Towards a more consistent picture of isopycnal mixing in climate models

Abstract:

The stirring of tracers by mesoscale eddies along isopycnal surfaces is often represented in coarse-resolution models by the Redi diffusion parameter A_Redi. Theoretical treatments of A_Redi often assume it should scale as the eddy energy or the growth rate of mesoscale eddies,. producing a picture where it is high in boundary currents and low )of order a few hundred m²/s) in the gyre interiors. However, observational estimates suggest that A_Redi should be very large (of order thousands of m²/s) in the gyre interior. We present results of recent simulations comparing a range of spatially constant values A_Redi (with values of 400, 800, 1200 and 2400 m²/s) to a spatially resolved estimate based on altimetry and a zonally averaged version of the same estimate. In general, increasing the A_Redi coefficient destratifies and warms the high latitudes. Relative to our control simulation, the spatially dependent coefficient is lower in the Southern Ocean, but high in the North Pacific, and so the temperature changes mirror this. We also examine the response of ocean hypoxia to these changes. In general, the zonally averaged version of the altimetry-based estimate of A_Redi does not capture the full 2d representation.