Interannual Variation of the Winter Rainfall in Malaysia Caused by the Activity of Rain-Producing Disturbances

Abstract

Abstract Heavy rainfall/flood (HRF) cyclones contribute close to two-thirds of the total rainfall PT in both parts of Malaysia [peninsular Malaysia (M) and west Borneo (B)]. Judging by the rainfall variance produced by these cyclones and its correlation (~0.9) with the interannual PT variation, this variation is caused primarily by HRF cyclones through two factors: 1) their westward propagation properties and 2) their rain-producing efficiency. The former is regulated by the change of the cyclonic shear flow around the near-equator trough, while the latter is determined by the change of the convergence of water vapor flux toward tropical Southeast Asia. During November–December of cold (warm) ENSO phases, the westward propagation of the cyclone's parent cold surge vortices (CSVs) from the Philippine (P) vicinity (Borneo) to peninsular Malaysia CSVPMs (CSVBMs) and intensified (weakened) convergence of water vapor flux toward tropical South/Southeast Asia act to enhance (reduce) the rain-producing efficiency of HRFPM (HRFBM) cyclones. During winter cold (warm) phases, the deepening (filling) of the near-equator trough crossing west Borneo allows some CSVs formed/trapped in Borneo CSVBBs to develop into HRFBB (HRFBBM) cyclones (to propagate westward to peninsular Malaysia). The rain-producing efficiency of HRFBB and HRFBBM cyclones is also increased (reduced) by the intensified (weakened) convergence of water flux toward tropical South/Southeast Asia. Interannual variations of both PT(M) and PT(B) caused by the impacts of the circulation pattern changes on occurrences of HRFPM and HRFBB/HRFBBM cyclones, respectively, and their rain-producing efficiency may pose a new challenge to simulate the weather–climate relationship in climate modeling.