Changes in the Relationship between Spring Precipitation in Southern China and Tropical Pacific-South Indian Ocean SST

The present study explores the changed relationship between the interannual variations in spring (April-May) precipitation over southern China (SPSC) and sea surface temperature (SST) anomalies in the tropical Pacific and south Indian Oceans during the 1960-2017 period. Observational analysis shows that the relation between SPSC and El Nino-Southern Oscillation (ENSO) was significant before the mid-1980s (P1) and after the early 2000s (P3) but insignificant in between, from the mid-1980s to the early 2000s (P2). In P2, positive anomalous SPSC was significantly correlated with negative anomalous SST in the south Indian Ocean. During this period, an anomalous anticyclone and intensified southwesterly winds tended to appear over tropical India accompanied by a negative anomalous south Indian Ocean SST, which caused anomalous low-level convergence over the western Pacific. As a result, the western Pacific subtropical high (WPSH) tended to weaken and retreat eastward. This resulted in anomalous moisture convergence in southern China, favoring enhanced SPSC. Further analysis shows that the negative south Indian Ocean SST anomalies tended to induce anomalous cross-equatorial vertical circulation where the south Indian Ocean and southern China are controlled by descending and ascending airflow. The ascending motion may also contribute to positive anomalous SPSC. The observed contribution of the south Indian Ocean SST anomalies to the SPSC variation is confirmed by numerical experiments using an atmospheric model. The intensified variance of SST in the south Indian Ocean and the eastward shift of the ENSO-related circulation anomalies over the western tropical Pacific may partly account for the changes in the SST-SPSC relationship.