Tropical Storm Rachel strong
After a quiet hurricane season from 2015-16, the Atlantic hurricane season is on track to be busy. Although Emily and Barbara were brief, the six-month season of a year or more that brings tropical cyclones to the Atlantic will be marked by several events: a marked warming of the oceans and the atmosphere, a jet stream that won’t stall, and a few large hurricanes that could produce catastrophic storm surge flooding. A few long-period storms are likely, too, including Arthur in late July and Joaquin in September, but many others are hard to predict at this point.
Tropical Storm Arthur took aim at the Carolinas, which were hit by Hurricane Fran in 1996 and Hugo in 1989.
Arthur might be the first winter storm to hit the South Atlantic. That’s because the Gulf Stream—a conveyor belt of warm water in the Atlantic Ocean—moves south in winter, helping hurricanes grow stronger, says Todd Fisher, a climatologist at the Colorado State University Cooperative Institute for Meteorological Satellite Studies. If storms form in the South Atlantic and moves up the East Coast in December or January, that can cause damaging coastal flooding. It’s not completely impossible: Hurricane Joaquin tracked along the same path in 2015, so it wasn’t such a clearcut case that a storm could in December.
Arthur was the closest of four winter storms that hit the Carolinas. It hit North Carolina on Sept. 14, followed by Hurricane Joaquin two days later, Sandy in late October, and Maria in early December. Many of the storms missed the coast and saw little or no damage. Arthur reached the coast just barely before going back out to sea and reforming again a few days later.
What’s been behind the rapid growth of the Atlantic? Climate change. The upper atmospheric winds, usually a surefire way to determine the storm track, have warmed at unprecedented rates, making the winds more powerful, says Colorado State University researcher Philip Klotzbach. That makes storms more difficult to predict. So far, the Atlantic hurricane season has been on the whole too calm. But Thomas Caldwell, editor of AccuWeather’s Atlantic hurricane outlook, says conditions are ripe for the beginning of a stronger hurricane season. “It would be foolish to think we are done with storm activity,” he says.
Five “regular” Atlantic hurricanes have been classified so far this season. They are:
— Gordon in July
— Matthew in October
— Maria in early December
— Katia in early October
— Andrea in early October
Each of these storms had an angle that stressed upper-level wind shear. They generally did more damage than many other Atlantic hurricanes in recent years, but scientists have seen stronger storms pass through the region on stronger winds shear and weaker southern constellations of land masses, but these more powerful storms failed to get power from these conditions.
So far, there are no six-month seasons of hurricanes around. However, the warmth of the tropics has created more fuel for more storms. Five or more hurricanes is an average for any Atlantic season, not a dramatic number. But when you have an anomaly like the relatively calm season of 2015-16, that five-month season becomes a surprising challenge to predict. “There is risk of underestimating what is likely to happen,” Caldwell says.
The Gulf Stream usually brings warm water to the South Atlantic in the spring, but this spring the flow didn’t reach its usual record levels because of the El Niño winter that reduced evaporation and helped keep the ocean pool cooler than usual. As that water has risen, however, it is becoming more hospitable to hurricanes because of increased wind shear. This summer the water level in the ocean in the South Atlantic reached its highest level ever, above the average for the year. Still, a few of the warmer upper-level winds stayed near the surface, and some storms were able to form without swamping land.
Interactive weather map.
— Associated Press
This story was produced by The Atlantic as part of the Climate Desk collaboration.