Posts Tagged ‘ocean salinity’

« Older Entries

Salinity Processes in the Upper Ocean Regional Study (SPURS): R/V Revelle SPURS-2 Epilogue

October 21st, 2016 by Maria-Jose Viñas

By Eric Lindstrom

Science party on the R/V Revelle.

Science party on the R/V Revelle.

Salinity Processes in the Upper-ocean Regional Study #2 is underway for the next year. Lots of science remains to be done, so it is very early to be writing an epilogue! However, the first big field campaign with a large research ship is completed and it seems right to sum up some of the operational conundrums be articulated as we plan for further operations with the Lady Amber over the coming year and the R/V Thomas Thompson in October 2017.

Conundrum#1: Finding “Just Right”

Like Goldilocks, the R/V Revelle team went in search of very special conditions. Conceptually, SPURS-2 is built around observing all the processes that lead to rainfall mixing into the ocean and the resulting large-scale variations of salinity that we detect from space.

One of the concepts is that rain falling into the ocean will reveal itself as low salinity “lenses” at the surface than mix into the ocean over time. Well, not unexpectedly, if the wind is anything but calm, the rain mixes quickly into the upper ocean mixed layer and cannot be seen as a lens – only tiny variations of salinity that build up over time. In order to see the strong signal of a lens of fresher water, one needs to observe the ocean during calm winds and rain. The difficulty with this is finding the simultaneous occurrence of rain and low wind conditions in a vast ocean with a platform with a top speed of about 12mph. During SPURS-2, it became apparent that the regions with more certain rainfall (those large cells and fronts visible from space) were also stormier and windier than ideal for measurement. In areas of calm winds, there are patches of rainfall but they don’t seem predictable (“It’s the tropics” says Jim Edson!)

So, the perfect observing condition for us were more difficult to find than we expected ahead of time. Probably no surprise that mother nature throws a great curve ball! In the end, we targeted the low-wind regime by following the surface wind forecasts and hoped to run into rain (which we did). After a few weeks at sea we learned and prevailed.

Conundrum#2: Risk

It is always tricky to balance risk and reward. Equipment deployed at sea is always at risk of loss. The reward for taking the risk is valuable data to expand our scientific understanding. We had several occasions to examine and balance these risks and rewards.

For example, the CODE drifter was an instrument that was modified ashore by addition of a salinity sensor but not tested prior to shipping five of them to R/V Revelle. In the parlance of oceanography, drifters float at the surface, while floats are neutrally buoyant and can sink and return to the surface to transmit profiles of the ocean. In a test deployment of one drifter, it appeared to stay at the surface (just), but when released from the ship it simply sank, never to be seen again! Not the behavior you like to see in a drifter! So, although modification to its four sister drifters were undertaken, it was eventually decided that these drifters were not ready for prime time. Lucky we tested one! The small experiment with CODE drifters will have to wait until next year.

The Lighter-Than-Air InfraRed System (LTAIRS) balloon deployments were quite complicated by variations in the balloon lift during rain and by variations in wind speed and direction. Dipping the balloon payload (expensive camera) in the water is a big risk. However, the data – infrared movies of the sea surface skin temperature in rain – are super interesting and scientifically novel. We learned a lot about making waterproof payloads, the best material and size of balloons, and weather characteristics unfavorable for ballooning. This knowledge was hard won with near misses and close calls and exhaustion of the helium supply. We all think it cost our chief scientist Andy Jessup some new gray hairs! However, the grin on his face when he shows you the data is priceless. Well worth the drama and risk in the process. LTAIRS work will go more smoothly next year on the R/V Thompson and the team will likely make some interesting discoveries about the thermal properties of the sea surface during rain.

Final Thoughts

It was a great pleasure to work with and support the scientists and crew on R/V Revelle. It is a capable ship and crew and the science party was very well prepared for the challenges and risks. Morale was high during the entire voyage – maintained by a busy schedule, everyone pitching in, and good food and fellowship. The teams from University of Washington Applied Physics Laboratory, University of Connecticut, Woods Hole Oceanographic, and Scripps Institution of Oceanography came to R/V Revelle super-prepared and ready for action. The schooner, Lady Amber, will be making periodic voyages during the coming year to service and renew out drifting (Lagrangian) experiment. As always, your blogger enjoyed every minute of our six weeks at sea. Its wonderful to watch scientists and engineers face the challenges of understanding the ocean while in its grip. I’ll keep you posted on developments during the coming year!

