The impact of climate change on the poleward movement of tropical cyclone-like vortices in a regional climate model

A regional climate model (DARLAM, is implemented over the Australian region and a 20-yr seasonally varying simulation is examined for the presence of tropical cyclone-like vortices (TCLVs). The horizontal resolution of the mo del is 125 km with nine vertical levels and is forced at its boundaries by the output of the Commonwealth Scientific and Industrial Research Organisat ion CCM using a mixed layer (or "slab") ocean. Additional simulations are p el formed with a horizontal resolution of 30 kin and with 18 vertical level s to examine the impact of increasing resolution on storm intensity. A samp le of TCLVs from the 125-km resolution simulation is simulated at 30-km res olution to determine whether they reach observed tropical storm intensity a t the finer resolution. It is found that stronger vortices in the 125-km re solution simulation are more likely to intensify when simulated at finer re solution than weaker vortices. In this way, a detection threshold for vorti ces generated in the 125-km resolution simulation is established and then u sed to detect TCLVs in that simulation. The regional climate model DARLAM p rovides a good simulation of both cyclogenesis and its seasonal variation u nder the current climate. The response of the model under enhanced greenhou se conditions is studied. Under 2 x CO2 conditions, there is. no statistica lly significant change in regions of formation of TCLVs, with only a slight southward shift simulated. Nevertheless, there are statistically significa nt effects on the poleward movement of TCLVs, with storms generally tending to travel farther poleward in a warmer climate once they have formed. An a nalysis is undertaken to determine the reasons for this behavior. While the dynamical constraints on the maintenance of TCLV intensity under 2 x CO2 c onditions (e.g., vertical wind shear) are similar to those in the current c limate, thermodynamic conditions (e.g., sea surface temperatures) are quite different and are likely to be at least partly the cause of this effect. O ther causes include the combination of the slight southward shift in format ion and a tendency for TCLV tracks to be more southward in enhanced greenho use condition,, a con sequence of more southward steering winds.