Wind Stress over the Arabian Sea from Ship Reports and Seasat Scatterometer Data

Abstract

Seasat scatterometer data over the Arabian Sea are used to build wind stress fields during July and August 1978. They are first compared with 3-day wind analyses from ship data along the Somali coast. Seasat scatterometer specifications of 2 m s?1and 20° accuracy are fulfilled in almost all cases. The exceptions are for winds stronger than 14 m s?1, which are underestimated by the scatterometer by 15%. Wind stress is derived from these wind data using bulk formula with a drag coefficient depending on the wind intensity. A successive correction objective analysis is used to build the wind stress field over the Arabian Sea with a 2°?2°and 6 day resolution. The final wind stress fields are not significantly dependent on the objective analysis because of the dense coverage of the scatterometer. The combination of scatterometer and coastal ship data gives the best coverage to resolve monsoon wind structures even close to the coast. The final wind stress fields show wind features consistent with other monthly mean wind stress fields. However, a high variability is observed on the 6-day time scale. In particular, a monsoon break is present on 16 July with a minimum wind stress intensity in the southern part of the Arabian Sea and and a drastic decrease over the whole region. Compared to climatology, the southwest monsoon is stronger and lasts longer in 1978: no clear decline is observed, but alternating strong increases and decreases in wind intensity takes place until the end of August. Scatterometer data are powerful to monitor such rapid and extensive wind variations.