Tropical cyclone history – part I: How reliable are past hurricane records?

(2) Sticking to ship tracks, Chang and Guo (2007) performed a different type of analysis: They compared the ship tracks of the years before the satellite era with TC tracks of recent years. For example, they took the ship tracks of the year 1917 and overlaid the TC tracks of 1999 and determined how many of the 1999 tropical cyclones would have been observed if the ships had navigated as in 1917. By comparing the years before the satellite era to all the ‘satellite’ years (after 1965) they obtained statistics for how many TCs would likely have been missed in earlier years if the distribution of TC tracks had been similar to that during the satellite period. In this way they estimated a TC undercount of about 2 per year in the period 1903-1914, 1-2 per year 1915-1925 and of less than 1 per year from 1925-1965.

The adequacy of this estimation depends on several assumptions: a) that the distribution of the hurricane tracks is about the same in the satellite period than in the periods before. As we have seen before (Holland 2007), there are shifts in the regional distribution over the 20th century. This could influence the estimation of underreporting both in a positive or negative way; b) that all landfalling storms have been detected correctly. It is likely that some of the landfalling storms (or their true strength) might have been missed, which would bias the estimate artificially low; c) that ships did not circumnavigate the storms (e.g. on the basis of predictions). If this was the case, the undercount estimate might be too conservative; d) that ships measured wind correctly. Because this error is random, a significant systematic bias of the undercount estimate is unlikely; and e) that all ship tracks are recorded in the database used. There may have been other, unrecorded ship tracks, which might have detected a storm missed by known ship tracks. This would lead to an exaggerated undercount estimate. Altogether these assumptions likely tend to somewhat underestimate the undercount.

(3) In a third study (Mann et al. 2007; in full disclosure, I was a co-author of this paper), an alternative approach was used employing the statistical relationship between Atlantic TC numbers and three climate variables influencing Atlantic TC activity (1. August-October sea surface temperatures over the main development region (“MDR”); 2. the El Niño/Southern Oscillation, and 3. the North Atlantic Oscillation) during the modern period of reconnaissance flights and satellite observations (1944-2006). This relationship was then used to predict TC numbers for the period 1870-1943 as would be expected from the behavior of the three climate variables used over that period. These estimates were then compared to the observed TC record, the difference providing an estimate for the underreporting. The results yielded an undercount before 1944 of 1.2 TCs per year (best estimate), with a range of 0.5-2 TC per year.

This analysis also relies on several assumptions. Namely, that a) the underlying climate variables do not contain artificial trends or other inhomogeneities that might bias the results. That the results were insensitive to using different alternative SST datasets, or switching the role of training period and prediction period (i.e. training on 1870-1943 and predicting for 1944-2006) was taken, however, as evidence against this being a significant issue. b) that there are no long-term trends in other climate variables not included in the statistical model, but that do influence TC numbers (e.g. wind shear or vertical stability) or might influence the relationships between TC numbers and the variables that are used. Although such an influence cannot be excluded, cross-checks that were performed such as statistical validation and switching the order of training and prediction intervals, seem to argue against this being a problem.

In summary, according to current knowledge, the best estimate for the underreporting bias in the hurricane record seems to be about one tropical cyclone per year on average over the period 1920-1965 and between one and three tropical cyclones per year before 1920. With only a few years of data available, the influence of Quikscat analyses after 2002 as discussed by Landsea, is difficult to as yet meaningfully estimate.

References:

Chang, E. K. M., and Y. Guo (2007): Is the number of North Atlantic tropical cyclones significantly underestimated prior to the availability of satellite observations? Geophys. Res. Lett., 34, L14801, doi:10.1029/2007GL030169.

Holland, G. (2007): Misuse of landfall as a proxy for Atlantic tropical cyclone activity. Eos Trans. AGU, 88, 349.

Holland, G.J., and P.J. Webster (2007): Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Philos. Trans. R. Soc. Ser. A, 365, 2695– 2716, doi:10.1098/rsta.2007.2083.

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