How the strongest recorded storm formed

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The warming of western Pacific waters to a depth of 100 meters generated the energy that fueled Typhoon Haiyan and enabled it to intensify into a massive 195-mph “super typhoon” that roared ashore at Guiuan on the Philippine island of Samar Nov. 8. It was the strongest tropical cyclone at landfall in recorded history.

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The preliminary death toll was 4,000; the Red Cross was reporting 25,000 people missing. Bloomberg Industries was estimating insured damage of $2 billion and total economic damage of $14 billion. Haiyan likely will be the most costly natural disaster in Philippines history. The Philippines’ deadliest typhoon was Thelma, which killed 5,000 to 8,000 people in 1991.

Based on an analysis of October’s average ocean temperatures, recorded by the Japanese Meteorological Agency, the “water temperature was 5 to 10 degrees above average down to 100 meters under the track of Haiyan,” says Jeff Masters, director of meteorology for the Weather Underground. “It’s not just the warm temperatures, but the deep warm water that make more Category 5 storms,” he says. “Upper winds were quite calm. There was nothing to interrupt the intensification process, no winds acting to slow the storm’s development.”

Masters says this 20-year warming trend in the western Pacific has elevated water temperatures to at least 79 degrees F to depths 17 percent greater than in the early ’90s.

Typhoons intensify as they move across the ocean and suck up warm, moist tropical air from the surface. That air cools as it rises until it is shucked off at the top of the system, creating a rotating circulation — wind — that strengthens or weakens, depending on conditions. Masters says conditions were just right for Haiyan to strengthen into the “perfect storm.”

“It doesn’t happen very often,” he says. “You have to have everything line up just right.”

There have been stronger ones — Nancy in 1961, with 215-mph winds, Violet, also in 1961, with 205-mph winds, and Ida in 1958, with 200-mph winds — but each weakened significantly to a Category 1 or tropical storm before landfall.

Haiyan, which came ashore with winds of 195 mph and gusts to 235 mph, stands at the head of a rogue’s gallery of deadly storms: Hurricane Camille, which made landfall in Mississippi in 1969 with 190-mph winds; Super Typhoon Joan, which hit Taiwan in 1959 with 185-mph winds; the Great Labor Day Storm of 1935, which slammed Florida with 185-mph winds; and Super Typhoons Megul and Zeb, which overran Luzan in the Philippines with 180-mph winds, Megul in 2010 and Zeb in 1998.

The aftermath of Typhoon Haiyan at Tacloban City in the Philippines

It’s no accident that the Philippines take much of the brunt of these superstorms. “The western Pacific gets half of all the major tropical cyclones on the entire Earth,” Masters says. The deep, warm water and the western Pacific’s enormous expanse are ideal for spawning these powerful storms, which in the Pacific are called typhoons but are meteorologically the same as hurricanes or, if you prefer, tropical cyclones.

Masters says strengthening trade winds have caused a southward migration and strengthening of warm-water equatorial currents, which have contributed to a 13 percent increase in the tropical cyclone heat potential in the western Pacific since the early 1990s. He says the strong trades also have pushed water up against the east coast of the Philippines during the past 20 years, resulting in a 10mm-per-year sea level rise — more than triple the global average and contributing to higher ocean surges when typhoons strike.

Logically, forecasters might expect this warming trend to have accelerated seasonal typhoon activity in the western Pacific, and, in fact, 2013 was an above-average year, with 30 named storms by mid-November, compared with the average of 27 for the period. However, “this is the first above-average season since 2004,” Masters says. He says upper-level winds have been inhibiting typhoon development and less active Asian monsoon seasons have resulted in fewer depressions in the region that can strengthen into major storms.

Although typhoon activity in the western Pacific was more active than usual in 2013, the Atlantic hurricane season was much quieter than forecast. NOAA’s August 2013 hurricane season update predicted 13 to 19 named storms, with six to nine of those becoming hurricanes and three to five developing into major hurricanes (Category 3 or higher). None of those forecast numbers were met. This was the first below-average season since 2009 and only the third below-average season since 1995, says National Hurricane Center spokesman Dennis Feltgen.

Although the 12 named storms recorded by mid-November was near the long-term average, the season produced few strong cyclones — only two, which was well below the long-term average of six, and neither strengthened above Category 1. (The last season that failed to produce a hurricane of Category 2 or higher was 1968.)

This satellite image shows the super typhoon shortly before it struck the Philippines.

All of the pieces were in place for an above-average season — warm water, an active African monsoon, lots of low-pressure systems rolling out of Africa and into the Atlantic, no El Niño effect and an ongoing cycle of elevated hurricane activity, Feltgen says. But more wind shear than usual and masses of dry, sinking air inhibited hurricane formation, he says. Wind shear disrupts air circulation in a storm system, and the sinking dry air stabilizes the atmosphere (rising air destabilizes it), he says.

Despite the weather pattern, Feltgen warned hurricane watchers not to become complacent. “What happened this season has no bearing on what happens next season,” he says.

January 2014 issue