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The age of Aquarius

Filed under: — gavin @ 12 June 2011

The international Aquarius/SAC-D satellite was successfully launched yesterday (thankfully!). Media coverage was good – except for the almost absolute avoidance of the term ‘salinity’ to describe the concentration of salts in the surface ocean that Aquarius will retrieve – oh well. But what is Aquarius going to see, and why is it important?

The most important factor governing the dynamics of any fluid is the equation of state that relates its composition to its density. That ultimately determines the pressure gradients and hence the velocity and circulation. In the atmosphere, the important constituents are temperature and water vapour, while in the ocean it is temperature and salinity. Making seawater warmer or making it fresher (by adding rain or river water) both decrease the density and make the water more buoyant. Similarly, cooling and evaporation both make seawater more dense. Salt also affects the freezing point of water (it is around -1.8ºC for normal seawater, colder still for seawater under pressure), so salinity can affect sea ice evolution too.

The specific definition of salinity itself has subtly changed over time. Originally it was the mass of dry solids left over after all the pure water was evaporated (measured in g/kg or similar), but that is difficult measurement to make on a routine basis. Then it was defined by measuring the Cl concentration (closely related since NaCl is the dominant salt). More recently, salinity was measured via conductivity (saltier water conducts electricity more readily) and reported in ‘practical salinity units’ (psu), and in 2009 the official definition changed again to a new ‘Absolute salinity‘ (see the TEOS-10 website for details). All of these are roughly coherent, but there are subtleties that have become more important as knowledge has increased (for instance, related to the (small) impact of varying carbonate concentrations), and realisation that very small changes in density can have important effects.

In the surface ocean, there are strong salinity gradients across the major currents – water is much fresher on the west side of the Gulf Stream extension than on the east side for instance. Warm core eddies in the South Atlantic are saltier than surrounding sea. The western pacific warm pool is fresher than water in the East Pacific since it rains more there. One important thing to note is that temperature and salinity anomalies are often balanced – that is cold water is often fresh, and warm water is often relatively salty. Since these two factors have opposite impacts on the density it is difficult to judge whether water is more or less dense (which is key to the ocean circulation) just from the temperature alone.

This is important, for instance, in attempts to predict ocean circulation based on current conditions – if you only use temperature information (because we don’t yet have good salinity data in real time), then you can incorrectly assume density anomalies that are much too large, or even the wrong sign (this was one of the problems in the Keenlyside et al paper from 2008).

Aquarius will hopefully give a much needed boost to attempts to track ocean density for these purposes. But there are a lot of other variations that will likely be seen that will give a lot of insight into important ocean dynamics. What is the role of salinity variations in the development of an El Niño event? Can we validate rainfall and evaporation estimates by looking at the change in ocean salinity? Can we quantify sea ice melt by its impact on salt? I’m pretty confident that Aquarius will reveal a far more dynamic picture of sea salt variations than we currently imagine.

Aquarius retrievals are based on passive microwave technology and rely on the fact that salinity affects the thermal emission properties of the ocean surface. This effects are quite subtle, and the range of variability is relatively small, so it has taken many years for the technology to catch up to the need. This is however a first attempt to do this from space, so challenges will undoubtedly remain. (Correction: The ESA SMOS mission (launched in 2009) was in fact the first salinity measurement from space.)

As an aside, NASA has a salinity quiz to test your knowledge. Rather unfortunately I only got 7/10 right, which (at least in two cases) I put down to ambiguities in the questions…. but I’ll be interested to see if others do better (we can discuss the details in the comments). There are also some minor errors in the education portion of the Aquarius site (e.g. sea ice has an average salinity of about 5 psu, not zero), so if anyone spots anything else, let us know and I’ll try and get it corrected.

65 Responses to “The age of Aquarius”

  1. 51
    William Ingram says:

    “There are also some minor errors in the education portion of the Aquarius site (e.g. sea ice has an average salinity of about 5 psu, not zero)”
    I think that’s another ambiguity, Gavin.
    Isn’t the ice itself pretty much salt-free, with the salt in hypersaline brine pockets within it, which are actually liquid & so by definition not ice?
    Of course since the brine pockets stay in the ice till it melts & are released then, your number is the relevant one for thinking about transports of the 2 components.
    And I may sound more than I want like President Reagan saying “I didn’t have cancer – something inside me had cancer”.

  2. 52
    Brian Dodge says:

    Gerry Quinn 14 Jun 2011 at 4:44 PM
    “Water at the seafloor must obviously be less salty on average because evaporation takes place at the surface.”

    You’re not accounting for thermohaline circulation. The saltiest surface waters are the Gulf Stream & N Atlantic, because of heating & evaporation in the tropics. The lower density because of the warmth counteracts the increase in density caused by the saltiness, and Eckmann transport carries these waters north. As this water cools in the North Atlantic (or freezes in the arctic), the salty water sinks (or cold brine is expelled from the forming ice and sinks). This cold, salty North Atlantic bottom water spreads, and is a major source of bottom water (after a long time, ~1500 years, and with some dilution from other mixing and diffusion of less salty waters) in oceans worldwide.

    Cold brines expelled from the annual freeze of Antarctic surface water also contribute salt to the bottom waters.

