Posts Tagged ‘salinity’

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Salinity Processes in the Upper Ocean Regional Study (SPURS): 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!

Salinity Processes in the Upper Ocean Regional Study (SPURS): The Thermosalinograph, The Bow Of The Knorr, And The Chase For Highest Salinity

September 26th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

The thermosalinograph (TSG) on Knorr is a shipboard instrument for measuring temperature and salinity of the near surface water . It is situated in a lab and receives a flow of water taken in near the bow and piped out near the stern.

Inlet and pump for the Knorr’s thermosalinograph. (Photo: Julius Busecke.)

It is quite an adventure going down to see the inlet for the TSG. You go to a storage locker in the bow, then down a long tunnel, then forward at the bottom of the ship. And you discover something cool when you get there. When Knorr was built, they put viewing ports in the bow. One could sit below the waterline in the very front of the ship and watch the ocean go by. Wow! They must have been really cool for watching bottlenose dolphins playing in the ship’s bow wave! Sadly, the viewing ports were sandblasted at some time in the past, and one can no longer see through the glass.

Start of downward journey to the thermosalinograph and the viewing port in the bow. (Photo: Julius Busecke.)

Looking forward to the viewing port in Knorr’s bow. (Photo: Julius Busecke.)

The viewing port in the bow of the Knorr. (Photo: Julius Busecke.)

On a cheerier note, an Aquarius project led by Prof. Arnold Gordon of Lamont Doherty Earth Observatory has been focused on surface salinity and the global record provided by TSGs on research vessels and ships of opportunity (commercial vessels that carry scientific equipment and take automatic measurements that scientists then use for their research).

Phil and Julius are aboard Knorr for SPURS from Lamont to assure the quality of the TSG data and provide ongoing analysis of the surface salinity from various instruments, including Aquarius. Chief Scientist Ray Schmitt set a contest in motion for a free dinner in the Azores for the group that documents and collects a bottle sample of the highest salinity of this SPURS expedition (most likely at the ocean surface). Game on!

Julius at work.

Phil at work.

The closer we look at the ocean with increasingly sophisticated satellite and ship-based instrumentation, the less it looks like the textbook images of large-scale circulation gyres, and more of an assemblage of around 100-km-sized eddies. What role do these eddy “swirls” play in the overall climate system? And more specifically, how effective are eddies in stirring freshwater into the evaporative salty regimes of the subtropics? During the SPURS field program the TSG and hull-mounted Acoustic Doppler Current Profiler (ADCP) observations of ocean currents are used to map the eddy field, to ascertain their potential to compensate the excess evaporation. What we learn will be applied along with the Aquarius satellite ocean surface salinity data, to other areas of the ocean, to more fully grasp the impact of eddy dynamics on the marine hydrological system.

The Lamont team prepared for this expedition by inspecting the archival surface layer temperature and salinity in the SPURS region, collected by Voluntary Observing Ships and transits from past research expeditions to estimate the eddy flux of freshwater into the North Atlantic evaporative subtropics. What they found is encouraging: the ocean eddies very well may be the primary force in compensating the net regional evaporation. The seasonal cycle of the eddy effect seems to set the seasonal swings of the ocean surface salinity, not net evaporation. The SPURS expedition data along with the Aquarius satellite data provides a far more quantitative data set to pursue this topic.

SPURS experimental design and measurement methods, consisting of sensor-laden moorings with an array of autonomous instrumentation, provide a beautiful 3-D view of the ocean stratification and circulation as it evolves over time. The R/V Knorr’s high resolution TSG and ADCP, as well as the data stream from the Aquarius satellite, link together the data from all these assets, to fully capture the eddy field and ascertain its role in the larger scale system. It’s exciting to see the complex factors influencing ocean surface salinity coming into focus!

Ocean surface salinity from R/V Knorr Thermosalinograph (color bar) vs latitude, longitude, and time.

While we can guess what we might observe, being there to actually see the data as it rolls in allows for the building of understanding and thus adjustment of the experimental design to maximum return. The observations from the Knorr will expand our quantitative knowledge of how the ocean, and its field of eddies, are coupled to the sea-air flux of water. That is oceanographer speak for “this is awesome!”

