It’s Christmas and a good time to visit Tiny Tim. The visit compares Atlantic storms in two periods and uses Tiny Tims to help in the comparison.
One is a modern period (the last twenty years, 1988-2007). This is a period of good (and ever-improving) detection tools, like advanced satellites, improved recon devices, denser buoy networks and so forth.
The modern period also matches the 1988-2007 list of Tiny Tim storms. Tiny Tims are storms so weak, small, remote and/or short-lived that there’s no record of ships or land experiencing storm-force winds, yet they were classified as tropical storms. By historical standards these modern Tiny Tims would have been regarded as depressions or disturbed weather, not tropical storms.
The second period is 1925-1944, which immediately precedes the start of aircraft recon and has reasonably similar AMM/AMO characteristics to the modern period. I’ll call this the “pre-recon” period. This pre-recon period had only ship and landfall information for knowledge of the existence and strength of tropical cyclones. It also had what I describe as little more than educated guesses about a storm’s strength, mainly based on ship data which was often sparse and distant from the center of the stronger storms. The weathermen of the day were detectives, as are those who reconstruct storm history, and they made the best of what little data they had.
Why look at 1925-1944? The 1925-1944 period was at or near the prior peak in Atlantic storm activity, so an examination of that period could be useful in comparison to the present elevated activity.
The data I use are the “ACE” values of the individual storms. ACE is a function of storm duration and intensity. The quality of storm ACE data for the modern period is pretty good but it is of highly questionable quality for 1925-1944. That cannot be emphasized enough. It’s important to not put too fine a point on any comparison.
So, with that background, here are several questions which I try to answer with graphs:
1. What do the distributions of storm ACEs look like in the modern era and the pre-recon era? How do they compare on a normalized basis?
3. How do they compare on an absolute basis?
4. How do the storm counts compare if Tiny Tims are removed from the modern period?
The figures below are bar charts which show the total count of storms in each ACE value (0 to 1, 1 to 2, etc). The overall visual impressions are (1) that the bulk of storms have fairly low ACE in both periods, (2) that there are more storms in the modern period than in pre-recon and (3) the pre-recon period seems to have an odd shortage of the weakest (ACE less than 1.0) storms.
On a normalized basis, the distributions look similar but with a couple of notable differences as shown below. The pre-recon “shortage” of very weak (0 to 1 ACE)storms is notable on the left side. Also, the pre-recon distribution seems to tail off more slowly (“fatter” around 15 and “thinner” around 30). Perhaps that is real or perhaps it reflects better ability to measure extreme events in the modern era or perhaps it’s a combination of effects – no way to know.
(Another item is the hint of a peak around 20 – maybe real, maybe noise, a topic for another day).
For questions #3 and #4 I’ll use one chart shown below. For this one I put the ACE values into buckets (0 to 2.99, 3.0 to 5.99, etc). The blue line is the as-is storm count data, which shows that the difference between 1925-1944 and 1988-2007 is largely concentrated in the weakest systems. This is not a surprise for those of us who think that improvements in detection of weak systems have played a major role in the increase in storm count.
What if the Tiny Tims are removed from the modern period, to put things on more of an apples-to-apples basis? The answer to that is the dashed red line. The removal of Tiny Tims makes a remarkable reduction in the weak-storm difference.
That’s the main thing I was exploring – the impact of the Tiny Tims. But, the curve also shows a tendency for the modern period to be somewhat more active at ACEs above 5.
It’s my conjecture that some weak-to-moderate storms (5 to 10 ACE) in the remote eastern Atlantic were missed in the pre-recon era, due to the likely low density of ship traffic in that region in the Great Depression and WW2 era. There is also a possible issue with the extra-tropical classifications in the pre-recon period which I won’t get into here.
Even with those, though, there still appear to be about 15 more strong storms (ACE above 10 to 15) in the modern era than in the pre-recon era. That’s 0.75 storms a year of moderate or strong ACE. Maybe that’s real and due to higher SST, maybe that’s real and due to a partial mismatch of periods, maybe it’s another measurement artifact, who knows.
As mentioned near the start of this, historical ACE values need to be used with great caution as they were almost always based on best-guesses rather than measurements.
The interesting thing to me in this exercise is the notable impact of removing the Tiny Tims in the fourth plot, which puts things more on an apples-to-apples basis.