Observed Rainfall Trends over Singapore and the Maritime Continent from the Perspective of Regional-Scale Weather Regimes

Abstract

Straddling the Asian?Australian monsoon region, the Maritime Continent (MC) experiences substantial rainfall variations from diurnal to interannual and longer time scales. In this study, rainfall over Singapore and the wider MC region are analyzed using objectively identified weather regimes. Eight regional-scale weather regimes are derived by k-means clustering of local vertical profiles of zonal and meridional winds, temperature, and specific humidity extracted over Singapore from ERA-Interim data for the period December 1980?November 2014. The composite synoptic flow and rainfall patterns over the region show that the weather regimes correspond to the seasonal migration of the intertropical convergence zone (ITCZ) across the equator. For Singapore, the regimes depict seasonal rainfall variability by capturing the alternating dry and wet phases of the prevailing local monsoon and transition periods associated with the regional-scale ITCZ movement. Following previous work, the regimes are used to examine the annual rainfall trend by calculating the contributions due to 1) changes in regime frequency, indicating regional-scale circulation changes, and 2) changes in within-regime precipitation, indicating altered thermodynamic conditions. The overall trend observed at Singapore and many other MC locations is overwhelmingly due to changes in within-regime precipitation. However, the overall trend masks the larger contribution resulting from regime frequency changes as these circulation changes tend to offset one another in reality. In many MC areas (including Singapore), summed rainfall changes due to regime frequency changes outweigh those due to changes in within-regime rainfall, when aggregated in an absolute sense.