Unpredictability of internal M₂

Abstract. Current observations from a shelf sea, continental slopes and the abyssal North-East Atlantic Ocean are all dominated by the semidiurnal lunar (M₂) tide. It is shown that motions at M₂ vary at usually large barotropic and coherent baroclinic scales, >50 km horizontally and >0.5 H vertically. H represents the waterdepth. Such M₂-scales are observed even close to topography, the potential source of baroclinic, "internal" tidal waves. In contrast, incoherent small-scale, ~10 km horizontally and ~0.1 H vertically, baroclinic motions are dominated around f, the local inertial frequency, and/or near 2Ω≈S₂, the semidiurnal solar tidal frequency. Ω represents the Earth's rotational vector. This confirms earlier suggestions that small-scale baroclinic M₂-motions generally do not exist in the ocean in any predictable manner, except in beams very near, <10 km horizontally, to their source. As a result, M₂-motions are not directly important for generating shear and internal wave induced mixing. Indirectly however, they may contribute to ocean mixing if transfer to small-scale motions at f and/or S₂ and at high internal wave frequencies can be proven. Also far from topography, small-scale motions are found at either one or both of the latter frequencies. Different suggestions for the scales at these particular frequencies are discussed, ranging from the variability of "background" density gradients and associated divergence and focusing of internal wave rays to the removal of the internal tidal energy by non-linear interactions. Near f and S₂ particular short-wave inertio-gravity wave bounds are found in the limits of strong and very weak stratification, which are often observed in small-scale layers.