A new study released by NOAA (National Oceanic and Atmospheric Administration) on Wednesday suggests that the forward movement of tropical cyclones is slowing down, thus increasing the impacts of flooding. This newly-released research has been conducted by scientist Jim Kossin. According to the report, “Globally, tropical cyclones slowed by 10 percent between 1949 and 2016. With additional water vapor in the atmosphere in a warming world, as little as a 10 percent slowdown could double local rainfall and flooding impacts caused by 1°C of warming.”
Tropical cyclones are more common in the Northern Hemisphere, thus the lack of them has been most substantial for that region. According to the map below, tropical cyclones have slowed down by 20% on average in the western North Pacific Ocean, followed by the Australian region at 15%. Closer to home in the North Atlantic, tropical cyclones over both water and land have slowed down by 6%.
Due to this slowing effect, Kossin asserts that “these trends are almost certainly increasing local rainfall totals and freshwater flooding, which is associated with very high mortality risk.” The main reason for this slowing is mainly due to the weakening of the steering winds, which guide and move these storms. This is a sign of the change in our Earth’s environment due to the warming temperatures. Kossin writes how “the observed 10% global slowdown occurred in a period when the planet warmed by 0.5°C, but this does not provide a true measure of climate sensitivity, and more study is needed to determine how much more slowing will occur with continued warming.” He goes on, saying how it’s still “entirely plausible that local rainfall increases could actually be dominated by this slowdown rather than the expected rain-rate increases due to global warming.”
This study is backed up by a relatively-recent event, Hurricane Harvey. This hurricane, making landfall on the Texas coast as a category four storm on August 26th of 2017, was the most significant rain-producing storm in the US on record. Some locations experienced over two feet of rain in just a matter of two days. According to NOAA, “while slower movement of tropical cyclones can most certainly lead to higher rainfall totals and devastating flooding, it was not the only factor that led to Harvey’s devastation. In addition to moving slowly, Harvey also reversed direction causing it to impact the same region for a particularly long period of time.”