NORTH-ATLANTIC CYCLONES IN CO2-INDUCED WARM CLIMATE SIMULATIONS - FREQUENCY, INTENSITY, AND TRACKS

The effect of CO2-induced climate change on the North Atlantic storm a nd cyclone tracks in winter is analysed using time slice experiments o f the Hamburg atmospheric general circulation model (ECHAM3) with tria ngular truncation at wave number 42 (T42) and 19 levels. The sea surfa ce temperature (SST) and sea ice boundary conditions for these experim ents are taken from a transient Intergovernmental Panel on Climate Cha nge (IPCC) scenario A run of ECHAM1/LSG at the times where the 1 x CO2 (control run), the 2 x CO2 and the 3 x CO2 concentrations are reached . Using a cyclone identification and tracking scheme, we detect the lo w pressure systems as relative minima in the 1000 hPa geopotential hei ght field and connect them to cyclone tracks. The results of the Euler ian analysis of the storm track using filtered variances and the Lagra ngian analysis of the cyclone trajectories from the three climate runs are discussed and compared with each other. In the 2 x CO2 experiment , the storm track shifts eastward, whereas the cyclone density shifts northeastward. In the 3 x CO2 experiment the storm track shows a south eastward shift, whereas the cyclone density shifts northward. The vari ability of the cyclone tracks is determined by a cluster analysis of t heir relative trajectories considering the first three days of the cyc lones. The relative cyclone tracks are grouped into stationary, zonal and northeastward travelling cyclones. This analysis provides a method to assess the model quality and to detect changes of the cyclone traj ectories in different climates. In the 2 x CO2 (but not in the 3 x CO2 ) run the occupation number of northeastward cyclones increases.