El Niño giveth, and the mountains taketh away.
The strongest hurricane ever recorded in the Atlantic or eastern Pacific made landfall Friday evening. The storm battered the Mexican coast in the state of Jalisco with estimated sustained winds of 165 mph.
Due to the drama around the unprecedented strength of Hurricane Patricia, many were likely confused come Saturday morning that the storm had already been dramatically downgraded. So what happened?
Because hurricanes fuel themselves with the heat of ocean water, the storm began to weaken quickly, as expected, after it came ashore and lost its source of energy. According to Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology, the mountains of Mexico made it weaken even faster than some hurricanes, due to the force of the drag exerted by their elevated surface on the structure of the storm.
But how did Patricia get so strong in the first place? When Peruvian fisherman named the phenomenon of warm waters off their coast El Nino – after the Christ child because it tended to occur around Christmastime – they likely didn’t know the full, global extent of its powers, and just how accurate their nomenclature was.
El Niño exerts its powers by redistributing a lot of energy. By bringing to the surface energy that is typically contained deeper in the ocean, it creates more storms along the equator. Most of these storms do not turn into landfall-ing hurricanes, but their presence influences atmospheric patterns around the world. Some of them, like Patricia, do become tropical cyclones, and with the extra heat available from El Niño they can be much stronger than typical hurricanes. Hurricane Linda, the strongest eastern Pacific storm before Patricia, had 185 mph winds and occurred during the strong 1997 El Niño. On the flip side, the Atlantic Ocean tends to get a break during El Niño, with fewer storms than an average year.
It’s not just temperatures at the surface that matter for fueling hurricanes – the energy available for a few tens-of-meters below the surface is also a factor. “We know that one of the factors that allowed Patricia to get so strong was the relatively high thickness of the warm ocean water layer near the surface,” said Kerry Emanuel in an email. This thickness is known to be a feature of El Niño, he said.
When I asked Emanuel about the influence of global warming, he wasn’t as confident. “It will not be possible to say if global warming really affected this or any other individual storms. To see global warming signals, we have to look at the statistics of large numbers of storms,” he said.
Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, also mentioned in an email the influence of the subsurface ocean temperatures from El Niño, but he went a little farther in his characterization of global warming’s influence on the intensity of the storm. First, the warm environment that the storm needs to form and strengthen consists not only of a substantial El Niño-related component but also of a background global warming component that influences the ocean’s heat content, he said. Trenberth also added that, “global warming accounts for about five percent more moisture in the atmosphere.” During storms, this five-percent-influence is amplified by around a factor of two, he said. Put simply, global warming could be making the storm about ten percent more powerful.
When you have a storm that’s already strong, this extra boost can make it record-breaking. Patricia now tops the lists for both lowest pressure (879 mb) and highest wind speed (200 mph) in the Atlantic and eastern Pacific record books. The two metrics are highly related – lower pressure means a stronger pressure difference between the center of the storm and the air mass surrounding it. The stronger that gradient, the faster the winds.
Although it weakened somewhat while still over the water, Patricia remained a powerful category five when it made landfall. Since the 1920s, fifteen storms have landed in North America as a category five, the last being Hurricane Felix in Nicaragua in 2007. The last category five to hit the United States was Andrew in 1992. The only other hurricane in the east Pacific to make landfall as a category five was in 1959.
The most exceptional record broken by Patricia? That 200 mph wind speed is the highest recorded wind speed in a hurricane or typhoon in any ocean basin since satellites began to measure wind speed accurately. Compared to other areas, the eastern Pacific does not typically spawn such strong storms. This is the first storm in the eastern Pacific recorded to fall below the 900 mb pressure mark, while a handful of storms in the Atlantic have achieved this strength. Thanks (or no thanks, maybe) to warmer waters, the western Pacific sees more strong hurricanes (there known as typhoons), with dozens of storms recorded to reach pressure levels below 900 mb.
A strong La Niña, which is the inverse of El Niño, followed the 1997-98 El Niño event. While forecasters can’t say for sure this will happen again, it is a possibility. If this is the case, the Pacific Ocean is likely to see fewer hurricanes than this year. Good news for those pummeled by storms this year, but what goes around comes around – during La Niña, the Atlantic basin is more favorable than usual for hurricane development.