Intraseasonal Variability in Tropical Surface Wind Field using Satellite Scatterometer Data

Satellite scatterometer data(ERS-2 and Qscat/SeaWinds) are used to construct gridded data set of surface wind/wind-stress vectors over the tropical Pacific and Indian Oceans and to investigate intraseasonal variabilities in the tropical wind field. Preliminary analyses by inter-comparison between our data set and in-situ data measured at the TAO buoys exhibit that our data set has a good reliability in the tropical region.

Our wind product by the ERS-2 data exhibits that some events of strong westerly wind anomalies covering the western Pacific and Indian Oceans occurred in October 1996 to September 1998. Spectral features for the zonal wind time series in these areas reveals high energies in the intraseasonal period band between 30 and 90 days, suggesting that the prominence of strong westerly wind are related to the enhancement of intraseasonal signal. In band-pass filtered (20-100 days) field, dominant area of the intraseasonal signals covers a wide area between 60E and 160W in the equatorial zone, and often shifts in the zonal direction along the equator. As similar to other studies, eastward migration of the dominant area is found in the end of 1996 to 1997, which might be related to the onset of 1997-98 El Nino event. Events of the westward signal migrations are also found in September to December 1997. The Complex Empirical Orthogonal Function(CEOF)s are calculated for the band-passed field of the zonal winds along some equatorial latitudes. Result reveals that the different zonally-migrating features can be decomposed into the lowest two CEOF modes; the first and second modes have characters of eastward and westward, respectively, phase lags.

A similar approach is made also for the wind product by the Qscat/SeaWinds data in August 1999 to September 2000. It is suggested that the two types of intraseasonal signals having different zonal structures are prominent in some years.