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Journal Club: 20-11-2017

Article Title: North Atlantic Oscillation amplifies orographic precipitation and river flow in upland Britain

Published in the Water Resources Research journal in June 2013. Authors: T. P. Burt and N. J. K. Howden.

This article assessed the relationship between both precipitation and river flow and the North Atlantic Oscillation (NAO) in Britain. For this, the authors got station data from all over the UK mainland and computed the seasonal totals of precipitation and river flow. Afterwards they correlated every season’s total of those variables with the NAO index. Although it was previously known that the NAO has considerable effect on European precipitation, the authors detailed this knowledge for Britain, finding that NAO variations cause large differences in seasonal precipitation and river flow totals. Additionally, because the stations were relatively spread out through the region – including low land and upland areas – the authors found that the influence of the NAO on precipitation and river flow is higher on upland areas, an effect they named “double orographic enhancement”, and which has relevant implication to water management in the region (both for flood and drought situations).

Projections of precipitation(a)/flow change (b) [%] per NAO deviation from the neutral conditions. The x axis contains the winter precipitation/flow total, which is correlated with altitude (proxy for altitude, as demonstrated previously in the paper). The y axis contains the percentage of enhancement. The (c) and (d) plots were estimated for the highest (blue) and lowest (red) NAO index on record, based on the regression coefficients found previously in the paper for the rate of change of precipitation/flow per unit change of the NAO index, for every precipitation/flow total.  

 The authors also performed similar preliminary analysis on other areas of the world both influenced by large scale patterns and with orographic arrangements (Pacific North West of the USA and Sri Lanka, both influenced by the El Nino Southern Oscillation). This was done to confirm that the “double orographic enhancement” effect was also present in other locations facing a similar set of conditions as Britain (high interannual variability of precipitation due to large scale patterns and high spatial variability of precipitation due to wind direction, orography and rain-shadow effects).

 

Conclusions:

The North Atlantic Oscillation has considerable influence on interannual variability of precipitation in Britain, an effect which is amplified with altitude.

Relevancy for our research:

To some extent, understanding precipitation variability is understanding solar radiation variability (as both depend highly on cloudiness), which is one of the focus of our group. This work, although not directly related to our research, has given insights on how the impact of a large scale variability pattern such as the North Atlantic Oscillation on some meteorological variables can vary drastically over small regions, as a result of interaction between atmosphere and topography.