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

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Ocean Motion

October 11th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

When on land, an oceanography brain usually associates the term ocean motion with the movement of seawater in the form of ocean currents.

Depictions of ocean surface currents derived from satellite data can be viewed at NASA’s web site.

However, when actually on the ocean, the biggest signal for our oceanography brains to cope with is the role of ocean motion in the pitch and roll of the ship (due to wind seas and swell). OK, so I am telling you that feelings matter. If you have ever been seasick, then you know what I am talking about!

So, if you spend 33 days on a small vessel at sea, you get a great appreciation of ship motion. There are actual six axes of motion of two kinds (lateral and rotational). Laterally there are surge (fore and aft), sway (port and starboard), and heave (up and down). Rotation also occurs around three axes that are more commonly discussed (maybe because we feel them more?). They are pitch (around port to starboard axis), roll (around bow to stern axis), and yaw (around the vertical axis). We can definitely feel all these motions but some do matter to our feelings more than others. Perhaps the most noticeable ones for land-loving creatures like humans are heave (the accelerating elevator feeling) and rotational motions largely absent in our daily lives (pitch and roll). Together, these can be combined into a kind of perfect storm of corkscrew-like motion that weakens the resolve of even veteran mariners. Lucky for us, the SPURS cruise has been pretty calm and no one has been suffering from seasickness.

By far the most noticeable and everyday motions on a ship are that of pitch and roll. Pitch is especially noticeable when navigating into a swell or into the wind. Going downwind or in the direction of the swell is much more gentle pitch-wise (sailors are wished good luck by offering them “fair winds and following seas”). Roll is especially diabolic when one is navigating nearly parallel with the troughs and crests of the swell. This is something you DO NOT do when the seas are large because you may very well roll over (not good; do not try this at home!). Faced with large seas, the safest course is to ride the weather facing into the wind and waves (and just suffer the horrible pitching).

Sorry to go on for so long about the ship motion, but its an ever present partner to our expedition and you deserve a share for sticking with us!

You may be wondering about the difference of wind seas and swell. Good question!  Swell and wind seas are both manifestations of surface gravity waves. Swell is caused by wind forcing of the ocean in some remote location. Wind seas are the jumble of more disorganized waves that come up directly under and within a wind event. There has been a good swell running in the SPURS region for the last two weeks due to Hurricane Nadine hundreds of miles away. On top of the swell, the wind waves cause whitecaps and noticeable roughness above wind speeds of about 10 miles per hour.

Well, on Knorr we are not out here to study wind waves (just feel them). I should get on to talking a little about ocean surface currents. Our main focus is the surface salinity and how it gets to be the way it is. Ocean currents moving patches of different salinity around is one key factor in accounting for salinity changes at any site in the ocean (we call this advection of salinity). The Ocean Motion web site produced for the NASA Physical Oceanography Program has a wealth of background information on ocean surface currents. Here in SPURS we believe there are only weak average currents (a year of moored measurements will determine that for sure), but that the wind-forced transient currents play an important role in the local water cycle (and surface salinity values). One obvious reason to think this is that the salinity maximum here is hundreds of miles north of the maximum in evaporation minus precipitation. The simplest explanation for this displacement is the northward movement of surface waters induced by the trade winds. On average, upper ocean waters forced by wind move 90 degrees to the right of the wind in the northern hemisphere so the easterlies of the trades give the ocean waters a push toward the north. This hypothesis will be tested by the SPURS observations over the coming year.

SPURS Argo float locations over time indicating flow at their parking depth of 3,300 ft. (Courtesy of Jessica Anderson.)

Out here it is extraordinarily difficult to fully appreciate the complexity of the currents around us in the upper ocean. The average current is small, but there are eddies and fronts with stronger flows around us that we observe sporadically as we move around from hour to hour.

Map of current vectors in the upper 167 feet (51m) along our track color coded by the surface salinity. (Courtesy of Phil Mele.)

We will only be able to make sense of our hour-to-hour estimates of ocean motion once we return to shore and can fully integrate the satellite and model information into the picture. We obtain details profiles of current vectors on each station but honestly have a tough time “connecting the dots” in real-time. It will take a good bit of analysis to understand the complex flow field.

Vertical variation of velocity vectors at three adjacent stations on the same day. (Courtesy of Julian Shanze.)

It feels a little like mapping Washington, D.C. in one day, from your car, driving around with a simple still camera. We get some great snapshots, but we can only place them in a larger context when be have full access to maps, satellite images, and historical records (stuff most people cannot examine in their car while driving around!)

