Characteristics and Mechanisms of Strong Precipitation in the Northern Bay of Bengal in Fall 2023

The El Niño events in the tropical Pacific usually suppress precipitation in fall over the northern Bay of Bengal through cross-basin air-sea interactions. However, a significant enhanced precipitation was observed during the fall 2023 super El Niño and triggered severe localized flooding. This study reveals that the anomalous precipitation event mainly originated from the synergistic effect of westerly wind activity and tropical cyclone in the Bay of Bengal. The anomalous westerly wind and anomalous tropical cyclone activity originated from the anomalous sea surface temperature (SST) anomalies in the Bay of Bengal mainly due to westerly anomalies-SST gradient positive feedback mechanism. In September 2023, the northern Bay of Bengal experienced significant westerly winds in the low-altitude atmospheric circulation field, which drove the accumulation of surface warm water towards the east coast, and the SST formed a westward temperature gradient, which affected the wind field by influencing the sea surface pressure gradient, allowing the anomalous westerly winds and anomalous SST distribution to be maintained. At the same time, tropical cyclone activity is obviously enhanced. The superposition of the southeasterly circulation of the tropical cyclone and the anomalous westerly winds formed a low-level rapid with a central wind speed of more than 20 m·s⁻¹, whose warm moisture transported from the ocean surface synergistically interacted with the tropical cyclone precipitation, ultimately leading to anomalous precipitation surge in the northern Bay of Bengal in September. Meanwhile, the tropical storms and typhoons in the Bay of Bengal in October-November were the strongest in the last 20 years in terms of duration and intensity, which in turn caused a significant increase in precipitation. These results demonstrate the complexity of the multi-scale air-sea interactions that regulate precipitation changes in the monsoon region of the Bay of Bengal, and provide a new perspective for predicting localized meteorological hazards under tropical climate change.