Impact of cross-equatorial Asian winter monsoon and the MJO on extreme precipitation over Western Java Island

摘要

The Indonesian capital of Jakarta experienced an extraordinary heavy precipitation/flood event in the middle of January 2013, as it did in 1996, 2002 and 2007. Heavy rainfall repeatedly occurred in a local area over western Java Island for 4 consecutive days 15-18 January. The central part of Jakarta city was inundated during 17-18 January 2013. This study examined the atmospheric circulations leading to the extreme precipitation event. Examination of ocean surface winds derived from the WindSat satellite measurements showed that a persistent, trans-equatorial, northerly wind took place 2-3 days prior to and during the heavy precipitation event, similar to that which occurred during the extreme precipitation event in Jakarta during late January into early February 2007. The strong, persistent, trans-equatorial Asian-winter monsoonal flow from the Northern Hemisphere was a main factor in the formation of extreme rain over western Java Island, as it induced an intensive low-level wind convergence along its leading edge over the island. In contrast to the extreme rain event of 2007, which was in an inactive MJO phase, the extreme precipitation/flood event of 2013 coincided with strong and coherent MJO activity with the enhanced convective phase centered on the western Pacific. The active phase of the MJO caused strong to moderate westerly to northwesterly winds at the levels from near the surface up to 400 hPa (~8 km) over Java Island in early to mid-January 2013. The westerly winds, in conjunction with the trans-equatorial monsoonal flow, strengthened the low level wind convergence near western Java Island, providing favorable conditions for precipitation. The results of the present study suggest that a persistent, trans-equatorial, northerly wind and the eastward propagation of an active phase of the MJO produced a great effect on the formation of extreme heavy rain over western Java Island.