Time Scale and Feedback of Zonal-Mean-Flow Variability

Abstract

The physical processes that determine the time scale of zonal-mean-flow variability are examined with an idealized numerical model that has a zonally symmetric lower boundary. In the part of the parameter space where the time-mean zonal flow is characterized by a single (double) jet, the dominant form of zonalmean-flow variability is the zonal index (poleward propagation), and the time-mean potential vorticity gradient is found to be strong and sharp (weak and broad). The e-folding time scale of the zonal index is found to be close to 55 days, much longer than the observed 10-day time scale. The e-folding time scale of the poleward propagation is about 40 days. The long e-folding time scales for the zonal index are found to be consistent with an unrealistically strong and persistent eddy-zonal-mean-flow feedback. A calculation of the refractive index indicates that the background flow supports eddies that are trapped within midlatitudes, undergoing relatively little meridional propagation.Additional model runs are performed with an idealized mountain to investigate whether zonal asymmetry can disrupt the eddy feedback. For single-jet states, the time scale is reduced to about 30 days if the mountain height is 4 km or less. The reduction in the time scale occurs because the stationary eddies excited by the mountain alter the background flow in a manner that leads to the replacement of zonal-index events by shorter-time-scale poleward propagation. With a 5-km mountain, the time scale reverts and increases to 105 days. This threshold behavior is again attributed to a sharpening of the background zonal jet, which arises from an extremely strong stationary wave momentum flux convergence. In contrast, for double-jet states, the time scale changes only slightly and the poleward propagation is maintained in all mountain runs.

Quantifying the Eddy Feedback and the Persistence of the Zonal Index in an Idealized Atmospheric Model

“…Additionally, the shift of upper-level jets is accompanied by the movement of surface westerly winds in all of our experiments. Despite the seeming artificiality of our perturbation, the jet regime transition and persistence in this model are analogous to those found in many other studies where the width of the baroclinic zone or the model resolution is changed (Lee 1997;Son et al 2008;Gerber and Polvani 2009;Chan and Plumb 2009). We further look at the climatological mean jet, the leading mode of the zonal wind, and the probability density function (PDF) of the jet latitude (Fig. 3).…”

“…Also, Chen et al (2008) found that the positive phase of annular modes is accompanied by an increase of eddy phase speed and a poleward shift of the critical latitude of eddy absorption, which may contribute to the positive feedback. Additionally, Son et al (2008) suggested that the eddy feedback can depend on the barotropic decay of baroclinic eddies that can be affected by the potential vorticity gradient in the subtropics. Despite all these possibilities, little progress has been made in quantifying the decorrelation time of the zonal index with respect to the physical parameters or climatological winds.…”

“…Also, the change of the dominant mode of zonal wind when the model is perturbed is often accompanied by a change of the climatological wind, making it difficult to understand the causal relationship between the variability and climatology. For instance, Gerber and Vallis (2007) and Son et al (2008) both found that the decorrelation time scale decreases as the time mean jet moves poleward, but there is not such a relationship as the model resolution is changed (Gerber et al 2008b).…”

Prediction of northern summer low-frequency circulation using a high-order vector auto-regressive model

“…9b). This time scale is consistent with that of most teleconnection patterns and wave-mean flow interactions (e.g., Feldstein 2000; Son et al 2008;Teng et al 2013), indicating wave propagation as a likely mechanism. The mechanisms of tropical heating impacts on forecast skill will be discussed in more detail in the next subsection.…”

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

“…Gerber et al (2008) showed that the coupled general circulation models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) report are too persistent compared with observations in both hemispheres and seasons. Some studies have addressed the sensitivity of zonal index persistence on external parameters using idealized models (Gerber and Vallis 2007;Son et al 2008a), but results must be interpreted with caution because changes in persistence when parameters are varied might be due to changes in the mean state rather than to changes in the feedback mechanism (Chen and Plumb 2009).…”