Some phases, such as the beginning and duration of the growing season in northern
and central Europe, correlate very well with the NAO index. The NAO is an
atmospheric circulation index describing a major driving force of the Northern
Hemisphere climate system, which largely determines the interannual variation of
winter temperatures in the northern Atlantic region (Hurrell, 1995). NAO reflects
the weather conditions in winter, as such a positive index is linked to warm wet conditions
in northern and central Europe. Phenological phases in these areas respond
to higher NAO indices by earlier spring onset (advancing onset of leaf unfolding,
flowering and a lengthening of the growing season) (e.g. Chmielewski & R¨otzer,
2001; Menzel, 2003a). Spatial studies for central (Scheifinger et al., 2002) as well as
central and eastern Europe (Aasa et al., 2004) indicate a decreasing influence of the
NAO with increasing distance from the North Sea. Menzel et al. (2005) also reveal
that the rate and the pattern of spring and summer progression through Europe is
linked to the NAO. Figure 4.15 demonstrates this influence of NAO index on the
spring progression in Europe: the patterns of mean late spring onset in Europe for
the 10 years with the highest NAO index and those for the 10 years with the lowest
NAO index (November–March) are clearly different. Progress of spring phenological
events run southwest to northeast in years with high NAO index and south to