News•April 15, 2015

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.

Super Typhoon Maysak seen by the Suomi NPP satellite on April 1, 2015, as it made its way toward the Philippines.
Credit: NOAA/NASA

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.

Areas that see more (orange and red) or less (green) tropical cyclone activity during an El Nino.
Credit: KNMI

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 northwestern Pacific drives the bus,” as Klotzbach put it.

Through April 6 of this year, the northwest Pacific’s ACE value was the second highest ACE to date on record, Klotzbach said.

Even though the El Niño didn’t fully emerge until February (by the reckoning of U.S. government forecasters), it is playing a part in this amped-up early season activity, which saw the formation of Category-5 Super Typhoon Maysak, one of only three storms of similar strength to form prior to April 1 in the records.

The most recent observations of sea surface temperatures across the tropical Pacific Ocean (top) and how different those temperatures are from normal (bottom).
Click image to enlarge. Credit: NOAA

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.

The most recent El Niño observations and climate model projections suggest the current event is looking a little more textbook and could last through summer, possibly even strengthening. Because of this, 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.

Of course, El Niño activity is notoriously difficult to predict in the springtime, and signs around this time last year of a strong El Niño in the works didn’t pan out. So forecasters are cautious.

Even if activity is below normal in the Atlantic, forecasters are quick to caution, there is still potential for a pocket to open up and bring the right ingredients together to form a devastating storm.

The best example of this is Hurricane Andrew, which barreled into Florida as a Category 5 storm in 1992, during what was otherwise a remarkably quiet year in the Atlantic. The considerable damage from that storm marked it as the costliest Atlantic hurricane on record until Katrina.