Feedbacks affecting the response of the thermohaline circulation to increasing CO2:: a study with a model of intermediate complexity

A three-dimensional ocean model with an idealized geometry and coarse resolution coupled to a two-dimensional (zonally averaged) statistical-dynamical atmospheric model is used to simulate the response of the thermohaline circulation to increasing CO2 concentration in the atmosphere. The relative roles of different factors in the slowing down and recovery of the thermohaline circulation were studied by performing simulations with ocean only and partially coupled models. The computational efficiency of the model allows an extensive and thorough study of the causes of changes in the strength of the thermohaline circulation, through a large number of extended runs. The evolution of the atmosphere-to-ocean surface heat fluxes is shown to be the dominant factor in causing the weakening of the circulation in response to an increasing external forcing as well as in controlling the subsequent recovery. The feedback between heat flux and the sea surface temperature is necessary for the ocean circulation to recover. The rate of the recovery, however, is not sensitive to the magnitude of the feedback, and changes in the atmosphere, while contributing to the recovery, play a secondary role. In the case of very strong feedback, substantial changes in the SST structure are shown not to be a necessary condition for the recovery of the circulation. Subsurface changes in the density structure accompany recovery despite nearly fixed SST in one of the uncoupled experiments. Changes in the zonal distribution of heat fluxes serve as a positive feedback for both decrease and recovery of the meridional overturning, and are as important as changes in the zonal-mean values of heat fluxes. The secondary role of the moisture fluxes is explained by a smaller magnitude of their contribution to the surface buoyancy flux. Imposing amplified changes in the moisture fluxes leads to a significant slow down of the circulation, accompanied, however, by changes in the heat flux. The changed heat flux, in its turn, makes a significant contribution to the future slow down. This feedback complicates the evaluation of the relative roles of the different fluxes.