The equilibration of short baroclinic waves

Abstract

The life cycles of short baroclinic waves are investigated with the intention of completing a simple classification of nonlinear equilibration scenarios. Short waves become important in moist environments as latent heating reduces the scale of maximum baroclinic instability. Long-wave life cycles (wavenumber 6) were previously found to depend on details of the low-level momentum fluxes established during the earliest stages of development. These fluxes also serve as a focal point for the present study. For a realistic, zonally symmetric jet on the sphere, the normal-mode life cycle of a short wave (wavenumber 8) under both dry and moist conditions is described. Latent heating intensifies the low pressure system and frontal zones but does not alter the broader details of the life cycle. The normal modes have predominantly equatorward momentum fluxes, in contrast to the mainly poleward momentum fluxes of long waves. The short waves are more meridionally confined. The equatorward momentum fluxes direct the waves toward cyclonic breaking. The feedback with the zonal-mean wind, the so-called barotropic governor, is less effective than in the standard long-wave life cycle, which ends in anticyclonic breaking. However, in contrast to long-wave life cycles that are "engineered" to produce equatorward momentum fluxes, relatively little potential vorticity and surface temperature anomaly roll up into isolated vortices. Therefore, the short wave undergoes protracted barotropic decay leading to complete zonalization. Short waves also have a brief period of baroclinic decay due to cold advection over the surface cyclones. Eliassen-Palm cross sections for the short-wave life cycles show the usual combination of upward and meridional propagation of wave activity. However, the meridional propagation is mainly toward the pole and there is a consequent zonal-mean deceleration at high latitudes. These details are included in the proposed classification of equilibration scenarios.