The Life Cycle of Baroclinic Eddies in a Storm Track Environment

Abstract

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 time-lagged regression that captures disturbances with large meridional velocities has been applied to the meteorological variables. This regressed solution is used to describe the life cycle of the baroclinic eddies. The eddies 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 manner similar to that of Charney modes. As the eddies evolve into a nonlinear regime, they grow deeper by fluxing energy upward, and the characteristic westward tilt exhibited in the vorticity vanishes by rotating into a meridional tilt, in which the lower-level cyclonic vorticity center moves poleward 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 principal 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 principal eddy in its evolution fluxes energy to the upper-level downstream eddy, whereas at lower levels the principal eddy receives energy fluxes from its downstream companion but grows primarily from baroclinic sources. This structure is critically dependent on the strong zonal variations in baroclinicity encountered within the storm track environment. A second important result revealed by this analysis is the fact that the low-level vorticity centers that migrate poleward tend to follow isotachs that closely correspond to the phase speed of the eddies. It is suggested that the maximum westward momentum that the eddies deposit at lower levels corresponds to the phase velocity, a quantity that can be estimated just from the upstream conditions. The intensity and direction of propagation of these waves will determine the overall structure of the storm track.