John Marshall

Cecil and Ida Green Professor of Oceanography, MIT

POTENTIAL VORTICITY CONSTRAINTS ON THE DYNAMICS AND HYDROGRAPHY OF THE SOUTHERN-OCEAN

POTENTIAL VORTICITY CONSTRAINTS ON THE DYNAMICS AND HYDROGRAPHY OF THE SOUTHERN-OCEAN.

(MARSHALL, J and OLBERS, D and ROSS, H and WOLFGLADROW, D), JOURNAL OF PHYSICAL OCEANOGRAPHY, vol. 23, no. 3, pp. pages, 1993.

Abstract

Constraints on the hydrography and geostrophic velocity shear of the Southern Ocean implicit in its potential vorticity field are discussed and illustrated by diagnostic study of observed and modeled potential vorticity fields. A stress-driven, thermodynamically inactive, eddy-resolving quasigeostrophic model of the Southern Ocean suggests that, through the systematic erosion of potential vorticity gradients by geostrophic eddies, the large-scale flow equilibrates toward a state in which interior potential vorticity gradients are small. Observations of the large-scale isopycnal distribution of potential vorticity (IPV), deduced from climatological hydrographic data, reveal a much richer structure. The most striking feature of the IPV field is the presence of large, near-surface gradients of IPV (a vortocline) coinciding with the axis of the Antarctic Circumpolar Current (ACC). To the south of this front potential vorticity (PV) is large; to its north, PV has low values, and relative to those found in the vortocline, PV gradients are indeed small. Beneath near-surface layers it is shown that a striking functional relationship exists between IPV and potential density suggesting that the broad density structure and baroclinic shear of the Southern Ocean can be found by inverting, consistent with boundary conditions, a potential vorticity distribution that is uniform on isopycnal surfaces. An analytical model of the zonally averaged ACC based on the uniform PV model is presented, which predicts a transport relative to the bottom of approximately 130 Sv (1 Sv = 10*6 m*3 s*-1), close to the observed value, and a meridional eddy heat flux of 0.25 PW independent of any ad hoc eddy closure assumptions. The model is then applied in a more realistic setting and the implied hydrography, relative baroclinic velocity, and dynamic topography of the uniform PV model favorably compared to the observations in the South Atlantic.

doi = 10.1175/1520-0485(1993)0232.0.CO;2