Influence of multidecadal hydroclimate variations on hydrological extremes: the case of the Seine basin

Abstract. The multidecadal hydroclimate variations of the Seine basin since the 1850s are investigated. Given the scarcity of long term observations of hydrological variables, a hydrometeorological reconstruction is developed based on an method that combines the results of a downscaled long-term atmospheric reanalysis and local observations of precipitation and temperature. This method improves the representation of daily flows as well as at longer time steps. This reconstruction provide therefore an interesting tool to study the multidecadal hydroclimate variability of the Seine basin, as well as its possible influence on extreme hydrological events. Based on this reconstruction, it is shown that the Seine river flows, groundwater and soil moisture, have been influenced by multidecadal variations from the 1850s. Spring precipitations play a central role by directly influencing the multidecadal variability of spring flows, but also soil moisture and groundwater recharge, which then modulate summer river flows. Groundwater controls a large part of the multidecadal variations in river flows, particularly in summer and fall. These hydroclimate variations seem to influence extreme hydrological events. The positive multidecadal phases indeed appear to be more conducive to flooding, with twice as many flood days as in the negative phases while the negative multidecadal phases seems to influenced the droughts intensity. These hydroclimate variations over the Seine basin are driven by anomalies in large scale atmospheric circulations, which themselves appear to be influenced by sea surface temperature anomalies over of the North Atlantic Ocean and the North Pacific Ocean.