Mechanisms responsible for interdecadal variation of the thermodynamic component of East Asian summer monsoon moisture transport

In this study, the East Asian summer (June-July-August) monsoon (EASM) atmospheric moisture transport variability under global warming are examined using ERA-Interim reanalysis data from 1979 to 2019. The total EASM atmospheric moisture transport is decomposed into the thermodynamic (delta TH) and dynamic (delta MCD) components, which are related to specific humidity and wind velocity, respectively. Based on empirical orthogonal function (EOF) analysis, both the second and the fourth modes of delta TH are linked to delta MCD, indicating that delta MCD can impact the moisture distribution and consequently influence delta TH indirectly. Both the first (delta TH_EOF1) and the third (delta TH_EOF3) modes of delta TH show considerable interdecadal changes. It is noted that their principal components exhibit significant increasing trends in recent decades. The response of EASM moisture transport to global warming is probably reflected in these two modes. delta TH_EOF1 is associated with negative phases of the Interdecadal Pacific Oscillation (IPO)/Pacific Decadal Oscillation (PDO). This causes the western North Pacific subtropical high (WNPSH) to extend westward and the strengthening of Pacific trade winds. As a result, positive precipitable water anomalies dominate East Asia and delta TH_EOF1 is characterized by anomalous southwesterly moisture transport along the East China Sea to southern Japan. delta TH_EOF3 corresponds to anomalous moisture transport over the Indochina Peninsula and Japan. The precipitable water associated with delta TH_EOF3 in general represents a meridional dipole. Anomalous southward wind induced by the eastward extension of the WNPSH is the main reason for positive (negative) precipitable water anomalies in southern (northern) East Asian. The anomalous circulation associated with the anomalous east-west sea surface temperature gradient over the Indian Ocean affects atmospheric moisture in southern East Asian and can dramatically impact delta TH_EOF3.