So global warming makes tropical storms more and more intense

Climate models predict that due to global warming, the strength of tropical cyclones, hurricanes and typhoons will increase. The future, however, is perhaps not the right tense. According to a study published in naturesIndeed, this trend would have already begun and would be evident in the data collected on the oceans over the last thirty years.

Because cyclones form where it’s warm

Cyclones are born in tropical regions at a perturbation, i.e. in a low pressure region, when the ocean water temperature exceeds 27 degrees. Temperature is a key variable, because the energy that powers a cyclone comes from the heat released by water vapor as it condenses. The first ingredient, therefore, is the presence of a thick layer of warm, humid air above the ocean. The second is the creation of air currents that move at the same speed and direction from the surface to the highest layers of the atmosphere. Again, warm, humid air is more unstable and therefore favors the formation of convection currents that reach the upper atmosphere. Finally, the last ingredient is rotation: this is imparted by the earth’s rotation through a force, called the Coriolis force, which is null at the equator but sufficient to trigger a vortex motion of the air already 500 km from it, and therefore in tropical regions.

The increase in intensity with global warming

Measuring the strength of the wind in the region of formation of a cyclone, and following its evolution, is not simple. Not with instruments that are used under normal conditions near the surface of the water, or with satellites. Firstly, because cyclones often form far from the coasts, secondly because direct measurement tools are limited to some oceanic regions and therefore do not allow the collection of sufficient statistics and finally because, often, they are unsuitable with respect to the atmospheric conditions and the force of the wind reached.

In the study, therefore, the relationship between the speed of ocean currents and the forces that winds exert on the ocean surface. Ocean currents near the surface, precisely in response to the rapid onset of strong winds, reach extremely high speeds in a tropical cyclone. The researchers measured wind speeds at the ocean surface using a vast network of floating devices called drifters, which move with ocean currents, collecting a record 84,110 measurements during tropical cyclones that occurred between 1991 and 2020.

It was clear that the ocean currents under tropical cyclones have strengthened in recent decades. An increasing trend in line with rising global temperatures and affecting all ocean basins where these storms occur. Thanks to good drifter coverage in the North Pacific Ocean, the authors also concluded that global warming would intensify cyclones of all intensities, although those classified as “weak” are the most common in this area. They are the ones that become tropical storms or category 1 hurricanes as they approach land.