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.

5. Model mechanisms of SST change during 1982-1983

The ocean model heat equation for the upper level includes all three temperature advection components--zonal, meridional and vertical--as well as a surface heat flux, a vertical diffusion between the first and second model levels, and a horizontal "eddy" diffusion between adjacent grid boxes; if surface cooling produces a gravitationally unstable situation, convective adjustment as it has been done by Bryan and Cox is used to produce a neutral density stratified upper water column. There is no formal model mixed layer. Because the model vertical mixing to some degree minimizes gradients right at the surface, the roles of vertical diffusion and vertical advectioin must be carefully considered. Any process that increases the vertical temperature gradient between the first and second model levels will tend to increase the downward diffusion of heat; upwelling (upward advection of colder water) into the second vertical grid box, or surface warming will each increase the gradient. Although the vertical mixing coefficient increases with decreasing Richardson Number (Pacanowski and Philander 1981), deeper in the water column it is constant between the first and second levels, thus vertical diffusion of heat is determined there by the vertical temperature gradient. Physically correct interpretation of increased vertical diffusion of heat generally requires detailed examination of the model flow changes as well as of the thermal field changes.

Time series plots of the zonal wind stress (multiplied by ten for clarity), the time rate of change of SST, zonal advection of SST and meridional advection of SST are presented for some of the regions and some of the hindcasts, in Figs. 5, 6 and 7. (Note that the units for stress are dyn cm, not C/mo.) The text below will be intelligible without reference to these figures, but they are offered to support the remarks made here. and especially the comments concerning the relative importance of local and remote forcing.

Figure 5. Some SST equation terms for the 155W region, together with the zonal wind stress over this region for the SADLER (a) and FNOC (b) hindcasts. See text for discussion.

Figure 6. As for Figure 5, except the 135W region and the SADLER (a) and ECMWF (b) hindcasts.

Figure 7. As for Figure 6, except the 115W region and the SADLER (a) and NMC (b) hindcasts.

a. The 1982 ENSO warming period

In the 155W and 135W regions each hindcast produced the major warming at a different time between July and December (Fig. 4(a-c; d and e)), but in each case the predominant warming mechanism was zonal advection. Under normal summer conditions when the surface flow is westward and the zonal SST gradient is negative, the zonal advection SST tendency is to cool; however, at some point in this period of 1982, the zonal advection became strongly positive in each hindcast and warming usually closely paralleled the zonal advection change. Eastward surface currents were the cause of the change in zonal advection tendency.

In none of the hindcasts in either the 155W or 135W region was the warming a result of increased surface heat flux. In fact, in almost every instance the surface heat flux was decreasing as the warming took place. Because of the simple parameterization of surface heat flux used in the model no inference concerning the role of surface heat fluxes in the oceanic event can be drawn. The model result does, however, establish that warming comparable to that observed in 1982 can result from purely ocean dynamical processes.

In the 115W region, the hindcasts do not show as much agreement on the mechanisms of surface warming. In the FNOC hindcast positive zonal advection is the initial warming process, but a substantial reduction in cooling from meridional advection and a reduction in vertical cooling from heat diffusion subsequently contribute to maintain the warming. The meridional advection tendency never becomes positive, just less negative (as would be expected from diminished equatorial Ekman divergence resulting from weakening easterly surface wind stress). In the SADLER, ECMWF, and NMC hindcasts the strongest warming periods are dominated by positive zonal advection, but the initial warming tends to involve each of the three mechanisms described above. In the FSU hindcast, instability waves are present until late October 1982 and no dominant mechanism emerges.

In none of the surface wind stress fields used here does the surface stress become westerly in the 115W region during the agreed upon period of warming (Fig. 2). The zonal stress becomes very weak (less than 0.02 N m) in NMC and ECMWF but typically remains greater than 0.04 N m in SADLER and FSU. These stress differences and the large local circulation differences that result account for the different warming mechanisms.

In the 95W region there is no consensus on a dominant warming mechanism; vertical diffusion changes, diminished meridional-advection cooling tendency, increased surface heat flux and changes in zonal advection contribute to varying degrees in each of the hindcasts. Zonal advection change plays a significant role in each hindcast, but it is seldom the dominant process. Meridional advection changes can be as important as zonal advection changes in some hindcasts and are unimportant in others. The wind stress fields differ greatly in the 95W region (Figs. 2, 3), so these differences come as no surprise.

The relative importance of remote and local wind forcing in the advective warming episodes observed here is of interest. Remote forcing would be indicated if the flow were to accelerate eastward while the local zonal stress either remained constant or became more negative; it would also be indicated if the surface zonal pressure gradient were to become more positive while the local stress either remained the same or became less negative. Examination of the surface zonal momentum budgets indicates that remote forcing clearly was a significant factor in the warming in some regions of some of the hindcasts, but was not clearly of importance in others, at least relative to the locally induced surface current change. Examples of remotely forced warming are shown in Fig. 6b (ECMWF, 135W, August-September 1982) and Fig. 7a (SADLER, 115W, November-December 1982). Figure 5a (SADLER, 155W, July-August 1982) shows a case where local and remote forcing appear to be comparable. There simply is no hindcast consensus on the relative importance of the two types of forcing.

Results from the SADLER hindcast are included in Figs. 5, 6, 7 so that the reader can contrast the behavior of the warming in three sequential regions. Note that two episodes of warming occur in the 155W region in July-August 1982, and the other in September-October 1982. In the 135W region there is a significant episode only in September-October 1982. In the 115W region there is modest warming in September-October 1982 and substantial warming in November-December 1982. The earliest mid-Pacific pulse of warming was largely cut off west of 140W because of strong easterly stress east of the date line, with maximum near 150W in July-August 1982 (Fig. 2). The subsequent pulse of warming originated in forcing west of the date line but is substantially modified by local zonal stress changes east of the date line. Smoothing over several months in time, as has been done typically with historical studies of ENSO behavior (e.g., Rasmusson and Carpenter 1982) is likely to obscure important information about the mechanisms responsible for SST evolution if the ocean behaves at all as the model does.

b. The mid-1983 cooling

In the 155W region there is consensus that cooling begins with increased downward diffusion of heat. Examination of the zonal wind stress fields reveals that this takes place concurrently with either the reappearance of easterly stress or with reestablishment of the easterly stress at roughly 0.02 N m. The increased downward diffusion results primarily from an increase in the temperature gradient brought about by subsurface upwelling. Once the cooling has begun both zonal and meridional advection begin to contribute a cooling tendency (Fig. 5) (as they do in the climatological easterly wind stress regime), and the ocean cools until roughly climatological temperatures are reached. In eyery hindcast the surface heat flux is increasingly positive as the cooling takes place.

The situation in the 135W region is similar, except zonal and meridional advection sometimes enter the budgets more quickly than in the 155W region because west of this region the easterlies generally return to significant amplitude first; as a result significant SST gradients exist to be advected against as the local easterly stress increases.

In the 115W region there is hindcast consensus, except for FSU, that increasing easterly stress first caused increased downward heat diffusion, and that cooling from meridional advection followed. The role of zonal advection is not agreed upon. In some hindcasts it also provided cooling, but in others it tended to warm. The different roles of zonal advection result from the initial pattern of increase of easterly stress and the induced equatorial SST gradient; if there is warm water east of this region, zonal advection will provide a warming tendency, and vice versa. The surface heat flux increased throughout the initial cooling phase.

In the 95W region the hindcasts differ on the relative importance of meridional advection cooling and vertical diffusion cooling as the initial primary mechanism. Once cooling begins, meridional advection cooling becomes important in every case. Zonal advection never provides a cooling mechanism.


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