Such superstorms as Tropical Cyclone Ian, which menaced Fiji in January, are reaching their peak intensities closer to the poles, shifting the locations at which they pose the greatest threat. NASA image courtesy of Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team, NASA GSFC
A research team finds that the point at which tropical cyclones reach their maximum intensity is shifting toward the poles, with serious implications for coastal engineering projects.
June 17, 2014—A team of climate researchers who are examining how temperatures in the upper levels of the atmosphere are changing by examining infrared satellite data of tropical cyclones made an unexpected discovery recently. The data show that the latitudes at which tropical cyclones reach their maximum intensity have shifted toward the poles significantly in the past 30 years.
The discovery, if confirmed by further research, has the potential to impact engineers working on projects in coastal areas where return rates for hurricane-force winds could potentially change significantly, according to the study’s lead author, James Kossin, Ph.D., an atmospheric research scientist at the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center.
The research findings, “The Poleward Migration of the Location of Tropical Cyclone Maximum Intensity,” were published in May as a letter to the journal Nature. The letter was coauthored by Kerry A. Emanuel, Ph.D., a professor of atmospheric science at the Massachusetts Institute of Technology, and Gabriel Vecchi, Ph.D., the head of the Climate Variations and Predictability Group at NOAA’s Geophysical Fluid Dynamics Laboratory.
Global historical data on cyclones is inconsistent, in part because the technology used to track the storms has improved dramatically in the past century, moving from distant observations to manned flights into the storm to monitoring via satellite.
Because of this, the authors examined a combination of recent, more precise, data from 1982 to 2012 to chart the locations at which cyclones achieved their lifetime-maximum intensity (LMI) and calculated an annual mean of those figures. The position of a cyclone at the time of LMI is considered one of the more precise measurements of cyclones available because by the time a storm has reached full strength it has usually been monitored for several days.
The team found that over the three decades, the LMI of cyclones in the Northern Hemisphere migrated north by 53 km per decade. In the Southern Hemisphere, the LMI of cyclones has migrated south by 62 km.
Was this a surprise? “Yes. And then quickly, no,” Kossin says. “We tested it in a number of ways. It’s important to be skeptical when you find something like that. We really wrung it out. No matter what we did to try to make the trend go away, it wouldn’t. [Then] we said, ‘OK, what is going on here?’ ”
Kossin says a separate body of research has suggested that the tropics are expanding toward the poles in both hemispheres, and Kossin notes a correlation between those expansions and the shift in LMI that his team observed. “It’s very compelling that they seem to be working together,” Kossin says. “Then the question becomes why would one thing cause another?”
The development of a tropical cyclone is governed by the “potential intensity” of the environment in an area at a given time. This thermodynamic property dictates how strong a storm can possibly become. Balancing this is vertical wind shear. Cyclones become weaker as vertical wind shear conditions increase. Cyclones reach their LMI at the point at which the potential intensity is greatest and wind shear is weak.
“So what we found, when we looked at how the environment is changing, [is] that the potential intensity in the deep tropics was decreasing over time and increasing, relatively, in the higher latitudes,” Kossin says. “At the same time, the shear, which is the confounding factor that keeps it from reaching its potential, is increasing in the deep tropics and decreasing in the higher latitudes. And this is in both hemispheres.”
Why this is happening is the “million-dollar question,” Kossin says. The team is in the process of further research, analyzing numerical models. They will first determine if the numerical models can recreate the poleward shifts that have been observed. “Then we would do experiments where we turn up CO2 or turn down CO2 or [change other factors] that specifically target a change that’s related to humans or natural variability, and see how the tropical cyclones respond to that.
“At this point, what we have is some very compelling evidence that the expansion of the tropics is creating this change in shear and potential intensity and the tropical cyclones are responding to that by migrating,” Kossin says.
“All of these things need to be looked at much more carefully,” he adds. “This is kind of a call to arms to add even more motivation for a better understanding of what is causing the tropics to expand.”