Salinity Processes in the Upper Ocean Regional Study (SPURS): SPURS-2 Initial Results

September 21st, 2016 by Maria-Jose Viñas

By Eric Lindstrom

The SPURS-2 science party on the R/V Revelle.

The SPURS-2 science party on the R/V Revelle.

The R/V Revelle expedition has been the opening round in a yearlong effort to understand the upper ocean physical processes in the eastern tropical Pacific. We put moorings in place to collect a time series of upper ocean measurements over the year. We launched the first round of Lagrangian drifters and floats. Lady Amber will be servicing these assets at regular intervals and launching new drifters and floats into the array.

It is with some caution and caveats that I try to summarize our findings from the last six weeks of effort. Ideas are still young and data processing is still in its early phases. Only data from shipboard measurements are complete but still require careful screening, calibration, and validation. Despite the caveats, some of our questions and challenges are much clearer now than before we left Honolulu.

The Conductivity, Temperature and Depth (CTD) work, led by Janet Sprintall, charting the variations of temperature, salinity, and oxygen in the upper ocean, revealed some quite interesting features. Janet and her team will be looking at salinity-compensated temperature inversions in the upper ocean that seem to be closely associated with the edges of the east Pacific fresh pool.

Jim Edson and Raymond Graham have a wonderful three-week time series of 80 atmospheric profiles from their radiosonde launches via weather balloons every six hours. Ray will be working these up for his MS degree research project, so I won’t steal any of his thunder. Needless to say, we all think he will have an awesome project because it is such a rich data set!

Ray also assembled all the rainfall data daily and found that during SPURS-2 we had about 10 inches of rainfall in our 3 weeks on site near 10N, 125W.

The women of SPURS-2.

The women of SPURS-2.

Carol Anne Clayson has collected more than 1,500 air-sea flux estimates (20-minute averages) to be analyzed in conjunction with the upper ocean data sets from numerous platforms. She has already been able to run her mixed layer model using initial conditions from early in the expedition and the raw flux estimates to compare with later expedition measurements. It gives us great optimism for new discoveries out of this data set.

Michael Reynolds brought the Remote Ocean Surface Radiometer (ROSR, from Andy Jessup) and the Infrared Sea surface temperature Autonomous Radiometer (ISAR, from Carol Anne Clayson) to SPURS-2 to examine the surface skin temperature of the ocean during rainfall. Michael successfully engineered the instruments for this purpose (normally a fair weather measurement). He has put together some exciting compilations of data comparing his measurements with other temperature measurements made on the ship.

Julian Schanze from Earth & Space Research in Seattle is the man with the Salinity Snake. It has provided a virtually continuous record of the salinity in the top 2 inches of the water column. Julian has identified nearly 40 fresh lenses where salinity at the surface is significantly lower than ship intakes at 6.5 feet (2 meters), 9.8 feet (3 meters), and 16.4 feet (5 meters depth). These measurements, when further combined with the Surface Salinity Profiler data from the upper 3.3 feet (1 meter), will constitute a rich data set for analysis of fresh water lenses induced by rainfall.

Ben Hodges from Woods Hole Oceanographic Institution (WHOI) has been following the three Wavegliders deployed near the SPURS-2 central mooring. They have salinity sensors near the surface and at 19.7 feet (6 meters) depth and will be hard at work over the entire coming year. One of the Wavegliders had an experimental package of sensors on it called the salinity “Rake” with sensors at every 3.9 inches (10 centimeters) depth from the surface to 3.3 feet. The Rake, invented by Raymond Schmitt at WHOI, recorded data internally and was recovered after a few days of operation to check functionality (it has not been to sea before). The data is amazing; but it only lasted one day before mechanical failure and short circuits took it out of commission. However, that vertical resolution and the interesting features that it saw were a new peek at the ocean surface salinity that really has not been seen before. More engineering work needs to be done at home before it can be re-deployed. The team at Woods Hole will be highly motivated to get this instrument to sea again soon, based on what we saw in just one day. The Waveglider was re-deployed without the Rake.

The Surface Salinity Profiler (SSP) team led by Kyla Drushka has a very successful run with 18 deployments (not counting various tests and trials). The data is still being assembled and examined but, like the Rake measurements, the SSP focus on the top 1 m of the ocean is going to provide new insights on ocean processes. The SSP has the added benefit of microstructure probes to provide information on turbulent mixing. The biggest difference in salinity seen over 3.3 feet by SSP was 9 units!

