Wave-Mean Flow Interaction during the Life Cycles of Baroclinic Waves

Abstract

A two-layer quasigeostrophic β-plane channel model is used to examine the role of the wave-mean flow interaction during the life cycles of baroclinic waves. Two cases are examined: a wide and a narrow jet limit. These two limits are required to satisfy the property that their instability lead to a realistic baroclinic life cycle consisting of baroclinic growth and barotropic decay. In order to characterize the properties of the zonal-wind tendency in the two cases, scaling arguments based on a study by Andrews and McIntyre are used. This scaling procedure is then used to explain the nonlocal (local) zonal-wind tendency during the realistic baroclinic life cycle for the wide (narrow) jet limit.Several differences between the properties of the two jet limits are found. For the wide jet limit, the acceleration at the center of the jet is confined to the growth stage. This contrasts the narrow jet limit where the jet is accelerated throughout the entire life cycle. These differences depend upon the lower-layer potential vorticity fluxes, which exhibit the same timing properties as the zonal-wind tendency. In addition, for both the wide and narrow jet limits, irreversible potential vorticity mixing is shown to force nonlocal and local permanent changes to the zonal wind, respectively. A comparison is also made between the vorticity flux and potential vorticity flux to determine which is a better predictor of the zonal-wind tendency. It is shown that in the wide (narrow) jet limit, the vorticity (potential vorticity) flux does better at predicting the zonal-wind tendency. It is also argued that one can use a barotropic model to study the temporal evolution of the upper-layer flow for both the narrow and wide jet limits.Last, it is shown that the properties of the inviscid calculations are retained when thermal forcing and surface Ekman friction are included. Calculations are performed with different values for the surface Ekman friction coefficient and with the thermal forcing coefficient fixed. For the wide (narrow) jet limit, it is found that the disturbance grows to a larger (smaller) total energy as the Ekman friction coefficient is increased (decreased). This behavior for the wide jet limit is explained in terms of an enhancement of the baroclinic energy conversions that overcome the barotropic governor mechanism of James and Gray.