Reconstructions of Late Holocene Storminess in Europe and the Role of the North Atlantic Oscillation
Orme, Lisa Claire
Date: 14 August 2014
University of Exeter
PhD in Geography
Winter storms can have devastating social and economic impacts in Europe. The severity of storms and the region they influence (southern or northern Europe) is related to the index of the North Atlantic Oscillation (NAO). However recent findings indicate that over the last millennium the relationship between the NAO and storminess ...
Winter storms can have devastating social and economic impacts in Europe. The severity of storms and the region they influence (southern or northern Europe) is related to the index of the North Atlantic Oscillation (NAO). However recent findings indicate that over the last millennium the relationship between the NAO and storminess varied and the forcings over centennial timescales are debated. Therefore in this research storminess has been reconstructed from NAO-sensitive regions of southern Europe (Galicia, Spain) and northern Europe (Outer Hebrides, Scotland), to investigate the Late Holocene NAO-storminess relationship and the causes of observed variability. Reconstructions were based on measurements of aeolian sand deposits within ombrotrophic peat bogs and a lake sedimentary archive from the Hebrides. The elemental composition of the lake sediments were analysed (using ITRAX XRF core scanning) to identify aeolian/in-washed sediment resulting from storms, as confirmed by correlations with instrumental data. As this is a relatively new technique there was a methodological focus on assessing its applicability for storm reconstructions and the maximum resolution achievable. It is concluded the reconstruction had a 10-year resolution (equivalent to 2-5 mm sampling resolution). The peat bog reconstructions span 4000 cal yr BP to present and indicate that there was a Late Holocene northward storm track shift. The results suggest that storminess was high in Galicia between 4000-1800 cal yr BP, after which it decreased and then gradually increased in the Outer Hebrides after 1500 cal yr BP. Comparison with an NAO reconstruction supports a consistent NAO-storm relationship through the Late Holocene. Orbital forcing is suggested as causing a steepening of the latitudinal temperature gradient and increasingly zonal circulation. Superimposed on this trend are centennial variations, which spectral analysis and visual comparisons suggest are primarily the result of solar minima (suggested as causing a weakened latitudinal temperature gradient and meridional circulation patterns), with some additional forcing from volcanic and oceanic changes. Therefore there has been a consistent storm-NAO relationship through the Late Holocene; however there appear to have been millennial and centennial shifts as the result of hemispheric circulation reorganisations.
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