Monthly Archives: February 2015

Revelle: Dialed-In

T_Tide_Logo_2015TTIDE 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).

Fast CTD with altimeter

Figure 1. Fast-CTD Fish # 2 ready for an evening dip. The altimeter is the small protrusion in the bottom of the tail.

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).

Altimeter_Closeup_Photo Altimeter

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.

Station_180900EOS_Cas_Dens

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).

 

Clarence & Winney Sunny Sea Lion

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!

 

Rob Pinkel

Falkor: You, Me, and the Tasman Sea

Swirling, living ocean

I used to think the ocean felt vast – an endless empty expanse of water, deep and unchanging. Within a month of starting my graduate degree at the University of Washington School of Oceanography, I had learned that the ocean is neither empty nor static. The ocean is teeming with life, and not just `charismatic mega-fauna’ like whales and tropical fish, but also tiny single-celled photosynthetic phytoplankton. Plankton may seem insignificant, but there are billions and billions of them, and their combined effect is enough to change the amount of carbon stored in the deep ocean and the amount of oxygen available for us to breathe.

And the ocean water itself is alive with motion. It circulates slowly, for sure, especially compared to the atmosphere that gives us our ever-changing weather. The deep currents in the ocean travel across the sea-floor for hundreds of years, and surface currents skirting the edges of the continents move at a few miles per hour or less. The surface itself is a patchwork of eddies – a literal ocean of swirling vortices. And the internal tides we are studying criss-cross the ocean in a web of wave beams, spilling onto the slopes and breaking or bouncing off again.

Falkor_TBEAM_end

 

 

 

 

 Falkor cruise track overlaid on sea surface height map from satellite altimetry (AVISO), showing eddies in the survey region. Courtesy of Luc Rainville.

 

 

 

Our little patch of sea

So, the ocean is not empty and not quiet… but surely the ocean is huge? Surely it’s beyond our ability to ever fully understand, beyond our ability to change? You might think – floating out here on a ship with the same group of people for a month, with nothing in any direction but the endless rise and fall of the Tasman Sea – that the ocean would feel uncommonly big. I have found however, that at least for me, the opposite is true.

The longer I’m out here, the more this patch of ocean has begun to feel familiar, comprehensible, like a well-known neighborhood in the middle of a sprawling city. The Tasman Sea determines our days here – the science we can do, the measurements we can make, the birds and fish and plankton we come across, even how easy it is to sleep or shower. We are in constant motion with the waves beneath us. I feel connected to the ocean here, now that I know more about it – the creatures that live here, the moods of the weather, how to work with the Tasman Sea instead of against it.

Dissolving disaster

Truth is, we are all more connected to the ocean than we realize. And we have a greater impact on it than you might expect. Here’s a simple experiment: the next time you brush your teeth or wash your face, take a look at your toothpaste or face wash. Do you see any tiny colored specs? Chances are these rough little particles are actually plastic! They are great at polishing teeth and exfoliating skin, but they often end up in the ocean and are not so great for the plankton and other marine life, who cannot digest them and die. Here on the Falkor, Randall Lee is measuring the micro-plastics coming from Australia, from near Hobart where concentrations are higher to way out here in the Tasman Sea where the ocean is almost empty of plastic.

Microplastics aren’t the only threat facing sea critters, in fact they aren’t even the worst one. As we’ve mentioned here before, the ocean is taking up a lot heat, reducing the immediate impact of global warming. It’s also taking up quite a lot of CO2 – something like 30-40% of human emissions. This gas dissolves in the ocean at the surface, forming carbonic acid and making the ocean more, well, acidic.

‘Ocean acidification’ is a hot topic right now, with clear evidence of the problems it’s causing for everything from the corals in Australia’s Great Barrier Reef to the oysters on the West Coast of the United States. Many tiny sea creatures have shells made of calcium carbonate, and they are slowly dissolving away. It’s difficult to imagine the solution, especially to a problem that seems so huge.

IMG_1632
 Microplastic filtration system set up by Randall Lee on the Falkor. Photo credit: Judy Lemus.

