RESEARCH
Sources and pathways of Weddell Sea Deep Water (WSDW) and Weddell Sea Bottom Water (WSBW). These cold, dense waters are formed as a result of brine rejection (during sea ice formation). As they transit clockwise around the Weddell Sea, they mix with cold shelf waters waters, eventually forming a portion of the Antarctic Bottom Water (AABW) in the Scotia Sea. One important conduit for this dense water is the Orkney Passage.
Centrifugal instability in the Orkney Passage, Part 1: observations
This is the first in a three-part study of potential vorticity dynamics associated with centrifugal instability (CI). In this part, we examine moored observations, 2015-2017, over the OP sill, which nominally resolve mean flow characteristics within the Passage. We first document volume transport within the Passage, finding that it hovers around 2 Sv with bi-monthly variability of 1 Sv. In March 2016, we observe a reversal in the flow, which we attribute to changes in mesoscale eddying dynamics associated with the Antarctic Circumpolar Current. Second, we document an observed relationship between mean flow and mean potential vorticity (PV) within the Passage. Third, while long-term dissipation measurements are missing from the moored measurements, we nonetheless demonstrate from rapidly-sampling (0.1 Hz) thermistors that turbulence is enhanced during periods of strong mean flow, suggesting topographic enhancement of energy and buoyancy fluxes within the OP. A realistic high-resolution (1-km) model qualitatively captures these features, providing a basis for further study with the model.
Centrifugal instability in the Orkney Passage, Part 2: realistic simulations
This is the second in a three-part study of potential vorticity dynamics associated with centrifugal instability (CI). In this part, we examine energy and buoyancy fluxes in high-resolution (1-km and 300-m) numerical simulations of the Orkney Passage. These simulations make use of realistic simulations in a nested configuration to progressively examine fine-scale structure associated with CI in the Passage.
Centrifugal instability in the Orkney Passage, Part 3: down-scaling
To be continued ...
Strong mean flows interacting with topography can generate cyclones (purple) and anticyclones (yellow). If the vortices are characterised by low gradient Richardson numbers typically found in bottom boundary layers (BBLs), the cyclone must alter its Rossby number. This occurs either through cyclo-geostrophic adjustment or loss of energy from symmetric instability (i.e. centrifugal instability). In contrast, anticyclones do not alter their mean state appreciably. This may explain the dominance of anticyclones over cyclones in the interior ocean.
The role of curvature in modifying frontal instabilities
In order to apply some of the methods developed by Thomas et al. (2013), DSR-II, to fronts in which centrifugal forces are present, we first investigated how curvature modifies the non-dimensional criterion of Hoskins (1974). Taking into consideration the curvature of the front involves examination of both (1) absolute angular momentum and (2) Ertel PV, and ultimately leads to a generalization of the Rayleigh criterion valid for continuously stratified curved fronts on an f-plane. In non-dimensional form, it supersedes the criterion of Hoskins (1974). We then investigate this expression using idealized axisymmetric vortices in gradient wind balance. An interesting outcome is that, for gradient Richardson numbers near unity, anticyclonic flows are more stable than cyclonic flows characterized by the same gradient Rossby number. We propose this provides a possible explanation for why small-scale, coherent vortices (e.g. submesoscale coherent vortices and polar eddies) are almost universally anticyclonic.