The Principal Modes of Variability of the Boreal Summer Hadley Circulation and Their Variations

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 Niño3.4 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).