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Insight into marine science
Oceanography from space
Satellite technology has completely changed our view of the global oceans. For the first time, it has been possible to make global measurements without waiting for the years it would take a ship to travel the same distance. The disadvantage is that most satellites can only see the ocean surface, or at most the top hundred metres or so, since electromagnetic radiation from deeper in the water is absorbed before it reaches the surface.
Satellites measure a variety of quantities, including ocean surface temperature and colour (related to the kind and concentration of phytoplankton present in the surface layer). For those studying ocean currents though, satellite altimetry is the most important technology.
Satellite altimeters, such as the US/French TOPEX and Jason , and the
instruments on the European ERS and Envisat satellites, measure sea
surface height, which tells us about currents in the same way that
atmospheric pressure tells us about winds. For example, sea level drops
by about 1 metre on crossing the Gulf Stream from south to north.
Monitoring the movements of sea level allows us to see how the currents
change with time. One of the clearest examples of this is the way
patterns of northward and southward currents propagate westwards in the
deep ocean. Known as Rossby waves, these patterns represent the way in
which information is transmitted across the oceans about changes in
winds occuring at distant locations. In the Southern Ocean, where there
is a strong eastward current, the waves are carried to the east by the
current, and interact strongly with the eastward flow.
The next major improvement will come with the launch of satellites to
measure the earth's gravity field. At the moment we can only measure
changes in currents from satellite altimetry, but not the average
current (rather like using atmospheric pressure but not knowing whether
the measurement was taken at sea level or on top of a mountain). To
measure the average currents we would need to know where sea level
would be if there were no currents (a surface known as the geoid). This
is what gravity measurements can tell us.
GRACE is a US/German pair of satellites which was launched on March 17th 2002, which will map the gravity field. The satellites fly about 220 km apart, and track each other. As gravity accelerates first one, then the other satellite, their separation changes. This information can be used to calculate the gravity field.
This gravity field will be further refined with the launch (scheduled for 2006) of the European GOCE mission, which will measure the geoid with higher resolution, allowing narrower currents to be measured.