Award: OCE-1434352

Macquarie Ridge is an underwater mountain range as tall as the Rocky Mountains that extends Southwest of New Zealand. As tides flow up and down the slopes of this range, they pump the interface between cold deep water and warm shallow water, creating subsurface waves (also known as internal tides). These waves have an amplitude of more than 100 m and a wavelength of around 170 km. They propagate northwestward across the Tasman Sea and eventually crash into the Tasmanian continental slope, where they partially break (like surface waves on a beach) and partially reflect. Where internal tides dissipate, they mix the ocean vertically and transfer heat downward, cooling the surface waters and warming the deep waters. Globally, internal tides contain sufficient power to vigorously mix the ocean and drastically influence climate. For this reason, T-Beam and T-TIDE were carefully designed to observe how internal tides dissipate and mix the ocean. The Tasman Sea was chosen as the ideal test location because it has energetic internal tides and a relative weak background flow (i.e., it has a high signal-to-noise ratio). Using the Research Vessel Falkor (with shiptime donated by the Schmidt Oceans Institute), the T-Beam science team collected extensive measurements of the internal tide in the middle of the Tasman Sea during January-March, 2015. These measurements precisely quantified the strength of the internal tide and its rate of energy loss. The rate of energy loss was found to be very weak (less than 1 mW/m2), suggesting that open ocean mixing in the Tasman Sea may be negligible. However, the measurements also showed that the internal tide delivered lots of power (about 5 kW/m-coastline) to the Tasmanian continental slope (directly downstream of the T-Beam site), where T-TIDE investigators observed substantial wave-breaking and mixing (although some regions of the slope reflected most of the wave energy). Lastly, the open ocean measurements also revealed that the internal tide wiggles slightly in time because of weak interactions with ocean eddies. This subtle point has vast implications for the detection of internal tides via satellite altimeters, which rely on internal tides crests and troughs being in exactly the same place for years on end. In essence, the observations led to accurate regional and global models that predict exactly where internal tides will "disappear" in satellite observations because they are interacting with ocean eddies. The results emphasize that satellite observations alone can not determine where internal tides mix the ocean. In situ experiments, like T-TIDE and T-Beam remain critical to understanding how tides mix and redistribute heat in the ocean. The results of this work were shared with numerous K-12 and college classes. Some of these classes were even taken on virtual tours of the research vessel while we were at sea. The fieldwork was also documented extensively at the T-TIDE (and T-Beam) website: https://scripps.ucsd.edu/projects/ttide/ . Technical results have been presented in more than a dozen conference presentations and published in seven journal articles to date. Last Modified: 09/29/2020 Submitted by: Samuel M Kelly