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November 16th, 2006
Measuring the Shape of the Oceans

Optimistic scientists believe that the new generation of radar altimeters – not radar imagers — will give us our deepest understanding of the way that carbon cycles between water and air. This is significant, because it is one of the least understood parts of the climate system, and possibly one of the most significant for understanding climate change.

Radar altimetry is a relatively new science, dating back to the early 1990s. NASA’s Seasat satellite validated the concept, but monitoring began in earnest in the 1990s, when the European Space Agency launched ERS-1. ERS-2 and Envisat followed. All three satellites carried a radar altimeter as part of a multi-instrument package. NASA and the French space agency launched Topex/Posidon and its follow-on mission, Jason-1.

To celebrate this fifteen years of success, Earth scientists from around the globe gathered in Venice, 13-18 March, for a symposium. It was jointly organised by ESA and CNES, the French space agency.

Carl Wunsch set the tone of the meeting when he said that the biggest contribution of the technology had been conceptual rather than scientific. Wunsch is from the Massachusetts Institute of Technology, one of the pioneers of the altimeter

‘The greatest achievement of the altimeter is that it has shown us that the ocean system changes rather dramatically every day. The shape of the surface of the ocean is not a geological phenomenon, creeping along very slowly. It’s much more interesting. Fluid is moving in all directions at all times’, he said.

All current radar altimeters work on the same principle. They transmit a radar pulse towards the ocean surface, and measure the time needed to receive the reflected energy. This gives, to a high degree of accuracy, the distance between the spacecraft and the water surface. Current instrument technologies, combined with GPS-based orbit determination, allow observers to estimate the position of the water surface to within 20 mm.

Moreover, since the radar beam illuminates an area several tens of kilometres wide, the spacecraft measures the average height over that area, not the instantaneous – wave affected — height.

This level of accuracy enables scientists to observe phenomina that until now they had only surmised must exist.

The primary measurement goal of an altimeter is to measure the time-varying portion of ocean surface topography. But a multitude of factors influence instantaneous sea surface height. Some of these can be teased out by better examination of the return.

The shape of the return is related to the height of the ocean waves. Its magnitude is linked to the sea surface roughness. Since waves are a function of wind speed, it allows one to infer local surface wind speed.

Fortunately, wave height and wind speed observations are independent of the measurement of sea surface height. The former are derived from the characteristics of the return signal, the latter from the time of its detection.

The shape of the ocean surface is a matter of fundamental importance to geodesy. Geodesists are interested in the height of the sea surface above a reference Earth ellipsoid. Geodesists start from the assumption that the ocean surface generally follows a surface of constant geo-potential energy known as the geoid. Undulations of the geoid, reflected in differences between the height of the geoid and a reference ellipsoid, are quite large compared with the height of the oceanographic contributions to sea surface height.

These undulations range worldwide from -107 metres to + 85 metres using the WGS 84 datum used by navigation satellites.

But beyond this, the data are of interest to many other disciplines. Oceanographers are interested in tides, regional and global mean sea level, ocean currents, ocean variability, and even global maps of the seafloor.

Meteorologists might want to approximate the thermal structure of the upper portion of the ocean by measuring sea surface height and sea surface temperature. This is vitally important in achieving accuracy and resolution in ocean circulation models and carbon exchange.

The Venice meeting demonstrated that there has been success on all these fronts, but it seems that the greatest progress has been made in our understanding of the overall structure of the oceans.

The ability to measure such small variances in sea surface height permits oceanographers to measure changes in ocean currents and for the first time, to create a weather chart of ocean circulation. Pierre-Yves Le Traon of the French Research Institute for Exploitation of the Sea called it Envisat’s most outstanding achievement.

Ocean forecasting behaves much the same as weather forecasting. If there is high pressure – signified by higher sea levels – an anti-cyclonic ocean circulation takes place. This usually translates into good weather conditions. Low pressure – signified by lower sea levels – indicates that a cyclonic ocean current is present.

This type of forecasting has important potential for society and the world’s economies. For example, it allows scientists to forecast El Nino events and the flooding of low-lying areas (such as Venice), and to predict the trajectory of pollutants.

One completely new phenomenon to emerge from the work has been the discovery of planetary waves. Paolo Cipollini, of the UK National Oceanography Centre, says this is the real success story of radar altimetry.

Planetary waves, also called Rossby waves, were thought to have existed in the ocean as far back as 1930. It was impossible to know for sure, because they occur internally as changes of density. Their expression on the surface is of the order of about 10 centimetres. They are impossible to detect from an oceanographic research vessel. Radar altimetry offers proof of these waves for the first time.

‘It has been suggested that planetary waves are one mechanism which brings nutrients from the deep sea up to the surface, which would make them important for the carbon cycle’, Cipollini said. ‘In fact, they may be responsible for setting the main circulation patterns in the ocean.’

‘So it is possible that these mysterious waves, that up until 20 years ago we weren’t even able to see, are also important for biologists and for people studying how the ocean is reacting to global warming’, he added.

There is still much to be done. Scientists would like to be able to study the variation of sea surface topography on much shorter – hourly – timescales. They would also like much better data on littoral regions, which are difficult to image with current technology because of the influence of the shore on the radar signal.

Possibly the most urgent task is a better understanding of the ice-water interface in Greenland and Antarctica. As the oceans warm, this is where the best evidence of the impact global warming will be found.

This was the mission of ESA’s Cryosat-1. In October 2005, it was lost when the launcher that was carrying it into space went awry. We need a replacement.


Jonathon Powers is a freelance technical writer living in Sydney.