Experiments With Buoyancy-driven Ocean Circulation
Seven numerical experiments are described, in which both wind and sources of buoyancy drive a quasigeostrophic model of the ocean circulation. The 'thermal' gyres develop upon the potential vorticity field, Q, set by the wind-gyres, and the thermaland wind-gyres interact significantly. The model is quasigeostrophic with three layers in the vertical and 200 x 200 horizontal resolution, representing a 4000 x 4000 km square, mid-latitude ocean. Free slip and 'hyper'-slip are applied at the
... coasts, with high-order viscosity in the interior. Typical duration of the experiments is 10 to 20 model years, which allows the average flow fields and energies to equilibrate. The mean Q-field develops 'islands' (that is, maxima and minima) in the vicinity of the buoyancy sources, which contrast the 'plateaus' and 'escarpments' that dominate purely wind-driven simulations. Although the buoyancy-driven circulations have the same qualitative nature as that predicted by steady, nonlinear theory (Rhines, 1985) , there are many differences: eddies induce a non-Sverdrup component of the mean depth-integrated circulation; the intensity of the flow is reduced, relative to laminar theory (due to the vertical eddy-transport of the horizontal momentum of the counter-rotating thermal gyres); the pattern of mean vertical velocity is greatly altered with respect to that of steady eddy-free theory; with strong forcing, the flow 'collapses' into meridional inertial boundary currents and zonal jets, rather than following smooth, broad interior currents predicted by Sverdrup theory. iv Meridional flow in the interior ocean, far from boundaries, is difficult to achieve in the simple steady theories. With thermal forcing but in absence of wind forcing, meridional flow is confined to boundary currents and regions of direct external forcing. Here either (i) interaction with the wind-driven gyres, or (ii) eddy induction, make possible broad regions of north-south flow. As diagnostics we show: spin-up histories of both linear and nonlinear cases; mean fields after spin-up; time-averaged vorticity balances along selected latitudes and longitudes; time-averaged buoyancy balances; 'Sverdrup' balance diagnostics, in which f&w/bz and 3 v are compared; scatter plots of mean Q and mean i, which show the transition from weak, non-functional dependence to strong, functional dependence Q = Q( ), as the strength of the forcing is increased, and energyhistories of the spin-up of the circulations. Owing to the great strength of the eastern boundary currents in the strongly driven cases, we include one run with enhanced grid resolution near both eastern and western boundaries (a total of 280 gridpoints east-west, rather than 200). TACTICAL NOTE We have put together this record of a set of experiments to transmit more or less the full experience of the simulations to the reader. Because research is pulse-like rather than steady it may be that investigations in this area will one day cease, and it seems important to record more than can be accommodated by the journals. One day, perhaps, there will be computer archives of this kind of work, in which the results and the model itself will be preserved. ACKNOWLEDGEMENTS Peter B.