Post-tropical cyclones, such as Fiona, will hit “much harder” on Canada’s Atlantic coast in the coming decades for a very simple reason: sea levels are rising due to global warming.
Destructive winds, torrential rains, gigantic waves: as we have seen in recent days, post-tropical cyclones generate devastating weather cocktails. However, the greatest damage caused by these events often results from “storm surges”, a kind of swelling of the sea that inundates the coast.
“The most significant impacts of Fiona were related to storm surges. And that, it’s clear that it will increase with the rise in sea level”, explains climatologist Alejandro Di Luca, professor at the University of Quebec in Montreal and member of the Intergovernmental Panel on the Evolution of the climate.
Storm surges are caused by the winds, which push the water towards the coast, and by low atmospheric pressure, which “sucks” the sea towards the sky. Both conditions were met last weekend in the Atlantic provinces. Winds of over 165 km/h were recorded. In addition, the lowest atmospheric pressure ever measured in Canada (less than 933 millibars) was recorded.
Record water levels were recorded at two locations: in Escuminac, New Brunswick, it reached 2.82 meters; and in Port-aux-Basques, Newfoundland, it was 2.75 m. Storm surges have added to the “base” sea level, which continues to rise due to the melting of the ice caps and the expansion of the water in the oceans.
Global sea level, which has already risen by about twenty centimeters since the beginning of the 20the century, will certainly continue to increase in the coming decades. Climatologists estimate that even under the most optimistic greenhouse gas emission reduction scenarios, sea levels will still rise about 40 cm by 2100.
Scientists are convinced that tropical cyclones (known as “hurricanes” in the Atlantic) will intensify due to climate change. Hurricane winds will therefore be more powerful. They will also climb a little further north, as far as New York for example, in their “tropical” form.
Things get more complicated, however, when it comes to understanding the effect of storms of tropical origin on Canada’s Atlantic coast. Because, at these more northern latitudes, hurricanes turn into “post-tropical cyclones”. It is no longer the heat of the ocean that supplies them with energy, but the north-south gradient of air temperature. Since the mechanisms involved are not the same, it is not yet known whether these post-tropical cyclones will, like hurricanes, bring stronger winds.
floods
However, says Raphaël Rousseau-Rizzi, a Quebec postdoctoral researcher at the University of Bern, Switzerland, the amount of rain associated with extratropical cyclones will increase because warm air can hold more moisture. “Extreme rainfall will become more extreme,” says this tropical cyclone specialist.
Notwithstanding this more abundant precipitation, it is really the rise in sea level that will increase the risk of flooding posed by extratropical storms on the Atlantic coast of North America, underlines Mr. Rousseau-Rizzi, who comes from to complete a research contract on this subject at the Massachusetts Institute of Technology.
With his colleague Ali Sarhadi, the young researcher assessed the risks of coastal flooding for different cities on the east coast of the United States. “We see that, for extratropical cyclones, the main cause of increased risk of flooding – by far – is the rise in sea level”, he summarizes.
“We should expect to have events of a magnitude at least equivalent to that of Fiona in the future, but these same events will produce much greater damage because the sea level will be higher”, repeats Professor Di Luca. These forecasts should encourage us to adapt now, he observes.
Adapting the coasts includes restoring the shoreline with plants, building rock structures to break the waves or, in certain particularly at-risk areas, outright relocating infrastructure and communities, explains a recent report on the impacts of climate change. climate in the Atlantic provinces.