As in rest of life, context is important in oceanography. We strive for it, but as with most things, the perspective of hindsight provides the best context. Feeling seasick is no exception! It’s so awful in the moment, and such a trifle in the glory of the entire expedition.

How Do You Spell Physical Oceanography?

October 10th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

How do you spell physical oceanography? I know my answer for that question! It’s easy: CTD.

CTD art in the blue Sargasso Sea.

CTD into the water.

For about the last 40 years, the mainstay of physical oceanography has been profiling the ocean with sensors for Conductivity, Temperature, and Depth (CTD). The SPURS cruise is no exception. These are key properties to derive salinity and the physical state of the seawater is described through the “Equation of State” which depends on temperature, salinity and depth.

During SPURS, we are doing CTD stations from four to six times per day (to  around 3,300 feet depth) and using an underway CTD as frequently as every 15 minutes (to about 1,300 ft). Computers aboard transform the raw sensor data into temperature, salinity, and depth information at regular levels (say every 3 feet) depending on the need. From there, it is relatively easy to use the equation of state to derive density and to estimate the geostrophic currents between the various CTD casts. Those are the currents caused by pressure variations resulting from the spatial density differences (like the winds between high and low pressure systems in the atmosphere). So CTD helps us unlock a major piece of ocean circulation, right there.

The CTD is the basic instrument embedded in most of our platforms – on gliders, moorings, autonomous vehicles, and our Knorr thermosalinograph. All use similar sensors in slightly different configurations. All the incoming data is converted to temperature and salinity data spread through space and time. Different CTDs on different platforms are specialized to measure different time and space scales of the temperature and salinity fields. So, some CTDs measure the microstructure at inches and smaller, and some better measure the variations over the full ocean depth. Some work best while moving rapidly through the water, while others are specialized for stationary measurements. Any way you cut it, you still spell physical oceanography with a CTD.

The underway CTD team.

Underway CTD unit clips to tip with 1,300 ft (400 m) of line.

Dropping the underway CTD over the stern.

Over the course of one SPURS expedition the number of CTD stations may number in the hundreds. And since we launch the underway CTD as often as possible as we go between stations, as many as a thousand underway CTD profiles are collected over a month at sea. The usual arrangement for CTD profiles is to mount the instrument inside a metal frame and pairing it with a rosette. On the rosette, one can mount Niskin bottles ( ) for collecting water samples. The Niskin bottles are cocked open before the CTD cast and are tripped in sequence.  Samples are then extracted on deck for calibration of salinity (or any other purposes).

A top view of an open Niskin bottle.

A bottom view of an open Niskin bottle.

All in all, the CTD is THE instrument of SPURS and the mainstay of physical oceanography since the 1970s. It was used to map the properties of the global ocean in the World Ocean Circulation Experiment in the 1990’s. In SPURS it will provide us with an unprecedented view of the salinity variations here in the highest salinity region of the Atlantic Ocean.

Hello, Knorr? It’s The International Space Station Calling

October 9th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

Friday, 5 October 2012 — We got a call today from the Commander of the International Space Station (ISS), Sunita “Suni” Williams. Suni, who was calling while the ISS was passing over Eastern Russia, wanted to congratulate us on our SPURS expedition.

Sunita Williams, Commader of International Space Station.

Eric Lindstrom, Adam Seamans, and Ray Schmitt, during call with the ISS.

We had 30 minutes of wide-ranging conversation about life at sea and life in space. We talked about the beautiful sunrises and sunsets viewed from both Knorr and the space station. Suni has the added bonus of seeing the aurora from ISS and the green flash of the sun lasting for an extended period.

We talked about the naming of the new WHOI research vessel after astronaut Neil Armstrong, which will provide yet another link between space and oceanography for years to come. We talked about superstitions on ships and the ISS — no whistling on the ship seems to be a Navy tradition common to both our vessel and the ISS.

The call was a big boost to morale on the Knorr as we suffered through a day of windy and bumpy conditions inflicted by Tropical Storm Oscar. On the good side, we are finished with our primary science tasks and we are riding the wind and waves toward the end of mission in the Azores.

The talk with ISS also is symbolic of the many and growing links between the exploration of the solar system (outer space) and the exploration of the ocean (inner space). As the Physical Oceanography Program Manager at NASA Headquarters, I see the links every day. However, I suspect that, for starters, many of you were unaware that NASA had anything to do with exploring the ocean. So let me fill you in!

I think there are at least three categories of relationships between inner and outer space to tell you about – first is astronaut skills and training, second is the naming and use of spacecraft and ships as exploration and observation platforms, and third are the intersections in science, remote sensing, and robotics for exploration.

