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.

6. Summary and discussion

The 1982-83 ENSO SST changes were among the largest in the historical record but their detailed structure was not well observed. Comparison of several different monthly averaged SST datasets indicates that only when SST changes exceed 1 C/mo is agreement on the sign of SST change likely to be found; quantitative agreement at the level of 0.5C/mo is almost never found. However, the SST fields agree that there was warming east of the date line sometime between July and December 1982; the more eastern the region, the later the warming began, and the later it ended. There is also agreement that there was strong cooling in June-July 1983, which represented the end of this warm event. There is no consistent consensus on the behavior of SST in the two months prior to June 1982, or on the months between January and June 1983; only the major changes have reasonably clear signals.

The ocean circulation model hindcasts produced net warming over the period June-December 1982 in each of the regions east of the date line, but they differed significantly with each other on the timing of the warming. Also, in any particular region, model warming occurred more abruptly than is indicated by the SST analyses, and the total warming was somewhat greater than observed; plausible reasons for these differences will be discussed below. The main model cooling period fell within May-July 1983, in each hindcast and for each region, again in qualitative agreement with observations.

Examination of the model SST equation balances reveals that the central Pacific 1982 warming occurred primarily because the surface currents became eastward, causing the SST tendency due to zonal advection to become positive. The physics of the surface current reversal is not the same in each hindcast, but in general it involves a combination of some remote forcing and some local response to diminishing local easterly trades. Neither surface heat flux changes nor meridional advection changes play a significant role in this part of the warming.

The differences in the timing of the warming in the different hindcasts result clearly from the differences in the wind fields. The spatial character of the appearance of westerly winds in the western Pacific, their subsequent evolution, and the evolution of the central Pacific easterlies all contribute to creating different surface current fields in the model hindcasts. Different surface current fields produce different SST advection tendencies. It should be noted that the magnitude of the eastward surface flows that cause most of the warming is generally consistent in each hindcast with that observed at 160W by Firing et al. (1983). The rapid model warming results from the fact that the initial model zonal SST gradient is larger than indicated in the SST analyses (Harrison et al. 1989), and that we have presented model rates of change from three-day snapshots, rather than from monthly means.

In the more eastern regions there is less consensus on the mechanisms of warming; meridional advection changes can be nearly comparable in importance to zonal advection changes. However, in every case the zonal advection tendency became positive, while the meridional advection tendency only very seldomly became weakly positive. In general, it remained negative at least through the main period of warming.

Thus the model hindcasts indicate that the very warm surface waters of the NECC were not an important factor in near equatorial warming. When meridional advection does play a significant role, it appears more in the form of reduced negative tendency that results from reduced meridional flow, that in turn results from reduced wind-driven Ekman divergence.

The main 1983 cooling begins in each hindcast in the central Pacific where the easterlies first return or regain an amplitude of about 0.02 N m, through increased downward diffusion of heat out of the surface grid box. This increased diffusion results from an increase in the temperature gradient below the surface resulting from upwelled colder water. Of course, the upwelling is simply forced by the surface Ekman divergence required by the easterly stress.

Once the surface begins to cool anywhere, significant SST gradients are quickly established and the surface current field subsequently plays a major role in cooling the rest of the central and eastern basin. The relative importance of meridional advection, zonal advection and increased downward diffusion (i.e., increased upwelling caused by increased local easterly stress), varies greatly from hindcast to hindcast.

Little can be said about SST change mechanisms west of the date line; the various SST analyses differ greatly about the patterns of observed SST change, and the hindcast results also show great diversity of behavior. At least from the large-scale perspective adopted here, the SST changes are too small and the differences between wind stress fields too large to permit useful inferences to be made.

