Much work has demonstrated the influence of global warming on the intensity of tropical cyclones. However, very few have looked at the effect on hurricane dissipation when they make landfall. A question greatly illuminated by work published in the scientific journal Nature on November 11.
Hurricanes that are more resistant to entering the land
After analyzing 50 years of data in the North Atlantic region, the researchers found that hurricanes were weakening at an increasingly slower rate. More specifically, on the first day they land, eddies decrease almost half as quickly today as in the 1960s. The trend is not linear as shown in the graph below. Nonetheless, it is undeniably on the rise and closely correlated with sea surface temperature in the tropical Atlantic.
To go beyond a simple association, the authors used very high-resolution numerical simulations to understand how a warmer sea could lead to slower decay systems. Indeed, the question is not trivial since once a vortex enters the land, contact with the ocean is abruptly cut off. One might therefore think that the two become relatively independent. However, simulations have confirmed that cyclones associated with warmer water weaken more slowly. But what is the mechanism involved?
Larger fuel tank in warmer climates
“Hurricanes are heat engines, just like the engines of cars. In-car engines, fuel is burned and this thermal energy is converted into mechanical work, ”says Lin Li, lead author of the study. “For hurricanes, moisture taken from the ocean surface is the fuel that intensifies and sustains the destructive power of a hurricane – the heat energy of moisture being converted into strong winds. Landing on earth is like shutting off the fuel supply to a car engine. Without fuel, the car will decelerate and without its source of moisture, the hurricane will decompose. ”
The evaluation of the models showed that in warmer climates, the vortices move with a greater stock of moisture. A phenomenon directly related to the fact that warmer air can contain more water vapor. The evaporation at the ocean surface is therefore increased – and so is the size of the reservoir feeding the engine. Once inland, it will therefore take longer for this reservoir to empty. As a result, the cyclonic circulation will absorb less quickly.
“The implications are very significant. Especially when you consider the policies put in place to deal with global warming, ”notes Pinaki Chakraborty, co-author of the paper. “We know coastal areas need to prepare for more intense hurricanes, but inland communities, who may not have the know-how or infrastructure to cope with such intense winds or heavy rains, should also be prepared. The destruction will no longer be confined to coastal areas, causing higher levels of economic damage and costing more lives ”. Finally, the next step will be to check whether these results can be generalized to other cyclonic basins around the world.