A Coupled Atmosphere-Ocean Model of Intermediate Complexity for Climate Change Study

A three-dimensional ocean model with an idealized global geometry and coarse resolution coupled to a two-dimensional (zonal-mean) statistical-dynamical atmospheric model is used to investigate the response to the increasing CO2 concentration in the atmosphere. Long-term present-day climate simulations with and without asynchronous integration in the ocean have been carried out with and without flux adjustments, and with either the Gent-McWilliams (GM) parameterization scheme or horizontal diffusion (HD). The results show that a moderate degree of asynchronous coupling between the oceanOs momentum and tracer fields still allows an accurate simulation of transient behavior, including the seasonal cycle. The use of the GM scheme significantly weakens the Deacon Cell and eliminates convection in the Southern Ocean. The deep ocean temperatures systematically decrease in the runs without flux adjustment. We demonstrate that the mismatch between heat transports in the uncoupled states of two models is the main cause for the systematic drift. The global warming experiments are sensitive to the parameterization of the sub-grid mixing. The penetration of anomalous heat in the Southern Ocean is noticeably weaker in the case with the GM scheme. As a result, the transient surface response exhibits less inter-hemispheric asymmetry than in the HD case. The equilibrium surface warming of the Southern Hemisphere is noticeably larger than that of the Northern Hemisphere in the GM case; the equilibrium warming is more uniform in the HD case. Use of the GM parameterization also leads to smaller decrease and faster recovery of the sinking in the North Atlantic. The increase in the surface heat fluxes is shown to be the dominant factor causing the weakening of the circulation. Results of the simulation with different rates of increase in the forcing are also presented.

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