What Happens When Two Hurricanes Collide?

Every June through November, the National Oceanic and Atmospheric Administration reminds the public that it only takes one hurricane striking the coast to make it an active hurricane season. But can you imagine two hurricanes striking simultaneously? On rare occasions, two tropical cyclones can actually track close enough to each other to pair up—an event known as the Fujiwhara effect.

The name for this phenomenon comes from Japanese meteorologist Sakuhei Fujiwhara, who was credited with first describing this hurricane interaction around 1920. (Although, according to the Japan Meteorology Agency, it's possible Diro Kitao was one of the first scientists to conceptualize the idea in the late 1800s.) 

History buffs will appreciate the fact that one of the first observed instances of two hurricanes merging came in the World War II era when typhoons Ruth and Susan delayed General McArthur's plans to land occupation forces in Japan in 1945. Today, though, the Fujiwhara effect remains a rarity. It only occurs about one to two times per year in the waters of the western North Pacific, and even less often—about once every three years—in the North Atlantic basin.

One of the most recent Fujiwhara interactions was observed in April 2021, when Tropical Cyclone Seroja fully absorbed Tropical Cyclone Odette just off the coast of Western Australia.

How Does the Fujiwhara Effect Happen?

A number of serendipitous events can encourage Fujiwhara interactions. If a basin is especially active, for example, tropical cyclones can crowd a particular region of the ocean. Troughs and ridges in the upper atmosphere, which act as barriers in a hurricane’s track, can also steer storms along similar paths, thereby increasing the chance that they’ll cross paths.

Even the speed of individual storms can lead to meet-ups. Fast-moving hurricanes can race ahead, catching up to storms that formed days earlier, while slow or stationary hurricanes can churn in place, waiting for passersby.

While each of the above situations helps position two tropical cyclones side-by-side, it’s the physical distance between them that determines whether or not they’ll interact. For the effect to take place, they must pass close enough to each other—a distance of roughly 900 miles or less, which is about as far apart as the state of California is long. Once two hurricanes become adjacent and spin this close together, one of several scenarios could unfold.

When Storms of Equal Intensity Meet

If the binary cyclones are fairly equal in strength, they’ll usually rotate around the area of ocean centered between them, spinning in a ring-around-the-Rosie manner.

Eventually, the pair will either have an “elastic interaction” in which they fling away, continuing on their own personal paths, or they’ll merge into a single storm. 

When a Strong and a Weak Storm Meet

If one hurricane dominates the other in intensity and size, the two storms will still "dance," however, the weaker storm will generally orbit the stronger storm.

As this rotation occurs, the larger cyclone can tear away part of its smaller neighbor, causing it to weaken slightly (a process known as “partial straining out”). Such was the case during the 2010 Atlantic Hurricane season when Hurricane Julia, which was a Category 1 storm at the time, flanked major Hurricane Igor too closely. Igor’s outflow battered Julia for a couple of days and eventually weakened it to tropical storm intensity.

The larger cyclone can also weaken the smaller cyclone to the point of dissipation (“complete straining out”). When this happens, the smaller cyclone is usually lost to the atmosphere, but the dominant storm can also partially absorb the weaker storm, growing slightly as a result.

Potential Impacts

As unsettling as the thought of twin tropical cyclones is, meteorologists stress that a megastorm scenario shouldn’t be expected—at least, not a megastorm the likes of which are portrayed in “Geostorm,” “The Day After Tomorrow,” and other disaster films. A low number of hurricane interactions result in the two storms merging. And even when storms do merge, the effects are rarely additive. That is, a Category 2 and a Category 3 storm won't necessarily combine to form a Category 5.

What coastal residents and vacationers should be aware of, though, is the possibility that Fujiwhara storms could trigger a last-minute change in a storm's path, since each storm affects the other's movement. And this means less of an opportunity to prepare before the storm, or storms, make landfall.