U.S. Dept. of Commerce / NOAA/ OAR / PMEL / Publications

Mechanisms of SST Change in the Equatorial Waveguide during the 1982-83 ENSO

D.E. Harrison

Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington

B.S. Giese

Department of Oceanography, University of Washington, Seattle, Washington

E.S. Sarachik

Department of Atmospheric Science, University of Washington, Seattle, Washington

Journal of Climate, 3(2), 173-188 (1990)
Copyright ©1990 American Meteorological Society. Further electronic distribution is not allowed.

1. Introduction

The 1982-83 El Niņo-Southern Oscillation (ENSO) episode was among the most intense in the historical record and was the best observed, but it is not well understood. While substantial ocean and atmosphere datasets have been collected and studied, many of the most basic questions about the physical processes responsible for the event cannot yet be answered.

Central to the ENSO problem is the evolution of near-equatorial sea surface temperature (SST) before and during the event. All existing coupled ocean-atmosphere models attribute a central role to SST variation, and atmospheric general circulation models have been able to reproduce many aspects of the low frequency ENSO changes when they are forced by imposing observed monthly mean SST fields. Unfortunately, the ocean-atmosphere coupled interaction processes that control SST evolution are among the most difficult to observe because there is great high frequency and small space-scale structure in the various heat equation term fields and because so few observations of the needed data are available.

Understanding the mechanisms of surface warming and cooling in ENSO events is important, both in its own right, and in order to validate coupled ocean-atmosphere models. Several recent coupled model studies seem to be converging on a mechanism for the existence of ENSO events; we need as much information as possible in order to judge the plausibility of this result. Even the most basic qualitative questions have not been answered so far from the observations. For instance, we are not able to determine from observations the extent to which anomalous eastward advection of warm water was remotely or locally forced; or whether anomalous southward advection of warm water from the NECC played any significant role in the warming; or the degree to which variation in the amount of upward advection of cold water played an important role in the major warming of 1982.

Lacking sufficient data, it is necessary to turn to model systems of the individual fluids themselves and explore possible physical scenarios for the event. The work reported here concerns modes of oceanic near equatorial sea surface temperature evolution during 1982-83, using the best available analyses of the surface wind stress field and estimating the surface heat flux using ocean-surface temperature, wind speed and other specified parameters. The ocean circulation model is that described by Philander and Seigel (1985), and will be described below. Because the surface wind is only imperfectly known over most of the tropical Pacific, and because model SST changes will be seen to be very sensitive to the surface wind in these studies, it is difficult to make definitive inferences about the ocean's behavior from model results like these. However, it will be seen that the largest model changes generally are consistent with the largest observed changes and that there is general model hindcast consensus on the mechanisms of warming and cooling over some regions and times, despite the large differences in surface wind stress fields. The present model results can speak to a number of issues of general interest, but authoritative determination of the SST change mechanisms at work in the ocean will not be possible until surface conditions are better known than they were in 1982-83.

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