Special Issue—Bathymetry from Space

considered to be a solved problem in oceanography. The general physics of tides were understood, and they could be modeled with reasonable fidelity using relatively simple differential equations solved on computers. In general, it was thought that improvements in tide models would be made only by having higherresolution models run on faster computers. The advent of satellite altimetry by GEOSAT, more recently provided to a higher precision by TOPEX/Poseidon, reinvigorated interest in the tides. But the resulting research was largely an “engineering” issue related to perfecting the numerical models of the tides so that they could be used to remove tidal effects from the altimetry observations. However, in the process of refining tidal models, new and interesting physics were revealed. In particular, the application of data assimilation methods revealed that dissipation of the tides was more complicated than previously understood. Egbert and Ray (2000) constrained their tide model with sea surface height data observed by the satellite altimeters. Using the residuals derived from the modeldata misfit, they were able to find regions where the model physics were incomplete. In particular, they found that their inverse model implied an enhanced dissipation in areas of the deep ocean. This dissipation could not be accounted for by the traditional bottom boundary layer physics. And while they could not use the residuals to determine definitively the mechanics of the missing dissipation, spatial maps of the dissipation were strongly suggestive. Most of the implied deep dissipation was concentrated along regions of rough bottom topography, such as mid-ocean ridges and island arcs, suggesting that these were regions where tidal energy was being converted to internal waves generated by the tides flowing over rough topography. The novel result of Egbert and Ray (2000) was that roughly 1 terawatt (TW) is dissipated in the deep ocean, which The seafloor is one of the critical controls on the ocean’s general circulation. Its influence comes through a variety of mechanisms including the contribution of mixing in the ocean’s interior through the generation of internal waves created by currents flowing over rough topography. The influence of topographic roughness on the ocean’s general circulation occurs through a series of connected processes. First, internal waves are generated by currents and tides flowing over topographic features in the presence of stratification. Some portion of these waves is sufficiently nonlinear that they immediately break creating locally enhanced vertical mixing. The majority of the internal waves radiate away from the source regions, and likely contribute to the background mixing observed in the ocean interior. The enhancement of vertical mixing over regions of rough topography has important implications for the abyssal stratification and circulation. These in turn have implications for the storage and transport of energy in the climate system, and ultimately the response of the climate system to natural and anthropogenic forcing. Finally, mixing of the stratified ocean leads to changes in sea level; these changes need to be considered when predicting future sea level.