THE EQUILIBRATION OF SHORT BAROCLINIC WAVES

The life cycles of short baroclinic waves are investigated with the in tention of completing a simple classification of nonlinear equilibrati on scenarios. Shea waves become important in moist environments as lat ent heating reduces the scale of maximum baroclinic instability. Long- wave life cycles (wavenumber 6) were previously found to depend on det ails of the low-level momentum fluxes established during the earliest stages of development. These fluxes also serve as a focal point for th e 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 d etails of the life cycle. The normal modes have predominantly equatorw ard momentum fluxes, in contrast to the mainly poleward momentum fluxe s of long waves. The short waves are more meridionally confined. The e quatorward momentum fluxes direct the waves toward cyclonic breaking. The feedback with the zonal mean wind, the so-called barotropic govern or, 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 anomal y roil up into isolated vortices. Therefore, the short wave undergoes protracted barotropic decay leading to complete zonalization. Short wa ves 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 m ainly toward the pole and there is a consequent zonal-mean deceleratio n at high latitudes. These details are included in the proposed classi fication of equilibration scenarios.