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

Abstract

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

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