85th AMS Annual Meeting

Thursday, 13 January 2005: 1:45 PM
Role of TRMM daily rain forcing of the Indian Ocean onto simulated intraseasonal-to-interannual climate changes in the tropics.
C. Perigaud, California Institute of Technology/JPL, Pasadena, CA
This study relies on the results of a previous study of the role of the seasonal-to-interannual rain and wind forcing of the Indian Ocean onto the Monsoons and ENSOs simulated by a coupled ocean/atmosphere/land model, using monthly averaged FSU winds and GPCP rains between 1980 and 2000. This new study is now focused on the role of the daily wind and rain changes on top of the seasonal-to-interannual forcing, using QuikSCAT wind and TRMM rain daily data between July 1999 and December 2000. First, daily data are analyzed to identify a series of Westerly Wind Bursts and Madden Julian Oscillations which show up in the (7-to-60 days) range over the Indian Ocean. Then, the TRMM and QuikSCAT daily variations are added to GPCP and FSU to force the Indian Ocean model and study the impact of high-frequency events onto this ocean. Using the sea level data provided by TOPEX-Poseidon, it is found that the simulated response of the Indian ocean is clearly improved by the use of daily forcing. The oceanic response is shifted towards the intraseaonal range (30-to-90 days). This shift is explained by the asymmetry of the ocean response between rain excess and rain deficits. It is then found with the atmospheric model that the MJOs are sensitive to the Indian SST changes resulting from these nonlinear processes. Finally, these results are used in the tropical Pacific model component. When forced by FSU winds, the model simulates ENSOs that are not sensitive to MJO-s in the western Pacific. But coupling the Pacific ocean to the atmosphere allows to demonstrate a great sensitivity of ENSO to MJO-s. This sensitivity is not a permanent feature of the coupled system, it is highly dependant on the combination between the intraseaonsal and the decadal changes of the ocean and the atmosphere.

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