(CNSNews.com) – Climate factors known to influence the formation of hurricanes, including a possible multi-decade cooling trend in the Atlantic Ocean, are causing “uncertainty about whether the high-activity era of Atlantic hurricanes has ended,” according to Kathryn Sullivan, administrator of the National Oceanic and Atmospheric Administration (NOAA).
“This year, there is strong variability in several key climate factors greater than in past years. And so there is uncertainty as to whether these factors will be reinforcing each other or competing with respect to tropical storm formation.
“More specifically, there’s uncertainty about whether the high-activity era of Atlantic hurricanes has ended,” Sullivan told reporters during a press conference Friday at NOAA’s Satellite Operations Facility in Suitland, Maryland.
“This high activity phase began in 1995. It’s associated with an ocean temperature pattern that is called the warm phase of the Atlantic Multi-Decadal Oscillation, or AMO. A warm phase of the AMO leads to warmer Atlantic Ocean temperatures, and a stronger West African monsoon, and these contribute to the formation of hurricanes.
“However, during the past three years, weaker hurricane seasons have been accompanied by a shift towards the cool signature of the AMO, cooler Atlantic Ocean temperatures, and a weaker West African monsoon,” Sullivan continued.
“If this shift proves to be more than short-lived, if it’s not just a temporary blip, then it could be signaling the arrival of a low activity era for Atlantic hurricanes. Possibly, that’s already begun, possibly we’re just in a transient,” she said, adding that “high and low phases tend to run 25 to 40 years."
“When we’re looking at these ocean temperature patterns, we’re not looking at month to month or year to year changes. We’re looking at patterns that last for multiple decades at a time,” explained Dr. Gerry Bell, head of NOAA’s hurricane forecasting team.
“So while we’re seeing the warm phase of the AMO possibly switching to the cold phase, this couple of year transition we’re seeing may just reflect the normal year to year signals and not really a multi-decadal pattern. So what we’ll be looking for to see if this actually is a multi-decadal shift is the duration and also its duration during the year.
“Right now, we’re seeing the cold AMO signal more in the winter and in the cool season, but really not very much in the summer and into the hurricane season. So we would expect this pattern to develop more through the year and the next couple of years. It may take a few years to really know if we’re in the cool phase of the AMO or not.”
The last time there was a transition to the cool phase of the AMO was in the early 1970s, Bell continued, and “we didn’t have any of the capabilities we have now to monitor this.”
CNSNews.com asked Bell what effect the cool phase of the AMO would have on hurricane activity over the next two decades.
“If and when [the AMO] does switch back to its cool phase, that is associated with a weaker African monsoon and also weaker hurricane seasons,” he replied.
“The last time we had a cold phase of the AMO, it was during 1971 to 1994. That was a low activity era for Atlantic hurricanes, and during that 25-year period, we only had two above-normal seasons and half were below normal. So that’s how strong this AMO signal is. It really is a powerhouse as far as controlling the hurricane season for decades at a time.”
However, Sullivan also told reporters that the upcoming 2016 Atlantic hurricane season, which runs from June 1 through November 30, is likely to be “near normal” - with more hurricane activity than last year’s “below normal” season,
“NOAA’s outlook for this season indicates that it is most likely to be a near-normal year. In the Atlantic this season, it will likely produce a range of between 10 to 16 tropical storms. Those are systems with top sustained winds of at least 39 miles an hour,” Sullivan said.
“Four to eight of those are expected to become hurricanes, with top winds sustained at 74 miles an hour or greater. And between one and four of those hurricanes are expected to grow to major strength of Category 3 or higher [on the Saffir-Simpson Hurricane Wind Scale], which translates to wind speeds of at least 111 miles an hour.
“Near normal may sound sort of encouraging, relax, things are okay, but I want to emphasize that the predicted level of activity that I just read off, compared to the past three years that we’ve experienced, actually suggests we could be in for more activity than we’ve seen in recent years,” Sullivan warned.
She noted that NOAA’s 2016 Atlantic Hurricane Season Outlook predicts the number of storms likely to form, not their tracks or possible landfalls.
Last year, NOAA's updated 2015 Atlantic Hurricane Season Outlook predicted “a 90 percent chance of a below-normal season… the highest given by NOAA for any such season since their seasonal hurricane outlooks began in August 1998.”
The agency based its 2015 prediction on a strengthening El Nino, which created “atmospheric conditions that are exceptionally non-conducive to tropical storm and hurricane formation."
NOAA predicted that six to 10 named storms would form in the North Atlantic, Caribbean Sea and Gulf of Mexico last year, with one to four becoming hurricanes, and at least one developing into a major hurricane.
In its 2015 hurricane season summary published in December, NOAA reported that 11 named storms formed in the North Atlantic, Caribbean Sea and Gulf of Mexico last year, with four reaching hurricane strength. Two were classified as major hurricanes: Danny and Joaquin. Neither storm struck the U.S. mainland.
“While the number of named storms, hurricanes, and major hurricanes was only a little below the long-term average activity levels of 12, 6, and 3, respectively, many of the named storms were relatively weak and short-lived…. This makes 2015 a below-average season in terms of ACE (Accumulated Cyclone Energy),” NOAA said.