ALBERT

Description: The sensitivity of the ocean’s climate to the diapycnal diffusivity in the ocean is studied for a global warming scenario in which CO2 increases by 1% yr−1 for 75 yr. The thermohaline circulation slows down for about 100 yr 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-yr integrations, are variable, but a direct relation with the diapycnal diffusivity cannot be found. At year 70 (when CO2 has doubled) an increase of the diapycnal diffusivity from 0.1 to 1.0 cm2 s−1 leads to a decrease in surface air temperature of about 0.4 K and an increase in sea level rise of about 4 cm. The steric height gradient is divided into thermal component and haline component. It appears that, in the first 60 yr 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 deep-ocean heat uptake is due to reduced upward isopycnal diffusive flux and parameterized-eddy advective flux. Surface warming, induced by enhanced CO2 in the atmosphere, leads to a reduction of the isopycnal slope, which translates into a reduction of the above fluxes. The amount of reduction is directly related to the magnitude of the isopycnal diffusive flux and parameterized-eddy advective flux at equilibrium. These latter fluxes depend on the thickness of the thermocline at equilibrium and hence on the diapycnal diffusion. Thus, the increase of deep-ocean heat uptake with diapycnal diffusivity is an indirect effect that the latter parameter has on the isopycnal diffusion and parameterized-eddy advection.

Description: The diapycnal diffusivity in 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 suggest 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. In this paper the sensitivity of the current climate to the diapycnal diffusivity is studied, focusing on the changes occurring in the ocean circulation. To this end, a coupled model with a three-dimensional ocean with idealized geometry is used. The 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 based on scaling arguments for uncoupled models 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 in closer accordance with the theoretical value. The vertical heat balance in the global ocean is controlled by, in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) parameterized mesoscale eddy [Gent–McWilliams (GM)] advection. The size of the latter three fluxes increases with diapycnal diffusivity, because 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. Thus larger diapycnal diffusion is compensated for by changes in isopycnal diffusion and GM advection. Little change is found for the advective flux because of compensation between downward and upward advection. Sensitivity results are presented 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 is consistent with the findings of two-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 this model.