Subject: APCC Seminar

1. Presenter: Prof. Bin Wang, IPRC, USA

2. Title: Why has Arabian Sea storm intensified recently?

3. Time: 11:00 a.m. on March 14, 2012

4. Place: The Edelweiss room on the second floor, APCC

5. Abstract

Tropical cyclones (TCs) over the Arabian Sea in pre-monsoon season (May and June) have intensified since 1997 primarily due to a significant reduction in storm-ambient vertical wind shear (VWS), which was attributed to the dimming effect of increasing anthropogenic black carbon and sulfates emissions. The climate model-simulated aerosol-induced VWS reduction, however, is moderate (~1 m/s over 30-years) and marginally significant. We show that the TC intensification is due to advanced TC occurrences, which result in a marked reduction of the VWS that is five times larger than the aerosol-induced change. The earlier TC occurrence is a consequence of advanced summer monsoon, which is evidenced by a significant trend in the 850 hPa relative vorticity, rainfall and westerly wind. The earlier development of summer monsoon is caused by enhanced land-ocean thermal contrast between South Asian landmass and Indian Ocean. Although the root cause of the increased land-ocean thermal contrast remains elusive, we contemplate that both internally-generated interdecadal variation and anthropogenic forcing-induced warming may be contributing factors.