Abstract
Abstract The major mechanisms of the oceanic poleward heat flux in the Southern Ocean are still in debate. The long-standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This belief has recently been challenged by several numerical modeling studies, which stress the importance of mean flow for the meridional heat flux in the Southern Ocean. Here, this study analyzes moored current meter data obtained recently in the Fawn Trough, Kerguelen Plateau, to estimate the cross-stream heat flux caused by the time-mean flow and transient eddies. It is shown that the poleward eddy heat flux in this southern part of the ACC is negligible, while that from the mean flow is overwhelming by two orders of magnitude. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task.
Highlights
The poleward oceanic heat flux in the Southern Ocean constitutes an important component of climate, connected with the meridional overturning circulation
There are some minor differences among models, most of them consistently show that the poleward heat transport across the Antarctic Circumpolar Current (ACC) south of 458S is predominantly carried by the time-mean horizontal circulation and that the heat flux due to transient eddies amounts to approximately one-third of that due to the former (Meijers et al 2007; Volkov et al 2010)
We have shown that across the Southern ACC Front (SACCF) passing through the Fawn Trough over the Kerguelen Plateau, the poleward heat flux due to transient eddies is negligible (0.3 kW m22) while that due to time-mean flow is abnormally large (219 kW m22) and associated with strong upwelling and anticlockwise turning with decreasing depth
Summary
The poleward oceanic heat flux in the Southern Ocean constitutes an important component of climate, connected with the meridional overturning circulation. To locate the vertical and horizontal positions of each current meter relative to the frontal structure of temperature across the Fawn Trough, Fig. 6 shows the observed time-mean currents and velocity variance ellipses, which are superimposed on a temperature section drawn from the top-to-bottom CTD data gathered during the deployment and recovery TRACK cruises.
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