Changing cyclones and surface wind speeds over the North Atlantic and Europe in a transient GHG experiment

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 north- and 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.