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Observations of Warm Water Volume Changes in the Equatorial Pacific and Their Relationship to El Niño and La Niña

C. S. Meinen1 and M. J. McPhaden2

1Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington
2Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington, 98115

Journal of Climate, 13(20), 3551–3559 (2000).
Copyright ©2000 by the American Meteorological Society. Further electronic distribution is not allowed.

1. Introduction

The El Niño-Southern Oscillation (ENSO) cycle dominates interannual variability in the equatorial Pacific and has been the subject of a great deal of study over the past several decades. Philander (1990) provides a review of many of the results of these studies; some of the more recent studies can be found in Barnett et al. (1991), Schneider et al. (1994, 1995), and Wang et al. (1999). Several studies have indicated that variability in the volume of warm water (WWV), and hence heat content, in the tropical Pacific is related to the ENSO cycle (Wyrtki 1985; Cane and Zebiak 1985; Zebiak 1989; Springer et al. 1990; Jin 1997a). At the simplest level, warm water builds up in the equatorial Pacific prior to El Niño and then is transported to higher latitudes during El Niño. WWV at the equator then slowly builds up again before the cycle repeats. It has been suggested, based on proxy measurements and modeling results, that this buildup of the WWV in the equatorial Pacific is a necessary precondition for the development of an El Niño (Wyrtki 1985; Cane et al. 1986).

Recently, Jin [1997a (hereafter J97)] and Jin (1997b) have produced a theoretical "recharge oscillator" paradigm to describe how changes in WWV are related to the timing of El Niño and La Niña events. In this theory, the depth of the main thermocline, and hence the WWV above it, plays an important dynamical role in the oscillation of the ENSO cycle by controlling the temperature of the waters upwelled in the eastern equatorial Pacific (i.e., deeper main thermocline results in the upwelling of warmer waters, and vice versa). Changes in the temperature of the upwelled waters in turn control the sea surface temperature (SST) anomalies in the eastern equatorial Pacific, which then impact the zonal winds via changes in the patterns of deep atmospheric convection and sea level pressure gradients. The result is a positive feedback (SST gradients create anomalous winds and winds amplify SST gradient) in either the El Niño or La Niña phases of the ENSO cycle until enough water has been discharged from or recharged into the equatorial region to end the event. Furthermore, J97 shows that this oscillator hypothesis is consistent with the "delayed oscillator" (Schopf and Suarez 1988; Suarez and Schopf 1988; Battisti and Hirst 1989), which is currently viewed as a leading paradigm to explain the ENSO cycle. The essential features of the delayed oscillator and the recent proposed refinements of it (e.g., Picaut et al. 1997; Weisberg and Wang 1997; Boulanger and Menkes 1999; McPhaden and Yu 1999) include equatorial wave processes, which affect thermocline depth and SST, and ocean-atmosphere feedbacks mediated by wind and SST variability.

Historical studies, which have endeavored to quantify the recharge and discharge of warm water from the equatorial Pacific, have been limited by the lack of available subsurface temperature measurements. Wyrtki (1985) estimated changes in the main thermocline depth using widely scattered island-based sea level measurements as a proxy. Zebiak (1989) looked at the problem using a dynamical model but had limited observational data with which to confirm his results. J97 attempted to demonstrate the validity of his theoretical oscillator by using widely scattered sea level measurements from island stations and, alternatively, using the output of the Zebiak and Cane model (Zebiak and Cane 1987). While these and other studies (e.g., Li and Clarke 1994; Mantua and Battisti 1994) have advanced the understanding of how the WWV changes during the ENSO cycle, there has, as of present, been no systematic test of the relevant concepts using subsurface ocean data. Also, no study to date has addressed the question of whether WWV variations during the very strong 1997-98 El Niño (McPhaden 1999) are similar to those of other events or whether they are consistent with the recharge oscillator theory of J97.

The theoretical oscillator proposed by J97 involves a number of testable hypotheses. Thus, the purpose of this paper is to determine to what extent available subsurface temperature observations confirm or refute this theoretical recharge oscillator and what further information these observations can provide regarding the El Niño-La Niña cycle. Our study utilizes a dataset of monthly gridded ocean temperatures developed using an optimal interpolation technique, which incorporates hydrographic and moored temperature data over the interval of 1980 to the present. These data span several large El Niño events, including the very strong 1997-98 event, and they represent the best available observational information with which to study the changes in WWV in the equatorial Pacific.


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