Bimodal Behavior in the Zonal Mean Flow of a Baroclinic β-Channel Model

Abstract

The dynamical origin of midlatitude zonal-jet variability is examined in a thermally forced, quasi-geostrophic, two-layer channel model on a β-plane. The model's behavior is studied as a function of the bottom-friction strength.Two distinct zonal-flow states exist at realistic, low and intermediate values of the bottom drag; these two states are maintained by the eddies and differ mainly in terms of the meridional position of their climatological jets. The system's low-frequency evolution is characterized by irregular transitions between the two states.For a given branch of model solutions, the leading stationary and propagating empirical orthogonal functions are related to eigenmodes of the model's dynamical operator, linearized about the climatological state on this branch. Nonlinear interactions between these modes are instrumental in determining their relative energy level. In particular, the stationary modes' self-interaction is shown to vanish. Thus, these modes do not exchange energy with the mean flow and, consequently, dominate the lowest-frequency behavior in the model. The leading stationary mode resembles with the observed annular mode in the Southern Hemisphere.The bimodality is due to nonlinear interactions between nearly equivalent-barotropic, stationary and propagating modes, while the synoptic eddies play a modest role in determining the relative persistence of the two states. The role of synoptic eddies is very substantial only at unrealistically high values of the bottom drag, where they give rise to ultra-low-frequency variability by modifying the jet in a way that reinforces generation of the eddy field. This type of behavior is related to the presence of a homoclinic orbit in the model's phase space and is not apparent for more realistic, lower values of the bottom drag. 1 . IntroductionIn this paper, we study the origin of the zonally symmetric component of extratropical atmospheric variability. Midlatitude atmospheric behavior is characterized by a variety of spatial and temporal scales. Major weather phenomena are associated with fast baroclinic waves, whose breaking forms synoptic eddies; the spatial structures of these baroclinic phenomena vary with height, and their time scales are on the order of a week and shorter. In contrast, the midlatitude low-frequency variability (LFV), whose time scale is longer than that of synoptic eddies, is predominantly equivalent barotropic (Wallace 1983). The annular modes consist of meridional displacements of the zonally averaged zonal jet.The next EOF of the zonal-mean flow in both hemispheres is associated with irregular weakening and strengthening of the jet (Lorenz and Hartmann 2001, 2003). These modes have been also obtained in idealized numerical models (Robinson 1991(Robinson , 1996(Robinson , 2000Yu and Hartmann 1993; 2 Feldstein and Lee 1996;Lee and Feldstein 1996;Koo and Ghil 2002;Kravtsov et al. 2003).The mechanisms that govern this behavior are not fully understood: observational and theoretical results give rise to controvers...

Weather Regime Prediction Using Statistical Learning

“…This state dependence may also be connected to the often discussed eddy feedback on the large-scale flow (Robinson 2000;Kravtsov et al 2005). Non-Gaussianity could then arise from the interaction of multiplicative stochastic noise with linear, or quasi-linear, large-scale dynamics.…”

A mechanistic model of mid-latitude decadal climate variability

“…The sign and magnitude of this SST feedback are like those in other conceptual models. In contrast, though, to the ad hoc formulation of the feedback by the latter authors, ours is based on the dynamics of a fairly realistic atmospheric model (Kravtsov et al 2005a). …”

“…The corresponding probability density function (PDF; not shown) is skewed, with the main peak near the location of the dominant high-latitude atmospheric state, and a secondary shoulder indicative of the presence of the less occupied low-latitude state. This nongaussianity is an intrinsic nonlinear atmospheric phenomenon, as it is present in uncoupled, atmosphere-only simulations (Kravtsov et al 2005a). It may also characterize some of the observed atmospheric variability in the Northern Hemisphere (Kravtsov et al 2006a), although these results are still a subject of ongoing scientific debate (see section 5.2).…”

Low-Frequency Variability in the Midlatitude Baroclinic Atmosphere Induced by an Oceanic Thermal Front

“…In the basic experiment ϭ Ϫ1, which corresponds to the values H 1 H ϭ 7 230 m and H 2 H ϭ 2770 m, given H ϭ 10 000 m (see Table 1); values of Ӎ Ϫ1 or H 2 : H 1 Ӎ 3/7 are often used in atmospheric models (e.g., Kravtsov et al 2005). In section 4a, the influence of the relative layer depth on the dynamics will be examined.…”