TTIDE Leg II is entering the home stretch, with great weather & even better data coming in. Aside from a brief appearance to steal one of Capt. Dave’s fishing lures. Bruce the shark has been kind enough to stay away from our deep-profiling Fast-CTD fish. We spent last Tuesday doing a series of water sampling stations for Nicole Jones & the T-Shelf team and checking that Drew Lucas’ wave-powered Wirewalkers were indeed walking (No problem: 0-100 m every 6 minutes).
Then it was back to work with the Fast CTD, looking at deep internal waves propagating up the slope. This “work” has been a lot more fun, given the host of improvements to the system implemented by Mike Goldin and our technical team. In addition to a radical improvement in reliability and user-friendliness, the game-changing addition to the system is a newly created acoustic altimeter. This gadget is a cigar-sized version of a ships echo sounder , (Figure 2a). Shoe-horned into the tail of the fish, it can tell us the distance to the sea-floor as the fish plummets downward. We’re really interested in measuring as close to the bottom as possible, as the turbulence we’re hoping to measure can be strongly influenced by the friction of the sea-floor.
The altimeter can detect the bottom at distances up to 75 m, and we typically hit the brakes and reverse at 15-20 m, giving us a 5-10 second cushion before impact (Figure 2b).
Figure 2 a. The altimeter transducer being installed in the tail of the fish. b), The lab readout shows a noisy band at 80 m range. When the bottom gets within range, we get to watch the final approach. Here the fish is turned around 15 m above the seafloor, 1900 m down.
The precise turn-arounds, as well as all other aspects of the operation, have been achieved by a mixed team of tech-savvy veterans and young volunteers, with homes ranging from the University of Texas to the University of Suva. No two of the students were born in the same country. All are getting into watching the deep ocean evolve in real-time. Working 12 hours on-12 off, 7 days a week, this great group has collected roughly 2500 CTD profiles without (shark excepted) mishap.
Figure 3. A 36 hour record of the deep slope, with 1100 -1600 m depth zone sampled every ~10 minutes. The horizontal blue lines indicate the depths of constant density surfaces , as they are vertically heaved by the internal waves. Every 12 hours, a ~100m tidal crest passes, bringing cold waters from below up the slope. The red dots indicate locations where more dense water is found above less dense water: internal breakers. Massive breaking is found in the crests, but it initially occurs above the sea floor & works its way downward.
The view has been definitely worth the price of admission. A 36-hour record taken in 1630 m of water on the slope is shown in Figure 3. For classical internal waves in a flat-bottomed ocean, the biggest vertical motions are found in mid water column. At the sea-floor, vertical motion dies completely away. But if you add just a few degrees of tilt to the sea floor, as we have here on the Tasman Slope, suddenly the biggest vertical motions are found just above the bottom. The big excitement is associated with the 100 m tall tidal crests that take the form of up-slope propagating “hills” of cold water. There’s a lot of turbulence associated with these shoaling crests, and it has a somewhat unique behavior. Unlike a classical turbulent bore (e.g. wave run-up on the beach), where the turbulence is initiated by friction over the sea-floor, these shoaling crests first go turbulent well above the bottom. The turbulence then spreads downward to the sea-floor as the crest passes. Developing a dynamical understanding of this new phenomenon will be a priority of our post-cruise data analysis.
All lab activities have been presided over by our cruise mascot and in-resident Tasmanian Devil, Clarence, (Figure 4a) who was shanghaied onto the cruise by our Leg I predecessors. Attempts to enlist local fishermen to stand shark-watch for us have been unsuccessful: they’re just too laid back (Figure 4b).
Figure 4 a. Clarence and his demure companion Winnie the wombat preside over the lab. Clarence is proving to be somewhat of a party animal. He won’t get a real chance to strut his stuff(ing) till we get back to the beach. Rules are rules. b. Ground Hog Day, Tasman Sea style. Only six more weeks of winter this summer!