Internal tides. global field of internal tides and mixing caused by internal tides

Abstract

Different approaches to the study of internal waves in the ocean are analyzed. Generation of internal tides over submarine ridges is considered on the basis of numerical models and measurements in the ocean. Energy fluxes from submarine ridges exceed many times the fluxes from continental slopes because the dominating part of the tidal flow is directed parallel to the coastline. Submarine ridges if normal to the tidal flow form an obstacle that can cause generation of large internal waves. Internal tides are extreme when the depth of the ridge crest is comparatively small with respect to the surrounding depths. Energy fluxes from most submarine ridges were estimated. They account for approximately one fourth of the total energy loss from the barotropic tides. Model estimates were compared with the measurements on moorings at 30 study regions in the oceans. Combined calculations and measurements result in a map of global distribution of internal tide amplitudes. The study is extended to the Arctic region. Extreme internal tides were recorded near the Mascarene Ridge in the Indian Ocean, Mid-Atlantic Ridge in the South Atlantic, Great Meteor bank and the Strait of Gibraltar. A high correlation has been found between bottom topography and vertical wavenumber spectra of vertical displacements calculated from potential temperature profiles measured by CTD instruments. Increased spectral densities of vertical wavenumber spectra are related to the presence of fine structure. The latter is caused by vertical motions in the ocean, which lead to mixing. Hence, vertical wavenumber spectra are an integral characteristic of many processes, which induce mixing: breaking of internal waves, intrusions, upwelling, frontal dynamics, etc. Spectral densities (dropped spectra) near submarine ridges are several times greater than in the regions far from abrupt topography where they are close to the background spectra described by the Garrett-Munk model. We found regions of enhanced mixing near the bottom in the deep Equatorial and Vema channels. This method also indicated strong mixing of Mediterranean and Atlantic waters west of the Strait of Gibraltar, mixing of North Atlantic Deep Water with Antarctic Intermediate and Antarctic Bottom waters, and mixing at the front of the North Atlantic Current.