AN ISOPYCNIC MODEL STUDY OF THE NORTH-ATLANTIC .1. MODEL EXPERIMENT

This paper describes a 30-yr spinup experiment of the North Atlantic O cean with the Miami isopycnic-coordinate ocean model, which, when comp ared with previous experiments, possesses improved horizontal resoluti on, surface forcing functions, and bathymetry, and is extended to high er latitudes. Overall, there is a conversion of lighter to heavier wat er masses, and waters of densities 1027.95 and 1028.05 kg m(-1) are pr oduced in the Greenland-Iceland Norwegian basin, and of density 1027.7 5 kg m(-3) in the Labrador and Irminger basins. These water masses flo w primarily southward. The main purpose of this present study, however , is to investigate the ventilation of the subtropical gyre. The role of Ekman pumping and lateral induction in driving the subduction proce ss is examined and the relative importance of the latter is confirmed. The paper also illustrates how the mixed layer waters are drawn south ward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging between 1026.40 and 1027.30 kg m(-3 ). These are organized into regular fashion by the model from a relati vely disorganized initial state. The evolution of the model gyre durin g spinup is governed by mixed layer cooling in the central North Atlan tic, which causes the ventilation patterns to move southwestward, the layers to rise, and surprisingly, to become warmer. This warming is ex plained by thermodynamic considerations. Finally, it is shown that the rate of change of potential vorticity following a fluid pathway in th e subtropical gyre is governed by the diffusion of layer thickness, wh ich represents subgrid-scale mixing processes in the model. This leads to increasing potential vorticity along pathways that ventilate from the thickest outcrop regions as fluid is diffused laterally and to dec reasing potential vorticity along neighboring trajectories.