Zonally localized storm tracks in an idealized AGCM

The observed storm tracks in the Northern Hemisphere winter develop in regions of high baroclinicity off the east coasts of the Eurasian and North American continents, and terminate near the quasi-stationary ridges on the other side of the oceans. Atmospheric general circulation model (AGCM) experiments with a variety of idealized surface boundary conditions are performed to investigate the relationship between this zonal localization of the storm track, near-surface baroclinicity and extratropical stationary eddies. We here define storm tracks as regions of high kinetic energy of high-frequency (<10 days) eddies. In an aqua planet experiment, zonally varying sea surface temperatures (SSTs) in the tropics make a rather zonally uniform storm track but forces robust stationary eddies in the extratropics. By contrast, zonal variations in midlatitude SST, while exciting only weak stationary eddies, force a zonally localized storm track just at the sharp SST gradient. In another experiment with zonally uniform SST and an extratropical continent, the storm track is almost zonally uniform in spite of a strong maximum in near-surface baroclinicity off the east coast. Thus, neither large stationary eddy nor high baroclinicity automatically leads to a zonally localized storm track. An analysis of the energetics of high-frequency eddies in the storm track suggests that the primary energy source for these eddies is baroclinic conversion, and that the nonlinear interaction between high- and low-frequency eddies is important for the termination of the storm track. We plan to examine the decay process of baroclinic eddies in light of this interaction.