Cross wavelet analyses of annual continental freshwater discharge and selected climate indices

It is now well recognized that large scale atmospheric and oceanic circulations fluctuations have a strong impact on the global hydrological cycle. Because of the complexity of the hydrological processes over continental surfaces, it is often proposed to deduce this variability from insightful analysis of river discharge time series since it integrates a large number of climatological parameters such as precipitation, temperature and evapotranspiration but also land cover evolutions. However, up to now these studies remain restricted to basin or regional scale and no larger scale of these relationships between climate and continental hydrology has yet been proposed. Such relationships are helpful in the global understanding of the non-stationary relationships that exist between ocean and atmosphere mean conditions and freshwater discharge integrated at a continental scale. Based on a recent estimation of annual continental freshwater discharge over the 1876–1994 interval, a cross wavelet analysis (including cross wavelet spectrum and wavelet coherence) together with selected climate indices (North Atlantic Oscillation – NAO, Arctic Oscillation – AO, Southern Oscillation Index – SOI, Pacific Decadal Oscillation – PDO and NINO3.4 – Pacific mean Sea Surface Temperature) is exposed. The relationships are characterised by a high temporal unstationarity but three main bands of variability are identified and analyzed: 2–10-year, 10–20-year and 20–30-year variability. The five continents exhibit a temporal correlation with the five indexes sometimes on the entire interval but more often on restricted intervals. NAO and AO impact the 4–15-year variability of the five continents discharge. SOI impacts the 2–7-year variability of all continent discharge variability except Europe. NINO3.4 impacts the 2–8-year variability of Africa, Asia and South America discharge but also the 10–20-year variability of Africa, Asia, North America and South America discharge variability. Finally, PDO impacts the ∼10–30-year variability of Asia, Europe, North America and South America discharge variability. These results could allow a first order prediction of the future evolutions of continental water resources from a climate change point of view in relationship with climate scenarios of evolution of atmospheric and sea surface temperature conditions.