Saturday, September 23, 2017


The Cape Verde Conveyor Belt Warms Up

Hurricanes are immense heat engines. They can’t exist except in the warm waters of late summer.  They are like canaries in a coal mine, showing us the heat signatures that exists in nature.  This summer has been the hottest summer of the hottest year on record, and these hurricanes, while not the most numerous, are the largest, most powerful storms we have yet seen.

Three days ago Hurricane Maria plunged Puerto Rico into total darkness after striking the Virgin Islands and Dominica a day earlier.  It is the most powerful storm to hit these islands in 80 years.  Meanwhile, storm Jose was off the coast of New England beating the coastline with tropical force level winds.  In Florida we are still picking up after Hurricane Irma and dealing with floods and storm damage.  What do all these storms have in common? They were all Cape Verde hurricanes that developed off the coast of Africa and made the long trip across the Atlantic to our shores.

In a typical summer, we get hurricanes from two sources in the Western Hemisphere-the warm waters of the Gulf of Mexico-like this year’s Harvey-or as storms that make the long trip from Africa and pass the Cape Verde islands of its coast.  It’s not easy for a Cape Verde storm to make it all the way across the Atlantic without breaking up or going off course. In a typical season, usually only two Cape Verde storms make it across, and they are generally the largest and most powerful storms. This year we have three already and the season is just past half over.

The long trip a Cape Verde storm takes is like bowling a strike. The ball takes time to get to the pins, and there are obstacles.  If you didn’t pitch it just right, your ball might go to one side or the other, or it might hit the lane hard and slow down. It might even land in the gutters, and you get nothing.  Only if you guide it with sufficient power, speed, and accuracy will you get what you are trying for-a strike.

The cosmic bowling alley that is the eastern Atlantic pitches hurricanes our way in late summer, when the waters are sufficiently warm.  The lane starts at Cape Verde, off the western coast of Africa.  Hot Sahara winds, combine with African wet season rains to blaze across upper Africa, fed by heat and sand.  The most powerful of the storms make it off the coast of Africa where they encounter the warm moist waters of the Eastern Atlantic. The two powerful forces merge and begin to spin, forming a depression. As the storm begins to cross the Atlantic, if it is shoved by favorable winds and conditions, the tropical wave becomes a depression, which becomes a tropical storm, and then a full-fledged hurricane as it picks up energy from warmer waters.

Lots of things can derail it-contrary winds, cooler waters it might intrude into, gouts of dry air, but if it navigates these potential barriers, it grows stronger and earns a name for itself.  It’s still not out of the woods at this point.  It can impact and be shredded by the mountainous islands of Puerto Rico, Hispaniola, and Cuba.  It can be pushed away by fronts of high pressure air over the continental US, or contrarily can be pushed into the US by the Bermuda High, a ridge of high pressure air that ebbs and flows all summer.

Why is this year such a good year for Cape Verde storms?  No weak highs near Bermuda to steer them harmlessly out to sea.  No cross winds blowing off the top of the spinning hurricane.  And no cold water to choke off a storm’s power.  This year the water in the Caribbean and the Gulf and the Gulf Stream off the coast of Florida is the warmest water ever recorded.  The unprecedented warm water in the Eastern Caribbean took a weak Tropical Storm Maria and turned her into a Category 5 storm in just 27 hours.  That’s some serious fuel.

The scientists who study climate have measured the ocean temperatures and they are warming fast, as predicted by global warming theory. So with warmer water on tap, we are likely to continue to see the Cape Verde conveyor sending the hemisphere’s most powerful, most dangerous hurricanes our way.

Sources:

“What is a Cape Verde Hurricane?” http://www.aoml.noaa.gov/hrd/tcfaq/A2.html




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