Astronaut Training and Skills

Astronauts must train to live and work in a weightless environment. That is quite difficult to find on Earth, but working under water provides a useful analogue because diving can simulate both weightlessness and working in pressure suits.

The Neutral Buoyancy Laboratory at Johnson Space Center is an enormous pool of water for training astronauts. The NBL contains full-sized mock-ups of the International Space Station modules and payloads.

The NOAA Aquarius Reef Base is an underwater habitat used by NASA on a regular basis for astronaut training.

Few humans posses the construction skills needed to build and maintain the International Space Station. To find qualified astronauts for that work, NASA looked to the deep-sea diving community for “the right stuff”.

Naming of Ships and Spacecraft

Naming spacecraft after ships of exploration (for example, the Discovery and Challenger) has been popular and it seems appropriate that oceanographic ships are named after explorers. The replacement for R/V Knorr was just announced and it will be named the R/V Neil Armstrong after the first astronaut to walk on the moon. Maybe this symbolizes a renewed decades-long commitment by NASA to seagoing exploration and discovery?

The space shuttle ushered in a new regular opportunity for astronauts to view and photograph the Earth and ocean. Similarly, oceanographers on ships can observe the same phenomena at sea level to provide in-depth views.

Science, Remote Sensing, and Robotics

NASA supports the development and use of technologies to explore remote hostile environments across the solar system. Many of these technologies are road tested on Earth in analogue environments, such as deserts, the Arctic, Antarctica, and deep ocean hydrothermal vents. Since we believe there are other oceans in the solar system to explore, like Europa, there is a growing interest in using the Earth’s ocean as a proving ground for remote sensing and robotic technology. This has been going on for more than 30 years.

In 2011, NASA launched the Aquarius mission to study variations in the surface salinity to the ocean. Aquarius senses the ocean at L-band (microwave emissions) in a frequency protected for radio astronomy. There are numerous galactic sources for L-band radiation and these have been well mapped by NASA astronomers. The fact that the radio astronomer’s map is reflected off the sea surface, makes for and interesting connection between the inner and outer space.

For years, oceanographers have been training astronauts for visual observation of the ocean and photography from space, and in turn the astronauts have produced many images of interesting oceanographic phenomena  from above.

In the coming years, ISS will be home to a NASA Physical Oceanography/JPL-constructed instrument to measure winds over the ocean. It’s a follow-on from the QuikSCAT project of 1999 that is still flying today. The new project is known as Rapid Scat and is under development for the Columbus module on ISS.

So that’s a quick tour of some of the inner and outer space connections. The longer this oceanographer works at NASA, the more he discovers. It’s a quite fruitful relationship. It was great to have a Navy Captain in space talking to oceanographers at sea today. Suni William, best of luck on the rest of your mission on International Space Station – and no whistling!

Sunset on the Sargasso Sea.

Measurements by long-term autonomous platforms in SPURS

October 6th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

A team of scientists from University of Washington is focusing on long-term observations in SPURS region with autonomous instruments: Argo floats (Drs. Stephen C. Riser, Jeffrey Nystuen, and student Jessica Anderson), Seagliders (Drs. Craig Lee, Charlie Eriksen, and Luc Rainville), and Lagrangian floats (Drs. Eric D’Asaro and Andrey Shcherbina). Their sentinel instruments will provide a larger-scale context for the SPURS moorings for the next year. Jessica (Jesse) and Andrey are aboard Knorr to look after all the deployments.

Andrey Shcherbina.

Jessica Anderson.

In order to connect the salinity that Aquarius sees at the ocean surface with processes throughout the ocean, SPURS is devoting resources to understanding of the processes happening in the near-surface layer. This area from the surface “skin” of the ocean to depths of 10-20 feet is where salinity measurements are most lacking.

In SPURS we added salinity sensors to normal global surface drifters. Enhanced Argo floats have extra sensors for the top 20 feet of the ocean. These will help to fill this gap in our salinity observations and help us better interpret Aquarius satellite data. SPURS data will be used to better characterize the near-surface variability, structure, and response of temperature and salinity to rain, wind, and ocean currents on time scales of hours to months.

Some of the details are fascinating. Twenty-six Argo-type profiling floats are being deployed over the course of the SPURS expedition and will remain in operation for four to five years. These floats will join the network of over 3,500 Argo floats currently monitoring the world’s oceans. Their additional sensor suites include the Surface Temperature and Salinity (STS) and Passive Acoustic Listener (PAL) packages. To ensure float longevity, typical Argo floats quit collecting data about 20 feet below the sea surface. The STS sensor, however, makes high vertical resolution (about 4 inches, or 10 centimeters) measurements of temperature and salinity from 100 ft depth all the way to the sea surface. The PAL sensor uses a hydrophone to acoustically measure wind speed and rainfall while the float is drifting between vertical profiles.