Eric Chan had the rare opportunity to focus a study on the air-sea exchange of carbon dioxide during rain events. The simultaneous analysis of his gas exchange data with the salinity snake and meteorological data will be enlightening. Certainly the raw carbon dioxide data show the dip in pCO2 expected during rain events when the salinity drops.

Andy Jessup, voyage chief scientist, at work.

Andy Jessup, voyage chief scientist, at work.

Finally, I’d like to heap praise on our chief scientist, Andy Jessup, who managed execution of all the projects and requirements with great skill and diplomacy. The whole SPURS-2 team owes Andy a great deal for making the R/V Revelle expedition such a successful initiation of the SPURS-2 Program.

Next blog post: PORT!

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

September 19th, 2016 by Maria-Jose Viñas

By Eric Lindstrom

Our mixed-up Monkey looks everywhere for microstructure probes.

Our mixed-up Monkey looks everywhere for microstructure probes.

One of the challenges of oceanography (and many other sciences) is telling a coherent story of the environment across a vast space of space and time scales. For example, cosmologists tell the story of the universe from subatomic particles to the breath of the visible universe and from first nanoseconds of the big bang to billions of light years.

For SPURS-2, mixing is obviously an important factor in telling the story of how rainwater combines with seawater to bring about the east Pacific fresh pool that we see from space. That mixing primarily happens in the upper ocean at scales of centimeters, where turbulence is caused by phenomena  such as wave breaking, current shears, convection in unstable layers, and the rain itself hitting the sea surface. In order to understand the big picture, we need to estimate the magnitude and location of the centimeter scale turbulent mixing.

Over many decades oceanographers have become quite adept at estimating mixing from measurements of centimeter scale temperature, salinity, and velocity variations – otherwise known as microstructure. Now, along with our standard instruments for measuring temperature and salinity, we also deploy microstructure probes – very fast sampling sensors of ocean variables.

It is beyond the scope of this blog to explain how microstructure measurements are transformed into ocean mixing estimates, but it is one of the more helpful developments in modern oceanography.

Caitlin Whalen, working on her review article for the Bulletin of the American Meteorological Society.

Caitlin Whalen, working on her review article for the Bulletin of the American Meteorological Society.

Caitlin Whalen of the University of Washington Applied Physics Laboratory (APL) is an expert in ocean mixing. While on the SPURS-2 expedition, she has contributed to a review paper on the subject for the Bulletin of the American Meteorological Society. During SPURS-2 she oversaw the addition of the Seagliders to the Lagrangian experiment.

The Seagliders will provide us with a picture of how the turbulence in the SPURS-2 region varies at deeper depths and over a longer time period than we will learn form our ship-based measurements. Over the next six months the Seagliders will repeatedly travel between the ocean surface and a depth of one kilometer, collecting data during each trip. From this data we will be able to determine how the turbulence in the ocean varies with depth and how it is related to other events such as heavy rain and the continuously changing density patterns of the water. Knowing where and when turbulence occurs will help us understand how the fresh rainwater eventually mixes deep into the salty ocean.

Dan Clark and Kyla Drushka of APL make final adjustments of the SSP microstructure probes.

Dan Clark and Kyla Drushka of APL make final adjustments of the SSP microstructure probes.

Kyla Drushka and Suneil Iyer, also from APL, have deployed microstructure probes on the Surface Salinity Profiler (SSP). They will try to determine, from these measurements and the vertical structure of salinity in the upper meter of the ocean, how quickly rain-formed low salinity lenses are mixed into the upper ocean during individual events. Their big challenge during SPURS-2 has been to get a snapshot of salinity lenses at various stages of their lifetime. Being in the right place (low wind conditions) at the right time (just as rain begins to fall) with the right gear (SSP) actually deployed has been challenging. Still, I am sure we have collected more such data this expedition than previously existed. Finally enough data to find great exemplars for discussion in the scientific community! This is a wonderfully “fresh” topic for a graduate student like Suneil to tackle.

Kyla Drushka and Caitlin Whalen from APL, hard at work in the rain.

Kyla Drushka and Caitlin Whalen from APL, hard at work in the rain.

