Abstract
AbstractMesoscale eddies are ubiquitous dynamical features that tend to propagate westward and disappear along ocean western boundaries. Using a multiscale observational study, we assess the extent to which eddies dissipate via a direct cascade of energy at a western boundary. We analyze data from a ship‐based microstructure and velocity survey, and an 18‐month mooring deployment, to document the dissipation of energy in anticyclonic and cyclonic eddies impinging on the topographic slope east of the Bahamas, in the North Atlantic Ocean. These observations reveal high levels of turbulence where the steep and rough topographic slope modified the intensified northward flow associated with, in particular, anticyclonic eddies. Elevated dissipation was observed both near‐bottom and at mid depths (200–800 m). Near‐bottom turbulence occurred in the lee of a protruding escarpment, where elevated Froude numbers suggest hydraulic control. Energy was also radiated in the form of upward‐propagating internal waves. Elevated dissipation at mid depths occurred in regions of strong vertical shear, where the topographic slope modified the vertical structure of the northward eddy flow. Here, low Richardson numbers and a local change in the isopycnal gradient of potential vorticity (PV) suggest that the elevated dissipation was associated with horizontal shear instability. Elevated mid‐depth dissipation was also induced by topographic steering of the flow. This led to large anticyclonic vorticity and negative PV adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation. Our results provide a mechanistic benchmark for the realistic representation of eddy dissipation in ocean models.
Highlights
Mesoscale eddies are ubiquitous in the world's oceans
This led to large anticyclonic vorticity and negative potential vorticity (PV) adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation
To explain the anomalous layers observed to the west of 76.92°W, we suggest that, in order to balance the generation of negative PV at the boundary and restore marginal stability, fluid is redistributed laterally along isopycnals
Summary
Mesoscale eddies are ubiquitous in the world's oceans. They typically have a diameter of the order 100 km and generally propagate westward, transferring energy and momentum, affecting budgets of heat, salt, and carbon (Chelton et al, 2011), and accounting for over 80% of global oceanic kinetic energy (Ferrari & Wunsch, 2009). Through a broad range of potential interactions, when the geostrophically near-balanced mesoscale eddy flows encounter steep topography they may become unstable, generating turbulence that dissipates mesoscale energy In this way, eddy-topography interactions at western boundaries may play an important role in the dissipation of eddy energy via a direct cascade to small scales. The relative roles of internal wave radiation and near-boundary processes in energy dissipation were addressed by Klymak and Gregg (2004), in the context of a comprehensive observation-based study of tidal flow over a sill in a coastal fjord These authors found that around one third of the energy lost from the impinging flow was dissipated locally to the sill, in the form of turbulence associated with shear instability and a hydraulic jump. Intense internal wave generation occurs where geostrophic flow impinges on rough and steep topography This process may play an important role in the dissipation of mesoscale eddy energy at western boundaries. Some of the processes we observe are compatible with, and some are distinct from, those highlighted in the theoretical and modeling studies reviewed above
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