Atypical anticlockwise internal tidal motions in the deep ocean

In the ocean, horizontal motions associated with freely propagating semidiurnal tidal inertia-gravity waves mainly describe an ellipse that is traversed in a clockwise direction in the Northern Hemisphere. In this article, rare observations of anticlockwise polarised semidiurnal motions are presented from deep North-Atlantic mooring sites. Anticlockwise motions are found to occur 1–20% of the time, irrespective of latitude and (weak) vertical density stratification. In a deep narrow channel like Kane Gap however, anticlockwise motions dominate over clockwise. The statistics of turbulence match the occurrence of polarisation change. It is unclear whether these observations represent theoretical predictions of reflected baroclinic Poincaré waves, or those involving the effects of the horizontal Coriolis parameter on internal wave propagation in weak stratification. The observed switching between clockwise and anticlockwise polarisation with time at a fixed position cannot be explained by the internal wave theory using the traditional approximation. Instead, the polarisation switching points at baroclinic effects involving varying background conditions like stratification and shear-induced mixing. It is suggested the polarisation statistics may be used as a diagnostic tool for such varying conditions.