Variability in the Western Equatorial Pacific Ocean during the 1986–87 El Niño/Southern Oscillation Event

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  • 1 Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington
  • | 2 Woods Hole Oceanographic Institute, Woods Hole, Massachusetts
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Abstract

We describe variability in the western Pacific Ocean during the 1986–87 El Niño/Southern Oscillation (ENSO) event, with emphasis on time series measurements of currents, temperature, sea level and winds near the equator at 165°E. Zonal winds were anomalously westerly from mid-1986 to late 1987 and were punctuated by 2–10 m s−1 episodes of westerlies lasting about 10 days to 2 months. Zonal current in the upper 100-m surface layer responded to these wind variations typically within a week, in some cases with speeds exceeding 100 cm s−1 to the east. Zonal current variations in the thermocline below 100 m were generally less coherent with the local wires than currents near the surface. They were also generally less variable, although the Equation Undercurrent disappeared for 3–4 weeks in October-November 1987 at a time when the normal eastward directed zonal pressure gradient force reversed along the equator. Periods of intense and prolonged eastward flow in the surface layer were associated with a decrease in sea level by 10–20 cm at the end of 1986 and in May-August, 1987. Similarly, significant westward flow near the surface and in the thermocline in September-November 1987 was accompanied by rising sea level and a westward migration from the date line of surface waters >30°C. These results suggest that wind-driven zonal currents at the equator were important in the evolution of the mass and heat balance of the western Pacific during the 1986–87 ENSO, Conversely, meridional wind stress and meridional velocity energy levels at periods longer than 100 days on the equator were 5–10 times weaker than in the zonal direction and less obviously related to the evolution of the 1986–87 ENSO.

Abstract

We describe variability in the western Pacific Ocean during the 1986–87 El Niño/Southern Oscillation (ENSO) event, with emphasis on time series measurements of currents, temperature, sea level and winds near the equator at 165°E. Zonal winds were anomalously westerly from mid-1986 to late 1987 and were punctuated by 2–10 m s−1 episodes of westerlies lasting about 10 days to 2 months. Zonal current in the upper 100-m surface layer responded to these wind variations typically within a week, in some cases with speeds exceeding 100 cm s−1 to the east. Zonal current variations in the thermocline below 100 m were generally less coherent with the local wires than currents near the surface. They were also generally less variable, although the Equation Undercurrent disappeared for 3–4 weeks in October-November 1987 at a time when the normal eastward directed zonal pressure gradient force reversed along the equator. Periods of intense and prolonged eastward flow in the surface layer were associated with a decrease in sea level by 10–20 cm at the end of 1986 and in May-August, 1987. Similarly, significant westward flow near the surface and in the thermocline in September-November 1987 was accompanied by rising sea level and a westward migration from the date line of surface waters >30°C. These results suggest that wind-driven zonal currents at the equator were important in the evolution of the mass and heat balance of the western Pacific during the 1986–87 ENSO, Conversely, meridional wind stress and meridional velocity energy levels at periods longer than 100 days on the equator were 5–10 times weaker than in the zonal direction and less obviously related to the evolution of the 1986–87 ENSO.

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