Centennial trends in Northern Hemisphere winter storm tracks over the twentieth century

Abstract In this study changes in Northern Hemisphere winter storm tracks during the twentieth century are investigated based on the individual 56 ensemble members of the twentieth century re‐analysis dataset. It is found that the twentieth century trends in storm‐track activities exhibit large discrepancies between the upper and lower troposphere. In the upper troposphere, a substantial intensification is identified at the poleward and downstream regions of the North Pacific and North Atlantic storm‐track activities, indicating a large northeastward expansion of storm tracks in the late twentieth century. However, in the lower troposphere the synoptic eddy activities, especially in terms of the eddy kinetic energy and meridional eddy heat flux, tend to be significantly weakened over the high latitudes of central‐western North Pacific and the upstream regions of the North Atlantic storm tracks. Further inspections find that such strengthening (weakening) of storm tracks in the upper (lower) troposphere are attributed mainly to an increase (decrease) of baroclinic instability, which is predominantly determined by the meridional temperature gradient changes. Moreover, from a local energetic perspective, the baroclinic generation and barotropic damping of the synoptic eddies are found to be substantially enhanced at the upstream and downstream regions of the two storm tracks in the upper troposphere, respectively, while in the lower troposphere the baroclinic energy conversion to eddies is generally decreased.