Abstract : This PhD analyses variability of sea surges and extreme sea-level in the Gulf of Lion and its relationship with atmospheric conditions at different spatio-temporal scales from 1905 to 2100. Sea-level height time series at four stations (Port-Vendres, Sète, Grau-de-la-Dent and Marseille) are used, as well as wind speed and directions at these stations and sea-level pressure (SLP) fields observed during the 20th century and simulated by ARPEGE-climat for the 21st century, according to two SRES scenarios (A2 and B2). Numerical time series of sea heights and surges at Grau-de-la-Dent, Sète and Port-Vendres have been digitized and validated on the 1986-1995 common period using NUNIEAU software, and daily sea level observed at 6h UTC at Grau-de-la-dent and monthly mean sea levels at Marseille have been used to analyse long-term variability on the 20th century.
Extreme sea surges (> 40 cm) in the Gulf of Lion are consistent at regional-scale, i.e. their temporal phase and amplitude are almost the same everywhere else, and mostly forced by southerly and south-easterly winds, associated with a strong zonal barometric gradient between a depression between UK and Iberian Peninsula and high pressure over Central Europe. This atmospheric circulation is mainly associated with “Greenland Above” (GA) and “Blocking” (BL) weather type and with the negative phase of the North-Atlantic Oscillation (NAO), which lead to a southerly shift of the main stormtrack across the North Atlantic. Local-scale sea surges occur when wind blows from N to NW, according to the local-scale coastal orientation (as in Marseille and Port-Vendres).
During the 20th century, the increase of extreme sea level heights at Grau-de-la-Dent (+ 0.31 mm/yr) corresponds to the superimposition of the mean sea-level rise (+ 0.20 mm/an) and of the increase of sea surge heights (+ 0.12 mm/an), linked with the long-term increase of the frequency and speed of southerly and south-easterly winds in the Gulf of Lions. Large-scale atmospheric anomalies (as the SLP rise over Central Europe and eastward shift of the SLP anomalies linked to NAO) increase the surge probability during GA weather regime and the negative phase of the NAO from 60s.
The frequency of sea surges in the Gulf of Lions is almost stationary during the 21st century in climate change scenarios A2 and B2, consistent with the quasi-constant frequency of low pressure travelling across the Bay of Biscay. But the rising mean sea levels, mainly forced by the sea warming will increase the risk of extreme sea levels. The frequency of sea surges > 40 cm (which is currently the geomorphological active level in the Gulf of Lions) could be over than 20% (30%) of days per winter in B2 (A2) at the end of the 21st century, if sea level rises according to the lower tail of the scenario. The current centennial sea level (> 1 m) could occur several times per winter at the end of the 21st century, even in the less pessimistic scenario of climate change, if sea level rises according to the upper tail of the scenario.