Just about everyone doing analysis of SPURS-2 data will use estimates of mixing in some way or another in the telling of their part of the story of salinity in the eastern tropical Pacific. It will be part of any salinity budget calculations and used in the description of salinity fronts. It will be an essential part of the story in explaining the seasonal patterns of salinity we see from space. The story of the smallest scales in the ocean meets up with the story of the planet as seen from space!

Salinity Processes in the Upper Ocean Regional Study (SPURS): Chasing the Elusive Surface Salinity Profile

September 2nd, 2016 by Maria-Jose Viñas

By Eric Lindstrom

The R/V Revelle and the Lighter-Than-Air InfraRed System, as seen from the Surface Salinity Profiler.

The R/V Revelle and the Lighter-Than-Air InfraRed System (on the left corner), as seen from the Surface Salinity Profiler. Credit: Dan Clark.

Kyla Drushka from University of Washington Applied Physics Laboratory (APL) received a National Science Foundation grant to participate in SPURS-2. It is one cornerstone of our work and is entitled “Rain-Formed Fresh Lenses in SPURS-2.” The idea of rainfall resulting in freshwater puddles or lenses at the sea surface is (perhaps) easy to imagine but is very tricky to observe and study in the real world. SPURS-2 will provide a wealth of new data on this subject with which to test the fidelity of our scientific imagination.

A beauty shot of the Surface Salinity Profiler.

A beauty shot of the Surface Salinity Profiler.

Kyla and the APL crew have a variety of instruments aboard R/V Revelle but the Surface Salinity Profiler (SSP) is central to SPURS-2 science objectives. When you want to know the fate of rainwater after it hits the sea surface, measuring salinity (or equivalently the amount of freshwater) in the upper meter of the ocean is crucial. The profile of salinity in that top meter of the ocean is elusive for a number of reasons – technical and logistical. Technically, salinity sensors require some time and bubble-free water flow to make the measurement. However, normally the sea surface is rapidly moving up and down (because of waves!) so a sensor is not always in the water and away from bubbles until it is several meters below the surface. Finding a way to tow salinity (and other) sensors at fixed, shallow depths following the motions of the sea is another approach. APL scientists and engineers including Andy Jessup, Bill Asher, and Dan Clark designed the SSP to apply this approach.

How the Surface Salinity Profiler runs outboard the ship -- photo taken from the Lighter-Than-Air InfraRed System balloon camera.

How the Surface Salinity Profiler runs outboard the ship — photo taken from the Lighter-Than-Air InfraRed System balloon camera.

Dan Clark, APL engineer extraordinaire.

Dan Clark, APL engineer extraordinaire.

The SSP platform is a converted paddleboard with a keel and surfboard outrigger. It is tethered to the ship so it skims the sea surface outside the wake or influence of the ship. Below the paddleboard, on the leading edge of the keel, are salinity (and temperature) sensors at depths of 10 centimeters (4 inches), 30 centimeters (1 foot), 50 centimeters (1.64 feet), and 100 centimeters (3.28 feet), microstructure sensors (to estimate turbulence), and cameras. The Salinity Snake (see my last blog entry) supplements the SSP by providing temperature and salinity in the upper few centimeters of the ocean. The SSP can be towed for many hours before the instrument batteries need refreshing.

Deployment of SSP is most informative when there is a rain event, leading to stratification of the near-surface ocean with less dense fresher water on top of denser saltier water. If there is a rain event ahead of the ship, the SSP goes in the water. Then it can measure how the ocean changes over the periods from before rain, during rain, and recovering from rain. Salinity from several depths and simultaneous turbulence estimates are then used to determine how rainwater mixes into the ocean.

Suneil Iyer with Surface Slainity Profiler on deck (sensor to bottom left on keel)

Suneil Iyer with Surface Salinity Profiler on deck (sensor to bottom left on keel)

Suneil Iyer is a new graduate student at University of Washington who will work with Kyla on interpretation of the microstructure measurements from the SSP data. He just arrived in Seattle from his hometown of Kansas City, Kansas, one week before the R/V Revelle departure from Honolulu. He just had time to find an apartment before jumping on a plane for SPURS-2. Graduate school in oceanography can be a blast of new experiences! This is not his first exposure to physical oceanography: Suneil worked on tides in estuaries during his undergraduate work at University of South Carolina.

-Kyla Drushka working on the Surface Salinity Profiler.

