The first map of salinity, or saltiness, of Earth's ocean surface produced by NASA's Aquarius satellite since it became operational on 25 August 2011.
Tasting the salt of the Earth's oceans
11 May 2012
New satellite images are providing unprecedented global views of the distribution of salt and fresh water across the surface of the Earth's oceans. Chris Banks describes how scientists at the National Oceanography Centre in Southampton are helping discover how accurate this new information is, and what it can tell us about the global water cycle.
If you have ever tasted the water while swimming in the Mediterranean or Red Sea, you might have noticed how much saltier it tastes than the water around the coast of the UK. Even though seawater is much saltier than rivers and lakes, the salt content varies significantly across the Earth's oceans. These variations have profound effects on the climate system.
For example, the UK's climate is much milder than that found at other parts of the world at similar latitudes, like Canada or Russia. This is because of the influence of warm Atlantic water transported northwards from the tropics by what is known as the Atlantic Meridional Overturning Circulation (MOC).
In fact, the Atlantic MOC is just one aspect of a global ocean circulation known as the thermohaline circulation. 'Thermo' comes from the Greek for heat or temperature, and 'haline' relates to salt. Differences in temperature and salt content change the density of seawater, and this in turn drives the global ocean circulation.
In oceanography, the saltiness of seawater is known as its salinity. Salinity represents the amount of salt dissolved in the water and is expressed as a ratio. The oceans' salinity varies globally between 30 and 40 parts per thousand, roughly equivalent to 30-40 grammes of salt dissolved in a litre of water. With salinity around 40, the Red Sea holds some of the saltiest seawater on Earth, while the English Channel, with salinity around 35, is more typical of the oceanic average.
Until recently, the only way to measure salinity was by direct sampling, using salinity sensors on ships or buoys (thankfully not involving tasting!). However, even at the surface, there are vast regions of the oceans that are rarely visited, and this leads to an incomplete global picture.
The first map of salinity, or saltiness, of Earth's ocean surface produced by NASA's Aquarius satellite since it became operational on 25 August 2011. Click to enlarge.
The situation has improved markedly in the last few years, thanks to the international Argo programme. Today, over 3000 Argo floats drift freely across the ocean. Every few days, each float provides measurements of salinity, temperature and depth over the top 2000m of the ocean. Even so, with Argo floats typically spaced more than 300km from each other, our knowledge of global ocean salinity is still incomplete.
Earth-orbiting satellites give a unique global view of our planet. Satellites have been measuring sea-surface temperature for the last 40 years with increasing accuracy. Measuring ocean salinity from space is altogether more challenging, and, until the launch of the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) satellite, was thought to be all but impossible.
To achieve the detail required to make out ocean eddies and river plumes using a conventional design, the satellite's antenna would have to be prohibitively large, making it impossible to fit it inside a rocket.
In November 2009, ESA launched the innovative SMOS satellite - as you might guess from its name, this is designed to measure both the water content of land and the salt content of the ocean surface. SMOS was small enough to fit inside a rocket because of its unique unfolding Y-shaped antenna. Spanning 8 metres when deployed, the antenna comprises 69 small receivers. By combining the signals from these receivers, a much larger antenna can be synthesised - mimicking a technique commonly used in radio-astronomy.
Remote sensing sea salt
But how can SMOS measure salinity from space? The answer relies on a property known as electrical conductivity. Essentially, how well seawater conducts electricity depends on its salt content; the saltier the water, the more easily electricity travels through it. SMOS measures the natural radiation emitted by the Earth's surface at a particular electromagnetic wavelength of around 21cm, known as L-band. The strength of this radiation, known as the brightness temperature, is related to the conductivity of the surface.
As a passive sensor, SMOS only measures natural radiation, and does not itself emit any signals, unlike lasers or radars. Unfortunately, this means that its measurements are sensitive to many other factors that also influence the radiation at that microwave frequency.
Soil Moisture and Ocean Salinity (SMOS) satellite.
This includes effects from surface temperature, wind, waves and other related features (for example, whitecaps and foam), all of which must be removed in order to extract the information about surface salinity. In addition, in the same way as we can see the sun, moon or stars reflected from the surface of the ocean, radiation at L-band is also reflected, so we need to make corrections to the measured brightness temperature.
One of the reasons the L-band wavelength was chosen is that it is 'protected' - broadcasting signals in this band is not permitted, so ideally there should not be any man-made sources.
Unfortunately, there are a few sources that create radio frequency interference. These sources can be strong, making measuring salinity like looking for a small candle flame on a bright, sunny day. Some of the sources can and have been identified (for example, due to incorrectly working television broadcasts) and removed but others are more problematic as they are the result of powerful military radars that spill into the measurement range of SMOS.
All these issues lead to errors in single measurements of salinity that are too large for the data to be used in many applications. The aim is for accuracy of 0.1 - accurate to about 0.1g of salt in a litre - over 10-30 days and 100-200km of ocean.
SMOS takes measurements over the entire ocean surface every three days, so by averaging many individual measurements we can improve the accuracy of the salinity observations. At the National Oceanography Centre (NOC), we average SMOS data over 1° (latitude) by 1° (longitude) boxes, and over a period of one month.
These gridded datasets have been compared with measurements from Argo buoys and output from a UK Met Office model of ocean circulation. Initial results show that, away from coastal areas (that is, more than 100km from land), SMOS can reproduce variations in salinity across the oceans. We always knew that coastal regions would be problematic as land emits much more L-band energy than water, so measuring salinity is even harder - like looking into a dark corner on a sunny day.
In recent months, SMOS measurements of coastal salinity are markedly improved as ESA has implemented an improved processing system following feedback from NOC and other international investigators.
As well as these on-going improvements in data from SMOS, the future of measuring ocean salinity from space is bright. In June 2011, SMOS was joined by another satellite measuring salinity when NASA launched the US/Argentine Aquarius mission. Aquarius uses very different technology from SMOS, but shares the same scientific goals, which benefit from good interaction between the scientists and engineers working on the two satellites to analyse and interpret these novel satellite datasets.
Results from these two innovative satellites along with data from Argo and other in situ measurements will improve our knowledge of ocean salinity. In turn, this will enhance our understanding of ocean circulation, and hence our ability to model climate and weather.
MORE INFORMATION
Dr Chris Banks is a member of the Marine Physics & Ocean Climate research group at
the National Oceanography Centre in Southampton. Email: chris.banks@noc.ac.uk.
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