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, monsoon precipitation surpasses climatological values, presenting a tripole spatial pattern, whereas Y1 is characterized by below-normal precipitation with a dipole pattern. In certain regions, the difference in precipitation between Y0 and Y1 reaches up to 3 mm day^–1. This work provides further insight into the key tropical ocean regions driving the precipitation distinction, and elucidates their 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 elevated sea surface temperature (SST) in Y0 supports an anomalous Western North Pacific (WNP) anticyclone via a Kelvin-wave-induced Ekman divergence mechanism. This anomalous anticyclone intensifies the 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, the easterly winds associated with the strengthened equatorial latitude SST anomaly (SSTA) gradient and the anomalous anticyclone intensified, transporting a large amount of water vapor to the west. The combined moisture and dynamic conditions support the enhanced precipitation in the South Asian region.