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STUDY SHOWS POLEWARD SHIFT IN TROPICAL CYCLONE MAXIMUM INTENSITY
MADISON – The latitude at which tropical cyclones reach their maximum intensity is migrating away from the tropics, shifting in the direction of the poles at a rate of about one-half degree of latitude per decade in both the Northern and Southern Hemispheres.
A new study published today (05/14/14) in the journal Nature documents this poleward migration by analyzing global historical tropical cyclone data for the past 30 years.
According to lead author Jim Kossin, NOAA National Climatic Data Center atmospheric scientist stationed at UW-Madison’s Cooperative Institute for Meteorological Satellite Studies, “we’ve identified changes in the environment in which the deep tropics have become more hostile to the formation and intensification of tropical cyclones and the higher latitudes have become less hostile. This seems to be driving tropical cyclones out of the deep tropics and toward the poles.”
Evidence for this migration of storms away from the tropics is based on observed trends in the latitude where storms reach their maximum intensity – this is the moment when a storm is strongest relative to its own lifetime. Intensity estimates can vary from dataset to dataset, but the latitude of tropical cyclone maximum intensity is a much more stable value and less likely to be influenced by data discrepancies or uncertainties.
The 30-year trend is not necessarily indicative of a longer-term trend, explains Kossin.
However, says Kossin, the results are important because they may forewarn of changes to come in the regions where storms make landfall. Places closer to the equator could experience a reduced risk for landfalling tropical cyclones. And places farther away from the equator, including Northern and Southern Hemisphere coastal cities, could experience an increased risk. This increased risk is particularly important given the devastating loss of life and property that can follow in the wake of a tropical cyclone.
Further, regions that depend on precipitation from tropical cyclones to replenish local water supplies may, in the future, have less water available to them. At the same time, regions that are less prepared may experience more frequent flooding.
From a scientific standpoint, Kossin notes, “the more compelling aspect is that the rate of migration fits very well into independent estimates of the observed expansion of the tropics,” a phenomenon that has been widely studied by other scientists.
And one that is attributed, in part, to increasing greenhouse gases, stratospheric ozone depletion, and particulate pollution that are by-products of human activity.
Whether the observed movement of tropical cyclone maximum intensity toward the poles is a result of the expansion of the tropics and its links to human activity requires more and longer-term investigation, says Kossin. Both phenomena, however, exhibit very similar behavior over the past 30 years, lending support to the idea that the two are closely related.
Co-authors on the Nature paper are Kerry A. Emanuel, Program in Atmospheres, Oceans and Climate at the Massachusetts Institute of Technology and Gabriel A. Vecchi, NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey.
Featured image on SSEC home page: Global montage of satellite images of tropical cyclones. Each image represents the strongest storm recorded in that area for the period 1980-2008. Images are from infrared satellites with colors indicating the intensity of the convection, which relates to storm intensity. Grays are weakest, blues-greens are stronger, and yellow-reds are strongest. The graphic shows the overall equator-to-poles distribution of tropical cyclone intensity. Image credit: Ken Knapp, NOAA National Climatic Data Center, Asheville, NC.