Climatologists at the Climate Prediction Center, part of the American Weather Service, have been warning for two or three months that a new El Nino event is beginning to develop. Those warnings have so far been muted, but during the last two weeks sharp sea-surface temperature changes have occurred off the Pacific coastline of South America which lend credence to them. However, Australian climatologists are still more cautious and consider the likelihood of an event this year no higher than 50:50.

El Nino (more precisely referred to in the meteorological community as an 'El Nino Southern Oscillation' event, usually shortened to ENSO) is a reversal of the prevailing wind and temperature distribution in tropial latitudes of the Pacific Ocean south of the equator. Once it is under way, you can expect our news media to blame every unusual or extreme weather event around the world on it. It is so easy to point the finger at El Nino - or whichever particular climatological phenomenon happens to be flavour of the month - whenever the weather makes the news. This obscures the truth about El Nino which certainly does have a substantial effect on the climate of countries bordering the Pacific Ocean, and a limited and unquantifiable effect on more distant regions.

Given the geographical location of ENSO events, it is logical that the most clear-cut repercussions will be in countries bordering the ocean in these latitudes. Thus floods and mudslides in Ecuador and Peru, and drought in Indonesia and New Guinea, are generally regarded as being inevitable consequences of each ENSO event.

Knock-on effects are not quite as reliable. The most often quoted of these are major winter storms at relatively low latitudes in the north Pacific leading to gales and floods along the western seaboard of the USA, especially in California, a reduced number of hurricanes in the Caribbean in late summer and autumn, drought and bush fires in eastern Australia, reduced reliability of monsoon rains in the Indian sub-continent, heavy rains leading to widespread flooding in the Horn of Africa, and prolonged drought in southern Africa. During the last major ENSO event in 1997-98 most of these came to pass, the exceptions being the drought in southern Africa and the weak monsoon rains in India.

Like any other major atmospheric fluctuation, one El Nino is never quite the same as another. There are differences in intensity, geographical extent, and length of life, so the repercussions are likely to vary from event to event. As far as western Europe is concerned we are about as far away from the source of ENSO events as it possible to get on the planet; like the ripples which gradually diminish as they spread out after a stone has been dropped in a pool, the effects of El Nino have almost died away by the time they reach us, and they are easily swamped by other phenomena. Thus in our part of the world El Nino is just one small contributor among many to our weather, and to try to isolate any direct influence is a painstaking exercise. So if you read in your newspaper or see on your television that abnormal weather events in Europe this summer are caused by El Nino, write a letter to the editor.

Related features:
Weather Facts - El Nino
North Atlantic Oscillation
Southern Oscillation