Regional features of Boreal summer intraseasonal variability and their relationship with rainfall extremes over Southeast Asia

Abstract

A regionally focused index of Boreal summer intraseasonal variability (RISV) modes for southeast Asia is used to study their impact on regional rainfall extremes. Four phases to characterise the northward/northeastward propagation of active and suppressed convective regions are defined, which show a clear and distinct impact on the mean and extreme precipitation over various sub-regions of Southeast Asia. Northward propagation of convection is strongly coupled to wind convergence at lower atmospheric levels. A threshold-based precipitation feature detection method has been used to describe the changes in precipitation regimes contributing to rainfall extremes. During the convectively active (suppressed) phases over the region, most of the precipitation is linked to enhanced (suppressed) widespread large-scale convective systems and suppressed (enhanced) isolated smaller-scale intense convective systems. Strong links between the slow-evolving RISV and rainfall extremes suggest that improved prediction of the likelihood of rainfall extremes may be feasible beyond the weather prediction time scales.