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Tropical cyclones and climate

Causes of changes in tropical cyclones have been explored in several studies. Trenberth and Shea (2006) analyzed the origins of the record breaking 2005 Atlantic hurricane season. SSTs in the 10? to 20?N North Atlantic region critical for hurricanes were at record high levels in spite of all the hurricane activity. However, about half of the anomaly is related to global SST changes, and thus global warming, ENSO accounts for another important portion, but North Atlantic SST variations, as given through the Atlantic Multi-decadal Oscillation, contributed in only a minor way. The latter was important for the lull in activity from 1970 to about 1990, however. Santer et al. (2006) have used 22 climate models to study the possible causes of SST changes in Atlantic and Pacific tropical storm cyclogenesis regions. The observed SST increases in these regions range from 0.32°C to 0.67°C over the 20th century. The climate models examined suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system. For the period 1906-2005, they suggest there is an 84% chance that external forcing explains at least 67% of observed SST increases in the two tropical cyclogenesis regions. Model "20th-century" simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. In experiments in which forcing factors are varied individually rather than jointly, human-caused changes in greenhouse gases are the main driver of the 20th-century SST increases in both tropical cyclogenesis regions. Fasullo (2006) has analyzed the observed best track hurricane record for sampling biases that might account for trends in the frequency of major storms, but the spurious influences of improvements in both the temporal sampling of storms and peak wind speed measurements are found to be weak. Anthes et al. (2006) jumped into the fray on the debate and outline issues related to hurricane changes with climate change. This article provides some much needed balance to counter some quite misleading articles and claims that natural variability alone is responsible for the observed changes.

Another major topic has been the energy and water cycles of hurricanes and their role in the climate system. A further study, has computed how much moisture that ends up as rain in hurricanes comes from local evaporation in the storm versus large-scale convergence (Trenberth et al. 2006b). This has been analyzed in a model framework using WRF at high resolution for simulations run for observed storms, in particular Ivan in 2004 and Katrina in 2005, that are realistic. Models sensitivity runs have also been made with SSTs increased and decreased by 1?C. Results demonstrate the overwhelming dominance of moisture convergence into the storms, in spite of the critical role of the surface evaporative source; see Fig. 2, and have implications for the changing environment on hurricanes as climate changes. These model results have been related empirically to the maximum sustained wind in the model and the results used with the “best track” global observed data on tropical cyclones to deduce how surface fluxes and precipitation in hurricanes have changed since 1970 (Trenberth and Fasullo 2006), see Fig. 3. Hurricanes play a key role in climate and that role is increasing over time as SSTs rise.

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