As noted in section 5, surface heat fluxes in the hindcasts almost always provide a negative feedback process for SST. The model surface heat flux parameterization is very simple; only variability of the latent and sensible heat flux components is included, and their variability is limited by assuming a fixed relative humidity and air-sea temperature difference. As there are large cloudiness changes in the western and central Pacific during ENSO events, and as cloudiness can substantially alter the short- and longwave radiation flux components, the model parameterization is unlikely to be faithful to reality. In fact, the present parameterization appears to introduce too much heat into the ocean when the winds are light and SST is high; maximum model SST values can be 32C when observations suggest that 30C is seldom exceeded. However, it has been shown that the major signals of the 1982-83 event can be reproduced in the model without recourse to more sophisticated surface heat flux parameterizations. The result of Reed (1986), that the surface data available to him suggest that heat fluxes estimated from bulk formulae are either uncorrelated or weakly negatively correlated with SST changes in the central and eastern equatorial Pacific in both the 1972 and 1982 ENSO events, should be noted; this type of behavior is not inconsistent with the behavior found in the model results. Meyers et al. (1986) have suggested that the western Pacific cooling observed from XBT data around 160E, roughly between July 1982 and February 1983, arose largely from an increase of latent heat flux cooling; we do not find consistently such behavior in the hindcasts, but note that because the model parameterization includes a background level of evaporative cooling it is not well suited to produce such behavior.

Because near equatorial surface current variations are so large during ENSO events and because SST gradients generally are large in the eastern central and eastern Pacific, it should not be a surprise that horizontal advection of SST plays such a dominant role in the warming process. Wherever there are climatological surface easterly stresses, the surface ocean is forced into a state much colder than would otherwise exist, and one which is maintained substantially by vertical and horizontal advective processes. Any diminution of these easterlies will permit the system to warm, whether by local processes (reduced upwelling, reduced zonal and meridional advection cooling) or by remote processes (e.g., Kelvin wave induced warming). Harrison and Schopf (1984) have previously pointed out the possibility that remotely forced anomalous zonal advection could have accounted for much of the observed central and eastern Pacific 1982 warming, while Philander and Seigel (1985) noted that anomalous zonal advection resulting from local wind stress changes appeared to be a major element of the 1982 warming in their hindcast (which used the NMC stresses). The hindcasts presented here suggest that both local and remote forcing played a role in the main 1982-83 warming, but there is much disagreement on the specifics of the warming in any given region; the wind stress differences are so large as to produce quite different surface current fields and, therefore, quite different patterns of horizontal advection of SST.

The problem of detailed model-data comparison with respect to upper ocean temperatures, was considered by Harrison et al. (1988, 1989). None of these hindcasts yielded what they define to be "quantitative hindcast skill" for SST. However, the range of hindcast SST values almost always bracketed the XBT observations that provided the validation dataset. Although the differences are larger than desired, no systematic inconsistency in model behavior is found. From this we infer that the hindcast errors introduced by the uncertainty in the imposed surface wind stress and heat flux fields must be reduced before more thorough model validation can be undertaken.

Of necessity, we have here confined our attention to the large amplitude periods of the 1982-83 ENSO event; the surface data are not adequate for discussion of the smaller amplitude periods. More, and better, SST data are needed if we are to study the small amplitude SST change periods of ENSO events. From this work, it appears that data adequate to get reliable rates of change of SST at the level of 0.5C/mo are needed to study the small amplitude phases of ENSO events (or, it must be noted, the annual cycle of SST). These periods may be the crucial ones for validation of coupled ocean-atmosphere models because all existing ocean and atmosphere models give similar behavior in response to the anomalies that exist in the large amplitude phases of ENSO events. It is understanding the transitions between normal behavior and ENSO behavior that will bring the ability to predict ENSO events.

Acknowledgments. The programming assistance of Steve Hankin, Mark Verschell and Jerry Davison is gratefully acknowledged. The hindcasts were performed on the CYBER 205 at the NTIS/ERL CS facility. Financial support from the NOAA/EPOCS program and from Dr. E. Bernard, Director NOAA/PMEL is acknowledged with thanks. This is PMEL publication No. 1052.

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