Maintaining the North Atlantic storm track

The maintenance of the storm tracks relies on maintaining the baroclinic zones from which mid-latitude cyclones develop. By expressing baroclinicity (a standard measure of baroclinic growth) in terms of dry entropy and constructing an entropy budget for the North Atlantic storm track, we find that the climatological maintenance of the storm track is due to large-scale advective processes in the free troposphere. We find the most important factor contributing to the maintenance of the baroclinic zone to be the import of cold continental air from North America towards the storm track, characterised by the zonal advection of lower entropy air masses. For eddy timescales, however, these advective processes weaken baroclinicity as they are dominated by the growth of weather systems. Our findings suggest that local diabatic effects, dominated by latent heating, are of secondary importance and may even damp the strength of the baroclinicity on average.

Our results indicate that the storm track in the N. Atlantic is essentially governed by the “discharging condenser” mechanism proposed by Jerome Namias in 1950. In that picture, the diabatic effects ultimately responsible for the maintenance of the N. Atlantic storm track are remote rather than local.

Namias, J. 1950. The index cycle and its role in the general circulation, Journal of Atmospheric Sciences 7, no. 2,
130 –139.