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Effects of Global Climate Oscillations on Intermonthly to Interannual Variability of Sea levels along the English Channel Coasts (NW France)

Turki Imen, Massei Nicolas, Laignel Benoit, Shafiei Hassan

Oceanologia

2020

No. 62 (2)

226--242

This work examines the multiscale variability in sea level along the English Channel coasts (NW France) using a wavelet multiresolution decomposition of water level values and climate oscillations in order to gain insights in the connection between the global atmospheric circulation and the local-scale variability of the monthly extreme surges. Changes in surges have exhibited different oscillatory components from the intermonthly (~3-6-months) to the interannual scales (~1.5-years, ~2-4-years, ~5-8-years) with mean explained variances of ~40% and ~25% of the total variability respectively. The correlation between the multiresolution components of surges and 28 exceptional stormy events with different intensities has revealed that energetic events are manifested at all timescales while moderate events are limited to short scales. By considering the two hypotheses of (1) the physical mechanisms of the atmospheric circulation change according to the timescales and (2) their connection with the local variability improves the prediction of the extremes, the multiscale components of the monthly extreme surges have been investigated using four different climate oscillations (Sea Surface Temperature (SST), Sea-Level Pressure (SLP), Zonal Wind (ZW), and North Atlantic Oscillation (NAO)); results show statistically significant correlations with ~3-6-months, ~1.5-years, ~2-4-years, and ~5-8-years, respectively. Such physical links, from global to local scales, have been considered to model the multiscale monthly extreme surges using a time-dependent Generalized Extreme Value (GEV) distribution. The incorporation of the climate information in the GEV parameters has considerably improved the fitting of the different timescales of surges with an explained variance higher than 30%. This improvement exhibits their nonlinear relationship with the large-scale atmospheric circulation.

Linking sea level dynamic and exceptional events to large-scale atmospheric circulation variability : a case of the Seine Bay, France

Turki Imen, Massei Nicolas, Laignel Benoit

Oceanologia

2019

No. 61 (3)

321--330

In this study, the multi-time-scale variability of the South English Channel (case of the Seine Bay, North France) sea level and its exceptional events have been investigated in relation with the global climate patterns by the use of wavelet multi-resolution decomposition techniques. The analysis has been focused on surges demodulating by an envelope approach. The low-frequency components of the interannual (2.1-yr, 4-yr, 7.8-yr) and the interdecadal (15.6-yr and 21.2-yr) time- scales, extracted from 46-years demodulated surges, have been correlated to 36 exceptional stormy events according to their intensity. Results have revealed five categories of storms function on their correlation with the interannual and the interdecadal demodulated surges: events with high energy are manifested at the full scales while moderate events are only observed at the interannual scales. The succession of storms is mainly carried by the last positive oscillations of the interannual and the interdecadal scales. A statistical downscaling approach integrating the discrete wavelet multi- resolution analysis for each time-scale has been used to investigate the connection between the local dynamic of surges and the global atmospheric circulation from SLP composites. This relation illustrates dipolar patterns of high-low pressures suggesting positive anomalies at the interdecadal scales of 15.6-yr and 21.3-yr and the interannual scales of 4-yr while negative anomalies at 7.8-yr should be related to a series of physical mechanisms linked to the North-Atlantic and ocean/ atmospheric circulation oscillating at the same time-scales. The increasing storm frequency is probably related to the Gulf Stream variation and its weakening trend in the last years.