INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 30: 2146–2154 (2010) Published online 29 September 2009 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.2022 Long-term hydrological changes of the Seine River flow (France) and their relation to the North Atlantic Oscillation over the period 1950–2008 † Nicolas Massei,* Benoit Laignel, Julien Deloffre, Johanna Mesquita, Anne Motelay, Robert Lafite and Alain Durand CNRS, UMR 6143 “Continental and Coastal Morphodynamics”, University of Rouen, Department of Geology, 76821 Mont-Saint-Aignan Cedex, France ABSTRACT: The variability of Seine River flow (France) was investigated using daily time series over the period of 1950–2008. The study aimed at characterizing the dominant modes explaining the variability of flow and at investigating their possible link with the North Atlantic Oscillation (NAO) as the dominant climate regime in the region. The Seine River flow was affected by a statistically significant increasing trend across the period. The hydrologic regime of the Seine River was found to be highly variable and seem to occur later in the year, approximately, since the end of the 1980s. Annual minimum, maximum and mean flow time series exhibited an obvious change around 1970. Continuous wavelet transform (CWT) of flow revealed energetic peaks highly localized in time, defining three time periods: before 1970, between 1970 and 1990 and after 1990. Two interannual modes (17 and 5–9 years) occur, respectively, around 1970 and in the early 1990s. The same observations on precipitation CWT ensure a climatic origin to these interannual modes. The annual oscillation was always strongly represented in flow and was affected by an increase in power from 1990 until the end of the time series. Comparison between NAO and annual flow anomalies revealed similar trend behaviour characterized by a change around 1970 towards more positive NAO indices and positive flow anomalies. In addition, CWT of NAO revealed similar modes of variability around 17 and 5–9 years as flow interannual modes. This highlighted a possible link between NAO and hydrometeorological processes in the region. The flow fluctuations (interannual modes and trend) expressing this potential link would explain approximately 23% of total variance of Seine River flow and would even reach 35% of the variance after removal of the annual cyclicity. These results would then emphasize the potential strong control of NAO on the variability of Seine River discharge. Copyright  2009 Royal Meteorological Society KEY WORDS NAO; Seine River flow; long-term trend; interannual variability; wavelet analysis; LOESS Received 15 April 2009; Revised 17 August 2009; Accepted 19 August 2009 1. Introduction In the present context of climate and environmental changes, the study of the modifications in hydrological characteristics on the long-term has become one major issue. More specifically, increasing knowledge on large- scale variations of hydrological variables and water cycle parameters and their implications on water resources availability is becoming crucial. In Europe, the North Atlantic Oscillation (NAO) is the dominant mode of the climate regime (Hurrell and Van Loon, 1997; Hurrell et al., 2003): the NAO refers to a north–south oscillation in atmospheric mass with centres of action near Iceland * Correspondence to: Nicolas Massei, CNRS, UMR 6143 “Continental and Coastal Morphodynamics”, University of Rouen, Department of Geology, 76821 Mont-Saint-Aignan Cedex, France. E-mail: nicolas.massei@univ-rouen.fr † This manuscript is a contribution to the special issue on hydroclima- tology organized by Dr. Glenn McGregor and Dr. Scott Curtis based on papers presented at the special session ‘Spatial and Temporal Trends in Hydrometeorological Records as Indicators of Climate Variability and Change’ during the 2008 Fall AGU meeting. and over the subtropical Atlantic from the Azores across the Iberian Peninsula. It can be defined as the normalized sea level pressure (SLP) difference between the Azores high and the Iceland low. It reflects the main fluctuation of climatic conditions in Europe and also affects the east- ern/northeastern coast of North America. In Europe, pos- itive phases of the NAO correspond to higher than usual high and lower than usual low, implying storm tracks to be localized to north: northern Europe undergo wetter weather conditions and conversely for negative NAO. In Northern France, Massei et al. (2007) investigated the potential links between rainfall variability and NAO fluctuations at a daily time-step. The authors pointed out the complexity of the relationship between the variations of NAO and precipitation, and associating the occurrence of a ∼6-year fluctuation in precipitation to a shift of the quasi-biennial component of NAO. Bradbury et al. (2002) in their study of the relationships between monthly NAO and streamflow in New England (East coast of the USA), suggested that such relationships would be better expressed in the lower frequency part of their respective Copyright  2009 Royal Meteorological Society