Microplastic

 Simulation of microplastics released from a flood plume into Port Phillip Bay, and moved by the winds and tides, slowly spreading towards the Bay entrance. Courtesy of Randall Lee

 

Global connections

I imagine that it’s kind of like the ocean though. You start with what is right in front of you, you work to understand your little patch of the world and treat it with respect, and it will connect you to everything and everyone and everywhere else! And, just like our internal tides, spreading out across the globe, local changes often have an unexpected global ripple effect. We are working to understand one tiny part of the ocean in order to improve our understanding of the entire climate system. But it’s the everyday choices we all make that have the best chance of protecting it.

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 Sunrise over the Tasman Sea looking out past the A-frame, from the aft deck of the Falkor. Photo credit: Hayley Dosser.

 

 

 

 

 

– Hayley Dosser, Falkor

Revelle: Breaking Undersea Waves Make Fish a Sandwich

T_Tide_Logo_2015It’s never good news when the phone rings in the chief scientists cabin. It was 7:30 am on Tuesday. We had just lost electrical communication with our Fast-CTD fish. I arrived on the fantail to find the worst-case scenario unfolding. Fifty meters of wire were being coiled on the deck. There was no instrument on the end. The fish and 800 m of wire were lost.

It didn’t take long to see what had happened. Nearly 20 m of the cable was filleted by razor sharp cutters (Figure 1). Then a few meters of undisturbed wire…. then “Chomp”!

 

SharkBite 1

 

 

About  once a cruise we have a deep encounter with a shark or big fish (Figure 2). Our recent cruises have been in the South China Sea or in equatorial waters, where sharks are expected. Only the depth of the encounters has been a surprise. Our old fish design had a lead nose, which over time got decorated with the gouge marks of sharks teeth (Figure 3). Once we brought up a tooth stuck in the lead. Off Tasmania, in the “Roaring Forties”, the waters are surprisingly warm, due to the south-flowing East Australia Current. As the local surfers confirm, there are sharks here too.

Figure 1. A close up of roughly 20m of “flossed” cable.

We were profiling between 1300 and 1900 m when the incident happened. “Bruce” (Nemo, Finding, W. Disney Inc, 2003), while swimming through the blackness at around 500 meters depth, apparently bumped into our wire. The instrument was near the top of its profile, 800 meters below. He struck at the moving wire, using the first 20m to floss his teeth. Then he bit down hard.

Crash Guard Before Bent Crash Guard II

Figure 2. The nose of our 2012-vintage Fish showing the orange micro-conductivity cell, before (left) and after

(right) an encounter with a UFO (Unidentified Fishy Object).

It’s tough when you lose an instrument. In over forty years of playing this game, well over 100 thousand profiles, we have previously lost only one CTD. It’s part of the cost of doing business in field science, and the cost unfortunately is getting to be quite high. The value of this instrument is equivalent to the operational cost of our ship for roughly 12 hours.

Close-up Chomp mark

Wasting time bemoaning lost equipment is expensive too. By 11 am we were ready to go again with our back-up CTD fish, using an old, well-worn, cable from a previous experiment. Hopefully we’ll learn a lot more about ocean mixing & no more about sharks as the cruise continues to unfold.

 

Figure 3. When the lead beneath the black tape is deeply gouged and there is a matching gouge on the opposite side of the fish, you know that you’ve failed the Bruce taste-test.

 

 

Rob Pinkel

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

 

Falkor: Thanks for watching, part 2

We are all happy to be home, but had a great time doing science at sea on the Falkor. In the remaining hours of our T-Beam cruise, we were able to put together a video using one of our favourite songs: Tassie Whalers by The Overlanders (Pete’s uncle is one of the members!).

When not working, we were also hard at work trying to win the fitness challenge between the Revelle and Falkor crew. As the Revelle Leg-1 crew mentioned, it is important to keep healthy when at sea for such a long time. We tried our best and we even had a rowing competition with medals! Overall, the Falkor science and ship’ crew:

  • Ran 375km
  • Cycled 671km
  • Rowed 194.5km
  • Pullups 351
  • Pushups 7091
  • Situps 6347
  • Squats 1766
  • Dead lifts 70
  • Yoga 405mins
  • Burpees 850
  • Gun sculpting 23
  • Skipping 8400
  • Hoola hoop 30mins
  • Cheese 42
  • Bacon 20

Well done team!

