Archive for ‘Salinity Processes in the Upper Ocean Regional Study (SPURS)’

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What is ocean microstructure and why study it?

October 4th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

“The techniques I developed for studying turbulence, like weather, also apply to the stock market.”
Benoit Mandelbrot

If Mandelbrot’s statement is true, maybe oceanographers studying ocean microstructure (caused by turbulence), besides writing journal articles about mixing in the ocean also work on padding their retirement accounts with stunning stock market acumen?

Jokes apart, how does ocean microstructure work? The microstructure component of SPURS features measurements of very small-scale (inch or smaller) variations of temperature, salinity, and velocity, used to infer mixing rates due to turbulence and convection in the upper ocean.

Profiles of Temperature, Salinity, two channels of small-scale temperature gradient and two channels of small-scale shear from a dive of the Vertical Microstructure Profiler (VMP). The depth intervals with the strongest variations in temperature gradient and vertical shear are the places where the ocean is mixing the strongest. These raw data are carefully processed to tell us how much of the salinity change that we see is due to the vertical mixing rates. They can also tell us which processes are causing the mixing, such as shear instability or salt fingers.

Lou St. Laurent, together with Woods Hole Oceanographic Institution colleagues Carol Anne Clayson and Ray Schmitt, are the scientists principally focused on these issues. Their work is supported by the National Science Foundation and is another good example of the inter-agency cooperation spanning SPURS.

Ken Decoteau, a WHOI engineer, leads the microstructure group’s sea-going effort. Together with James Reilly, a research technician from UMass-Dartmouth/SMAST, Oliver Sun, a WHOI postdoctoral investigator, and Alec Bogdanoff, a graduate student in the MIT/WHOI Joint Program for Physical Oceanography, they run 24-hr operations of two instrument systems, a vertical microstructure profiler (VMP) and turbulence-sensing gliders (T-gliders).

Ken Decoteau.

Jim Reilly.

Oliver Sun.

Alec Bogdanoff.

The first, a vertical profiler (Rockland Scientific VMP5500), is lowered into the water and released. In its pressure housing, it carries batteries and electronics. The data collected by the external sensors is recorded to a disk drive inside the instrument. The steel ballast weights it carries cause it to sink to a pre-set depth. Upon reaching this depth, a trigger mechanism is fired, causing the weights to be released, and the instrument to float back to the surface. The instrument is then recovered, the data downloaded, and preparation for the next deployment begins. Although the instrument is capable of profiling to a depth of 18,000 feet (about 5,500m), most of the profiles on this expedition are being done to a depth of 4,000 feet (1,200m). These deployments take about one hour from the time we release the instrument until it re-surfaces.

The Vertical Microstructure Profiler, ready for deployment.

A second kind of measurement (of longer duration) is conducted using two Teledyne-Webb Slocum Gliders (T-gliders), each carrying a Rockland Scientific MicroRider. The MicroRider is similar to the VMP, but is much smaller in size since it is not designed to go as deep, and does not carry its own batteries and CTD; it draws battery power from the glider. The T-glider does not utilize direct propulsion (as in a motor), instead changing its buoyancy to move up and down in the water column and using wings to translate this motion from vertical to horizontal (a.k.a. gliding). This low power design means microstructure data can be collected continuously over a time period covering weeks, even months, with minimal intervention.

Small boat crew filming a T-glider in trial deployment.

During SPURS, one T-glider is covering the upper 230 feet (70m) of the water column while stationed in a small area close to the highly-instrumented WHOI mooring, while the other T-glider is covering the upper 660 feet (200m) of the water column while doing continuous transits between the WHOI mooring and PMEL-N mooring.

So far, more than 30 VMP microstructure profiles have been taken by two twelve-hour shifts working 24/7. With the minimal additional amount of effort required to deploy and to periodically checkup on the two gliders, we have collected over 700 additional profiles of the upper ocean from the gliders. Utilizing these two different platforms simultaneously has a dramatic positive effect on our sampling efficiency, and illustrates why autonomous sampling techniques have become much more common in recent years.

The measurements of microstructure variations in temperature, salinity, and velocity are crucial to SPURS estimating the salinity balance.  We need to know how the less saline waters to the south and south are mixed with the saltier waters near the salinity maximum in the North Atlantic.

Maybe, after all, it is like the stock market! The “value” (measurement) of our “commodity” (salinity) depends on a complex global “marketplace” (a salinity balance equation) forced by wind, evaporation and precipitation and “trading” in complex oceanic processes (circulation and eddies) acting in the end through “derivatives” (turbulent mixing) to determine the values we see.  Maybe these oceanographers are in fact sitting on a gold mine!

