August 29, 2007

Hurricanes Down Under

Filed under: Climate Extremes, Hurricanes

First things first – the title is wrong. We have presented many essays recently showing that hurricane activity is not increasing in terms of frequency and intensity or that any increase is simply a return to what was commonly observed decades or centuries ago. When we were writing about hurricanes in the Atlantic Ocean, Caribbean Sea, or Gulf of Mexico (or even the Northeast Pacific), our use of the term “hurricane” was correct. These severe tropical storms appear in many other parts of the world, and when they occur over the Northwest Pacific Ocean and west of the International Date Line, they are called typhoons. “Hurricanes” near Australia and in the Indian Ocean are sometimes called Willy-Willies by the locals. However, in the scientific community, hurricanes, typhoons, and Willy-Willies are all called tropical cyclones.

For a variety of reasons, most of the research we have reviewed has been conducted in the Northern Hemisphere on tropical cyclone trends that have occurred in the Northern Hemisphere. However, the Southern Hemisphere gets its fair share of tropical cyclones, and the global warming supporters do not differentiate hemispherically in their never-ending claims that elevated concentrations of greenhouse gases will cause an increase in tropical cyclone frequency and/or intensity.

An interesting article has appeared in Earth and Planetary Science Letters regarding tropical cyclone activity in northeastern Australia over the past eight centuries. Eight centuries? A researcher would need to be very clever to figure out the number of large tropical cyclones that occurred every year from AD 1226 to AD 2003 in northeastern Australia.

A team of scientists from institutions in Australia and New Zealand figured a way to do it, and here is the trick. The water that condenses in the clouds of tropical cyclones is slightly different from the water that forms in regular tropical clouds. Due to the enormous amount of water in a tropical cyclone and the height at which water condenses, the water is depleted in a particular isotope of oxygen (called the “oxygen 18 isotope” and denoted as “δ18O”). The Nott et al. team explain “An isotope gradient occurs across the cyclone with the eye wall region experiencing lowest levels of δ18O and low levels also occur within the zones of uplifted air around the cyclone known as spiral bands.” It is likely that the most intense tropical cyclones will have “cloud tops at greater altitude around the eye and in spiral bands. The longevity of the system and hence the amount of rain that has occurred prior to the system crossing the coast also plays a role in the extent of isotope depletion.” So if we had a water sample from each event, we could examine the δ18O level and have an index of the severity of the storm – finding the water samples sounds like a problem.

The miracle in this game is in the Chillagoe limestone karst terrain area near Cairns (see Figure 1) where caves are full of stalagmites growing upward from the floor. The water from the tropical cyclones contributes to the growth of each year’s layer in the stalagmites, and sure enough, years with big tropical cyclone events have a signature preserved in the δ18O levels within the annual growth layers.


Figure 1. Location of the study site (from Nott et al., 2007)

Nott et al. had an excellent observational record of tropical cyclone activity in the region from 1907 to 2003, and they statistically compared the actual record to what they found in the stalagmite. They report that “Each of the peaks in the δ18O depletion curve corresponds to the passage of a cyclone within 400 km of Chillagoe. Twenty of these twenty seven cyclones passed within 200 km of Chillagoe, 22 within 230 km, 23 within 270 km and the other four within 400 km. Our record accounts for 63% of all cyclones that passed within 200 km of Chillagoe since AD 1907.” Before you think the error is too great for serious scientific inquiry, be aware that “Despite the absence of many cyclones it is important to note that every intense cyclone (i.e. AD 1911, 1918, 1925, 1934, 1986 as determined by barometer or damage to urban infrastructure and loss of life) to make landfall in the region (400 km region) since AD 1907 is registered by a peak in the isotope depletion curve.”

The plot below (Figure 2) is the estimate of tropical cyclone intensity from AD 1226 to AD 2003, and global warmers will not be happy as Nott et al. write “it is clear that the period between AD 1600 to 1800 had many more intense or hazardous cyclones impacting the site than the post AD 1800 period. Seven events that were more intense/hazardous than the 1911 event occurred during this 200 yr period. Indeed the cyclone registering the lowest isotope difference value, hence the most intense or hazardous, of the entire record occurred here during this time.”


Figure 2. Isotope index showing intense tropical cyclone events from AD 1226 to AD 2003 (from Nott et al., 2007)

Mother Nature speaks, and once again, we see little evidence whatsoever of any increase in tropical cyclone frequency or intensity during the past 100 years when greenhouse gas concentrations increased – as noted by Nott et al., in northeastern Australia anyway, there has, in reality, be a decrease.

Reference

Nott, J., J. Haig, H. Neil, and D. Gillieson. 2007. Greater frequency variability of landfalling tropical cyclones at centennial compared to seasonal and decadal scales. Earth and Planetary Science Letters, 255, 367–372.




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