A nonstationary analysis for investigating the multiscale variability of extreme surges: case of the English Channel coasts

Abstract. This research examines the nonstationary dynamics of extreme surges along the English Channel coasts and seeks to make their connection to the climate patterns at different time-scales by the use of a detailed spectral analysis in order to gain insights on the physical mechanisms relating the global atmospheric circulation to the local-scale variability of the monthly extreme surges. The variability of extreme surges highlights different oscillatory components from the interannual (~ 1.5-years, ~ 2–4-years, ~ 5–8-years) to the interdecadal (~ 12–16-years) scales with mean explained variances of ~ 25–32 % and ~ 2–4 % of the total variability, respectively. Using the two hypotheses that the physical mechanisms of the atmospheric circulation change according to the timescales and their connection with the local variability improves the prediction of the extremes, we have demonstrated statistically significant correlations between ~ 1.5-years, ~ 2–4-years, and ~ 5–8-years and 12–16-years with the different climate oscillations of Sea-Level Pressure, Zonal Wind, North Atlantic Oscillation and Atlantic Multidecadal Oscillation, respectively. Such physical links have been used to implement the parameters of the time-dependent GEV distribution models. The introduced climate information in the GEV parameters has considerably improved the prediction of the different time-scales of surges with an explained variance higher than 30 %. This improvement exhibits their nonlinear relationship with the large-scale atmospheric circulation.