Revisiting: Hurricanes are slowing down and becoming more dangerous

Hurricane Laura, August 26, 2020

In 2017, Hurricane Harvey moved very slowly across parts of Texas (US) dropping “more than 30 inches of rain in two days and nearly 50 inches over four days.” “Harvey’s rainfall exceeded every known flooding event in American history since 1899.”

The reason for the high rainfall totals was the slow movement of the storm–and a 2018 study (Kossin) reports that “between 1949 and 2016, tropical cyclone translation speeds (how quickly a storm moves) declined 10 percent worldwide.” While a 10% change may not seem significant, the reality of Hurricane Harvey and other recent storms show the effect–“devastating flooding and billions of dollars of damage” as well as death, trauma, stress, widespread and long-lasting environmental damage, and the list goes on.

What is the cause of the slower storm movement? “Broader evidence suggests that climate change is playing a role.” The winds that push hurricanes along are influenced by temperature differences between the tropics and the Poles. That temperature difference is getting smaller. As a result, the steering winds are weakening, and the hurricanes and other storms are moving more slowly dropping more rain and causing more destruction.

Hurricanes are becoming more dangerous.

Additional scientific analysis indicates that as the winds that push hurricanes weaken, the winds inside hurricanes strengthen. And, the danger from hurricanes is not just along the coasts. Inland flooding and mudslides caused by the intense rainfall “poses the highest mortality risk nowadays in certain regions.”

Read the article (Kendra Pierre-Louis, New York Times, June 6, 2018).

Sources:

*Kossin, J. P. (2018). A global slowdown of tropical-cyclone translation speed. Nature, 558(7708), 104-107,107A-107H. [Cited by]

*Kossin, J. P. (2018). Author correction: A global slowdown of tropical-cyclone translation speed. Nature, 564(7735), E11-E16. [Cited by]

*Hall, T. M., & Kossin James, P. (2019). Hurricane stalling along the North American coast and implications for rainfall. NPJ Climate and Atmospheric Science, 2(1). [PDF] [Cited by]

The average speed of tropical cyclone (TC) translation has slowed since the mid 20th century. Here we report that North Atlantic (NA) TCs have become increasingly likely to “stall” near the coast, spending many hours in confined regions. The stalling is driven not only by slower translation, but also by an increase in abrupt changes of direction. We compute residence-time distributions for TCs in confined coastal regions, and find that the tails of these distributions have increased significantly. We also show that TCs stalling over a region result in more rain on the region. Together, increased stalling and increased rain during stalls increased coastal rainfall from TCs, other factors equal. Although the data are sparse, we do in fact find a significant positive trend in coastal annual-mean rainfall 1948–2017 from TCs that stall, and we verify that this is due to increased stalling frequency. We make no attribution to anthropogenic climate forcing for the stalling or rainfall; the trends could be due to low frequency natural variability. Regardless of the cause, the significant increases in TC stalling frequency and high potential for associated increases in rainfall have very likely exacerbated TC hazards for coastal populations.”

*Kossin, J. P., Knapp, K. R., Olander, T. L., & Velden, C. S. (2020). Global increase in major tropical cyclone exceedance probability over the past four decades. Proceedings of the National Academy of Sciences of the United States of America, 117(22), 11975-11980. [Cited by]

“Theoretical understanding of the thermodynamic controls on tropical cyclone (TC) wind intensity, as well as numerical simulations, implies a positive trend in TC intensity in a warming world. The global instrumental record of TC intensity, however, is known to be heterogeneous in both space and time and is generally unsuitable for global trend analysis. To address this, a homogenized data record based on satellite data was previously created for the period 1982–2009. The 28-y homogenized record exhibited increasing global TC intensity trends, but they were not statistically significant at the 95% confidence level. Based on observed trends in the thermodynamic mean state of the tropical environment during this period, however, it was argued that the 28-y period was likely close to, but shorter than, the time required for a statistically significant positive global TC intensity trend to appear. Here the homogenized global TC intensity record is extended to the 39-y period 1979–2017, and statistically significant (at the 95% confidence level) increases are identified. Increases and trends are found in the exceedance probability and proportion of major (Saffir−Simpson categories 3 to 5) TC intensities, which is consistent with expectations based on theoretical understanding and trends identified in numerical simulations in warming scenarios. Major TCs pose, by far, the greatest threat to lives and property. Between the early and latter halves of the time period, the major TC exceedance probability increases by about 8% per decade, with a 95% CI of 2 to 15% per decade.”

Questions? Please let me know (engelk@grinnell.edu).

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