On the variability of the tropical Pacific thermal structure during the 1979-96 period, as deduced from XBT sections

Abstract

The thermal structure variability of the tropical Pacific is investigated using an objective analysis of about 250 000 temperature profiles (mainly XBT) collected during the 1979-96 time period. Mean conditions and seasonal variability are briefly described to set the context, and temperature anomalies are constructed relative to a mean seasonal cycle to focus on the ENSO (El Niño-Southern Oscillation) timescale. Heat content anomalies (0-450 m) built up in the western equatorial basin prior to the 1982-83, and 1997-98 El Niño events but not clearly prior to the 1991-92, 1993, and 1994-95 events, which are thus found "atypical" Low-frequency migration of temperature anomalies located at the mean thermocline depth is evidenced eastward in the equatorial band, as well as westward along a narrow zonal band located slightly north of the mean position of the intertropical convergence zone (say, 10°-20°N). This indicates that ENSO-related temperature anomalies in the subsurface ocean are present generally in the western equatorial Pacific about 1-2 years before the appearance of temperature anomalies in the eastern equatorial Pacific. Similarly, this indicates that subsurface temperature anomalies tend to be present in the eastern Pacific basin around 14°N about 1-2 years before the appearance of temperature anomalies in the western Pacific basin at the same latitude. The likely mechanisms responsible for these migrations and the possible link between the eastward and westward migration are discussed.