[1] Ten years of sea-surface height (SSH) fields constructed from the merged TOPEX/Poseidon (T/P) and ERS-1/2 altimeter datasets are analyzed to investigate mesoscale variability in the global ocean. The higher resolution of the merged dataset reveals that more than 50% of the variability over much of the World Ocean is accounted for by eddies with amplitudes of 5 -25 cm and diameters of 100-200 km. These eddies propagate nearly due west at approximately the phase speed of nondispersive baroclinic Rossby waves with preferences for slight poleward and equatorward deflection of cyclonic and anticyclonic eddies, respectively. The vast majority of the eddies are found to be nonlinear.

Planetary or Rossby waves are the predominant way in which the ocean adjusts on long (year to decade) timescales. The motion of long planetary waves is westward, at speeds Ն1 cm s Ϫ1. Until recently, very few experimental investigations of such waves were possible because of scarce data. The advent of satellite altimetry has changed the situation considerably. Curiously, the speeds of planetary waves observed by TOPEX/Poseidon are mainly faster than those given by standard linear theory. This paper examines why this should be. It is argued that the major changes to the unperturbed wave speed will be caused by the presence of baroclinic eastwest mean flows, which modify the potential vorticity gradient. Long linear perturbations to such flow satisfy a simple eigenvalue problem (related directly to standard quasigeostrophic theory). Solutions are mostly real, though a few are complex. In simple situations approximate solutions can be obtained analytically. Using archive data, the global problem is treated. Phase speeds similar to those observed are found in most areas, although in the Southern Hemisphere an underestimate of speed by the theory remains. Thus, the presence of baroclinic mean flow is sufficient to account for the majority of the observed speeds. It is shown that phase speed changes are produced mainly by (vertical) mode-2 east-west velocities, with mode-1 having little or no effect. Inclusion of the mean barotropic flow from a global eddy-admitting model makes only a small modification to the fit with observations; whether the fit is improved is equivocal.

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A mechanism by which long planetary waves in the ocean may propagate significantly faster than the classical long baroclinic Rossby waves is investigated. The mechanism depends on the poleward thickening of intermediate density layers and the concomitant thinning of near-surface and deep layers. These features of the mass distribution are associated with the well-known homogenization of potential vorticity in intermediate density layers and with significantly elevated meridional potential vorticity gradients near the surface and somewhat at depth. The mechanism is explored in a simple three-layer model, in which the middle layer has zero potential vorticity gradient and is sandwiched between a surface layer with large potential vorticity gradient and a bottom layer with modest potential vorticity gradient. The effective phase speed of the planetary waves is merely the sum of the phase speeds of virtual baroclinic Rossby waves propagating on the individual layer interfaces as though the other interface were not there and as though there were no mean vertical shear. The mechanism is also examined for a continuous model with zero potential vorticity gradient throughout the interior and large virtual potential vorticity gradients near the surface and bottom. Planetary waves in these models can propagate westward up to twice as fast as baroclinic Rossby waves would through an ocean with the same vertical stratification, but no mean vertical shear. This explanation of the Rossby wave speedup complements a recent detailed theoretical calculation of planetary-wave phase speeds based on geostrophic velocity profiles from archived hydrographic data.