Why this Atlantic hurricane season is predicted to be unusually stormy

THE ATLANTIC hurricane season runs from June 1st until November 30th—the months during which tropical storms are most likely to form and wreak havoc on their roughly westerly course through the Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea. In the past few years it has been common for the first storms to appear before the season officially opens; this year none has, and none looks likely to form in the Atlantic in coming days, though the remnants of a Pacific storm currently passing over Mexico could become a tropical storm again when they reach the Gulf. Read nothing into this slow start. Most forecasters predict the season will be a dangerous one.

America’s National Oceanic and Atmospheric Administration (NOAA) thinks there is a 65% chance that there will be an above-average amount of tempestuous weather. It expects its National Hurricane Centre, which allocates a name to any storm that has a rotating circulation pattern and wind speeds above 63 kilometres per hour, to have to do so around 17 times (the average over the past 30 years has been 14). Britain’s Met Office is betting on 18 named storms for this season; the well-regarded forecasting group at Colorado State University reckons there will be 19. More storms overall normally means more that rise to the level of hurricanes (wind speeds over 119kph) and major hurricanes (over 179kph). Compared with an average of seven hurricanes and three major hurricanes, NOAA predicts around eight and four this season; the other forecasters say nine and four.

Why so many? One important reason is La Niña conditions prevalent since June 2020. Interactions between the currents of the Pacific Ocean and the atmosphere above it influence each other in ways that can rock back and forth between extremes. This phenomenon, called the El Niño-Southern Oscillation (El Niño being the opposite extreme to that of La Niña), has big impacts on weather patterns throughout the tropics and beyond.

La Niña normally results in weaker winds high in the atmosphere above the Atlantic, which means the amount that the wind’s velocity changes as it gets higher—“vertical wind shear”—is lessened. Hurricanes grow better when there is less wind shear, so La Niña makes it easier for storm systems to form in the Atlantic basin. (El Niño, conversely, suppresses hurricanes in the Atlantic but drives them in the central and eastern Pacific.) La Niña also encourages Atlantic storms by increasing the difference between Atlantic temperatures and Pacific ones; that leads to more thunderstorms in the Atlantic, and it is from thunderstorms that hurricanes are built.

Absolute water temperatures matter too; hurricanes are engines for taking energy out of the ocean and putting it into the atmosphere, and the more energy there is to fuel them the more such work they can do. Sea temperatures in the Gulf of Mexico are already high. Many of the most devastating hurricanes to hit America, including Hurricane Katrina in 2005, occurred in years when a monster current of warm water known as the “Loop” extended particularly far north in the Gulf of Mexico, meaning that storms crossed it on their way towards land. This year the Loop is in a notably similar position to the one it was in in 2005.

The role of climate change in all this is “a messy one”, says Phil Klotzbach of Colorado State University. Models do not suggest that storms necessarily become more common. But more heat in the oceans should allow them to intensify more easily and to pump more water into the sky, which would make hurricanes more powerful; the consensus points towards this being the case, says Mr Klotzbach. A study published recently in Nature Communications, a journal, found that the unusually destructive Atlantic hurricane season of 2020 had more extreme rainfall than it would have had without climate change. Rising sea levels mean storm surges are more dangerous.

If forecasters are right, this will be the seventh consecutive year with above-average hurricane activity in the Atlantic. Nothing is guaranteed, Mr Klotzbach says, pointing to the season of 2013, which was quieter than anyone expected. But there is a good chance that one or more of the names which the National Hurricane Centre assigns will, by the end of the year, be associated with a serious amount of loss and damage.

More from The Economist explains:
What is La Niña?
The increasing frequency of fatal wet-bulb temperatures
What would different levels of global warming look like?

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