January 17, 2008

Cycles in Landfalling U.S. Hurricanes?

Filed under: Climate Extremes, Hurricanes

To many global warming alarmists, every disastrous weather event becomes yet another piece of evidence of the coming man-made apocalypse. One only needs to look at their exploitation of Hurricane Katrina victims in the furtherance of the global warming crusade. Most average citizens are shocked to find out that at landfall, Katrina was not a record-setting Category 5 monster hurricane, but really a Category 3 storm—hardly unprecedented in intensity but devastating with respect to the landfall location and timing.

Most reasonable people believe that there exist natural cycles in climatic events, perhaps even hurricanes. But to the aforementioned alarmists, cycles are anathema—with increasing greenhouse gases, temperatures (and the related disastrous repercussions) must only trend inexorably upward. The declining (cooling) limb of any cycle is simply unacceptable.

This background is at the heart of the ongoing debate on trends in Atlantic hurricanes. While the alarmists see only trends in hurricane numbers and strength arising from warmer ocean waters, others point to evidence of cycles in Atlantic sea-surface temperature tied to something called the Atlantic Multidecadal Oscillation, a multi-decadal scale shifting in Atlantic temperatures between cold and warm phases.

A 2007 paper in the Journal of Climate by Louisiana State University scientists Barry Keim, Robert Muller, and Gregory Stone adds another bit of evidence for debate. Most of the previous cycle work has examined total numbers of hurricanes over a large scale, such as the entire North Atlantic Basin. But Keim et al. elected to plot local tropical cyclone strike patterns from northern Maine through Brownsville, Texas. The authors divided the Atlantic and Gulf coasts into segments (see Figure 1) and tallied the number and strength of tropical storms and hurricanes at the time of landfall over the period 1901–2005.

Figure 1. Demarkation of the Atlantic and Gulf of Mexico coastlines into segments for examining the spatial and temporal changes in landfalling tropical cyclones from 1901–2005 (from Keim et al., 2007).

Apart from being an update of earlier studies, this kind of approach is certainly not new. Similar maps have been used for many years to study the vulnerability of specific locations and of course are a critical component in the assessment of insurance risk.

What is most interesting in the Keim study is their examination of the changes in storminess over time and space (see Figure 2). In this kind of time vs. space plot, it’s easy to visually integrate the information to identify patterns and trends. In essence, this shows the long-term pattern for each coastal segment.

Figure 2. Time and space plot of each landfalling tropical storm, weak hurricane, and severe hurricane from 1901–2005. Time is increasing toward the bottom of the plot, while space is plotted across the top, from south Texas (left) to northern Maine (right) (from Keim et al., 2007).

One of the first things to note from Figure 2 is the lack of any kind of obvious long-term trend. While the 2000s were certainly active in the Gulf of Mexico. there were plenty of other active periods, including the 1940s and 1960s. In fact, the data hardly are randomly organized. There are periods of time when certain coastal regions are heavily impacted multiple times within the same year and over consecutive years. There are good climatological reasons for this kind of behavior related to spatial patterns in factors both favorable and unfavorable to tropical cyclone development.

It’s also possible to identify broad evidence of cycles in landfalling storms in some coastal locations. According to the authors:

The most dramatic interdecadal variability has occurred in southern Florida on both east and west coasts: Vero Beach on the Atlantic around to St. Petersburg and Cedar Key on the Gulf of Mexico. There is a clustering of events for the 25 yr between the mid-1920s and 1950, and several events during the mid-1960s. However, with the exception of Hurricane Andrew in 1992, there are almost no hurricane strikes, and only a few tropical storm strikes over the most recent 40 yr until 2004 and 2005. Along the Outer Banks of North Carolina, there has also been two temporal clusters of strike events, in the 1950s and again in the 1990s, with relatively few strikes in between.

Although the purpose of their analysis was not to evaluate hurricane landfall cycles, Keim et al. conclude this portion of their analysis by noting that:

“The dataset in its totality depicts a temporal pattern beginning in the mid-1920s, extending into the mid-1960s, that was very active for landfalling storms in the United States. A relatively inactive period is evident from the mid-1960s through the mid-1990s, which again becomes active from 1995 to 2005. These patterns appear consistent with documented fluctuations in the [Atlantic Multidecadal Oscillation] (Elsner 2006; Vermani and Weisberg 2006)…”.

Despite increasing greenhouse gas concentrations over their entire period of records, patterns of landfalling storms are more fairly characterized as cyclic rather than by an increasing trend. Although it would be inappropriate for us to suggest that there is no linkage between global warming and hurricanes, it’s similar inappropriate to summarily dismiss the role of natural cycles in hurricane behavior. The patterns show by Keim et al. for the Atlantic and Gulf coasts is one in which the conditions favorable for hurricane formation seem to vary in time and space in a complex manner that cannot be easily linked to greenhouse gas levels. So hopefully the next time you read another news story on the “obvious” global warming–hurricane linkage, you’ll remember to look back at Figure 2 to see just how “obvious” this connection really is.


Elsner, J.B., 2006: Evidence in support of the climate change-Atlantic hurricane hypothesis. Geophyscial Research Letters, 33, L16705, doi:10.1029/2006GL026869.

Keim, B.D., Muller, R.A. and G.W. Stone, 2007: Spatiotemporal patterns and return periods of tropical storm and hurricane strikes from Texas to Maine. Journal of Climate, 20, doi:10.1175/JCL14187.1.

Vermani, J.I., and R.H. Weisberg, 2006: The 2005 hurricane season: An echo of the past or a harbinger of the future? Geophysical Research Letters, 33, L16705, doi:10.1029/2006GL026869.

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