18 Impact of Cross-equatorial Asian Winter Monsoon and the MJO on Extreme Precipitation over Western Java Island Peiming Wu 1 *, Ardhi Adhary Arbain 2 , Shuichi Mori 1 , Jun-ichi Hamada 3 , Miki Hattori 1 , Fadli Syamsudin 2 , Jun Matsumoto 1,3 and Manabu D. Yamanaka 1 1 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan, 2 Agency for the Assessment and Application of Technology (BPPT), Indonesia, 3 Tokyo Metropolitan University, Japan * Correspondence to: pmwu@jamstec.go.jp Keywords: monsoon, heavy rainfall, the MJO 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 [1]. 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 [2]. 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 (Fig. 1). 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. References [1] Wu, P., M. Hara, H. Fudeyasu, M.D. Yamanaka, J. Matsumoto, F. Syamsudin, R. Sulistyowati, and Y. S. Djajadihardja, 2007: The impact of trans-equatorial monsoon flow on the formation of repeated torrential rains over Java Island. SOLA, 3: 93−96. [2] Wu, P., A.A. Arbain. S. Mori, J. Hamada, M. Hattori, F. Syamsudin, and M.D. Yamanaka, 2013. The effects of an active phase of the Madden-Julian Oscuillation on the extreme heavy precipitation event over western Java Island in January 2013. SOLA, 9: 79-83. brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by JAMSTEC Repository