Tropical cyclone history – part II: Paleotempestology still in its infancy

Figure 1. Major hurricanes outside the MDR. The annual number of major hurricane tracks where major hurricane status is only reached north of 25ºN or north of 20ºN between 50-75ºW (red line) and the total annual number of major hurricanes in the Atlantic (blue line), shown both as 5-year-running mean. Data is from the NOAA National Hurricane Center best track data set (HURDAT).

Although the correlation patterns presented by Nyberg et al. suggest a good correlation of their Caribbean proxies to wind shear over a large part of the Northern tropical Atlantic, this does not mean automatically that the correlation of the proxies to total Atlantic major hurricane activity is representative for the whole Atlantic as well. Firstly, hurricane activity is influenced by other factors than wind shear (which might vary over different areas), and secondly the effect of wind shear evidently is opposite in different regions.

There is a correlation of the proxies to major Atlantic hurricane activity over the period 1946-1995, however, the large activity increase already starts more than 5 years earlier in the proxies than in the hurricane record. The authors explain this time lag by the El Niño in the early 1990ies, however, since the influence of El Niño partly works over wind shear, this influence should be already included in the proxies.

Neither the reconstruction nor the calibration period covers the strong increase in major hurricane activity in the 1990ies, but it is obvious from observed wind shear, and recognized by the authors, that this increase cannot be explained by wind shear and is probably due to the increase in SSTs as several studies have pointed out (e.g. Hoyos et al. 2006). Since the training period of their reconstruction model covers a phase, where the main variation of hurricane activity seems to be mostly through wind-shear (1946-1990), but not the following phase, where SST changes are the most important factor, the model probably does not represent the influence of SSTs very well.

Now we come back to the question of the reliability of the hurricane record. One of the big problems of the Nyberg et al. reconstruction is, that it is far off the hurricane record before 1944 (Figure 3 in Nyberg et al). For the period 1850-1944, the reconstruction shows more than 3 (about 3.3) major hurricanes per year on average, while the hurricane record has less than 1.5 per year. Nyberg et al (2007) explain this discrepancy by the unreliability of the hurricane record. This explanation seems too simple. There are, as discussed above, uncertainties in the hurricane record. These are, however, not infinitely large. I have tried to estimate an upper error bar for major hurricane activity. Since there are no estimations of underreporting biases of major hurricanes, I assumed a constant proportion of major hurricanes to total tropical storm number. As Holland and Webster (2007) have shown, this proportion has some multidecadal variation, but these variations are not very large on the 50 year time scale, and there seems to be no significant long-term trend. I used the highest reporting bias estimations of tropical storms by Landsea (2004, 2007) – which are probably too high, as discussed in ‘Tropical cyclone history – Part I’, i.e. 1851-1885 plus 3 tropical storms per year on average; 1885-1965 plus 2; 2003-2006 minus 1; see Figure 2, black line).

Figure 2. Major hurricanes compared to the total number of tropical storms. The red line shows the 5-year moving average of the ratio between major hurricanes and the total number of tropical storms calculated from the best track data of the NOAA National Hurricane Center (HURDAT). The black line shows the ratio after correction of the observational bias proposed by Landsea et al. The green line shows the ratio after correction for a possible observational bias of major hurricanes before 1910.

Nyberg et al. claim that Landsea suggests 0-6 additional storms per year before 1885 and 0-4 per year before 1900 and thus the ‘upper limit’ correction should be 6 and 4 storms, respectively. However, the range given by Landsea represents the annual variation of the bias, not an uncertainty range of the average (I very much doubt that Landsea thinks a zero correction on average to be possible).

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