Linkage between Southeast and South Asian Summer Monsoons: Relative Roles of the Pacific–Japan Pattern and El Niño–Southern Oscillation

Abstract The various regional summer monsoons over Asia have been known to be highly interactive, but their interrelationships are still under debate. The current study investigates the linkage between the South Asian summer monsoon (SASM) and the Southeast Asian summer monsoon (SEASM) and the involved physical mechanisms. A robust positive correlation is found between the first empirical orthogonal function (EOF) modes of SASM and SEASM precipitations. However, asymmetric features emerge between the mutually enhanced (ME) and mutually weakened (MW) SEASM–SASM cases. Apparent Pacific–Japan pattern occurs in the ME SEASM–SASM cases under a relatively weak sea surface temperature (SST) background, causing a southwest–northeast precipitation dipole with a positive center over the northern Bay of Bengal and a negative center over the eastern Arabian Sea and southern India induced by westerly anomalies with anticyclonic curvature associated with the anomalous heating over the tropical western North Pacific and cooling over the Maritime Continent. While in the MW SEASM–SASM cases, the El Niño–Southern Oscillation–related SST anomalies (i.e., a transition from the equatorial eastern Pacific warming in winter to cooling in summer) are substantial, which cause simultaneous weakening of the first modes of SEASM and SASM through anomalous western North Pacific anticyclone induced by the Pacific SST anomalies and the capacitor effects of other tropical oceans. In addition, intensified diabatic heating over the Maritime Continent driven by the anomalous zonal circulation related to the eastern Pacific cooling can enhance the precipitation over the eastern Arabian Sea and southern India via the westward-propagating Rossby wave. Significance Statement The South Asian summer monsoon (SASM) and the Southeast Asian summer monsoon (SEASM) can be independent from each other at one time but interactive with each other at another time. However, many features of their relationship remain unclear. This study is aimed at better understanding their linkage and the involved physical mechanisms. A robust positive correlation exists between the first modes of SASM and SEASM precipitations, but asymmetric features emerge between the mutually enhanced (ME) and mutually weakened (MW) SEASM–SASM cases. The Pacific–Japan pattern and El Niño–Southern Oscillation play a relatively important role in the ME and MW situations, respectively. A better understanding of the SEASM–SASM linkage can provide useful theoretical basis for improving seasonal prediction of Asian summer climate.