Did you know that Tropical Cyclones sometimes go dancing? This interesting phenomena is called the Fujiwhara effect*, which can be seen this week in the Eastern Pacific Ocean off the coast of Mexico.

Hurricanes Hilary (right) and Irwin (left) in Eastern Pacific at 1400 UTC on 26 July 2017. Image courtesy of NOAA

Sometimes, when tropical cyclones get close to one another (within about 1,200km), they rotate around each other in an anti-clockwise direction (in the northern hemisphere). They tend to rotate around a point between them, rather like two dancers joining hands and spinning around, this is the Fujiwhara effect. If the Tropical Cyclones are of a similar size, then they can move around one another for perhaps up to a few days, then release and move away on their own paths, like the dancers letting go of each other’s hands.

If the Tropical Cyclones are of different sizes, then the larger of the two will tend to dominate, with the smaller one orbiting around it, similar to the way in which the moon orbits around the Earth. Sometimes, the smaller Tropical Cyclone will be “eaten up” by the larger one. The two systems then essentially merge, with the smaller storm dissipating and the larger storm remaining and moving away on its own. This is what is expected to happen through Thursday and Friday this week, with Hurricane Irwin being consumed by the stronger and larger Hurricane Hilary. Hurricane Hilary is then expected to move away to the northwest, before finally dissipating later this weekend.

Forecast track showing Hurricane Hilary and Hurricane Irwin as they dumbell round each other

When Tropical Cyclones dance around each other due to the Fujiwhara effect, it can be challenging for numerical weather prediction models to accurately represent the interactions that the two Tropical Cyclones have with each other. In fact, if you get three Tropical Cyclones coming within close proximity of each other, they can all interact in a complex fashion, making forecasts even more difficult. The accurate forecast of Tropical Cyclones is very important as these storms can sometimes have huge impacts on tropical regions if they make landfall in populated areas. Fortunately, in this case, Hilary and Irwin are forecast to stay well away from land, remaining instead over the open waters of the eastern Pacific, so are not expected to cause any significant impacts.

Further Information

For more information on Hurricanes Irwin and Hilary, including the official forecast tracks and warnings, please see the US National Hurricane Center’s website.

The Met Office routinely supplies predictions of cyclone tracks from its global forecast model to regional meteorological centres worldwide, which are used along with guidance from other models in the production of forecasts and guidance. We also provide updates on current tropical storms via @metofficestorms on Twitter.

*The Fujiwhara effect was named after Sakuhei Fujiwhara, who described it in his 1921 paper about symmetrical motions in the atmosphere (http://onlinelibrary.wiley.com/doi/10.1002/qj.49704720010/pdf – QJRMetS, October 1921, pages 287-292)