Ac. Hirst et Wj. Cai, SENSITIVITY OF A WORLD OCEAN GCM TO CHANGES IN SUBSURFACE MIXING PARAMETERIZATION, Journal of physical oceanography, 24(6), 1994, pp. 1256-1279
The sensitivity of a coarse-resolution model of the World Ocean to par
ameterization of subgrid-scale mixing is examined. The model is based
on the GFDL code. Results are presented from a series of model runs wh
ere the subsurface mixing parameterization is sequentially upgraded to
ward a more physical representation. The surface forcing is the same f
or all principal model runs and features a strong relaxation of surfac
e temperature and salinity toward perpetual wintertime observed values
. One model version is rerun with a full annual cycle of surface forci
ng and verifies that use of the perpetual winter surface relaxation in
troduces only minor biases in the essential characteristics of the sol
ution. Runs 1 and 2 feature the diffusivity tensor in the traditional
horizontal/vertical orientation, and examines the effect of different
vertical diffusivity profiles on the solution. Results are compared wi
th those of previous studies. In both cases, the water mass properties
(especially the salinity fields) are rather poor. In runs 3-5, a stan
dard parameterization is introduced that allows for enhanced diffusion
along the isopycnal surfaces. Each of these runs feature a different
prescribed profile of isopycnal diffusivity, though with the same prof
ile of vertical diffusivity as for run 2. Introduction of isopycnal mi
xing considerably improves the water mass structure, in particular by
freshening and cooling water at intermediate depths toward realistic l
evels. However, the vertical stratification and density fields are lit
tle changed from mn 2. Likewise, the current structure and meridional
overturning are little changed. Thus isopycnal mixing has a major effe
ct upon the temperature and salinity fields, but very minor effect on
the ocean dynamics. Isopycnal mixing is found to modestly increase pol
eward oceanic heat transport in the midlatitudes via enhanced quasi-ho
rizontal mixing of warm salty subtropical and cold fresh subpolar wate
rs. In run 6, the isopycnal diffusivity of run 4 is retained, but the
vertical diffusivity is instead allowed to vary as the inverse of the
local Brunt-Vaisala frequency. However, the resulting solution is litt
le changed from that of run 4. Reasons for this small change are discu
ssed. Also discussed are the impact of numerical problems associated w
ith the use of realistically small vertical diffusivity, and problems
inherent in deep water formation in coarse-resolution models.