Ocean tides

Abstract

According to their spatial scales and to the generation mechanisms applying, tidal phenomena in the sea are presented together with hydrodynamic models explaining their existence and appearance. The astronomical tide generating forces, to which the tidal variations of the ocean state variables can finally be traced, have planetary scale and therefore can directly excite tidal oscillations in the open ocean. Applying models of schematic ocean basins and of the real global ocean elucidate the dependence of theses forced oscillations on the free oscillation properties of ocean basins and on physical processes. Introducing data information into hydrodynamic-numerical tide models of the global ocean by employing a data assimilation procedure turns out to considerably contribute to the computation of realistic tidal fields and of realistic dependent geophysical quantities. The dynamics governing co-oscillating tides in shelf and adjacent sea areas are discussed subsequently. Having in view analytical wave solutions to the tidal equations, in many cases allows to recognize the importance of specific waves for the formation of tidal regimes. The increasing importance of frictional effects in shelf and adjacent sea areas becomes particularly apparent in current field properties, and quasi-resonantly amplified diurnal and semidiurnal tidal waves originating from the open ocean make clear the decisive role these areas are playing in the tidal energy budget. Proceeding to tidal phenomena characterized by further reduced spatial scales, the existence of over- and compound tides with in parts remarkable energy contents is assigned to nonlinear interactions between tidal waves in shallow water areas. The energy transfer from the astronomical tides to over- and compound tides becomes obvious when applying hydrodynamic-numerical models which include such areas as well as the shelf edge and slope. Also internal tidal motions cannot directly be excited, instead corresponding variations of currents and stratifications with often considerable amplitudes are due to energy transfer from the barotropic tides referred to above.