Impact of the North Atlantic Subpolar Front on Near-Inertial Wave Propagation

Recent studies emphasize the strong influence of oceanic fronts on the strength of diapycnal mixing and on the generation and propagation of near-inertial waves. Here we analyze the temporal variability of near-inertial wave energy in relation to changes in flow speed, the presence of eddies and the wind field at the North Atlantic subpolar front using three-year velocity timeseries at three moorings covering the depth range from 240m to 3000m. Along the mooring array the influence of the North Atlantic Current (NAC) is decreasing northward, inducing pronounced differences in the internal wave field. Other studies e.g. at the Subtropical Front in the North Pacific showed that near-inertial waves radiate off the front. In the region of the NAC however, near-inertial waves are most energetic at the northernmost mooring (least influenced by the NAC) whereas the energy level at the southern moorings is lower. Thereby variability in near-inertial wave energy is dominantly induced by changes in the wind field.

Within the upper part of the NAC data are consistent with low Richardson numbers, likely inducing enhanced internal wave breaking. Additionally a stronger stratification within the lower part of the NAC compared to north of the front results in a shift of wavenumber aspect ratio and a longer residence time of the internal waves in the upper water column. Both processes would result in the observed reduction of near-inertial internal wave energy at greater depths within the region of the NAC.