A Tale of Two Hurricane Seasons: Atlantic vs. Pacific

It was the busiest of hurricane seasons, it was the quietest of hurricane seasons.

The end of November means that both the Atlantic and East Pacific hurricane seasons have drawn to a close, and storm activity in the two ocean basins was quite different, indeed, but for the same reason. A strong El Niño muffled activity in the Atlantic, but boosted it in the Pacific, as forecasters anticipated back in the spring.

Tropical Storm Ana as it headed toward landfall in South Carolina on May 9, 2015, a somewhat unusual pre-season Atlantic storm.

Credit: NASA

But both seasons still had some surprises, from the easternmost hurricane known to have formed in the Atlantic to the highest winds ever measured in any hurricane in the Pacific’s Hurricane Patricia.

The 2015 Atlantic hurricane season got off to an early start, with Tropical Storm Ana forming in early May, weeks before the official June 1 start to the season. Such early bird storms aren’t unheard of, as tropical systems can form whenever conditions are right. Those conditions just tend to be more favorable during the officially recognized six-month season, which encompasses about 97 percent of tropical cyclone activity in the Atlantic.

That “right place, right time” storm occurrence was a theme of the Atlantic season. While El Niño meant that winds across the primary storm formation areas were hostile to burgeoning systems, tropical Atlantic waters warmed considerably through the summer. So when a disturbance in the atmosphere happened to hit an area where winds were favorable, it often had the ocean heat to help fuel it.

Those unexpectedly warm waters were what caused some of the seasonal forecasts to slightly underestimate the amount of storm activity in the Atlantic, Philip Klotzbach, a hurricane researcher and seasonal forecaster at Colorado State University, wrote at the Capital Weather Gang blog. But they all correctly predicted a muted season.

Overall, the Atlantic recorded 11 named storms, including four hurricanes and two major hurricanes. An average Atlantic season has 11.5 named storms, 6.1 hurricanes, and 2.6 major hurricanes (defined as Category 3 or higher).

Another measure used to rate hurricane activity, called Accumulated Cyclone Energy, or ACE, takes into account the winds of a storm throughout its lifetime. Storms with higher winds and those that are longer-lived have a higher ACE. The ACE for the entire 2015 Atlantic season was 62, compared to the 1981-2010 average of 92, according to Klotzbach.

Hurricane Joaquin as it strengthened to a Category 4 storm over the Bahamas.

Credit: NOAA

Half of that ACE came from just one storm, Hurricane Joaquin. Joaquin formed at the end of September and found a pocket over the Bahamas where it was able to strengthen considerably, reaching Category 4 status, becoming the strongest Atlantic hurricane since 2010’s Igor.

While there was some worry that Joaquin might make a Sandy-like strike on the East Coast of the U.S., the storm eventually turned out to sea. In fact, the 2015 season stretched the streak of years without a major hurricane landfall in the U.S. to 10. Many quibble with the measure, though, since other damaging storms, most notably Sandy, did hit during that time, even if they were not technically classified as major hurricanes.

One of the few Atlantic hurricanes that formed this year, Fred, did so further east than any other on record and provided the Cape Verde islands off Africa with their very first hurricane warming.

Another notable storm for the Atlantic, particularly given the overall below-average activity, was early November’s Kate, the latest hurricane to form in the Atlantic since the blockbuster 2005 season.

Preliminary tracks and intensities of each East Pacific storm that formed during the 2015 hurricane season.

Credit: NHC/NOAA

While the Atlantic was relatively sleepy this season, the East Pacific was downright hyper.

There were 18 named storms to form in the East Pacific this year, of which 13 became hurricanes and a record-breaking 11 became major hurricanes. The previous record for major hurricanes for the basin was 10 in 1992. (An average East Pacific season has 15 named storms, 8 hurricanes and 3 major hurricanes.)

This season’s ACE was second only to that from the 1992 season (288 compared to 292), according to Klotzbach.

The East Pacific saw two other impressive records: In late August, three Category 4 storms (Kilo, Ignacio and Jimena) spanned the basin, the first time that has ever happened in any ocean basin.

And in October, Hurricane Patricia became the strongest hurricane ever recorded in the Western Hemisphere and had the highest wind speeds ever directly measured, clocking in at 200 mph.

The flurry of activity in the East Pacific is also linked to El Niño, which has the opposite effect there as in the Atlantic, tending to make winds more favorable to storm formation and spiking ocean temperatures in some areas.

While both seasons are now officially over, as ocean waters cool with the onset of winter, that doesn’t mean storms can’t still form if conditions are right. The chances of that happening this year are very small, though. In records going back to 1950, there has never been a December storm recorded in an El Niño year in the Atlantic, Klotzbach said. And in the East Pacific, December storms are exceedingly rare.

Looking ahead to next season, activity could again depend on what is happening with El Niño. Currently, forecasters expect it to peak in strength this winter, then taper off through the spring, likely resulting in neutral conditions in the tropical Pacific during summer. But the exact timing of that shift is still up in the air, as is what next hurricane season might bring.