The 2017 Atlantic Hurricane Season in Perspective

When I was doing graduate work for doctoral studies, my advisor caught me up on the use of the word “significant” in a paper I was writing. He observed that this term had an explicit statistical meaning that was quantifiable and I hadn’t supported its use with the necessary analysis.

Today, a paper came to my attention that reminded me of the use of descriptors that are emotionally loaded while failing to provide an analytical basis for their use. The paper, Causes and Predictability of the Exceptionally Active 2017 Atlantic Hurricane Season, uses words such as “exceptionally active” and “extreme ACE anomalies”. Other terms such as “active” become meaningful only with a similarly precise definition.

We will comment on the use of these terms in the context of the two graphs from the paper, presented in Figure 1 below.

Figure 1. Data on the 2017 US hurricane season

The statistical means of analyzing the data in these two charts would be to calculated the mean and standard deviation (STD) for the data sets. One can then calculate how many STDs the outlier points are from the mean. In describing these points one would then define how many STDs represent an “extreme” event.

The data covers a period of 98 years. The paper seems to use the word “active” as an expression of the amount of accumulated cyclonic energy (ACE) shown in Figure 1A. Inspecting the graph 1A we see that in the last 22 years from about 1995, there were 4 years with the same or greater ACE as 2017, or 18%. If, however, we go back to 1920 where the data begins, the number is 7 out of 98 or about 7%.

So to describe 2017 as an exceptionally active year depends on the period over which one observes storms. ACE is a relatively recent invention. The degree of accuracy with which storms of the past can be modeled, I would question. Certainly proper analysis does not provide simply a single number but a number with a confidence interval – the range of error associated with the caluclation. Thiss is easily graphed by associating an error bar with each point.

A hundred years from the perspective of earth’s climate is such a minute period, conclusions based on it must be considered to be inconclusive.

The data in Figure 1B appears even less “exceptional”. Over the 98 year period, 24 years exhibited the same or greater number of landfalls as 2017. That’s 24% or roughly once every 4 years on average.

In conclusion, 2017 was certainly more active than normal where “normal” is the mean annual ACE for the period examined and “active” is actual ACE for the year. And there were certainly more landfalls than normal where “normal” refers to the mean number of 1.5 as cited in the paper. I would not, however, call 2017 an “exceptional” or “extreme” year, particularly since with the recent “hiatus” in temperature rise, the political dialogue has shifted to “extreme weather” as the justification for the carbon taxation being imposed across the globe.

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