Baroclinic wave breaking and the internal variability of the tropospheric circulation

A simple model of the tropospheric circulation, based on a 10-level primiti ve equation model, is forced by linearly relaxing the potential temperature toward an idealized, zonally symmetric equilibrium held. The model equatio ns are integrated in time until a statistically steady state is obtained. T he local relationship between the state of the background flow, the directi on of wave propagation, and subsequent wave breaking at the tropopause leve l is then investigated. Maps of potential vorticity (PV) on isentropic surf aces are analyzed and all four different types of wave breaking described r ecently by Peters and Waugh are shown to occur. It is found that cyclonic w ave breaking events are usually initiated by poleward fluxes of wave activi ty, and anticyclonic events by equatorward fluxes. Composites are then used to show that equatorward fluxes are associated with a jet that is locally broad and weak, with relatively strong isentropic PV gradients to its equat orward flank. By contrast, poleward fluxes are associated with a narrow, st rong jet, with very weak or even negative PV gradients on its equatorward s ide. It is argued that this result is consistent with nonlinear critical-la yer theory, as under certain conditions an isolated region of homogenized p otential vorticity must remain a perfect reflector of wave activity for all time. The variability exhibited by the zonal flow field is then investigated usin g a cross-sectional EOF method. The first EOF is found to have similar stru cture in the latitude-height plane to the baroclinic waves themselves, and describes much of the variability associated with them. The second EOF has structure that corresponds to a sharp, narrow jet in its positive phase and a weak, broad jet in its negative phase. Its phase is shown to be well cor related with the wave activity flux index, with the maximum occurring at a space and time lag, with the phase of the EOF preceding the index. Most of the variability associated with this EOF occurson the scale of zonal wavenu mbers 2-4, suggesting that the direction of meridional propagation of the b aroclinic waves is determined locally. Strikingly, the phase of the second EOF propagates in a wavelike manner, with wavenumber and period (approximat e to 11-14 days) quite distinct from those of the baroclinic waves. Individ ual phase maxima of these long waves can persist for up to approximate to 2 0-25 days, as they do not decay rapidly due to downstream radiation.