The Impact of Baroclinic Eddy Feedback on the Persistence of Jet Variability in the Two-Layer Model

Abstract

Although it is well known that the persistence of extratropical jet shifts is enhanced by a positive eddy feedback, the dynamics of this feedback is still debated. Two types of mechanisms have been proposed: barotropic mechanisms rely on changes in upper-level propagation and baroclinic mechanisms rely on the coupling between barotropic and baroclinic flow. Recent studies have suggested that barotropic models can capture key aspects of the observed jet variability but the role of baroclinic dynamics has been less explored.This study investigates the temporal relations between barotropic and baroclinic anomalies and their eddy forcings during the internal variability of the simple two-layer quasigeostrophic model. A large correlation is found between barotropic and baroclinic anomalies and between the meridional and vertical components of the Eliassen-Palm divergence, especially at low frequency. The low-frequency variability is consistent with the baroclinic mechanism: persistent upper-level eddy momentum convergence is associated with (and precedes) persistent anomalies in the poleward eddy heat flux. In contrast, at high frequency, poleward heat flux anomalies are associated with eddy momentum divergence aloft and both eddy forcings have same-sign contributions to the upper-level eddy potential vorticity (PV) flux. In this limit the eddy PV flux is associated with wave activity transience as effective diffusivity is too small to dissipate the wave-mean flow interaction term.The large correlation between barotropic and baroclinic anomalies implies that the low-frequency variability of barotropic flow may be affected by thermal damping when this damping is sufficiently strong. For example, zonal index persistence drops drastically in our model when baroclinicity shifts are prevented by strong thermal restoration.

Applying the Fluctuation–Dissipation Theorem to a Two-Layer Model of Quasigeostrophic Turbulence

“…The values of the model parameters are nondimensionalized versions of those used by Zurita-Gotor et al (2014), with the velocity scale set by the strength of the baroclinically unstable jet and the length scale set by the radius of deformation. With these parameters the jet width s is 3.5, b is 0.196, t f is ;15, t d is ;100, and n is 0.01.…”

“…With these parameters the jet width s is 3.5, b is 0.196, t f is ;15, t d is ;100, and n is 0.01. We choose to follow Zurita-Gotor et al (2014) in the choice of parameters because they result in qualitatively realistic annular-mode variability. With this nondimensionalization, Phillips's stability criterion for uniform vertical shear in the absence of dissipation is b , 1.…”

“…Finally, several papers have investigated the complex structure of the autocorrelation of the annular modes (Lorenz and Hartmann 2001;Gerber et al 2008;ZuritaGotor et al 2014;Zurita-Gotor 2014). In particular, Zurita-Gotor et al (2014) showed that this autocorrelation includes high-and low-frequency components that reflect the effects of positive and negative eddy feedbacks.…”

“…In particular, Zurita-Gotor et al (2014) showed that this autocorrelation includes high-and low-frequency components that reflect the effects of positive and negative eddy feedbacks. This may explain the lack of quantitative agreement in Ring and Plumb (2008), as simplified methods that effectively assume the structure of the correlations are unlikely to give accurate estimates and also justifies our use of the full FDT, which integrates over different time scales.…”

A Short-Term Negative Eddy Feedback on Midlatitude Jet Variability due to Planetary Wave Reflection

“…However, there is still a debate on the exact nature of the positive synoptic eddy feedback (e.g., Robinson 2000;Lorenz and Hartmann 2003;Robinson 2006;Barnes and Hartmann 2011;Zhang et al 2012;Zurita-Gotor et al 2014): some studies emphasize the role of barotropic mechanisms whereby a jet shift creates a change in the horizontal propagation of the waves (e.g., Barnes and Hartmann 2011;Lorenz 2014). Some others focus on baroclinic mechanisms in which the jet shift induces a change in the latitude of the baroclinicity and thus a change in the latitude of wave stirring, which in turn impacts the latitude of momentum flux convergence (e.g., Robinson 2000;Zurita-Gotor et al 2014). Others again propose mixed barotropic-baroclinic mechanisms in which the latitude of the lower baroclinicity determines the nature of wave propagation at upper levels (Rivière 2009).…”