 

Falkor: Back from the Tasman Sea

After 26 days at sea, the T-Beam crew has sailed back into port along the Derwent Estuary, and tied up on land. We never expected this work to be easy – battling with the constant barrage of storms headed directly from the southern ocean – the Tasman Sea did not disappoint. We did, however, have a successful cruise.

Our part of the T-TIDE experiment was tasked with identifying the internal tide beam as it races across the Tasman Sea, through a sea of eddies to understand what the beam looks like before it slams into the continental slope of Tasmania. After starting out with some coordinated operations with the Revelle at the beginning of our cruise, we headed east and set up camp in the middle of the Tasman Sea.

 

Gabi Pilo (right), Spencer Kawamoto and Ryan McDougall-Fisher taking instruments off the CTD-cage on a rare sunny day during the transit to Hobart. Credit: Judy Lemus

Gabi Pilo (right), Spencer Kawamoto and Ryan McDougall-Fisher taking instruments off the CTD-cage on a rare sunny day during the transit to Hobart. Credit: Judy Lemus

We conducted a set of nine 25-30 hour CTD profiling stations two different lines spanning the internal tide beam. Our CTD was equipped two ADCPs to get vertical profiles of horizontal velocity as well as chi-pods which are able to measure temperature (and turbulence) at a very high rate. T-TIDE PIs have been working tirelessly over the months & years leading up to these cruises to determine, to the best of their ability, where the internal tide beam might be. This has involved using satellite altimetry data as well as mathematical models designed to simulate what the internal tide characteristics look like and guided us in picking our CTD stations. Although our work has only just begun, and we will spend the next months (and years!) untangling the data set that we have obtained, our initial findings suggest that we found the beam!

Preliminary estimates of the internal tide energy flux (arrows) as measured from the Falkor. Pink triangles (F2-F9) are the locations of our CTD stations, and the red/grey arrows show our initial estimates of the semi-diurnal internal tide energy flux headed towards the Tasman slope right towards the T-TIDE moorings (blue circles). Credit: Sam Kelly, Luc Rainville and Amy Waterhouse.

Preliminary estimates of the internal tide energy flux (arrows) as measured from the Falkor. Pink triangles (F2-F9) are the locations of our CTD stations, and the red/grey arrows show our initial estimates of the semi-diurnal internal tide energy flux headed towards the Tasman slope right towards the T-TIDE moorings (blue circles). Credit: Sam Kelly, Luc Rainville and Amy Waterhouse.

Although we will all be scattering our separate ways very soon, there is really not enough that can be said about the incredible science crew on our T-Beam cruise. This includes the hard work of Captain Heiko and his ship’s crew. Everyone worked long hours and endured some of the worst weather – yet everyone was always ready for more, determined to get as much science as we could while being a great, fun group of people!

T-Beam science crew enjoying the last sunset on the RV Falkor. Credit: Danielle Mitchell
T-Beam science crew enjoying the last sunset on the RV Falkor. Credit: Danielle Mitchell

Hayley Dosser, a PhD candidate at UW-APL, interviewed the science party yesterday asking us simply ‘Why you study the ocean?’ After thinking for a few minutes, I realized that this is a very profound question. I study the ocean because I love it – I love the complex physical signals that lie beneath the ocean surface, and as scientists, I feel that we are responsible for doing our part in understanding how these complex signals will change or affect our changing climate. The internal tide in the Tasman Sea is a process that we need to understand in order to broaden our understanding and ability to predict the future ocean & climate, and with these small steps, such as the T-TIDE experiment, we are making that process.

Fair seas to the Leg-2 and 3 Revelle T-TIDE contingent who are still out on the slope! We are looking forward to your updates.

