The Role of diapycnal mixing in coupled atmosphere-ocean general circulation models

The value of ocean diapycnal diffusivity (v) sets the rate at which dense bottom water can be mixed<br/>up through the stratified water column and thus plays an important role in the meridional overturning<br/>circulation (MOC). Previous idealised experiments and simplified theory suggest that the strength of<br/>the MOC and the ocean heat transport scale with the v. This study investigates the dependence of<br/>the MOC and other parameters on v using atmosphere-ocean general circulation models (AOGCM).<br/>Firstly, the dependence of the MOC strength on v is studied using a low resolution AOGCM with<br/>realistic geometry, FORTE, with spatially constant v values ranging from 0.1 cm2/s to an unrealistic<br/>high value of 5 cm2/s. At the cyclostationary state, global MOC strength is found to scale with v<br/>(in agreement with previous studies) according to a power law of 0.5. No power law is found for the<br/>MOC in the individual basins. The increase in MOC strength in the Atlantic and Pacific Oceans is<br/>associated with an increase in the ocean heat transport. The atmosphere responds to the change in<br/>the ocean state by a decrease of its energy transport and surface winds. Only a partial compensation<br/>is found between the ocean and atmosphere energy transport. The strength of v is found to have a<br/>strong impact on coupled phenomena, such as a cessation of El Niño at high v.<br/>Secondly, similar experiments are conducted with a state-of-the-art AOGCM, ECHAM5/ MPIOM.<br/>In this model, v is derived from a constant background diapycnal diffusion (b), wind induced<br/>mixing, the Richardson number and the convective adjustment. A set of 3 coupled experiments is<br/>conducted, with b = 0.1, 0.25 and 1 cm2/s. The scaling law from simple theory and the previous<br/>experiments with FORTE is not observed with this coupled model. At the cyclostationary state, the<br/>MOC strength weakens by 16% as b increases from 0.1 to 1 cm2/s. This behavior is not found<br/>when the experiments are repeated with an ocean-only model. The reduction in MOC in the coupled<br/>model is linked to a strong reduction in the convective mixing at high latitudes. The convective<br/>mixing is reduced by a continuous strong freshening in the Arctic region due to an increase in surface<br/>air temperature and melting of the sea-ice in the coupled experiments, which is not observed in the<br/>ocean-only experiments.<br/>The responses of the two coupled models show many similarities as b increases. Both models<br/>show convection in the Pacific for high values of b. The main difference is the response of the MOC<br/>in the Atlantic is linked to the different locations of the deep convection and their relative changes in<br/>the models.<br/>I conclude that the diapycnal mixing and the ocean-atmosphere interactions both control the strength<br/>of the MOC, and their influences cannot be considered separately.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2006

Author dc:creator

• Dubois, Clotilde