Salinity Processes in the Upper Ocean Regional Study (SPURS): Life in the Sargasso Sea

September 24th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

John snags a flying fish in mid flight.

There are not many places in the open ocean that get their own special name as a “sea.” Most seas are what we call marginal seas – offshoots of the major ocean basins.

The Sargasso Sea, as a vast track of the western subtropical North Atlantic Ocean is known, has a special characteristic – something noted by Portuguese sailors for centuries and even visible from space. It is the home waters of Sargassum, a genus of brown macroalgae (seaweed) that inhabit the open ocean. The sea is named after the seaweed and it seems that small clumps are nearly always within sight of the ship (we have yet to see giant mats of the stuff in the SPURS region). Anyway, the Sargasso Sea is special because of a plant. Well, it is more complicated than that!

A patch of Sargassum at the surface (Photo: Julian Shanze.)

Sargassum, up close.

Closeup of Sargassum.

In my opinion, the really cool thing about Sargassum is that each clump can be a teeming ecosystem by itself. Several varieties of fish (e.g. Sargassum fish and flying fish), crabs , and nudibranchs live in close association with the weed. Each clump is a complex island of life floating free at the surface of the deep ocean. When you are out here in the vast emptiness of the open ocean, it is just hard to imagine how this intricate web of life came to be, survived, and actually thrives. Every time I am in the Sargasso Sea, it seems such a wonder.

A flying fish.

Barnacles on French glider recovered by Knorr.

We had spectacular sunset last night and I was reminded of the old adage: “Red sky at night, sailor’s delight. Red sky in morning, sailor’s warning.” Here we are still in proximity to Hurricane Nadine and is this saying true, or is it just an old wives’ tale? Like the answer to most questions, there is a web site for that. In fact I think it may be true for us; we are well south of the hurricane weather and forecasts have good weather for in days ahead.

Sunset sen from the Knorr.

An interesting sidebar to today’s blog topic is another kind of life we have found in great abundance at our SPUR study location. It was a mystery for a few days – we were seeing lots of floating microscopic reddish dusty particles. Some said it looked a little like sawdust (but where are the trees?), and some wondered whether it was floating dust from the Sahara. Well, thank goodness for the Web again. I discovered that it’s a bloom of an important nitrogen-fixing bacterium (Trichodesmium) also known as “sea sawdust.” It certainly reinforces the idea that a key to identification is a good description!

Trichodesmium in a bucket of sea water.

A Trichodesmium bloom in the Pacific Ocean seen from space.

Trichodesmium, it turns out, just loves these sea conditions – just as much as SPURS oceanographers love the North Atlantic salinity maximum!

Salinity Processes in the Upper Ocean Regional Study (SPURS): The Moods of Sea and Sky

September 21st, 2012 by Maria-Jose Viñas

By Eric Lindstrom

A beautiful day in the trade winds zone, with its typical cumulus clouds.

From the shipboard perspective, all we really see of the sea is the surface. Of course we can see into the water a short way, right close to the ship, but not very far. The horizon is 360 degrees and the great dome of sky seems endless.

Being that we are about a thousand miles from the nearest port, it is also fair to say that all we see from the ship is sea surface and sky. These are the shades of blue that I referred to in an earlier post. When this is all one has to see aside from the Knorr, our shipmates, and interesting oceanographic data (OK, we watch movies too!), it should come as no surprise that everyone becomes sensitive to the moods of the sea and sky. When you want a moment alone, the likely place to go is on deck, and there you are confronted with some new variation of the sea and sky.

We are working in what is called the trade winds zone. It is a belt of generally east and northeast winds north of the equator and east or southeast winds south of the equator that are quite steady and global in extent (in the days of sail, one’s trade depended on using routes through these reliable wind zones). In the trades zone, we might expect relatively steady 10-15 mph winds and fair skies with broken clouds. One characteristic of the fair weather cumulus clouds in the trades is that they lean over because of wind shear in the atmosphere. The blue sky dominates the evenly scattered puffy white leaning clouds that seem to all have the same base (maybe 3,000 feet above the sea).

The trade wind cumulus clouds break up the bright shades of blue on the sea surface by casting rapidly evolving shadows across the sea. The color of the sea surface certainly depends on the light reflected from the sky (a gray sky can give the ocean a grayer look) but also depends on the intrinsic color of water, which is blue.

