REPRESENTING THE GLOBAL-SCALE WATER MASSES IN OCEAN GENERAL-CIRCULATION MODELS

A hierarchy of coarse-resolution World Ocean experiments were integrat ed with a view to determining the most appropriate representation of t he global-scale water masses in ocean general circulation models. The largest-scale response of the simulated ocean to the prescribed forcin g in each model run is described. The World Ocean model eventually has a realistic approximation of continental outlines and bottom bathymet ry. The model forcing at the sea surface is derived from climatologica l fields of temperature, salinity, and wind stress. The first experime nt begins with a quite unrealistic and idealized World Ocean. Subseque nt experiments then employ more realistic surface boundary conditions, model geometry, and internal physical processes. In all, 16 changes t o the model configuration are investigated. A fundamental dynamical co nstraint in the Drake Passage gap appears to limit the outflow rate of bottom water in the Antarctic region. This constraint acts to decoupl e the extreme Antarctic waters from the rest of the World Ocean. In a similar manner, including a surface wind stress acts to decouple the t wo hemispheres by limiting near-surface meridional flows across the eq uator. In the Atlantic basin, this decoupling becomes negligible when North Atlantic Deep Water (NADW) production is simulated. It is found that the representation of low salinity Antarctic Intermediate Water ( AAIW) is sensitive to the level of horizontal diffusion employed by th e model, as well as the chosen geometry of the Drake Passage gap and t he amount of buoyancy provided by the model's deep water. For example, provided that lateral diffusion rates are not too excessive, a fresh tongue of AAIW is simulated if either sufficiently dense bottom water is formed off Antarctica. or if enough NADW outflows into the Southern Ocean. The inclusion of an isopycnal mixing scheme is shown to improv e the representation of AAIW in coarse-resolution models. The rate of horizontal diffusion and the relative location of the Drake Passage ga p to the polar westerlies determines the shape and strength of an inte nse meridional overturning cell in the Southern Ocean. The inclusion o f an isopycnal mixing scheme does not affect this circulation pattern significantly. On the other hand, the intensification of NADW producti on can substantially weaken the downwelling component of this cell by drawing more water of Southern Ocean origin northward. Accurately simu lating NADW production and outflow requires a complete seasonal cycle in thermohaline forcing in the North Atlantic. The return path of NADW is primarily via a ''cold water route'' (i.e., the Drake Passage), al though sufficiently strong NADW formation sees some return flow via th e Agulhas leakage (i.e., the ''warm water route''). By the last experi ment of the present study, the model reproduces the subtle vertical la yering of deep and intermediate water masses quite accurately. This re presents a major success for the coarse-resolution multilevel ocean mo del.