Signals of Spring Thermal Contrast Related to the Interannual Variations in the Onset of the South China Sea Summer Monsoon

Land-sea thermal contrast is one of the key factors modulating the onset and strength of summer monsoons. This study investigates the precursory thermal-contrast signal of the onset of the South China Sea summer monsoon (SCSSM) and reveals that the land-sea thermal contrast DT (T-ocean minus T-land at the surface) in March tracks the date of the monsoon onset very well. In an energy-balance framework, the monthly anomalies in this thermal contrast are decomposed into different components associated with the anomalies of various radiative and nonradiative dynamical processes over the South China Sea in early spring. It is found that the interannual variations of DT, and thus the onset date of monsoon, are mainly tied to the anomalies in surface heat fluxes, ocean-land heat storage rate, and oceanic dynamical processes. The variation of ocean-land heat storage rate and ocean dynamics is the largest positive contributor to the change in DT. Specifically, an early SCSSM onset tends to be associated with an amplified "cold-land, warm-ocean" pattern that can be further attributed to increased soil moisture content in March, which enhances land surface cooling and weakens oceanic heat storage rate that benefits ocean surface warming. Furthermore, the variations of DT and soil moisture content in March are positively related with the late-February precipitation over the Indochina Peninsula, which could therefore be regarded as a precursory signal for both the land-sea thermal contrast in March and the onset of the SCSSM.