Jb. Zhang et al., SENSITIVITY OF THE GFDL MODULAR OCEAN MODEL TO PARAMETERIZATION OF DOUBLE-DIFFUSIVE PROCESSES, Journal of physical oceanography, 28(4), 1998, pp. 589-605
The effect of double-diffusive mixing on the general circulation is ex
plored using the GFDL MOM2 model. The motivation for this comes from t
he known sensitivity of the thermohaline circulation to the vertical d
iffusivity and the earlier work of Gargett and Holloway, who studied t
he effects of a simple nonunity ratio between heat and salt diffusivit
ies in a GCM. In this work, a more realistic, yet conservative, parame
terization of the double-diffusive mixing is applied, with the intensi
ty depending on the local density ratio R rho = alpha T-z/beta s(z). A
background diffusivity is used to represent non-double-diffusive turb
ulent mixing in the stably stratified environment. The numerical model
is forced by relaxation boundary conditions on both temperature and s
alinity at the sea surface. Three control experiments have been carrie
d out: one with the double-diffusive parameterization (DDP) determined
by the local density ratio, one with constant bur different diffusivi
ties for heat and salt as previously considered by Gargett and Hollowa
y (GHD), and the other with the same constant diapycnal eddy diffusivi
ty for both heat and salt (GHD). The meridional overturning in run DDP
is 22% less than in run CDD, and the maximum poleward heat transport
is about 8% less. In comparison, the overturning rate and poleward hea
r transport in run GHD display reductions that are about half as large
. The interior temperature and salinity in run DDP and GHD are higher
than in run CDD, with the change in run DDP more than twice that in ru
n GHD. In addition, in DDP and GHD, the density ratio distribution bec
omes closer to unity than in run CDD, with the change in run DDP being
larger than in GHD. Interes :ingly, the double diffusion is stronger
in the western boundary current region than the interior, implying a c
lose relation between vertical shear and the intensity of double diffu
sion. These results indicate a greater sensitivity of the thermohaline
circulation to double diffusion than had previously been suspected du
e to the tendency of the double-diffusive mixing to generate self-rein
forcing flows. This effect appears to be more significant when the dou
ble-diffusive mixing is applied only when the stratification is favora
ble rather than uniformly applied. In addition, parameter sensitivity
experiments suggest that double diffusion could have stronger effects
on the meridional overturning and poleward heat transport than modeled
here since the parameterizations chosen are rather conservative.