Stalling and Dissipation of a Near‐Inertial Wave (NIW) in an Anticyclonic Ocean Eddy: Direct Determination of Group Velocity and Comparison With Theory. Issue 5 (2nd May 2021)

• Record Type: Journal Article
• Authors: Sanford, Thomas B.; Ma, Barry B.; Alford, Matthew H.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 5(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-02
• Subjects: NIW -- near‐inertial -- mesoscale eddy -- intrinsic frequency -- critical layer -- group velocity; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016742
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: A near‐inertial wave stalling and breaking in a critical layer was observed for a week by a pair of autonomous velocity and density profilers on concentric paths in an ocean mesoscale eddy. Profiler observations provide estimates of the eddy's vertical vorticity and shear, quantities needed to test theories of downward near‐inertial wave (NIW) energy flux and loss from inertial wave–eddy interactions. The unique observations of the wave's intrinsic frequency ω i and vertical wavenumber m provide a novel estimate of the vertical group velocity C gz from changes in ω i with respect to m . Shear, strain, energy flux convergence, and parameterized turbulence are all elevated near 140 m depth, near the bottom of the strongest eddy velocities. Our observations are consistent with a downgoing NIW's group velocity decreasing owing to wave–eddy interactions, providing important clues on global energetics of NIW mixing. Plain Language Summary: Internal gravity waves with frequency near the Earth's Coriolis frequency, called a near‐inertial wave, was observed stalling and breaking as it propagated downward from the sea surface. The wave stalled because its downward propagation speed slowed in the presence of a background eddy that weakened with depth. We believe this is by far the clearest observation of such a process, which is an important aspect of interactions between internal gravity waves and eddies. A key finding of the paper is the first‐ever direct determination of the wave's propagation speed from our observations without assumptions or reliance on theory. Key Points: A pair of EM‐APEX floats moving with a subsurface turbulent zone observe mesoscale vorticity (by Kelvin's circulation theorem) and vertical shear; observations resolve near‐inertial wave phase and group velocities, density, and shear to diagnose dynamics of NIW stalling and dissipation First known determination of NIW vertical group velocity computed from ∂ωi /∂m in ocean observations Analytic theory of mesoscale vorticity and vertical shear predicts NIW kinetic properties that compares well with observations