Mean and Eddy Dynamics of the Main Thermocline

Abstract

This paper reviews and discusses a selection of developments in the theory of the structure of the main thermocline, and the mesoscale eddies that inhabit it. In classical theories, that is theories that assume a steady, near-laminar flow and that are based on the planetary geostrophic equations, the upper thermocline (below the surface mixed layer) is conservative and advectively dominated — that is, the dominant balance in the thermodynamic equation lies in the advective terms, leading to a ventilated thermocline. Below this the internal thermocline is a diffusive transition region, and in the limit of small diffusivity it becomes an internal boundary layer between the ventilated thermocline and the abyss. The thermocline is, typically, baroclinically unstable and this leads to an upper ocean populated by vigorous mesoscale eddies. The eddies are strongest in regions of western boundary currents, ‘mode water’ regions, and in the circumpolar current, and in these regions the eddies significantly affect the structure of the main thermocline. Elsewhere, the structure of the upper (ventilated) thermocline is largely determined by mean-flow advection. Lower in the water column, eddies typically tend to thicken the isostads that form the internal thermocline, leading to a complex three-way balance between mean flow, eddy fluxes and diffusion, suggesting that the internal thermocline may have finite thickness even as diffusivity tends to zero. In the circumpolar current eddies are a dominant effect, and qualitatively change the structure of the stratification.