Quantitative analysis of sea surface temperature warming prior to the summer monsoon onset over the Bay of Bengal

The Bay of Bengal (BoB) serves as the primary region for the initial establishment of the Asian summer monsoon. Prior to monsoon onset, the sea surface temperature (SST) in the central BoB increases, potentially influenced by strong solar radiation absorption and shallow mixed layer depth, forming a spring warm pool. Its peak occurs approximately 2 pentads before the monsoon onset (denoted as 2P). A rapid warming period from 9P to 5P, characterized by a daily warming rate of 0.04 °C, is a key precursor signal for summer monsoon onset. Spring SST warming increases atmospheric specific humidity, promotes convective system development and decreases outgoing longwave radiation. This process serves as a crucial environmental factor in triggering the monsoon onset. The ability of climate model in simulating SST warming in the BoB is also evaluated. Multi-model analysis indicates that rapid SST warming occurs from 14P to 5P, with an earlier, longer and more intensive warming period than observations. It highlights systematic biases in both temporal duration and intensity of the pre-monsoon SST warming simulations. These findings increase the understanding of air-sea interaction processes before summer monsoon onset and provide new insights for improving climate model performance.