The Atlantic Ocean's biggest hurricanes are getting stronger more quickly than they were 30 years ago, according to new research. And they're now gaining power in different parts of the Atlantic than was previously the case. It's mainly down to a mysterious ocean cycle called the Atlantic Multidecadal Oscillation, which may be coming to the end of a period of warming.

The prime cause is the Atlantic Multidecadal Oscillation (AMO), a climate cycle that affects ocean temperature, which in turn influences the formation of hurricanes.This is the cycle of heating and cooling in the North Atlantic, each of which usually lasts at least a decade.

Rapid intensification

The researchers analysed 16 climate studies created to assess global warming, using data gathered between 1986 and 2015. They took a close look at the rapid intensification process of hurricanes. This is a crucial part of a major hurricane's development when its maximum wind speed increases by at least 25 knots (46 km/h) within 24 hours. Think of it as a signature of a big hurricane, evident in last year's Harvey, Irma, Jose and Maria. Almost all category 4 and 5 Atlantic hurricanes experience this intensification.

Though the researchers found that these rapid intensifications are not happening more frequently, they found that significant storms are growing more powerful more quickly in this 24 hour window than they were 30 years ago.

They found that this 24 hour increase is about 13 mph (21 km/h) greater than it was 30 years ago, an increase of 4.3 mph (6.9 km/h) per decade.

Interestingly, the researchers say that while rapid intensification has historically been associated with the Western Atlantic, the increasing strengthening is more evident in the Central and Eastern Atlantic, focusing on an area east of the lesser Antilles, encompassing the Virgin Islands and St Kitts. Researchers say this is where Irma Jose and Maria all underwent their intensification last year.

Causes

The team attributes this increase to a number of factors, but significant among them is the increasing heat in the upper layer of the ocean, which essentially fuels the hurricane.

Also significant is a decrease in wind shear, the competing pushes and pulls of winds blowing in different directions at different altitudes above the ocean. The less this is a factor, the easier it is for powerful hurricanes to form.

But ultimately, the researchers think the AMO is the biggest contributor. It plays a crucial part in the prevailing temperature of the ocean. The AMO has been on a warming cycle since the late 1990s.

""This was a surprise, that the AMO seems to be a bigger influence in rapid intensification than other factors, including overall warming," Karthik Balaguru said in a press release put out by the Pacific Northwest National Laboratory.

Global warming?

The research challenges the idea that major hurricanes are more frequent because hurricanes of all sizes are more frequent. It instead suggests that you can have more major hurricanes without the overall number of hurricanes going up.

Crucially, the research makes no comment on the broader influence of global warming, which is outside the scope of this research. The study notes that we could be on the verge of entering a cooling phase of the AMO, which should allow more research into the potential influences of global warming on hurricane intensification.

The study has been published in the journal Geophysical Research Letters.