Complex North Atlantic Oscillation (NAO) index signal of historic North Atlantic storm-track changes

Previous reports have suggested a link between increased storminess in the North Atlantic during recent years with a period of time during which the North Atlantic Oscillation Index has been strongly positive. New analyses of late nineteenth-century gale-day data for meteorological stations in northern Scotland and western Ireland indicate that the relatively high storminess that characterized this period was associated with monthly NAO Index values that rarely exceeded +2 and, on several occasions, were strongly negative. It is speculated that this difference may reflect the influence of an expanded sea-ice cover in the Greenland Sea that caused a considerable southward displacement of the North Atlantic storm track during the late nineteenth century. Such changes imply that the polar atmospheric and oceanic fronts in the North Atlantic were displaced southward during the late nineteenth century.