As Tropics Expand, Tropical Storms Follow

He discovered the trend by accident.

Jim Kossin, a NOAA scientist stationed in Madison, Wisconsin, started tracking tropical cyclones to settle a disagreement about the temperature at the bottom of the stratosphere. When he looked at all the data he’d gotten about cylcones’ positions, it was clear: they’ve been wandering.

Tropical cyclones, a category that includes hurricanes and typhoons, are rotating storms hundreds of miles wide. Once one forms, it gathers strength as water evaporates from the ocean’s surface; it hits a maximum intensity and then wanes and finally dissipates. What Kossin noticed was that over the last 30 years, even though there hasn’t been a change in the frequency or the peak strength of cyclones, they’ve been hitting that peak farther and farther from the equator. His research, published in Nature, showed that those points of maximum intensity are inching toward the poles at more than 30 miles per decade, putting communities at higher latitudes at greater risk of damage.

This slow exodus out of the tropics reflects changes in the climatic conditions that nurture these storms. Factors like water temperature, humidity, and the temperature difference between the bottom and the top of the lowest layer of the atmosphere combine to determine a cyclone’s maximum possible strength. Whether or not a storm ever reaches that theoretical ferocity, though, depends on the winds it encounters along the way.

“Think of them like a cylinder,” Kossin said. “They like to be vertical.” If there’s a lot of “wind shear,” meaning the wind’s speed or direction changes dramatically as you move upward, that cylinder gets disrupted. Kossin likens the effect of wind shear on cyclones to trying to move a phone book by pushing only on the top few pages. “You can push the phone book as a whole across the table and it's fine,” he explains, “but if you just push on the top and hold the bottom steady, it will shear.” That kind of shear will sap a cyclone’s strength.

In the deep tropics -- places like the southern Philippines -- the temperature and humidity are changing to discourage strong cyclones. Meanwhile, the wind shear is strengthening, weakening cyclones when they form. On the other hand, said Kossin, “if you move away from the equator towards higher latitudes, the opposite is true: the potential intensity is getting relatively stronger, and the shear is getting relatively weaker. The deep tropics are becoming less hospitable, and the higher latitudes are becoming less hostile to tropical cyclones.”

What’s really compelling to Kossin is that the cyclones’ shift seems to mirror another trend: the expansion of the tropics.

On a map, the tropics are defined by latitude lines. But their characteristic weather patterns are determined far above ground, by a tunnel of air called the Hadley circulation. Warm air rises at the equator and is propelled toward the poles before falling and curling back on itself at a northerly latitude near the Texas-Mexico border and a southerly latitude bisecting South Africa. Where this warm, dry air descends, deserts form, and the tropics end.

Since the late 1970s, though, the Hadley winds have been traveling a few degrees of latitude farther from the equator before dropping back to Earth. Explanations for this tropical expansion vary -- too much ozone pollution in the lower atmosphere, too little protective ozone in the upper atmosphere, greenhouse gases that heat up the air and allow it to travel farther -- but they’re all related to human activity.

It’s too early to conclusively demonstrate that this human-fueled expansion of the tropics is what’s propelling cyclones into new latitudes, but the two trends are strikingly similar. Cyclones are moving north and south at about the same rate as the tropics. When the tropical expansion temporarily sped up in the 1990s, cyclones hustled poleward more quickly, too. Expanding tropics will create more deserts, and the cyclones traveling with them will put new communities in harm’s way while depriving others of seasonal downpours. Together, these two trends mean that the lives of people living anywhere near the equator could look very different just a few decades from now.