Science & Environment

North Atlantic Oscillation 'amplifies' uplands rainfall

Rainbow (Image: BBC)
Image caption Westerly winds bring moist air over the UK, which in turn fall as rain - especially over the uplands

A climate system over the mid-Atlantic Ocean has a direct effect on rainfall and river flow level in UK uplands, a study has shown.

Data identified a link between a strong North Atlantic Oscillation (NAO) and up to 150% more precipitation.

The NAO influences the strength of winds over the Atlantic Ocean that blow warmer, moist air over the British Isles.

The findings appear in the journal Water Resources Research.

"The NAO is basically an index of how strong the atmospheric circulation is in the mid-latitudes of the Atlantic Ocean," explained co-author Tim Burt from Durham University's Department of Geography.

"It basically measures the pressure difference between the Azores and Iceland, so it is the pressure difference across what is known as the "polar front", which is where the jet stream is."

He told BBC News that a high index resulted in a "strong west to east circulation across the Atlantic, and that generates a lot of wet weather, particularly in the uplands."

He added: "When it is highly negative, as it was in March this year, then we tend to have high pressure tending to sit over Scandinavia and a pretty static circulation giving us cold, dry weather - in winter at least."

"The index is really telling you how strong or weak the circulation across the Atlantic is, and that's what brings us our weather in the UK and north-west Europe."

Seasonal variations

Fellow co-author Nicholas Howden from the University of Bristol explained that they were interested in identifying whether there was a link between the strength of the NAO and seasonal total of precipitation.

Image caption Climatic conditions over the ocean shapes weather conditions over UK uplands

Dr Howden added that they then looked to see if the increased rainfall affected river flows in the catchment areas.

"We were interested to find out whether there was any link between the strength of the NAO and the seasonal totals of precipitation across the UK, and whether that translated in to some effect on the seasonal river flow totals.

"What we showed was that in the presence of the NAO, precipitation levels increased markedly and that translates directly to river flow in upland areas," he told BBC News.

The pair said variations in the oscillation caused large differences in seasonal precipitation levels and triggered the meteorological conditions for "more frequent and intense winter rainfall" than NAO-neutral conditions.

The effects, the researchers said, were amplified with altitude, which they dubbed "double orographic enhancement".

Orographic enhancement is when air travels over the ocean, it gathers moisture. When it reaches land, the profile of hills and mountains forces the air upwards towards higher altitudes, causing it to cool and shed its moisture, falling back to the ground as rainfall.

Long-term data

The pair compared a range of long-running data sets to reach their conclusions, Dr Howden explained.

Image caption Low-lying developed areas can bear the brunt of sustained rainfall in nearby hills and mountains

"The longest record was 181 years, in Oxford. Most NAO data was available from 1825. [Many] of the records were about 100 years long. "

The study used precipitation records from 90 stations, with a mean length of data stretching back 72 years. The river flow records were gathered from 86 sites, which provided a mean data length of 45 years.

"This was looking at many of the longest records available to us, so it was not a little snapshot - it is a really long-term analysis," Prof Burt added.

Despite identifying a link between conditions over the mid-Atlantic and its effect on precipitation levels over the UK, Dr Howden said that it was not possible to use this information in short-term forecasts.

"There is a multi-decadal pattern within the NAO, but it is not predictable from one month to the next," he explained.

"So we cannot necessarily predict what the NAO is going to do for the next six months."

Prof Burt said the findings could prove useful to policymakers in a number of ways.

"One is really about water supply and the amounts of water available for public use," he observed.

"That will maybe be of greater interest in times of deficit when we are talking about droughts and reservoirs being empty.

"But clearly, if one has conditions that - say - provides a very wet winter then that inevitably will be under conditions that will be some flood events contained within that very winter.

"So I am sure that our results do have relevance for floods and flooding, and that is then exacerbated by inadequacies in planning systems and where people have been allowed to build."

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