Typhoon Haiyan: How Warm Western Pacific Waters Fueled a Record-Breaking Super Typhoon

The unusual warming of western Pacific waters to depths approaching 100 meters supplied the energy that allowed Typhoon Haiyan to explode into a catastrophic storm. On Nov. 8, Haiyan made landfall at Guiuan on the Philippine island of Samar with sustained winds of 195 mph and gusts up to 235 mph, making it the strongest tropical cyclone at landfall in the historical record.

Initial reports put the death toll around 4,000, while the Red Cross reported some 25,000 people missing. Bloomberg estimated insured losses near $2 billion and total economic damage at roughly $14 billion, which would make Haiyan likely the most costly natural disaster in the Philippines’ history. The country’s previous deadliest typhoon, Thelma in 1991, killed an estimated 5,000 to 8,000 people.

Analysis of October ocean temperatures by the Japanese Meteorological Agency showed sea temperatures 5 to 10 degrees above average down to 100 meters along Haiyan’s track. Jeff Masters, director of meteorology at Weather Underground, noted that it was not only unusually warm surface water but also unusually deep warmth that enabled such extreme intensification. Calm upper-level winds provided little to disrupt the storm’s development, allowing rapid strengthening.

Masters points to a roughly 20-year warming trend in the western Pacific that has pushed warm-water layers deeper. He estimates water temperatures of at least 79°F now extend about 17 percent deeper than in the early 1990s. That deeper heat reservoir helps sustain and energize very intense tropical cyclones.

Tropical cyclones amplify by ingesting warm, moist air from the ocean surface. As that air rises and cools, it releases latent heat and is expelled at the top of the storm, driving the rotating circulation of winds. When ocean heat is both abundant and deep and upper-level winds are weak, the environment becomes highly favorable for rapid intensification—conditions that aligned for Haiyan.

Such a combination is rare. Masters observes that everything has to “line up just right” for a storm to reach these extreme intensities.

Historically there have been storms with reported higher peak winds—Nancy (1961) at 215 mph, Violet (1961) at 205 mph, and Ida (1958) at 200 mph—but those systems weakened substantially before making landfall. Haiyan’s exceptional strength at landfall places it among the most devastating storms to strike populated coastlines, alongside Hurricane Camille (190 mph at U.S. landfall, 1969), Super Typhoon Joan (185 mph, Taiwan, 1959), the Great Labor Day Storm of 1935 (185 mph, Florida), and Super Typhoons Megi (2010) and Zeb (1998), both of which produced sustained winds around 180 mph when they struck Luzon.

The Philippines’ frequent exposure to powerful storms is no accident. The western Pacific basin generates roughly half of the world’s major tropical cyclones. Its vast expanse and persistently warm, deep waters create ideal conditions for the formation and strengthening of typhoons—the same meteorological phenomena that are called hurricanes in the Atlantic.

Changes in large-scale circulation have also played a role. Masters explains that strengthening trade winds have shifted and intensified warm equatorial currents, contributing to a roughly 13 percent increase in tropical cyclone heat potential in the western Pacific since the early 1990s. Those same trade winds have pushed water against the eastern shores of the Philippines, contributing to localized sea level rise on the order of 10 mm per year—more than three times the global average—and increasing the risk of higher coastal storm surges during typhoons.

While these oceanic trends might suggest a corresponding increase in seasonal typhoon activity, the picture is complex. The western Pacific was above average in 2013, with 30 named storms by mid-November compared with a 27-storm average for the period, and this was the basin’s first above-average season since 2004. Nonetheless, variations in upper-level winds and a less active Asian monsoon reduced the number of systems that could intensify into major storms.

By contrast, the 2013 Atlantic hurricane season was unexpectedly quiet. NOAA’s August forecast had projected 13 to 19 named storms, including six to nine hurricanes and three to five major hurricanes. Those numbers were not met. National Hurricane Center spokesman Dennis Feltgen noted this was the first below-average Atlantic season since 2009 and only the third below-average season since 1995.

Although the Atlantic produced about 12 named storms by mid-November—close to the long-term average—only two reached hurricane strength and neither strengthened beyond Category 1. A combination of stronger-than-normal wind shear and large masses of dry, sinking air limited cyclone development. Wind shear disrupts the vertical organization of a storm, while dry, descending air stabilizes the atmosphere and suppresses the rising motion needed for storm intensification.

Forecasters caution against drawing long-term conclusions from a single season. As Feltgen warned, one quiet or active season does not determine what will happen the next year. Nevertheless, the extreme example of Haiyan illustrates how deeper, warmer ocean heat content and favorable atmospheric conditions can combine to produce storms of unprecedented intensity and destructive potential.

January 2014 issue