An Argo float at the ocean surface awaiting descent to more than 3,000 feet.

Additionally, the floats use Iridium satellite telemetry so the mission (time, depth, frequency of profiles) of the float can be modified after launch. Currently, sixteen of the floats deployed in a grid around the SPURS central mooring are making synchronous profiles once a day at noon, providing a daily “snapshot” of the SPURS region. In the future, the mission of these floats will be altered once again, so that both diurnal and longer time scale processes can be resolved.

Three Seagliders have been deployed in SPURS. They are built for endurance and efficiency. They will slowly patrol the salinity-maximum region moving up and down through the water column and monitoring temperature, salinity, oxygen and chlorophyll content of the waters surrounding the moorings. These Seagliders will operate continuously until spring 2013, when they will be recovered and replaced with a new threesome.

SeaGlider 190 away.

SPURS Seagliders are specially equipped to monitor ocean mixing. The upper ocean is filled with turbulence – chaotic assemblages of eddies that mix warmer and saltier water near the surface with cooler and fresher water below. Seagliders will record minute fluctuations of temperature associated with the tiniest of the turbulent eddies, hundreds of times per second. The gliders will navigate a racetrack pattern around the moorings for the duration of the experiment, allowing us to see how mixing varies with depth and time in response to changing winds, weather patterns, and ocean currents.

The final awesome piece of technology from our Seattle team is the Mixed-layer Lagrangian floats (MLFs) that will characterize turbulent mixing from another perspective. A “Lagrangian” description of a fluid motion (named after mathemetician Joseph Lagrange) is taken from the perspective of particles moving with the fluid flow. In that sense, most floats and drifters are Lagrangian instruments to some extent since they follow ocean currents. MLFs are fully Lagrangian: they can also follow much weaker vertical motion of the water. They do it by closely monitoring surrounding water and changing their volume to remain neutrally buoyant.  They also have a set of six flexible wings that can be extended horizontally and “anchor” the float to the surrounding water. As a result, the MLFs can directly follow the large turbulent eddies in the mixed layer and observe their evolution. It is the interplay of the large and small eddies that controls ocean mixing, so MLF observations uniquely complement microstructure turbulence measurements obtained by Seagliders.

MLF deployment (A.Shcherbina on the line.)

MLF being carefully lowered during deployment.

Aboard Knorr, Jesse and Andrey work closely together to support then entire Seattle-based SPURS team. Jesse makes sure the Argo floats get deployed at their appropriate location and will continue to check up on the floats over the coming weeks/years.

Together they look after the gliders testing and deployment – making sure they are well trimmed – not too light, not too heavy – like Goldilocks, everything has to be just right! Once deployed glider operations go to a group of pilots back in Seattle, who will keep track of the gliders’ progress, collect the ocean data, and direct the instruments on their sampling track. We worked with them directly via satellite phone so we could do simultaneous profiles with the ship’s instruments and the glider. When they say “dive” to a glider from Seattle, we can watch from the ship as it slips beneath the waves just yards away!

Lagrangian floats are even more sensitive to the precise ballasting – even the amount of paint that they may wear off over the next year needs to be taken in consideration! However, some things are impossible to predict – such as algal growth or how much fish love the float. So the pilots in Seattle need to keep constant track of the float telemetry and instruct it to go through the ballasting routine on a regular basis. They also balance the sampling strategy to optimize battery life throughout the floats’ one-year deployment. Overall, the operators have remote control of about 300 variables that determine the float’s behavior. Keeping those in check will be Andrey’s job for the next year, once we get him back home.

Your blogger asked Jesse and Andrey to describe their feelings about SPURS. Their responses were superb, so I won’t paraphrase!

Jesse said: “It’s been great to be a part of such a large, well-planned field campaign. I am amazed at all the varied data we are collecting and can’t wait to start piecing a small part of it together to further unravel the ocean’s roll in the global freshwater cycle as part of my PhD work. I am also excited to finally deploy (after a failed attempt on my last research cruise) my first Argo float! Working with the data from these floats for years, it is about time I finally wave one off to begin its journey in the great wild blue yonder. Working in front of your computer screen, it is easy to feel disconnected from the dynamic environment I am trying to understand. So being out a sea is a great rejuvenator. There is a unique beauty that is found only in the middle of the ocean with no land in sight. The stars are that much brighter, the sky that much bigger and the color of the ocean – the most unreal crystalline blue!”

