Barotropic Regeneration of Upper-Level Synoptic Disturbances in Different Configurations of the Zonal Weather Regime

Gwendal Rivière

doi:10.1175/2008jas2603.1

Abstract

Barotropic dynamics of upper-tropospheric midlatitude disturbances evolving in different configurations of the zonal weather regime (i.e., in different zonal-like large-scale flows) were studied using observational analyses and barotropic model experiments. The contraction stage of upper-level disturbances that follows their elongation stage leads to an increase of eddy kinetic energy that is called the barotropic regeneration process in this text. This barotropic mechanism is studied through notions of barotropic critical regions (BtCRs) and effective deformation that have been introduced in a previous paper. The effective deformation field is equal to the difference between the square of the large-scale deformation magnitude and the square of the large-scale vorticity. Regions where the effective deformation is positive correspond to regions where the large-scale flow tends to strongly stretch synoptic disturbances. A BtCR is an area separating two large-scale regions of positive effective deformation, one located upstream and on the south side of the jet and the other downstream and on the north side. Such a region presents a discontinuity in the orientation of the dilatation axes and is a potential area where the barotropic regeneration process may occur. Winter days presenting a zonal weather regime in the 40-yr ECMWF Re-Analysis dataset are decomposed, via a partitioning algorithm, into different configurations of the effective deformation field at 300 hPa. A six-cluster partition is obtained. Composite maps of the barotropic generation rate for each cluster exhibit a succession of negative and positive values on both sides of the BtCRs. It confirms statistically that the barotropic regeneration mechanism occurs preferentially about BtCRs. Numerical experiments using a forced barotropic model on the sphere are performed. Each experiment consists of adding a synoptic-scale perturbation to one of the zonal-like jet configurations found in observations, which is kept fixed with time. The combined effects of the effective deformation and nonlinearities are shown to be crucial to reproduce the barotropic regeneration process about BtCRs.

Highlights

Synoptic disturbances in the upper troposphere take the form of coherent wave packets with intense mobile troughs surrounding the globe in the midlatitudes and play a crucial role in the dynamics of various weather systems
The initial stretching of the cyclone is less strong and the regeneration process is less pronounced than in the more realistic case of nonzonal flows that are characterized by nontrivial effective deformation fields. In the latter case we have shown that this process is likely to occur about barotropic critical regions (BtCRs)
Barotropic dynamics of upper-tropospheric midlatitudes disturbances evolving in different kinds of zonallike jets were studied using both observational analyses and barotropic model experiments

Summary

Introduction

Synoptic disturbances in the upper troposphere take the form of coherent wave packets with intense mobile troughs surrounding the globe in the midlatitudes and play a crucial role in the dynamics of various weather systems They are involved in the triggering of extratropical surface cyclogenesis via baroclinic interaction (Petterssen and Smebye 1971) and second, they act on the large-scale atmospheric circulation through eddy momentum fluxes (Hoskins et al 1983). Lee (2000) explains this result by the fact that meridionally elongated waves in the diffluent region of the Pacific jet, for example, are contracted when they arrive in the confluent region at the entrance of the Atlantic jet Another area of positive barotropic generation rate exists over the North African subtropical jet; eddies evolving along the extratropical Atlantic jet get a northeast–southwest tilt in its exit region, propagate equatorward, and are compacted on the cyclonic side of the subtropical jet. It is worth noting that this process is a transient nonmodal phenomenon that may occur without the presence of barotropic instability, as shown theoretically in simple linear barotropic models by Farrell (1989) and Lee (1995)

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