大西洋和印度洋北方半球和南半球的海温梯度具有明显的尺度非对称性，而JJA HC的高频年际变率则以QSM为主要特征，并与热带东太平洋海温和Nio3.4指数有较好的相关性，对HC体制变化的进一步研究表明，ENSO对HC的影响主要表现在高频年际时间尺度上。相关分析和合成分析表明，夏季风在东半球热带季风区上空的活动与赤道部分的环流减弱密切相关南海、南亚东部和西非受区域纬向环流的影响显著，因此，这些季风的减弱趋势可能受到JJA HC体制转变期间越赤道环流减弱的影响。然而，南亚西部夏季风与区域纬向环流没有明显的趋势和密切的联系，本文的结果证实了Li和Zeng（2002）将南亚夏季风区分为东西两部分的合理性。
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 Nio3.4 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 cross-equatorial 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).