Long-term variability of internal waves and diapycnal mixing: The role of the North Atlantic Deep Western Boundary Current

Abstract

Five years of continuous mooring data combined with CTD/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 16°N in the western Atlantic. As the temporal variability in current strength and corresponding flow speeds within the DWBC is very high, this geographic location provides an ideal setting to analyze the direct influence of strong currents or their interaction with topography on the generation of internal waves and the magnitude of vertical mixing rates. The mooring data include two-hourly rotor current meter measurements and temperature/conductivity time series with high temporal resolution of 5 - 20 minutes. Thus, the data resolve timescales ranging from the low-frequency variability of the large scale DWBC that generates internal waves due to interactions with the topography, to high frequency vertical mixing induced by breaking internal waves. Diapycnal diffusivities obtained from a finescale parameterization show elevated mixing rates of up to 10^(-3)m^2/ s in the bottommost 1500m during times of a strong DWBC where velocities at the mooring site reach up to 50cm/s. Enhanced shear to strain ratios during these times denote an increase in low frequency waves during phases of strong flow. Variability in the high frequency range calculated from mooring data, considered as a proxy for turbulent mixing, is significantly correlated with the DWBC strength above the continental slope which also indicates a pronounced increase of vertical mixing during strong flow. During these periods spectra of horizontal velocity and internal wave available potential energy change substantially at depths below 1200m and show a strong increase of energy in internal waves particularly in the near inertial frequency band. This increase is stronger at the mooring over the continental slope than slightly more offshore over the continental rise. The generation of low frequency, near inertial waves due to the interaction of the DWBC with the slope topography to the west of the moorings where the local water depth equals the depth of the DWBC core is the mechanism proposed for the generation of the observed intensification of low frequency waves and enhanced vertical mixing rates; ray paths estimated for internal waves generated at the continental slope agree well with the observed spectral changes at different depths. Furthermore the vertical energy propagation direction estimated from rotary spectra of shipboard LADCP measurements shows a divergence at depths approximately corresponding to the depth of the DWBC core. Above the core the energy propagation is dominantly upward whereas it is downward below. This is consistent with the estimated ray path for (near inertial) internal waves generated by the interaction of the DWBC core with the slope topography to the west of the moorings.