365 Days of Climate Awareness 125 – Extreme Events: Cyclones

 

As with other extreme events, tropical cyclones are difficult to ascribe directly to global warming. The AR6. WG1 report states that there is “low confidence in observed long-term (40 years or more) trends in TC [tropical cyclone]  intensity, frequency, and duration, and any observed trends in phenomena such as tornadoes and hail…” Yet while statistical analysis does not show an obvious trend in strength or frequency of tropical cyclones, extratropical cyclones are another story.

Weather patterns and storm tracks are governed to a large extent by global wind patterns known as cells, which are organized roughly by latitude (described in post #36). Tropical cells, known as Hadley cells (named after lawyer and meteorologist George Hadley), circulate upward about 10-15 km at the equator and then poleward, descending as cooler air at roughly 30° N/S latitude. Here the Hadley cells are met by the downward circulation of the Ferrel cells, another circulation band which extends from approximately 30-60° N/S. Tropical cyclones are generated within the Hadley cells.

Basic concept of Hadley and Ferrel cell circulations.

Over the past fifty years, the efficiency and energy content of the Hadley cells has increased with the increased heat in the atmosphere. Models show that they are likely to expand. This is consistent with observations of general wind patterns, as well as storm behavior. “[I]t is likely that extratropical storm tracks have shifted poleward in both the Northern and Southern Hemispheres and that heavy rainfalls and mean maximum wind speeds associated with TCs will increase with continued greenhouse gas (GHG) warming…”

Increased warming in the tropics is adding energy to the Hadley cell patterns, causing them to carry more heat more efficiently and stretch toward the poles, bringing tropical weather patterns like cyclones with them.

There is ample anecdotal evidence of disastrously strong extratropical cyclones [ETCs] in the United States: 2012’s Superstorm Sandy and 2021’s Hurricane Ida which tore into Pennsylvania after coming ashore in Louisiana are two. However, the statistical evidence is not as robust: “there is overall low confidence in recent changes in the total number of ETCs over both hemispheres [and] there is medium confidence in a poleward shift of the storm tracks over both hemispheres since the 1980s. Overall, there is also low confidence in past-century trends in the number and intensity of the strongest ETCs due to the large interannual and decadal variability.”

The sample size of annual hurricanes, given the year-to-year and decade-to-decade variability in their number and strength, is small enough that we can’t say confidently they are stronger or more common than before. We can, however, say that large cyclones are progressing farther toward the poles than before, bringing increased destruction to temperate zones, and increasing the transport of heat from the tropics to the higher latitudes.

Tomorrow: the US climate normals.

Be brave, and be well.