October 2, 2006

Hurricane Data Not Cooperating

Filed under: Hurricanes

During fall of 2005, the United States experienced an unusually high number of landfalling hurricanes and the global warming crusade was having a field day. Hardly a day could go by without some one making the claim that global warming was elevating sea surface temperatures in the Gulf of Mexico, Caribbean Sea, and Atlantic Ocean, and those warmer seas over vast areas provided the necessary fuel to heat up hurricanes. If you saw the Al Gore movie, you undoubtedly recall his explanation of the cause behind Katrina and other hurricanes of a year ago – global warming, of course.

Now that hurricane season is upon us (and things seem incredibly quiet), we can look more carefully at some recent literature on the subject.

This week, an article has appeared in Geophysical Research Letters with the title “Large increase in heavy rainfall associated with tropical cyclone landfalls in Korea after the late 1970s”, and at first glance, it would seem perfect for further arguing the horrors of global warming. Joo-Hong Kim of Seoul National University headed a team who collected actual rainfall data from stations throughout Korea, hurricane records from the region, and local temperature, specific humidity, horizontal wind, and vertical velocity data from the European Center for Medium-range Weather Forecast (ECMWF). The team found that in the 1970s, something happened and the amount of heavy rain in Korea nearly doubled in August and September.

In the Gore tradition, we could get some film footage of heavy rains in Korea and blame all of this on global warming. The problem is that the Kim et al. team never mentioned global, hemispheric, or regional warming as the cause. Instead, they blame a southward shift in the tropospheric jet stream in Asia that occurred in the 1970s. This shift has increased tropospheric divergence thereby increasing ascending motion in the local atmosphere which strengthens the frontal zone around Korea. The change in circulation altered the southerly moisture flux and provided more moisture to storms approaching the peninsula. They conclude that “All these consistent changes in the dynamic fields support the abrupt increase in the heavy rainfall in Korea.” Recall that the 1970s was a decade when we came to fear global cooling, not warming. Blaming the great circulation shift on global warming has one huge problem – the shift occurred before the planet warmed. The heavy rainfall in Korea is not a symptom of global warming, in fact, the shift in circulation that caused the heavy rain occurred near the end of a 30-year cooling period.

Kim et al. note that there are fears that hurricanes around the world are intensifying faster; indeed insurance firms (e.g., Swiss Re, Munich Re) have expressed concerns that “rapid intensification” of hurricanes will result in increased damage and loss of life. The Kim et al. team examined the 6-hourly tropical cyclone (TC) data for the Korean area and found “Overall, the lowest pressure value and the number of six-hourly observations do not show any significant interdecadal change with respect to the late 1970s.” They reinforced this conclusion at the end by stating “However, it was found in this study that there is no evidence of interdecadal change in the TC intensification when the TC approaches the Korean peninsula.”

In a recent paper published in Meteorology and Atmospheric Physics, Robert Balling and Randall Cerveny, of Arizona State University, addressed this very issue of rapid intensification of hurricanes. They collected six-hourly hurricane data for the North Atlantic Basin from 1970-2003 and produced various wind-based and pressure-based definitions of “rapid intensification”. They found that “rapid intensification” of hurricanes occurs far more in August to November than any other time of the year and it occurs far more often in the day than at night. In their own words, they stated “our analysis of this matrix revealed no statistically significant trend in any of the wind-based and pressure-based rapid intensification variables. There was no significant relationship between any rapid intensification variable and regional sea surface or low tropospheric temperature anomalies.” Balling and Cerveny found no link between intensification and temperatures, but they noted that rapid intensification is related to other complicated environmental variables such as relative angular momentum at 200 mb, encounters with warm core ring, low vertical shear, and interactions with upper-tropospheric troughs.

As with so many elements of the global warming debate, these two recent articles on hurricanes are reminders that the climate system is very complex, tropical storms respond to many variables other than sea-surface temperatures, and simple statements about global warming and hurricanes are often not supported by theory or by observations.

References

Balling, R. C., Jr. and R. S. Cerveny, 2006: Analysis of tropical cyclone intensification trends and variability in the North Atlantic Basin over the period 1970-2003. Meteorology and Atmospheric Physics, 93, 45-51.

Kim, J.-H., C.-H. Ho, M.-L. Lee, J.-H. Jeong, and D. Chen, 2006: Large increase in heavy rainfall associated with tropical cyclone landfalls in Korea after the late 1970s. Geophysical Research Letters, 33, 2006GL027430.




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