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.
