The Wind-driven Circulation: Quasi-geostrophic Simulations and Theory for Nonsymmetric Winds

Abstract

Simulations of the wind-driven Ocean circulation, carded out with an eddy-resolving quasi-geostrophic numerical model, and symmetric, idealized wind forcing have a large-scale structure that is predicted wen by the steady nonlinear theory of Rhines and Young. The sharp jet and inertial recirculation am often confined weft inside the region of closed hyperbolic characteristics, defined by that theory, and hence do not affect the Sverdrup-dynamics part of the gyre. The characteristics make possible simple predictions about the development of the circulation, including time dependence and eddy stirring. By tilting the line of vanishing Ekman pumping away from the east-west orientation (as it is tilted in the North Atlantic, and less so the North Pacific), we explore a family of circulations. As the tilt of the wind held is increased, characteristics originating at the eastern boundary begin to thread through the energetic region occupied by the free jet. Then, extensive new branches of eddy-driven flow occur, reaching poleward into the subpolar gyre. Lagrangian float trajectories are shown and Lagrangian mean circulation and diffusivity discussed. A Peclet number measuring the relative strengths of advection and eddy mixing of potential vorticity is defined and mapped. Its value, typically 3 to 5, suggests the dominant nature of mesoscale eddy mixing in the ocean. Stirring by mesoscale eddies arises in midocean from baroclinic instability. It leads to a loss of 'memory? of quasi-conserved properties over typically 300 to 1000 km. Eddies are essential to the transport of potential vorticity from subpolar to subtropical regions, across the limiting characteristic, finally determining the structure of the recirculating subtropical gyre. Permanent tongues of potential vorticity invade the subtropics from the subpolar gyre, entering where the characteristics of the theory form a stagnation point. The experiments exhibit several features of the observed circulation of the ocean. With increasing tilt of the winds we find: decreasing total energy of the circulation; great decrease in the length of the eastward-flowing free jet; increased concentration of the circulation in the upper ocean where it wore closely resembles the simple Sverdrup transport function, with broad regions of eastward flow., increased production of cutoff rings near the western boundary (rather than just at the eastern end of the jet, as with symmetric winds); shrinkage of the north-south extent of the subtropical gyre at the 300?1000 m level yet increase in its consent extent (so that it reaches 5000 km northeastward, to the eastern boundary); and displacement of the boundary current separation point poleward of the line of vanishing Ekman pumping. The subpolar gyre shrinks in size. The simulations help one to understand the differences between, on the one hand, the North Atlantic Ocean, with its very confined middepth circulation and NE-SW strike of the 1000-m potential vorticity contours, and a relative small region of penetration of the concentrated Gulf Stream jet into the interior, and, on the other hand, the North Pacific, where the subtropical anticyclone penetrates much deeper and the Kuroshio jet penetrates a greater distance eastward. A review of relevant observations of the North Atlantic is given, particularly to show that the regime of the model is realistic; as one moves toward the ?quiet? parts of midocean, the ratio of eddy to mean kinetic energy actually rises, suggesting that eddy mixing cannot be neglected there.