Slower Hurricanes Means More Dangerous Hurricanes

With the US Atlantic 2018 hurricane season already underway, many are wondering if we are about to witness more destructive events like the last year’s major Hurricanes Harvey, Irma, and Jose.

NOAA forecasters predict a 70 percent likelihood of 10 to 16 named storms, of which 5 to 9 could become winds of hurricane strength. Out of those, 1 to 4 could be major hurricanes, featuring winds of 111 mph or more.

Since an average season features 12 named storms, with 6 hurricanes and 3 major hurricanes, NOAA expects that the 2018 hurricane season will most likely be average (40% likelihood) or above average (35% likelihood). The 2017 season had similar pre-season outlooks. However, the outcome was above average – with 17 named storms, six major hurricanes, and at least 464 casualties.

Hurricanes Irma and Harvey were so severe that some recommended to update the five-category Saffir-Simpson scale to include Category 6 storms.

These Landsat 8 images, acquired August 25 and September 20, 2017 show the before and after effects of Hurricane Irma on the British and U.S. Virgin Islands. Source: NASA, public domain.

Why Have Hurricanes Become so Violent?

A study published in the scientific journal Nature can provide at least a partial answer to that question. The author, James P. Kossin from NOAA’s Center for Weather and Climate, has found that hurricane speed has slowed significantly in the past six decades – specifically, between 1949 and 2016.

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The estimated 10 percent slowdown enables storms to linger longer in certain areas and deliver more massive rainfalls. When the affected area is densely populated – like Houston – it can cause a catastrophic chain of events that we have witnessed during Hurricane Harvey.

To make things worse, in some cases the slowdown seems to become greater once a storm reaches the land – Kossin’s study found that in the Atlantic region, storms move 20 percent slower over land.

Why Are Hurricanes Slower and Heavier?

What is the mechanism that now makes storms slower and heavier? Kossin has implied that the global warming-driven changes in circulation of the Earth’s atmosphere play a significant role.

Polar regions warm up faster than equatorial regions, and the disturbance slows down the overall atmospheric circulation. Because tropical cyclones and storms are an integral part of this system, they also become governed by the new rules.

At the same time, the increase in global temperatures leads to more evaporation. More water in the atmosphere means that storms also become more saturated with water, leading to torrential rainfalls.

When slowdown and water vapor combine, it is easy to see that more destructive storms can be expected in the future, though it is not guaranteed that we are in for another exceptionally rough year.

Predictions remain the same, but the fact that the cyclone system is obviously changing should be counted in.

While global warming certainly plays a significant role in the surge of heavy storms, there may be other factors contributing to the increased hurricane strength that we don’t know of yet.

Colin Zarzycki, a climate scientist who reviewed Kossin’s study, added that there is a possibility that some unknown, decades-long cycle might exist, but couldn’t be noticed in the 60-year period that the study covers. As more seasons unfold and scientists gather more data, interesting findings about the nature of contemporary storms are sure to surface. In the Atlantic Ocean Basin, hurricane season spans from June 1st to November 30th.