Development of Synoptic-Scale Disturbances over the Tropical Western North Pacific Associated with the Boreal Summer Intraseasonal Oscillation and the Interannual Pacific–Japan Pattern

Abstract

The development mechanism of synoptic-scale disturbances over the tropical western North Pacific (WNP) associated with the boreal summer intraseasonal oscillation (BSISO) under different phases of the interannual Pacific–Japan (PJ) pattern is investigated. Intraseasonal convection is enhanced widely over the WNP for BSISO phases 5–7 in the positive PJ years, when seasonal-mean convective activity over the tropical WNP is enhanced. By contrast, developed convection is confined over the South China Sea in the negative PJ years. Similar features are also found in the horizontal distributions of eddy kinetic energy (K′) representing the activity of synoptic-scale disturbances and of tropical cyclone occurrences. The differences in location of intraseasonal convection and the activity of synoptic-scale disturbances between the positive and negative PJ years likely lead to different teleconnections to midlatitude East Asia. The non-PJ years exhibit mixed features of the positive and negative PJ years. A K′ budget analysis reveals that the energy conversion from eddy available potential energy to K′ (PeKe) associated with synoptic convection primarily contributes to K′ generation during the convectively active phases of the BSISO. The barotropic energy conversion from mean kinetic energy to K′, KmKe, is determined to be the second largest contributor to the K′ increase in the lower troposphere, especially during the early stage of development of synoptic disturbances. Large K′ produced by PeKe and KmKe in the tropics is advected to the subtropics by the mean flow in the late to mature stages of development. There are two factors that can determine the different locations where synoptic disturbances develop in association with PeKe and KmKe. One is intraseasonal sea surface warming during convectively suppressed phases of the BSISO preceding the active phases. The other is convergence or shear of seasonal-mean horizontal winds associated with interannual fluctuations of monsoon westerlies over the WNP.