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, WrzesiƄski & 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.