Respective and Combined Impacts of North Indian Ocean and Tropical North Atlantic SST Anomalies on the Subseasonal Evolution of Anomalous Western North Pacific Anticyclones

Abstract

Abstract The subseasonal variation of the anomalous western North Pacific anticyclone (WNPAC) has important implications for East Asian summer monsoon variability. How the WNPAC evolves on the subseasonal time scale under the different configurations of tropical North Atlantic (TNA) Ocean and north Indian Ocean (NIO) SST warming is elucidated in this study. The WNPAC forced by individual TNA SST warming shows an obvious subseasonal variation with a stepwise northward movement. In contrast, the WNPAC forced by individual NIO SST warming shows a weak subseasonal variation, being nearly stabilized at around 20°N from June to August and thereby causing long-lasting and intense positive mei-yu–baiu–changma rainfall anomalies. The physical mechanism for the different subseasonal variation of WNPAC is further investigated. The TNA SST warming generates a WNPAC via a Rossby wave–induced divergence/convergence chain response. In this process, the TNA SST warming-induced suppressed convection over the western Pacific moves northward with the northward movement of climatological intertropical convergence zone and summer monsoon region, which generates a northward shift of the WNPAC. However, the NIO SST warming produces a WNPAC via a Kelvin wave–induced suppressed convection over the western Pacific Ocean. This suppressed convection is stabilized at around 20°N because of the Kelvin wave activity scope being limited within 20°N, which finally produces a nearly stationary WNPAC from June to August. In addition, under the simultaneous occurrence of the TNA and NIO SST warming, the subseasonal variation of WNPAC bears a resemblance to that for the individual NIO SST warming condition, where the TNA SST warming fails to exert its impact.