Earth's most powerful storms—sometimes called hurricanes or typhoons but collectively known to scientists as tropical cyclones—remain dangerously unpredictable. And what's most mysterious about tropical cyclones is what we would most like to know: how strong they are likely to become. I'm not talking about whether climate change is going to make hurricanes stronger or not (although we'd like to know that too). I'm talking about whether any given storm is going to intensify into a monster Category 5, boasting wind speeds above 252 kilometers-per-hour, such as Hurricane Andrew 20 years ago, or remain relatively weak, with wind speeds crossing the 119 km/h threshold, such as Hurricane Ernesto this year.

The forecast factor that we've been overlooking could be freshwater.

A hurricane draws its force from the seas over which these weather behemoths form. When sea surface temperatures are warm, hurricanes swell. When sea surface temperatures cool, tropical cyclones diminish. And every typhoon can become self-defeating—if the storm grows powerful enough, it will increase deep water mixing that then cools the ocean's surface and saps the storm. So mixing is a key variable in a storm's strength.

Now, a study reveals that freshwater influx from rivers or even previous intense rainfalls over sea can create barriers to such mixing. The result: tropical cyclones intensify 50 percent more over such regions than those without freshwater barrier layers.

The research, published August 13 in Proceedings of the National Academy of Sciences, started with an observation made during Hurricane Omar of 2008. This tropical cyclone swelled as it passed over the warm waters of the Caribbean. A pair of Argo autonomous buoys for monitoring the ocean happened to be in the vicinity: one 12 kilometers from Omar's path and one slightly farther at 22 km. The floats revealed that a deep freshwater barrier layer helped Omar gain strength, and the storm ultimately became Category 4—wind speeds between 209 and 251 km/h.

But that was just one storm, and such barrier layers are relatively rare—the chances of a storm hitting such a region are less than 25 percent. So the researchers analyzed a decade's worth of the paths that tropical cyclones took in the tropical Atlantic, Indian and Pacific Oceans. A total of 587 such storms between 1998 and 2007 revealed that those that passed over typical ocean waters gained strength at roughly 1 meter per second during a 36-hour period, while those passed over freshwater barrier layers spun up by more than 1.5 meters per second over the same time span.

In other words, forecasters should pay more attention to the structure of the ocean waters when trying to predict the future strength of a tropical cyclone.

Image: Hurricane Omar in 2008, courtesy of NASA.