Differences in Summer Monsoon Rainfall over South Asia During Multi-year La Niña Events

This research analyzes the variations of the South Asian Summer Monsoon Rainfall Anomaly (SASMRA) between the first development year (Y0) and the following year (Y1) of all multi-year La Niña events from 1958 to 2022. During Y0, precipitation surpasses usual amounts, presenting a tri-pole pattern, whereas Y1 experiences below-normal precipitation, characterized by a dipole. In certain regions, the difference in precipitation between Y0 and Y1 reaches up to 3 mm/day. This work provides insight into the main tropical ocean regions responsible for this precipitation distinction, as well as its coupling mechanisms with large-scale atmospheric circulation anomalies. Influenced by the development of earlier ocean-atmosphere anomaly patterns, the tropical Indian Ocean and western Pacific (TIO-WP) warming (cooling) is significant during the summer of Y0 (Y1). The raised sea surface temperature supports an anomalous west-north Pacific (WNP) anticyclone via the Kelvin-induced Ekman divergence mechanism. This anomalous anticyclone intensifies suppressed convection over the WNP, which results in increased divergence in the upper-level troposphere over the Indian Ocean and South Asian regions, thereby boosting convection. Simultaneously, easterly winds associated with the strengthened equatorial latitude sea surface temperature anomalies (SSTA) gradient and the anomalous anticyclone intensified, transporting a large amount of water vapor to the west. The combined moisture and dynamic conditions support precipitation rise in the South Asian region.