3.1. River-Discharge Variability
shows the R values for the correlation between the winter mean climate indices and river discharges for the 18 river basins. The winter river-discharge variabilities in the Spey and Almond Rivers in the UK, at the more northerly latitudes, were well correlated with the NAO index (R = 0.56), and, in a lesser degree, with the SCAN index (R = 0.39–0.47); these R values dropped off dramatically south of 56° N, that is, south of Scotland. The WEPA index consistently showed the highest correlation with winter river discharge for the English, Welsh, and French rivers (Table 2
). The correlation with WEPA was particularly strong for rivers in the middle and south of the UK, with R > 0.72; the correlation was significant (p
< 0.01). In this region, winter river discharge variability also has a significant but weaker relationship with the EA (R = 0.46–0.59) and the POL index (R = −0.36–−0.43), except for the Thames Rivers.
French rivers were also characterized by a strong and significant correlation between WEPA and winter river discharge (R = 0.41–0.66). A significant but moderate correlation with the WR index (R = 0.27–0.38) was also found in this region. In contrast, there was an overall poor correlation between winter river discharge and the winter NAO index.
On the Iberian Peninsula, winter river-discharge variability was significantly and negatively correlated with NAO, with R increasing from the north (R = −0.44–−0.48 for the Ebro and Douro Rivers) to the south (R = 0.72 for Guadalquivir). In contrast, the relationship with WEPA and EA (significant for Douro and Tagus) decreased toward the south in this region, from R = 0.43–0.48 for the Dour River in the north to R = 0.07–0.12 for the Guadalquivir River in the south.
Therefore, the NAO and WEPA indices were the two main climate indices that best explained winter river-flow variability in western Europe, with WEPA proving the greatest R from 55° N (south Scotland) to 42° N (north Iberian Peninsula).
shows the temporal evolution of the normalized river discharge, WEPA and NAO for nine rivers. WEPA showed a large interannual variability, consistent with the alternating years of high and low discharge in the Welsh (e.g., Severn, Figure 2
a), English (e.g., Thames, Figure 2
a), French (e.g., Seine, Garonne, Rhône, Figure 2
c) and, to a lesser extent, north Iberian (e.g., Douro, Figure 2
e) rivers. Unlike NAO, WEPA (positive phase) captured the years characterized by important flooding events in these regions: the 2013/2014 floods in the UK (Figure 2
a), in which the Thames discharge reached its highest peak on record [46
]; the 2018 floods in France (Figure 2
c) that flooded the city center of Paris [48
]. NAO also showed some low-frequency variations but was mainly characterized by a marked trend toward the positive phase over the last few decades (Figure 2
b,d,f). This trend is in line with the general decrease of winter discharge of the Iberian rivers from the 1980s (Figure 2
f). In these southern latitudes, positive NAO phases corresponded to lower winter river discharges, while negative phases captured wet years (Figure 2
f) such as 2010 that was characterized by dramatic floods in the Guadalquivir basin [49
]. For rivers in the north of the UK, the positive (negative) NAO phases captured many, but not all, wet (dry) years (Figure 2
To gain further insight into the WEPA and NAO influences on the whole hydrological cycle, Figure 3
shows the monthly river flows from October to September of the years characterized by winters with positive (WEPA+/NAO+), neutral (WEPA0/NAO0), and negative (WEPA−/NAO−) phases of each index, calculated by averaging the river flow in the five years with the largest, closest-to-zero, and smallest values, respectively.
The Spey River in Scotland showed an obvious difference in river discharge for the winter months (DJFM) only between the NAO− years (44–65 m3
/s) and the NAO+ years (86–111 m3
/s) (Figure 3
a.1). In the case of the Welsh and English rivers, the differences in mean river discharge between the WEPA− and WEPA+ years (Figure 3
b.1–b.4) were remarkable in the winter months, and notable also in the spring months (April, May). For example, the mean discharge of the Thames River varied from 34 (WEPA−) to 234 m3
/s (WEPA+) in February; from 47 to 107 m3
/s in April (Figure 3
This was also the case for the French rivers; other than the major differences in the winter months (the mean discharge doubled between WEPA− and WEPA+ in most of the rivers), mean discharge was also slightly higher for the WEPA+ years in the autumn and spring months. This is related to the hydrological regime, characterized by secondary flow peaks from precipitation and snowmelt in these periods (Section 2.1
). The mean summer discharge of the Seine and Loire Rivers was also slightly lower during the WEPA− years (e.g., 177 m3
/s for Seine) than during WEPA+ years (348 m3
The mean river discharge of the Iberian rivers showed major changes between the positive and negative NAO phases from January to May (Figure 3
a.2,3). It is noticeable that the differences in mean discharge between the NAO+ years (e.g., 958 m3
/s for the Guadalquivir in February) and the NAO0 years (117 m3
/s) were much greater than the differences between the NAO− years (14 m3
/s) and the NAO0 years. WEPA can also explain the differences in the Douro discharge between wet and dry years in the winter and spring months. In summary, WEPA and NAO mainly control the multidecadal variability in the winter river discharge, but can also impact, to a lesser extent, the hydrological variability in the other seasons, depending on the hydrological regime of the river.
