Diurnal Cycle in Different Weather Regimes and Rainfall Variability over Borneo Island Associated with ENSO

The interannual variability of precipitation over Borneo Island in association with El Niño-Southern Oscillation (ENSO) has been studied by using the GPCC gridded rain gauge precipitation, the CMORPH satellite estimated precipitation, the QuikSCAT satellite estimated sea winds and the NCEP-DOE reanalysis data. Analysis of the GPCC precipitation shows a dipolar structure of wet southwest versus dry central and northeast in precipitation anomalies associated with El Niño over Borneo Island during the austral summer (December to February). By using the 0.25°-degree and 3-hourly CMORPH precipitation, it is found that rainfall over Borneo is strongly affected by the diurnal cycle of land-sea breezes. The spatial distribution of rainfall over Borneo depends on the direction of monsoonal winds. Weather typing analysis indicates that the dipolar structure of rainfall anomalies associated with ENSO is caused by the variability in the frequency of occurrence of different weather types. Rainfall is enhanced in the coastal region where sea breezes head against off-shore synoptic-scale low-level winds, i.e., in the lee side or wake area of the island, which is referred to here as the "wake effect." In the December-February of El Niño years, the northwesterly austral summer monsoon in South Borneo is weaker than normal over the Maritime Continent and easterly winds are more frequent than normal over Borneo, acting to enhance rainfall over the southwest coast of the island. This coastal rainfall generation mechanism in different weather types explains the dipole pattern of a wet southwest versus dry northeast in the rainfall anomalies over Borneo Island in the El Niño years.