Submesoscale Dynamics near a Seamount. Part I: Measurements of Ertel Vorticity

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  • 1 School of Oceanography, University of Washington, Seattle, Washington
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Abstract

The prevailing view that submesoscale fluctuations (horizontal wavelengths less than a few kilometers and vertical wavelengths less than a few hundred meters) are dominated by internal gravity waves is tested by measuring Ertel's potential vorticity, Π = (f + ∇ × V) · ∇B, where the buoyancy B = −gδρ/ρo. Unlike geostrophic or nonlinear Ertel vorticity-carrying motions, internal waves have no Ertel vorticity fluctuations. Velocity and temperature profile surveys beside Ampere Seamount reveal appreciable Ertel enstrophy, and thus a significant non-internal-wave component, on horizontal wavelengths of 6–15 km and vertical wavelengths of 50–380 m. The twisting terms are negligible and the relative vorticities less than 0.2f, so the anomalies are in geostrophic balance.

It is unlikely that the anomalies arise from stirring of the large-scale isopycnal gradients of stretching and planetary Ertel vorticity as this would require stirring lengths of thousands of kilometers. The most likely source appears to be forcing at the seamount, but generation by (i) dissipative 3D turbulence in the pycnocline or (ii) detrainment of the winter mixed layer cannot be absolutely ruled out. It remains to determine whether the coexistence of internal wave and Ertel vorticity-carrying fluctuations characterizes smaller scales (λ ≤ 50 m, λH ≤ 5 km) in the deep ocean away from topography as well.

Abstract

The prevailing view that submesoscale fluctuations (horizontal wavelengths less than a few kilometers and vertical wavelengths less than a few hundred meters) are dominated by internal gravity waves is tested by measuring Ertel's potential vorticity, Π = (f + ∇ × V) · ∇B, where the buoyancy B = −gδρ/ρo. Unlike geostrophic or nonlinear Ertel vorticity-carrying motions, internal waves have no Ertel vorticity fluctuations. Velocity and temperature profile surveys beside Ampere Seamount reveal appreciable Ertel enstrophy, and thus a significant non-internal-wave component, on horizontal wavelengths of 6–15 km and vertical wavelengths of 50–380 m. The twisting terms are negligible and the relative vorticities less than 0.2f, so the anomalies are in geostrophic balance.

It is unlikely that the anomalies arise from stirring of the large-scale isopycnal gradients of stretching and planetary Ertel vorticity as this would require stirring lengths of thousands of kilometers. The most likely source appears to be forcing at the seamount, but generation by (i) dissipative 3D turbulence in the pycnocline or (ii) detrainment of the winter mixed layer cannot be absolutely ruled out. It remains to determine whether the coexistence of internal wave and Ertel vorticity-carrying fluctuations characterizes smaller scales (λ ≤ 50 m, λH ≤ 5 km) in the deep ocean away from topography as well.

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