The variation of the Walker circulation is closely related to the onset of El Niño-Southern Oscillation (ENSO) events [Kousky, 1984], so observing the Walker circulation can lead to better understanding of ENSO events. Wind data from the NASA Scatterometer (NSCAT), Quick Scatterometer (QuikSCAT), and European Remote Sensing Satellite-1/2 (ERS-1/2) were merged into one data set in a 1°×1° grid covering a period from January 1992 to December 2000. The study region was confined to the tropical oceans between 20°S and 20°N. The vector empirical orthogonal function (VEOF) method [Hardy and Walton, 1978] was used to process the wind data set and extract principal modes of the wind fields.
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