ON THE ROLE OF FLUX ADJUSTMENTS IN AN IDEALIZED COUPLED CLIMATE MODEL

The effect of employing flux adjustments on the climatic response of a n idealized coupled model to an imposed radiative forcing is investiga ted with two coupled models, one of which employs flux adjustments. A linear reduction (to the planetary longwave flux) of 4 W/m(2) is appli ed over a 70 y period and held constant thereafter. Similar model resp onses are found (during the initial 70 y period) for global-scale diag nostics of hemispheric air temperature due to the nearly linear surfac e-air temperature response to the radiative forcing. Significant regio nal scale differences do exist, however. As the perturbation away from the present climate grows, basin-scale diagnostics (such as meridiona l overturning rates) begin to diverge between flux adjusted and non-fl ux adjusted models. Once the imposed radiative forcing is held constan t, differences in global mean air temperature of up to 0.5 degrees C a re found, with large regional-scale differences in air temperature and overturning rates within the North Atlantic and Southern Ocean. Two a dditional experiments with the flux adjusted model (beginning from poi nts further along the control integration) suggest that the eliminatio n of much of the coupling shock before the radiative forcing is applie d leads to results slightly closer to the non-flux adjusted case, alth ough large differences still persist. In particular a dipole structure indicating an enhanced warming within the Pacific sector of the South ern Ocean, and cooling within the Atlantic sector is not reproduced by the flux adjusted models. This disparity is intimately linked to the Southern Ocean overturning cell along with the flux adjustments employ ed as well as the drift arising from coupling shock. If a similar form of sensitivity exists in more realistic coupled models, our results s uggest: (1) perturbation experiments should not be undertaken until af ter the coupled model control experiment is carried out for several hu ndred years (thereby minimizing the coupling shock) (2) care should be exercised in the interpretation of regional-scale results (over the o cean) in coupled models which employ flux adjustments; (3) care should also be taken in interpreting even global-scale diagnostics in flux a djusted models for large perturbations about the present climate.