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

Guest Commentary from Urs Neu

When discussing the influence of anthropogenic global warming on hurricane or tropical cyclone (TC) frequency and intensity (see e.g. here, here, and here), it is important to examine observed past trends. As with all climate variables, the hurricane record becomes increasingly uncertain when we go back in time. However, the hurricane record has some peculiarities: hurricanes are highly confined structures, so you have to be at the right place at the right time to observe them. Secondly, hurricanes spend most of their life in the open oceans, i.e. in regions where there are very few people and no fixed observations. This means that the reliability of the long-term hurricane record is dependent on who was measuring them, and how, at any given time. The implementation of new observation methods, for example, might have altered the quality of the record considerably. But how much? This crucial question has been widely discussed in the recent scientific literature (e.g. Chang and Guo 2007, Holland and Webster 2007, Kossin et al. 2007, Landsea 2007, Mann et al. 2007). Where do we stand at the moment? This post will concentrate on the North Atlantic, which has the longest record.

The official Atlantic hurricane record provided by the U.S. National Hurricane Center (HURDAT) represents the reference data base for most of the studies and contains all observed TCs, their individual tracks and intensity. The record has been extended back to 1850, and the earlier periods (until 1914) have been re-analysed in recent years. Reanalysis work continues, and updates and corrections are regularly reported.

This record contains two important abrupt inhomogeneities. The introduction of air reconnaissance flights in 1944 and the launch of the first geostationary satellite ATS-I in December 1966 mark two important improvements of measurement facilities and thus the observational coverage of the area under examination. Landsea (2007) claims a third one in 2002, since the new advanced microwave sounding unit (Quikscat) has lead to the retrospective detection of additional tropical cyclones in the last few years. Some also argue for placing the start of the satellite area later, in the mid-1970s relying on the launch of the GEOS-satellites. Furthermore, there are some changes in ship track patterns after 1914 with the opening of the Panama Canal and during the two world wars.

In addition, there are also a number of gradual observational improvements over time, e.g. the increasing quality of satellite images, or in earlier times the increase of the number of ship tracks or the growing population density on the coastlines, both of which enhance the probability that a TC would have been observed. And last but not least, there might be inhomogeneities due to the subjective component in the classification of tropical storms (the so-called Dvorak method, Dvorak 1984) which might lead to systematic differences between different forecasters. However, the homogeneous reanalysis of the last 23 years has shown that this subjectivity and improved observations has not lead to a noticeable alteration of the long-term trend for Atlantic storms (Kossin et al. 2007).

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