The life cycle of baroclinic eddies in a storm track environment

The life cycle of baroclinic eddies in a controlled storm track environment has been examined by means of long model integrations on a hemisphere. A t ime-lagged regression that captures disturbances with large meridional velo cities has been applied to the meteorological variables. This regressed sol ution is used to describe the life cycle of the baroclinic eddies. The eddi es grow as expected by strong poleward heat fluxes at low levels in regions of strong surface baroclinicity at the entrance of the storm track, in a m anner similar to that of Charney modes. As the eddies evolve into a nonline ar regime, they grow deeper by fluxing energy upward, and the characteristi c westward tilt exhibited in the vorticity vanishes by rotating into a meri dional tilt, in which the lower-level cyclonic vorticity center moves polew ard and the upper-level center moves equatorward. This rather classical picture of baroclinic evolution is radically modified by the simultaneous development of an upper-level eddy downstream of the p rincipal eddy. The results suggest that this eddy is an integral part of a self-sustained system here named as a couplet, such that the upstream princ ipal eddy in its evolution fluxes energy to the upper-level downstream eddy , whereas at lower levels the principal eddy receives energy fluxes from it s downstream companion but grows primarily from baroclinic sources. This st ructure is critically dependent on the strong zonal variations in baroclini city encountered within the storm track environment, A second important result revealed by this analysis is the fact that the lo w-level vorticity centers that migrate poleward tend to follow isotachs tha t closely correspond to the phase speed of the eddies. It is suggested that the maximum westward momentum that the eddies deposit at lower levels corr esponds to the phase velocity, a quantity that can be estimated just from t he upstream conditions. The intensity and direction of propagation of these waves will determine the overall structure of the storm track.