Effects of vertical variations of thickness diffusivity in an ocean general circulation model

The effects of a prescribed surface intensification of the thickness (and isopycnal) diffusivity on the solutions of an ocean general circulation model are documented. The model is the coarse resolution version of the ocean component of the National Center for Atmospheric Research (NCAR) Community Climate System Model version 3 (CCSM3). Guided by the results of Ferreira et al. (2005) [Ferreira, D., Marshall, J., Heimbach, P., 2005. Estimating eddy stresses by fitting dynamics to observations using a residual-mean ocean circulation model and its adjoint. J. Phys. Oceanogr. 35, 1891-1910.] we employ a vertical dependence of the diffusivity which varies with the stratification, N², and is thus large in the upper ocean and small in the abyss. We experiment with vertical variations of diffusivity which are as large as 4000 m² s^-1 within the surface diabatic layer, diminishing to 400 m² s^-1 or so by a depth of 2 km. The new solutions compare more favorably with the available observations than those of the control which uses a constant value of 800 m² s^-1 for both thickness and isopycnal diffusivities. These include an improved representation of the vertical structure and transport of the eddy-induced velocity in the upper-ocean North Pacific, a reduced warm bias in the upper ocean, including the equatorial Pacific, and improved southward heat transport in the low- to mid-latitude Southern Hemisphere. There is also a modest enhancement of abyssal stratification in the Southern Ocean.