I read a recent article in the Guardian by Michael Mann saying that recent research had shown that tropical cyclones are slowing down and even stalling, in a similar way to how hurricane Dorian (2019) has recently done over the northern Bahamas and hurricane Harvey (2005) did over Texas. Never wanting to take things I read at face value, I decided to do some research of my own into the subject by means of the HURDAT2 database, and see what a bit of analysis would uncover. The software wasn’t as difficult to write as I had imagined and I displayed the results in three separate graphs. This first of these graphs (fig 1) shows the average speed of all hurricanes since 1945, I have ignored any 6 hourly reports when the tropical cyclone wasn’t classed at least category 1 strength. As you can see there is not a lot of variation since 1945, but does give you an idea of the spread of speeds.
The second graph (fig 2) is a bar chart of average annual speed of all hurricanes overlaid with a linear trend. Again there’s not a great deal of variation since 1945, but the linear trend does highlight a reduction in speed of around 10% or 1.4 knots during that time. This reduction more or less ties in with the first of the two studies mentioned in the Guardian article. So the answer is, yes, hurricanes do seem to be slowing down as the first report in Nature suggests.
- A global slowdown of tropical-cyclone translation speed
- Hurricane stalling along the north American coast and implications for rainfall
The second report concerns the stalling of hurricanes on the eastern seaboard of the United States, and is a little more tricky to verify. One way that I came up with was to count the 6 hourly speed of all hurricanes and simply keep a tally of when their forward speed was 5 knots or less. Here’s the data in the form of another bar chart with overlaid linear trend (fig 3).
What I found from this is that the number of times stalling occurred has decreased slightly since 1945. Admittedly, I have chosen to look at hurricanes over a much larger area (south of 40° north and west of 60° west) than the second study, but this simple analysis shows no increase in stalling of hurricanes, even if their average speed has decreased during that time.
I can’t imagine why anyone would think that hurricanes have become more slower and lethargic than they were in the past, and I can’t see how you could possibly tie this slowness to warmer sea surface temperatures. If anything you would have thought that as SST increased hurricanes would have become more energetic. And how you can link this claimed increase in the number of stalled hurricanes to recent increases in global temperatures sounds more like science fiction than science fact to me.