GCM Simulations of Eastern Australian Cutoff Lows

J.J. Katzfey and K.L. McInnes

J. of Climate
9, 2337-2355.

Abstract

The ability of the CSIRO-9 General Circulation Model (GCM) to capture surface cut-off 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 1xCO_2 and 2xCO_2 simulations are examined to determine the possible effects a doubling of CO_2 will have on their occurrence.

The GCM surface lows were found to develop from an upper level cut-off 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 cut-off lows was that the GCM surface lows tended to develop further to the east of the upper level cut-off, 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 cut-off lows was underpredicted in the GCM by about 45% in the 1xCO_2 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 2xCO_2 simulation produced even fewer cut-off 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 cut-off lows were found to be more intense in the 2xCO_2 simulation, suggesting the greater role played by latent heat effects once development has been initiated.