Revelle: Digging In

Screen Shot 2015-02-02 at 1.57.22 PMWe shoved off from Macquarie Wharf in Hobart on Friday afternoon. Departure is always a bit spooky on the Revelle. There’s no roaring of diesels or shuddering of massive driveshafts. The Z-drive propellers are vectored, the electric propulsion motors are energized, and the dock silently recedes. We spent some time swinging the compass (swinging the ship to calibrate the compass), & then headed down the Derwent Estuary. After a few hours we stopped to test the new acoustic altimeter in our Fast CTD fish. We found that the tiny gadget could see the sea-floor from a distance of 70 m-a big win for Mike Goldin & the technical team that created it as an over-the holiday Christmas project.

We rounded Tasman Island in a beautiful sunset and headed north into the gloom of the Tasman Sea (Figure 1).

Tasman Island

Figure 1. Tasman Island, an iconic landfall. It’ll look less somber when we’re passing it on the way in, three weeks from now.

Saturday morning found us well north along the coast, ready to deploy an array of eight moorings on the Tasman shelf in support of the T-SHELF component of TTIDE. Lead by Drs. Nicole Jones of The University of Western Australia and Drew Lucas of Scripps, T-SHELF aims to track the shallow water consequences of the deep tidal energy that arrives at the Tasman coast after propagating across the Tasman Sea from New Zealand. Starting with a tripod framed bottom lander (Figure 2a) and ending 18 hours later with deployment of the last Wirewalker wave-powered profiling instrument platform (Figure 2b), Saturday was a “fun” day. Fortunately, the weather cooperated.

UWA_Frame Deployment Wirewalker Deployment

Figure 2 a. The University of Western Australia’s Bottom Lander is prepared for launch, the initial T-Shelf mooring deployed on Saturday morning (left). b. The wave-powered Wirewalker is readied for a midnight deployment.

After a night spent mapping the local sea-floor topography using the ships multi-beam echo sounder, we headed offshore Sunday morning for the initial deployment of the Fast CTD. The ocean is stratified, with the lightest waters at the surface and water density increasing progressively with depth. Surprisingly, the difference in density between the top and bottom of the sea is only about 1%, so it takes a sensitive instrument to track density fluctuations within the ocean. The internal waves that we’ve come to study move these density layers vertically as they pass. By measuring the vertical motion of the density field we can track the passage of the waves.

The CTD is an instrument that measures ocean density electrically. To see waves passing through a large volume of the ocean, we profile the CTD vertically. The trick is to repeat these profiles very rapidly, so that the ocean barely has time to change between successive measurements. If you’re successful, the passage of internal waves is seen as smooth undulations of the density surfaces. If the profiling is too slow, you get to see a more jerky picture, or one that makes no sense at all.

CTD Fish 2015 Boom Deployed

Figure 3. The Fast-CTD can profile to depths of two km, suspended from the thin black PBO cable. The drag of the cable reduces the speed of the fish to ~2 m/s at great depth. The 8m boom keeps the fish from tangling in the Revelle’s propellors.

So our Fast CTD is packaged in a streamlined “fish” and raised and lowered at speeds up to 5 m/s (10 kts) by a powerful electric winch (Figure 3). This is about 4 times the profiling speed of the ship’s general-purpose CTD system. The trick will be to end the profiles very near the sea floor, where most of the turbulence that we’re hunting is expected. It’s like dangling grandma’s Mercedes by a thin string off the South Rim of the Grand Canyon, lowering toward the canyon floor at 10 mph, and seeing how close you can get without wiping out. Then doing it again…

Here’s where the new acoustic altimeter is paying off. On our Sunday initial run, we had no problem reversing the profiles within 20m of the sea floor. Down deep (where the fish falls slower), this is 5-10 seconds before impact. No time to be asleep at the switch.

Our initial Sunday-Monday run was in 1900 m water depth near a small hill protruding from the continental slope. The CTD system had few teething problems & provided a great initial 25-hour run. Very energetic internal tides were seen, culminating in the passage of a 100 m tall internal bore (Figure 4 a, b).

Site 1-080330_Dissipation

Real time data visualization by San Nguyen

Figure 4. The density layer cake of the ocean (top) with the layers undulating as the internal waves pass through. At 22:00, a vertical wall of water 100 m high passes under the ship. This bore is traveling along the seafloor at 1920 m and is an aspect of the dissipation of the incoming internal tide. When internal waves break, heavier water is temporarily swirled above lighter water (red dots, lower panel). Mixing and energy dissipation occur as this unstable situation settles out.

We’re now branching out to explore other sites. Guided by computer modeling studies of Profs. Harper Simmons (University of Alaska, Fairbanks) and Jody Klymak (University of Victoria), initial scouting measurements by robotic gliders, and the pioneering observations of Matthew Alford’s Leg I Revelle team, we hope to add a few new pieces to the puzzle of how the ocean mixes.

Rob Pinkel