Category Archives: Images

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.

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 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.

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 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: 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: 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

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

Falkor: Engineering a Career in Ocean Science

As you may have noticed, there is a lot of high-tech experimental equipment and instruments on an oceanographic research cruise.  We are in an era of ocean exploration that sometimes rivals the technologies used in space exploration.  While research scientists may be involved in the design or concept of research equipment, the majority of this design work, along with the development, construction, assembly, and testing of the equipment, is done by ocean engineers, specifically folks known as development engineers.

A critical role

Development engineers are a critical element for oceanography cruises.  When an investigator has a question they want to ask, or some parameter they want to measure, that requires special equipment, they engage the help of a development engineer.  Unlike other engineering professions that tend to specialize, development engineers will often be involved in all aspects of creating a piece of equipment, which may include skills in mechanical, electrical, and program engineering.  Once the equipment is tested and ready for work, the development engineer still plays a key role – they accompany the researchers on the cruise to ensure things are set up properly, work as they should, and troubleshoot when they don’t.  So the job offers opportunities for both lab and field work – a bonus for people who love variety in their career.

Spencer tagline

Spencer Kawamoto mans one of the tag lines during a CTD/LADCP deployment.  SOI/Judy Lemus
Amy and Spencer testing LADCP

Amy and Spencer make sure the LADCP and computer are able to communicate with one another. SOI/Judy Lemus

Spencer Kawamoto is the development engineer on board Falkor for the T-Beam cruise.  He trained as an electrical engineer at UCSD after taking courses at a community college in Saratoga, CA.  Although he was already thinking about engineering (his dad is also an engineer), an early job placement course at West Valley CC confirmed his aptitude for the field and he set off on his path. “I always liked putting things together”.  As a development engineer, he has a lot of flexibility to explore many aspects of his discipline and teach himself new skills.

Multi-talented

Spencer got his start at Scripps Institution of Oceanography through an internship while he was still an undergraduate.  His specialty at that time was programming and he developed software that helped translate “machine language” (binary code) into scientific language (actual scientific units of measurement).  From there he was offered other jobs and then a permanent position at Scripps.  He’s worked on systems for a variety of projects, mostly related to climate, but ranging from low cost weather stations deployed in Yosemite, CA to moorings that measure mixing between the Pacific and Indian Oceans.

Installing LADCP

Amy and Randall help Spencer reinstall the LADCPs onto the rosette (note the custom brackets) after changing the internal batteries. SOI/Judy Lemus

 Spencer using the ADCP brackets for an alternative purpose – to build up his guns. SOI/Judy Lemus
Spencer’s specific role on the T-Beam cruise involves the LADCPs that have been providing all the data on the internal tide current velocities.  As we’ve mentioned, the CTD rosette has two of these instruments, one pointing up and one pointing down.  The challenge was that the ADCPs were coming from Scripps, and the CTD rosette lives on Falkor.  Each CTD rosette is slightly different so there is no “one size fits all” solution for attaching the instruments to the rosette.  So Spencer designed the brackets that would hold the LADCPs on the metal frame of the rosette.  These have to be strong enough to hold the instruments in place AND withstand possible high wave action, bumping and jostling, AND also be as light as possible.  Lucky for the T-Beam team, Spencer is very good at his job and the brackets have worked exactly as they should!

Engineers often get right into the science duties – in this case helping to recover the CTD (note the angle of the ship deck to the ocean!).  SOI/Judy Lemus

– Judy Lemus, Falkor

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

Falkor: It’s getting hot in here, let’s mix it up!

Last week, the great dark turquoise waves rolling past the ship in the dark looked impressive, at 15 feet tall or more, but they are nothing compared to their giant cousins below. As you know, the internal tide we are chasing across the surging Tasman Sea is hundreds of feet tall, heaving water deep below the ocean’s surface up and down by over 100 feet every twelve hours.

Waves from outer space

These waves may not be visible to us, standing on the deck of Falkor, but that doesn’t mean we don’t know roughly where they are. Satellite altimeters floating in the blackness of space have been measuring the height of the sea surface for the past 25 years. They can track the motion of the ocean caused by the tide and by the internal tidal beam that we are hunting; the surface of the ocean moves up and down by only a few inches, but that’s enough for a satellite!

In fact, satellite altimeters can even measure the gradual sea-level rise caused by global warming, currently estimated to be about 3 millimeters per year, about the height of two stacked pennies. This may not sound like much, but some small island nations may already be suffering from the effects of rising ocean levels, including some of the South Pacific islands near Australia. So what causes sea-level rise, and what does any of this have to do with T-TIDE?

