Variability in the Internal Wave Field Induced by the Atlantic Deep Western Boundary Current at 16°N

Abstract

Five years of continuous mooring data combined with conductivity-temperature-depth (CTD)/lowered acoustic Doppler current profiler (LADCP) measurements from five cruises are used to investigate the influence of the deep western boundary current (DWBC) on the internal wave field and associated vertical mixing at the continental slope at 168N in the western Atlantic. The mooring data include 2-hourly rotor current-meter measurements and temperature/conductivity time series with a high temporal resolution of 5-20 min. Thus, the data resolve time scales ranging from the low-frequency variability of the large-scale DWBC that generates internal waves due to interactions with the topography to frequencies greater than that of internal waves that are associated with vertical mixing. Estimates of the vertical mixing induced by the breaking of the observed internal waves show elevated diapycnal diffusivities of up to 10 23 6 0.4 3 10 23 m 2 s 21 in the bottommost 1500 m during times of a strong DWBC (maximum velocities at the mooring site up to 50 cm s 21 ) whereas vertical mixing rates are about an order of magnitude lower (1.6 3 10 24 6 0.6 3 10 24 m 2 s 21 ) during weak flow. During periods of a strong DWBC, spectra of horizontal velocity and internal wave available potential energy change substantially at depths below 1200 m and show a strong increase in variance particularly in the near-inertial frequency band. Low-frequency, near-inertial waves generated by topography/DWBC interaction on the slope to the west of the moorings can potentially cause this observed wave intensification; ray paths estimated for these waves agree well with the observed spectral changes at different depths. Variability in the high-frequency range, considered as a proxy for turbulent mixing, is significantly correlated with the DWBC strength above the continental slope.

Internal wave-driven mixing: governing processes and consequences for climate

“…Observations in the western boundaries of midlatitude ocean basins also show evidence for the generation and propagation of internal lee waves there [182]. Lee wave radiation is highly intermittent due to changes in the intensity and position of the background flow, resulting in temporal variability of the lee wave field on time scales up to decadal [183,184,185].…”

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

“…Internal tides are generated in areas where the barotropic tide interacts with rough or steep topography, and the global pattern of internal tide generation is a product of topographic roughness, tidal strength, and stratification. On the global scale, the low‐mode waves (wavelengths of a few kilometers to about 100 to 200 km, group velocities in the order of 1 m s −1 ) carry a major part of the energy converted from the barotropic tide (e.g., Falahat et al, ), but in regions of rough small‐scale topography such as mid‐ocean ridges (Falahat et al, ; St. Laurent & Garrett, ; Vic et al, , ) or eddy‐slope interaction such as western boundaries (Clément et al, ; Köhler et al, ), more energy can be contained in higher modes. The higher mode waves break near the generation region and dissipate locally (e.g., Klymak et al, ; Vic et al, ), while low‐mode internal tides as well as the wind generated near‐inertial waves can radiate far away from their sources (e.g., Alford, ; Alford & Zhao, ).…”

Tracking Labrador Sea Water property signals along the Deep Western Boundary Current

“…Since those times, there have been many direct current measurements of this southward current using moored instrumentation: in the subpolar gyre at 53°N [ Dengler et al ., ; Fischer et al ., ], at 47°N [ Rhein et al ., ; Mertens et al ., ] and at the Grand Banks [ Schott et al ., ], close to the inter‐gyre boundary at Line W at 39°N (Figure ) [ Joyce et al ., ; Toole et al ., ; Peña‐Molino et al ., ], at Cape Hatteras [ Pickart et al ., ], at N [ Meinen et al ., ; Srokosz and Bryden , ], at 16°N [ Kanzow et al ., ; Köhler et al ., ] and in the Southern Hemisphere at 8°S and 11°S [ Dengler et al ., ; Hummels et al ., ] as well as 34°S [ Meinen et al ., ; Dong et al ., ].…”