Weather: Current ideas in climate research
Wednesday 19 November 1997
These are exciting times for the World Ocean Circulation Experiment (WOCE), a global research project that started in 1990. The idea behind it sounds simple: to improve ocean models for predicting climate change and to collect the data needed to test them. In practice, however, taking the necessary measurements has not been easy. Research ships could cover only a tiny sample of the vast area of the oceans, and measurements were generally restricted to conditions near the surface. So when the WOCE began, it was designed as a 12-year-long project, seven years of collecting data and five years of analysing it.
The first phase is now approaching its end, which is one reason for excitement, but the coincidental arrival of the biggest El Nino this century is a bonus that should provide the data needed to offer a stern test of the robustness of computer models of climate.
The oceans play a huge role in the way solar energy affects the earth. About 50 per cent of the sun's incoming radiation reaches the earth's surface, and the greater part of that energy is absorbed by the top layers of the oceans. The high thermal capacity of sea water ensures that it is an effective store of heat. Currents then ensure that the oceans play their part in distributing heat around the world.
At the highest levels of the oceans, currents are driven by the wind. The Gulf Stream, for example, is a consequence of winds caused by the Earth's rotation and the differential heating of Equator and poles by the sun. The result is that the oceans help in the spread of energy from the Equator to the poles. But it is what goes on at deeper levels that makes it more complicated.
Until comparatively recently, it was thought that the deeper parts of the ocean were generally still. One finding of the WOCE survey is that that is not the case. While the winds efficiently stir and mix the water to a shallow depth, there is also a slow and sporadic mixing taking place vertically. The "thermohaline circulation" converts masses of warm surface water at high latitudes to colder, denser masses which sink and return towards the Equator. The WOCE research has followed the paths of such masses of new deep water and has confirmed that it eventually returns to the surface, even decades or centuries later, on a sort of global conveyor belt.
Two diagrams in a recent publication from WOCE give an idea of the power of such systems. One shows the flow of ocean volumes of water; the other shows the transport of heat. The units employed are millions of cubic metres per second and Petawatts (1015 watts). One Petawatt is about 60 times the global consumption of energy.
Internet users can learn more about the WOCE at http://www.soc.soton.ac.uk/ OTHERS/woceipo/ipo.html
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