Seasonal to interannual modes of sea level variability in the western Pacific and eastern Indian Oceans

Almost six years of TOPEX/Poseidon altimeter data were analyzed to discern low frequency (monthly to interannual) variability in the region from the eastern Indian Ocean east to the dateline. An empirical orthogonal function (EOF) analysis shows that from September 1992 through April 1998 the dominant mode of sea level variability in the western Pacific/eastern Indian Ocean is on the ENSO time scale, and annual variability is secondary. The amplitude of the annual cycle becomes even less in the latter part of the time period (in the years prior to the 1997/98 El Nino event). The pattern that emerges on interannual time scales is one of western Pacific warm pool buildup (during non-El Nino years) and discharge (during El Nino years). Mixed with these signals is a large amplitude signal in the eastern Indian Ocean, south of the equator (off the south coast of Sumatra), which has both annual and interannual variations associated with it. Harmonic analysis shows the annual cycle is dominated by Rossby waves in both the equatorial Pacific and at higher latitudes in the eastern Indian Ocean.