Rectilinear and circular inertial motions in the Western Mediterranean Sea

Abstract

Kinetic energy spectra appeared similar at all observational depths in the 3000 m deep Algerian Basin, Western Mediterranean Sea. They were featureless except for a single peak at ∼2% above the local inertial frequency (f), even at depths (z) where stratification was very weak, N(z)=O(f). At great depths a second peak was observed at ∼0.99f. However, at a single point, we observed rotary spectra varying considerably with stratification and depth. Where N<f, that is between 600 and 1200 m in a series of homogeneous layers associated with a double diffusion process and below 2200 m in a dense homogeneous water formed nearby, all near-inertial horizontal motions were almost rectilinear. The observation of near-rectilinear inertial motions was in contrast with common observations of near-circularly polarized inertial motions, which were found, for example, from the same mooring at depths where N⩾2.5f, that is in continuously stratified waters encountered near the surface (100 m) and at intermediate depths (1800 m). Our observations suggested that the near-inertial motions in the Algerian Basin did not always represent internal gravity waves, but also dominant inertio (gyroscopic) waves. These waves could have been generated after excitation of the surface layer by atmospheric disturbances and the surface pressure gradient (barotropic) excitation of a simple 2-layer model via a coastal boundary condition, a process common in shelf seas.