Lifecycle of a Submesoscale Front Birthed from a Nearshore Internal Bore

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  • 1 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California
  • | 2 Oregon State University, Corvallis, Oregon
  • | 3 Oregon State University, Corvallis, Oregon
  • | 4 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California
  • | 5 Oregon State University, Corvallis, Oregon
  • | 6 Oregon State University, Corvallis, Oregon
  • | 7 Oregon State University, Corvallis, Oregon
  • | 8 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California
  • | 9 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California *
  • | 10 Naval Information Warfare Center Pacific, San Diego, California
  • | 11 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California
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Abstract

The ocean is home to many different submesoscale phenomena, including internal waves, fronts, and gravity currents. Each of these processes entail complex nonlinear dynamics, even in isolation. Here we present shipboard, moored, and remote observations of a submesoscale gravity current front created by a shoaling internal tidal bore in the coastal ocean. The internal bore is observed to flatten as it shoals, leaving behind a gravity current front that propagates significantly slower than the bore. We posit that the generation and separation of the front from the bore is related to particular stratification ahead of the bore, which allows the bore to reach the maximum possible internal wave speed. After the front is calved from the bore, it is observed to propagate as a gravity current for ≈4 hours, with associated elevated turbulent dissipation rates. A strong cross-shore gradient of along-shore velocity creates enhanced vertical vorticity (Rossby number ≈ 40) that remains locked with the front. Lateral shear instabilities develop along the front and may hasten its demise.

Current Affiliation: Department of Geosciences, University of Oslo, Oslo, Norway

Corresponding author: Jennifer A. MacKinnon,jmackinnon@ucsd.edu

Abstract

The ocean is home to many different submesoscale phenomena, including internal waves, fronts, and gravity currents. Each of these processes entail complex nonlinear dynamics, even in isolation. Here we present shipboard, moored, and remote observations of a submesoscale gravity current front created by a shoaling internal tidal bore in the coastal ocean. The internal bore is observed to flatten as it shoals, leaving behind a gravity current front that propagates significantly slower than the bore. We posit that the generation and separation of the front from the bore is related to particular stratification ahead of the bore, which allows the bore to reach the maximum possible internal wave speed. After the front is calved from the bore, it is observed to propagate as a gravity current for ≈4 hours, with associated elevated turbulent dissipation rates. A strong cross-shore gradient of along-shore velocity creates enhanced vertical vorticity (Rossby number ≈ 40) that remains locked with the front. Lateral shear instabilities develop along the front and may hasten its demise.

Current Affiliation: Department of Geosciences, University of Oslo, Oslo, Norway

Corresponding author: Jennifer A. MacKinnon,jmackinnon@ucsd.edu
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