While many extreme weather events are thought to be becoming more frequent as a result of rising global temperatures, new research suggests that at least one is actually declining.
By the end of this century, the number of so-called Arctic hurricanes could be cut in half. Arctic storms are not actually true hurricanes, given their birth outside of the tropics, but they can be intense.
For these severe storms to develop, the atmosphere over the North Atlantic Ocean must be unstable, explained Matthias Zahn of the University of Reading, in the United Kingdom, and lead researcher on the new study. But if the air warms more quickly than the ocean surface, which climate models suggest will be the case in the future, then the Arctic atmosphere will be come more stable and less hurricane-friendly. (When warmer air sits below cooler air, it rises, causing water to condense, releasing heat energy and creating storms; but when warmer air sits atop colder air, as would be the case in this scenario, no air rises, so no storms form.)
Such a change would be welcome news for seamen braving the potentially dangerous seas at the top of the world.
"It's particularly interesting to know how the climate and weather systems in the Arctic are changing because of the anticipated increase in oil exploration and new shipping routes through the region [as a result of melting Arctic ice]," Zahn told OurAmazingPlanet.
Unfortunately, due to their relatively small size and transient nature, predicting and detecting Arctic hurricanes is extremely difficult. The fierce storms only span up to about 310 miles (500 kilometers) across, compared to the larger-scale tropical cyclones that form further south and can reach more than 621 miles (1000 km) wide. They also tend to subside quickly within 12 to 36 hours whereas tropical hurricanes can last for days, or even weeks.
For a mariner, this heavy precipitation and high Arctic wind with gusts to over 60 mph (97 kph) can seem to appear out of nowhere.
Using regional climate simulations, and a series of global climate change scenarios from the Intergovernmental Panel on Climate Change, Zahn and his colleague, Hans von Storch, analyzed 30-year slices of weather from both the 20th and 21st centuries. The results predict that the annual number of Arctic hurricanes will gradually decrease over time from an average of 36, based on data from 1960 to 1989, to below 20 sometime in the last three decades of this century.
Further, their findings suggest that the future storms that do strike will be shifted north by about 2 degrees latitude. This would be consistent with the northward movement of the edge of Arctic sea ice, which often offers the necessary climatic conditions to trigger a storm.
The models did not have the resolution to forecast future storm intensity.
While other researchers are currently investigating some potentially parallel patterns in the North Pacific, Zahn has moved on to studying changes in rainfall, a weather phenomenon that is expected to increase with global warming. The increase in rainfall would be a result of the fact that a warmed atmosphere can simply hold more water, he said.
So far, Zahn added, "Arctic hurricanes are the only kind of extreme weather events I know of for which frequencies are projected to decrease in the future."
The study is detailed in the Sept. 15 edition of the journal Nature.