Measuring Salinity from Space

September 11th, 2012 by Maria-Jose Viñas
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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!

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12 Responses to “Measuring Salinity from Space”

  1. sangeeta deogawanka says:

    Hi,
    I wanted to know whether it is possible to check out salinity of groundwater or open water bodies using either of the two / Grace missions.

    In the meantime, I look forward to your post on the implications of mapping ocean salinity and temperature.

    • Eric Lindstrom says:

      Sangeeta,
      The GRACE satellites measure gravity variations only and therefore focus on changes in mass on the Earth. The detected changes in mass are related to many things including ground water changes, changing ice sheets, and ocean mass variations. GRACE does not detect changes in ground water salinity. The Aquarius and SMOS missions, using L-Band microwave remote sensing, are sensitive to changes in surface moisture over land and both missions are producing a soil moisture products. We cannot tell the salinity of surface water on land from either mission, just characterize the relative abundance of water.
      Eric

      • Sandy says:

        Per the discussion on using gravity sensors to measure salinity, Aquarius measures salinity of sea surface only, correct? If using a gravity sensor to measure salinity (the sensor would have to be incredibly sensitive), it would be measuring gravity of the total water column at a particular location. To calculate salinity at a location, percentage of other dissolved solids and particulate matter would need to be known, as well as the total mass below the water (crust to core) and depth of water column. Unfortunately detailed information on those other variables a priori is just not available to solve the equation…

        • Eric Lindstrom says:

          Sandy, Thanks for the added clarification. Aquarius is focused on measuring sea surface salinity. I realize that in non-oceanographic circles “salinity” is a big subject related to ground water and surface water on land. We are not examining that issue. Trying to get to salinity of ground water as a “mass” problem through GRACE seems darn near impossible (its such a small signal in the overall mass signal and its variation). Maybe it will be possible one day, but seems like it more directly accessible through in situ measurements.
          Eric

  2. ldavidcooke says:

    Hey Dr. Lindstrom,

    I am curious as to why they chose to apply microwave as opposed to Lidar, is it due to the optical depth issue at sodium resonate frequency? If so could a harmonic frequency not work? Oh, and while you are on campaign, are you also sampling for dissolved carbonic acid?

  3. Eric Lindstrom says:

    There is a good presentation on this question at the Aquarius mission web site.

    Check out: http://aquarius.nasa.gov/pdfs/LeVine_3Jun11b.pdf

    David Levine is better trained than this sea-going oceanographer to answer the hypothetical question on other potential choices for remote sensing of ocean salinity.

    Eric

    • David Le Vine says:

      Reply to the question of why microwave and not Lidar: The microwave response to salinity has a long history of research and validation dating back to the 1970’s. There were even reports of changes associated with salinity in the signal from the L-band radiometer on SkyLab. See Section D in Proc IEEE Vol 98 (5) pp 688 for a short history. Salinity changes the conductivity of water and the change in conductivity changes the thermal emission enough at L-band to be measured with modern instruments from space. The change in conductivity is also the basis for measuring salinity in situ. There may be a correlation between ocean color and salinity, but otherwise, I am not aware of passive optical or Lidar remote sensing of salinity. It would be nice, if possible, because the challenge now is to obtain the spatial resolution needed to address issues closer to the coast.

      • ldavidcooke says:

        Hey Dr. Le Vine,

        Thank you for your response, I was considering a reapplication of the Colorado State, atmospheric sodium lidar. (A reference here: http://lamar.colostate.edu/~lidar ) As this device is used as a ground based sensing device for Mesopheric atmospheric activity at 55 – 65 km elevation, it might be difficult.

        With the loss of the L band yellow slot below this altitude may suggest it would simply require a slight detuning or retuning for a Na/Cl emission band. I only mention it in passing as it may offer a separate; but, valuable alternative tool for the future of marine research.

  4. Eric Lindstrom says:

    We are not delving into the chemistry and biology of the ocean on this voyage. We have the Knorr labs full with people and various gear to focus on ocean salinity variability.
    EJL

    • ldavid cooke says:

      Hey Dr. Londstrom,

      As you may have surmised, wrt the question about dissolved CO2, is the relationship to ocean surface temperatures. I’m curious if it is possible that the CO2 could “trap” normal insolation near the surface resulting in spikes of salinity, similar to long term synoptic weather conditions of clear skies and “dry” high pressure centers.

      It would be very useful to know if there could be a correlation to the atmospheric “thermal blanket” theory. Along the lines of a hypothesis of where the CO2 creates a strong inversion layer both concentrating insolation near the surface and sheilding the water column below, hence, increasing SSTs.

  5. Eric Lindstrom says:

    I am not too sure if I understand this question. Let me just try some stating some facts.

    CO2 in water does not have greenhouse blanket properties as in the atmosphere. infrared (thermal) radiation that is “trapped” by CO2 in atmosphere does not penetrate very far in seawater and is more related to the properties of water than small changes in water chemistry (like the amount of trace gases). Low salinity at the surface (as might be caused by rainfall or land runoff) can cause stratification leading to enhanced warming at the surface. Surface water warmed by the sun during the day might not be mixed down due to the buoyancy imparted by the low salinity.

    The partial pressure of CO2 in seawater is related to both the temperature and salinity of the seawater, so chemists are interested in precise maps of surface salinity in order to better understand the fluxes of CO2 between the ocean and atmosphere.

    • ldavidcooke says:

      Hey Dr. Lindstrom,

      My apologies for not being clear, the tought I was trying to share was as the incoming UV, which I have read penetrates to over 40M, is absorbed, with the energy converted to IR by the increase in water molecule vibration which I suspect results in heating of the water or via increased collisions the emission of IR energy.

      The question then returns to the basic physics if there is an increase in carbon molecules, ie: carbonic acid or calcium carbonate, near the surface, would these molecules tend to trap incoming solar energy in the upper 10-30 meters of the ocean and shade the deeper water?

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