Abstract
Abstract. In this article, multi-decadal variations in the French hydroclimate are investigated, with a specific focus on river flows. Based on long observed series, it is shown that river flows in France generally exhibit large multi-decadal variations in the instrumental period (defined in this study as the period from the late 19th century to the present), especially in spring. Differences of means between 21 yr periods of the 20th century as large as 40% are indeed found for many gauging stations. Multi-decadal spring river flow variations are associated with variations in spring precipitation and temperature. These multi-decadal variations in precipitation are themselves found to be driven by large-scale atmospheric circulation, more precisely by a multi-decadal oscillation in a sea level pressure dipole between western Europe and the eastern Atlantic. It is suggested that the Atlantic Multidecadal Variability, the main mode of multi-decadal variability in the North Atlantic–Europe sector, controls those variations in large-scale circulation and is therefore the main ultimate driver of multi-decadal variations in spring river flows. Potential multi-decadal variations in river flows in other seasons, and in particular summer, are also noted. As they are not associated with significant surface climate anomalies (i.e. temperature, precipitation) in summer, other mechanisms are investigated based on hydrological simulations. The impact of climate variations in spring on summer soil moisture, and the impact of soil moisture in summer on the runoff-to-precipitation ratio, could potentially play a role in multi-decadal summer river flow variations. The large amplitude of the multi-decadal variations in French river flows suggests that internal variability may play a very important role in the evolution of river flows during the next decades, potentially temporarily limiting, reversing or seriously aggravating the long-term impacts of anthropogenic climate change.
Highlights
The role of internal low-frequency variations in the evolution of the climate system has recently received increasing attention, stemming from the societal need for relevant climate information on the few decades for planning and adaptation in the context of climate change
Multi-decadal variability in river flows over France in the instrumental period has been investigated
Multi-decadal variations in observed river flows have been noted. Those variations are clearly more important in spring, and have been shown to be associated with multi-decadal variations in precipitation that are themselves driven by large-scale circulation
Summary
The role of internal low-frequency variations in the evolution of the climate system has recently received increasing attention, stemming from the societal need for relevant climate information on the few decades for planning and adaptation in the context of climate change. Current models generally present some moderate deficiencies in capturing the exact spatio-temporal characteristics of the observed lowfrequency variations in the North Atlantic Ocean, and more serious difficulties in correctly capturing the associated hydroclimate impacts over land (Kavvada et al, 2013). This issue regarding hydroclimate variations is especially problematic since, due to the multiplicity of the uses for water and the tensions that often already exist between demand and resources, low-frequency fluctuations in continental hydroclimate, and in particular river flows, may have serious impacts for society
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