We had few days where there seemed to be a pink hue to the sky (especially at the low sun angles of morning and late afternoon). This is likely the result of having more Saharan dust suspended in the atmosphere. It is quite common for dust storms to carry thousands of miles out over the ocean.

Sunset in the Sargasso, with hints of Saharan dust.

The color of the water depends on the angle at which you view it and the height of the sun. One of the cool things I see in the open ocean is that when you look down into the deep waters in the middle of a sunny day there is a radiation of sunbeams seemingly coming back at you from the depths. It gives the blue ocean a kind of jewel-like quality.

Sun beams in the Sargasso Sea.

 

Salinity Processes in the Upper Ocean Regional Study (SPURS): NOAA Contributions to SPURS

September 20th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

A NOAA buoy in the water.

When we are doing work at sea, it hardly seems fair for NASA to hog the limelight. We are usually offering data from satellites, not ships, moorings, or gliders. There are partner agencies in the U.S. Government who make enormous contributions to the physical oceanography enterprise. In D.C., oceanographers know these agencies as “the four N’s” – NASA, the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), and the Navy. Because each, in its own way, contributes to the success of physical oceanography in the USA and of SPURS in particular, I am going to try to tell you about them through their contributions and through relevant posts from the field. With this post, I am going to focus on NOAA.

The 5-cent summary is that NOAA Pacific Marine Environmental Laboratory (PMEL) is proving two moorings for SPURS and the NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML)  is providing enhancement to their ongoing basin-wide observing system.

One way we divided us SPURS so that we could look at all the relevant time and space scales of salinity variation (minutes to years and inches to thousands of miles), was by looking at who was strong in particular areas. NOAA is the key agency when it comes to monitoring the global ocean with measurements in the water. They maintain moorings in the tropical oceans for seasonal climate prediction, Argo floats around the globe for monitoring of upper ocean temperature and salinity profiles, a global array of surface drifters for sea surface temperature and surface velocity maps….and the list goes on.

The ocean involves so many different interacting processes that no single observing tool captures the whole picture (the variety of instrumentation in SPURS is a good example). We find that different kinds of measurements used together give a more well-rounded vision that is fuller than the simple sum of the individual parts. A focused process study like SPURS takes advantage of this, and NOAA scientists were eager to be part of it. While NOAA maintains long-term monitoring arrays that record broad fluctuations, those don’t necessarily illuminate the processes that underlie the fluctuations, so the diverse measurements in SPURS add interpretive value to NOAA’s arrays. SPURS is also a testing ground to learn what particular instruments measure well (and what they don’t), and to hone sampling strategies. When we deploy Argo floats and surface drifters in SPURS, these measurements enhance our knowledge of salinity in our study area, but the instruments will also remain in place for years to come and contribute to the Atlantic Ocean monitoring array maintained by NOAA.

AOML started a new XBT transect between Cape Town and New York City (referred as AX08) on August 18, with XBTs deployed every 15.5 miles (25 kilometers). This is the third of five AX08 realizations that will be done on 2012. There are five realizations planned for 2013. A total of 550 XBTs are deployed on each realization.

Enhancement XBT line for SPURS.

A planned NOAA expedition to in September with some SPURS-related activity had to be postponed due to mechanical malfunction of the ship, the R/V Ron Brown.

It’s very exciting for us to help PMEL by deploying two of their “Prawler” (Profiling Crawler) moorings in SPURS. The Prawler uses the motion of the waves to provide lift for free: each time a wave lifts the mooring, the instrument holds tight to the wire with a ratchet and goes up. When the wave trough passes, the ratchet releases and the round fin keeps it from going down, so it “crawls” up the wire in steps. When it gets to the top, it free-falls back down the wire, making a profile of temperature and salinity. Since battery power is one of the main limiting factors in designing ocean instruments, the Prawler’s use of wave energy lets it work for much longer than if it had to carry a large battery pack and motor. A full and fascinating description is available here.

A Prawler on a wire.

To summarize, in order for NASA to advance the science of physical oceanography, we work closely with other federal agencies, such as NOAA, to bring the correct mix of measurements and technology to the field. SPURS is most definitely a team effort!

Notes from the Field