Interdecadal Variability Ofsouth-Southeastasian Rainfall And Cross-Equatorial Flows Duringapril-May

From April to May, South-Southeast Asian rainfall (SSEAR) and cross-equatorial flows (CEFs) off the East African coast have exhibited interdecadal variability over the past four decades. Both SSEAR and CEF strengthened based on comparative analysis for the periods 1979-1998 and 1999-2016. Associated modulating processes feature warm sea surface temperature (SST) anomalies in the tropical western North Pacific (WNP) and cold SST anomalies in the tropical eastern Pacific. These features lead to a low-level clockwise circulation anomaly across the North and South Indian Ocean. This intensifies CEFs via anomalous southerly flows along the circulation's western section and transports moisture via anomalous westerly flows on its northern side that enhances SSEAR. After 1998, SSEAR and CEFs vary coherently from 1999 to 2005, but incoherently from 2006 to 2016. For coherent cases (1999-2005), warm SST anomalies in the tropical WNP invoke a clockwise circulation anomaly over the tropical Indian Ocean. Meanwhile, cold anomalies in the tropical Indian Ocean and strong warm anomalies over Asian landmasses cause a meridional temperature difference (MTD) forming a strong cyclonic anomaly in the north that constrains the clockwise circulation anomaly south of 15 degrees N. The circulation's western section enhances CEFs, while anomalous westerly flows on its northern side enhance moisture supply over southern parts of the Asian landmasses thereby increasing SSEAR. For incoherent cases (2006-2016), SST anomalies are warm in both the tropical WNP and Indian Ocean, while anomalous warming over Asian landmasses is more moderate. A weakened MTD yields an isolated weakened cyclonic anomaly in the north allowing for the development of a northward-displacing clockwise circulation anomaly with anomalous westerly flows north of 20 degrees N. Rainfall increases a little in northern parts of South-Southeast Asia but decreases noticeably in southern parts. Intensified CEFs thus occur concurrently with decreased SSEAR.