What Happens When Two Hurricanes Collide?

Eager to get in on all the hurricane action, the British tabloid press screamed of “DOUBLE HURRICANE HELL” as weather models showed the remnants of Hurricanes Maria and Lee merging to form a “Giant Atlantic SUPERSTORM.”

Apart from the fact that, as you may suspect, this apocalyptic vision is somewhat oversold – it’ll just be a tad windy and wet over the UK for a day or two – it is actually possible for two hurricanes to merge.

There’s not much point in worrying about Maria and Lee, though; right now, they’re non-threatening Tropical Storms or just fairly insignificant depressions that will peter out over the next few days. What does happen when hurricanes collide, though?

Cyclonic mergers are described by the Fujiwhara effect, named after the Japanese meteorologist who first described such a phenomenon back in 1921, using water vortices as an example. 

When two cyclones get near each other, they will engage in a bit of ballet, pirouetting around each other either in a counterclockwise direction (in the Northern Hemisphere) or clockwise (in the Southern Hemisphere).

Eventually, they will begin to spiral in towards a central point. This happens for one of two reasons: either diverging winds essentially push them together, or because of something called positive vorticity advection, which describes how regions of high spin (hurricanes, for example) migrate towards areas of low spin (the space between the two).

Meteorological term-o-the-day: #Fujiwhara Effect - The tendency of two nearby tropical cyclones to rotate cyclonically about each other. pic.twitter.com/UCnsy0MGuc

— NWS Spokane (@NWSSpokane) July 27, 2017

The hurricane with a larger vortex – the more powerful one – will often dominate proceedings, and the smaller hurricane will dance around it for a bit, before falling into it. What happens afterwards, however, is uncertain.

Normally, the chaotic interaction of two entirely different tropical storms – whose outer winds are often going in different directions – means that the merger acts as a breaker force. This tends to reduce the singular entity’s overall strength and blasts it into weak remnants.

This is what appeared to have happened in 2001, when Hurricane Gill interacted with Hurricane Henriette; Gil was disrupted and broke up, but not before consuming the remnants of Henriette.

On other occasions, the hurricane merger can fail and ricochet both to completely different directions to the paths they were originally on. Back in 1974, Hurricanes Kristen and Ione met up, but resisted each other and ending up bouncing apart, with the former being dragged northwest and the latter heading northeast.

Infrequently, you get a mix of  both. Back in 1995, four tropical waves formed in the Atlantic, which all eventually became storms – Humberto, Iris, Karen, and Luis. To some extent, the first three interacted with each other and influenced their formations and paths. Iris ultimately merged with Karen.

It’s possible that two tropical cyclones can emerge to become briefly more powerful, as their combined warm water vapor loads could fuel more precipitation formation and lower the central pressure even more – but it’s unclear whether this has ever happened in real life.

What’s more likely is that a hurricane dances around a stronger one, fails to merge, and as a result gets stronger itself. This took place in 2014, when TropicalStorm Karina spun around Hurricane Lowell and became a hurricane for some time – before eventually being cannibalized by Hurricane Marie.

Meteorological term-o-the-day: #Fujiwhara Effect - The tendency of two nearby tropical cyclones to rotate cyclonically about each other. pic.twitter.com/UCnsy0MGuc

— NWS Spokane (@NWSSpokane) July 27, 2017

In any case, hurricane mergers are rare. It happens roughly once a year in the Western Pacific, but once every few years in the Atlantic. This rarity, along with the fact that they become far more unpredictable when they collide, makes mergers potentially very dangerous – so it’s a good thing plenty of them result in a disruptive, rather than an empowering, effect.