The modification of long planetary waves by homogeneous potential vorticity layers

A mechanism by which long planetary waves in the ocean may propagate signif icantly faster than the classical long baroclinic Rossby waves is investiga ted. The mechanism depends on the poleward thickening of intermediate densi ty layers and the concomitant thinning of near-surface and deep layers. The se features of the mass distribution are associated with the well-known hom ogenization of potential vorticity in intermediate density layers and with significantly elevated meridional potential vorticity gradients near the su rface and somewhat at depth. The mechanism is explored in a simple three-la yer model, in which the middle layer has zero potential vorticity gradient and is sandwiched between a surface layer with large potential vorticity gr adient and a bottom layer with modest potential vorticity gradient. The eff ective phase speed of the planetary waves is merely the sum of the phase sp eeds of virtual baroclinic Rossby waves propagating on the individual layer interfaces as though the other interface were not there and as though ther e were no mean vertical shear. The mechanism is also examined for a continu ous model with zero potential vorticity gradient throughout the interior an d 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 st ratification. but no mean vertical shear. This explanation of the Rossby wa ve speedup complements a recent detailed theoretical calculation of planeta ry-wave phase speeds based on geostrophic velocity profiles from archived h ydrographic data.