THE CLIMATE RESPONSE TO CO2 OF THE HADLEY-CENTER COUPLED AOGCM WITH AND WITHOUT FLUX ADJUSTMENT

Coupled atmosphere-ocean general circulation models have a tendency to drift away from a realistic climatology. The modelled climate respons e to an increase of CO2 concentration may be incorrect if the simulati on of the current climate has significant errors, so in many models, i ncluding ours, the drift is counteracted by applying prescribed fluxes of heat and fresh water at the ocean-atmosphere interface in addition to the calculated surface exchanges. Since the additional fluxes do n ot have a physical basis, the use of this technique of ''flux adjustme nt'' itself introduces some uncertainty in the simulated response to i ncreased CO2. We End that the global-average temperature response of o ur model to CO2 increasing at 1% per year is about 30% less without fl ux adjustment than with flux adjustment. The geographical patterns of the response are similar, indicating that flux adjustment is not causi ng any gross distortion. The reduced size of the response is due to mo re effective vertical transport of heat into the ocean, and a somewhat smaller climate sensitivity. Although the response in both cases lies within the generally accepted range for the climate sensitivity, syst ematic uncertainties of this size are clearly undesirable, and the bes t strategy for future development is to improve the climate model in o rder to reduce the need for flux adjustment.