Super Typhoon Haiyan's Intensification and Unusually Warm Sub-Surface Waters

A remarkable warming of the sub-surface Pacific waters east of the Philippines in recent decades, due to a shift in atmospheric circulation patterns and ocean currents that began in the early 1990s, could be responsible for the rapid intensification of Super Typhoon Haiyan. 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. It's well-known that tropical cyclones need surface water temperatures of at least 26.5°C (80°F) to maintain themselves, and that the warmer the water, and the deeper the warm water is, the stronger the storm can get. Deep warm water is important, since as a tropical cyclone tracks over the ocean, it stirs up cooler water from the depths, potentially reducing the intensity of the storm. When both Hurricane Katrina and Hurricane Rita exploded into Category 5 hurricanes as they crossed over a warm eddy in the Gulf of Mexico with a lot of deep, warm water, the concept of the total heat energy available to fuel a hurricane--the Tropical Cyclone Heat Potential (TCHP)--became one that gained wide recognition. 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. Super Typhoon Haiyan tracked over surface waters that were of near-average warmth, 29.5 - 30.5°C (85 - 87°F.) However, the waters at a depth of 100 meters (328 feet) beneath Haiyan during its rapid intensification phase were a huge 3°C above average, according to Professor I-I Lin of the Department of Atmospheric Science at the National Taiwan University. An analysis by the Japan Meteorological Agency for October showed ocean temperatures 4 - 5°C (7 - 9°F) above average during October (Figure 1). This analysis was from a model. When looking at actual measurements made by the Argo float data in early November, the temperatures in the layer 100 meters below the surface under Haiyan were about 3°C above average, not 4 - 5°C, according to Dr. Lin. 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.


Figure 1. Modeled departure of temperature from average at a depth of 100 meters in the West Pacific Ocean during October 2013, compared to a 1986 - 2008 average. The track and intensity of Super Typhoon Haiyan are overlaid. Haiyan passed directly over large areas of sub-surface water that were much above average in temperature, which likely contributed to the storm's explosive deepening. While this model showed 4 - 5°C departures from average in October, the actual values were closer to 3°C in early November, according to Argo float data. Image credit: Japan Meteorological Agency.

Why was there such unusually warm sub-surface water?
The sub-surface waters east of the Philippines have warmed dramatically over the past twenty years. According to Pun et al. (2013), "Recent increase in high tropical cyclone heat potential area in the Western North Pacific Ocean", the depth to where ocean temperatures of at least 26°C (79°F) penetrates has increased by 17% since 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). The strong trade winds have pushed a large amount of water up against the east coast of the Philippines in the past twenty years, resulting in a rate of sea level rise of 10 mm per year--more than triple the global average of 3.1 mm/yr (Figure 2.) This extra sea level rise contributed to the storm surge damage from Super Typhoon Haiyan. Sea level rise data from Legaspi in the Eastern Philippines shows a rise of about 305 mm (12 inches) since 1949. For comparison, global average sea level rose 7.5" (190 mm) since 1901. Part of the rise along the eastern Philippine coast is from tectonic processes--the subsidence of the Philippine plate under the Eurasian plate--but most of it is due to the stronger trade winds piling up warm water along the coast, and the fact that warmer waters expand, raising sea level.


Figure 2. Trend in sea level from satellite altimeter measurements in 1993 - 2010. Black lines are the Sea Surface Height (SSH) in cm from Rio et al. (2009.) Image credit: Qiu, B., and S. Chen, 2012, "Multidecadal sea level and gyre circulation variability in the northwestern tropical Pacific Ocean", Journal of Physical Oceanography 42.1 (2012): 193-206.

Why have the trade winds sped up?
The surface trade winds in the equatorial Pacific are part of the Walker Circulation--a pattern of rising and sinking air along the Equator that the El Nino/La Nina cycle influences. A strong Walker circulation means there is lower 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, they pull surface ocean waters away from South America, allowing cold water to upwell to the surface. This is a La Niña-like situation, which takes heat energy out of the atmosphere, putting it into the ocean, keeping global surface temperatures cooler than they would otherwise be. A weakened Walker circulation is the reverse, resulting in weaker trade winds, and a more El Niño-like situation with higher global surface temperatures. As long as the stronger Walker circulation that has been in place since the early 1990s holds, global surface temperatures should stay cooler than they otherwise would be, prolonging the slow-down in global surface warming that has received much attention this year. There may also be a greater chance of super typhoons and higher storm surges affecting the Philippines, due to the warmer sub-surface waters and re-arranged ocean currents. A 2013 paper by L’Heureux et al. notes that the climate models predict that the Walker circulation should weaken (a more El Niño-like situation)--the reverse of what has been observed the past twenty years. The researchers took the observed pressure patterns over the Pacific in recent decades and removed the atmospheric response to the El Niño/La Niña cycle. The resulting pattern they found showed a steady strengthening of the Walker circulation, in concert with global rising temperatures. So, are we seeing a failure of the climate models? Or is the recent speed-up of the Walker circulation a decades-long temporary "speed bump" in the climate system? Time will tell. It is worth pointing out that a just-released paper by British and Canadian researchers shows that the global surface temperature rise of the past 15 years has been greatly underestimated. As discussed at realclimate.org, "The reason is the data gaps in the weather station network, especially in the Arctic. If you fill these data gaps using satellite measurements, the warming trend is more than doubled in the widely-used HadCRUT4 data, and the much-discussed “warming pause” has virtually disappeared."

I appeared on PBS Newshour last night to discuss the linkages between stronger tropical cyclones and climate change, video here.

References
L’Heureux, Michelle L., Sukyoung Lee, and Bradfield Lyon, 2013, "Recent multidecadal strengthening of the Walker circulation across the tropical Pacific", Nature Climate Change 3.6 (2013): 571-576.

Pun, Iam‐Fei, I‐I. Lin, and Min‐Hui Lo, 2013, "Recent increase in high tropical cyclone heat potential area in the Western North Pacific Ocean", Geophysical Research Letters (2013).

Qiu, B., and S. Chen, 2012, "Multidecadal sea level and gyre circulation variability in the northwestern tropical Pacific Ocean", Journal of Physical Oceanography 42.1 (2012): 193-206.

Jeff Masters