Last weekend, Hurricane Harvey put an end to a lucky streak: It became the first major hurricane to make landfall in the United States since 2005. The Category-4 storm barreled into Texas on 25 August, lashing the coast with 200-kilometer-per-hour winds, and deluging Houston with more than a meter of rain. As the third hurricane of the season, Harvey also gave weight to predictions from the National Oceanic and Atmospheric Administration (NOAA) that 2017 will be an above-average year for Atlantic storms. For decades now, storms have been getting a boost from a powerful but still mysterious long-term cycle in North Atlantic sea surface temperatures, which appears to be holding steady in its warm, storm-spawning phase.
This cycle, called the Atlantic Multidecadal Oscillation (AMO), swings between warm and cool phases every 20 to 60 years, shifting North Atlantic temperatures by a degree or so and setting the backdrop for hurricane season. Since about 1995, the AMO has been in a warm state, but researchers aren’t sure where it’s headed next. The AMO has traditionally been attributed to natural shifts in ocean currents, and some think it’s on the cusp of shifting back toward a cool, quiescent phase. But others propose that human activities—a combination of declining air pollution and greenhouse warming—might prolong the current warm period, keeping hurricane activity high.
“It’s important to understand the mechanism,” says Rong Zhang, an oceanographer at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. “The projections are opposite.”
Researchers first detected the AMO in ocean temperature measurements spanning the past 150 years. But tree rings and other climate records from places strongly influenced by the AMO show evidence of temperature variations going back centuries.
Shifts in the AMO reverberate through the climate system, affecting rainfall in Europe, drought in the Amazon, and Atlantic hurricanes. The warm phase fuels storms by warming the tropical Atlantic and intensifying the West African monsoon. A stronger monsoon, like La Niña (a cooling of the eastern tropical Pacific), reduces wind shear, vertical changes in wind direction that tend to break up embryonic storms. The monsoon also spins up low-pressure systems that enter the hurricane nursery of the tropical Atlantic. “This wind pattern allows these storms to very quickly develop rotation and energize,” says Gerry Bell, lead hurricane forecaster at NOAA’s Climate Prediction Center in College Park, Maryland.
By NOAA’s metrics, the AMO remained in a warm phase this year, but some see hints of a change. “The waters in the far North Atlantic, up by Greenland, have been really cold—much colder than normal,” says Phil Klotzbach, a meteorologist at Colorado State University in Fort Collins. The pattern potentially upset tropical conditions from afar, causing quieter than average hurricane seasons in recent years, he says.
The cold anomaly may herald a transition toward a cool phase, especially if the AMO is mainly driven by natural variations in a “conveyor belt” of Atlantic Ocean currents. This circulation draws warm surface water northeast along the Gulf Stream until it cools and sinks in the seas surrounding Greenland, returning south in the deep Atlantic. Stronger circulation brings more warm water north and leads to a positive AMO; when the circulation flags, cooling begins in the far North Atlantic and moves south, culminating in a negative AMO, Zhang says. According to her estimates, the AMO is now close to neutral. Klotzbach’s approach, which factors in high-latitude temperatures, suggests that the AMO has already shifted negative.
However, recent research indicates that factors outside the ocean may also trigger changes in the AMO. Natural climate records suggest that, for centuries, volcanic eruptions and small changes in the sun’s output warmed and cooled the ocean, helping pace the AMO. In past decades, humans have added their own influences, such as aerosol particles from burning coal, which reflect sunlight back to space and cool the ocean, says Ben Booth, a climate scientist at the Met Office Hadley Centre in Exeter, U.K. Booth thinks skyrocketing aerosol emissions in the second half of the 20th century were the primary cause of the most recent cold phase of the AMO, which lasted from 1970 to 1994. A subsequent drop—thanks to clean air regulations in the United States and Europe—may have instigated the current warm phase.
The role of greenhouse gas emissions is another story. Hotter oceans are generally thought to boost the intensity of storms, but not necessarily their frequency, and researchers subtract out this long-term warming when calculating the AMO. However, research by Lisa Murphy Goes, an atmospheric scientist at the University of Miami in Florida, suggests that greenhouse emissions may still help trigger swings in the AMO. As greenhouse gases keep rising and aerosols fall, Murphy Goes says the AMO should remain slightly positive for at least the next decade.
Understanding what lies ahead depends on whether natural variability or human influences win out. Most likely, both play a role; Booth suggests that their impacts could vary by region. Changes in ocean circulation might matter most in the northern Atlantic—where the cold anomaly has hunkered down—whereas external factors, such as aerosols, might impact the tropics most. These forces might also play off of one another over many decades in unexpected ways, or evolve under the long-term effects of climate change. “We hold many of the pieces,” Booth says, “but we don’t yet have a holistic picture.” Harvey could be a tragic culmination to the current hurricane era—or a sign that it’s not over yet.
*Correction, 30 August, 3 p.m.: An earlier version of the story gave the incorrect location for the Climate Prediction Center.