Large-scale circulation with small diapycnal diffusion: The two-thermocline limit

Abstract

The structure and dynamics of the large-scale circulation of a single-hemisphere, closed-basin ocean with small diapycnal diffusion are studied by numerical and analytical methods. The investigation is motivated in part by recent differing theoretical descriptions of the dynamics that control the stratification of the upper ocean, and in part by recent observational evidence that diapycnal diffusivities due to small-scale turbulence in the ocean thermocline are small (-0.1 cm2 s-i). Numerical solutions of a computationally efficient, three-dimensional, planetary geostrophic ocean circulation model are obtained in a square basin on a mid-latitude B-plane. The forcing consists of a zonal wind stress (imposed meridional Ekman flow) and a surface heat llux proportional to the difference between surface temperature and an imposed air temperature. For small diapycnal diffusivities (vertical: