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

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Measuring Salinity from Space

September 11th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

Recent years have seen significant developments in satellites for oceanographers. The European Space Agency launched the Soil Moisture and Ocean Salinity (SMOS) mission and NASA launched the Aquarius instrument on the Argentine SAC-D mission.

Artist’s concept of the Aquarius/SAC-D spacecraft. (Credit: NASA)

Salinity has always been a challenging but critically important measurement for oceanographers. The small changes in salinity that we sea in the ocean are important in determining the density of seawater and density differences are partly responsible for ocean circulation (direct forcing by the wind being another big factor). It’s a challenging measurement because we estimate salinity by simultaneously measuring the temperature and conductivity of seawater and using a well-established formula to calculate salt concentration. The salinity of the open ocean ranges from about 33 to 38 parts per thousand and the target accuracy of satellites is to measure differences of 0.2 parts per thousand. That is like measuring the change in salinity from adding a pinch of salt to a gallon of waters. I challenge you to taste the difference!

It’s the relationship between conductivity and salinity that allows for its remote sensing of salinity from space. As the conductivity of ocean surface waters change (with salinity) there are minute detectable changes in the “brightness” of the surface in microwave emissions. So, in theory, if we have a sensitive enough microwave radiometer we should be able to detect these variations from low earth orbit and translate them into salinity. In fact, the scientific and technical complexity of this task is enormous (and luckily space agencies feast on such challenges!)

SMOS and Aquarius are two very different solutions to the technical problem of measuring a small microwave signal from Earth orbit. But they both face the daunting task of making adjustments for the sea surface temperature and roughness, the intervening atmosphere and ionosphere, and galactic signals reflected off the sea surface (there are strong sources for the microwaves that astronomers have mapped for decades: in fact we measure salinity at a frequency band 1.4 Ghz that is “protected” for astronomical research.)

Well, you can surf the web sites if you want to go into all the details… but let’s get back to the ocean, since I am out here! The advent of SMOS and Aquarius has renewed interest in the details of surface salinity properties of the ocean. Never before had oceanographers had weekly maps of the global salinity field. We have been looking at temperature maps for decades, but salinity is new. It’s worth a whole posting on what it means to know both the temperature and salinity maps of the surface of the ocean year-round, and I’ll do that another day.

Aquarius’s first map of global ocean surface salinity, released in September 2011. (Credit: NASA)

SMOS and Aquarius drive scientists to wonder exactly how the variations of salinity seen from space come to be the way they are. That’s where detailed study of a few key ocean sites is so important.

Meanwhile, we are continuing to enjoy beautiful weather despite the hurricanes out and about in the Atlantic. The blue waters of the Sargasso Sea are delightful!

Of ship meals and science

September 10th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

Saturday’s lunch menu at Chez Knorr.

Saturday, 8 September 2012 — Given the talk around the Knorr’s dinner table, you would have to say that oceanographic expeditions run on their stomachs. Key to surviving weeks away from home and family is good food and good moods. The food on the Knorr is excellent, so we are likely to have happier people and better results. Most people will come back a few pounds heavier. There is a gym on the ship but it can be dangerous to be on a treadmill or lifting weights in rolling seas…so, more than likely most will lose the weight battle to the dining experience!

Bobby, preparing for lunch.

Everyone has a mug.

Today was the day we passed in front of the predicted track of Hurricane Leslie. We felt a bit of the swell coming from the storm, but otherwise we had fine conditions. In the early afternoon, we slowed for a few minutes to deploy the first Argo float of the voyage. Deploying the floats is pretty easy since they have all been powered up and are ready to go right out of the box. We just have to slow the ship and lower the instrument over the stern and gently into the water.

We plan to deploy two Argo floats in front of the predicted hurricane track. They will measure temperature and salinity profiles before, during, and after storm passage. In addition the profiling instruments “listen” to ocean acoustics and can make an estimate of wind speed and rain rate based on the noise of those actions on the sea surface (pretty cool!) So, we especially wanted to see how the acoustic sensors deal with a big signal like a hurricane. Steve Riser at University of Washington is the Principal Investigator for the Argo floats in SPURS.

First Argo float deployment.

The float disappears over the stern.

I also received my first official science training of the voyage. We have a small device called an underway CTD  for measuring a profile of conductivity, temperature, and depth of seawater. Two people will work together to deploy and recover this instrument on an hourly basis throughout most of the experiment. The device is tethered to a small winch and boom (like a fishing rod and reel) at the stern of the vessel and dropped in the water to make measurements from the surface to 400 meters (about 1312 feet) depth while the ship is steaming at 10 knots. It finishes a profile in 100 seconds and is then reeled back aboard. It is kind of like hauling in a fish with an electrically-powered rod and reel. It takes 20 minutes to haul in the beast and then another few minutes to download the data from its internal memory to a laptop computer. We plan to deploy this instrument every hour once we begin normal operations in the SPURS region next Friday.

The latest wind and wave information from Hurricanes Leslie and Michael suggest that our path to the SPURS site is just about perfect for seeing the minimum impact from the storms. The seas are still lumpy, but Knorr looks to be slipping through the gap between the storms. Fingers crossed!

Hurricanes, make way for Knorr!

September 7th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

Cloudy skies over the Atlantic, with Hurricane Leslie in the distance.

We knew when we left Woods Hole yesterday that we had two hurricanes (Leslie and Michael) standing between us and our study site, far southeast of Woods Hole in the mid-Atlantic. How Captain Adam of the Knorr chooses to deal with this over the next days is a study in weather and ocean forecasting – and calculated risks. We considered at least three options: delay departure until the coast is clear, make a significant departure to the cruise track (sail far south, then east), or leave on time but travel quickly to the east and then south to miss the worst of the hurricane weather.

The last option is what we are doing. We abandoned plans to do test stations and do science training for a few days so we can make hast to the east, to the east side of Leslie’s track. Then we can make our way south and around Michael. Tom Farrar and Fred Bingham of the SPURS science team have provided the bridge with Gulf Stream analyses that we hope will help us take best advantage of ocean currents in our race eastward. We could easily have found ourselves in an eddy that would hinder our progress, and no one wants that with bad weather coming toward us! It was a fabulous start to the voyage to have the crew of the Knorr and the SPURS team working so closely on the strategy to both keep us safe and on schedule.

The Knorr's new track to avoid Hurricane Leslie.

It’s worth mentioning too that satellite data and the shore-based team play a crucial role in our cruise planning. Zhinjin Li and Michelle Gierach at the NASA Jet Propulsion Lab have been feeding us the latest satellite images from the Atlantic so we can better understand the wind, waves, and currents in our path. We rely on these contributions because it is difficult to surf the web from sea – we have Internet access, but it can be quite slow.

Today we feel the enormous forces of the tropical storms on the western Atlantic. There is a good swell running – speaking clearly of some strong wind at a distance. Some aboard are feeling seasick, but the mess is still full of hearty eaters and smiling faces!

SPURS scientist Fred Bingham goes through safety training.

One of the things you notice in these conditions are all the items that can shake, rattle, squeak, groan, or otherwise move and make noise. I have been up and down in my cabin a number of times during the night to silence rattling cupboard doors, toppling books, sliding glasses – it’s almost an endless battle. At sea you don’t just put something down – you secure something, wedge something, pad something, or it becomes something broken, lost, dangerous or just plain noisy! Sometimes one even has to wedge oneself in a place and just stop moving around! All of us have greater appreciation than most for the stability of land and the silence that stability brings. When things are shipshape, all is secure and in its place. We are living shipshape today.

Off to sea!

September 7th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

Loading a SPURS Mooring on the R/V Knorr.

We had a whirlwind of preparations after the Labor Day Weekend.  All the gear was loaded on the ship and lashed down. The scientific party (22 people) arrived and set up in various spaces around the ship.  Bill Ingalls, a NASA Headquarters photographer, captured many great shots of the Knorr and the equipment. His photos are online at NASA Headquarter’s Flickr account.

Bill Ingalls, NASA photographer.

SPURS researchers held a press conference the day before departing. From left to right: Eric Lindstrom, Dave Fratantoni and Ray Schmitt,

I also had the pleasure of meeting Naomi Harper, a teacher from Will Rodgers Middle School in Fair Oaks, CA. She was visiting Woods Hole and stopped by the ship. I am looking forward to interacting with her students via this blog during the voyage.

We cast off and set sail at 8:30 a.m., on Thursday, Sept. 6. While it’s a very beautiful calm day, we know that we have storms in the Atlantic between us and our destination (Hurricanes Leslie and Michael). The cruise plan is being adjusted a little so we spend the first few days focused on “outrunning” the storms. We can do out test stations and other such preparations after we have reached calmer waters west of the hurricanes. So, we are expecting rough seas at some points during the next few days and the first order of business as we set sail is to get all our gear secure on deck and in our cabins. Everything that can move needs to be secured before we face the storm waves.

There was a nice gathering of friends and family at the wharf to see us off on our voyage. We are lucky to have such a beautiful sunny morning and calm winds for the send-off.

Family and friends of the SPURS researchers gather at the Woods Hole dock to see the cruise off.

Thirty minutes after setting off from Woods Hole, we have our first meeting of the entire scientific party. There are people from several institutions and we all need to put names to faces. There are also scheduled safety meetings and boat drills during our first day and before we leave the calm waters of Massachusetts behind.

The first SPURS science meeting aboard the R/V Knorr.

The only scientific work planned for the first few days, as we race past the storms, is regular sampling of surface waters to calibrate the ship’s thermosalinograph. This instrument monitors ocean temperature and salinity continuously from water coming in through a port in the ship’s hull. It will operate continuously during the voyage, but we need to collect calibration samples every 4 hours for precision salinity determination.

We are all learning about the rhythms of shipboard life. We will have people on many different watch schedules. The watches will operate on local time. That shifts as we move east across time zones. As your blogger on the SPURS cruise, I plan to float across different groups and schedules as the expedition progresses to give you multiple perspectives. On the ship, meal times are very important. We will have breakfast every day from 7:30 to 8:15 a.m., lunch from 11:30 a.m. to 12:15 p.m., and dinner from 5:00 to 5:45 p.m. I’ll let you know more about the delicacies we eat at sea!

SPURS supporters beyond the R/V Knorr

August 29th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

The Surface Salinity Profiler from University of Washington’s Applied Physics Lab, shown here while it was being tested at sea, is now deployed on R/V Thalassa.

The SPURS experiment involves not only sea-going oceanographers but modelers, and of course, remote sensing scientists using satellite data. As part of the daily posts from R/V Knorr I will go into more detail on the role of the models and satellite data in the overall scientific enterprise. I will profile the modeling teams and their interaction with the ship-based team during the voyage.

Numerical models of the ocean, run on supercomputers, can provide high-resolution animations of the physical environment for us sea-going scientists. In order to actually simulate the environment, the models need to take in data from the expedition and make sure the model solution is one that best fits the available data. This process is called data assimilation. (You are more familiar with data assimilation in models than you think – daily weather maps are the result of models and forecasts are done using data assimilation into sophisticated models of atmospheric physics.)

An objective of SPURS is to provide a high-resolution, near-real-time stream of data that can be assimilated into ocean models. Because the ocean is so big and complex, and our at-sea capacity to measure it so puny, we rely on ocean modeling and data assimilation to help us interpret the environment. The model results can be used in planning work at sea and to diagnose the balance of salinity in the upper ocean. The observations are essential to locking model results into the real oceanographic environment. The model is essential to estimating things we cannot measure directly and expanding upon the interpretations provided by the observations.

In SPURS, we hope that we scientists at sea and those ashore can collaborate through the exchange of data and model results. This was not possible as late as a decade ago. Now we have Internet at sea and can exchange information and results for daily planning and analysis.

I will discuss the synergy of models and data much more during posts from the ship in September.

SPURS as an International Enterprise

The SPURS experiment involves a number of expeditions and nations working in cooperation at the single location in the North Atlantic.

The French R/V Thalassa is in the SPURS region now with one U.S. team aboard from University of Washington’s Applied Physics Laboratory. I will give more details in future posts.

Follow-on U.S. expeditions will use R/V Endeavor from University of Rhode Island to follow-up the measurements of R/V Knorr. The first of these two expeditions is in the spring of 2013 and will service SPURS equipment that requires maintenance after 6 months at sea. That expedition will also do more of the “feature” characterization. A second expedition in the fall of 2013 will recover all the moored equipment and gliders, among other duties.

A Spanish cruise is also planned for next spring. The details of this expedition will be subject of a post.

What’s up next?

Next up will be the loading of R/V Knorr on 5 September and a media event at 10 am that day. We set sail on 9/6 at 10 am.