Truth be told, their mathematical wizardry is focused on understanding the ocean and the water cycle, not the stock market and the business cycle. However, old Benoit Mandelbrot was entirely correct! A few brilliant men like him can live in both worlds and we all profit.

The Bridge Of The Knorr

October 3rd, 2012 by Maria-Jose Viñas

By Eric Lindstrom

The bridge of the Research Vessel Knorr. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution.)

Almost everyone can imagine the bridge of a ship – from the movies, a tour of a ship, or maybe you are the master of your own vessel. It is the place where control of all ship operations is commanded. On the bridge of the Knorr, an officer and a seaman are always present and in full control of the vessel, for the safety of all aboard and effectiveness of the ship as a research platform. It’s amazing what they can do and the various skills that are required to command a research vessel.

The bridge and chart room house all the ways and means needed to safely navigate Knorr, maintain her stability, maneuver her on station, and to communicate with anyone as necessary. Like the dashboard of a (sophisticated) car, they have a display of key engineering functions and to operate the ship in both manual (i.e., you driving) and autopilot (car cruise control) modes.

One key function of the master that you may not know about is to complete calculations assessing the stability of the ship and to ballast her correctly as fuel is consumed or heavy scientific gear is deployed. Seawater ballast tanks are available to compensate for changing conditions (Knorr uses between 1,800 and 3,800 gallons of fuel per day).

There is an overarching sense of situational awareness that permeates life on the bridge of any research vessel. The captain and the mates are always alert of changing conditions like weather, location, time, ship traffic, and science plans.  With the rhythm of the ship, they are also keenly attuned to watch changes, meals, rest periods, and fresh coffee brewing. Also, there is all the important watch for fish. The chief engineer must be informed 24/7 of any sighting.  We all need our protein…

That leaves the oceanographers to focus on the science bits – like what the tiny variations of salinity are in the top 3,000 feet of the ocean! That’s sweet. The bridge looks after us, we look after the ocean, and everyone is, well… working. Not much else to do out here.

Knorr has amazing capability to stay on station. In gentle sea conditions, it’s quite possible to stay on location to within a few feet. The officer on watch can set the ship to keep position, so their attention can be shifted to observing deck operations further aft on the ship.

The track of the Knorr while on station. The orange circles have radii of 4 feet and 8 feet.

The view from the starboard bridge station aft.

Captain Adam Seamans has been a Master of Knorr since 2008. He started with Whoods Hole Oceanographic Institution (WHOI) as an Able Seaman in 2000, where one of his first duties was to clean a science cabin after a head overflowed. My, how times have changed! Now he commands the 22-member crew of Knorr and navigates her to all corners of the globe. Maybe he still has to deal with a lot of crumby jobs, but they are mostly of the cleaner variety!

Captain Adam Seamans holding the Knorr’s heading box, which contains three wooden blocks with ten digits inscribed in each. One can set the heading of the ship 0-360 degrees and place this in front of the seaman steering the ship’s course. For the photo, the heading is set at 279 – the length of Knorr in feet.

The Knorr has enormous capability for communication – via satellite phone or internet or radio. In close quarters with foreign vessels, Knorr can communicate the old-fashioned way via flags. Sequences of brightly colored flags have universal meanings to mariners.

The Knorr’s signal flags box.

Finally, I thought I would call your attention to training. All the officers aboard Knorr have great experience and through my decades of oceanography, I have found maritime officers always passing their knowledge and experience forward to others. Teaching and sharing, honestly, what a great concept! It makes for safer seas.

On this voyage we have a learner, a cadet named Anna, from Sweden. She is just 18 and approached a WHOI captain about being a cadet aboard one of their vessels. Sometimes you need to be careful what you wish for! Anna has been working all the jobs on the ship for our expedition. Just today, she was driving a winch by herself for the first time, looking happy and proud! Her initiative and hard-working nature are an example for all of us. Sometimes the teachers become the students… Anna, Bravo Zulu!

Cadet Anna drives the CTD winch.

Managing SPURS Data

October 2nd, 2012 by Maria-Jose Viñas

By Eric Lindstrom

A high-level concept for SPURS data flow whereby data collected by the many devices will be relayed via satellite to shore and integrated with numerical ocean models and satellite observations.

