What happens when two hurricanes combine?

typhoon melor and ts parma 2009, nasa
Typhoon Melor and Tropical Storm Parma in the Pacific, via NASA in 2009.

When hurricanes Maria and Lee were hanging out in the ocean together a little over a week ago, many people were wondering what would happen if the two storms combined. In fact, British tabloids were warning about “double hurricane hell” if the two storms merged and made their way toward the Isles. Rain, wind, destruction – the tabloids weren’t painting a pretty picture for our friends in the UK.

Fortunately (or unfortunately), it’s not time to call George Clooney – the perfect storm isn’t likely to form anytime soon. Two hurricanes merging into one storm, which is called the Fujiwhara Effect in meteorology, is very rare. This effect was named after the Japanese scientist who first discovered the phenomenon in 1921.

Fujiwhara used water vortices to demonstrate that when two cyclones collide, they’ll “dance” around each other until they reach a central point. Scientists have speculated that this happens due to one of two reasons: either their diverging winds will push them together, or one high-spinning vortex will gravitate toward a low-spinning vortex in a process known as positive vorticity advection. Basically, the two cyclones function similarly to a magnet, eliminating the space between them and creating one cohesive storm.

As you might have guessed, the larger storm will typically dominate during this “merger.” The smaller hurricane may “dance” around the larger storm until finally being “eaten” by the bigger storm. Yes, hurricanes can be cannibals. Do you think they’d go for a chianti?

Most the time, a hurricane merger results in a weaker final storm. While it could be possible that two cyclones could combine and become stronger, it’s far more likely to see a smaller storm travel around a larger storm, fail to merge, and then become more powerful itself due to water vapor consumption.

The Fujiwhara Effect happens approximately once a year in the western Pacific, and it occurs only once every few years in the Atlantic. This year, hurricanes Hilary and Irwin engaged in this dance, creating high surf in parts of Southern California. Because a hurricane merger does have the potential to create an extremely hazardous storm, we’re fortunate that most mergers result in disorganization, not strengthening.



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