Observing & Measuring: The Space Revolution

The International Geophysical Year (IGY) in 1957/1958 saw the emergence of a new subject of research: ' Planet Earth '. For the first time, scientists sought to understand and analyze the Earth as a whole. It was also the first time that the oceans became the subject of international cooperation.

By a coincidence, this was also the beginning of the space age: the first satellite was launched in 1957. From then on, they would be used to observe the whole of the Earth from space, just as Earth-based observatories are used to observe the other planets. It was to be a genuine revolution for the study of the oceans. Like the atmosphere, the oceans are in constant flux, as if they were an enormous jigsaw puzzle whose pieces continually change shape at different rates. Satellites, which are able to encompass the entire ocean and can take continuous measurements over long periods of time, have made it possible to discover and analyze the complexity and variability of its structures on all spatial and temporal scales.

Ocean dynamics are at the heart of the climate change problem. For this reason, it is important to understand ocean currents and quantify the forces that drive them so that they may be modelled. Seasat, the first satellite to be completely given over to the study of the oceans, was launched in 1978. It only lasted for 105 days, but it proved that the currents and their main driving force, the wind, could be observed from space. First of all, the currents themselves. Any motion of the sea re-shapes its surface. This is true both for waves, whose height can be measured by satellite, and for tides. It is also the case for currents: they all cause a slope in the sea surface, and the stronger the current, the steeper the gradient.

In the Gulf Stream, variations in sea level of 1 metre over 100 kilometres have been observed. By measuring such differences in sea level, the strength of the corresponding currents can be inferred. This is done with satellite-borne altimeter radars which are used to draw up maps of the surface topography of the oceans, enabling ocean currents to be measured ( JASON-1, ENVISAT, ERS-2, GEOSAT ). Then the forces which drive the currents. First, there is the main force that drives ocean circulation, the wind. This is measured from space using radar scatterometry. The wind causes ripples on the sea surface. In calm weather you can see how a flurry of wind causes ripples to spread across the surface. It is these ripples that refl ect the waves emitted by the satellite-borne radar back to it. Their intensity depend on the ripples, and therefore on the wind. In this way, we can observe wind fields, which are the main driving force of ocean currents ( ERS-2, ASCAT on MetOp, SeaWinds on QuikSCAT ).

Lastly, there are thermodynamic exchanges with the atmosphere (heat exchanges, evaporation, precipitation) which, by causing variations in the density of sea water, drive the deep ocean circulation (thermohaline circulation). Such exchanges are highly dependent not only on the wind but also on the sea surface temperature, which can be estimated from space by measuring the infrared radiation emitted by the ocean. Such measurements were long carried out by weather satellites (the first of which, TIROS 1, was launched on 1 April 1960). This is now done by geostationary satellites (GOES, Meteosat, etc) and by polar orbiting satellites (NOAA, ENVISAT, MetOp, etc).

In the near future, satellites will provide measurements of surface salinity which will complete the assessment of evaporation/ precipitation. To do this, they will use microwave radiation, sensitive to the emissivity of sea water, which varies with salt concentration. This will be the case for SMOS - Soil Moisture and Ocean Salinity, which is due to be launched in 2009, and Aquarius, planned for 2010. Fields measured in this way are two-dimensional surface fields: satellite-based sensors are no better than the human eye at probing the interior of the ocean, which remains stubbornly opaque to them. To observe ocean circulation below the surface layers it is therefore necessary to carry out “soundings”, just as we do in the atmosphere. Space comes to the rescue again, thanks to satellite-borne systems such as Argos, which can be used to locate in situ measuring platforms and transmit the data they collect. Currently, 3 000 floats deployed right across the world's oceans are sounding the top 2 000 metres, measuring temperature and salinity. We thus have a permanent observation system for the interior of the oceans which, together with remote sensing measurements, gives us a pretty complete picture of it.

Bruno Voituriez
Les Argonautes