Abstract :
Simulations of global warming over the coming century from two CSIRO GCMs are analysed to assess changes in
the intensity of extratropical cyclones, and the potential role of increased latent heating associated with precipitation
during cyclones. A simple surface cyclone detection scheme is applied to a four-member ensemble of simulations from
the Mark 2 GCM, under rising greenhouse gas concentrations. The seasonal distribution of cyclones appears broadly
realistic during 1961–1990. By 2071–2100, with 3 K global warming, numbers over 20◦N to 70◦N decrease by 6%
in winter and 2% annually, with similar results for the south. The average intensity of cyclones, from relative central
pressure and other measures, is largely unchanged however. 30-yr extremes of dynamic intensity also show little clear
change, including values averaged over continents.
Mean rain rates at cyclone centres are typically at least double rates from all days. Rates during cyclones increase by
an average 14% in the northern winter under global warming. Rates over adjacent grid squares and during the previous
day increase similarly, as do extreme rates. Results from simulations of the higher-resolution (1.8◦ grid) Mark 3 GCM
are similar, with widespread increases in rain rates but not in cyclone intensity. The analyses suggest that latent heating
during storms increases, as anticipated due to the increased moisture capacity of the warmer atmosphere. However, any
role for enhanced heating in storm development in the GCMs is apparently masked by other factors. An exception is a
5% increase in extreme intensity around 55◦S in Mark 3, despite decreased numbers of lows, a factor assessed using
extreme value theory. Further studies with yet higher-resolution models may be needed to examine the potential realism
of these results, particularly with regard to extremes at smaller scale
