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Structure of Intraseasonal Kelvin Waves in the Equatorial Pacific Ocean

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  • 1 Department of Marine Sciences, University of South Florida, St. Petersburg, Florida
  • | 2 National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington
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Abstract

Previous studies have shown that intraseasonal Kelvin waves are a prominent mode of variability in the eastern and central equatorial Pacific. These waves appear to be remotely forced by wind variations in the western Pacific and propagate eastward at approximately first baroclinic mode phase speeds. In this study, moored temperature and velocity time series data from 1984 to 1987 between 110° and 140°W are studied to isolate the Kelvin wave structures and to document their wave-mean flow interactions with the South Equatorial Current/Equatorial Undercurrent (RUC) system. Observed structures at periods of 59–125 days have rms amplitudes of about 10 cm s−1 for zonal velocity and about 1°C for temperature in the thermocline. These structures exhibit significant departures from linear no-mean-flow theory: zonal velocity is intensified below the EUC core, and temperature variability in the thermocline is weaker on the equator than 1.5° off the equator. Additionally, the wave temperature signal at the surface is nonzero and lags both zonal velocity and deeper temperatures by about 90°. These observed structures are compared with those from a linearized wave-mean flow interaction channel model, with the model mean flow specified to match the observed mean flow as closely as possible. The model qualitatively reproduces both the intensification of wave zonal velocity below the EUC and the weakening of temperature variability an the equator where the mean stratification is weaker. The model cannot reproduce the observed SST signal since it is zonally invariant in its basic state, and hence, perturbation SSTs are zero. However, an estimate of temperature advection due to wave zonal velocities advecting the observed sea surface temperature gradients accounts for the phase and the O (0.1°C) rms amplitude of observed SST variations at intraseasonal periods.

Abstract

Previous studies have shown that intraseasonal Kelvin waves are a prominent mode of variability in the eastern and central equatorial Pacific. These waves appear to be remotely forced by wind variations in the western Pacific and propagate eastward at approximately first baroclinic mode phase speeds. In this study, moored temperature and velocity time series data from 1984 to 1987 between 110° and 140°W are studied to isolate the Kelvin wave structures and to document their wave-mean flow interactions with the South Equatorial Current/Equatorial Undercurrent (RUC) system. Observed structures at periods of 59–125 days have rms amplitudes of about 10 cm s−1 for zonal velocity and about 1°C for temperature in the thermocline. These structures exhibit significant departures from linear no-mean-flow theory: zonal velocity is intensified below the EUC core, and temperature variability in the thermocline is weaker on the equator than 1.5° off the equator. Additionally, the wave temperature signal at the surface is nonzero and lags both zonal velocity and deeper temperatures by about 90°. These observed structures are compared with those from a linearized wave-mean flow interaction channel model, with the model mean flow specified to match the observed mean flow as closely as possible. The model qualitatively reproduces both the intensification of wave zonal velocity below the EUC and the weakening of temperature variability an the equator where the mean stratification is weaker. The model cannot reproduce the observed SST signal since it is zonally invariant in its basic state, and hence, perturbation SSTs are zero. However, an estimate of temperature advection due to wave zonal velocities advecting the observed sea surface temperature gradients accounts for the phase and the O (0.1°C) rms amplitude of observed SST variations at intraseasonal periods.

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