We just had a hurricane in the Atlantic Ocean. That wouldn’t be an unusual statement during the summer, but no, it’s the middle of January—the middle of winter!—and there was a full-blown hurricane in the Atlantic Ocean. (As of Friday morning, Alex weakened to a tropical storm as it made landfall in the Azores, off the coast of Portugal.) Stranger things have happened, but this one’s pretty high up there. Why did Hurricane Alex develop at such an odd time of the year? The dynamics came together just right, like nature winning the tropical lottery.
(National Hurricane Center’s forecast track for Hurricane Alex, January 14, 2016. | Source: Dennis Mersereau)
The National Hurricane Center (NHC), the official tropical forecasting branch of the U.S. National Weather Service, said that Hurricane Alex had 85 MPH winds on the afternoon of Thursday, January 14, 2016, with a minimum central pressure of 981 millibars. (Standard sea level pressure is 1013 millibars, so 981 millibars is decent for a category one hurricane.)
The low that would become Alex had a long life before turning into a hurricane a few hundred miles northwest of the African coast. The NHC first issued a forecast for soon-to-be-Alex back when it was still an extratropical cyclone near Bermuda on January 7, tracking it across the ocean until it formed into a subtropical storm (I’ll explain that below) and then an unlikely hurricane seven days later.
While the storm certainly didn’t take forecasters by surprise, it’s surprising in that hurricanes aren’t really supposed to happen in the Atlantic in the middle of the winter.
(Tropical cyclone climatology in the Atlantic Ocean. | Source: NHC)
The Atlantic Ocean’s hurricane season runs from June 1 through November 30. The climatological peak in hurricane activity is September 10; once fall cold fronts start sweeping through the United States and cold air pushes over the Atlantic Ocean, tropical activity drops off dramatically and it gets harder and harder for new storms to develop.
A “hurricane season” is little but a human construct—a range of dates on the human-made calendar that delineates a period of the year when our observations show that tropical systems are most likely to form in a certain ocean basin. But nature doesn’t always play by our rules, and nature does not define itself with our boxes. It is for this reason that nature sometimes leaves us scratching our heads, as it did when Hurricane Alex formed on January 14.
We’ve seen tropical storms or hurricanes form in the Atlantic in every month of the year—they’re least common in February and April (with only one system each on record)—and there have been a handful of systems in January. Two of the most well-known January storms actually formed on December 30 of the previous year. Tropical Storm Zeta, the last storm of the hyperactive 2005 Atlantic hurricane season, formed just before New Year’s Eve and dissipated on January 7, 2006. Alex is only the third known hurricane to exist in the Atlantic Ocean during the month of January, the other two being 1954’s Hurricane Alice (which was the other storm to form on December 30, surviving the first six days of 1955) and a brief, unnamed hurricane in the eastern Atlantic at the beginning of January 1938.
EXTRATROPICAL VS. SUBTROPICAL VS. TROPICAL
Visible satellite loop of Hurricane Alex on January 13 and 14, 2016. Image credit: NOAA
There are three types of large-scale cyclones (low pressure systems) that we deal with in the weather world. The most common type is called an extratropical cyclone, or one of those low-pressure systems that has cold and warm fronts swirling around its circulation. These systems, sometimes called “mid-latitude cyclones,” feed their energy from the jet stream, which creates strong lift in the atmosphere across a large area, leaving less air and lower air pressure at the surface. Most of our exciting weather is the result of extratropical cyclones.
A tropical cyclone, on the other hand, is one that’s completely warm and moist throughout the entire storm. Tropical cyclones are known as warm-core storms, as the cyclone gathers its strength from the powerful thunderstorms around the eye—the eyewall—which in turn feed off of the warm water below. Air rapidly rises through the thunderstorms in the eyewall, leaving very low air pressure at the surface. If dry air, strong winds, or cool water disrupt the thunderstorms in the eyewall, the storm quickly weakens.
A subtropical storm is sort of in between the other two types of cyclones, existing as a cyclone that’s shed its cold/warm fronts and consists of a warm core that still has some cold air left in the upper levels of the atmosphere. When conditions are favorable, a majority of subtropical storms in the Atlantic will fully transition into tropical entities, as Alex did. If you ever find yourself in the path of a subtropical storm, you won’t notice much of a difference—the difference between subtropical and tropical is mostly a concern for meteorologists and hardcore weather buffs.
WHY DID ALEX FORM?
(Sea surface temperatures, in °C, for January 13, 2016. | Source: NOAA/ESRL/PSD)
A disturbance over the water needs three main ingredients in order to turn into a tropical cyclone: warm water, low wind shear, and ample moisture. In this case, there was very little wind shear over the eastern Atlantic Ocean, and the low that would become Alex managed to ingest enough tropical moisture from the south that it insulated the system from dry air to its north and west.
That left the issue of warm water. The water isn’t all that warm in the northeastern Atlantic Ocean even in the middle of the summer, let alone the middle of January. Water temperatures are far below the levels one would typically expect to sustain a hurricane, but there’s a catch. Sharp temperature differences between air at the surface and air in the upper levels led to strong instability, allowing air to rise very quickly through the atmosphere. According to the NHC, yesterday the air high in the atmosphere above the hurricane was -60°C, which is extremely cold. This strong vertical temperature gradient is compensating for the lack of warm water, allowing thunderstorms to blossom and permitting Alex to defy the odds and become a hurricane.
What does this storm say about this summer’s hurricane season? Not much. The ongoing El Niño in the eastern Pacific Ocean is expected to wear off by this summer, bringing along the potential for a more active hurricane season than we saw in 2015. Fortunately, however, Hurricane Alex isn’t an omen for storms to come. This historical oddity was a one-off event, and it’s something we’re unlikely to see again for many years.