Mechanisms for the Anomaly of Local Meridional Circulation during Early and Delayed Onsets of the South China Sea Summer Monsoon

The onset of the South China Sea summer monsoon is strongly associated with the variations of meridional circulation over East Asia, particularly during 1-15 May. Using a linear diagnostic model, numerical experiments are performed that successfully capture the local meridional circulation and its anomaly relating to early and delayed onsets of the summer monsoon. It is shown that, in years with an early monsoon onset, anomalous ascending (descending) motion tends to occur over the South China Sea (central China), in clear contrast to the pattern in years with a delayed monsoon onset. Such a difference of meridional circulation is found to result from the meridional gradient of latent heat release, thermal and momentum advection, and the forcing effect of the southern boundary. An analysis of the related synoptic patterns suggests that the southward-shifted subtropical upper-level jets, westerly disturbances, frontal activities, and the retreat of the subtropical high from the South China Sea can trigger a direct circulation between low and middle latitudes through an effective transport of heat and momentum. The anomalous northerly wind from middle latitudes and the westerly wind from tropical regions work together to strengthen the low-level moisture convergence and convection over the South China Sea, driving strong upward motion. The results highlight that a strong interaction between midlatitude and tropical weather systems may play an important role in regulating the timing of the onset of the summer monsoon.