Storminess in northwest Europe: an evaluation of correlations between the meteorological wind record and the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) characterises the latitudinal pressure gradient between the Arctic and subtropical North Atlantic and its index is calculated as the difference in normalised sea level pressures between Iceland and the Azores. The NAO index, and in particular the winter index (which refers specifically to winter months between November and March), is used widely as a measure of regional climate in the North Atlantic. Since the NAO can be calculated back to the 1500s (Luterbacher et al., 2002) it provides a convenient substitute for the instrumented wind record, which is often limited to the 20th century. There is a general consensus that sustained positive phases in the winter index correspond to wetter, stormier weather over Europe, whilst negative phases are associated with drier, calmer weather. As such, the winter index is increasingly employed as a proxy for storminess where an evaluation of storm-forcing, such as that associated with the analysis of coastal change, is required. Although several studies have considered the NAO in the context of North Atlantic storminess, there has been little systematic analysis of the associations between NAO indices and the instrumental wind record. Process-based studies are usually most interested in the influence of specific high energy events that are capable of effecting significant geomorphic change. But it is not clear how appropriate it is to use the NAO winter index as a measure of local forcing over the longer time scale. Here, we analyse the wind record from various northwest European sites to examine the robustness of the association between storminess, wind climate and the NAO. We calculate storminess on the basis of a number of previously used measures, such as winter gale day frequency or storm event frequency, and assess their correlation with the NAO winter index. Further, we examine the relationship of the whole winter wind record with the NAO winter index. Results show that associations are spatially variable, but more importantly that the links between wind climate and the NAO are fundamentally driven by the frequency of winds from the southwest and northeast. Measures of storminess can be strongly linked to the NAO, but only where wind direction is predominantly from the southwest or northeast. The results of this study have important implications for the reconstruction of wind climate and storminess over the historical timescale, the use of such proxies in the examination of wind-forced geomorphic behaviour, and for our understanding of coastal responses to climate variability and change.