Working Group I: The Scientific Basis

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Any changes in ENSO as the climate changes will impact the distribution and tracks of tropical cyclones (including intense storms such as hurricanes and typhoons). During an El Niño, for example, the incidence of hurricanes typically decreases in the Atlantic and far western Pacific and Australian regions, but increases in the central and eastern Pacific (Lander, 1994). Thus it should be recognised that decreases in one area may be offset by increases in another area because of the global connectivity of the tropical atmospheric circulation. Global warming from increasing greenhouse gases in the atmosphere suggests increased convective activity but there is a possible trade-off between individual versus organised convection. While increases in sea surface temperatures favour more and stronger tropical cyclones, increased isolated convection stabilises the tropical troposphere and this in turn suppresses organised convection making it less favourable for vigorous tropical cyclones to develop (Yoshimura et al., 1999). Thus changes in atmospheric stability (Bengtsson et al., 1996) and circulation may produce offsetting tendencies (e.g., Royer et al., 1998). General circulation models of the atmosphere (see Chapter 8, Section 8.8.4) do not resolve the scales required to properly address this issue; for instance, moist convection and hurricanes are not resolved adequately.

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