The 2020 Atlantic Ocean hurricane season – from June 1 to November 30 – will see above-average activity, according to the tropical storm season outlook just released by Riskpulse and Resilience360, DHL's supply chain management platform. Riskpulse is an analytics and risk management firm focusing on weather impacts to supply chains.
There are two primary factors that contribute to above-average activity in an Atlantic hurricane season – sea surface temperatures and wind shear.
Warm waters provide the generation that fuels storm; the warmer the surface temperature of the water – especially warm and deep water – the more quickly tropical storms intensify and the more powerful they can become. Partly because the winter was so mild, and the oceans didn't cool off as much as in previous years, sea surface temperatures are unusually high in the Gulf of Mexico, the Caribbean Sea and the tropical Atlantic, the band of ocean connecting Africa to Florida that forms the pathway for many hurricanes.
In fact, Jon Davis, chief meteorologist at Riskpulse, told FreightWaves that sea surface temperatures are currently just as warm or warmer than they were at this point in 2017, a devastating year of hurricanes that included Harvey, Irma, Jose and Maria.
"What we have right now, if you add up the formation zone – the tropical Atlantic from Africa to Florida, the Caribbean, the Gulf of Mexico – if you add that up, we're well above normal from an ocean temperature standpoint," Davis said. "Normally when you're on the warm side in May, you stay on the warm side. The Gulf has been exceptionally warm due to a really mild winter, and waters in the Atlantic zone are usually warm."
Warmer waters in general have led to more frequent "rapid intensification" of storms, Davis said, pointing to Hurricane Michael's rapid development in 2018 and Hurricane Dorian in 2020 as examples of that phenomenon.
The second important macro factor for hurricane development is wind shear – high winds in areas where storms develop tend to shred the storms before they can organize into larger systems. Interestingly, what happens in the equatorial Pacific actually determines wind shear conditions in the region where Atlantic hurricanes form, due to the jet stream.
When the waters of the equatorial Pacific region where the jet stream originates is warmer, the jet stream is more powerful, and higher winds prevail in the Atlantic basin. When the equatorial Pacific waters are colder, the jet stream weakens, and there's less wind shear to disrupt hurricane formation in the Atlantic.
During a La Niña cycle, as opposed to an El Niño, cool waters in the Pacific weaken the jet stream.
"We're trending toward a La Niña; there is not one yet but we're trending there," Davis explained. "Water temperatures in the equatorial Pacific are cooler than normal. Because they're cooler than normal, we don't tend to have as strong of a jet stream developing there. Eventually that weakened jet – the subtropical jet – travels to the Gulf of Mexico and we tend to have lower mean winds during the season."
Davis continued, "If you're a potential tropical system, you don't want outside influence, you don't want strong winds, you don't want high pressure, you want warm waters, but you don't want high winds in any portion of the track you're going through."
Due to both high sea surface temperature and low wind shear, the Atlantic basin is set up for a hurricane season with above-average activity.
Riskpulse forecasts an ACE score, which stands for Accumulated Cyclone Energy, of 125 to 150 for the 2020 Atlantic hurricane season, above 2018 and 2019 levels.
The forecast for 2020 does not look nearly as bad as the ACE score for 2017, but Davis explained that 2020 has the potential to reach those measured ACE levels. What made 2017 such a devastating season was an intra-seasonal factor. A large region of rising motion, or upward-moving air settled over an important area for storm formation in the first week of September, right at the peak of hurricane season. That rising motion triggered the organization of multiple storms and led to one of the very few periods with three named storms simultaneously in the Atlantic basin.
The Atlantic Basin on September 8, 2017.
But that sort of short-term, intraseasonal factor that made an impact in September can't be predicted from where we are now in May, just before the official beginning of the season. Rising motion and other meteorological phenomena that will play a role in increasing or decreasing 2020's ACE score will have to be tracked as they happen.
Davis left us with a final word of warning about the upcoming hurricane season.
"If we look at the last two or three years, I think one of the takeaways is how lucky the U.S. has been," Davis said. "If Dorian was 100 miles west it would have hit Florida instead of the Bahamas, and you have a disaster on your hands. Michael made landfall in one of the least populated areas of the Gulf Coast; it couldn't have had a better scenario. In the past couple of years, the U.S. has gotten lucky."
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