Short-Term Climatic Variability in the Thermal Structure of the Pacific Ocean during 1979–82

Abstract

Short-term climatic variability in both sea surface temperature (SST) and vertically averaged temperature over the upper 400 m of ocean (Tav) is mapped over the Pacific from 20°S to 50°N each bimonth for four years from 1979 to 1982, leading up to the 1982?83 ENSO (El Nino?Southern Oscillation) event. This mapping was made possible by the collection of approximately 85 000 temperature/depth observations in the Pacific Ocean by volunteer observing ships. Anomalies of SST and Tav were approximately the same magnitude at midlatitude as in the tropics, with the exception of large changes occurring in the tropics during the 1982?83 ENSO event. During the ENSO event, (Tav variability was largest in the western tropical North Pacific and SST variability was largest in the eastern equatorial Pacific. Both parameters had spatial patterns which were of opposite phase on either side of the ocean, indicating an eastward shift of warm waters during the ENSO event. Correlation studies determined that on average during the four years extraequatorial Tav anomalies propagated from east to west at baroclinic long-wave speeds, travelling faster near the equator (i.e., 25?75 cm s?1 at 10°N and 10°S) and slower at higher latitudes (i.e., 2 cm s?1 new 40°N). At the equator, Tav anomaly propagation was to the east at internal Kelvin wave speeds (i.e., 50?250 cm s?1). On the other hand, SST anomalies propagated on average during the four years in the direction of mean surface currents, so that, for example, at 5°N anomalies propagated to the east with the North Equatorial Countercurrent. Beginning in 1981, anomalously high Tav propagated westward from the central North Pacific near 16°N and approached the maritime coast of Asia late in the year. During early 1982 it propagated rapidly down the coast toward the equator, then along the equator to the west coast of South America. This initial development was similar to the development of the 1968?69 and 1972?73 ENSO events in the western North Pacific observed by White and others. In mid-1982, this initial development was followed by a reduction in the Southeast Trade Winds over the western and central equatorial Pacific and a rapid intensification of the ENSO episode.