Abstract
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.
Highlights
Ocean interior observations by Leaman and Sanford (1975), Pinkel (1983) and van Haren (2006) showed relatively short vertical scales O (100 m) for near-inertial and tidal motions
In comparison with the near-rectilinear inertial horizontal motions observed in the deep Mediterranean, the present effects of polarisation change in tropical deep-ocean internal tidal motions provide a slightly ambiguous proof of the nontraditional approximation’ (non-traditional approximation (TA)) influence of the
Near deep topography like relatively small hills and channels and above ridges like the Reykjanes, this would be an upgoing beam. Over such deep topography and relatively close to the equator, N can be sufficiently small for non-TA to take effect
Summary
Ocean interior observations by Leaman and Sanford (1975), Pinkel (1983) and van Haren (2006) showed relatively short vertical scales O (100 m) for near-inertial and tidal motions. They showed a strong asymmetry in vertical propagation at particular sites where about 90% of the energy propagated down-/upward (as evaluated from phase propagating up-/downward), respectively. In the open-ocean inertia-gravity IG-wave domain, anticlockwise superinertial motions are predicted for particular wave characteristics under relatively weakly stratified conditions (Gerkema and Exarchou, 2008) This prediction is made using ‘nontraditional approximation’ (non-TA), that is, by not using the traditional approximation (TA) of omitting the terms. It is important to better understand the precise mechanisms behind vertical exchange, which is important for sediment and nutrient redistribution
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