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 comparing composites of ten GCM cases with ten observed lows. The lows are also studied individually to compare their development and movement, as well as synoptic features, which may have been smoothed out in the compositing process. Finally, the incidence of all such lows in the 1 x CO2 and 2 x CO2 simulations are examined to determine the possible effects a doubling of CO2 will have on their occurrence. The GCM surface lows were found to develop from an upper-level cutoff low in a manner similar to the observed lows. In both sets, this development took place between the upper-level subtropical and polar jets in all seasons except summer, where only one jet was evident. Latent heat release appeared 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-level cutoff, the upper-level features were less intense and occlusion did not take place. As a result, the GCM lows had a greater eastward translation compared to the observed lows, which often meander along the east coast for several days while they intensify. These features appear to be related to the low resolution of the GCM. The frequency of east 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 aloft, which may account for this problem. The 2 x CO2 simulation produced even fewer cutoff lows over eastern Australia. This was probably caused by the reduced baroclinicity due to increased warming of polar regions, which resulted in an even weaker dual jet structure. The east Australian cutoff lows were found to be more intense in the 2 x CO2 simulation, suggesting the greater role played by latent heat effects once development has been initiated.