Weather: The physics of a catastrophe

The transference of heat about the globe may be brought about either by winds in the atmosphere or currents in the ocean. When these huge forces operate against one another, the consequences can be both violent and unpredictable - as El Nino is now showing.

The oceans over more than two-thirds of the earth's surface, with over 60 per cent of of total ocean area in the Southern hemisphere. The seas, like land, absorb heat from the sun and radiate it back into the atmosphere. However, while the land has the decency to remain stationary, the waters are constantly mixing and moving, which makes their effect on the weather at best difficult, and at times impossible, to calculate.

Curiously, the simple, regular motion of the seas - the tides, caused by the moon - appear to have only a minute effect on weather patterns. Compared with the effect of the earth's own rotation - which causes the temperature differences between day and night, the effect of the rotation of the moon about the earth is insignificant.

What does make a difference, however, is the slow changes of temperature that occur in vast masses of water in the oceans. The biggest of these is the El Nino southern oscillation (ENSO) which begins as a large-scale warming of the eastern tropical Pacific. It is irregular - occuring at intervals of between two and seven years - and its effects may be minor or catastrophic. We still do not even know whether it starts in the oceans or in the atmosphere - the interaction of winds and currents, and air temperature and sea temperature, are simply too complex to be certain about such things.

The latest suspect in this complex chain of events is the Indian Ocean. This year, while El Nino was bringing drought to Indonesia and Papua New Guinea, the Indian Ocean was sending huge amounts of cloud across Australia, bringing rain that saved the crops from the predicted disaster. Australian scientists believe that the simultaneous occurrence of the two unusual weather systems is more than just a coincidence. Ever since the last great El Nino of 1982-83, the Australians have been monitoring the transfer of heat between the Indian and Pacific Oceans through the Indonesian flow- through.

The El Nino events of 1982-82, and 1994, both brought bad droughts to Australia, but the 1992 event had no bad effect, and the difference has been ascribed to the behaviour of the Indian Ocean. This year's El Nino has led to the launch of a new effort to study the Indian Ocean. Global Ocean Data Simulation Experiment, sponsored by Australia, France and the US, hopes to build theworld's first global ocean analysis facility.

Until its work is completed, however, we shall still have to muddle through, taking each El Nino as it comes, watching it reverse the normal pattern by sending cool ocean currents west and warm ones east, and turning the Pacific trade winds around and starving many countries of the southern hemisphere of the normal rains that are drawn from warm seas and bringing unusual warmth to japan, Korea and Canada. Or it could all be quite different this year. With El Nino, you can never be sure.