Observations of Warm Water Volume Changes in the Equatorial Paciic and Their Relationship to El Ni~ No and La Ni~ Na

This paper describes observed changes in surface winds, sea surface temperature (SST) and the volume of water warmer than 20 C (WWV) in the equatorial Paciic Ocean for the period 1980{99. The purpose is to test recent hypotheses about the relationship between variations in WWV and the El Ni~ no/Southern Oscillation (ENSO) cycle. Our results connrm inferences based on theory, models, and previous empirical analyses using proxy data (namely sea level) that ENSO involves a recharge and discharge of WWV along the equator, and that the cyclic nature of ENSO results from a disequilibrium between zonal winds and zonal mean thermocline depth. We also nd that the magnitude of ENSO SST anomalies is directly related to the magnitude of zonal mean WWV anomalies. Furthermore, for a given change in equatorial WWV, the corresponding warm El Ni~ no SST anomalies are larger than the corresponding cold La Ni~ na anomalies. This asymmetry between the warm and cold phases of the ENSO cycle implies diierences in the relative importance of physical processes controlling SST during El Ni~ no and La Ni~ na events.