Atlantic Tropical Cyclone Records – Trends and Ephemerality

Guest Commentary from Urs Neu

To understand the influence of climate change on tropical cyclone and hurricane activity, it is crucial to know how this activity has varied in the past. There have been a number of interesting new studies of Atlantic tropical cyclones (TCs) and hurricanes (tropical cyclones with maximum sustained winds exceeding 74 miles per hour) since my review of the topic a couple years ago (see here and here). These newer studies underscore that, while our knowledge continues to improve in this area, key uncertainties persist. In particular, it remains very difficult to confidently estimate trends in the past.

In assessing past trends, one must distinguish between two distinct time intervals: 1) the period of historical observations (mainly after 1850), and 2) the earlier period for which TC activity can only be reconstructed using proxy data. Furthermore, we have to distinguish between trends in tropical cyclone (TC) number and TC intensity–the latter measure is particularly important from the standpoint of impacts. There is no a priori reason to expect these quantities to vary in concert, either in the past, or in the future. Unfortunately, uncertainties are much greater for intensity than for counts.

In this article, I will review our current understanding of Atlantic TC and hurricane trends with respect to: A) the historical record of basin-wide TC numbers; B) the historical record of hurricanes and TC intensity; C) distant past proxy estimates of TC (primarily, hurricane only) counts; and D) distant past proxy measures of TC/hurricane intensity. I will conclude with a discussion of current methods for forecasting Atlantic hurricane activity.

A) Historical record of TC counts

The historical record of Atlantic tropical cyclones of the U.S. National Hurricane Center (HURDAT) goes back to 1850. However, only since the start of the satellite area in the 1970s has an area-wide observation system been available. Before that, the density of observations increased with time, either gradually (e.g. density of ship tracks or settlements on coasts), or stepwise (e.g. the introduction of reconnaissance flights in 1944 and launch of GEOS satellites in 1975). There was also subjectivity in various analysis methods, be they the interpretation of local observational data from stations and ships, or of satellite pictures (Dvorak method). Some improvements are obtained by the reanalysis of the whole data set with the same method (e.g. Kossin et al. 2007) or by the analysis of additional data (past meteorological observations not yet included in the analysis, e.g. Landsea et al. 2008).

However, the inhomogeneity caused by a changing and incomplete areal coverage of measurements before the start of the satellite era in 1975 will never be completely eliminated. The only way to correct for this is to estimate the number of TCs ‘missed’ by the observations in earlier times, using indirect methods. One way to do this is to estimate average numbers of ‘missed’ TCs for certain periods using the relationship of general parameters which are known in the past to the TC numbers in the satellite era. While such estimates won’t reveal the ‘right’ TC numbers for individual years, the average ‘missings’ (“undercount bias”) will improve the analysis of long-term trends.

In recent years there have been a number of attempts to estimate the undercount bias. A first attempt of Landsea (2007) using the percentage of landfalling TCs as a basis has been argued to be implausible, as the percentage of landfalling TCs has multidecadal variations and is thus not constant as assumed (Holland 2007). A second approach, by Mann et al (2007), used the statistical relationships of seasonal TC counts to climate variables (NAO, El Niño, and Main Development Region SST). A third method analyzed past ship tracks (Chang and Guo 2007, Vecchi and Knutson 2008 (“VK08”)). These estimates used a new reanalysis of the ICOADS ship track data. Reanalysis of the ICOADS data for 1911-1925 has led to the detection of about one additional TC per year (Landsea et al. 2008), which suggests that the same reanalyis of pre-1911 data might also lead to the detection of additional TCs.

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