The Annual Wind-driven Rossby Wave in the Subthermocline Equatorial Pacific

Abstract

The annual cycle of temperature in the subthermocline equatorial Pacific is studied using a new compilation of historical hydrographic profiles. The observations have several characteristics suggestive of a vertically propagating, first meridional mode (l=1) long-wavelength Rossby wave: phase lines that slope downward from east to west indicative of upward and westward phase propagation amplitude maxima parallel to phase lines, and nearly symmetric off-equatorial maxima of annual amplitude. Estimates of zonal wavenumber, vertical wavenumber, and the location of maxima of isotherm displacements are consistent with those of the l = 1 Rossby wave. A solution to a linear continuously stratified model, driven by a version of the observed annual wind field, confirms this interpretation. The solution is dominated by a vertically propagating, l = 1 Rossby wave. The wave is generated primarily by the westward-propagating component of the equatorial zonal wind field; it carries energy along WKB ray paths into the deep ocean. Both amplitude and phase of the model density field agree well with the observations. There are, however, two prominent differences between the observations and the solution: first, in the solution a boundary-reflected l = 3 Rossby wave is present in the deep eastern Pacific but is apparently absent in the data; second, the model solution is nearly symmetric about the equator, while the observations are symmetric in phase but have larger amplitude in the Northern Hemisphere. Thus, efficient vertical propagation of Rossby wave energy through the thermocline into the deep ocean appears to be an important oceanic process. The lack of this process in single active-layer models may explain the unrealistically high amplitudes of off-equatorial variability that are produced in them, since such models necessarily trap all energy in the surface layer.