A Mechanism for the Poleward Propagation of Zonal Mean Flow Anomalies

Abstract

Observational studies have shown that tropospheric zonal mean flow anomalies frequently undergo quasi-periodic poleward propagation. A set of idealized numerical model runs is examined to investigate the physical mechanism behind this poleward propagation.This study finds that the initiation of the poleward propagation is marked by the formation of negative zonal wind anomalies in the Tropics. These negative anomalies arise from meridional overturning/breaking of waves that originate in midlatitudes. This wave breaking homogenizes the potential vorticity (PV) within the region of negative zonal wind anomalies, and also leads to the formation of positive zonal wind anomalies in the subtropics. Subsequent equatorward radiation of midlatitude waves is halted, which results in wave breaking at the poleward end of the homogenized PV region. This in turn generates new positive and negative zonal wind anomalies, which enables a continuation of the poleward propagation. The shielding of the homogenized PV region from equatorward wave propagation allows the model's radiative relaxation to reestablish undisturbed westerlies in the Tropics, while extratropical westerly anomalies arise from eddy vorticity fluxes.The above process indicates that the poleward zonal mean anomaly propagation is caused by an orchestrated combination of linear Rossby wave propagation, nonlinear wave breaking, and radiative relaxation. The importance of the meridional wave propagation and breaking is consistent with the fact that the poleward propagation occurs only in the parameter space of the model where the PV gradient is of moderate strength. Implications for predictability are briefly discussed.

Persistent Circulation Regimes and Preferred Regime Transitions in the North Atlantic

“…However, it might influence the preferred transitions in the sense that some of the preferred transitions are more dominant in certain seasons. But the fact that our study finds a preferred poleward motion that is consistent with previous studies (Riehl et al 1950;James and Dodd 1996;Feldstein 1998;Lee et al 2007) suggests that our results are reliable and robust.…”

“…This is consistent with the preferred transitions in Kimoto and Ghil (1993), which show evidence for a poleward movement of geopotential height anomalies. The preferred poleward movement of the regimes and the role played by wave breakings in the regime transitions is consistent with the mechanism put forward by Lee et al (2007). They find in model experiments that wave breaking and linear Rossby wave propagation with a relaxation process drive the poleward zonal mean flow motion.…”

“…3 one can see that this corresponds to a preferred transition cycle with a northward movement of the jet. This might be the same phenomenon as described in Riehl et al (1950), James and Dodd (1996), Feldstein (1998), and Lee et al (2007). The transition into the southern jet regime is particularly dramatic, comprising a shift from the northern regime rather than the geographically closer central regime.…”

The Atmospheric General Circulation and Its Variability

“…If the wave propagation is preferentially equatorward, this displaces the wave breaking and consequent residual sinking motion to the warm side of the jet. A preference for equatorward wave propagation and breaking provides a mechanism for poleward propagation of jet anomalies (Lorenz and Hartmann 2001;Lee et al 2007) that has been observed in some studies (Feldstein 1998).…”

Annular Variability and Eddy–Zonal Flow Interactions in a Simplified Atmospheric GCM. Part I: Characterization of High- and Low-Frequency Behavior

“…However, the tropospheric jet itself exhibits natural variability and, if fluctuation-dissipation theory holds, understanding tropospheric flow regimes may be important for determining the response to such forcings. Numerous previous studies have examined zonal-mean tropospheric flow regimes in both models and observations (e.g., James and James 1992;James et al 1994;James and Dodd 1996;Lorenz andHartmann 2001, 2003;Son and Lee 2006;Lee et al 2007;Gerber and Vallis 2007;Vallis and Gerber 2007). James et al demonstrate that, for a simple global circulation model, wave-mean flow interactions give rise to an intrinsic long time scale and may be implicit in the generation of low-frequency variability with time scales greater than a year.…”