Typhoon Haiyan, which devastated large portions of the Philippines in November 2013, was one of the strongest ocean storms ever recorded, killing at least 6,300 people. It set records for the strongest storm ever at landfall and for the highest sustained wind speed over one minute ever, hitting 315 kilometers per hour (194 miles per hour) when it reached the province of Eastern Samar.
Now a new analysis of what controls the peak intensity of typhoons suggests that under climate change this century, storms like Haiyan could get even stronger and more common. The NASA, National Science Foundation, and NOAA-funded study, led by researchers at Scripps Institution of Oceanography, UC San Diego, projects the intensity of typhoons in the western Pacific Ocean to increase as much as 14 percent – nearly equivalent to an increase of one category – by century’s end even under a moderate future scenario of greenhouse gas emissions.
Unusual upper ocean warming rates over the low-latitude northwestern Pacific have already intensified storms in the region since the warmer water provides more fuel for storm intensity. The study projects that by the year 2100, the temperature of the upper ocean will be more than 1.6 degrees Celsius higher than the baseline average of the 50-year period from 1955-2005.
The study appears in the journal Science Advances on May 29.
“Accurate projections of changes in tropical cyclone intensity can benefit society and inform policy making. Previous projections are based on theory and numerical modeling. We are tackling this issue from a different angle,” said Wei Mei of Scripps, the lead author of the study. “We first study the physical mechanisms for the observed variations in cyclone intensity, and then build a statistical model to project the future changes.”
"NOAA is pleased to be a co-sponsor of this study, which sheds further light on the oceanic conditions that control cyclone intensification," said Craig McLean, NOAA's Assistant Administrator for Oceanic and Atmospheric Research. "These results will help us better understand and prepare for the impacts of anticipated changes in storm strength and behavior."
In an effort to understand the variability of the intensity of tropical cyclones including typhoons, the authors employed a new approach by decomposing intensity into two components: intensification rate (akin to acceleration) and intensification duration. This methodology allows them to gain more insights into which climate conditions were most strongly associated with the peak amount of intensity from one year to another by influencing one or both of the two factors. They found that compared to atmospheric factors such as wind shear, atmospheric pressure, and vorticity, ocean temperature most strongly correlated to the rate of cyclone intensification.
Specifically, how strongly and quickly a cyclone can grow depends on two oceanic factors: pre-storm sea surface temperature and difference in temperature between the surface and subsurface. A warmer sea surface generally provides more energy for storm development. A large change in temperature from the surface to subsurface, however, can disrupt this flow of energy, because strong winds drive turbulence in the upper ocean, bringing cold water up from below and thereby cooling the sea surface.
The authors showed that in the current climate, changes in sea surface temperature and in vertical temperature difference contribute nearly equally. But under climate change, sea surface temperature change dominates, because the subsurface ocean warms at a similar pace as the surface, resulting in a modest change in vertical temperature gradient.
“Our findings provide the first observational evidence for the dominance of upper ocean thermal structure in the interannual-to-decadal variability of typhoon lifetime peak intensity,” said co-author Shang-Ping Xie, the Roger Revelle Chair in Environmental Science at Scripps. “The observation-based empirical relationships allowed us to make the first statistical projection of typhoon intensity in a warming climate based on projected changes in upper ocean temperatures.”
Besides Mei and Xie, study co-authors include Francois Primeau of UC Irvine, James McWilliams of UCLA, and Claudia Pasquero of the University of Milano-Bicocca in Milan, Italy.
Scripps Institution of Oceanography at the University of California San Diego, is one of the oldest, largest, and most important centers for global science research and education in the world. Now in its second century of discovery, the scientific scope of the institution has grown to include biological, physical, chemical, geological, geophysical, and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today on every continent and in every ocean. The institution has a staff of more than 1,400 and annual expenditures of approximately $195 million from federal, state, and private sources. Scripps operates oceanographic research vessels recognized worldwide for their outstanding capabilities. Equipped with innovative instruments for ocean exploration, these ships constitute mobile laboratories and observatories that serve students and researchers from institutions throughout the world. Birch Aquarium at Scripps serves as the interpretive center of the institution and showcases Scripps research and a diverse array of marine life through exhibits and programming for more than 430,000 visitors each year. Learn more at scripps.ucsd.edu and follow us at Facebook, Twitter, and Instagram.
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