Hurricane Patricia: The role of El Niño, climate change and good fortune

The strongest hurricane ever seen in the western hemisphere came ashore in southwest Mexico at 6:15pm local time on Friday. Looming menacingly off the coast, US scientistsbrandedit “potentially catastrophic”. But a stroke of luck meant it caused far less damage than feared.

Why did Patricia spin up so fast? What factors helped lessen the blow? Carbon Brief looks at the role of climate change and other factors in understanding what made this hurricane so unusual.

Extraordinary spin-up

At her peak, Patricia’s winds reached an unprecedented200mph. This isa long wayabove the threshold of 156mph that classifies a hurricane as Category 5 – the highest possible rating.Some scientistswere saying Patricia would have been Category 7, if the scale extended that far.

Low air pressure in the core – a measure of storm strength – bottomed out at 879 millibars, making Patricia, officially, the most powerful hurricane ever measured in the western hemisphere.

科学家们用卫星探测飓风和keep a close eye on how they develop. This image from NOAA shows Patricia hurtling towards Mexico’s coast on Friday.

Satellite images showed Hurricane Patricia spinning up to a Category 5 storm with startling speed. Peak wind speed increased from85mph to 200mph in just 24 hours.Wunderground, an online weather service founded by Dr Jeff Masters, explains:

This places Patricia among the most rapidly intensifying tropical cyclones ever witnessed anywhere in the world since the advent of modern meteorology.

Incidentally, scientists know exactly how strong Patricia was because once satellites spot it and can track its course, stormchaser pilotsfly directly to the centreto take measurements.

The crew of#NOAA43(@NOAAP3) from the historic flt on 10/23/15 into hurr#Patricia. 879 refers to lowest pressurepic.twitter.com/WFiuZoNDcO

— NOAAHurricaneHunters (@NOAA_HurrHunter)October 24, 2015

So, what caused Patricia to spin up so quickly?

Warm water fuels storms

Hurricanes feed off warm water and the theory that rising ocean temperatures aremaking them strongerthan they would otherwise have been has been around for a long time.

While pinpointing a trend has so far been hampered by poor data records,scientists expectto seemore storms of this strengthas ocean temperature rises further.

As well as the ongoing influence from climate change, this year has been a bit special.

The hugeEl Niño happening in the Pacifichas made water in the region where Patricia formed even warmer than usual, adding further fuel to any storm that passes through.

While there are more hurricanes off the west coast of America during an El Niño year, there are fewer off the east coast. Of the record-breaking22 hurricanes recorded this yearin the northern hemisphere, 21 have been in the western, central or eastern Pacific basins, with Hurricane Joaquin the only one to reach Category 4 in the Atlantic this season, explainsThe Weather Channel.

Making landfall

A few hours before Patricia hit landfall, scientists werewarningof “potentially catastrophic” damages. A public advisory issued by the US National Hurricane Centre:

Patricia is expected to remain an extremely dangerous category 5 hurricane through landfall…Preparations to protect life and property should be rushed to completion.

.@NASANPPVIIRS imagery of#Patricia. Landfall as a Catagory 5 storm expected within 12hrs.https://t.co/JBalL2pAXMpic.twitter.com/4iGmes0dA4— NOAA Satellites (@NOAASatellites)October 23, 2015

While the storm still caused a lot of damage – with reports of heavy rain,flooding, landslides and several casualties – Hurricane Patricia causedfar less destruction than feared. Why?

Patricia was still a Category 5 when she hit landfall in Mexico, but, by then, peak wind speed haddropped to 165mph.The strongest winds were alsoconcentrated close to centre, limiting the amount of damage the storm could wreak.The storm’s course took it through the middle of the coastal resorts of Manzanillo and Puerto Vallarta, which were relatively quiet at the time.

Patriciaweakened rapidlyas she hit the mountainousregions of central Mexico. BySaturday morning, the US National Hurricane Centre haddowngradedPatricia to a tropical storm, falling further later that day to what’s called a “remnant low“.

A number of other factors contributed to Patricia causing less damage on impact.

Rising sea levelsmean that when a hurricane hits land, thestorm surgeit produces can overwhelm coastal defences and cause flooding. This is part of what made other storms, such asHurricane Katrina, so damaging.

Though scientistswarnedabout “large and destructive” waves, Patriciaspun up so fast that there wasn’t sufficient time,Jeff Weber, a scientist at the National Centre for Atmospheric Research in the US,tellsthe New York Times. He says:

It was born so quickly and so close that it didn’t really have time to build up the storm surge.

Scientists know that a warmer atmosphere can hold more moisture, meaning there’s more water available to fall as rain when a storm hits. But Patriciamoved at such a speed that rainfall didn’t concentrate in any one area for too long.

Between 20-25 October, total rainfall reached over 409mm (16.1 inches), but was distributed across a large area, as themap belowfrom NASA shows.

Quick action

Accurateforecasts,quick actionby the Mexican authorities and a fair amount of luck meant fewer people were hurt than might have been.Early estimatesput insurance losses at$500m, alsoconsiderably lower than expected.

Does this mean the storm wasoverhyped, as some media outlets suggested?

Emphatically “no”, says Dr Marshall Shepherd, professor of atmospheric science at the University of Georgia. The fact that it didn’t cause a lot of damage doesn’t means the emergency calls weren’t warranted, he writes inForbesmagazine:

I have no issue with the National Hurricane Center using language like “potentially catastrophic storm” because it certainly had that potential…A slight last minute shift in track or eyewall size would have changed everything.

Brad Plumer over atVox回应这个观点,他说:

It’d be wrong to say Patricia was overhyped…Rather, Patricia ended up mercifully passing through a sparsely populated area. And, crucially, Mexico is getting much better at dealing with tropical storms…This time around, orderly evacuation efforts likely saved lives.

Scientific evidence points towards Patricia’s strength likely being bolstered by climate change and tipped over the edge into record breaking territory byEl Niño. While every storm has its own characteristics and quirks, this one being particularly unusual, a warming global ocean means we’re likely to see more storms of this strength in future.

Main image: Hurricane Patricia’s Eyewall on October 23, 2015. Credit:NOAA/NASA.