Science

 What do internal waves in the ocean actually look like?

Internal waves are generated in many different places in the ocean, one of them is in the Strait of Gibraltar which is located between Spain and Morocco. Here is a nice animation from the Physical Oceanography Group at the University of Malaga showing the generation of internal waves at a sill, and then their propagation into the Mediterranean with each change of the tide. These are even visible from satellite images!

Where are the internal tides generated in the Tasman Sea?

The Macquarie Ridge is a mostly underwater mountain range that extends south west from New Zealand. The ridge includes a few islands, including Macquarie Island, which was used as a support base by Sir Douglas Mawson during the Australasian Antarctic Expedition from 1911-1914. Measured from the sea floor, the peaks along the ridge are slightly taller than the Rocky Mountains.

As tidal currents flow back and forth over the Macquarie Ridge, cold dense deep water is pushed up the slopes for 6 hours and then pushed back down the slopes when the tidal currents reverse. This process converts about 2 gigawatts of tidal energy into waves along the internal density layers of the ocean–waves we call internal waves, or more specifically, internal tides. Two gigawatts of power is the same amount of electricity the Hoover Dam can generate during optimal conditions. However, unlike the raging Colorado river, the tidal currents over Macquarie Ridge are usually quite calm. As a result, the internal waves that are generated there don’t break right away, they propagate away from the ridge transporting their tidal energy toward Tasmania.

Below is 3D map of the Macquarie Ridge as seen from Antarctica. In numerical simulations, lots of tidal energy is converted to internal tides in the blue regions, and the biggest internal tides propagate away from the ridge in the red regions.

CSW_bathy_web_v1

 

Now watch a realistic simulation of the internal waves as they are generated at the Macquarie Ridge south of New Zealand and propagate across the Tasman Sea. On the left panel is Kinetic Energy and on the right is the distance in meters that the density surfaces are pushed up or down below the surface. Water in these internal tides moves up and down by more than 130 ft!


 

How much of the internal tide is reflected at Tasmania?

One goal of T-TIDE is to determine how much energy dissipates when the internal tide hits Tasmania and how much energy reflects back to the open ocean. To help decide where to make observations, we’ve used idealized models to predict the fraction of the internal tide that reflects at each location along the west coast of Tasmania. One model suggests that reflection varies between 40% and 90% over distances of less than 100 miles, indicating that small differences in local topography may greatly impact where the internal tide breaks.

R25_500_web

Now watch another model of an internal wave packet as it propagate across the Tasman Sea, hits the continental slope and reflects off to the northwest!


 

How do oceanographers measure the ocean?

One way is by lowering an instrument that measures conductivity, temperature and depth, which is usually referred to as a CTD. These instruments are standard on all oceanographic vessels. This is what a CTD profile looks like if you were attached to the CTD! Thanks to Matt Durham, a Marine Technician on the R/V Revelle, a research vessels at the Scripps Institution of Oceanography that will be participating in the Tasman Tidal Dissipation Experiment.

Now, watch Saskia Madlener’s great documentary following observational oceanographers at work in Greenland. During the Tasman Tidal Dissipation Experiment, we will be deploying many of the same instruments to understand the fate of internal waves in the Tasman Sea. To find out more about this amazing work in Greenland, more information is here Greenland field program