The ability of the CSIRO-9 General Circulation Model (GCM) to capture
surface cutoff lows over eastern Australia is investigated by comparin
g composites of ten GCM cases with ten observed lows. The lows are als
o studied individually to compare their development and movement, as w
ell as synoptic features, which may have been smoothed out in the comp
ositing process. Finally, the incidence of all such lows in the 1 x CO
2 and 2 x CO2 simulations are examined to determine the possible effec
ts a doubling of CO2 will have on their occurrence. The GCM surface lo
ws were found to develop from an upper-level cutoff low in a manner si
milar to the observed lows. In both sets, this development took place
between the upper-level subtropical and polar jets in all seasons exce
pt summer, where only one jet was evident. Latent heat release appeare
d to play an important role in the intensification of the surface lows
. The main difference between the two sets of cutoff lows was that the
GCM surface lows tended to develop farther to the east of the upper-l
evel cutoff, the upper-level features were less intense and occlusion
did not take place. As a result, the GCM lows had a greater eastward t
ranslation compared to the observed lows, which often meander along th
e east coast for several days while they intensify. These features app
ear to be related to the low resolution of the GCM. The frequency of e
ast Australian cutoff lows was underpredicted in the GCM by about 45%
in the 1 x CO2 simulation, with the greatest underprediction occurring
in autumn and winter. Analysis of upper-level jet structure indicated
that the GCM produced a poor simulation of the dual jet structure alo
ft, which may account for this problem. The 2 x CO2 simulation produce
d even fewer cutoff lows over eastern Australia. This was probably cau
sed by the reduced baroclinicity due to increased warming of polar reg
ions, which resulted in an even weaker dual jet structure. The east Au
stralian cutoff lows were found to be more intense in the 2 x CO2 simu
lation, suggesting the greater role played by latent heat effects once
development has been initiated.