Category Archives: Revelle

Back to the barn // TTIDE ends in Hobart port

The R/V Revelle has headed “back to the barn” and has just docked in Hobart, Tasmania.

Back to the Barn. Photos: Thomas Moore

Back to the barn. Photos: Thomas Moore

All hands on deck are now in the usual post-cruise full-court-press to get all the remaining scientific gear packed, off the ship, and into containers for the return journey.

It’s a morning of mixed emotions, we’re all happy to be back in port and heading home but it’s bitter-sweet to be leaving behind the R/V Revelle, her Captain and crew, and our days on the Tasman Sea.

Albatross of the Tasman Sea. Photo: Thomas Moore

Albatross of the Tasman Sea. Photo: Thomas Moore

 

Good morning from the Tasman Sea

A special dawn for the R/V Revelle off southeastern Tasmania.  Photo: Thomas Moore

A special dawn for the R/V Revelle off southeastern Tasmania. Photo: Thomas Moore

Good morning! The daytime watch awoke to a spectacular sunrise here aboard the R/V Revelle, stationed about 40 kms off southeastern Tasmania.

All 14 moorings are aboard and we have been settling in for the final few days of ocean observations. We have 48 hours left on this final leg and the TTIDE team is making as many “yoyo” and “towyo” operations as we can.

T-TIDE student profile: Madeleine Hamann

PhD student Madeleine Hamann from the Scripps Institution of Oceanography is aboard the R/V Revelle for the T-TIDE leg 3 cruise and here she talks about her work and how studying math, science, and engineering has opened up a world of opportunity for her.

The final countdown

The Final Countdown

Albatross fill the air, Tasman Island in the distance - the location for the TTIDE southern moorings.  Photo: Thomas Moore

Albatross fill the air, Tasman Island in the distance – the location for the TTIDE southern moorings. Photo: Thomas Moore

It’s the second weekend out here on the Tasman Sea for the TTIDE leg 3 crew aboard the R/V

Revelle. Today we are pushing hard to finish recovering all four remaining moorings still in the water.  If the team can make all that happen before darkness falls tonight that will keep the TTIDE project ahead of schedule and give the scientists extra time to conduct “yoyo” and “towyo” operations – filling important gaps in our view of the internal wave energy pulsing across the Tasman Sea.  There are only a few days left until the R/V Revelle steams “back to the barn” and for all aboard it’s starting to feel a bit like the final countdown.

Dawn broke gray and chilly but the howling westerly wind and the short, steep windswell it generated has mercifully laid down.  It was a great way to start the recovery of TTIDE “M4”, a 2300 metre tall mooring designed to capture the energy of internal waves breaking in the shallowing waters of the continental slope using two highly specialised McLane profilers.

The McLane profilers are “wire-crawlers”, programable robots that climb and descend the mooring line over and over and over again, one million metres worth of travel in every one of their large internal lithium battery packs.  These profilers come jammed with an array of instruments that

A McLane profiler breaks the surface.  Photo: Thomas Moore

A McLane profiler breaks the surface. Photo: Thomas Moore

measure pressure, temperature, salinity, and most importantly current velocity at a finer scale and across a longer vertical reach than any other tool in our oceanographic toolbox.

The McLane data are invaluable, they are costly acquire, and each profiler runs on hardware and software that takes great skill and experience to operate.  The McLane profiler, often abbreviated as “MP” in casual conversation on the back deck, is the star of the show and everybody quietly anticipates the outcome each time one of these yellow beasts breaks the surface of the sea under the tug of our winches.  Did the wire-crawler survive the pressures of the deep and what data will it hold for the TTIDE team?

