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Maintenance of western North Pacific anomalous anticyclone in boreal summer by wind-induced moist enthalpy advection mechanism


Western North Pacific anomalous anticyclone (WNPAC) is the most important interannual circulation anomalies over the western Pacific warm pool, which can persist from boreal winter to following summer and has great impacts on the East Asian monsoon. The maintenance of the WNPAC in boreal summer was explained by Ekman divergence anomalies over the western North Pacific (WNP) induced by the equatorial easterly Kelvin waves from the tropical Indian Ocean and the Maritime Continent. In this study, we propose a new mechanism for the maintenance of the WNPAC in the season, referred to as “wind-induced moist enthalpy advection” (WIMEA) mechanism. Warm anomalies over the tropical Indian Ocean enhance local deep convection and thus excite atmospheric easterly Kelvin waves to the east. Climatological moist enthalpy decreases eastward over the WNP due to the local northward extension of the intertropical convergence zone (ITCZ) with the establishment of the WNP summer monsoon. Hence, the easterly anomalies transport low moist enthalpy (dry) air westward to the WNP. The induced negative moist enthalpy advection anomalies drive downward motion anomalies and thus suppresses deep convection over the WNP under the constrain of the moist static energy budget balance. This moist teleconnection mechanism does not rely on the Ekman processes. Numerical experiments using a dry linear baroclinic model indicate that the WIMEA and the conventional Kelvin-wave-induced Ekman divergence mechanism has nearly equal relative contributions to the maintenance of the summer WNPAC, with the former mainly driven by the enhanced convection over the tropical Indian Ocean, while the latter by that over the Maritime Continent.