Hurricane Ida, which crosses the Gulf of Mexico, became a category 4 hurricane overnight from Saturday to Sunday. It is approaching the coast of Louisiana and continues to strengthen, with winds up to 209 km / h.
According to forecasts from the National Hurricane Center (NHC), "Ida is expected to be a major and very dangerous hurricane by the time it makes landfall along the Louisiana coast." The state had suffered in 2005 the ravages of Katrina, which had left more than 1 dead.
Given the devastating potential of such phenomena, it is imperative for societies to protect themselves from them, if they cannot be avoided. And the first of the protections consists in predicting as best as possible their intensity and their trajectory. This is what the cyclone forecasting services responsible for the affected areas do.
Hurricane Ida, the rapidly strengthening storm barreling toward Louisiana, could be one of the strongest to hit the state in more than a century, meteorologists and state officials warned on Saturday. https://t.co/pq1oHy40a7
- The New York Times (@timestimes) August 28
Hurricane, Typhoon, Tropical Cyclone
Tropical cyclones are the most devastating meteorological phenomena on the planet, due to the intensity of the winds they generate and the size of the areas they devastate; tornadoes can reach much stronger winds, but over areas of only a few tens of meters.
These phenomena take the name of hurricane in the North Atlantic and Northeast Pacific, typhoon in the Northwest Pacific and quite simply tropical cyclone everywhere else. If, in French, the term "cyclone" well designates these tropical phenomena, in English, it includes all the vortex systems, and in particular the usual depressions which regularly affect the middle latitudes.
Tropical cyclones belong to the large family of tropical depressions, their specificity resulting only from the intensity of the winds associated with them: above 63 km / hour, the system is identified as a tropical storm and takes a name in a alphabetical list. It is only from 118 km / h that the system enters the classification of tropical cyclones (category 1 out of 5 according to theSaffir-Simpson scale).
There are about 90 tropical storms per year on the planet, about half of which reach the tropical cyclone stage. Not all tropical cyclones have the potential to destroy a Katrina or Nargis (Burma 2008, 138 dead and 000 billion dollars). This depends primarily on the population density in the affected area. The cyclones which touch the emerged lands will see the number of victims and the most numerous damages. Only about fifteen annual tropical cyclones "land", to quote an English translation landfall.
- NASA Watch (@NASAWatch) September 2, 2019
The hurricane season for an ocean takes place between summer and autumn in the hemisphere concerned, with a peak of activity at the end of summer. For the North Atlantic, the official hurricane season runs from 1er June to November 30, for the Northeast Pacific, from November 15 to November 30. For the southwestern area of the Indian Ocean - including Météo-France at the Responsibility Law - it covers the period from November 15 to April 30.
As for the Northwest Pacific, there isn't really an official season as hurricane activity can occur throughout the year. The North Indian Ocean has a small peculiarity since the season is cut in two by the Indian monsoon which is rather unfavorable to cyclonic activity.
Note that these dates do not prevent the appearance of cyclonic phenomena outside the periods indicated, like Alex, who crossed the Azores archipelago from January 13 to 15, 2016. This case is interesting because, no only it occurred out of season, but it also formed in relatively cool water of 22 ° C, which is rather rare for a tropical cyclone. It is therefore one of the cyclones that form on surface water cooler than 26 ° C, a limit often invoked as a necessary condition for the formation of a cyclone.
The issue of climate change
In fact, if the line that delimits waters above 26 ° C does indeed envelop the zone of formation of tropical cyclones, it is purely factual. There is no physical reason preventing a cyclone from forming in colder water. It was during the 1950s that the Palmèn calculations (1948) on the stability of the atmospheric column led to popularize this threshold which characterizes the vertical temperature profile, rather than the simple sea surface temperature, necessary to trigger cyclones.
This question of the threshold of 26 ° C is not trivial. Indeed, with man-made global warming, sea surface temperatures are expected to increase by several degrees during the XNUMXst century.e century. If the surface were solely responsible for the conditions of cyclone formation, then one would expect a significant increase in cyclones in the future climate.
However, this is not what the most popular climate simulations suggest to us. more reliable. These simulations, although few in number because of the digital cost they generate (it is necessary model the atmosphere and the ocean on very fine grids), seem to agree on the fact that the global number of tropical cyclones should remain stable, or even register a slight decrease.
In fact, with warming, it is not only the surface, but the entire troposphere that warms up, leaving the vertical temperature profile almost identical which is the real catalyst for cyclonic phenomena.
Thus, the climatic indices which integrate the temperature of 26 ° C in the triggering conditions of tropical cyclones are doomed to give an erroneous picture of the impact of warming on cyclonic activity.
Nothing to fear from warming?
Yes, all the same, because the risks associated with cyclonic activity do not depend only on the number of phenomena identified; you only have to remember the 45 annual cyclones to see that not all of them cause damage, nor do the fifteen or so systems that "land".
Le 2013 IPCC report reports a tendency to intensify the most violent phenomena with global warming. Indeed, when the atmosphere heats up, its capacity to contain humidity also increases according to the Clausius-Clapeyron formula, which, during an extreme event, makes it possible to mobilize more energy linked to latent heat.
In the first place, this results in more rain, but in the case of cyclones, the release of heat feeds back to the winds to strengthen them. This does not necessarily translate into a systematic increase in the intensity of the systems, but major cyclones can take full advantage of this mechanism to strengthen themselves.
Another cause for concern relates to the storm surge associated with the cyclone. The cyclonic depression tends to raise the sea level. This can be accentuated by accumulation of oceanic water pushed by strong winds. This rise, combined with tidal phenomena, can cause major flood damage. Average sea levels are expected to rise over the next century, making storm surges even more devastating.
The extension of the hurricane domain
Apart from all the changes that have just been mentioned, the modification of the trajectories of cyclones could also become a matter of concern for the future because it would make vulnerable areas which are not today affected and therefore little prepared for this type of event. phenomenon.
To date, too few scientific results have been obtained to be able to give a reliable answer to the question of changing trajectories. We will have to wait a few more years to study this subject in more depth. Among the fears raised by warming, the extension to the poles of the tropics could extend the playground of tropical cyclones.
A recent study thus points to a trend observed in this direction over the past thirty years. It is too little to be able to support the hypothesis of the expansion of the zone of activity of the cyclones, but it is a subject which will have to be studied in depth in the near future.
To conclude, let us insist on the need for the robustness of the trends observed or forecast to allow an opinion on the evolution of a climatic phenomenon. The study of the evolution of tropical cyclones during the XXIe century is complicated by the gigantic need for computing resources necessary to represent these phenomena of small size on the scale of the planet.
With the rise of current computers, future simulations should multiply to answer all our questions.
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