Extreme Weather: Changes in Storm Surges in Southern Europe Under Climate Change Scenarios

Marta et al. (2011) used a 2D (2-dimensional) ocean model to investigate changes in storm surges due to future climate change induced by increased concentrations of greenhouse gases. Three different climate scenarios (A2, A1B and B1, IPCC 2000, Special Report on emission scenarios) for the period 2000-2099, together with a control run for 1950-1999, have been simulated to study differences in intensity and duration of future storm surges in the region of the Mediterranean Sea and the northeastern sector of the North Atlantic Ocean. The ocean model was forced by atmospheric pressure and wind derived from a global variable resolution atmospheric model forced by three emission scenarios as described above.

The model was first used in a hind-cast mode for the period 1958-2001 to identify extreme events over the Mediterranean Sea. This hind-cast run, together with a control run, allowed past extreme events to be identified and provided a criteria to define moderate or strong events based on a rise of water level of 15cm or 25cm. Based on these runs, the moderate-to-strong event climatology shows a NE-SW gradient, with maximum values of 4-5 events/year in the Aegean and North Adriatic Sea. A minimum value of just 1 event/year shows up on the Libyan coasts. Localized maxima of moderate events due to water piling by wind (typically westerly to southwesterly flow) are found in coastal areas of the NW Iberian Peninsula.

Results projected for the 21st century suggest a generalized decrease in the number of positive (storm surge) events under all three emission scenarios. The spatial distribution of differences shows maximum frequency decrease in the northern part (Northern Adriatic and the Bay of Biscay) of up to -2 events/year and smaller changes in southern areas (up to -0.5events/year). Changes in negative storm surge events show an increase for all emission scenarios. Further, the simulation shows a progressive decrease in the 50-year return level of (moderate-to-extreme) storm surge events. Finally the location parameter of extreme distribution is found to depend on the NAO (North Atlantic Oscillation) index values for winter, especially for positive surges.

It must be noted here that storm surge events over most of the Mediterranean are modest, compared to storm surges in open oceans like the Atlantic or the Gulf of Mexico, where hurricane-induced storm surges can be very high and extremely dangerous. For example, the storm surge associated with hurricane Camille in July 1969 was over 4 meters at Pascagoula (Mississippi). This extreme storm surge led to the death of over 100 people along the Gulf coast of Mississippi in July 1969. Storm surges associated with tropical cyclones in the Bay of Bengal are also extremely dangerous for many low-lying areas of Bangladesh. In November 1970, a tropical cyclone (category 3 or 4) hit Bangladesh, killing an estimated 200,000 people! (largest human fatalities in a single weather related disaster).