Rainy season onset over the Southeast Asia low-latitude highlands: objective definition and relation with spring drought

Based on multiple datasets and different methods, this manuscript objectively determined the rainy season onset (RSO) over the Southeast Asia Low-Latitude Highlands (SEALLH) and investigated the relationship between RSO and spring drought. The RSO date is defined as the minimum of the rainfall departure over the SEALLH, with a mean value of 11 May and a standard deviation of 15 days. The RSO demarcates the transition between the dry and wet seasons, which is an effective indicator of the sudden emergence of rainfall and convection over the SEALLH. A delayed RSO is characterized by a reduction in April-May rainfall, accompanied by low-level divergence, descending motion and adiabatic warming, decreased cloud cover and increased solar radiation. Consequently, a delayed RSO is frequently associated with spring drought over the SEALLH due to the resulting water deficiency, warmer temperature, and increased evaporation.