Examining the ATL hurricane data in another thread, I pointed out that there was a very substantial increase in coverage further to the east. In the entire data set, the median westing for a track measurement was 69W.
To attempt to minimize count bias resulting from increased eastward coverage, I did storm and hurricane count calculations restricting measurements to tracks west of 69W. If there is a robust trend, then obviously any robust trend would also exist in the restricted domain west of 69W.
CA readers are used to novelties, but this one is pretty neat.
In the following two graphics, I restricted the track data to measurements west of 69W and then took a count of the number of tropical cyclones in each year. Holland and Webster said that the ATL data had three regimes: from 1905-1930; from 1931-1994 and post 1995 with a sharp demarcation between each “regime”. Does it look to any of you like the post-1994 counts are off the charts, especially when the 2006 season is included?
Figure 1. ATL cyclone count (west of 69W)
But there’s more.
The next figure shows the hurricane count restricting the data in the same way. 2005 is a big season (like 1933 also a very hot year), but where’s the overwhelming statistical trend?
Figure 2. Same with hurricane (GT 65 knots). Red – runnning 9-year mean.
UPDATE. Here are the same graphs restricted to the east of 69W. Figure 3 below is calculated parallel to Figure 1. Some storms will occur in both counts – we’re exploring the data here. There’s a lot in common between Figure 1 and Figure 3, but obviously Figure 3 has a trend that doesn’t exist in Figure 1. Is this climatological – Judith Curry suggests that it might be something to do with the AMO – or is it an artifact of changing measurements? I don’t know, although I’m inclined to think that it’s probably to do more with measurements than climatology. However, isn’t this the sort of thing that the specialists should be doing before publishing articles announcing that the trends are “strong” and “statistically significant”?
Figure 3. As with Figure 1, restricted to east of 69W.
Now for the same thing for hurricane-strength wind measurements showing the results E of 69W. Here the trend is not as strong as for storms, lending further support for the suspicion that there is a methodological bias in eastern “small” storms. However, there’s something else interesting in this plot – if one is seeking “regimes” in this particular chart, one would be inclined to assign a regime to the post-1950 period i.e. the modern aircraft surveillance period. But what is one to make of the similar counts in the late 19th century? Is there a “regime change” associated with a changeover from sailing vessels to steam vessels? Looks possible to me. (I’m old enough that high school readings for me included novellas like Conrad’s Typhoon and Youth, so the memories of hurricanes and sailing vessels were still alive in literature in the 1950s and 1960s.)
Maybe this is all to do with the AMO as Judith Curry is now proposing, but again I think that the authors should try to work these things out before rather than after publishing.
Figure 4. Same as Figure 2 (hurricane strength measurements), restricted to east of 69W.