The Coolest Meteorological Term You'll Learn This Week

Two tropical cyclones orbiting around each other in the northwestern Pacific Ocean on July 25, 2017.
Two tropical cyclones orbiting around each other in the northwestern Pacific Ocean on July 25, 2017.
RAMMB/CIRA

What happens when two hurricanes start to invade each other's personal space? It's easy to picture the two hurricanes merging into one megastorm that tears across the ocean with twice the fury of a normal storm, but what really happens is less dramatic (although it is a beautiful sight to spy on with satellites). Two cyclones that get too close to one another start to feel the pull of a force called the Fujiwhara Effect, a term that's all the rage in weather news these days.

The Fujiwhara Effect occurs when two cyclones track close enough to each other that the storms begin orbiting around one another. The counterclockwise winds spiraling around each cyclone force them to participate in what amounts to the world's largest game of Ring Around the Rosie. The effect is named after Sakuhai Fujiwhara, a meteorologist who studied this phenomenon back in the early 1900s.

The extent to which storms are affected by the Fujiwhara Effect depends on the strength and size of each system. The effect will be more pronounced in storms of equal size and strength; when a large and small storm get too close, the bigger storm takes over and sometimes even absorbs its lesser counterpart. The effect can have a major impact on track forecasts for each cyclone. The future of a storm completely depends on its new track and the environment it suddenly finds itself swirling into once the storms break up and go their separate ways.

We've seen some pretty incredible examples of the Fujiwhara Effect over the years. Hurricane Sandy's unusual track was in large part the result of the Fujiwhara Effect; the hurricane was pulled west into New Jersey by a low-pressure system over the southeastern United States. The process is especially common in the northwestern Pacific Ocean, where typhoons fire up in rapid succession during the warmer months. We saw a great example of the effect just this summer when two tropical cyclones interacted with each other a few thousand miles off the coast of Japan.

Weather Channel meteorologist Stu Ostro pulled a fantastic animated loop of two tropical cyclones named Noru and Kulap swirling around each other at the end of July 2017 a few thousand miles off the coast of Japan.

Typhoon Noru was a small but powerful storm that formed at about the same latitude as Kulap, a larger but much weaker storm off to Noru's east. While both storms were moving west in the general direction of Japan, Kulap moved much faster than Noru and eventually caught up with the latter storm. The Fujiwhara Effect caused Typhoon Noru to stop dead in its tracks, completely reverse its course and eventually perform a giant loop over the ocean. Typhoon Noru quickly strengthened and became the dominant cyclone; the storm absorbed Kulap and went on to become a super typhoon with maximum winds equivalent to a category 5 hurricane.

What Caused Pangea to Break Apart?

iStock.com/alfimimnill
iStock.com/alfimimnill

Emily Devenport:

There's another way to look at this question. People tend to think in terms of supercontinents forming and then breaking up again due to convection currents in the mantle, hot material rising and causing rifts in weaker spots, possibly in old sutures where the continents were shoved together—but what is really happening is that ocean basins are opening and closing, and the ocean has an active role in subduction.

The opening and closing of an ocean basin is called a Wilson Cycle. It begins when hot material rising from the mantle stretches the overlying crust. As molten material rises, a rift is formed. The rift is widened as material continues to squeeze into it. If that rifting goes on long enough, through a broad enough swath of a continent, ocean water will eventually flow into it, and an ocean basin begins to form. The upwelling of hot material will continue to rise through that thinner area of crust, pushing the plates apart. The Atlantic Ocean is an example of a basin that is well along in the Wilson Cycle; eventually subduction is going to begin at its margins, and the whole shebang will pivot.

This will happen because at the edge of continents, sediments accumulate. The weight of those sediments, combined with the weight of the water, drives the heavier, denser edge of the oceanic plate under the continental crust, which is fatter and lighter. Eventually subduction begins, and the basin begins to close again. The Pacific Ocean is an example of a basin that's closing.

If you look at a map of the oceanic rift zones, you'll notice that the one in the Atlantic is pretty much in the middle of that ocean, but the Pacific rift zone has been pulled all the way over to North America above Central America. Subduction is actively occurring on all margins of that plate.

The simple picture is that the continents are moving toward each other across the Pacific Ocean while the Atlantic Basin continues to widen. The truth is more complicated. When plates subduct, the water in the crust lowers the melting point of those rocks, so partial melting occurs. The partially melted material begins to rise through the overlying rocks, because it's less dense, and decompression melting occurs. Eventually, the upwelling of hot material forms plutons and volcanoes above the subduction zones. Fore-arc and Back-arc [PDF] basins can form. As the oceanic crust is pulled under the continental plate, island chains and other chunky bits get sutured to the edge of the continent along with sediments, making it larger. Our world is ~4.6 billion years old, so our continents are really large, now. They're unlikely to rift through the ancient cratons that formed their hearts.

What will happen if subduction begins on the eastern side of North America before the Pacific Basin closes? The margin next to California is a transform fault; it's not subducting. Will it eventually push itself under that part of North America again, or will the transform zone get bigger? The hot spot that was driving the ancient Farallon Plate under North America was eventually overridden by the southwestern states (Arizona, New Mexico, etc.) forming a rift zone. Will it continue to rift or poop out?

There are computer models predicting what supercontinent may form next. They will continue to change as our understanding of tectonic processes gets more accurate.

This post originally appeared on Quora. Click here to view.

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