Internal tides in the ocean

Abstract

Theory and observations of internal tides in the oceans are reviewed. Internal tides are simply a form of internal wave with quasi‐tidal periodicity, though north of their inertial latitudes they should be trapped to bottom topographic features, including continental shelves. In oceans of constant depth, their theory is reducible to that of ordinary (surface) tides in very shallow equivalent oceans. The short wavelengths appropriate to internal tides, however, mean that localized nonglobal solutions are most useful. Theory and observation both support the notion that internal tides are generated by the passage of the surface tide over topographic features, though it is not yet possible to say with certainty that any of several mechanisms dominates. A tabulation of worldwide observations suggests that internal semidiurnal tides contain 10–50% of the energy of the surface tide. Consideration of both the rate of input of energy into the internal tides and the rate of loss from them leads to the conclusion that the internal tides are probably not a significant sink for energy extracted from the surface tide. The energy does appear significant compared with the rates of input of energy to the general circulation by wind and thermal forcing. Unlike other internal waves the tides are dominated by the lowest vertical modes.