SHOP PBS

Planet Earth

22
Jan

Anatomy of a Super Typhoon

By any measure, Super Typhoon Haiyan was one of the most extraordinary tropical cyclones in world history. Tropical cyclones rarely reach Category 5 strength, when wind speeds exceed 155 mph. In fact, an average of just four Category 5 storms occur globally each year. But as Haiyan hurtled westwards towards the Philippines across the warm waters of the West Pacific Ocean during the first week of November 2013, the storm fed off the deepest region of warm waters anywhere on the planet, rapidly intensifying into a Category 5 storm.

typhoon-haiyan-satellite-composite
A composite satellite image of Super Typhoon Haiyan bearing down on the Philippines

As it swept over the open ocean, it continued to draw strength from the unusually warm waters below. Three hours before landfall, the Joint Typhoon Warning Center said Haiyan’s winds were blowing at a sustained 195 mph and gusting to 235 mph, making it the fourth strongest tropical cyclone in world history.

Satellite data suggests that Haiyan weakened only slightly, if at all, in the two hours after that advisory. The super typhoon likely made landfall with winds near 195 mph. The next JTWC intensity estimate, made about three hours after landfall, showed the winds had only slowed to 185 mph. If we take the average of these readings, we can estimate that Haiyan’s winds were a devastating 190 mph just one hour after landfall.

With winds of 190–195 mph at landfall, Haiyan is the strongest landfalling tropical cyclone in world history, a record previously held by the Hurricane Camille of 1969, which made landfall in Mississippi with 190 mph winds. (The margin may be even greater because meteorologists now recognize that the maximum sustained winds estimated for typhoons were overestimated during the 1940s to 1960s.) It is also the deadliest and most costly tropical typhoon in the history of the Philippines. Haiyan has killed over 6,000 people and caused $13 billion worth of damage, or about 5% of the Philippines’s GDP.

With Haiyan, the Philippines has experienced their three most expensive natural disasters in history in a 12-month time span: Super Typhoon Haiyan in November 2013 ($13 billion), Super Typhoon Bopha in December 2012 ($1.7 billion), and the torrential rains in and around the capital city of Manila in August 2013 ($1.7 billion). Among them, Haiyan stood out not only for its devastation, but also for the way it gathered strength over the ocean. So what made the storm so powerful? And can we expect more like it in the near future?

A Long Climate Shift

Haiyan may have been decades in the making. Since the early 1990s, sub-surface waters east of the Philippines have warmed remarkably due to a shift in atmospheric circulation patterns and ocean currents. Hurricanes are heat engines, which means they take heat energy out of the ocean, and convert it to kinetic energy in the form of wind.

Tropical cyclones need surface water temperatures of at least 26.5 ˚C to maintain themselves. The warmer the water, and the deeper the warm water is, the stronger the storm can get. Deep warm water is key. As a tropical cyclone moves over the ocean, it stirs up cooler water from the depths, which can reduce the intensity of the storm.

Super-Typhoon-Haiyan
Super Typhoon Haiyan was the most destructive storm in the history of the Philippines.

Both Hurricane Katrina and Hurricane Rita crossed over an eddy in the Gulf of Mexico with a lot of deep warm water. Both storms, too, grew into Category 5 hurricanes. Even before the conditions that produced Haiyan, the Pacific Ocean east of the Philippines has the largest area of deep warm water of anywhere on Earth, and these waters have historically fueled the highest incidence of Category 5 storms of anywhere on the planet.

But while Super Typhoon Haiyan tracked over surface waters that were close to average warmth, 29.5–30.5 ˚C, the waters at a depth of 100 meters were a huge 3 ˚C above average, according to Professor I-I Lin of the Department of Atmospheric Science at the National Taiwan University, who studied data from Argo floats which collect temperature and salinity information on the world’s oceans. As the typhoon stirred this unusually warm water to the surface, the storm was likely able to feed off the heat, allowing Haiyan to intensify into one of the strongest tropical cyclones ever observed.

Warm Down Deep

The average temperature of those sub-surface waters east of the Philippines has risen dramatically over the past twenty years. Warm, 26 ˚C water now penetrates 17% deeper than in the early 1990s, and the tropical cyclone heat potential has increased by 13%. The warm-up is due to an increase in the surface winds blowing across the region—the trade winds—which have caused a southward migration and strengthening of the North Equatorial Current (NEC) and the North Equatorial Counter Current (NECC).

Haiyan-visible
Stronger trade winds have allowed warmer waters to extend deeper, which contributed to Super Typhoon Haiyan's power.

Over the past 20 years, strong trade winds have piled a large amount of water up against the east coast of the Philippines, resulting in a rate of sea level rise of 10 mm per year—more than triple the global average of 3.1 mm per year. Sea level rise data from Legaspi in the Eastern Philippines shows a rise of about 305 mm, or 12 inches, since 1949. For comparison, global average sea level rose 190 mm, or about 7.5 inches, since 1901. This extra sea level rise added to the storm surge damage from Super Typhoon Haiyan. Part of the rise along the eastern Philippine coast is from tectonic processes—the Philippine plate is subsiding under the Eurasian plate—but most of it is due to the stronger trade winds and the fact that warmer waters expand, raising sea level.

Quickening Trade Winds

The swifter trade winds are the result of a series of complex processes. The surface trade winds in the equatorial Pacific are part of the Walker Circulation—a pattern of rising and sinking air along the Equator influenced by the El Niño/La Niña cycle. A strong Walker circulation lowers pressure over Indonesia, which pulls in more air at the surface along the Equator from the east, increasing the easterly trade winds. As these trade winds strengthen, their friction against the surface of the ocean pulls water away from South America, which allows cold water to well up to the surface. This is a La Niña-like situation, which takes heat energy out of the atmosphere and puts it into the ocean. This keeps global surface temperatures cooler than they would otherwise be. The opposite is a weakened Walker circulation, which produces weaker trade winds and a more El Niño-like situation with higher global surface temperatures.

haiyan-before-after
These false-color satellite photos show region of and around Tacloban before Haiyan in 2004 (right) and after (left). Red hues represent vegetation like trees, many of which were knocked down by Haiyan's powerful winds.

A strong Walker circulation has existed since the early 1990s. As long as that holds, global surface temperatures should stay cooler, prolonging the slow-down in global surface warming that has received much attention recently. But even that may be the result of gaps in weather station data. A paper released late last year fills in gaps left by the lack of weather stations in the Arctic. When the researchers filled those holes using satellite data, they discovered the global surface warming trend is more than twice that predicted by a commonly used data set.

With global temperatures on the rise, Haiyan may be a harbinger of things to come for the Philippines. Meteorologists and climatologists suspect the dual trends of warmer sub-surface water temperatures and shifting ocean currents will likely subject the island nation to more super typhoons and higher storm surges.

Climate models have been predicting that the Walker circulation should weaken, creating a more El Niño like situation. But that’s the opposite of what’s been happening over the past 20 years. Though if you remove removed how the atmosphere responds to the El Niño/La Niña cycle, as another recent paper did, the resulting pattern they found showed a steady strengthening of the Walker circulation in concert with global rising temperatures.

The question this raises is, are we seeing a failure of the climate models? Or is the recent speed-up of the Walker circulation a decades-long, but ultimately fleeting, shift in the climate system? Time will tell.