Space-Based Observations of Anomalies in the Equatorial Pacific

by Timothy Liu and Wenqing Tang

From the start of the ocean observing mode on 15 September 1996, the NASA Scatterometer (NSCAT) has observed stronger than normal easterly winds (negative) in the tropical Pacific, which may have piled up surface warm-water in the western Pacific, as reflected in the anomalous high sea level and sea surface temperature observed by the Topex/Poseidon altimeter and the Advanced Very High Resolution Radiometer (AVHRR). At the same time, lower than normal values of the two parameters are observed in the central and eastern Pacific. NSCAT observes a number of episodes of strong westerly wind anomalies (positive). The first two wind anomalies, in the December/January and February/March periods, generate two downwelling Kelvin waves which propagate across the Pacific, as represented by positive sea level anomalies observed by the altimeter. Such strong waves have been absent for more than a year. The waves are followed by anomalous warming in the central and eastern equatorial Pacific and anomalous cooling in the west. Some of the observed conditions are known precursors of El Niño. The mechanism which relates intraseasonal wind anomalies and Kelvin waves to interannual El Niño episodes are not sufficiently known. However, by May of 1997, anomalous warming was experienced through the central and eastern equatorial Pacific. (W. Timothy Liu and Wenqing Tang are scientists at the Jet Propulsion Laboratory of the California Institute of Technology).

Figures are time-longitude variations of the anomalies, along the equator, of (left) zonal wind component observed by NSCAT, (center) sea level observed by the Topex/Poseidon altimeter, and (right) sea surface temperature observed by AVHRR, showing atmospheric forcing and oceanic responses. Climatological seasonal cycles have been subtracted. Longitude (horizontal axis) covers most of the Pacific from Indonesia to the Galapagos; time (vertical axis) starts when NSCAT data became available.