— Amy Waterhouse, Falkor

Falkor: Rollin’ in the Deep

This morning I was handed a small vial of water from the deepest reaches of the Tasman Sea (4800 meters deep, to be exact).  So what, you ask?  Well, this water has not seen the light of day for about 600 years.  That’s right, 600 years ago this water became very cold, salty, and dense, gradually sank to the bottom of the ocean in either the North Atlantic or the Southern Ocean and has not been in contact with Earth’s atmosphere since.  These water samples were collected by Pete Strutton on our last CTD profile – the Niskin bottles on the CTD were opened at 4800 meters and the cold, salty water poured in.  We’ve previously mentioned Pete’s work on nutrients in surface waters and the effect on phytoplankton.  In this case, he wanted to be able to compare the abundance of those nutrients at the surface with the very deepest water in the Tasman Sea (which would tell him a bit of how much the deep nutrients are getting mixed into the upper layers).  Back at the surface, Danielle Mitchell partitioned the samples out into vials as souvenirs for everyone on board.
Shrunken cups

Decorated Styrofoam cups brought down to 4000 meters deep become doll-sized due to the efects of pressure (the paper towels inside are helping them retain their shape until they dry). SOI/Judy Lemus
Deep water

A vial of water that has spent 600 years in the deep ocean. SOI/Judy Lemus

What goes down, must come up
Our retrieval of this deep ocean water gives me the opportunity to talk about the important role of deep water circulation around the globe and the water cycle.  The water that we brought up in our samples actually has a 50% chance of being either from the North Atlantic or the Southern Ocean (Matsumoto, 2007).  These two areas are responsible for creating the large masses of deep ocean water because the surface water in these two areas becomes very, very cold.  Water becomes more dense as it cools and therefore it starts to sink. In addition, the surface seawater may even freeze (especially in the Antarctic) and as it does, the dissolved salts precipitate out, making the seawater below even more salty, and even more dense. The sinking water masses, called “North Atlantic Deep Water” and “Antarctic Bottom Water”, initiate a deep ocean circulation pattern (called thermohaline circulation) that carries water from these two regions towards the equator where wind-driven upwelling brings it to the surface, gets warmed again, some of which is evaporated into the atmosphere, becomes clouds, and rains back down into the ocean again in higher latitudes.  Surface waters are entrained in large ocean gyres that move in a cyclonic fashion around the ocean basins in the northern and southern hemispheres.

A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, while red paths represent surface currents. This collection of currents is responsible for the large-scale exchange of water masses in the ocean. Figure by NASA
Chi-pod work

Working in the deep ocean means instruments must be constantly maintained.  Here Amy and Sam tend to one of the Chi-pod temperature sensors. SOI/Judy Lemus

The Dating Game
A familiar characteristic of the deep ocean is the immense pressure, due to the thousands of meters of water pressing down from above.  Students at Salmon Bay school in Seattle, WA will get a first hand demonstration of this pressure when the Styrofoam cups they decorated come back to them after being taken down to 4000 meters in a mesh bag attached to the CTD.  But what is ecologically important about the deep ocean is its storage capacity.  In addition to nutrient storage covered in a previous blog, the deep ocean can store a large amount of heat and carbon.  It is this storage of carbon, as dissolved CO2, that gives scientists the ability to know how long the water we collected on our CTD is about 600 years old.  The earth’s atmosphere has a very small amount of naturally occurring 14C, also called radiocarbon.  14C is an unstable form of carbon and undergoes radioactive “decay” (one of its neutrons becomes a proton and the molecule becomes a stable nitrogen atom, 14N).  This decay happens at a very predictable and measurable rate.  There is no source of 14C in the ocean, so all of the 14C in the ocean has come from the atmosphere.  Once 14CO2 is dissolved in seawater, any decay of the 14C can only be replaced by new 14C if the water remains in contact with the atmosphere.  So radiocarbon dating can tell us how long it has been since a particular sample of deep ocean water has been at the surface.  And that can give us an estimation of how long it takes for water to move through the “conveyor belt” of global thermohaline circulation.  From one small atom to a big picture of how the ocean affects Earth’s climate – science is pretty amazing.

– Judy Lemus, Falkor

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

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

Falkor: Using Nebula to Solve Nebulous Problems

Out here in the Tasman Sea, chasing the internal tide involves a lot of detective work, piecing together clues from water velocity, temperature, and density to determine where exactly the tide beam is heading.  Scientists know from 20 years of satellite altimetry (sea height) data where it is most likely to be, based on estimations of energy flux.  But there are a lot of factors that come into play that can affect the direction of the beam, including the physical characteristics and shape of the deep sea ridge where the internal tide is generated.

