On the Buoyancy Forcing and Residual Circulation in the Southern Ocean: The Feedback from Ekman and Eddy Transfer

Abstract

The effect of buoyancy forcing on the residual circulation in the Southern Ocean is examined in two different ways. First, the rates of water-mass transformation and formation are estimated using air?sea fluxes of heat and freshwater in the isopycnal framework developed by Walin, which is applied to two different air?sea flux climatologies and a reanalysis dataset. In the limit of no diabatic mixing and at a steady state, these air?sea flux estimates of water-mass transformation and formation are equivalent to estimating the residual circulation and the subduction rates in the upper ocean, respectively. All three datasets reveal a transformation of dense to light waters between σ = 26.8 and 27.2, as well as positive formation rates peaking at σ = 26.6, versus negative rates peaking at σ = 27. The transformation is achieved either by surface heating or freshwater inputs, although the magnitude of the formation rates varies in each case. Second, an idealized model of a mixed layer and adiabatic thermocline for a channel is used to illustrate how changes in ocean dynamics in the mixed layer and freshwater fluxes can modify the buoyancy fluxes and, thus, alter the residual circulation. Increasing the Ekman advection of cold water northward enhances the air?sea temperature difference and the surface heat flux into the ocean, which then increases the residual circulation; an increase in wind stress of 0.05 N m?2 typically increases the surface heat flux by 8 W m?2 and alters the peaks in formation rate by up to 8 Sv (1 Sv ≡ 106 m3 s?1). Conversely, increasing the eddy advection and diffusion leads to an opposing weaker effect; an increase in the eddy transfer coefficient of 500 m2 s?1 decreases the surface heat flux by 3 W m?2 and alters the peaks in formation rate by 1 Sv.