The diapycnal diffusivity of the ocean is one of the least known parameters in current climate models. Measurements of this diffusivity are sparse and insufficient for compiling a global map. Inferences from inverse methods and energy budget calculations suggests as much as a factor of 5 difference in the global mean value of the diapycnal diffusivity. Yet, the climate is extremely sensitive to the diapycnal diffusivity, as shown by studies using single-hemispheric ocean General Circulation Models (GCMs) and 2-dimensional coupled models. In this thesis we study the sensitivity of both the current climate and the climate change to the diapycnal diffusivity - using, for the first time, a coupled model with a 3-dimensional global ocean component and idealized geometry.

Our results show that, at equilibrium, the strength of the thermohaline circulation in the North Atlantic scales with the 0.44 power of the diapycnal diffusivity, in contrast to the theoretical value of 2/3. On the other hand, the strength of the circulation in the South Pacific scales with the 0.63 power of the diapycnal diffusivity. The implication is that the amount of water upwelling from the deep ocean may be regulated by the diapycnal diffusion in the Indo-Pacific ocean.

The vertical heat balance in the ocean is controlled by: in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) bolus velocity (GM) advection. The size of the latter three fluxes increases with diapycnal diffusivity. The thickness of the thermocline also increases with diapycnal diffusivity leading to greater isopycnal slopes at high latitudes, and hence enhanced isopycnal diffusion and GM advection. Larger diapycnal diffusion compensates for changes in isopycnal diffusion and GM advection. Little changes are found for the advective flux because of compensation between changes in downward and upward advective fluxes.

We present sensitivity results for the hysteresis curve of the thermohaline circulation. The stability of the climate system to slow freshwater perturbations is reduced as a consequence of a smaller diapycnal diffusivity. This result confirms the findings of 2-dimensional climate models. However, contrary to the results of these studies, a common threshold for the shutdown of the thermohaline circulation is not found in our model.

In our global warming experiments, the thermohaline circulation slows down for about 100 years and recovers afterward, for any value of the diapycnal diffusivity. The rates of slowdown and of recovery, as well as the percentage recovery of the circulation at the end of 1000-year integration, is variable but a direct relation with the diapycnal diffusivity cannot be found. The steric height gradient is divided into a temperature component and a salinity component. It appears that, in the first 70 years of simulated global warming, temperature variations dominate the salinity ones in weakly diffusive models, whereas the opposite occurs in strongly diffusive models.

The analysis of the vertical heat balance reveals that, in global warming experiments, deep ocean heat uptake is due to reduced upward isopycnal diffusive flux and GM advective flux. Surface warming, induced by enhanced CO2 in the atmosphere, leads to a reduction of the isopycnal slope which translates to the magnitude of the isopycnal diffusive flux and GM advective lux at equilibrium. As mentioned above, the latter fluxes depend on the thickness of the thermocline at equilibrium, hence on the diapycnal diffusion. Thus, the increase of deep-ocean heat content with diapycnal diffusivity is an indirect effect that the latter parameter has on the isopycnal diffusion and GM advection.

Lastly, we analyze results from three climate GCMs involved in the Coupled Model Intercomparison Project. These GCMs have similar climate sensitivity to the MIT Earth Model of Intermediate Complexity (MIT-EMIC) but different rate of deep-ocean heat uptake. We find that the rates of change of surface air temperature and sea level rise are comparable to those derived from the MIT-EMIC with different values of the diapycnal diffusivity. At the year of doubling CO2, we estimate that an increase of the diapycnal diffusivity from 0.1 cm2/s to 1.0 cm2/s leads to a decrease in surface air temperature of about 0.4K and an increase in sea level rise of about 4 cm.