Tracking the Tasman Sea’s Hidden Tide

A Steep Slope

Luc Rainville and Amy Waterhouse
Feb. 06 2015

The continental shelf of Tasmania is pretty steep. If you were on the bottom of the Tasman Sea, it would be like driving in a car across a desert and running into a mountain 4,000 meters high (13,000 feet).

Map figure showing modeled reflectivity of the T-Beam off the Tasman slope. The lines indicate different cross-sections of the continental slopes that were tested with the model. The color indicates the fraction of incoming energy that bounces off the slope without dissipating. The depth contours are every 1000 m. The black arrow indicates the angle of the incoming T-Beam.
Map figure showing modeled reflectivity of the T-Beam off the Tasman slope. The lines indicate different cross-sections of the continental slopes that were tested with the model. The color indicates the fraction of incoming energy that bounces off the slope without dissipating. The depth contours are every 1000 m. The black arrow indicates the angle of the incoming T-Beam.Sam Kelly
Google Earth image showing the cruise track of Falkor, as well as the previous transect line C1-C2 and location of new transect at which CTD/LADCP profiles will be conducted at sites F6 and F5.
Google Earth image showing the cruise track of Falkor, as well as the previous transect line C1-C2 and location of new transect at which CTD/LADCP profiles will be conducted at sites F6 and F5.Luc Rainville and Amy Waterhouse

The grade up this mountain road would be about 8%, the steepest of any interstate in the U.S. The mountains that generate the T-Tide over on the New Zealand side of the basin (called the Macquarie Ridge) are also steep,  about 4,000 meters high from the sea floor. Scientists have observed that steep slopes generate larger internal tides with higher velocities than more gradual slopes; much like a bigger stone (i.e., a bigger disturbance) generates larger ripples. So the 100 meter tall internal wave generated at Macquarie Ridge moves across the Tasman basin at a fairly fast velocity, as far as internal waves go, about 1.5 m/s (3.4 miles/hour).

It’s complicated

Scientists like Sam Kelly have studied and modeled internal tides and waves for several years. Because internal tides are very regular and their locations are pretty well known from satellite data, scientists can use mathematical models to predict how a particular internal tide behaves once it is generated. Waves of any kind will always follow well-understood principles of physical laws. That’s the simple part. The more difficult piece is that these waves will interact with all kinds of structures and processes in their path, some known but many unknown. And that complicates things considerably. We’ve already mentioned how eddies might interact with the propagation of the T-Tide beam. Other elements include seamounts, seasonal density differences in the water, and even interaction with other internal waves, such as the remnants of the T-Beam itself after it hits Tasmania.

Current models predict that even though the T-Beam may be partially refracted and dissipated by various processes, the majority of the wave’s energy still propagates all the way across the Tasman Sea to Tasmania. So the question becomes, “What happens to it next?”. There are two possibilities: 1) the wave is completely dissipated on the Shelf, much like a surface wave crashing on a beach; or 2) the wave is reflected off the Tasman Shelf, either at a different angle with somewhat less energy, or back into the pathway of the incoming internal wave. In fact, both of these processes may occur for any given wave.

Sam Kelly with a pelagic tunicate (sea squirt) that got caught up on the small CTD rig one evening.
Sam Kelly with a pelagic tunicate (sea squirt) that got caught up on the small CTD rig one evening.Randall Lee

Seeing the reflection

Sam Kelly believes there is good evidence that reflection may be particularly prominent for the T-Beam. In the southern region of Tasmania around Hobart, the continental shelf is especially steep. Remember that steep slopes can generate very high internal tides – which means that they can also create high reflection of an internal tide as the wave bounces off the vertical face. Data from underwater gliders (a type of autonomous vehicle) – built by Shaun Johnston at Scripps (and another T-Tide investigator) – collected over several months also indicate there is a lot of reflection of the internal tide around southern Tasmania. That is, the wave patterns are similar to those created under controlled conditions when a primary wave is reflected off a vertical wall. Further north, the picture changes and it appears that there may be more scattering of the wave, changing it from a wave with large energy and large wavelength (a “low mode” wave), to one with less energy and smaller wavelength (a higher mode), which then dissipates more quickly.

Doing science is always a team effort. Team Falkor gets it done.
Doing science is always a team effort. Team Falkor gets it done.Judy Lemus

After dodging some crummy weather over past two days (again), the data that we collect at our next site along the beam will hopefully give Sam and the rest of the T-Team a better picture of how strong the beam is closer to the Macquarie Ridge. Then they can estimate any energy lost in the beam over the 100km distance between these two transects. Knowing that will get them one step closer to understanding the total energy budget (energy coming from the ridge + energy dissipated by eddies + energy coming back from Tasmania) of the T-Beam, and one step closer to understanding the role of internal tides in contributing to Earth’s climate.


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