Some characteristics of internal gravity waves in the equatorial Pacific

Abstract

One‐year moored current and temperature measurements made in the central equatorial Pacific Ocean at subthermocline depths display spectral characteristics at internal gravity wave frequencies which are distinct from those found at mid‐latitudes. These characteristics are meridional orientation of current ellipses, kinetic energy scaling proportional to (buoyancy frequency)1.2, and coherence of zonal current and temperature at phases significantly different then 0° or 180°. Characteristics vary over as little as 100 m vertically but not over 40 km horizontally. Low‐frequency flow is predominantly zonal and shares similar spatial structure, suggesting internal waves are influenced by it. Although horizontal internal wave fluxes cannot be calculated directly from the observations, models provide a framework in which to interpret internal wave spectral characteristics. Models of internal waves near the equator in a resting ocean, an ocean with a uniform zonal current, and an ocean with depth‐dependent zonal current all require wave flux to be horizontally anisotropic in order to obtain current ellipses and zonal current‐temperatures similar to those observed. The model with vertical shear is the most realistic of these three. Observed shears are sufficiently strong to allow the possibility of internal wave critical layers, which in turn are a likely source of both horizontal anisotropy and vertical asymmetry of wave fluxes.