Unpredictability of internal M2

[摘要] Current observations from a shelf sea, continental slopes and theabyssal North-East Atlantic Ocean are all dominated by the semidiurnal lunar(M₂) tide. It is shown that motions at M₂ varyat usually large barotropic and coherent baroclinic scales, >50 kmhorizontally and >0.5 H vertically. H represents the waterdepth. SuchM₂-scales are observed even close to topography, the potentialsource of baroclinic, "internal" tidal waves. In contrast, incoherentsmall-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 solartidal frequency. Ω represents the Earth's rotational vector. Thisconfirms earlier suggestions that small-scale baroclinicM₂-motions generally do not exist in the ocean in anypredictable manner, except in beams very near, <10 km horizontally, totheir source. As a result, M₂-motions are not directlyimportant for generating shear and internal wave induced mixing. Indirectly however, they may contribute to ocean mixing if transferto small-scale motions at f and/or S₂ and at high internal wave frequencies can be proven. Also farfrom topography, small-scale motions are found at either one or both of thelatter frequencies. Different suggestions for the scales at these particularfrequencies are discussed, ranging from the variability of "background"density gradients and associated divergence and focusing of internal waverays to the removal of the internal tidal energy by non-linear interactions.Near f and S₂ particular short-wave inertio-gravitywave bounds are found in the limits of strong and very weak stratification, which areoften observed in small-scale layers.

[学科分类] 海洋学与技术