Climate Drift in a Global Ocean General Circulation Model

Abstract

A global version of the GFDL modular ocean model is forced using conventional restoring boundary conditions (BCs), mixed BCs (i.e., restoring the upper-level temperature but specifying a fixed salt flux), and stochastic fluxes of both heat and freshwater. The climatology of the model is found to drift if stochastic freshwater fluxes are applied at high latitudes under mixed BCs. The drift is global in extent: the ocean is generally warmer in the North Pacific and Weddell Sea but cooler and fresher at depths elsewhere in the Southern Ocean and in the North Atlantic. There is a slight reduction (by about 5%) in the meridional 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 Current. The drift occurs within a few decades, suggesting that it may be important in enhanced greenhouse scenarios for early next century that have been obtained using coupled atmosphere-ocean GCMS. It is also possible that some of the intrinsic variability identified in the same models is actually a residual drift. The drift depends upon convective adjustment to occur but can be amplified by the surface heat flux parameterization, 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 North Atlantic Deep Water Formation. A similar mechanism underlies the drift that can occur when the switch from restoring to mixed BCs is made. The heat flux feedback represents the atmosphere-ocean coupling in the 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. The existence of such modes makes some climatic drift in coupled models inevitable, if the individual components are equilibrated separately prior to coupling. The applicability of these results to more sophisticated coupled models depends, in part, upon how well the restoring BC on temperature captures the heal flux feedback they exhibit.