1. College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
2. NASA Goddard Space Flight Center, Code 926, Greenbelt, Maryland 20771, USA

Correspondence to: G. D. Egbert¹ Correspondence and requests for materials should be addressed to G.D.E. (e-mail: Email: egbert@oce.orst.edu).

How and where the ocean tides dissipate their energy are long-standing questions¹ that have consequences ranging from the history of the Moon² to the mixing of the oceans³. Historically, the principal sink of tidal energy has been thought to be bottom friction in shallow seas^{4, 5}. There has long been suggestive evidence^{6, 7}, however, that tidal dissipation also occurs in the open ocean through the scattering by ocean-bottom topography of surface tides into internal waves, but estimates of the magnitude of this possible sink have varied widely^{3, 8, 9, 10, 11}. Here we use satellite altimeter data from Topex/Poseidon to map empirically the tidal energy dissipation. We show that approximately 10¹² watts—that is, 1 TW, representing 25–30% of the total dissipation—occurs in the deep ocean, generally near areas of rough topography. Of the estimated 2 TW of mixing energy required to maintain the large-scale thermohaline circulation of the ocean¹², one-half could therefore be provided by the tides, with the other half coming from action¹³ on the surface of the ocean.