Cyclone Drift

Cyclone Drift Hurricane Isabel, 2003.
Photo Credit: NASA

Cyclones have been peaking in intensity closer and closer to the poles over the last 30 years.


Cyclones’ latitude adjustment. I’m Bob Hirshon and this is Science Update.

Tropical cyclones, which include hurricanes, are gradually peaking farther and farther from the Equator. This according to research led by the National Oceanic and Atmospheric Administration. MIT atmospheric scientist Kerry Emanuel says they analyzed satellite data back to 1980.

In most parts of the world, the latitude at which storms reached their peak intensity is expanding poleward: that is, it’s moving northward in the northern hemisphere and southward in the southern hemisphere, at a pretty good clip.

It’s not yet clear what’s causing this, but it seems to relate to shifting air currents in the tropics, which may be driven by climate change. Emanuel says the consequences for people on land will probably vary. But possible risks include stronger storms in higher-latitude coastal cities, and a shortage of heavy rain in tropical areas that depend on it. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

When you think of cyclones (or, more commonly, hurricanes, which are a kind of cyclone), the prime targets that come to mind include tropical islands, like those in the Caribbean or South Pacific, or southeastern American states like Florida, Georgia, and Louisiana. Certainly, cyclones can cause widespread damage in these places. But when intense tropical storms hit land closer to the poles, which aren't accustomed to them—as 2012's superstorm Sandy did on the New Jersey Shore and elsewhere in the Northeast—the effects can be especially devastating. This study suggests that storms like these may, in fact, be gradually drifting away from the tropics over time.

Gathering data for the study was difficult. Ground-based measures of cyclone intensity are inconsistent, since the storms are generally tracked much more closely when they make landfall in a populated area. So instead, the researchers turned to satellite data, which, at least since roughly 1980, are detailed enough to judge the peak of a storm's intensity even when that peak happens far offshore. 

The researchers here found that during that time, cyclones have been peaking in intensity closer and closer to the poles in both hemispheres, at the rate of about 35 miles per decade. The amount of poleward migration varies by region. The greatest migration was found in the northern and southern Pacific and South Indian Oceans. On the other hand, despite Sandy, there was no evidence that the peak intensity of Atlantic hurricanes has migrated in the past 30 years.

As to what's causing this, it may be related to a general phenomenon known as “tropical expansion.”  Since the 1970's, the area of the Earth that can be officially classified as “tropical” by climate standards has grown beyond its traditional borders. Scientists are still figuring out the causes of the expansion, but evidence suggests that human-generated greenhouse gases and air pollutants may contribute. The shift in tropical cyclone patterns may be one result of this particular aspect of climate change. 

Now try and answer these questions:

  1. What exactly has changed about cyclones in the past 30 years?
  2. How did the researchers measure this? Why did they choose that method?
  3. What are some of the risks posed by the changes in cyclone patterns?
You may want to check out these related resources:
Hear how hurricanes cycle through long periods of high and low activity in the Hurricanes podcast.
Go through the development of a hurricane with the Animated guides: Hurricanes tool.

Going Further

For Educators

The Science Netlinks collection Hurricanes points to lessons and resources about hurricanes, wind, and other weather.

The collection The Science of Weather focuses on different types of weather and the natural forces that cause them.

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