Watch the Fujiwhara Effect in the Northwest Pacific Ocean

Meteorologists worldwide are watching two cyclones in the Northwest Pacific begin their “dance” around each other. Tropical storm Kulap and typhoon Noru are beginning to endure a weather phenomena known as the Fujiwhara Effect.

The Fujiwhara Effect occurs when two cyclones rotate or “dance” around each other in a counter-clockwise direction (for the Northern Hemisphere). The two cyclones are actually rotating around a single point, or center, on an axis that connects the cyclone centers. Binary interaction of the cyclones can occur only if the cyclone centers are separated by a distance of roughly 1400km, or nearly 870 miles. The National Weather Service defines Fujiwhara Effect as the tendency of two nearby tropical cyclones to rotate cyclonically about each other.

Fuji July 24 Noru Kulap
Typhoon Noru (26.2N, 154.9E) and Tropical Storm Kulap (33.1N, 159.1E) dancing around each other in the Northwest Pacific just after midnight local Japan time on July 24, 2017.

The Fujiwhara effect is named after Dr. Sakhuhei Fujiwhara, who was the Chief of the Central Meteorological Bureau in Tokyo, Japan after the First World War. In 1921, he performed and studied the interaction and movements of water vortices. Dr. Fujiwhara concluded that if two water vortices spinning counter-clockwise came close enough to each other, each vortex would rotate around the other. In the atmosphere, if one vortex (tropical cyclone) is stronger than the other, eventually the smaller vortex (tropical cyclone) could get caught in the circulation of the larger one.

When the Fujiwhara Effect occurs, the intensity and track of a tropical cyclone can be greatly altered, creating a complicated and difficult forecast. In the Northwest Pacific, tropical storm Kulap will weaken and dissipate as it is engulfed by typhoon Noru over the next 24 hours.

According to Dong (1983), archived data from the National Hurricane Center portrayed the western Pacific Ocean to have a higher frequency of binary interaction than the Atlantic Ocean. During a 36 year period, 1946-1981, two spatially proximate storms averaged 1.5 annually over the western Pacific compared to .33 annually over the Atlantic. This is likely due to the Pacific having an overall higher tropical cyclone frequency compared to the Atlantic Ocean. Furthermore, Dong studied the Intertropical Convergence Zone (ITCZ) over the Pacific Ocean versus the Atlantic Ocean. He concluded the ITCZ is better defined in the Pacific basin, leading to higher tropical cyclone activity, compared to the Atlantic basin. A good majority of tropical cyclones in the Atlantic spawn from tropical waves that emerge off the West African coast.

1994 Ruth
Photo, via The Merger of Two Tropical Cyclones, of Pat and Ruth orbiting each other, approaching a distance of 200 nautical miles.

Another example of a classic Fujiwhara Effect occured during the last week of September 1994 with tropical cyclone Pat and tropical storm Ruth.

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