April 8, 2008

Another Hurricane Update

Filed under: Climate Extremes, Hurricanes

A few months have passed since our last hurricane update, and sure enough, two more interesting articles have appeared recently in leading scientific journals. Despite a relative calm over the past few years on the hurricane front, the global warming crowd continues to insist on thousands of websites that hurricanes are becoming more frequent and intense due to the ongoing buildup of greenhouse gases. Their claims are looking more silly every day, but they fully understand that some other Katrina-like disaster is always in our future, so they seem to be patiently waiting for the next major photo opportunity.

The first of the recent articles was published in the Journal of Climate by William Briggs who unlike virtually everyone else we feature does not seem affiliated with any recognizable research institution (he is William Briggs, New York, New York). If he is working by himself without the baggage that comes along with any research group, we applaud the effort. Furthermore, the Journal of Climate is published by the American Meteorological Society, and authors are held to very high standards of scientific scrutiny – we congratulate him for the effort.

Briggs reviews the recent literature on the subject of changing hurricane activity in recent decades focusing on the highly celebrated results of Emanuel suggesting that hurricanes in the North Atlantic have become more destructive in the past 30 years. Briggs notes that others “criticized the data analysis method used to demonstrate that the index was increasing by pointing out that the smoothing method used on the raw time series data was slightly flawed, that errors in the observations should lead to a less certain statement about increases, and that the wind speed adjustments used by Emanuel were too aggressive.”

Briggs collected popular and widely-used hurricane data for the North Atlantic and applied advanced (Bayesian) statistical methods to the analyses of trends in the data. Briggs reports “The conclusion to be drawn here is that there is good evidence that the number of tropical cyclones has increased, but only if one chooses the right date at which to start one’s analysis. Using start dates before around 1975 but after 1966 shows that there has been a definite linear increase. But using any start date from 1966 to about 1974 shows no increase. The rate at which hurricanes evolved from storms does not appear as sensitive to the start date in the data, and there is some evidence that this rate has decreased since at least 1966.” Briggs presents the box plots (Figure 1) for the log of storm number, track length, and power dissipation index, and concludes “There is no apparent trend.”


Figure 1. Time series of box plots, for each year of the log of number of storms (top), the log of track length (middle), and the log of the power dissipation index (bottom) (from Briggs, 2008).

Briggs elaborates noting “We find no evidence that the distributional mean of individual storm intensity, measured by storm days, track length, or individual storm PDI, has changed (increased or decreased) through time. Any increase in storm intensity at the conglomerated yearly level, as for example found by Emanuel, is likely due to the increased number of storms and not a result of the increased intensity of individual storms. We also repeated our analysis on the distribution of each storm’s (log) maximum wind speed over its lifetime and came to the same conclusion as was reached for the other measures of intensity.” Along the way, Briggs also found that “There is some evidence that the rate at which storms evolve into hurricanes has decreased.”

Our second gem is from the Journal of Geophysical Research published by a pair of scientists from the University of Missouri and the Malaysian Meteorological Service. Zuki and Lupo note that Malaysian suffered damage from several severe hurricanes (typhoons) between 1996 and 2001, and that “These storms have caused concern in Malaysia as to whether or not this was part of a long-term trend toward more frequent tropical cyclone strikes.” Malaysia lies lose to the Equator, and generally, the low latitude location does not favor tropical cyclones. Zuki and Lupo note that only 3% of the storms in the Pacific Ocean occur near Malaysia, but they can nonetheless prove to be quite destructive in the region.

As seen in the figure below (Figure 2), one could make the case that the number of tropical cyclones has increased around Malaysia in recent decades. However, serious scientists would note that Zuki and Lupo state “However, when this trend was tested for significance, it was found that the trend was not statistically significant at even the 90% confidence level by using the F-test and assuming that there should be no trend a priori. This agreed with the findings of many other studies of the active tropical regions.”


Figure 2. The time series of (a) all tropical cyclones and (b) tropical cyclones of local origin occurring near Malaysia from 1960 to 2006. The dashed line is the insignificant linear regression line.

We will stay tuned on this issue, we fully expect many more articles on the subject in 2008, and we believe that despite gobs of evidence to the contrary, greenhouse advocates will continue to insist that mankind’s burning of fossil fuels have acted to hurricanes make hurricanes detectably more frequent and intense.

References

Briggs, W.M. 2008. On the changes in the number and intensity of North Atlantic tropical cyclones. Journal of Climate, 21, 1387-1402.

Zuki, Z. M., and A. R. Lupo. 2008. Interannual variability of tropical cyclone activity in the southern South China Sea. Journal of Geophysical Research, 113, D06106, doi:10.1029/2007JD009218.




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