More Stalled Hurricanes And Less Wind Shear – Bad News For U.S. Coasts

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The 2019 Atlantic hurricane season has started even as impacts of the 2017 and 2018 seasons are still being felt in the United States. Over the past 2 years, lives, property, and economies from the Caribbean to the mainland U.S. were destroyed by hurricanes bearing names like Harvey, Maria, Michael, Irma, and Florence. Questions always arise about whether we have entered a generation of naturally-varying hurricanes “juiced” by a climate change steroid. The NOAA Geophysical Fluid Dynamics Laboratory website is, in my view, a definitive source on where the science stands on climate change – hurricane linkages. There is so much inaccurate information swirling around questions like “Is climate warming causing more frequent or stronger hurricanes?” I strongly urge the reader to visit the website, which is continuously updated by hurricane experts, rather than play “Twitter tennis” with people on whether changes in hurricane activity are natural or anthropogenically-impacted (By the way, it is almost always “and” not “or”). Four recent studies in the peer review literature show that hurricanes and their environments along the U.S. East Coast are changing. Here is my breakdown of why that is bad news.

Hurricane Florence

USA.gov

The study that originally caught my eye was a new study published in Nature. The paper is entitled, “Hurricane stalling along the North American coast and implications for rainfall.” The study found that average speed of tropical cyclone movement has slowed since the middle of last century. Specifically, North Atlantic tropical cyclones (hurricanes) are more likely to “stall” near coastlines.  This is precisely what we saw with Hurricane Florence in the Carolinas in 2018 and Hurricane Harvey in 2017. Both storms produced record-breaking, life-altering, and economy-devastating flooding. Jim Kossin, one of the authors of the study, told me by email:

There’s nothing good that comes with a hurricane hanging around your neighborhood for a long time. You get much more rainfall flooding, which is a leading cause of hurricane-related deaths, as well as more wind damage. The recent hurricanes Harvey and Florence, and Cyclone Idai in Mozambique show this to devastating effect

Kossin, an atmospheric research scientist at NOAA’s Cooperative Institute for Meteorological Satellite Studies

at the University of Wisconsin, also points out that their analysis found evidence of increased coastal rainfall over the period 1948 to 2017. The paper concludes that the stalling is related to slower movement of the storms and abrupt changes in direction. It was beyond the scope of the study to attempt attribution of the stalling tendency to natural variability or climate warming.  Another paper published by Kossin recently found a 17% slowdown of hurricane translation speed over the conterminous US since 1900.

Flooding from Hurricane Harvey in Houston, Texas

NWS

A third study published by Kossin in Nature, one of the most selective and rigorous scientific journals in the world, found that the natural climate barrier to hurricane intensification along the U.S. coast is being degraded by greenhouse gas (GHG) forcing. The study abstract points out that vertical wind shear (VWS) and sea surface temperatures (SST) “form a protective barrier along the United States during periods of heightened basin-wide hurricane activity.” It is well-known that warm SST is favorable for hurricane intensification, on average, and oceans generally are warming in response to climate change. Scientific understanding of how VWS will change due to climate warming has lagged.

Typically, vertical wind shear serves to mitigate hurricane development as the storms move out the tropics and toward the U.S. East Coast (graphic below). Kossin and colleagues from Columbia University’s Lamont-Doherty Earth Observatory recently published a paper in Scientific Reports that used climate models to show that rising greenhouse gases (atmospheric carbon dioxide is currently at 413.5 ppm) will weaken vertical wind shear along the East Coast. Some of the simulations suggests that this could happen within the next couple of decades. Weaker wind shear, coupled with warmer SSTs, would be favorable to intensification if a hurricane moves into the region. Unfortunately, separate studies by scientists like Kerry Emanuel and colleagues have found that hurricanes are intensifying, on average, further away from the tropics and at latitudes closer to U.S. coastal cities.

Kossin sums up the threat in the email, “We’re finding long-term changes in hurricane behavior that are heightening risk along the U.S. East Coast.”

Deep Layer Wind Shear along the U.S. East Coast on June 7th

CIMSS/NOAA

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