Winds' effect on winters gauged

March 6, 1998

A transatlantic collaboration could soon be under way as scientists try to shine new light on the weather phenomenon responsible for bringing Britain its recent run of wet and warm winters.

American and European scientists are to meet in Florence in May to start investigating the North Atlantic Oscillation (NAO), the name given to the pressure patterns and associated winds over the north Atlantic. They want to see if recent changes in the system are the result of natural variability or human activity.

Pressure patterns over the north Atlantic have changed markedly over time. Some years the high pressure associated with the Azores has been unusually high and the Icelandic low unusually low; so the westerlies blowing between the two are abnormally strong and Britain experiences particularly warm wet winters.

Other years, the reverse occurs. Neither pressure pattern is marked, so the westerly winds are weaker and do not bring warm air from the Atlantic, making Britain's winters colder.

Richard Washington, of Oxford University's department of geography, says that since 1972 the NAO has been predominantly in positive mode, with extremes of pressure and strong westerlies, bringing warm and wet winters to Britain. This contrasts with the period from 1955 to 1972, when it was predominantly negative.

Mr Washington said: "It has always been variable, but it has not been as clearly separated into positive and negative before as in recent years. At the moment we do not know how much of the change in the NAO is due to natural variability or how much is due to increased carbon dioxide."

No one knows what makes the NAO change. Some believe it is forced by ocean temperature changes, others a result of ocean-atmosphere interactions.

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