This is where mathematical modeling is again helpful. Modeling allows scientists to create a visualization of a complex phenomenon that integrates multiple sources of data with theoretical physics. The current state of the art model to describe ocean circulation, including internal waves, is called the MITgcm model (created at MIT in Boston, MA).  On board Falkor, Dmitry Brazhnikov is using this model to generate a hypothesis that will describe the pathway of the T-Tide beam after it is generated on the Macquarie Ridge.  Having an accurate picture of the direction of the beam is very important for understanding how it will interact with and reflect off of the Tasman Continental Slope. Starting with the satellite altimetry data to define one geographic section (domain) of the beam, Dmitry then began adding velocity and temperature data from this cruise into the model to re-prescribe the parameters of the MITgcm model to fit the specific conditions for the Tasman internal tide.  This process is called data assimilation and is extremely useful for improving the accuracy of numerical models, and for exploring the behavior of a system/phenomenon when you don’t have the luxury of making direct observations on every aspect of it.

Dmitry with clouds

Internal waves can occur in any medium where there is a strong difference in densities. Dmitry Brazhnikov admires the cloud formations created by an internal wave in the atmosphere. SOI/Luc Rainville

A Bending Beam
Although the weather has not been entirely cooperative, the T-Tide science team have still been able to collect a good amount of velocity and temperature data that offer a glimpse of where the T-Beam is.  As we mentioned previously, the highest water velocities at the depth we would expect to see the internal tide seem to be located somewhat north of where the original model using satellite altimetry data had indicated.  Dmitry is now using those data to create new simulations of the beam, and is getting some very interesting preliminary visualizations.  Rather than simply taking a more northward course directly from the point of origin, the model shows the beam “bending” toward the north as it progresses across the Tasman Sea. Physicists categorize waves into “modes” according to their vertical structure, which defines the energy, wavelength, and stability. Dmitry’s model shows that as the T-Beam wave moves toward and encounters the Tasman Shelf, the energy from the internal wave transitions from a long wavelength, stable mode 1 wave (which can be propagated over long distances), to a mode 2 wave (shorter wavelength and less stable) and then to a mode 3 wave (even smaller wavelength and stability).  The energy from the mode 3 wave is likely entirely dissipated on the continental shelf.

T-Beam model

Dmitry’s model illustrating the “bending” of the T-Beam across the Tasman Sea. Figure by Dmitry Brazhnikov
Model of beam reflection

This model shows the primary beam of the internal tide (mode 1: large energy with a major single direction) transitioning to a reflected wave that is mostly mode 2 (with some dispersed energy), and finally to mode 3 (dissipated energy without any general direction).  Figure by Dmitry Brazhnikov

Modeling the Future of Oceanography
None of these simulations would be possible during the course of a research cruise without the help of a “supercomputer”, which is essentially a computer with very high computing power and processing speed. This cruise, the T-Beam team is fortunate to be able to use the Nebula system, a cloud-based supercomputer recently installed on Falkor. Dmitry believes that supercomputers will help advance oceanographic research by allowing scientists to adapt a cruise plan while at sea, depending on what the real-time data are indicating.  This could have real implications for biological oceanography where scientists might want to predict the location of, say, zooplankton communities, over the course of a cruise.  Or use this dynamic modeling to help locate hydrothermal vents based on chemical and temperature signals in the water column.  In the future, there is the potential for substantial savings of time and money by being able to identify ocean features and phenomena while researchers are actually at sea, rather than waiting to analyze and visualize the data back in their laboratories.

Dmitry doesn’t just work with models at a computer – here he handles one of the tag line poles to recover the CTD. SOI/Judy Lemus

– Judy Lemus, Falkor

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

Falkor: Stormy weather

With most of our field work on the RV Falkor located in the middle of the Tasman Sea, we have had our fair share of rough weather. We were able to hunker down (or escape) the biggest storms that rolled through, but sometimes we had to stop collecting data for a few hours. Below is footage of one time we were glad not to be on deck.

– Sam Kelly & Amy Waterhouse, Falkor

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

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