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

Variability in the Eastern Equatorial Pacific Ocean During 1986-1988

Michael J. McPhaden and Stanley P. Hayes

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Geophysical Research, 95(C8), 13,195-13,208 (1990)
Not subject to U.S. copyright. Published in 1990 by the American Geophysical Union.

7. Summary and Discussion

The eastern equatorial Pacific was characterized by warmer than usual SSTs from mid-1986 until early 1988. Weak initial anomalies in mid-1986 increased to more than 1C in September-November 1986 in association with a 30 cm s reduction in speed of the South Equatorial Current and a 20- to 40-m depression of the thermocline. These warm anomalies persisted until early 1988, after which cold anomalies of >3C developed in response to large-scale, remotely forced upwelling. Interannual fluctuations observed in the eastern equatorial Pacific appear for the most part to be determined by year-to-year variations in the trade winds over 40-50 of longitude in the central and western Pacific.

The importance of remotely forced variability in the eastern Pacific during ENSO events is well known [e.g., Cane, 1983]. However, it is interesting to note that both interannual and intraseasonal time scales contribute to the observed variability, and that for several variables, extrema during the 1986-1987 ENSO coincided with the occurrence of remotely forced intraseasonal Kelvin waves. Specifically, the highest dynamic heights, deepest thermocline depths, and largest zonal current transport anomalies were observed in early 1987 as a result of the strong 2-month westerly wind event west of the date line at the end of 1986.

The dynamical relevance of these intraseasonal fluctuations to the evolution of ENSO is not well understood. Statistically, for example, the frequent occurrence of western wind events may result in a seasonally averaged westerly wind anomaly [Luther et al., 1983]. However, the variations themselves do not appear to be essential to the generation of ENSO events based on simple coupled ocean-atmosphere model analyses [e.g., Zebiak and Cane, 1987; Schopf and Suarez, 1988; Battisti, 1988; Zebiak, 1989]. Also, intraseasonal time scale variations are not as prominent in SST as in subsurface temperature, dynamic heights, and thermocline currents (for which, see Figures 2 and 4). Nonetheless, in general, one might expect a degree of nonlinear coupling between high- and low-frequency phenomena because intraseasonal waves like those excited by the November-December 1986 wind event can have very large amplitude. It is also possible that the timing of these waves relative to the seasonal cycle could be important in the development of ENSO, if, for example, the coupled system is particularly sensitive to perturbation at certain times of the year.

In section 6 we showed that the 1986-1987 ENSO was less intense and evolved differently than the 1982-1983 ENSO. Particularly noteworthy is the fact that the Equatorial Undercurrent did not disappear in the eastern Pacific during 1986-1987 as it did during 1982-1983, which probably reflects the smaller amplitude and scale of the zonal wind and pressure gradient anomalies along the equator during 1986-1987. In contrast, a feature common to both events was a weakening of the South Equatorial Current by about 30 cm s in the latter part of 1982 and 1986. Sea level differences across the South Equatorial Current in the central and western Pacific have previously shown this weakening, albeit inferentially by geostrophy, for ENSO events from the 1950s through 1982-1983 [Wyrtki, 1974; Philander, 1990]. Direct measurements on the equator suggest that the duration and/or intensity of this weakening may be related to the magnitude of SST anomalies via anomalous zonal advection during the latter part of the year when zonal SST gradients are strongest.

The 1986-1987 ENSO, like the 1982-1983 ENSO, evolved differently than the Rasmusson and Carpenter [1982] composite. Warm SSTs of >1C developed in the eastern Pacific during September-November 1986 prior to South American coastal warming and persisted for 18 months (through December 1987 up to February 1988) instead of for only 12 months as in the composite. Moreover, Rasmusson and Carpenter found that westerly wind anomalies develop as early as January of the ENSO year near the date line, intensify in the summer, and last until the following February. In contrast, the observations for 1986-1988 show weak westerly anomalies developing near the date line in mid-1986 and persisting until the end of 1987. Lander [1989] discusses the differences between the 1986-1987 ENSO and the Rasmusson and Carpenter [1982] composite in more depth. It is clear, however, that although there are similarities between ENSO events, there is no single scenario that describes them all. One must be cautious, therefore, when generalizing about ENSO on the basis of observations from a single event.

Acknowledgments. Special thanks to Paul Freitag, Margie McCarty, and Nancy Soreide for processing the mooring time series. We also appreciate the technical assistance of Andy Shepherd, Carol Coho, Doug Fenton, Ben Moore, and Rick Miller for instrument preparation and mooring operations at sea. Linda Mangum and Linda Stratton were responsible for CTD data processing. Thanks also to the anonymous reviewer who was very detailed and thorough in his critique. This work has been supported by NOAA's Equatorial Pacific Ocean Climate Study (EPOCS) program and the U.S. TOGA Project Office. NOAA PMEL contribution 1120.

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