CLIMATE DRIFT IN A GLOBAL OCEAN GENERAL-CIRCULATION MODEL

A global version of the GFDL modular ocean model is forced using conve ntional restoring boundary conditions (BCs), mixed BCs (i.e., restorin g the upper-level temperature but specifying a fixed salt flux), and s tochastic fluxes of both heat and freshwater. The climatology of the m odel is found to drift if stochastic freshwater fluxes are applied at high latitudes under mixed BCs. The drift is global in extent: the oce an is generally warmer in the North Pacific and Weddell Sea but cooler and fresher at depths elsewhere in the Southern Ocean and in the Nort h Atlantic. There is a slight reduction (by about 5%) in the meridiona l overturning of the Southern Ocean and the North Atlantic. The drift of the barotropic flow is most pronounced in the Southern Ocean and is associated with a permanent meandering of the Antarctic Circumpolar C urrent. The drift occurs within a few decades, suggesting that it may be important in enhanced greenhouse scenarios for early next century t hat have been obtained using coupled atmosphere-ocean GCMs. It is also possible that some of the intrinsic variability identified in the sam e models is actually a residual drift.The drift depends upon convectiv e adjustment to occur but can be amplified by the surface heat flux pa rameterization, both locally and by an additional feedback associated with large-scale flow changes. In an extreme case, the latter leads to a total collapse of the thermohaline circulation associated with Nort h Atlantic Deep Water Formation. A similar mechanism underlies the dri ft that can occur when the switch from restoring to mixed BCs is made. The heat flux feedback represents the atmosphere-ocean coupling in th e model, so this aspect of the drift can be regarded as a coupled mode that actually contributes to the mean state of the coupled system. Th e existence of such modes makes some climatic drift in coupled models inevitable, if the individual components are equilibrated separately p rior to coupling. The applicability of these results to more sophistic ated coupled models depends, in part, upon how well the restoring BC o n temperature captures the heat flux feedback they exhibit.