Sunset

Surface waves roll past the Falkor at sunset, giving little clue to the 100 meter high wave below. SOI/Judy Lemus
Ryan XBT

Ryan McDougall-Fisher launches an XBT off the aft-deck of the Falkor. SOI/Luc Rainville

Expanding ocean

Let’s start with some simple physics, because hey, I love simple physics! If you want sea levels to rise, you need more ocean, which means you need a bigger volume of ocean water. There are two ways to increase volume – increase mass or lower density – and each one contributes about half of the total observed sea-level rise.

The first thing you can do to raise sea levels, if you are really keen on having a lovely ocean beach near your house in central Oregon, is to melt some ice to increase the total mass of water in the ocean. Sea-ice won’t work, because it’s already floating, but the Antarctic and Greenland ice sheets are perfect. Greenland alone has been losing hundreds of cubic kilometers of melt water – that’s like 100 times the amount of water in Lake Washington – each year!

The second way to increase sea levels, and turn Seattle into an expensive island chain, is to add a lot of heat to the ocean. Warm salt water is less dense than cold salt water, which means it expands and takes up more volume. This is called thermal expansion. Thermal expansion doesn’t happen equally everywhere in the ocean, because heat isn’t added evenly everywhere.

Satellite altimetry

Satellite altimetry map of the sea surface height associated with the internal tidal beam in the Tasman Sea. Zhongxiang Zhao, University of Washington, Applied Physics Lab.

All mixed up

And that is where T-TIDE comes in! As we’ve mentioned here before, one reason we are studying the internal tide is to better understand how internal waves cause mixing, so we can improve how that mixing is integrated into climate models. This is all mixed up (heh) with sea-level rise and thermal expansion because mixing is really good at moving heat through the ocean.

Heating the ocean is not as simple as warmer oceans = nicer swimming (although being from Seattle, I wish it worked like that). Ocean locations with more heat being added to the surface (like the sunny tropics where I secretly suspect the Revelle has been hanging out based on their photos) will experience more sea-level rise unless that heat is mixed or transported elsewhere. And while heat carried down into the deeper ocean may seem good in the short run, it also means that ocean water near the surface absorbs even more heat, resulting in higher seas in the long run.

Meltwater flows off the Greenland ice sheet, Credit: Roger J. Braithwaite, The University of Manchester, UK. Image Source: NASA.

The good news is that every day we learn more about the oceans and the Earth’s complicated climate, we get better at predicting what will happen in the future. That means we can plan for and adapt to a changing planet. So hopefully no one will have to be up to their ankles ocean waves unless they really want to be!

– Hayley Dosser, Falkor

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

Revelle: TTIDE Leg II shoves off

T_Tide_Logo_2015

Well, Folks,

We’ve just shoved off from Macquarie Wharf in Hobart The Captain is spending some time calibrating the ships compass. Then we’ll head down the Derwent Estuary and into the Tasman Sea.

Matthew Alford and his Leg I Team brought the ship onto Hobart Tuesday morning. They had their gear well organized for offloading, and after a few hours of work by the Revelle’s crew, our container was aboard and we started to assemble the Fast CTD system.

On Wednesday, Nicole Jones and her team from the University of Western Australia arrived and loaded the T-SHELF mooring arrays and bottom landers that we’ll be deploying tomorrow. Preparing instruments and assembling electronics has kept us busy till now.

Revelle image

Figure 1 a) The RV Revelle. B) Output of Harper Simmons global model of semi-diurnal baroclinic tidal generation and propagation. The beam heading from the Maquarie Ridge toward Tasmania motivates our study.

As we head out to sea, it’s hard to believe that the initial TTIDE proposal was written back in 2009. It had just been noted that both satellite evidence and numerical simulations suggested that an energetic (5 kW/m) beam of semi-diurnal baroclinic (internal) tidal energy is heading WNW from the Macquarie Ridge toward the east coast of Tasmania. When these mode-1 waves impact the Tasmanian continental slope, significant turbulent mixing, scattering to higher mode internal waves, and reflection are anticipated. On Leg II, we’ll focus on documenting the phenomena associated with mixing processes and quantifying the scattered high-vertical-mode waves. Preliminary modeling of the eastern continental slope’s response to the incoming tidal beam has been undertaken by Jody Klymak at U. Victoria using the MIT GCM in 2-D and 3- D (Fig 2a,). The results suggest that dissipation is concentrated in a number of regions along the coast, where bottom topography is favorable to the generation of lee waves or upslopebores.

