Seasonality of internal gravity waves kinetic energy spectra in the Ligurian Basin

Abstract

In the central Ligurian Basin, Western Mediterranean Sea, dense water formation occurs in winter so that near-surface stratification displays a marked seasonality. Consequently, kinetic energy spectra from the surface to intermediate depths are entirely different in summer and winter. In summer, the spectra show a peak near the local inertial frequency (f). The polarization of currents in the internal gravity wave frequency (σ) band (f < σ < N, N the buoyancy frequency) resembles internal wave theory (for N >> f). In winter, however, no significant near-inertial (or any other) spectral peak is observed and motions are not polarized at any frequencies, whatever the observation depths. With respect to the energy levels observed in summer, those in winter are enhanced at all frequencies (except near f). The seasonal pycnocline determines the summertime development of near-inertial and higher frequency internal waves, also at great depths (~1100 m at least). In winter, no near-surface pycnocline does exist and no (gravity nor gyroscopic) near-inertial internal waves occur; the observed currents are associated with strong mesoscale activity and dense water formation, not with local N. For the ‘internal wave band’ this implies a lack of scaling of horizontal kinetic energy with N that is reflected in changes of the spectral slope. In the coastal zone of the Ligurian Basin, especially near Corsica, where the alongslope circulation and an almost permanent stratification prevail, near-inertial currents are found yearlong, with only moderate changes in amplitude in the near-surface layer.