Stationary Sea Surface Height Anomalies in Cyclonic Boundary Currents; the Role of PV-Conservation Along a Topographic Slope

In northern high-latitude sub-polar seas, such as the Nordic Seas and the Labrador Sea, time-mean geostrophic currents mediate the meridional oceanic heat transport. These currents are often found on the continental slopes as intense cyclonic boundary currents, which, due to the relatively weak stratification, are strongly steered by the bottom topography. However, analysis of hydrographic and satellite altimetric data along depth contours exhibit some remarkable stationary along-stream variations in the depth-integrated buoyancy. A closer examination reveals that the variations seem to be linked to changes in steepness and curvature of the topography beneath.

In order to examine the underlying dynamics, a steady-state model of a cyclonic stratified boundary current over a topographic slope is developed in the limit of small Rossby numbers. To the lowest order, the bottom velocities are aligned with the bottom topography. Based on the conservation of potential vorticity, equations for variations of the first-order pressure and buoyancy fields along the depth contours are derived. These show that the pressure and the depth-integrated buoyancy tend to increase (decrease) where the lowest order flow increases (decreases) its relative vorticity. Along-isobath variations in relative vorticity, in turn, tend to be most pronounced for cyclonic anomalies and occur where the topography is steep and/or curves. The thus predicted variations in pressure and buoyancy are comparable in magnitude to the ones found in the data.