Regions of active mixing should extend 200-300 m above the steeply sloping sea floor, with forward-scattered waves potentially extending even further into the water column (Fig. 2b). Our challenge is to resolve rapidly evolving phenomena like bores as they pass under the ship, held on station 1-2 km above. We have had past experience observing shoaling tides in the South China Sea (Fig.3). Here, a very energetic tide is arriving from Luzon Strait. The near sea floor isopycnals become distinctly non-sinusoidal, assuming a bore-like form. Breaking occurs in regions of high strain (isopycnal separation), where density gradients are low. The challenges in collecting time series such as this one include maintaining the ship on station for extended periods of time, developing a rapid vertical profiling CTD and turbulence sensing system, and operating it continuously so as to obtain a continuous time record of the deep phenomena.

VerticallyDissZoom Jody'sShoaling simulation

Figure 2. a) The eastern slope of Tasmania with expected levels of depth-integrated turbulent dissipation indicated by color. TTide mooring lines will be concentrated around km 110 and km 305. b) A cross-section of the slope showing cross-slope currents (top) and anticipated dissipation rates (bottom). Model output courtesy of Jody Klymak, using the MIT GCM.

VelandOvrtrns

Figure 3. Observations of a shoaling internal tide on the western slope of South China Sea.The sea-floor is at ~700m (black line),appearing to vary as the ship shifts in position. Theblack lines in the top figure indicate the depths of surfaces of constant water density, as they get lifted up & down by the waves. Colors indicate cross-slope flow, as seen from the ship’s ADCP. Strong turbulence initiates (lower figure) as the density surfaces diverge from the sea floor, producing a region of low density-gradient. Roughly 200 CTD profiles were obtained during this 30-hour observation to document the shoaling tide and resulting turbulence.

The instruments we’ll be using on Leg II include the Hydrographic Doppler Sonar System (HDSS) on the Revelle, a nested set of 140 and 50 kHz Doppler sonars built into the Revelle’s hull, and the Fast-CTD, a Seabird 49 CTD that is vertically profiled at ~5 m/sec, to document the evolution of the density field as the waves pass by. To profile rapidly, the Fast CTD (Fig. 4) is housed in a streamlined lead-nosed package. The instrument is suspended from a thin spectra cable to minimize cable drag. Falling at 5 m/s (10kts), we will profile through the lower 200 m of the sea (the turbulent region) in ~40 sec. We’ll try to get within 20 m (4 sec) of the sea floor on each drop. This requires extreme vigilance on the part of the operators. Along with the CTD, the profiler contains a micro-conductivity cell to detect turbulence (0.1 m vertical resolution) and an inertial package to determine the spin-rate/ trajectory of the fish. New for this trip is an acoustic altimeter that should detect the sea floor from ranges up to 50 m. If it works, we’ll get ~10 seconds of advanced warning as the bottom approaches.

The Fast CTD is operated off a custom boom that will be mounted on the Revelle’s stern (Fig 4b). The boom is needed to keep the CTD cable from fouling the ship’s hull or propellers during operation. To manage the cable payout at high speeds, a motorized sheave is used at the end of the boom. The sheave can operate at extreme wire angles, which are common when the CTD is used in strong currents. The system has been extensively modified for the rough conditions expected off Tasmania. We’ll see how it all works in the next few weeks.

CTD Fish 2015 Boom Deployed

Figure 4. a) the Fast-CTD fish. The micro-conductivity probe is protected by the circular guard. The CTD and altimeter are housed within the fish. b) The motorized sheave manages cable tension on descent. The boom and the motorized sheave work in tandem to keep the fish clear of the Revelle’s hull & screws.

Control Display

Figure 5. The control screen for the Fast-CTD. Winch status, control, and CTD raw data are displayed in real time.

The Fast CTD is operated from a console in the Revelle’s aft lab. A lab-view display controls the winch (Fig 5). Several video monitors and a CTD data display also are located at the operator’s station.

On station, the plan is to operate 24 hours a day. Typically we divide into two watches, each of which stands alternating 12-hour shifts (3 am-3 pm). Within each watch, crew rotates between hourly stints controlling the CTD, standing watch on the fantail to look for cable fouling, and serving as back-up / reserve.

Tomorrow we’ll start to deploy Nicole Jones’ “T-Shelf” moorings and Drew Lucas’ Wirewalker profilers on the Tasman shelf. We’ll then spend a day profiling near these moorings. Following this initial effort, we’ll move offshore and launch the Fast CTD. We’ll spend 24-36 hours at each site, exploring the various wave and turbulent phenomena associated with different topographic features. We’ll decide on sites to explore based on our initial findings and those of Matthew Alford’s team on Leg I.

– Rob Pinkel, The Revelle

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

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