Role of Strong Sea Surface Temperature Diurnal Variation in Triggering the Summer Monsoon Onset Over the Bay of Bengal in a Climate Model

The earliest Asian summer monsoon onset (SMO) occurs in the Bay of Bengal (BoB), heralding the coming of the rainy season. In late April or early May, the strong sea surface temperature (SST) diurnal variation accompanied by ocean surface warming triggers the SMO. However, this observed diurnal cycle intensity cannot be reasonably simulated by state-of-the-art climate models, resulting in a spurious delayed SMO. To address this issue, the SST diurnal cycle parameterized by a diagnostic sublayer scheme was incorporated into a climate model named FIO-ESM v2.0. The large diurnal amplitude of SST contributes to surface warming and changes atmospheric circulation. Consequently, the high-pressure anomaly at high levels and an inverted trough at low levels promote more convective activity, triggering an earlier SMO. Our findings improve the ability of climate models in simulating the evolution of the Asian monsoon system. Asian summer monsoon onset (SMO) accompanied by the coming of the rainy season has a significant impact on the lives and livelihoods of more than half of the world's population. However, state-of-the-art climate models struggle to capture the major features of SMO accurately. The simulated diurnal amplitude of sea surface temperature (SST) is much smaller compared with observation, resulting in a cold bias in SST and a delayed SMO in the Bay of Bengal (BoB). Here, we incorporated a diagnostic sublayer parameterization into the climate model to account for the effect of a strong diurnal cycle. The SST in the BoB increases, and more convective activities are promoted in response to the warmer underlying surface. Changes in monsoon circulation create favorable environmental conditions for an advanced SMO. Considering the effect of strong diurnal variation provides a solution to reduce common biases of the climate model in simulating SMO. The spurious delayed Asian summer monsoon onset (SMO) is improved by including the effect of large sea surface temperature (SST) diurnal amplitude in a climate model Strong diurnal variation of SST contributes to upper ocean warming in the Bay of Bengal before the Asian SMO Large-scale circulation with favorable conditions for convection activity in response to ocean warming triggers an advanced monsoon onset