The forward speed of tropical cyclones (which includes all hurricanes, tropical storms, and tropical depressions) has decreased globally by about 10% since 1949, according to a paper published in Nature on Wednesday by University of Wisconsin hurricane scientist Dr. Jim Kossin. As a result of their slower forward motion, these storms are now more likely to drop heavier rains, increasing their flood risk. Most significantly, the study reported a 20% slow-down in storm translation speed over land for Atlantic storms, a 30% slow-down over land for Northwest Pacific storms, and a 19% slow-down over land for storms affecting the Australia region. A storm moving 20% slower over land has the opportunity to dump up to 20% more rain over land, increasing the flood risk for flood defense systems designed for a 20th Century climate with less extreme precipitation events. The paper concluded that “these trends have almost certainly increased local rainfall totals in these regions.” Another increased hazard slower storms bring is increased wind damage, due to an increase in the duration of damaging winds structures are exposed to.
The slow-down was observed in both the Northern and Southern Hemispheres, and in every ocean basin except the North Indian Ocean. Tropical cyclones affecting land areas in the Eastern Pacific, Northern Indian Ocean, and western South Indian Ocean showed no significant trends in translation speed while they were over land.
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