Impact of Madden-Julian Oscillation on Philippine Rainfall during Boreal Winter

Intraseasonal variability in the tropics is modulated by Madden-Julian oscillation (MJO), a large-scale convective disturbance which propagates eastward across the equatorial Indian Ocean and western-central Pacific at a rate of 5 ms^-1. Impact of MJO on Philippine rainfall during boreal winter of 1987-2017 was studied using composite analysis of rainfall, convective and circulation anomalies. MJO events has been defined using the RMM index. Compositing analysis used strong MJO events (amplitude greater than 1) to obtain the mean for each MJO phases. Normally, Philippines experiences a northeasterly wind with convection mostly occurring in the Eastern Philippines. High amount of rainfall is experienced in the windward direction while places in the leeward direction receives no or less rainfall. However, rainfall was found to deviate from normal winter values coherent with the MJO phases, with generally negative anomalies during phases 1,2,3,7 and 8 (MJO convective center in Indian Ocean and western Hemisphere) and positive anomalies during phases 4-6 (MJO convective center in Maritime Continent and Western Pacific). The greatest impact of MJO occurred in type I and III climate regions (dry climate during boreal winter) which experienced enhanced (suppressed) rainfall during rainy (dry) phases of MJO. MJO's influence on rainfall can be explained not just by the direct influence of tropical convective anomalies but also be the result of enhancement (suppression) of the winter monsoon, induced anomalous low (high) sea level pressure, rising (sinking) air and troughs (ridges). "Active" (days of strong MJO events with positive rainfall anomaly) and "break" (days of strong MJO events with negative rainfall anomaly) during ENSO years has also been studied. There was observed higher number of break (active) days during El Nino (La Nina) and also that several non-typhoon associated floods coincided with MJO rainy events.