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Qiong Zhang, Karin Holmgren, and Hanna Sundqvist

Abstract

A rainfall dipole mode characterized by negative correlation between subtropical southern Africa and equatorial eastern Africa is identified in instrumental observation data in the recent 100 years. The dipole mode shows a pronounced oscillation signal at a time scale of about 18 years. This study investigates the underlying dynamical mechanisms responsible for this dipole pattern.

It is found that the southern African rainfall dipole index is highly correlated to the land–sea contrast along the east coast of Africa. When the land–sea thermal contrast strengthens, the easterly flow toward the continent becomes stronger. The stronger easterly flow, via its response to east coast topography and surface heating, leads to a low pressure circulation anomaly over land south of the maximum easterly flow anomalies and thus causes more rainfall in the south.

On a decadal time scale, an ENSO-like SST pattern acts to modulate this land–sea contrast and the consequent rainfall dipole. During a “wet in the south and dry in the north” dipole, there are warm SSTs over the central Indian Ocean and cold SSTs over the western Indian Ocean. The cold SSTs over the western Indian Ocean further enhance the land–sea contrast during austral summer. Moreover, these cold western Indian Ocean SSTs also play an important role in regulating land temperature, thereby suppressing clouds and warming the land via increased shortwave radiation over the less-cloudy land. This cloud–SST coupling acts to further strengthen the land–sea contrast.

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