Hurricanes Harvey, Irma and now Jose, and Katia: It’s been an extraordinary couple of weeks for highly destructive hurricanes and tropical storms.
But why? Call it a storm of factors conspiring to batter the Atlantic basin.
“It’s a progression of conditions going across the Atlantic that are conducive to hurricanes,” explains Gerry Bell, lead scientist of the season hurricane outlook team for the National Oceanic and Atmospheric Administration.
Oscar Schofield, chair of the department of marine and coastal sciences at Rutgers, agrees.
“Both the atmosphere and ocean are just really primed to fuel big, large storms right now,” Schofield says. “And we still have a period of time this year left for peak storm season.”
Hurricane season in the Atlantic spans June 1 to Nov. 30. The affected region includes the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. By the beginning of September in an average year, we would expect to have had two hurricanes, only one of which would be category 3 or greater in strength, according to the National Hurricane Center.
Already, we’ve seen Harvey make landfall as a Category 4 and Irma reach Category 5, the top of the Saffir-Simpson hurricane wind scale. Jose, once a tropical storm in the Atlantic, grew into a Category 1 hurricane Wednesday. Tropical Storm Katia, now off the Gulf of Mexico, also grew into a Category 1 as of late Wednesday night.
For this hurricane season, Bell and NOAA issued an updated outlook in August that predicted above-normal activity with the possibility it would be an extremely busy hurricane year. The agency predicted only a 10 percent chance of a below-normal season.
Here are five reasons conditions are so ripe for hurricanes:
1. West Africa
Hurricanes that strike the Caribbean, Mexico and the United States start in West Africa during monsoon season, which runs from June 15 to Sept. 30. But this year, West Africa is experiencing higher than normal activity, said Bell.
For example, on Wednesday, the Red Cross in the West African nation of Togo reported it was making a second delivery of humanitarian supplies such as mosquito netting, sleeping mats, and soap as heavy monsoon rains continued to pound villages on the Mono River.
Families had been driven from their homes because of rising water levels. Some were being evacuated by canoes over fears a dam could burst.
Monsoons such as those hitting Togo can set up conditions that launch big hurricanes here.
2. Warmer waters
The path in the Atlantic Ocean from Africa to the tropical Atlantic and the Caribbean is running about one to two degrees warmer than normal. That warmth helped generate huge amounts of moisture in Harvey as it pounded the Texas coast.
Warmer waters, which fuel hurricanes, accounted for about 3 percent to 5 percent more moisture in the atmosphere. That seemingly small amount can mean massive amounts of rain in a storm the size of Harvey. Climate scientists believe the long-term trend will be warmer waters in the future because of greenhouse gases.
But for now, the Atlantic is in a long-term period of warm surface waters as part of a pattern of weather conditions known as the Atlantic Multidecadal Oscillation, or AMO, Bell explained.
“It’s a climate pattern that can take place 25 to 40 years at a time,” Bell said. “It’s been in its warm phase since 1995, so we’re seeing a lot more hurricanes. That’s opposed to the ’70s, ’80s and early ’90s when the pattern was in its cold phases. This is just a naturally occurring climate pattern.”
It is uncertain whether human activity is a factor in the AMO.
But, as Schofield notes, warm water is not the only indicator of climate change. Sea-level rise also plays a role, because it leads to increased flooding as storms hit.
3. Wind patterns
Winds are among the most complex factors for hurricane formation. For hurricanes to pick up strength as they move from Africa to the tropical Atlantic, the winds must be favorable. And that’s what’s happening right now as trade winds form a super conduit across thousands of miles of ocean.
The trade winds, so called because trading ships took advantage of them to sail west to east, are weak right now. Conversely, strong trade winds blow in cooler, drier air that don’t help storms gain strength.
But weak trade winds allow warmer tropical air in. That’s also fuel for hurricanes.
At the same time, there is little vertical wind shear, which is the change in wind speed and direction between 5,000 to 35,000 feet above ground. Strong vertical wind shear can rip a developing hurricane apart, or even prevent it from forming. It essentially knocks the storms from their vertical formation and steals their energy.
Hurricanes tend to form only when there’s weak wind shear.
4. Jet Stream
The African Easterly Jet Stream now also is encouraging hurricanes, Bell said. The jet stream, essentially a river of air, sees its strongest winds in September and is a critical factor in whether a storm strengthens.
The cloud systems associated with the Easterly Jet Stream move east to west across Africa, about 10,000 feet in the atmosphere. The weaker trade winds allow the cyclonic counter-clockwise rotation to begin. The jet stream fosters the spin.
From there, the spin can intensify into tropical cyclones or hurricanes, especially when the storms encounter very warm ocean temperatures.
5. Lack of an El Niño
Earlier this year, forecasters saw conditions forming for a possible El Niño. But conditions remained more neutral and an El Nino did not form. An El Niño, caused by a warming of the waters in the Pacific, can have far-reaching impacts,.
An El Niño can cause stronger wind shear in the Atlantic and other conditions. Simply put, El Niño favors stronger hurricane activity along the Pacific, but suppresses it in the Atlantic basin.
The lack of an El Niño has meant weak wind shear.
So favorable winds, warmer water and the lack of anything to impede tropical cyclones have all conspired to produce monster storms.
Conditions will begin to change by late October when the water starts to cool, Schofield said. However, that does not mean the eastern seaboard, including New Jersey, will be out of the woods.
NOAA data show that by October, the path of a tropical storm or hurricane is more likely to sweep up along the eastern seaboard. The most recent example, Sandy, struck the east coast on Oct. 29, 2012.
By November, as the water continues to cool, most hurricanes simply head northeast, away from the coast, and are drawn out to sea.
“We are finishing up summer and will still have warm water in October,” Schofield said. “So we have still got weeks of prime conditions that might fuel other storms,”