Science, caught fresh from the sea

The MP’s have indeed brought a data harvest, fresh from the sea.  As moorings have been brought onboard the attached instruments are cleaned and logged before TTIDE team members get busy up forward in the analytical labs extracting the data onto a dizzying collection of hard drives.

the first analyses of MP data is underway

the first analyses of MP data are underway

Time is always short aboard ship but TTIDE scientists have started to look at the new MP data in the past 24 hours, building the initial analyses of what an underwater robot has learned from crawling a mooring wire for many months deep under the surface of the Tasman Sea.  This first look at the MP data shows the clear fingerprints of the daily tide, lunar cycle, and the passing of swirling mesoscale eddies as they swept over the slope 20 kilometres or so off St Helens, Tasmania.

When the TTIDE scientists finally return home* they will bring all the fresh science they have caught into their data kitchen and cook up a better understanding of our earth, climate, and ocean.

Thomas Moore, for the TTIDE team

Prof. Matthew Alford works on a MP back in the ship's lab.  Photo: Thomas Moore

Prof. Matthew Alford works on a MP back in the ship’s lab. Photo: Thomas Moore

* “home” means many things for the diverse TTIDE team, made up of experts from the University of Minnesota – Duluth, the University of Alaska – Fairbanks, Oregon State University, the University of Washington, and the Scripps Institution of Oceanography.

Revelle: The Pulse of the Ocean

 

T_Tide_Logo_2015    After several days of exploring nearby sites, we’ve turned attention to a small hill on the side of the continental slope where computer simulations have suggested that large internal waves and energetic turbulence would be found. Using the computer controlled guidance system in the ship, we’ve taken a number of repeated transects across the top of this hill, watching the waves and turbulence evolve 1900 meters below the somewhat angry sea surface. Guided by an acoustic altimeter, our Fast-CTD is profiling to within 20 m of the sea floor. We can repeat profiles between 1500 and 2000 m every 8 minutes.

The view has been worth the price of admission: We are seeing large rhythmic internal wave crests radiating downward every tidal period, with near-vertical breaking fronts (color images, left below) almost 200 m tall. The actual turbulent regions associated with the breaking propagate downward also, at a fixed phase of the wave. Determining why the waves break where they do is one of the goals of the experiment. These data provide a great clue.

Fast CTD_Feb 13 0800 UTC copy

Figure 1. The Fast CTD real-time data display showing three crossings of the hill. The downward propagation of the wave crests is best seen in the center section of the left color panel. In the beginning and end segments, the crests are still propagating downward, but the Revelle is simultaneously driving “up-slope”, so the combined effect doesn’t show the motion as well. Breaking sections of the wave are indicated by the red dots in the upper-right panel. Check-out http://santvn.ucsd.edu/FCTD/ to see the Fast CTD data come in in real time.

 

Night Ops            Figure 2. The TTIDE Leg II team model the latest in resort wear, appropriate for a summer cruise on the Tasman Sea.

 

Our Bi Hemispherical (?) team of veterans and volunteers has done a great job of keeping the Fast CTD running 24-7 since the initial deployment. We’ve taken over 1400 CTD profiles so far. We’re in the sweet spot in the cruise right now, with the gear midway between “dialed in” and “worn out”.

 

 F-CTD_Sunset          

 

Next, we’ll move on to the shelf for a day and take a 24-hour record of ocean density, salinity, and suspended sediment data for Nicole Jones, Drew Lucas, and the T-SHELF team. Then it’s back out to deeper water to see what happens when the waves generated at this hill crash into the upper continental slope.

 

Figure 3. Proof: It’s still up there! A nice sunset on February 12.

 

Rob Pinkel

 

Revelle: Breaking undersea waves make you a fish sandwich

The giant subsurface waves the T-team are studying are triggered thousands of kilometers away. After beaming through the Southern Ocean, the waves break against the continental slope, mixing the deep ocean. But, like bath-time with a hyperactive toddler and an especially slippery rubber ducky, these waves occasionally slosh up and over the edge of the tub. In the relatively shallow waters of the adjacent continental shelf of Tasmania, a whole new set of phenomena takes place, one that ultimately influences both the growth of sea life and the carbon dioxide content of the atmosphere.

