Eddy Heat Flux in the Southern Ocean: Response to Variable Wind Forcing

Abstract

The authors assess the role of time-dependent eddy variability in the Antarctic Circumpolar Current (ACC) in influencing warming of the Southern Ocean. For this, an eddy-resolving quasigeostrophic model of the wind-driven circulation is used, and the response of circumpolar transport, eddy kinetic energy, and eddy heat transport to changes in winds is quantified. On interannual time scales, the model exhibits the behavior of an "eddy saturated" ocean state, where increases in wind stress do not significantly change the circumpolar transport, but instead enhance the eddy field. This is in accord with previous dynamical arguments, and a recent observational study.The instantaneous response to increased wind stress is to cool temperatures through increased northward Ekman transport of cool water. But, in the longer term, the enhanced eddy state is more efficient at transporting heat, leading to a warming of the ocean. The total eddy heat flux response is greater than the Ekman transport heat flux in this model by a factor of 2, indicating that coarse (non eddy resolving) models may fail to adequately capture the key processes. The authors also test the model response to long-term changes in wind forcing, including steadily increasing circumpolar wind strength over a 30-yr period. The model shows a response in eddy heat flux, and a change in ocean temperature not dissimilar from observed Southern Ocean warming. These findings suggest that eddy heat flux, energized by increasing wind stress, may be a significant contributor to the observed warming of the Southern Ocean.

Configuration of a Southern Ocean Storm Track

“…Current views of Southern Ocean equilibration (e.g., Hogg et al 2008) suggest that transient eddies compensate an increase in wind-driven Ekman overturning. Here, stationary fluxes respond to changes in meander size, which accompany a change in zonal-mean flow.…”

Exchange across the shelf break at high southern latitudes

“…ACC frontal jets are known to be baroclinically unstable (Inoue, 1985;Moore et al, 1997Moore et al, , 1999Sprintall, 2003;Dong et al, 2006) and there are suggestions that the ACC is actually in an "eddy saturated" (Hogg et al, 2008) state where wind increases do not significantly change the transport but instead increase the eddy kinetic energy (Meredith and Hogg, 2006).…”

Chapter 1 Impacts of the Oceans on Climate Change

“…The exact causes of this warming are not yet understood, though most theories seek to relate it to the intensification and southward shift of the band of westerly winds that overlie the circumpolar Southern Ocean. Potential candidate mechanisms include a latitudinal shift in the ACC, greater airsea heat fluxes, an intensification of the circumpolar eddy field, and possibly other processes also, almost certainly in some combination (Fyfe, 2006;Fyfe and Saenko, 2006;Gille, 2002Gille, , 2008Hogg et al, 2008;Meredith and Hogg, 2006). The large-scale circulation patterns of the Southern Hemisphere atmosphere over the past few decades reveal changes that are reflected in the leading mode of Southern Hemisphere climate variability, the SAM (Thompson and Wallace, 2000).…”