Surface Layer Temperature Balance in the Equatorial Pacific during the 1997–98 El Niño and 1998–99 La Niña*

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Abstract
The surface layer temperature balance in the equatorial Pacific has been examined for the period of 1996–99 by using data obtained from Tropical Atmosphere–Ocean buoy array at four locations along the equator, namely, 165°E, 170°W, 140°W, and 110°W. Time rate change of SST, horizontal temperature advection, and heat fluxes at the sea surface are estimated directly from the data. Vertical fluxes of heat out of the base of the mixed layer, related to changes in vertical mixing and upwelling, are calculated as a residual. Results indicate that all terms in the temperature balance contributed to SST variations during different stages of the 1997–98 El Niño and the 1998–99 La Niña. Zonal advection was particularly important during the onset of the El Niño in the central Pacific. The inferred vertical heat flux out of the base of the mixed layer was important most of the time, and was especially pronounced in the eastern Pacific during the termination phase of the El Niño. Meridional advection due to eddies was a negative feedback on SST change in the eastern Pacific during both the El Niño and La Niña, tending to counteract the development of warm and cold anomalies. Likewise, the net surface heat flux generally represented a negative feedback, tending to damp SST anomalies created by ocean dynamical processes. The relevance of these results to recent theoretical developments concerning ENSO timescale physical processes is discussed.