Investigating possible links between the North Atlantic Oscillation 3 and rainfall variability in northwestern France 4 over the past 35 years

In this paper, the temporal dynamics of precipitation in northwestern France in 8 relation to the dominant climatic pattern in Europe were investigated. The general trends 9 and the nonstationary behavior of the North Atlantic Oscillation (NAO) were described 10 using fractal analysis and Fourier spectral and continuous wavelet analysis of a NAO 11 index time series over the 1865–2004 period. The 44-year and 8-year multidecadal 12 components showed a clear increasing power during the second half of the last century. 13 The possible link between rainfall variability and the NAO was then investigated. The 14 links between the NAO and rainfall were not as obvious, as NAO-like components were 15 not easily retrieved in the precipitation records: Relationships between the NAO and 16 rainfall were very likely at certain timescales but were not systematically very obvious. 17 For instance, the characteristic quasi-biennal oscillation (QBO) of the NAO was barely 18 detected in precipitation, but a 6-year fluctuation beginning in the 90s was found to be 19 statistically significant at a 95% confidence limit After investigating a possible link 20 with the Southern Oscillation Index, the occurrence of this fluctuation in the beginning of 21 the 90s could be related to the observed shift of the QBO toward slightly higher 22 frequencies in the NAO time series. On the other hand, a modulation of the QBO by a 23 6-year interannual fluctuation would suggest the existence of a 6-year climate pattern 24 that could affect precipitation and, to a lesser extent, the NAO. Cross-wavelet analysis 25 between the NAO and precipitation revealed a loss in correlation across the 1970–2005 26 period, which seemed to be the fact of a QBO-like fluctuation. This loss of correlation 27 was related to the above-mentioned shift of the QBO and 6-year rainfall interannual 28 band since the 90s.