Wintertime wind forcing and sea surface cooling near the south India tip observed using NSCAT and AVHRR

This report addresses a case of topographic air-sea interaction in the Gulf of Mannar, near the Indian tip, for the winter monsoon of 1996-1997. Using high spatial resolution NASA-Scatterometer (NSCAT) wind data, a 1 degrees x1 degrees region of strong wind is identified in the Gulf during the peak of the winter monsoon. The characteristic topography of South India and Sri Lanka and their orientation to the monsoon wind tend to channel this stron g wind. Air-sea heat flux analyses, using the NSCAT wind and the European C enter for Medium-Range Weather Forecast objective analyses surface data, re veal that the strong winter monsoon bursts cause large latent heat loss (18 0 W/m(2)) from a wind-fetch region centered on 7.5 degrees N, 77.5 degrees E during January. Weak air-sea temperature gradients result in weak sensibl e heat loss (<15 W/m(2)) from the ocean. The ocean response to this forcing is examined using weekly and monthly mean satellite-derived sea surface te mperature (SST) maps; these indicate a sea surface cooling of about 1 degre es C along the axis of the wind flow. A by-product of this cooling is an em ergence of a SST front along the periphery of the strong wind-stress region . Time-series analyses of the surface meteorology reveal that this forcing has a periodicity of about 15 days, with a peak during the last week of Dec ember. Wind stress curl derived from the NSCAT wind data exhibits high nega tive values (anticyclonic Ekman pumping) on a 2-week time scale, concomitan t with strong wind bursts. The features observed in the Gulf of Mannar are similar to those reported form some other locations. (C) Elsevier Science I nc., 2000.