Mesoscale subduction at the Antarctic Polar Front driven by baroclinic instability

A study of mesoscale subduction at the Antarctic Polar Front (PF) is conduc ted by use of hydrographic data from a high-resolution, quasi-synoptic surv ey of the front. The geostrophic velocity and isopycnal potential vorticity (PV) fields are computed, and the ageostrophic flow diagnosed from the sem igeostrophic omega equation. It is found that the ageostrophic circulation induced by baroclinic instability counteracts the frontogenesis and frontol ysis effected by the confluence and difluence, respectively, of the geostro phic velocity field. Though the sense of the ageostrophic circulation is re versed repeatedly along the front, the existence of PV gradients along isop ycnals leads to a net cross-front "bolus'' transport. In response to a reve rsal of this gradient with depth (a necessary condition for the onset of ba roclinic instability), the bolus transport is northward at the protruding t emperature minimum layer that characterizes the PF, and southward above. Th is net cross-front overturning circulation acts to flatten the isopycnals o f the front and results in a subduction of the temperature minimum layer as it progresses northward along isopycnals. Consistently, a net baroclinic c onversion rate of approximately 1 cm(2) s(-2) d(-1), corresponding to a net subduction rate of O(20 m yr(-1)), is calculated in the survey area. The s imilarity between the PV field of the PF and other Southern Ocean fronts su ggests that the authors' results may also be applicable there. This has pro found implications for the understanding of the zonation of the Antarctic C ircumpolar Current.