Observations of annual and El Niño thermal and flow variations at 0°, 110°W and 0°, 95°W during 1980–1985

Abstract

Moored measurements of the surface wind and of the current and temperature fields within the uppermost 250 m were recorded from March 1980 to September 1985 at 0°, 110°W and from July 1981 to November 1983 at 0°, 95°W. At these sites the annual mean thicknesses of the equatorial undercurrent (EUC) and of the surface portion of the south equatorial current (SEC) were about 200 m and 20 m, respectively. The annual mean EUC core speed was 0.9 m s−1. Annual cycles of temperature and zonal current at 110°W were most distinctive near the surface: at nominal depths of 15 m and 47.5 m, the average annual amplitudes were 1.5°C and 0.5 m s−1; at about 100 m the annual amplitudes were about 0.5°C and 0.10 m s−1. At 15 m the annual mean zonal current was 0.05 m s−1, which was not representative of the monthly mean zonal current; in April the monthly mean direction was eastward, and in October it was westward with speeds of about 0.5 m s−1 in both months. Warmer water occurred in April and colder water in October. The direction of the zonal wind component was westward throughout the year, and the annual amplitude of the zonal wind was only about 2.5 times larger than that of the zonal current. The average amplitude of the annual cycle of the meridional current component was less than 0.05 m s−1 and not distinguishable from year‐to‐year monthly mean variations. Monthly mean anomalous or El Niño currents at 110°W were defined as the difference between the observations and the annual cycle. During the August‐December 1982 onset phase of El Niño, the strength of the normally westward flowing near‐surface SEC was greatly reduced to near‐zero speed by an El Niño eastward flowing zonal current of about 0.3 m s−1. The largest vertically averaged (15–250 m) zonal current anomaly (≈−0.5 m s−1) occurred in January and February 1983 and represented a substantial decrease in the eastward transport of the EUC, causing a disappearance of the undercurrent. The duration of the EUC disappearance was longer at 95°W than at 110°W. A surface‐intensified, eastward jet with a maximum daily speed of 1.75 m s−1 at 15 m occurred in May 1983 at 110°W but not at 95°W. A massive redistribution of heat throughout the mixed layer and thermocline was associated with the 1982–1983 El Niño. Beginning in August 1982, the thermocline at 110°W and 95°W sank 100 m, reaching its maximum depth in December 1982; temperatures at 100 m and 150 m, which normally were about 15°C and 14°C, reached 27.5°C and 22.5°C. The time rate of change of the 15‐ to 200‐m heat content was 500 W m−2 during the August‐December 1982 warming phase and −460 W m−2 during the January‐February 1983 subsurface cooling phase. The EUC disappeared during the subsurface cooling phase. A second, less intensive mixed‐layer deepening occurred in May 1983 at 95°W but not at 110°W.