Sensitivity of the ocean's climate to diapycnal diffusivity in an EMIC. Part II: Global warming scenario

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 cm(2) 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.