North Atlantic Potential Vorticity and Its Relation to the General Circulation

Abstract

Maps and sections of the large-scale North Atlantic potential vorticityq are presented. Here q is fd?/dz, where f is the Coriolis frequency,? the potential density and z the vertical coordinate. They bearon the general circulation, and on geostrophic waves, instabilityand turbulence in many ways; both Eulerian and Lagrangian mean circulationsproceed along isostrophes, q=constant, in a zero-dissipation region.In a resting fluid q varies simply as the sine of the latitude, butwe show here that the wind-driven circulation reshapes the q-field,creating ?bowls? and ?plateaus? which allow the flow to cross latitudecircles. The implied nature of the western boundary current is verydifferent than in classical frictional theory. The maps show a regionof uniform potential vorticity in the wind gyre (??=26.5?27.0) whichfills the ocean between 15?37\,^{\circ}N and 20?80\,^{\circ}W.Such regions were prominent features of a circulation theory of Rhinesand Young (1982a,b). At deeper levels, and close to surface outcropsof the density layers, the isostrophes are ?open,? extending overa vast latitude range in mid-ocean. They provide flow paths, forexample, which connect the Labrador Sea and the subtropical deepocean without the need of dissipation of potential vorticity.The maps of q show where the North Atlantic is susceptible to baroclinicinstability. The generalized Rayleigh criterion for instability issatisfied in a large region south of the center of the wind gyre,between 10 and 32\,^{\circ}N. This supports the idea that eddyproduction is a strong feature of the subtropical mid-ocean regions.