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

  • reconstruction of the observational bias by relating past observation density (e.g. ship tracks) to modern storm tracks
  • using the relation between total TC number and better known subsets of the TC record (e.g. landfalling storms)
  • using relationships of known underlying variables (e.g. relevant climate indices) to annual TC numbers to create a ‘predicted’ TC record, and compare it to the observed record.

All these approaches have a common caveat, namely the assumption that the relationships they rely upon are constant over time. The validity of this assumption therefore has to be examined in any studies using such approaches. Let us consider some recent such studies:

(1) Landsea [2007] performed a simple analysis to estimate the observational bias for the time from 1900 until the begin of the satellite period in 1966. He examined the percentage of tropical cyclones that struck land (PTL) and notices a considerable difference between the time periods 1900-1965 (pre-satellite period, PTL=75%) and 1966-2006 (PTL=59%). He suggests that this difference indicates an underestimation of about 2 tropical cyclones per year before 1965.

Unfortunately, Landsea does not discuss the evolution of PTL before 1900 (left side of the red dashed line in Figure 1). If PTL really is a proxy for underreporting due to decreasing observation density, PTL should further increase before 1900. However, there is a decrease. The period 1851-1899 has an average PTL of 67%, the period 1851-1885 even has an average PTL of 61%, which is not significantly different from the satellite period after 1966 (59%).

Figure 1. Percentage of all reported tropical storms, subtropical storms, and hurricanes that struck land 1851-2006. Extension of Fig. 2b in Landsea [2007].

Therefore it is questionable if PTL really is a reliable proxy of underreporting. One might argue, as Landsea implicitly does, that after 1900 the population density on the coasts and islands was high enough to catch all tropical cyclones, and the underreporting is only due to decreasing density of shipping tracks, while before 1900 also some tropical cyclones that struck land were missed. However, before 1900 not only population density but also shipping track density was lower and therefore PTL likely should be about at the same level as after 1900 but not significantly lower. In addition, Landsea contradicts that argument himself by stating that even in 2005 a retrospective analysis reveals that there was a tropical cyclone that made landfall in a sparsely populated area and was therefore not initially included as a landfalling storm.

Moreover, in another study (Holland, 2007) it has been shown, that the natural variability of TC numbers is different for different regions of the tropical Atlantic. Therefore, the proportion of TCs over the open sea also varies naturally, altering the landfall proportion for reasons unrelated to observation bias. Thus PTL seems not to be a good proxy for observational biases. The study showed that the decrease in PTL in the 1960s is mainly due to a decrease of TC number in the Caribbean and the Gulf of Mexico. Since these regions are well observed by dense ship tracks and a number of islands, this decrease is very unlikely to be mainly an observational bias.

(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.

Page 2 of 3 | Previous page | Next page