Revelle: How did we get here?

Credit: Julia Calderone

John Mickett and Eric Boget discuss a double-anchor mooring drop. Credit: Julia Calderone

Studying the ocean is similar, in a way, to studying the farthest reaches of outer space. Just as we don’t have a full grasp on the physical boundaries that separate deep space from whatever else may be beyond, we don’t, as of yet, have a complete topographical map of the bottom of the ocean. Since light can only penetrate the first layer of the ocean’s surface, many of the physical and biological processes churning about the deepest depths are a mystery.

Oceanographers, as a result, don’t attempt to document each and every swirl, wave or eddy in the ocean. Instead, they practice inquiry-based science. They think about interesting problems in general, and then try to answer them by studying specific chunks or processes in this massive body of water.

An interesting problem for the Tasman Tidal Dissipation Experiment (TTIDE) is climate change. And the project’s goal is to understand how the ocean’s internal dynamics—specifically its hidden yet powerful and enormous tides and waves—interact with the atmosphere to grab carbon dioxide and heat from it to influence our climate.

Selecting a site to study such processes in the ocean is an enormous task. It’s cliché to say, but there really is an “ocean of possibilities.” Oceanographers rely mostly on simulations and models pieced together from data from satellite images and forces from the sun and moon to pick spots in the ocean that they think will be interesting enough to elucidate certain processes. And then they zero in on those sites to study them and then extrapolate those results to other similar parts of the ocean.

Harper Simmons, an oceanographer at the University of Alaska, Fairbanks and one of the head scientists for TTIDE, developed a global simulation of internal tides to identify optimal regions to study these underwater swells. His model (see video below) shows that the ocean is saturated with propagating waves, as represented by pink and blue beams. The Hawaiian Ridge acts as a major generator of these tides, but this region doesn’t harbor much mixing—a phenomenon TTIDE is interested in where crashing internal waves circulate cold, dense water from the bottom of the ocean to the top and pull the warm, lighter water from the top down to the bottom.


The mooring team after a successful evening deployment. Credit: Julia Calderone.

Enter the Tasman Sea. Ideally, the team would measure these huge internal waves throughout their full life cycle—from their birth to their “death” when they break. This is challenging since these massive, 30-story-high waves can travel hundreds of kilometers before breaking. But Simmons’ model shows a unique river of tidal energy flowing between New Zealand and Tasmania, a region that is fairly manageable to navigate. So here we are. The team has picked three spots to scrutinize in the region—the point of wave birth near New Zealand, the point of wave propagation in the middle of the ocean, and the point of wave “death,” or where they break, on the Tasman Slope.

By catching this flow of internal wave energy from start to finish, the team hopes to document the physics for the first time, and better understand how this process may play out in other similar regions of the ocean. Even further, such calculations could add another crucial data point to climate models to make them even better than they are.

So here we are. Breaking down complicated physical processes in the ocean, one bit at a time.

Julia Calderone, The Revelle

The Tasman Tidal Dissipation Experiment//Supported by the National Science Foundation