High Resolution Isotope Records of Tropical Cyclones

The sedimentary evidence of past tropical cyclones tends to record the most extreme events. Smaller ridges will be built by weaker cyclones but these are either destroyed or overridden by more intense events and tend not to be as well preserved. As a consequence the frequency of events recorded is lower and only one spectrum of the climatology of these events remains. This together with the inherently moderate resolution of the dating techniques (radiocarbon and luminescence) limits the ability to identify trends at decadal to centennial scales. In order to overcome this problem high resolution records (annual) have been recently derived from isotopic analysis of annually layered carbonate stalagmites extending back nearly 800 years near Cairns.

Tropical cyclone rain, compared to normal tropical rain, is strongly depleted in 18O because of extensive fractionation during condensation of uplifted air and the continuous and higher levels of rainfall (amount effect) (Lawrence and Gedzelman, 1996). Tropical cyclone rain typically contains 18O levels between —5 and —15 %o (vSMOW) (Lawrence and Gedzelman, 1996; Lawrence, 1998). These isotope values have been measured as far as 400 km from the centre of the cyclone (Lawrence and Gedzelman, 1996). An isotope gradient occurs across the cyclone with the eye wall region generally experiencing lowest levels of 18O and low values also occur within the zones of uplifted air around the cyclone known as spiral bands (Lawrence and Gedzelman, 1996). While the levels of 18O appear to be inversely proportional to the altitude of uplifted air within the one cyclone there have not been any systematic studies to date that focus specifically on the relationship between isotope depletion and cyclone intensity. It is possible that this relationship does exist since more intense cyclones 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 (amount effect) (Dansgaard, 1964; Hendy and Wilson, 1968; Hendy, 1971). Because of these factors, rain water falling in tropical coastal regions with 818O below —6 % (vSMOW) can be regarded as a signature of its origin in a tropical cyclone (Lawrence and Gedzelman, 1996; Lawrence, 1998). Dilution of this rain water will occur when mixed with ground and surface waters and this would also be expected when such waters are utilized in the formation of terrestrial faunal carbonate shells and limestone speleothems in caves. These diluted water isotope values have been measured between —5 and —10 % (vPDB) (Lawrence, 1998). To date however this isotopic signature has not been used as a measure of the long-term history of tropical cyclones in well preserved carbonates.

An annually layered stalagmite was sampled from Chillagoe approximately 130 km west of Cairns. Oxygen isotope (818O) analyses were undertaken on each of the 777 layers (AD 2004 to AD 1228) (Fig. 14). (Nott et al., 2007). Comparisons were made between the isotope and historical records of tropical cyclones (starting AD 1907) passing through the region (Fig. 15). Each of the peaks in curve corresponds to the passage of a cyclone within 400 km of Chillagoe. Twenty

Fig. 14 Annual S18O % (vPDB) AD 1226 to 2003

year AD

Fig. 14 Annual S18O % (vPDB) AD 1226 to 2003

Fig. 15 Timing of tropical cyclone occurrences, high wet seasonal rainfalls and no cyclone years relative to S18O % (vPDB) record for period AD 1907 to 2003

of these twenty seven cyclones passed within 200 km of Chillagoe, 22 within 230 km, 23 within 270 km and the other three within 400 km. The record accounts for 63% of all cyclones that passed within 200 km of Chillagoe since AD 1907. It is not reasonable to expect all cyclones to be registered in the stalagmite for a number of reasons including a) many cyclones in this region are 'midgets' (very small diameter) and even tracking within 200 km these cyclones will not produce 18O

depleted rain at Chillagoe, b) the cyclones were short lived and hence had higher levels of 18O because the amount effect (Dansgaard, 1964; Hendy and Wilson, 1968; Hendy, 1971) had not had time to develop, or c) the cyclones were of low intensity. 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) (Callaghan, 2005) to make landfall in the region (400 km region) since AD 1907 is registered by a peak in the isotope depletion curve.

