El Niño Is Hanging On: What that Means for Hurricanes

Over the next few months, the globe might see an uptick in tropical cyclone activity thanks to an El Niño that is showing signs of asserting itself more forcefully.That doesn’t mean more hurricanes everywhere, though: While El Niño tends to boost activity in the Pacific Ocean, it clamps down on storm formation in the tropical Atlantic. That link has at least one hurricane forecaster calling for a very quiet Atlantic hurricane season this year — possibly the quietest since the mid-20th century.

While El Niño is a cyclical climate phenomenon in the Pacific Ocean — marked by warmer ocean temperatures in the tropics and a weakening of the usual easterly trade winds — it can impact weather around the globe. During an El Niño, a domino of atmospheric effects causes a large area of stable, subsiding air to form over the tropical Atlantic. Exactly the opposite of what a fledgling tropical cyclone, which thrives on instability, needs to grow and strengthen. El Niño also tends to bring more wind shear to the region, meaning the speed and direction of the wind changes more between different altitudes, putting the kibosh on a burgeoning storm.

In the Pacific, it’s a different story: There, tropical cyclone activity ramps up during El Niño events. That doesn’t necessarily mean more storms, said Philip Klotzbach, a hurricane researcher at Colorado State University; instead, it might just mean storms form in areas that leave them with plenty of ocean to traverse before they peter out (and can shift the areas more likely to be hit by storms). Such longer-lasting storms are something frequently seen in the Pacific during an El Niño, Klotzbach said. Forecasters combine the strength of a storm and its duration into a measure called Accumulated Cyclone Energy, or ACE, which can be used to gauge the impact of a single storm or a season’s worth of them. Stronger, longer-lasting storms have higher ACE values.

In an El Niño year, fewer Atlantic storms mean a lower ACE for that basin, while in the eastern and northwest Pacific, longer-lasting storms raise the ACE value. But the changes aren’t equal: The increase in Pacific activity is larger than the decrease in the Atlantic, so “your global activity actually increases in El Niño years,” Klotzbach said.In particular, activity in the northwest Pacific is a big determinant of global ACE for a given year. There, the tropical cyclone (or typhoon, to use the regional term) season lasts virtually all year, and so there is more opportunity for storms to form. The tendency towards El Niño-like conditions last year still boosted Pacific activity and tamped down on the Atlantic. Several storms reached Hawaii, while Japan saw several landfalls — other signatures of El Niño-year activity. The global ACE value was also higher than in the previous two years, when neutral conditions were in place.

Klotzbach thinks that ACE “should actually be high this year,” though he expects only seven tropical storms to form in the Atlantic, compared to the average of about 12. Of those, his forecast calls for only three to become hurricanes, compared to an average of six.

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