The ocean’s food web begins with the phytoplankton. These tiny, autotrophic organisms harvest energy from sunlight to produce sugars through the process of photosynthesis. The energy harvested from the sun by the uncountable swarms of natural solar cells is transferred on to the zooplankton, the tiny, heterotrophic grazers of the ocean. After chowing down on the phytoplankton, the zooplankton in turn become food for small fish, and small fish for bigger, and on and on all the way up to your tuna sandwich.

The team aboard the Revelle deploys the anchor weight of one of the T-Shelf moorings. Photo credit: San Nguyen

The team aboard the Revelle deploys the anchor weight of one of the T-Shelf moorings. Photo credit: San Nguyen

Photosynthesis is one of the great marvels of nature: the delicate, complex biochemical process ultimately responsible for 99% of life on the planet. The machinery that harvests energy from photons zipping past, called chlorophyll, as well as the precursors necessary for fixing inorganic carbon dioxide into organic carbohydrates, must be synthesized from compounds acquired from the environment. Since the demand is high in the sunlit surface ocean, those necessary nutrients are always in short supply. The rate of nutrient supply thus controls the productivity of the phytoplankton, and indirectly influences the ocean’s food web and it’s ability to take up atmospheric carbon dioxide. But what controls the supply of nutrients? That’s where the giant subsurface waves, and the T-Shelf project, come in.

In much the same way that turbulence in the ocean’s abyss mixes cold water with warm, controlling the ocean’s ability to transport heat, mixing at the boundary between the sunlit surface waters and the deeper, dark waters below controls the supply of nutrients necessary for photosynthesis. This happens because the vast, deep zones of the ocean are nutrient reservoirs, created by the biological recycling of organic material that rains down from above. These nutrients can be brought into the sunlit surface waters through a number of physical mechanisms. Recently, we have come to appreciate that an important, and poorly studied, pathway is through mixing and transport driven by breaking internal waves.

The T-Shelf program aims to understand the fate of internal tide breakers as they slosh onto the flat continental shelf. At less than 150 m, the depth of the shelf means that much of the water above receives adequate sunlight for photosynthesis. The phytoplankton there are limited by the supply of nutrients, and we think that much of the nutrient supply comes from the transport and mixing associated with these undersea breakers.

We have deployed a series of moorings to measure the breakers as they cross the continental shelf break and the continental shelf. These moorings are in many ways like the much longer and deeper moorings that make up the T-TIDE mooring array. But they differ in two significant aspects: first, the moorings carry instruments to measure the quantity of phytoplankton and the amount of sediment in the water. Second, we are measuring phenomena occurring on small scales relative to the deep array, and have instruments that measure the currents, temperature, salinity, phytoplankton, sediments, turbulence, and nutrients with very fine vertical resolution. The combination of the T-TIDE, T-Beam, and T-Shelf data will allow us to discriminate between mixing and transport from remotely generated internal waves, tracked by T-TIDE and T-Beam from south of New Zealand, and internal waves generated at the Tasman shelf break itself.

We ultimately hope to unravel the complicated puzzle of the relationship between breaking internal waves, nutrient supply, and the biological character of the local ocean offshore Tasmania. These same processes are likely to be responsible for driving the food web in many places in the ocean, and are yet another important, fundamental process in the wonderful, interconnected planet we call home.

Drew Lucas and Nicole Jones, Revelle

The team aboard the Revelle deploys the anchor weight of one of the T-Shelf moorings. Photo credit: San Nguyen

The team aboard the Revelle deploys the anchor weight of one of the T-Shelf moorings. Photo credit: San Nguyen

The team afixes instruments to a T-Shelf mooring. Pictured (L-): Josh Manger, Nicole Jones, Drew Lucas, and Tyler "Slappy" Hughen

The team afixes instruments to a T-Shelf mooring. Pictured (L-R): Josh Manger, Nicole Jones, Drew Lucas, and Tyler Hughen. Photo credit: San Nguyen

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

The Tasman Shelf Flux Experiment / Supported by the Australian Research Council and the University of Western Australia

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_AnnotatedIsopys
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

scripps oceanography uc san diego