The influence of boreal summer intraseasonal oscillations on precipitation extremes and their characteristics in Southeast Asia

Abstract

The regional characteristics of the boreal summer intraseasonal oscillations (BSISO) over southeast Asia are presented. The northeastward transition of the BSISO is characterised by 4 phases, such that convection is enhanced over the Philippines and Indochina in phase 1 and suppressed over Peninsular Malaysia, Borneo and Java. The opposite is true in phase 3. The role of BSISO in modulating extreme precipitation is highlighted, showcasing how its phases impact both the frequency and intensity of extreme precipitation events across the region. Using a method to detect and characterise precipitation features in terms of precipitating areas and their associated object properties, this study shows distinct shifts in precipitation regimes during different phases of the BSISO. Phase 1 exhibits increased large-scale convective activity, particularly affecting regions like the South China Sea and northern Philippines, linked to increased tropical storm frequency but reduced localised extreme precipitation events. In contrast, Phase 3, with active convection over Peninsular Malaysia, Borneo and Sumatra, shows intensified extreme precipitation from smaller to medium-sized areas. BSISO phases also modify the distribution of small and large precipitation objects over land, ocean, and coastal regions. This classification of precipitation regimes provides detailed insights into how the BSISO’s large-scale envelope modifies regional precipitation extremes through various precipitation properties. This information could benefit probabilistic predictions of regional extreme precipitation events at subseasonal time scales.