OBJECTIVE IDENTIFICATION OF CYCLONES IN GCM SIMULATIONS

An objective routine for identifying individual cyclones has been deve loped. The procedure was designed with the aim to keep the input expen diture low: The method ensures a complete collection of cyclones and a n exclusion of short time fluctuations attributed to numerical effects . The cyclones are identified as relative minima of the geopotential h eight field in 1000 hPa. The initial stages of the cyclones are found by locating relative maxima in the 850-hPa vorticity field. Further on the temporal development of the extrema is taken into consideration. An individual cyclone is regarded only if it exists for at least 24 h and if it attains a mature stage at least once, where a certain margin of the geopotential gradient to the surroundings is exceeded. The ide ntification routine is applied to simulations with the Hamburg general circulation model ECHAM in T21 resolution. Also, cyclone tracks based on ECMWF analyses are evaluated, to which the model results are compa red. The effect of different climate conditions, for example, global w arming, on cyclone frequency and track location is investigated. It is found that a warmer SST distribution leads to a slight reduction of c yclone frequency in the Southern Hemisphere in fall (March, April, May ) and winter (June, July, August); elsewhere the differences are not s ignificant.