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.