On the Sensitivity of Thermocline Depth and Meridional Heat Transport to Vertical Diffusivity in OGCMS

Abstract

Aimed at a further understanding of the role of vertical diffusivity in determining the vertical structure of the thermocline circulation and meridional heat transport in ocean general circulation models (OGCMS), sensitivity of a box-basin isopycnal ocean model to vertical diffusivity is examined. In constant diffusivity experiments, the model e-folding depth for potential density is proportional to the ? power of vertical diffusivity, which is in agreement with previous sensitivity studies with the GFDL OGCM. In the experiments with an N?1 type of stability-dependent vertical diffusivity, we found that the minimum diffusivity in the vertical is a relevant scale for the stability-dependent diffusivity in describing the sensitivity of the thermocline depth. With the choice of this vertical diffusivity Scale, the ? power law for the constant diffusivity cast can be extended to the stability-dependent diffusivity case. Contrary to the previous sensitivity studies, meridional heat transport in the present model is rather insensitive to vertical diffusivity. It is shown that the low sensitivity is mainly due to the different reference temperature and surface anomaly damping rate used in the model's Newtonian cooling parameterization for the surface heat flux. Since sensitivity of the thermocline depth is rather OGCM independent and that of meridional heat transport is dependent on the way surface heat flux is parameterized, the author suggests that vertical diffusivity be tuned to obtain a realistic thermocline. In this regard. the stability-dependent vertical diffusivity is recommended.