No abstract

Northern Europe can be strongly influenced by winter storms driven by the North Atlantic Oscillation (NAO), with a positive NAO index associated with greater storminess in northern Europe. However, palaeoclimate reconstructions have suggested that the NAO-storminess relationship observed during the instrumental period is not consistent with the relationship over the last millennium, especially during the Little Ice Age (LIA), when it has been suggested that enhanced storminess occurred during a phase of persistent negative NAO.To assess this relationship over a longer time period, a storminess reconstruction from an NAO-sensitive area (the Outer Hebrides) is compared with Late Holocene NAO reconstructions. The patterns of storminess are inferred from aeolian sand deposits within two ombrotrophic peat bogs, with multiple cores and two locations used to distinguish the storminess signal from intra-site variability and local factors. The results suggest storminess increased after 1000 cal yrs BP, with higher storminess during the Medieval Climate Anomaly (MCA) than the LIA, supporting the hypothesis that the NAO-storminess relationship was consistent with the instrumental period. However the shift from a predominantly negative to positive NAO at c.2000 cal yrs BP preceded the increased storminess by 1000 years. We suggest that the long-term trends in storminess were caused by insolation changes, while oceanic forcing may have influenced millennial variability.

Future anthropogenic climate forcing is forecast to increase storm intensity and frequency over Northern Europe, due to a northward shift of the storm tracks, and a positive North Atlantic Oscillation. However understanding the significance of such a change is difficult because the natural variability of storminess beyond the range of instrumental data is poorly known. Here we present a decadal-resolution record of storminess covering the Late Holocene, based on a 4-m-long core taken from the peat bog of Cors Fochno in mid-Wales, UK. Storminess is indicated by variations in the minerogenic content as well as bromine deposited from sea spray. Twelve episodes of enhanced storm activity are identified during the last 4.5 cal ka BP. Although the age model gives some uncertainty in the timings, it appears that storminess increased at the onset and close of North Atlantic cold events associated with oceanic changes, with reduced storm activity at their peak. Cors Fochno is strongly influenced by westerly moving storms, so it is suggested that the patterns were due to variations in the intensity of westerly airflow and atmospheric circulation during times when the latitudinal temperature gradient was steepened. Document embargo 03/07/2016.Peer reviewe

21Changes in the strength and location of winter storms may cause significant societal and economic 22 impacts under future climate change, but projections of future changes in Northern Hemisphere storm 23 tracks are highly uncertain and drivers of long term changes are poorly understood. Here we develop a 24Late Holocene storminess reconstruction from northwest Spain and combine this with an equivalent 25 record from the Outer Hebrides, Scotland, to measure changes in the dominant latitudinal position of 26 the storm track over the past 4000 years. The north-south index shows storm tracks moved from a 27 southerly position to higher latitudes over the past 4000 years likely driven by a change from 28 meridional to zonal atmospheric circulation, associated with a negative to positive North Atlantic 29 Oscillation (NAO) shift. We suggest that gradual polar cooling caused by decreasing solar insolation 30 receipt in summer and amplified by sea-ice feedbacks, and mid-latitude warming caused by increasing 31 winter insolation, drove a steepening of the winter latitudinal temperature gradient through the Late 32Holocene, resulting in the observed change to a more northerly storm track. Our findings provide 33 palaeoclimate support for short-term observational and modelling studies linking changes in the 34 latitudinal temperature gradient and sea-ice extent to the strength and shape of the circumpolar vortex. 35