Internal wave band eddy fluxes above a continental slope

Abstract

Three weeks of velocity and temperature measurements from the bottom 45 m above the continental slope in the Bay of Biscay are used to evaluate the role of the internal wave band in boundary mixing near a sloping bottom. Utilizing acoustic Doppler current profilers and thermistor strings, internal wave band eddy fluxes of momentum and heat are estimated. The instrumentation is specifically designed to resolve internal wave band processes. Due to unresolved Doppler shifting, this wave band may include turbulence as well as internal waves. A very energetic and highly variable near-bottom environment is found. Periods of mixing and restratification alternate at the M 2 tidal frequency. Interpreting the observations in an Ekman sense, the three-week mean current is downwelling-favorable, which is consistent with existing boundary layer theories. However, a bi-directional flow associated with sloping boundary mixing is not found in the near-bottom layer, possibly due to observed strong stratification all the way to the bottom. We evaluated boundary layer dynamics and the effect of internal wave-band fluxes from two frequency ranges (Ն 15 cpd and Ն 1.9 cpd, including tides) on the three-week mean flow. The high-frequency range (Ն 15 cpd) of the internal wave band supports significant momentum and buoyancy fluxes while the low-frequency range (Ն 1.9 cpd) only supports significant momentum fluxes. Mean bottom-normal eddy diffusivities associated with anisotropic, nonlinear internal waves, are negative and O(Ϫ10 Ϫ2 m 2 s Ϫ1). Interpreting these negative eddy diffusivities as indication of a restratification process, high mixing efficiencies are expected throughout the mixing layer, which extends typically 20 m above the bottom. Mean eddy viscosities are positive in cross-slope direction and negative in alongslope direction, implying a strong anisotropy in the interaction between internal wave band eddies and the mean flow. Alongslope momentum is transferred from the internal tide to the mean flow. Buoyancy and pressure gradient forces, which we could not determine directly, may generate a buoyancy-driven secondary flow. The buoyancy equation is dominated by advection, possibly balanced by divergence of cross-slope and alongslope internal wave band fluxes.

Dissipation of Internal Wave Energy Generated on a Critical Slope

“…Efforts over the Mid-Atlantic Ridge (Polzin et al 1997) and Hawaii (Klymak et al 2006) have demonstrated increased dissipation over deep, rough topography. Similar observations have been made elsewhere in a growing body of literature (e.g., Gemmrich and van Haren 2002;Nash et al 2007;Alford et al 2011Alford et al , 2014van Haren and Gostiaux 2012). Progress has also been made in predicting the dissipation observed near topography.…”

“…Observations have revealed elevated near-bottom mixing at midslope regions that are near critical to the tidal forcing. This enhanced mixing is associated with internal wave propagation onto a critical slope by the setup of borelike structures due to wave reflection (White 1994) or through the collapse of unstable stratification associated with oblique internal waves (Gemmrich and van Haren 2001).…”

On the influence of a surface coastal front on near‐inertial wind‐induced internal wave generation

“…As the model is fully nonlinear, the transfer of energy from the fundamental forcing frequency (namely the inertial frequency) to higher harmonics is also briefly examined. Recent measurements [ van Haren et al , 1999, 2002; Gemmrich and van Haren , 2002] and modeling have shown that this process is important in transferring energy to higher frequencies and eventually turbulence [ Inall et al , 2000]. This process of transferring energy from a fundamental forcing frequency to higher harmonics is well established in tidal modeling in shallow sea regions [e.g., Pingree and Maddock , 1978; Davies and Kwong , 2000, and references therein] and occurs here particularly in the frontal region.…”

On the interaction between internal tides and wind‐induced near‐inertial currents at the shelf edge

“…Recent calculations using a three‐dimensional prognostic model forced with the barotropic tide and incorporating a turbulence energy closure scheme have shown that it can reproduce the internal tide and enhanced shelf edge cooling [ New and Pingree , 1990] associated with it [ Xing and Davies , 1996a]. Besides the tides, wind forcing is a major source of mixing, particularly in shallow seas [ Van Haren et al , 1999; Van Haren , 2000] and shelf edge regions, where the continental slope influences the internal wave spectrum [ Van Haren et al , 2002; Gemmrich and Van Haren , 2002]. Recent ideas [ Munk and Wunsch , 1998] suggest that a major part of the mixing in the ocean is due to boundary layer mixing of tidal and wind origin at the shelf edge, which then diffuses into the ocean.…”