National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1993

Effects of a three-dimensional mean flow on intraseasonal Kelvin waves in the equatorial Pacific Ocean

Johnson, E.S., and M.J. McPhaden

J. Geophys. Res., 98(C6), 10,185–10,194, doi: 10.1029/93JC00759 (1993)

Intraseasonal Kelvin waves observed in the central and eastern equatorial Pacific Ocean have first vertical mode structures which are significantly different from first vertical modes in an otherwise motionless ocean. The observed mode has doubled temperature amplitudes relative to zonal velocity in the upper thermocline and shorter vertical scale structures in both zonal velocity and temperature. These departures are observed in the shallowest 200 m, where the mean currents are strong, and are related to interactions between these mean currents and the wave field. Modeled linear waves interacting with a zonally invariant zonal mean flow qualitatively explain some of the observed wave structures. However, the best quantitative assessment of wave variability is one that takes into account the full mean circulation and its gradients in all three dimensions. Specifically, by diagnosing the linearized budgets of temperature and zonal velocity for the observed and model waves we find that the observed doubling of wave temperature amplitudes is largely produced by the effects of mean vertical advection of wave temperatures. The increased wave temperature amplitudes lead to a wave pressure field which decays more rapidly with depth. Thus, for the same surface amplitudes the observed wave's pressure and velocity fluctuations in the deep ocean should be half the amplitude expected for a wave in the absence of mean flow. The effects of mean vertical advection are frequency dependent and will be stronger for longer-period waves. We conclude that models of low-frequency variability in the equatorial oceans must include the effects of a fully three-dimensional mean flow in order to realistically reproduce the structures of first vertical mode Kelvin waves.

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