Long Term Tropical Cyclone Trends
By Paul Homewood
Jamal Munshi of Sonoma State University has sent me a copy of his latest paper, A General Linear Model for Trends in Tropical Cyclone Activity, published last June:
The ACE index is used to compare tropical cyclone activity worldwide among seven decades from 1945 to 2014. Some increase in tropical cyclone activity is found relative to the earliest decades. No trend is found after the decade 1965-1974. A comparison of the six cyclone basins in the study shows that the Western Pacific Basin is the most active basin and the North Indian Basin the least. The advantages of using a general linear model for trend analysis are described.
His conclusions are:
In a general linear model for global mean annual Accumulated Cyclone Energy (ACE) in six basins and seven decades from 1945 to 2014 we find some evidence of a rising trend in tropical cyclone activity relative to the early part of the study period prior to the decade D03 [1965-1974]. No global trends are found after this decade. The same pattern is found in three of the six cyclone basins studied, namely, EP [Eastern Pacific], SI [South Indian], and SP [South Pacific]; with each basin showing a rising trend relative to the decades prior to D03 and none since D03. No trends could be detected in the other three cyclone basins in the study, namely, NA [North Atlantic], NI [North Indian], and WP [Western Pacific].
The global model found significant differences in mean overall ACE index among the six basins. The Western Pacific Basin was the most active and the North Indian Basin was the least active. Not much separates the other four basins except that the South Indian Basin was more active than the South Pacific Basin.
The “best track” cyclone data were used as received from the NCDC11 without corrections, adjustments, additions, or deletions with the exception that the years 1848-1944 were not used because they did not contain data for all six basins. It is generally assumed that these data may contain a measurement bias over time and across basins because of differences in data collection methods and procedures (Kossin, 2013). If an undercount bias exists in the oldest data it may explain why a rising trend in cyclone activity was detected only against the early part of the study period. The findings presented here are entirely empirical and their utility depends on the validity of the ACE index as a measure of tropical cyclone activity. All data and computational details are available in the online data archive for this paper (Munshi, Global cyclone paper data archive, 2015).
His comment that “it is generally assumed that these data may contain a measurement bias over time and across basins because of differences in data collection methods and procedures (Kossin, 2013). If an undercount bias exists in the oldest data it may explain why a rising trend in cyclone activity was detected only against the early part of the study period” is particularly pertinent. It is really only since the 1970s, and arguably 1980s, when satellite monitoring became common, that any proper comparisons can be made.