GCM SIMULATIONS OF EASTERN AUSTRALIAN CUTOFF LOWS

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.