4.3 Relations with large-scale atmospheric circulation
As the large scale climate drivers have high- and/or low-frequency time
variability, the correlation analysis should consider long time series.
That is why only rivers with at least 30 years of data during 1970-2010
have been examined. AMO and NAO are the most influential. At the annual
scale, the AMO was the driver with positive correlations for a
considerably high percentage of catchments, 21% for the longest annual
no-flow event and 23% for the annual sum. Results for both metrics in
seasons are presented in Table 2. The summer and the winter AMO are the
drivers with the strongest links to the summer and winter metrics. The
cluster of rivers positively associated with the winter AMO is well
recognizable in northern Europe (Norway, Finland) for both winter
metrics. The belt comprising Great Britain and south Sweden with a
negative link between the winter AMO and the winter sum is noticeable. A
similar pattern is observed for the summer and the annual AMO (Figure
7). Additionally, the southern, Mediterranean part of Europe might be
positively influenced by the AMO. The conclusion can be drawn that the
AMO is a potential driver of the intermittence and that its influence
can be stretched to the following season. The associations with the NAO
are less frequent. However, the apparent cluster of rivers with a
negative link to the NAO DFJ can be observed in the Scandinavian
Peninsula for both winter metrics (Figure 7). The rivers positively
linked to JJA NAO are scattered over Europe. It is worth emphasizing
that this large scale climate effect can be hidden by more local climate
conditions.
The relationship between climate drivers and hydrological
characteristics in Europe has been documented by many researchers (e.g.,
Valty et al. 2015, WrzesiĆski & Paluszkiewicz 2011). Results of the
analysis are to a large extent consistent with results obtained by
Giuntoli et al. (2013) where positive links between the annual discharge
volume deficit and the annual NAO and AMO were revealed in many rivers
in France, providing insights into the increase of drought severity.
Results are also consistent with Linderholm et al. (2009) who
reconstructed the summer NAO since the 19th century
and revealed strong links to droughts in the Eastern Atlantic region
from a multi-century perspective. Results are similar to Hurrell and
Folland (2002) who showed that the NAO, apart from its worldwide
importance in shaping the winter hydrological conditions, can affect the
summer hydrological conditions and might force droughts.