Observations of long period waves in the tropical oceans and atmosphere

Abstract

The existence of resonant, baroclinic, equatorially-trapped inertia-gravity waves (discovered by Wunsch and Gill (1976)) is confirmed in the mid-Pacific by spectral analysis of long sea level records. The energy of the low-mode inertia-gravity waves is found to decrease toward the meridional boundaries. A simple spectral model, acknowledging the dispersive characteristics of the equatorial waves, adequately reproduces the observed mid-Pacific sea level spectra in the 1-6 day band. Model spectra computed at latitudes outside the equatorial waveguide of the gravest meridional modes suggest the presence of "inertial" peaks in several observed sea level spectra. Resonant, low-mode inertia-gravity waves may also exist in the Indian Ocean. Sea level fluctuations along the Pacific equator are found to have Kelvin wave characteristics in the 35-80 day band, and, in particular, propagation from the western Pacific to the coast of South America is observed. The Kelvin waves are atmospherically-forced in the central- western Pacific and have a computed equivalent depth corresponding to the first-baroc1inic mode.

Outside of the equatorial mid-Pacific, a non-static ocean response to air pressure in the 4-6 day band is dominated by a basin-wide, barotropic, planetary mode. The low Q of this mode suggests that the ocean is viscous with respect to large-scale barotropic oscillations. The dynamical components of the observed long-period tides have been isolated for the first time using the "self-consistent" equilibrium tide of Agnew and Farrell (1978). The tides are slightly non-equilibrium with large horizontal scales. The relatively short-scale Rossby modes predicted by Wunsch (1967) are not observed, perhaps because of the poor spatial coverage of the dataset. Considering the low Q of the 4-6 day planetary basin mode, it is suggested that the long-period tides are frictionally-controlled.

The 4- and 5-day equatorial inertia-gravity waves, the 35-80 day Kelvin waves and the 4-6 day planetary basin mode are clearly atmospherically forced, and, perhaps surprisingly, they are forced by atmospheric waves that have similar horizontal structures, i.e., 4-5 day Rossby-gravity waves, 40-50 day Kelvin waves and a 5-day global barotropic mode. The surface expressions of these atmospheric waves are determined in order to understand the nature of the oceanic response, e.g., resonant or forced. Much of the information about the surface atmospheric fields that has been collected, including frequency-wavenumber descriptions, awaits an accurate model of the coupling between wind stress and internal ocean waves.