Ekman Et Al 1983

[Blanche Bunnell]

Fourdifferent datasets of monthly mean near-equatorial Pacific sea surfacetemperature for 1982-83 are compared, and the space-time regions for which there wasconsensus that cooling or warming took place, are determined. There was consensus thatwarming took place east of the date line, averaged over the period July-December 1982, andthat the warming progressed eastward from the central Pacific. There was also consensusthat weak cooling took place in April 1983, and that substantial cooling occurred inJune-July 1983, generally over the central and eastern Pacific. However, the analyses tendto agree on the sign of SST change only in periods of cooling or warming in excess of1C/month; quantitative agreement at the level of 0.5C/month or better is almost neverfound.

SST changes in five ocean-circulation model hindcasts of the 1982-83 period(differing only in that each used a different analyzed monthly mean surface wind stressfield to drive the ocean), are compared with the observations and with each other. Thereis agreement that net warming occurred in the July-December 1982 period and cooling inmid-1983. The heat budgets of these experiments indicate that the major model centralPacific warmings occurred primarily from anomalous eastward surface advection of warmwater. Further, east zonal advection remains significant, but a diminished coolingtendency from meridional advection can also be important; different hindcasts differ onthe relative importance of these terms. Surface heat flux changes do not contribute to thewarmings. The reduced cooling tendency from meridional advection is consistent withdiminished surface Ekman divergence, suggesting that southward transport of warm northequatorial counter current water was not a major factor in the model warmings. Thehindcasts do not agree on the relative importance of local or remote forcing of theeastward surface currents; while there is clear evidence of remote forcing in somehindcasts in particular regions, local forcing is also often significant. The main 1983midocean cooling began because of increased vertical advection of cool water; but oncecooling began horizontal advection often contributed. Further cast, where the easterliesgenerally return later than they do in midocean, upwelling and horizontal advection allcan be important. Again no model consensus exists concerning the details of SST evolution.

Because the observations do not agree on the sign of SST change during much of the1982-83 period, improved SST data is needed in order to document the behavior of the oceanthrough future ENSO periods. Better forcing data will be needed to carry out improvedocean-model validation studies, and to explore the mechanisms likely responsible for SSTchange through entire ENSO cycles.

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

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 attributea central role to SST variation, and atmospheric general circulation models have been ableto reproduce many aspects of the low frequency ENSO changes when they are forced byimposing observed monthly mean SST fields. Unfortunately, the ocean-atmosphere coupledinteraction processes that control SST evolution are among the most difficult to observebecause there is great high frequency and small space-scale structure in the various heatequation term fields and because so few observations of the needed data are available.

Understanding the mechanisms of surface warming and cooling in ENSO events isimportant, both in its own right, and in order to validate coupled ocean-atmospheremodels. Several recent coupled model studies seem to be converging on a mechanism for theexistence of ENSO events; we need as much information as possible in order to judge theplausibility of this result. Even the most basic qualitative questions have not beenanswered so far from the observations. For instance, we are not able to determine fromobservations the extent to which anomalous eastward advection of warm water was remotelyor locally forced; or whether anomalous southward advection of warm water from the NECCplayed any significant role in the warming; or the degree to which variation in the amountof 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.

What do we know about how tropical Pacific SST changed during 1982-83? Fields of monthly mean SST have been obtained from Fleet Numerical Ocean Central ("spot obs", made available to us by Scott Woodruff), the NASA THEP (TOGA Heat Exchange Project) fields (made available by Tim Liu), and the NOAA Climate Analysis Center (CAC) "in situ" and "blended" analyses (made available by Dick Reynolds). The FNOC and NMC "in situ" fields were based primarily on surface marine observations, while the NMC "blend" product made considerable use of satellite (MCSST) SST spatial gradient information, together with surface data. The THEP product was based primarily on satellite (SMMR) data. The large-scale pattern of SST evolution during 1982-83, according to the NMC "blend" analysis, has been presented in a number of places (e.g., Rasmusson and Wallace 1983), and is assumed to be familiar to the reader. This event differed from the composite post-1950 ENSO event described by Rasmusson and Carpenter (1982) in several respects: there was no significant warming along the South American coast in Spring 1982; the first substantial warming occurred along the equator in late Summer 1982; substantial warm anomalies appeared along the South American coast late in 1982, and the midocean remained significantly warmer than usual into late Spring 1983. Our interest is to understand the mechanisms responsible for these changes.

The heat equation relates the time rate of change of SST to the various advective,diffusive and forcing processes, so it is necessary to concentrate not on the patterns ofSST, but of SST change from month to month. In order to limit this discussion, theequatorial Pacific has been divided into five regions, each extending from 2N to 2S,and across 10 degrees of longitude in zonal extent. Region center longitudes are 95W,115W, 135W, 155W and 165E. Forward time differences of SST from each SST datasetaveraged over each region are displayed in Fig. 1(a-c; d and e); climatological SST changes, based on the post-1950surface marine data, are also included for reference.

Figure 1. Monthly average rates of change of SST for the five analysis regions,according to the NMC "in situ," NMC "blend," FNOC spot obs, and NASATHEP SST datasets (see text for more information on these SST fields, and for discussion).Note that the different fields generally differ on the sign of the month-to-month change,except when the rate of change exceeds roughly 1 degree C per month.

In the 95W region the climatological cycle of cooling between March and September,followed by warming between October and February, is followed roughly during 1982; thereis less cooling in May, July and August 1982 than is inicated in the climatology, andthere is warming instead of cooling in September 1982. Nineteen hundred and eighty-threewas also similar in pattern to climatology until July, when much stronger than normalcooling took place; this cooling was sufficiently strong to bring SST values back tonormal. These is consensus among the SST datasets that cooling took place in March, Mayand June 1982 and April, June and July 1983; there is consensus that warming took placebetween October 1982 and March 1983.

In the 115W region, where there are fewer surface observations, the pattern of cooling and warming is similar to that in the 95W region, but with more month-to-month variability and generally with smaller values. The climatological cycle is also noisier than at 95W. The strongest warming that is agreed upon occurs in September 1982; the strongest cooling was in May and June 1983. Both are sharp departures from climatological behavior. These departures are consistent with the near surface 110W mooring data reported by Halpern (1987).

At 135W the normal seasonal cycle pattern roughly holds until around late August 1982when there is arguably a two month period of substantial warming. (Two of the analysesshow very anomalous August 1982 warming, while the others show no temperature change; allshow subsequent warming.) There follow many months of uncertain temperature tendency,until in June-July 1983 there is agreement that the ocean cooled much more strongly thanindicated by climatology. Averaged over the period August-November 1982 there is consensusthat warming took place. No moored or other densely sampled dataset is available forcomparison in this region for this period.

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