  3. 53
    jkga says:

    I’m pretty sure (having floated in the Dead Sea) that buoyancy goes up with increasing salinity and that buoyancy goes down with lower density, in which case a word should be changed in the second paragraph.

    [Response: No. *You* are relatively more buoyant in denser water so you float more in the Dead Sea than in the Atlantic or a swimming pool. Saltier water is less buoyant than fresh. – gavin]

  4. 54
    jkga says:

    Of course – silly me.

  5. 55
    ardavenport says:

    I got 9/10 right on NASA’s quiz, but a few I got right because I had just read the article here. And one can disagree on what a ‘large’ variability in salinity is.

  6. 56
    spyder says:

    Thank you Brian for your response. I suppose i am wondering if the satellite could gather some data of the circulation, although, as you point out, the timelines are exceedingly long.

    One side note, the icy brine of which you speak, especially that under the iceshelf (s), is a major resource for krill and other tiny but abundant creatures. Changes in salinity will result in changes in the oceans’ food carrying capacity.

  7. 57
    Max says:

    You only got the same score as a podunk like me (and that’s with misreading the question on most common constituents)? Shame Gavin, shame. :p

  8. 58
    Carl Mears says:

    I got 8/10 and I am part of the Aquarius/SACD team. I agree with
    Gavin that some of the questions are ambiguous. I said the salinity
    variations are large, given that they are so important for circulation.

    Also, since deep water tends to form at the surface, I do not think it
    is right to say that the deep water is more saline than the surface.

    Maybe its a case of reading to much into the questions.

    Thanks, Gavin, for highlighting the mission.

  9. 59
    M says:

    “I said the salinity variations are large”

    One of my professors used to campaign against imprecise words like “large”, “small”, etc.: he would state, “large compared to an atom? Or to a planet?”

    For some purposes, salinity variations are large (I also answered that), but for others, they are small. This is the same issue which confused some people when the AIRS satellite produced CO2 maps and there was “lumpiness” – but for the purposes of GHG forcing, those lumps are small. Similar lumpiness in salinity for the purposes of circulation, on the other hand, would be considered large. All in the eye of the beholder, and the purpose for which the comparisons are being made.

  10. 60
    Stuart says:

    “The last question is the one I got wrong. Won’t all that melting of glaciers, ice sheets and sea ice decrease salinity? It seems obvious to me,”

    Conversely though, we are talking about a small amount of extra water when compared to the total volume of the oceans, so other effects could cancel it out or overwhelm in the other direction.

    Water at the seafloor must obviously be less salty on average because evaporation takes place at the surface

    Conversely dense water sinks, which includes water with high salinity.

  11. 61
    llewelly says:

    On the question of what effect climate change will have on the salinity of the oceans, one effect I do not see mentioned in the comments or the article is that intensification of hydrological cycle will result in more runoff, and therefor more sediment – and thus more salt in the oceans. I have no idea how this would compare to the other effects – such as melting glaciers – which would decrease salinity.

  12. 62
    CRS, Dr.P.H. says:

    8/10….not bad for a biologist! Mind you, I might have received 10/10, except for the distraction of my lovely wife demanding I put the laptop down and get to bed! Cheers!

  13. 63
    Charlie says:

    10/10, with the help of your figure. Being able to read the mind of the person making the quiz was a key skill, because they weren’t all that clear. And c’mon, “chloride” is an ion, not an element. Get on the ball.

  14. 64
    Brian Dodge says:

    @llewelly 17 Jun 2011 at 9:24 AM re hydrologic cycle and salinity

    “With a transport of some 16,000 m3 s-1 the Mississippi River is the third largest of all rivers of the world (after the Amazon River with a transport of 175,000 m3 s-1 (or in oceanographic units 0.175 Sv) and the Congo River with 38,000 m3 s-1), and its freshwater supply to the ocean is comparable to the transport of many ocean currents.”
    “Salinity in the mixing region of the Amazon River on the continental shelf of South America. River water has a salinity 35.” Fig 13.2 “Shelf and Coastal Zone Lecture Notes, Chapter 13”
    “The Mississippi River end-member near New Orleans had a salinity of 0.02 in August 1998 and 0.2 in September 1998. The higher salinity in September is most likely a result of diminished freshwater discharge.”
    “Riverine inorganic carbon flux and rate of biological uptake in the Mississippi River plume” Wei-Jun Cai GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 2, 1032, doi:10.1029/2002GL016312, 2003

    World ocean volume is ~1.3e18 m^3 (wikipedia)

    Totaling numbers from, the current (heehee, pun intended) flow of rivers is ~563000 m^3/sec, so that would require ~73000 years to completely turn over the oceans, and add less than 0.2 PSU. Increased rainfall would decrease salinity, which would tend to counteract the increased flow, so even if there were large changes in the hydrologic cycle, the increase in salinity would be slow.

  15. 65
    Joseph O'Sullivan says:

    I got 8/10. If remember correctly from my undergrad days the salinity gradients, as shown in the diagram in this post, in the open oceans that have major effects on ocean circulation are not large enough to themselves have an effect on the biotic community.

    I am correct on this? I received a BS in marine biology in ’95, but I did not work in the sciences after this. I would like to think that I still have some sound scientific knowledge!