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Why This Year's Hurricane Season Has Been Quiet Until Now
Published: September 14, 2018
This year’s Atlantic hurricane season has been relatively tame until this week. So far, Florence is the lone landfalling U.S. hurricane in an otherwise slumberfest of a year where most of the storms that have formed have appeared on satellite images as weak, unorganized blobs as compared to the stunningly beautiful, perfectly symmetric buzzsaw monster storms of 2017.
In comparison, the 2017 tropical season was a brutal one. The first in a trio of ferocious hurricanes to hit the United States was Hurricane Harvey in late August, which blasted the Texas coast with winds in excess of 130 mph. Portions of the Houston metropolitan region recorded over 50 inches of rain. Harvey was followed by major hurricanes Irma and Maria, both of which achieved Category 5 intensity. Irma pounded the Caribbean before setting its sights on the U. S., bringing 130 mph wind gusts to Florida and damaging winds inland as far as north Georgia. Finally, Maria scoured Puerto Rico with a ferocity that killed nearly 3,000 people and left much of the island without power and running water for months. These three storms inflicted more than $200 billion in damage and brought a striking end to a several-years-long drought of major hurricane landfalls in the United States.
A few factors can be cited for the slow start to the 2018 season.
The tropical Atlantic, the stretch of sultry ocean between the Caribbean and the West Coast of Africa where many of history’s most destructive hurricanes have formed, has been relatively cool as compared to normal, and notably cooler than during last year’s blockbuster season.
This cool anomaly is important since warm water temperatures in the tropical Atlantic act as the steam engines that allow thunderstorm clusters moving off of west Africa to develop into the remarkable storms such as Irma, Maria, Andrew and Camille that fill the meteorological history books.
Cooler waters mean less energy for storms to develop, and often, fewer storms. The map below from NOAA/ESRL illustrates the relatively cool ocean temperatures in the tropical Atlantic during the 2018 hurricane season, represented by the area in blue extending from the west coast of Africa toward the Caribbean Islands.
It’s important to note, however, the warmer than average temperatures in the Gulf of Mexico and North Atlantic. These areas are where many of the storms that have occurred during the 2018 season have formed.
A second important factor is the occurrence of frequent dry air intrusions into the tropical Atlantic region.
Dry air inhibits the development of thunderstorms that are the precursors to hurricanes. In the graph to the right, which displays an experimental index of dry air intrusions over the Atlantic, the blue line represents episodes of dry air intrusion during the 2018 hurricane season.
The magnitude of dry air intrusion in 2018 is clearly greater than the long-term average values shown in black, at times by a factor of two.
Stronger Wind Shear
Wind shear, or wind at different altitudes blowing from different directions or at different speeds, has been stronger than average this year. While hurricanes are powerful, brutal storms, they are paradoxically quite fragile to strong background wind fields. Even a background wind field of 15 mph – the intensity of a pleasant afternoon breeze – can significantly weaken the strongest hurricane. In fact, wind shear and dry air intrusion played a role in Hurricane Florence’s weakening before landfall.
In 2017, a strong La Nina pattern in the Pacific Ocean, typically associated with weak wind shear in the Atlantic and a corresponding active hurricane season was in place. In 2018, the La Nina pattern ended, contributing to increased shear over the Atlantic and playing a role in limiting of the potency of the storms that have formed. In this image from July, strong shear is evidenced by red contours over the Caribbean and western Atlantic.
Clearly, up until this point, the 2018 hurricane season has been a shadow of the 2017 season. But in recent weeks, the atypically cool temperatures over the tropical Atlantic have warmed a bit and shear has decreased toward average levels.
Consequently, tropical activity has picked up. Indications heading into the latter half of September are for conditions to become a bit more hostile for development over the central Atlantic, but with relatively warm waters over the north Atlantic and Gulf of Mexico, residents shouldn’t let their guard down yet .
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