From a hydroclimatic point of view, western Europe can be divided into three regions: river flow variability correlated with winter NAO in the southern and far-northern latitudes, and with winter WEPA in the middle and northern latitudes, from 42° N to 55° N; this is consistent with previous work. Burt and Howden [29
] evaluated the interannual variability of seasonal discharge at 86 river-gauge stations in the UK but only found strong correlations with NAO at the northwest region for the winter discharges and, to a lesser extent, for the spring discharges. Lorenzo-Lacruz et al. [24
] found a significant response of river discharge to the variability in the NAO index across the entire Iberian Peninsula in winter and autumn, particularly in the Atlantic watershed. This result was corroborated by Trigo et al. [23
] for the Douro, Tagus, and Guadiana rivers. Chevalier et al. [27
] found a strong coherence between river discharges in the four main French basins (Seine, Loire, Garonne, and Rhone), suggesting a common external forcing on hydrological variability They also found common modes of variability at multidecadal scales between the NAO and river discharge in these basins. However, the WEPA largely outperformed all the other climate indices tested in the previous studies in explaining discharge interannual variability in the UK and French rivers [22
] and is the only index able to capture the wettest years. All these previous studies agreed that the river-flow variability is a direct consequence of the precipitation variability, which depends on the interannual shifts in the atmospheric dynamics on the North Atlantic region [7
3.2. Precipitation Variability
To help further understanding the climate influence on river discharge and precipitation in western Europe, Figure 4
illustrates the spatial distribution of correlations between the winter mean precipitation anomaly and the four main climate indices that best correlated with river discharge in this region (Table 2
), the WEPA, NAO, SCAN, and EA indices. The spatial patterns were consistent with the correlations found for winter river discharge. NAO and SCAN showed higher R with winter precipitation in northern Europe (up to 0.65 and −0.5, respectively, in Scotland). WEPA had the highest correlation in middle-to-south UK (R increased southwards from 0.40 to 0.85) and in France (R decreased southwards from 0.85 to 0.70); EA showed only moderate correlations in this region. On the Iberian Peninsula, WEPA (R ≈ 0.4–0.6) and NAO (R ≈ −0.4–−0.6) explained winter precipitation variability in the northwest, while NAO showed the highest correlation in the south (R up to 0.75). These results confirm the robust relationship between river discharge, precipitation, and the modes of large-scale atmospheric variability.
The spatial correlations between these main climate indices and the precipitation minus the evapotranspiration is given in Supplementary Materials
. Results show similar patterns than correlations with precipitation. The correlation with the NAO index in the Iberian Peninsula and east of France was nonetheless enhanced. This is probably due to the effect of air temperature on the evapotranspiration. According to previous studies (e.g., [16
]), the NAO explains the temperature variability over Europe by the advection of heat and the modulation of short-wave and long-wave radiation by cloud-cover variations.
The correlations with precipitation were slightly stronger than the corresponding correlations with river discharge, particularly in France, and showed clear north–south gradients. For example, the discharge of the Seine River had a lower correlation with WEPA than the southern French rivers (Table 2
) while the precipitation correlation in France decreased toward the south (Figure 4
a). The Thames River also broke the increasing correlation gradient toward the south between WEPA and river discharge (Table 2
). These results suggest that water-storage variability and other catchment characteristics, such as the altitude [29
] can also impact the interannual variability of river discharge.
To further emphasize the importance of WEPA to the hydrological regime of the western European rivers, Figure 4
e–g shows the spatial distributions of the climate indices that best explain the winter precipitation variability in western Europe. The optimum climate index was defined as the index with the greatest |R| related to the local winter mean precipitation. Consistent with previous studies [22
], if we disregard the WEPA index (Figure 4
e), the two optimum climate indices explaining winter precipitation in western Europe were NAO (north of 56° N and south of 43° N) and EA. When WEPA was included (Figure 4
f), this index outscored the other three indices in the west of Europe, from 41–42° N to 56° N. Compared to EA, WEPA had an increased correlation with inter precipitation, up by 0.3 and 0.45 in South UK and France, respectively (Figure 4
g), corresponding in an increase of R locally exceeding 180%.
Given that most of the previous studies used the NAO index to explain the precipitation variability in these regions [28
], Figure 5
shows the increase in R between the correlations of NAO (Figure 4
b) and WEPA (Figure 4
a) with winter precipitation. Compared to NAO, WEPA increased R by 0.8 in the southern UK, in the west of France, and in southern Ireland. The correlation with WEPA was also slightly higher in the Alps region (increase in R of up to 0.6), Belgium (up to 0.45), southern Germany (up to 0.30), the Cantabrian coast (northern Spain, up to 0.50), Tunisia (up to 0.30), and Ukraine (up to 0.30), which demonstrates that WEPA can also be used in these regions to address river-discharge variability.
To further gain insight into the hydroclimate patterns for both WEPA and NAO, Figure 6
shows the mean river-flow and precipitation anomalies for the years characterized by positive (WEPA/NAO+) and negative (WEPA/NAO−) phases of both indices, calculated by averaging the five years characterized by the greatest and smallest values, respectively. Storm tracks, SLP, and wind surface during these same years are available in Castelle et al. [34
] and help to provide physical insight into the atmospheric phenomenon. During the NAO+ years, weaker and stronger river discharges were observed at southern and northern latitudes, respectively (Figure 6
c). The opposite situation was observed during the NAO− years (Figure 6
d). The positive phase of the NAO has long been associated with deep low-pressures crossing between Greenland and Scotland, higher W–SW wind over 60° N [16
], and stronger precipitation over the northern latitudes (Figure 6
]. During the NAO− years, there was higher precipitation over the southern Europe and dry winters in the north (Figure 6
]. During the WEPA+ years, higher river discharges were observed through all western Europe, particularly from 41° N to 56° N (Figure 6
a). As shown in Castelle et al. [34
], the positive phase of WEPA reflects a southward-shifted and intensified Icelandic-Low/Azores-High dipole driving increased W–SW winds near 45°N across the Atlantic Ocean, bringing wetter winters at these latitudes (Figure 6
e). In contrast, during the WEPA− years, fewer storms [34
], lower precipitation, and river discharges were observed in this region (Figure 6