heating in the upper 250 m averaged over the region 152°W to 110°W.U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Improved understanding of the El Niño-Southern Oscillation (ENSO) phenomena in the Pacific Ocean has received increased attention in recent years as the importance of these signals in the year-to-year variations of the global climate has become apparent [Philander, 1990]. Crucial to this understanding is an accurate description of the processes which maintain and change the tropical Pacific sea surface temperature (SST) and the interaction of these oceanic changes with the atmosphere. The important processes are likely to vary with location and at least three regimes in the equatorial Pacific can be identified based on atmospheric and oceanic conditions. In the western Pacific the surface warm layer is thick (although salinity may provide an important source of shallow stratification), SST is high and changes little seasonally, and the mean winds are weak but their variability is large. In the central Pacific, SST has a weak seasonal signal but relatively large interannual changes which are climatically significant; the southeast trade winds are generally strong except during ENSO events. In the eastern Pacific the thermocline is very shallow, near-equatorial waters are relatively cool, and SST gradients are large. The prevailing winds have a strong cross-equatorial component and both the winds and the SST vary annually.
Several recent studies address the upper ocean heat budget and SST changes
near the equator. Wyrtki
[1981] used climatological data to estimate the mean heat budget for a 50-m-deep
box from 170°E to 100°W, 5°N to 5°S. Meridional and vertical diffusion was negligible
in his calculation. The net surface heating was balanced by zonal and meridional
advection which included the contribution from upwelling. Enfield
[1986] expanded this study to include the entire equatorial band and to
examine seasonal variations. Based on Enfield's "best guess" mean
heat budget, all of the possible terms in the upper ocean heating appear to
be significant. In the western Pacific the balance was primarily between surface
heat gain and vertical diffusion. In the central Pacific, atmospheric fluxes
and meridional diffusion contribute nearly equal warming; this heat is removed
by zonal and meridional advection (which includes upwelling) and by vertical
diffusion. In the eastern Pacific near 110°W, the surface heating (about 30%
of which is due to meridional diffusion) is primarily offset by meridional advection
and vertical diffusion. Bryden
and Brady [1989] emphasized the importance of the meridional eddy diffusion
which in their calculation contributed 245 W m
heating in the upper 250 m averaged over the region 152°W to 110°W.
In the western Pacific, Meyers et al. [1986] pointed out the importance of variations in latent heat flux in cooling the warm pool during the 1982-83 ENSO episode. Similarly, McPhaden and Hayes [1991] found that evaporative cooling was important in the day-to-month SST changes and also noted the role of zonal advection. Stevenson and Niiler [1983] examined the heat budget in the central Pacific and concluded that on seasonal time scales, the surface heat flux was a major factor determining the evolution of SST. Zonal advection was not found to be as important. In the eastern Pacific zonal advection appears to be an important contribution to the warming during ENSO [Halpern et al., 1983; Mangum et al., 1986; Philander and Hurlin, 1988; McPhaden and Hayes, 1990].
In this paper we present the results of a study of the surface mixed layer heat budget estimated from moored array data along 110°W during 1986-88. The analysis focuses on the seasonal and lower frequency variability. This period encompasses an ENSO cycle and includes the onset and development of the 1986-87 warm event and the onset of the subsequent cool event. The near equatorial variability at 110°W during these years and its relation to the evolution of conditions throughout the equatorial Pacific are described in McPhaden and Hayes [1990] (hereinafter referred to as MH). Briefly, MH noted the onset of warm SST anomalies at 0°, 110°W in mid-1986; an increase in these anomalies and a concomitant decrease in the strength of the South Equatorial Current (SEC) in September-November 1986; and persistence of the warm SST throughout 1987. In early 1988 the SST rebounded rapidly, and cold anomalies in excess of 3°C appeared in association with a large scale shoaling of the thermocline. Much of the year-to-year variability at 110°W appeared to result from zonal wind variations in the central and western Pacific.
In the present study, the moored array data used in MH are combined with other nearby measurements and climatology to describe the mixed layer variability on seasonal and interannual time scales and to estimate terms in the heat budget. The purpose is to attempt to quantitatively compare the relative importance of various physical processes in the evolution of the SST in the eastern Pacific. This region is of particular interest in simplified coupled models of ENSO [Cane and Zebiak, 1985; Battisti, 1988]. The shallow thermocline contributes to large SST changes associated with local and remote wind stress variations and these SST fluctuations can in turn yield changes in the local surface wind [Wallace et al., 1989; Hayes et al., 1989b]. Understanding the mechanisms of SST change in the eastern Pacific is fundamental in order to accurately model the coupled ENSO phenomena.
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