USING CHLOROFLUOROCARBONS TO ASSESS OCEAN CLIMATE MODELS

The potential for using chlorofluorocarbons (CFCs) to assess ocean cli mate models is explored within a series of global ocean experiments. T he ocean model simulations are run under identical wind stress and the rmohaline forcing, but with three different formulations of subgrid-sc ale mixing. Two passive tracers are included in the equilibrated model s, representing the dissolved concentrations of CFC-11 and CFC-12 in s eawater. The resulting model simulations are then compared directly wi th observed CFC levels in key areas for deep and bottom water formatio n. CFC-II uptake is found to be overestimated in the Southern Ocean wh en mixing rates are defined in traditional Cartesian co-ordinates. The inclusion of an isopycnal mixing scheme (which is often used in coupl ed ocean-atmosphere models) actually degrades the CFC-11 simulation by blending water masses too strongly, particularly in the Southern Ocea n. The spurious uptake of CFC-II at 55 degrees-70 degrees S suggests t hat certain climate models might overestimate the role of the Southern Ocean in moderating climate change. A more sophisticated mixing param eterization that simulates the effects of subgrid-scale eddies on the mean ocean flow (and allows for zero lateral diffusion) is seen to gre atly reduce CFC-11 uptake in the Southern Ocean, Climate models that a dopt this new mixing scheme are likely to predict a more rapid CO2-ind uced warming over the Southern Hemisphere.