Feature article
Review of Recent Tropical Cyclone Climatological Research
Howard J. Diamond, NOAA/National Climatic Data Center and the University of Auckland
Figure 1. Tropical Cylone intensity in the South West Pacific
Averaged across all global ocean basins, the tropical cyclone (TC) season of 2005 saw an above normal (1981–2000 base) number of tropical storms, but fewer hurricanes/typhoons/cyclones (HTC) or major HTCs than average. Globally, 103 tropical storms (> 33 kt) were recorded, with 53 becoming HTCs, and just 28 attaining major/intense (> 95 kt) status, compared to an average of 97.25, 55, and 25.35 storms respectively (Diamond, 2006)1. While the Atlantic had its all time busiest season ever with 28 total storms, the other basins were characterized by near normal to below normal levels of activity. A question for all basins, including the Southwest Pacific, is, have TCs increased in number, accompanied by a possible increase in the intensity and duration of the storms? TCs require an ocean temperature of 26°C and above to sustain their formation, and increased global sea surface temperatures coupled with an increase in lower tropospheric water vapour, provide the necessary dynamic for increased intensity of TCs, (Trenberth, 2005). In essence, a new science has been created to assess whether TC activity has increased as a result global warming (Kerr, 2006), and some the results of studies on trends are now starting to be published (Klotzbach, 2006).
It is possible that an increase in TC frequency and intensity may be occurring (Webster, et al., 2005); and TCs may be more destructive due to possible increases in storm life (Emanuel, 2005). A few examples in the Southwest Pacific include the 2005 season in which five TCs struck the Cook Islands, and the very severe events of TC Waka which struck Tonga in 2002 and TC Heta which struck Niue in 2004. However, for the Southwest Pacific there is no discernible increase in frequency or intensity over the period of satellite monitoring (1970–2006). Another factor to consider is the quality of the data itself, and recent TC activity may be equal or even greater to that during the last active period in the mid-20th Century (Landsea, 2005). Determining trends in TC intensity is more complicated simply because there are many possible metrics of intensity (e.g., maximum potential intensity, average intensity, average storm lifetime, average wind speed, maximum sustained wind speed, maximum wind gust, accumulated cyclone energy, power dissipation), that have not been closely studied from the standpoint of historical trends due to data limitations (Pielke, 2005).
Two distinct, active, and intense schools of thought have emerged related to TC climatology, and for lack of a better nomenclature I have named them after the most prominent authors of papers representing each school of thought. The Emanuel/Trenberth/Webster school focuses primarily on how global warming, as manifested in increased Sea Surface Temperatures, over the past few decades has modified or increased the intensity of TCs. While, the Gray/Landsea/Pielke school, attributes natural variability associated with TCs independent of global warming, along the dearth of reliable historic global or basin scale data to explain the present situation.
The number of storms as well as the number of strong Category 4/5 storms2 appears to be on the increase. However, there is also greatly improved monitoring and satellite data analyses over the past few years. Re-analyses of older satellite data from the 1960s and 1970s shows that past TCs in the Indian Ocean for example, were categorised at a lower level, and would now be upgraded, thus boosting levels of extreme TCs in the 1970s and 1980s to those levels seen more recently (Landsea, et. al., 2006). The 28 TCs in the Atlantic in 2005 was an all-time record, breaking the previous mark of 21 set in 1933. However, without any satellite data it is very possible that there may have been more than 21 storms in 1933, as well as in some other years before the period of satellite coverage, but they likely were just not identifi ed. As such, it is interesting to note that the 28th storm of the 2005 Atlantic season was not discovered until April 2006 (6 months after it occurred) by a reanalysis of the satellite data from the season. It will be interesting to see how this research progresses, and what affect the results will have on the Southwest Pacific; the author will continue to monitor and provide updates on new developments as appropriate.
1 See www.ncdc.noaa.gov/oa/usgcos/ICUReferences.pdf
2 See www.ncdc.noaa.gov/oa/usgcos/Saffir-Simpson.pdf