Variability Patterns and Eddy Feedbacks in Different Flow Regimes of the Jet Stream

Meridional shifting of the eddy driven jet is the most prominent variability pattern of the zonal mean zonal wind in both hemispheres (Lorenz and Hartmann 2001, 2003). However, other variability patterns are seen over certain limited zonal sectors, such as the extension - retraction of the North Pacific jet during Northern Hemisphere winter (Eichelberger and Hartmann 2007, Athanasiadis et. al. 2010) and the seesaw between the subtropical and eddy driven jet over the South Pacific during Southern Hemisphere winter (Codron 2007). Besides the different variability patterns, there are also observed differences in the persistence of the variability modes and the strength of the eddy feedbacks that maintain them in different seasons and locations (Barnes and Hartmann 2010a, b). There seems to be a relation between the latitude of the jet and its variability characteristics (Eichelberger and Hartmann 2007, Barnes et. al. 2010).

In this study, the variability patterns of the zonal mean flow and the eddy feedbacks that maintain them are analyzed for different flow regimes of the jet stream, using a two-layer spherical quasi-geostrophic model. The flow regime is controlled by model parameters affecting the degree of baroclinic instability. As the parameters are varied to allow for stronger instability, the statistically steady state jet shifts polewards and the flow transitions from a subtropical jet regime, to a merged (subtropical-eddy driven) jet regime and then to an eddy driven jet regime. The variability characteristics of the zonal mean flow change with the flow regime and are found to be closely related to the wave spectrum.

In the subtropical jet regime the maximum zonal wind in the upper layer is located at the subtropical edge of the Hadley cell around latitudes 30◦ - 35◦, and a weak eddy driven jet exists at midlatitudes. Most of the variability is concentrated far poleward of the subtropical jet and is characterized by meridional shifting of the eddy driven jet and strengthening or weakening of the high latitude zonal winds associated with a polar meridional circulation. The mechanism driving the variability in the midlatitudes is related to cyclonic wavebreaking of a zonal wavenumber 4 low phase speed wave on the poleward flanks of the jet. At high latitudes the variability is driven by the interaction of the mean flow and the polar cell with long (zonal wavenumbers 1-3) westward propagating waves. The variability signal is weak and has a time scale of a few tens of days.

In the merged jet regime the jet is sharp and located inside the Ferrel cell around latitudes 40◦ - 45◦ with surface westerlies below the jet. The spectrum is dominated by zonal wavenumber 5, which grows baroclinically at the jet latitude and maintains it by its momentum flux convergence. The variability of the upper layer zonal wind is characterized by weak meridional shifting of the jet with very long time scales, that can reach 100 days. The weak signal and high persistence of the variability pattern are related to the strong positive feedback between wavenumber 5 and the mean flow. The variability of the vertical shear of the zonal wind is characterized by oscillations of strengthening and weakening with a well defined period of around 10 days, which are driven by a negative feedback between the vertical shear and the eddy heat flux.

In the eddy driven jet regime the jet is wide and its maximum is located inside the Ferrel cell around latitude 50◦. The wave spectrum is wide, with medium scale waves growing baroclinically and transferring energy to long waves by nonlinear interactions. The variability is strong and characterized by fluctuations between single and double jet states, with a time scale of a few days. The short time scale of the variability is related to a negative feedback between the waves and the mean flow, where anomalies of the zonal mean zonal wind lead to meridional shifting of the critical latitude on the equatorward flanks of the jet, which in turn leads to eddy momentum flux convergence anomalies forcing opposite signed zonal wind anomalies.

The analysis of the variability mechanisms in the different flow regimes of this idealized model serves as a reference for interpreting the variability properties of the observed flow. We suggest a few simple criteria for categorizing the observed flow regimes and their type of variability, as a step towards understanding the mechanisms controlling the internal variability of the extratropical atmospheric flow.