For the past week and change, we've watched Hurricane Edouard swirl its way through the central Atlantic. Its track is thousands of miles long at this point, yet it's affected nobody but some ships and some planes and some fish. What causes hurricanes to make such grand curves out in the ocean?
The answer lies in two places: high pressure systems in the Atlantic or storm systems coming off the East Coast. We'll look at high pressure systems first, since this is what's causing Edouard to make its big oceanic curl.
Storms Recurving Around a High
Edouard began its life as many Atlantic tropical cyclones do, forming from a disturbance that made its way across sub-Saharan Africa and emerged into the Atlantic Ocean. Thanks to warmer waters and weakened wind shear, the disturbance developed a closed low at its center and it developed into a tropical depression. The depression continued to grow until Edouard reached major hurricane status a couple of days ago—the basin's first since Sandy back in 2012.
Edouard is currently at or close to its northernmost extent as it weakens and heads east towards the Azores, and it's expected to make a sharp turn to the south before it completely dissipates in a couple of days. The reason for Edouard's curvy, thousands-of-miles-long track is a well-developed high pressure system:
This particular high pressure system is known as an Azores high, and it can often dictate the steering for tropical cyclones if they develop far enough north after coming off the coast of Africa.
High pressure systems out in the Atlantic don't always protect the United States — sometimes if a high sets up near Bermuda, it could actually prevent a storm from turning out to sea and force it to hit the U.S. instead.
Troughs and Cold Fronts
Thanks in large part to good timing, a good number of storms that curve away from the United States are repelled by troughs and cold fronts—think of it like the atmosphere's citronella candle keeping away mosquitoes named Bill. Speaking of which, here's a look at 2009's Hurricane Bill, which turned away from the U.S. and grazed Atlantic Canada as a result:
In Bill's case, as with many other storms, the hurricane gets caught in between a trough over the East Coast and a high over the Atlantic, causing the storm to travel north until it has a chance to wrap east around the top of the high. Sometimes that doesn't happen in time, and that's often when the Canadian Maritimes gets hit by a storm.
At the moment, there's nothing of concern threatening the United States except for the remnants of Odile giving the southwest some heavy rain. Aside from that slight incident coastal North Carolina had with Hurricane Arthur this year, we've been fairly lucky. Hopefully that streak will continue.
[satellite image via NASA, maps by the author, surface pressure map by WeatherBELL with annotations by the author]