The complex job of managing the data from SPURS is daunting because of the number of different platforms and data streams that need to be logged and cross-calibrated. Overall, by NASA standards, we are dealing with relatively small quantities of data (Gigabytes) but there are many,many different sensors and platforms involved.  The value of the SPURS dataset comes from analyzing together the measurements made by all the different instruments at a variety of time and space scales. Such an analysis would not be credible without detailed comparison and calibration between all the instruments. SPURS interpretation is dependent on all the salinity measurements being comparable to one another and that one ocean salinity field is derived from the variety of measurements. Let me go into a little more detail on these points (forgive me for getting nerdy about this, but its important to the expedition, so I gotta tell you!)

To give you an example of the range of platforms and instruments measuring salinity in SPURS, consider that we are trying to measure surface salinity from space using Aquarius, surface drifters, Argo floats, Wavegliders, Seagliders, thermosalinographs on ships, moored salinity sensors, underway and station CTD, and bucket samples (it’s just what it sounds like!). NASA data systems are not generally working the entire buckets-to-satellites range of seawater analysis, so we are learning as we go — and doing just fine so far!

Salinity is a difficult variable to measure properly. It is determined by measuring the ability of seawater to conduct electricity. Though this sounds simple in principle, in practice, the measurement of salinity requires exacting standards of quality. The relevant variations in SPURS are one part in ten thousand (for comparison, a pinch of salt in a gallon of water changes its salinity by about 2 parts in ten thousand). Our instruments can measure salinity differences down to a few parts in a million.

The ocean salinity standard by which all of our ocean salinity measurements are compared is created by using a laboratory salinometer and a standard for seawater salinity, the “IAPSO Standard Seawater”. Operating the salinometer is a critical but tedious job, taking place daily in a temperature-controlled room just off the Knorr’s main laboratory.  We collect dozens of water samples every day, all of which need to be examined by the salinometer with exacting adherence to procedures and recording of results. The salinometer is calibrated for each batch using the standard seawater with known salinity of exactly 35 parts per thousand.

The salinometer on Knorr.

Bottles of seawater collected during CTD profiles and from the ship’s thermosalinograph system are analyzed on a regular basis and compared with the Standard Seawater. The fully-calibrated measurements from the CTD and the ship’s underway system are then used to standardize all of the other instruments being operated in SPURS (by comparison of nearby simultaneous observations).

A SPURS project led by Prof. Frederick Bingham of University of North Carolina Wilmington and Yi Chao at Remote Sensing Solutions in California has been focused in part on cross-calibration of the different data streams. Fred and student Andrew Whitley are on the Knorr from UNCW.

Fred and Andrew at work.

Fred is an observational physical oceanographer who has worked with many different kinds of oceanographic data over the years, including CTD, drifter, mooring, velocity, float and satellite data. In addition to his PhD in Oceanography, he has a Masters degree in computer science and information systems. This helps him manage and understand the sometimes complicated and confusing data structures the different oceanographic instruments produce.

Andrew Whitley is a graduate student at UNCW in Physical Oceanography studying large-scale coastline modeling. He has undergraduate degrees from UNC Chapel Hill (in music) and UNCW (in physics).

Andrew is a first-timer to long oceanographic expeditions.  He told me “I’ve always been fascinated with shipboard operations and research, but being a student of modeling rather than observing the ocean, I had not had opportunities to get aboard a research vessel. Thankfully, Dr. Bingham heard about my desire to go to sea, and I am very grateful that he asked me to come along. This has truly been one of the most extraordinary experiences of my life!”

I think he might have nightmares about seawater sampling for years to come! 

At sea, Andrew is also helping Fred with his data processing efforts. Andrew works up daily reports and plots on sea surface temperature and height, and drifter positions.  He is processing daily CTD data for general shipboard use. Like many first-timers Andrew is soaking in the experience. It seems like no matter the hours or the watch to stand, he is loving every minute!

Another one of Fred’s duties on Knorr is to add to our “situational awareness” of ocean conditions and our sensor web. The SPURS instrument array is complex and constantly evolving in time as instruments drift or maneuver through the water. Satellites overhead provide information about sea surface salinity, temperature, currents, winds and weather. Onshore models make forecasts of ocean conditions 24-48 hours ahead. Much of this real-time data flow is unique to SPURS. In the new world of real-time oceanography, the ship can be directed to features of interest, instruments can be deployed based on analysis of satellite data or autonomous vehicles directed to travel to where they are needed away from the ship. Fred’s job is to keep the chief scientist aware of what observing assets are where and what the status of the array is.

