Two paradigms of baroclinic‐wave life‐cycle behaviour

Abstract Two idealized baroclinic wave‐6 life cycles examined here suggest a framework of opposite extremes (a) in which to view the behaviour of real synoptic‐scale disturbances in middle latitudes, and (b) in which to interrelate the synoptic and wave‐theoretic viewpoints, using the ‘saturation‐propagation‐saturation’ (SPS) picture of wave‐6 life‐cycle behaviour. The two life cycles. dented by LC1 and LC2, are higher‐resolution versions of the Simmons‐Hoskins ‘basic’ and ‘anomalous’ cases (showing strong and weak late decay of eddy kinetic energy, EKE). They illustrate, in varying degrees. Two extreme types of behaviour here disignated ‘anticyclonic’ and ‘cyclonic’, and epitomized by strongly contrasting upper‐air trough behaviour. ‘Anticyclonic’ behaviour dominates the late stages of LC1 and is characterized by backward‐tilted, thinning troughs being advected anticyclonically and equatorward, as in the commoner cases of planetary‐scale mid‐stratospheric ‘Rossby‐wave breaking’. ‘Cyclonic’ behaviour dominates LC2 and is characterized by forward‐tilted, broadening troughs wrapping themselves up cyclonically and poleward. producing major cut‐off cyclones in high latitudes. These morphologies are visualized by upper‐air maps of potential temperature on the nominal tropopause, defined as a constant‐potential‐vorticity surface. Some atmospheric mid‐latitude disturbances examined here, using the same visualization applied to operational analyses, show the same two extreme types of trough behaviour together with intermediate cases. The SPS picture is re‐examined, using Eliassen‐palm and refractive‐index cross‐sections. It is shown, in particular, by reference to a wave‐activity theorem of Haynes, that the late stages of LC2 can be looked upon as a remarkably clear, and morphologically novel, large‐amplitude counterpart of the nonlinear reflection scenario of Rossby‐wave critical‐layer theory. The late stages of LC1, by constrast, look more akin to an nonlinear critical‐layer absorption scenario. LC2 exhibits region of largely undular PV contours adjacent to a nonlinear of irreversiby deformen PV contous. In the latter region PV rearrangement, and hence absorption of Rossby‐wave activity, has largely ceased. This accounts for the more persistent EKEE in the LC2 case.