Tropical cyclones (TCs), also known as hurricanes and typhoons, generally form at low latitudes and have their impacts in tropical regions to the north and south of the equator. They start as clusters of individual thunderstorms that rotate weakly around a common axis but transition into a cyclone over weeks, days or even hours. Their formation is associated with the warm waters of tropical oceans, but far enough away from the equator to allow the Earth’s rotation to cause them to spin up into the spiraling vortices so typical of a cyclone.
Once formed, tropical cyclones generally move westward and poleward before they interact with mid-latitude westerly winds, known as jet streams, that circle the planet. But as the climate warms, temperature differences between the equator and the poles will decrease, which may cause weakening or even a split in the jet streams; this may allow conditions to arise in the mid-latitudes where tropical cyclones can form and intensify.
In a review article published recently in the journal Nature Geoscience, a team of scientists led by Joshua Studholme from Yale’s Department of Earth and Planetary Sciences suggests that the 21st century will see an expansion of hurricanes and typhoons into mid-latitude regions, which includes major cities such as New York, Boston, Beijing, and Tokyo. These are some of the most populous cities in the world and tropical cyclones that reach them could cause untold death and damage.
For the study, Studholme and colleagues analyzed numerical simulations of warm climates from Earth’s distant past, recent satellite observations, and a variety of weather and climate projections, as well as the fundamental physics governing atmospheric convection and planetary-scale winds.
They noted that the configuration of the continents during the Eocene epoch (56 to 34 million years ago), along with the warmer climates, extratropical humidity, CO2 concentrations and shifts in the jet streams would have favored poleward expansion of the areas favorable for TC formation. Eocene simulations thus showed that TCs most probably formed in the subtropics (~25° latitude) in both hemispheres, in contrast with the situation in the modern era.
According to the authors, contemporary TC distributions were probably established towards the end of the warm Pliocene epoch, some 3 million years ago. This has remained largely stable since that time but today, as the planet warms due to anthropogenic greenhouse gas emissions, the conditions for a poleward movement of tropical cyclones could once again be established.
“This research predicts that the 21st century’s tropical cyclones will likely occur over a wider range of latitudes than has been the case on Earth for the last 3 million years,” Studholme said.
In fact, the 2020 subtropical storm Alpha, the first tropical cyclone observed making landfall in Portugal, as well as this year’s Hurricane Henri, which made landfall in Connecticut, may already provide evidence of the trend predicted in the study.
Although scientists disagree about whether the number of tropical cyclones will increase as the planet warms, evidence certainly suggests that they will become more intense.
“There are large uncertainties in how tropical cyclones will change in the future,” said Alexey Fedorov, a professor of oceanic and atmospheric sciences at Yale. However, multiple lines of evidence indicate that we could see more tropical cyclones in mid-latitudes, even if the total frequency of tropical cyclones does not increase, which is still actively debated. Compounded by the expected increase in average tropical cyclone intensity, this finding implies higher risks due to tropical cyclones in Earth’s warming climate.”
“The core problem when making future hurricane predictions is that models used for climate projections do not have sufficient resolution to simulate realistic tropical cyclones,” explained Studholme. “Instead, several different, indirect approaches are typically used. However, those methods seem to distort the underlying physics of how tropical cyclones form and develop. A number of these methods also provide predictions that contradict each other.”
The new study derives its conclusions by examining connections between hurricane physics on scales too small to be represented in current climate models and the better-simulated dynamics of Earth’s jet streams and north-south air circulation, known as the Hadley cells.
The authors of the review conclude that, since at least the 1970s, the planet’s TC distribution has moved unambiguously poleward, with latitudes of peak TC intensity migrating poleward at a rate of around 0.5° latitude per decade. Should this continue as predicted, these deep-tropical TCs will most certainly become a critical feature of Earth’s climate and a potential source of devastation to human life and property.
“This represents an important, under-estimated risk of climate change,” said Studholme.
By Alison Bosman, Earth.com Staff Writer