In his every day work on board, Fred keeps a constant watch on the ship’s position and the positions of all of the other assets in the water. (An example of the type of display he uses can be seen on this website — click on “SPURS Data” and then on “Visualization”). When the need arises, he downloads and plots satellite imagery. He extracts data from the different data streams that are updating regularly and analyses them. It requires daily interaction with the modeling team at NASA’s Jet Propulsion Laboratory. All of the scientific team aboard Knorr use the daily updates on the environment and observing system provided by the UNCW team. It is an essential element for SPURS success!

Modeling And The Dry Side of SPURS

October 1st, 2012 by Maria-Jose Viñas

By Eric Lindstrom

SPURS dry team members. From left to right: Peggy Li, Yi Chao and Gene Li.

Yi Chao is one of the “spiritual leaders” of our “dry” team in SPURS (those people who help from land). He is an ocean modeler in California who has been involved with Aquarius and SPURS for many years. He long ago decided to be on the dry team because of the seasickness that he experienced ten years ago during the tests of the Aquarius precursor instrument. Worse of all, he tells me, he had to stay on deck for days during the entire expedition because his seasickness got much worse if he went inside. He decided right then that it would be the last voyage in his oceanographic career. Since then, Yi has been an “armchair” oceanographer, working on satellites and computer models of the 3-D changing ocean. With outstanding results, I might add. Stay dry, Yi!

Other key members of the dry team include Zhijin “Gene” Li, Peggy Li, Quoc Vu, and Fred Bingham (on the Knorr to supply dry humor). Gene has dedicated more than a decade to formulate, develop and refine an ocean data assimilation system at JPL. This system integrates measurements from satellite, underwater autonomous vehicles and other ocean measurements into sophisticated computer models to depict the ocean. To support SPURS, this system is based on the community Regional Ocean Modeling System (ROMS), configuring as a nested set of three spatial domains and zooming in using horizontal resolutions of 9,3 to 1 kilometer to represent flow systems from hundreds of to a few kilometers in size.

Sea surface salinities from the three model domains.

Before Knorr began its voyage, using the ROMS model, the dry team predicted for the “wet team” (as we the expeditioners are fondly known) that the SPURS area of research would be warmer and saltier than normal this year. Since then, the model system has been providing nowcast and forecast of the conditions of the SPURS ocean on a daily basis. The forecasts have helped to refine our sampling strategies aboard Knorr. Only time and more observations will determine whether their prediction is correct!

As the data stream from SPURS comes in, the model system will synthesize the observations and deliver more accurate forecasts of the ocean conditions, which will in turn help adjust the measurement strategy as needed. In essence, we use ROMS at NASA’s Jet Propulsion Laboratory (JPL) as our virtual ocean. It’s an estimate of the three-dimensional evolution of the ocean in the region given the data and physics of the ROMS model (both of which we try to improve).

Peggy Li is a dry team specialist in computer science (not an oceanographer) who is interested in building real-time visualization systems, convert data into useful information and present said systems in the fastest and most intuitive way. She has been providing outstanding support to SPURS investigators. She makes sure the information is flowing smoothly from the observation and model sources to the SPURS Data Management System. She is “on watch,” like those aboard Knorr. She initiates her day by checking to see if all the data are present on the SPURS visualization page. Only then she allows herself to have some coffee… such dedication! She has supported several field campaigns, but claims that this one is the most interesting by far.

A goal of the dry team is to integrate all the salinity data sets into a one-stop shopping web site for SPURS situation awareness. This includes data collected by both in situ and satellite platforms as well as output from computer models. Gathering the data from a distributed in situ (in place) sensor network is a challenge. All the in situ sensors send the collected data to their home institutions through satellite communication, where a quick first round of quality control is done by technicians who are experts in those instruments. For example, the ship R/V Knorr, flux mooring, wave glider, and Slocum glider send data to Woods Hole Oceanographic Institution in Massachusetts, the surface drifting buoys send data to the Scripps Institution of Oceanography in La Jolla, California, and the profiling floats and Seagliders send data to University of Washington in Seattle. The dry team computer at JPL in Pasadena, California, is checking all these computer servers around the country on the hourly basis, transferring and updating the data on the SPURS web site.

A screen shot for the SPURS data management system visualization using the Google Earth interface for monitoring the location for various instruments deployed from Knorr.

