Ocean Science www.ocean-sci.net 1812-0784 1812-0792 3 2 2007 10.5194/os-3-337-2007 http://www.ocean-sci.net/3/337/2007/ http://www.ocean-sci.net/3/337/2007/os-3-337-2007.html http://www.ocean-sci.net/3/337/2007/os-3-337-2007.pdf 337 344 2007-06-15 Unpredictability of internal M₂ H. van Haren hansvh@nioz.nl Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, The Netherlands 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. 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