Space-borne instruments have revolutionized research on the circulation patterns and strengths of the oceans. For instance, a satellite can observe all of the world's oceans in less than ten days. With radar technology, it can measure the shape of the sea surface and provide observations on the surface flows, including major currents or eddies — phenomena that are much like weather systems in the atmosphere.
The TOPEX/POSEIDON mission, a US–French undertaking, has been prominent in providing precision observations of this sort. But a single satellite has its limitations in detecting variations both in space and time. Although there has been more than one radar instrument in space for several years now, the task of merging satellite data is formidable and requires considerable statistical skill. Yet that is what N. Ducet and colleagues have done, as they describe in the Journal of Geophysical Research (105, 19477–19498; 2000). The result is the best picture yet of the ocean and its vigorous eddy component.
The authors have combined data from TOPEX/POSEIDON and the European ERS missions over the period 1992–98. The picture here shows in stunning detail the regions where currents fluctuate vigorously (units are in cm2 s−2). The regions of high (red) amplitudes indicate particularly turbulent currents that are involved in transporting and redistributing heat and other properties, and so affect global climate.
What of the future? The next aim is to fly satellites routinely in a configuration and with technology that will allow current and eddy flows to be measured directly without the need for statistical estimation. The ultimate objective is to incorporate the data into models of ocean circulation, much as is done in weather forecasting. Physical oceanographers will not be the only beneficiaries. Other prospects include identifying local variations in flows that affect fisheries, the distribution of pollutants, and fuel-saving routes for ships. On a larger scale, such studies should also tell us more about the transport of heat from low latitudes towards the poles, where it is released to the atmosphere and has a large influence on mid-latitude climate.