Northern Hemisphere climate variability during winter: Looking back on the work of Felix Exner

We present a brief commentary on the landmark study of Exner (1913), in which an analysis of the correlation between monthly anomalies of "polar pressure" and sea level pressure from numerous stations around the Northern Hemisphere during winter was presented. Exner's analysis still stands today as an excellent description of the spatial structure of the leading pattern of atmospheric variability over the extratropical Northern Hemisphere (NH); namely, the North Atlantic Oscillation (NAO). Moreover, Exner provided an accurate commentary on regional changes in surface air temperature driven by NAO variability. His quantitative analysis of these aspects represented a major step forward in the history of research on NH climate variability that, prior to his study, was primarily qualitative.