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

Climate change impacts on hurricanes generally focus on two key quantities, the frequency and intensity of storms. There is no reason to believe that both quantities will change similarly. In the discussion of past activity, the frequency is described by the number of tropical storms that occur annually in each basin. The maximum intensity (or even more complex metrics such as the Power Dissipation Index which integrates intensity over space and time; Emanuel 2005) is harder to measure, because it requires detailed information about the storm along the storm track over its entire lifetime. Therefore most discussions of historical trends have focused on tropical storm frequency/number or in some cases the number of intense storms (e.g. the number of major hurricanes).

Thus the key question is: how many storms did we miss in the past? Recently there have been a number of attempts to estimate this ‘undercount bias’ in the tropical storm record. These attempts have included:

  • 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.

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