National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1990

Aspects of the Kelvin wave response to episodic wind forcing

Giese, B.S., and D.E. Harrison

J. Geophys. Res., 95(C5), 7289–7312, doi: 10.1029/JC095iC05p07289 (1990)

Episodes of westerly wind are an important aspect of surface stress variability in the western Pacific. During El Niño-Southern Oscillation periods, the presence of such wind episodes comprises much of the low-frequency relaxation of the trades over the central and western Pacific. In this paper we describe the oceanic Kelvin pulse response to a single idealized episode of westerly wind stress, using results from linear theory as well as from a 27-level general circulation model. Linear theory predicts that an episode of westerly wind will excite a train of equatorial trapped Kelvin pulses. The amplitude and longitudinal structure of the forced ocean Kelvin pulses varies as a function of baroclinic mode and the wind patch properties. Because of changing vertical thermal structure across the Pacific, the vertical structure of Kelvin pulses is altered as they propagate away from the forcing region. When stratification typical of the western and eastern Pacific is used, the conservation of energy flux predicts a reduction of surface currents associated with the first baroclinic mode and an enhancement of surface currents associated with the second baroclinic mode. The idealized wind anomaly is also used to drive an ocean general circulation model. When the wind anomaly is weak the model Kelvin response agrees with predictions of linear theory. For more realistic strong forcing there are three important deviations from linear theory: the amplitude of low baroclinic modes increases; the amplitude of higher baroclinic modes decreases; and the phase speed increases. In the presence of realistic oceanic background conditions, response in the equatorial waveguide is complicated by the equatorial undercurrent, a sloping thermocline and instability waves. Model sea surface temperature warming at the coast of South America is dominated by the second baroclinic mode, consistent with the results derived from linear theory.

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