Well, have you heard the one about how if it rains in Africa, a tropical storm hits Hawaii?
It turns out that one is true.
Last week, hurricanes Iselle and Julio trekked thousands of miles across the Pacific Ocean to a point where they had Hawaii in their sights.
Iselle lost power and struck Hawaii’s Big Island as a tropical storm, while Julio veered away in its final days to pass just north of the island chain.
The public’s first inkling of the storms was when the National Hurricane Center in Miami put them on its tracking maps in the Pacific Ocean well off the west coast of Mexico.
Both storms had histories a lot longer than that. The seeds for what would become Iselle and Julio were sown over Africa weeks before and reached the oceans with a push from an annual weather event called the African jet.
As spring arrives in the Northern Hemisphere each year, the difference in temperature between the Saharan Desert in North Africa and the cooler region along the Gulf of Guinea off the central part of the continent provides the ingredients for a river of air to form, according to the Hurricane Research Division of the U.S. Atlantic Oceanographic & Meteorological Laboratory.
From April to October, this jet becomes a conveyor belt of sorts, dragging thunderstorms and low pressure troughs west every few days and shoving them into the Atlantic Ocean.
Many meteorologists call this the “wave train.” And when it’s particularly active, satellite photos show large clumps of clouds lining up across Africa and the Atlantic. These waves create more than half of all tropical storms and weak hurricanes in the Atlantic and about 85 percent of all the “major” storms of Category 3 or greater, according to research by Chris Landsea, science and operations officer at the National Hurricane Center.
About 60 of these tropical waves form every spring and summer, and most never become tropical systems. The 30-year average is for 12 tropical storms and hurricanes to form each season in the Atlantic. However, some waves get a second chance after they cross Central America and find themselves in a new ocean -- the Pacific.
Most hurricanes that form off Mexico in the Pacific Ocean may have their start in Africa, according to research by Lixion Avila and Richard Pasch, senior hurricane specialists at the center.
This is what happened with Julio and Iselle, two storms born in Africa that failed to gain traction in the Atlantic only to become threats in the Pacific.
So, while it may be hard to measure the impact of a butterfly flapping its wings, it’s quite possible for a thunderstorm in Africa to ruin a perfectly good day in Hawaii weeks later.
To contact the reporter on this story: Brian K. Sullivan in Boston at firstname.lastname@example.org