Why this weather feature has so much influence on a hurricane’s path
If you’ve ever paid close attention to an Atlantic hurricane forecast, odds are you’ve heard chatter about the “Bermuda high.” In essence, it’s a large, sprawling zone of high pressure splayed across the Atlantic Ocean. It’s a key feature in steering hurricanes - and it can be a good thing or a very bad thing.
The position and strength of the Bermuda high are critical in hurricane track forecasts. Will a hurricane be able to recurve out to sea? Or will it be shunted farther west, threatening the Caribbean or entering the Gulf of Mexico?
Once in a while, the Bermuda high can even help storms swing up the Eastern Seaboard, impacting the entire East Coast with wind, rain or storm surge.
Let’s break down the Bermuda high and how it works.
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What is the Bermuda high?
The Bermuda high, sometimes called the Azores high, is a large, semi-stagnant, high-pressure system that spans most of the Atlantic Ocean. It brings sinking air and supports fair weather. That subsidence, or sinking, is what warms and dries out the Sahara. In the summertime, the Bermuda high strengthens and swells, shifting west and anchoring near Bermuda.
That induces southerly flow across the eastern U.S., scooping warmth and moisture northward. (Ever been to D.C. in the summertime and noticed the humidity? Blame this zone.)
The high acts as a force field, of sorts. Storm systems are deflected around the high, steered away by expanding air fanning away from high pressure.
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When the Bermuda high is weak
A weak Bermuda high tends to mean a weaker force field. Tropical cyclones that form in the Atlantic’s Main Development Region (MDR), or the zone between the Caribbean and the Cabo Verde Islands west of the African mainland, can slip out to sea more easily. The storms might be suppressed south and steered west initially around the Bermuda high’s periphery, but they recurve northwards sooner. That’s because a weaker Bermuda high is usually smaller or more compact; the force field doesn’t spread as far west, so the high’s zone of influence is smaller.
With a weak Bermuda high, the risk of impacts to the Caribbean or U.S. wanes. Storms often begin to ride north and out to sea even before they would impact Bermuda, missing the British overseas territory to the east.
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An average Bermuda high
The Bermuda high is constantly meandering; it wobbles west and east, and pulses in strength. If the Bermuda high is in an average position, Bermuda can find itself in the crosshairs of hurricanes.
Usually, a run-of-the-mill Bermuda high will allow a westward-moving hurricane to slip north just before impacting Puerto Rico or the Leeward Islands. That’s around the point when the system would start to curve northward. But Bermuda could get slammed as the hurricane pinwheels around the west side of the Bermuda high.
Bermuda is about 650 miles east of the Outer Banks; it occupies just 21 square miles of land. Since 1850, the centers of at least 57 hurricanes have passed within 60 nautical miles of Bermuda. At least six hurricanes have made a direct landfall on the archipelago.
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When the Bermuda high is strong
When the zone of high pressure is strong, that’s when we have issues. A stronger Bermuda high keeps a hurricane shunted south for longer, driving it farther west and threatening land.
Puerto Rico, the Lesser Antilles and the Bahamas all quickly come into play during a setup with a strong Bermuda high. And if the Bermuda high’s western edge is draped far enough west, a hurricane can make it all the way to the Southeastern U.S. before beginning its northward curve.
Sometimes, the Bermuda high might keep a hurricane suppressed southward long enough that a second high-pressure force field works in from the northwest. That can keep steering the storm westward, sometimes directly into the Lower 48. That’s what happened with Hurricane Hugo in 1989 and Hurricane Florence in 2018, which both made landfall in the Carolinas.
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Sneakier storms
Not all storms form over the open Atlantic. Some spin up in the Caribbean and Gulf.
Those in the Gulf don’t really depend on the Bermuda high. They spin up usually as a result of leftover lobes of vorticity, or spin, at the end of dying cold fronts. Other times, there’s a broader area of spin in the atmosphere (called a gyre) that can pinch off a more concentrated pocket of spin to cook up a hurricane.
In the western Caribbean, it’s similar. For the eastern Caribbean, it’s not terribly uncommon to have African easterly waves, or the same tropical waves that roll off the coast of Africa, make it all the way westward and then start forming. If the Bermuda high is extra-strong and a system doesn’t curve north early, it can surf the trade winds westward and then become a problem in the Caribbean.