Kyla Drushka working on the Surface Salinity Profiler.

Kyla Drushka joined UW in 2014 after PhD work at Scripps Institution of Oceanography. She is now an active member of NASA’s Ocean Salinity Science Team and the mission science team for the Surface Water Ocean Topography satellite that is planned for launch in April 2021.

This blog entry goes to press near an auspicious moment on R/V Revelle SPURS-2 voyage. Saturday at 2 am (local time) is the halfway point of our expedition! In our oceanography lexicon we would call this the “hump day” for our voyage. For all you land-loving 5-day-a-weekers hump day might mean Wednesday. For the 24/7 work at sea, we really have only a “hump moment” to mark the halfway point of a continuous work schedule. Like any good NASA activity, people aboard have countdowns running to both our hump moment and our arrival back in Honolulu (8 am on Sept. 23). Amusement is in short supply!

Salinity Processes in the Upper Ocean Regional Study (SPURS): Packing and Departure

August 15th, 2016 by Eric Lindstrom
The Lady Amber and R/V Revelle in Honolulu.

The Lady Amber and R/V Revelle in Honolulu.

By Eric Lindstrom

Two ships in Honolulu were abuzz with action this last week preparing for SPURS-2, a detailed study of ocean salinity in the eastern tropical Pacific Ocean. The Roger Revelle, upon which all the scientific party sails, had to be loaded with many tons of scientific equipment and installations completed all over the ship. Lady Amber, the 20-meter (66-feet) schooner, was also readied for action with the installation of meteorological and oceanographic gear.

It was amazing to see what was completed in only a week under the supervision of chief scientist Andy Jessup (University of Washington). Containers from Seattle, Woods Hole, and San Diego were unloaded and equipment hauled to the ships. Many scientists and technicians dedicated the entire week prior to Revelle’s departure on August 13 for stowage, assembly, installation, testing, and securing of instruments and gear. It was a very quiet ship over the last 24 hours after we departed as everyone had a well-deserved rest and acquired their sea legs!

The Lady Amber crew successfully tested its new installation of scientific gear but suffered a schedule setback when the crew discovered that the ship required a new engine prior to departure from Honolulu. These arrangements are underway and we are appreciative of the support provided by the University of Hawaii as the Lady Amber work gets completed. Lady Amber is expected to leave Honolulu in about a week and catch up with Revelle at the SPURS central work site, eight days southwest of Hawaii.

Argo the cat, relaxing in the Lady Amber.

Argo the cat, relaxing in the Lady Amber.

Weather was fine on Saturday afternoon for Revelle departure. It was a quick trip out of the harbor with a great view of Waikiki skyline and Diamond Head. We passed by the Big Island of Hawaii on Sunday morning, August 14, giving people one last chance at cell phone calls. The island, really the largest mountain on our planet (from ocean bottom to summit), is a phenomenal sight.

Goodbye to Honolulu!

Goodbye to Honolulu!

On Sunday we had our first of weekly safety drills. Everyone got to learn how to decode the various horns that call for assembly, all clear, and abandon ship. We learned about survival suits and life raft deployment. Especially, we learned how to be safe on the Revelle and watch out for one another. We were encouraged to see the expert knowledge and conduct of the crew in their drills. It is over 2000 miles from Hawaii to the site where we begin work in earnest (deployment of moorings that pack the after deck). In the coming week, as we make the transit, the primary occupation will be testing and checking out various systems and training everyone for the complex operations to come. Once we are on site, we will quickly enter 24/7 scientific operations and we hopefully will have worked out all the glitches!

On a sad note, we had to leave our colleague Fred Bingham standing on the wharf in Honolulu, when we had expected him to sail with us. He was struck down with an illness that required he remain ashore and head home to Wilmington, North Carolina. His primary role in SPURS is as data manager. We are all assured that he will be on the mend very soon but we will miss his sunny manner and organizational skills.

The scientific party aboard Revelle is quite diverse – from first timers to old veterans. I’ll try to introduce you to the team in the weeks ahead. I am happy to report that I have seen no severe cases of seasickness during our initial day at sea. No doubt a few people are feeling a bit “green” but all are adapting well.

As always, I welcome your comments and suggestions for this blog. Send your messages to ejlindstrom@rv-revelle.ucsd.edu (case sensitive!). The expected frequency of the blog will be about one every two days.

Notes from the Field