Andrey said: “Taking part in a concerted effort to understand the physics behind the formation of the saltiest water in the open ocean is truly fascinating. The latest technology and support from NASA enables us to study this unique area at an entirely new level. I am looking forward to observing turbulent mixing of heat and salt in the upper ocean as it varies daily, monthly, seasonally. Relating these observations to the pathways of salt and fresh water in the ocean is a fascinating puzzle to unravel. But the best thing is that it is nearly impossible to predict what we will discover, as each new voyage brings in something totally unexpected. I’m sure every scientist aboard Knorr would agree that it is this sense of wonder that keeps us going out to sea again and again.”

Prawlers, Engineers, and the Future of Oceanography at Sea

October 5th, 2012 by Maria-Jose Viñas

By Eric Lindstrom

A number of the instruments deployed during SPURS are “works in progress.”  They work well, but need exercise in new or more challenging environments to perfect them. The whole of SPURS is an experiment, taking salinity measurement in the ocean to an entirely new level. Extended deployment of sensor webs in hostile and distant environments is something NASA needs to perfect for the future of Earth System Science research and for planetary exploration.  One of the overarching goals for those involved is to advance the use of autonomous measurement platforms for real-time global oceanography.

The Prawler (Profiler + Crawler) instrument from NOAA Pacific Marine Environmental Laboratory (PMEL) is one example of SPURS exercising a work in progress. It is also a place where we can see common goals between NASA and NOAA in physical oceanography.

Prawler closed on the laboratory bench.

The Prawler is a wave-powered subsea instrument that eliminates the need for multiple sensors on a mooring line. During descent, it makes a profile using whatever sensors have been installed and communicates those via inductive modem to the surface buoy (from there they are communicated via satellite to shore). Once the Prawler falls to the pre-determined bottom depth (arount 1,640 feet, or 500 meters), a micro-processor activates a ratcheting mechanism and harnesses the wave motion of the mooring to crawl back up the mooring line.

Diagram showing the ratchet mechanism on Prawler that allows it to climb the mooring wire.

The Prawler has been years in development and testing. This expedition we deployed two NOAA moorings where the Prawler is the primary instrument. The Prawler will make from five to 30 profiles per day (the average is about 20 profiles per day). For SPURS, the Prawler measures temperature, conductivity, and pressure. Those are core variables that are easily conversed to temperature, salinity, and depth.

An example (from a prior test of Prawler in the Pacific Ocean) of how many profiles one can expect to make using one Prawler during the course of a year. Provided by Billy Kessler, NOAA PMEL.

Billy Kessler at NOAA PMEL and University  of Washington is leading a NOAA SPURS project to test the Prawler technology. Billy and I were in graduate school at University of Washington together in the early 1980s and shared the same PhD mentor, Prof. Bruce Taft. It’s wonderful to be working with Billy again after all these years!

John Shanley and Andrew Meyer (aboard the Knorr) are two NOAA PMEL engineers who have been involved with Prawler mooring design tests for the last 3 years. They have already participated in some major tests of Prawler on moorings (a 7-month and a 4-month deployment in Hawaii, each time with two Prawlers, and many tests in Puget Sound). SPURS offered a great opportunity for a full year deployment near the heavily-instrumented Woods Hole mooring whose deployment I described in an earlier post.

John Shanley.

Andrew Meyer.

John and Andrew are fortunate to work in a small group of engineers where they get to work on all aspects of the Prawler, from design input to the actual fabrication of the instruments, to testing materials and components, to full ocean deployments. “From art to part,” as the boss likes to say! The NOAA PMEL mooring shop has a long history of excellence providing products that meet both researcher and operational requirements.

Seeing the Prawler used in SPURS after years of development is the light at the end of the tunnel for John and Andrew. This project has been a cumulative effort involving their entire engineering group. There have been four radically different versions, countless numbers of modifications, long days and weekends of machining parts. They have seen Prawler grow from just a few scribbles on a white board to deployment of the “finished” product over the stern of Knorr on this voyage. They both describe persistence as the key to success.  It almost brings a tear to your eye to hear Andrew describe to me “the last few touches before deployment as we assemble and ballast, to bolting it on the mooring line and dropping it into the ocean, bring a great sense of accomplishment.”

For Andrew and John being out here on Knorr to deploy the Prawler is just icing on the cake. The interaction with people from other institutions and seeing many different ways and means of measuring salinity truly puts the Prawler capability in a new perspective. They are both standing watches and working with the other teams to expand their ocean instrumentation expertise. They certainly now know that they too are on the leading edge of global real-time ocean observing in the 21st century. From interactions aboard ship they go home energized with ideas for the next innovation!

Notes from the Field