The magnitude of each of the depletion peaks is likely to be a function of the level of 18O in the cyclone rain and the amount of 18O in soil water from the previous wet season rainfall or previous rainfall events in the same season. The step function appearance of the raw isotope data plot seen in Fig. 14 is likely to be due to the relative increase in soil water 818O content, as seen following approximately AD 1400, and decrease following approximately AD 1800. Because of the 18O soil water lag effect, Nott et al. (2007) suggested that a more appropriate measure of the level of isotope depletion for a cyclone event is the difference in 18O between the depletion peak and the preceding curve trough. A curve of 818O differences is presented in Fig. 16.

Multiple regression analysis showed that isotope depletion levels have their closest relationship to the intensity of the cyclone divided by the distance (closest point on path) of the cyclone from Chillagoe (R2 = 0.6, p < 0.001). A very low isotope value (high peak on Fig. 16) is therefore more likely to be a product of an intense event that tracks very close to the site which as a consequence experiences a

Year AD

Fig. 16 Detrended S18O %% (vPDB) values (crest - preceding trough from Fig. 15) to account for S18O soil water dilution for landfalling tropical cyclones from AD 1228 to 2003. Critical value for moderate to severe hazard impact at-a-station is —2.50 %. Note no events above this value between AD 1200 to 1400 and AD 1500-1600, 2 events between AD 1400-1500, 7 events between AD 1600-1801 and 1 event (just above this value) since AD 1801

Year AD

Fig. 16 Detrended S18O %% (vPDB) values (crest - preceding trough from Fig. 15) to account for S18O soil water dilution for landfalling tropical cyclones from AD 1228 to 2003. Critical value for moderate to severe hazard impact at-a-station is —2.50 %. Note no events above this value between AD 1200 to 1400 and AD 1500-1600, 2 events between AD 1400-1500, 7 events between AD 1600-1801 and 1 event (just above this value) since AD 1801

severe hazard. A higher isotope value could be due to an intense event at a greater distance or a low intensity event very close to the site. Either way the site experiences a lower severity hazard. The higher isotope values could also be due to some other environmental variable (i.e. slight variations in isotope values in evaporated source waters) and not necessarily be due to a distant or weaker tropical cyclone. However, as discussed it is unlikely that the lower isotope values are due to anything else besides tropical cyclone rainfall.

Only one cyclone event (AD 1911) during the historical period (post AD 1907) has an isotope difference less than —2.5 %o (i.e. a more negative value) (Fig. 15). A barometric pressure measurement of 959 hPa was made approximately 50 km from the estimated landfall point of this cyclone; the central pressure of this cyclone is not known but must have been considerably lower. This cyclone registers as the highest historical peak on Figure 16 because of its intensity and also because it passed within 30 km of Chillagoe. This cyclone also produced the highest peak during the period AD 1802-2004. In comparison, the period AD 1600-1801 registers 3 peaks with a lower than —2.5 %o; isotope difference value, 3 more were below —3.0 %o, and 1 was below —4.0 %% from AD 1500-1600 there were no peaks with an isotope difference value lower than —2.5 %; the period AD 1400-1600 had 1 event below —3.0 % and one below —3.5 %.

Using the isotope difference value of 2.5 % for the 1911 cyclone as a reference 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 year period. Indeed the cyclone registering the lowest isotope difference value (<—4.0 %), hence the most intense or hazardous, of the entire record occurred during this time. There were no comparatively hazardous cyclones during the period AD 1500 -1600 yet the period from AD 1400 to 1500 had two events with considerably lower isotope difference values and hence were presumably more hazardous than the AD 1911 event. The two centuries from AD 1200 to 1400 had no events equal in intensity to the AD 1911 event.

These results suggest that there may be centennial scale regimes in landfalling tropical cyclone activity in this region. Comparisons with sea surface temperature (SST) data from geochemical proxies in corals from the GBR (Hendy et al., 2002) show that these centennial scale phases of heightened landfalling cyclone frequency occurred during both warmer (AD 1700-1800) and cooler (AD 1600-1700) than present SSTs in this region (Nott et al., 2007). This is the first time that centennial scale variations in landfalling cyclone activity have been recognised for any ocean basin and such information can be invaluable in decoupling human induced changes in cyclone behaviour from natural variability. Before such goals can be properly realised however it is imperative that further high resolution, long-term records of this hazard are investigated from both this region and other ocean basins globally. While this single station record is insufficient on its own to draw any conclusions about natural variability versus global climate change issues it is very useful for better assessing risk from this hazard for this region.

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