Mesoscale eddy dissipation by a 'zoo' of submesoscale processes at a western boundary

Date: 
Wednesday 20 November 2019 - 11:00
Location: 
NOC Southampton - Node Room (074/02) (Waterfront Campus).
Speaker: 
Dr. Gwyn Evans (NOC, Southampton)

 

Oceanic mesoscale eddies are ubiquitous. They propagate westward and disappear along the western boundaries of ocean basins. We assess whether eddies disappear via a direct cascade of energy involving turbulent processes at the boundary, using a multi-scale observational study. We analyse data from a ship-based microstructure survey and an 18-month mooring deployment to assess the dissipation of energy in mesoscale eddies impinging on the slope east of the Bahamas in the north Atlantic Ocean. The ship-based observations revealed 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. Near-bottom turbulence was observed in the lee of a protruding escarpment where elevated Froude numbers suggest hydraulic control. Energy is also radiated in the form of upward propagating internal waves. Elevated dissipation at mid-depths occurred in regions of strong vertical shear where the slope modified the vertical structure of the northward eddy flow. Low Richardson number and a local change in gradient of potential vorticity suggests the elevated dissipation may be associated with horizontal shear instability. Elevated mid-depth dissipation is also associated with topographic steering of the flow, leading to southward flow adjacent to the slope and associated negative potential vorticity. Here, centrifugal instability appears to drive the observed enhancement in dissipation. The importance of these dissipative processes to mesoscale eddy decay are explored by comparing eddy energy to these energy sinks. Implications for the global ocean energy budget are discussed.

 

Seminar category: 
POC seminars
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