Internal tide dissipation by an anti-cyclonic mode water eddy in the western-boundary of the North Atlantic subtropical gyre

• Main Authors: Fernández-Castro, B., Evans, D. Gwyn, Vic, C., Frajka-Williams, Eleanor, Naveira-Garabato, Alberto
• Format: Still Image
• Language: English
• Published: 2019
• Subjects: Canada; Cierva; North Atlantic
• Online Access: http://hdl.handle.net/10261/206128

Poster.-- 27 IUGG General Assembly, 8-18 Julio 2019, Montreal (Canada) The MerMEED project seeks to determine the mechanisms of mesoscale energy dissipation in the western boundary of the subtropical North Atlantic. Here, we present results from a 4-month glider survey between November 2017 and March 2018, within 300 km of the shelf break at ~26ºN. An anticyclonic mode water eddy that remained in the study area until late January. During this time, the glider performed three transects across the eddy (13-23 Nov, 11-31 Dec, and 1-13 Jan). At the eddy core we observed a mode of 18 ºC water with weak stratification (N<0.001 s-1), low potential vorticity (<0.5·10-9 s-3) and elevated oxygen concentration (AOU < 20 μmol kg-1) between 150 and 500 m (25.9-26.2 kg m-3); but its signal on isopycnal slopes extended down to the maximum sampling depth (1000 m). Turbulent kinetic energy dissipation rates were inferred from the fluctuations of glider-derived seawater vertical velocity. Energy dissipation was reduced at the eddy core (~0.5·10-9 W kg-1), and enhanced below it (1–1.3·10-9 W kg-1), particularly in the Northwestern flank, with respect to background levels away from eddy influence (≤1·10-9 W kg-1). Wavy structures in pressure perturbations, strain and vertical velocity, with a vertical wavelength of 100-200 m, suggest that dissipation below the eddy core could be related to eddy-internal wave interactions. Ray tracing experiments indicate that the most plausible explanation for this observation is that semi-diurnal, high mode (>3-4) internal-tides generated at the shelf break undergo a critical layer when impinging on the eddy The MerMEED project is funded by the U.K. Natural Environment Research Council. B. Fernández-Castro was supported by a Juan de La Cierva Formación postdoctoral fellowship (FJCI-641-2015-25712) and a José Castillejo travel grant (CAS18/00017) by the Spanish Government. Peer reviewed