Using the well-known Google Earth interface, users on both the wet and dry teams can zoom in and out of the SPURS region to locate the most updated ship position, to read the salinity measurements from both in situ sensors and the Aquarius instrument in space, and to compare observations against the most recent model forecast.

A closer look at the SPURS instruments’ deployment.

Communicating between the wet and dry teams has also been a challenge. The Internet link to shore is a shared resource between many U.S. research vessels at sea. It works well, but it is not like having a cable running right into your house!

All in all, the wet team on Knorr is much better informed because of the dry team’s efforts at JPL. Likewise, since one objective of SPURS is to improve our modeling of ocean salinity variations, the dry team is reaping great rewards from interaction with the wet team.

Starting A Career In Oceanography And The Global Water Cycle

September 27th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

The SPURS work has renewed interest in the broader community in studying the ocean to better understand the global water cycle, heating and cooling of the oceans, and oceanic mixing.

Julian Schanze of Woods Hole Oceanographic Institution/MIT is about to complete his Ph.D. in physical oceanography under the supervision of Ray Schmitt. Julian and Ray are on the Knorr to study ocean salinity, the water cycle and mixing in the ocean.

Julian Schanze at work.

Raymond Schmitt on the Knorr.

For his Ph.D. project, Julian is trying to estimate how much mixing occurs in the ocean. For this, he is using satellite datasets on surface fluxes of heat and fresh water and a concept known as power integrals. This is a mathematically complex subject, so let’s avoid the technical details and consider it in simpler terms.

Consider that, broadly speaking, the Earth is heated near the equator and cooled near the poles. For the equatorial and polar regions to not heat up or cool down, respectively, the excess heat must be transported away from the equator and towards the poles through mixing. The same approach can be used for the water cycle (which creates salt differences) as well such as ocean density and other, let us say “weird” ocean variables. For example, oceanographers consider the “spiciness” of ocean water as a measure of how warm and salty it is. So Julian is doing something really cool and looking at not just at heat moving through the ocean, but density and spiciness fluctuations as well. These are directly related to vertical and horizontal mixing in the ocean.

The equations that govern these power integrals relate the production of heat and salt variance (in our example, heating at the equator and cooling at the poles) to the destruction of variance (mixing) in the interior ocean. However, Ray and Julian found something curious: Under the right circumstances, the ocean interior can produce density variance rather than destroy it. The reason for this is double diffusion or salt fingers. When warm, salty water is found atop cool, fresh water, heat is diffused faster than salt in the ocean, leading to the formation of cold and salty “salt fingers”. These salt fingers transport salt downward and can create sharp density gradients. The SPURS region is top heavy in salt and therefore a likely place to find salt fingers.

On this SPURS cruise, Julian is trying to extend his understanding of mixing in the oceans from theoretical studies to hands-on work with the data. He is hoping the data will help him constrain uncertainties in the global maps of the water cycle and the heat budget that he has assembled. But while the approach he has taken in his dissertation allows him to calculate the total sum of mixing in the ocean, it does not constrain where the mixing occurs. This is where instruments deployed in SPURS enter the picture. Some of the SPURS instruments specifically allow for mixing in the ocean interior to be estimated by recording miniscule changes in temperature, salinity, and velocities in the ocean. The Vertical Microstructure Profiler and several of the gliders equipped with similar technologies allow oceanographers to estimate mixing quite precisely.

Night deployment of Velocity Microstructure Profiler.

Sensor package on VMP.

On board, Julian is in charge of the Lowered Acoustic Doppler Profiler (LADCP), an instrument lowered on a wire that records horizontal velocities in the ocean by pinging sound waves off small particles that float in the water. This requires him to prepare the instrument for deployment, charge its batteries and process the data after the retrieval.  The LADCP helps identify good sites for mixing by measuring where the velocity changes most rapidly with depth.

How did Julian get to be such an intelligent man? He tells me that at age seven, he moved close to the North Sea in Germany and became fascinated by the ocean. He soon became a keen sailor and decided to complete a four-year B.S./M.Sci. degree in oceanography at the University of Southampton in England. While his research has been largely focused on using satellite data to estimate the global water cycle (80-90 percent of which occurs over the ocean), he is thrilled at being able to go on a month-long research cruise to get in touch with the subjects he has been studying for the last 9 years. His fascination with satellite remote sensing and his research in oceanography are perfectly combined in NASA’s work on SPURS and the advent the Aquarius satellite, to measure sea surface salinity from space.

Everyone on Knorr believes that Julian has a stellar career in front of him!

Notes from the Field