Multi-scale climate processes and rainfall variability in Sumatra and Malay Peninsula associated with ENSO in boreal fall and winter

摘要

The impact of the El Nino-Southern Oscillation (ENSO) on seasonal rainfall variability over Sumatra and the Malay Peninsula (SMP) has been investigated from the perspective of multi-scale physical processes, especially the diurnal cycle and daily weather types (WTs). Observations indicate that the rainfall variability associated with ENSO is not spatially coherent in the SMP in both September-November (SON) and December-February (DJF). Among the five WTs, there are more WT2 and fewer WT3 in the SON period of El Nino years, thus the seasonal rainfall anomaly pattern is similar to that of WT2, with wet anomalies in the western Malay Peninsula and northern Sumatra, and dry anomalies in southern Sumatra. In DJF of El Nino years, there is an area of wet anomalies in the southern tip of the Malay Peninsula and in central Sumatra, and slightly dry anomalies in the central Malay Peninsula and in northern Sumatra due to more days of WT4 and fewer days of WT3 and WT5. In the early northeast monsoon season (December-January) when the Inter-Tropical Convergence Zone (ITCZ) is still north of or nearby the equator, the El Nino-enhanced easterly wind anomaly at 850 hPa favours more frequent WT4 with more zonally oriented east-northeasterly winds. This facilitates the propagation of rainfall from the area of above-normal SSTs in the southern South China Sea towards Singapore and the adjacent seas, thus producing above-normal rainfall. In February, however, more frequent WT4 will not enhance rainfall in the vicinity of Singapore because the ITCZ has already moved to the Southern Hemisphere, and the upstream area in the South China Sea is quite dry.