'SSW to NNE' - North Atlantic Oscillation affects the progress of seasons across Europe

Three European plant phenological network datasets were analysed for latitudinal and longitudinal gradients of nine phenological 'seasons' spanning the entire year. The networks were: (1) the historical first European Phenological Network (1882-1941) by Hoffmann & Ihne, (2) the network of the International Phenological Gardens in Europe (1959-1998), founded by Schnelle & Volkert in 1957 and based on cloned plants, and (3) a dataset (1951-1998) that was recently collated during the EU Fifth Framework project POSITIVE, which included network data of seven Central and Eastern European countries. Our study is most likely the first, for over a century, to analyse average onset and year-to-year variability of the progress of seasons across a continent. For early, mid, and late spring seasons we found a marked progress of the seasonal onset from SW to NE throughout Europe, more precisely from WSW to ENE in early spring, then from SW to NE and finally from SSW to NNE in late spring, as exhibited by the relationship between latitudinal and longitudinal gradients. The movement of summer was more south to north directed, as the longitudinal gradient (west-east component) strongly declined or was even of opposite sign. Autumn, as shown by leaf colouring dates, arrived from NE to SW. Possible reasons for the differences among the three datasets are discussed. The annual variability of latitudinal and longitudinal gradients of the seasons across Europe was closely related to the North Atlantic Oscillation (NAO) index; in years with high NAO in both winter and spring, the west-east component of progress was more pronounced; in summer and autumn, the pattern of the seasons may be more uniform.