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A 240 yr run of the ECHAM4/OPYC3 coupled ocean-atmosphere model with transient greenhouse gas (GHG) forcing according to the IPCC IS92a scenario is examined with respect to simulated changes in boreal winter cyclone activity and 10 m wind speeds over Europe, the North Atlantic and Eastern North America. It is found that simulated cyclone activity undergoes a pronounced northand eastward shift over Europe and the Northeast Atlantic. This shift is accompanied by a decrease in the number of weak cyclones and an increase in deep cyclones (with core pressures below 970 hPa) in this area. The cyclone signal corresponds to the changes in storm track activity and upper-tropospheric baroclinicity. Increases of mean wind speeds and of wind speed extremes are identified over Northern Europe and parts of the East Atlantic. The wind signal is due to an increase in wind speed variability and an intensification of the westerly mean current connected with an enhanced mean pressure gradient. It is shown that the rising number of extreme wind events in the GHG simulation is connected to the augmented occurrence of deep cyclones over Northern Europe and the adjacent ocean areas. There are also strong wind speed increases over Hudson Bay and the Greenland Sea. They are restricted to the planetary boundary layer and appear to be connected to the reduction in winter mean sea-ice cover, which leads to locally decreased static stability and -over the Greenland Sea -also to a reduction in surface roughness.KEY WORDS: GCM · GHG scenario · Cyclones · Low-level wind extremes · Storm tracks · Baroclinicity Clim Res 15: [109][110][111][112][113][114][115][116][117][118][119][120][121][122] 2000 Sea for the recent decades, but according to their estimates the current intensity is not larger than it was at the beginning of this century.Assuming a trend in storm and cyclone occurrence is detectable in observational data, it would be interesting if the observed changes are part of natural variability or if they can be assigned to rising greenhouse gas (GHG) concentrations. This question is generally addressed by investigating numerical climate simulations with a prescribed increase in GHG forcing. There is a certain agreement between different simulations of this kind concerning the signal in the mean state of the climate system, particularly with regard to the vertical meridional structure of temperature change (Mitchell et al. 1990, Karoly et al. 1994, IPCC 1996, Cubasch et al. 1997. Warming maxima in the low-latitude upper troposphere and in the polar lower troposphere are common features of many GHG simulations. These temperature changes lead to increased upper tropospheric and reduced lower tropospheric zonal mean baroclinicity and to a global increase in atmospheric water-vapour content.Although studies with simplified conceptional models have shown that baroclinic wave activity is more sensitive to lower than to upper level changes (Held & O'Brien 1992, Pavan 1996, it is not a straightforward task to predict the net effect ...

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