Distinguished Intraseasonal Characteristics in East Asian-western North Pacific summer monsoon

Abstract:

There are varied phases in the East Asian summer monsoon (EASM) and western North Pacific summer monsoon (WNPSM) within intraseasonal time scale. Linear analysis shows low predictability of EASM-WNPSM precipitation which is simply represented by Meiyu-Baiu rainbelt. Similar studies on nonlinear monsoon phases and dynamical interaction between local and large-scale phenomenon have not been performed previously because of difficulties in separating the irregular monsoon intraseasonal system. By applying a cluster analysis to daily monsoon circulation indices, we found four major intraseasonal phases named as pre-Meiyu&Baiu, Changma&Meiyu, WNPSM, and Monsoon Gyre modes. The dynamic-thermodynamic structure of four modes indicates that pre-Meiyu&Baiu and WNPSM are dominated by thermal advection resulted in midlatitude upper-level jet while Changma&Meiyu and Monsoon gyre are dominated by low-level moisture transport along the western North Pacific subtropical high which links tropics to midlatitude. Moreover, this method allows us to estimate the asymmetric ENSO response to intraseasonal monsoon variability providing a source of predictability. It is identified that Changma&Meiyu and Monsoon Gyre mode are closely linked to tropics through the subtropical pressure system while pre-Meiyu&Baiu and WNPSM mode have compartmental midlatitude system. The model experiments further support the interactive air-sea coupling over the Indian Ocean and western Pacific have a key role in generating WNPSH.