She found that the M 2 tide generates 0.7 TW of internal waves at depths grater than 1000 m. An early direct computation of the tidal generation of internal waves over the global ocean was done by Sjöberg and Stigebrandt, and later refined by Gustafsson. In order to overcome these difficulties, direct computation of the wave generation is necessary. Moreover, the energy flux to internal waves is distributed very inhomogeneously, with much structure on very small scales, while the spatial resolution of inverse calculations is inherently low, approximately 5° for the results of Egbert and Ray. Such inverse calculations do not distinguish between internal wave generation and other dissipation mechanisms, although this mechanism is thought to be the dominating one in the deep ocean. They found that 0.6–0.8 TW is dissipated in the deep ocean, representing 25–30% of the total dissipation of the M 2 tide. Recently, however, Egbert and Ray have inferred the geographical distribution of the dissipation of the M 2 tide from satellite altimeter data by inverse tidal modelling. Historically, it was believed that the tidal dissipation is dominated by bottom friction in shallow seas. This value is based on astronomical observations. The total dissipation of the M 2 tide, which accounts for about two thirds of the energy of all tidal components combined, is known to be 2.4 TW. It is therefore essential to determine the energy flux from tides to internal waves. These waves are generated by winds and by tidal currents that encounter rough topography on the bottom of the ocean. In the upper 1000 m of the ocean the winds play a dominating role through direct Ekman pumping, but the deep circulation is probably mainly driven by vertical mixing caused by internal waves. While the atmosphere can be regarded as a heat engine, the ocean circulation is driven mechanically by winds and tides.
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