On the Coupling between Barotropic and Baroclinic Modes of Extratropical Atmospheric Variability

Abstract

The baroclinic and barotropic components of atmospheric dynamics are usually viewed as interlinked through the baroclinic life cycle, with baroclinic growth of eddies connected to heat fluxes, barotropic decay connected to momentum fluxes, and the two eddy fluxes connected through the Eliassen–Palm wave activity. However, recent observational studies have suggested that these two components of the dynamics are largely decoupled in their variability, with variations in the zonal mean flow associated mainly with the momentum fluxes, variations in the baroclinic wave activity associated mainly with the heat fluxes, and essentially no correlation between the two. These relationships are examined in a dry dynamical core model under different configurations and in Southern Hemisphere observations, considering different frequency bands to account for the different time scales of atmospheric variability. It is shown that at intermediate periods longer than 10 days, the decoupling of the baroclinic and barotropic modes of variability can indeed occur as the eddy kinetic energy at those time scales is only affected by the heat fluxes and not the momentum fluxes. The baroclinic variability includes the oscillator model with periods of 20–30 days. At both the synoptic time scale and the quasi-steady limit, the baroclinic and barotropic modes of variability are linked, consistent with baroclinic life cycles and the positive baroclinic feedback mechanism, respectively. In the quasi-steady limit, the pulsating modes of variability and their correlations depend sensitively on the model climatology.

Frequency‐Dependent Behavior of Zonal Jet Variability

“…This is consistent with the idealized modeling results of Sparrow et al (), who found that the horizontal and vertical eddy terms of EP flux reach a quasi‐equilibrium at low frequencies, dominated by positive eddy feedback. The overall picture of the eddy signal associated with EOF1 at low frequency, therefore, is like that described by Robinson () and Robinson (), and found by Boljka et al () in model and reanalysis data: a quasi‐steady response of increased baroclinic wave generation, and locally increased EKE, in association with a poleward shift of the jet. In other words, baroclinic fluxes act to support the barotropic structure of the SAM at low frequencies.…”

“…All of the above shows (1) that the first zonal wind mode describes latitudinal shifts of both the midlatitude jet and its associated storm tracks at long timescales; (2) that EKE decreases in the direction of the jet shift and increases in the opposite direction at short timescales; and (3) that these two behaviors add up to the correlation seen in the unfiltered data, indicating that cancellation between low-and high-frequency processes produces the weak structural correspondence seen in Figure 3e. The first 10.1029/2019GL086585 two of these points are consistent with the strong correlation (anticorrelation) between the SAM and EOF2 of EKE under low-pass (high-pass) filtering shown by Boljka et al (2018).…”

“…Thompson and Woodworth () and Thompson and Barnes () have argued that Southern Hemisphere extratropical flow can be described by the SAM (a barotropic mode) and the first EOF of EKE (a baroclinic mode) and that the SAM accounts for a very small fraction of EKE and heat flux variability. However, Boljka et al () showed that strong correlations between the SAM and EOF2 of EKE can be found under time filtering. Here we confirm the results of Boljka et al () by showing that the correlations between the SAM, EKE, and heat flux depend strongly on the timescale one considers.…”

“…However, Boljka et al () showed that strong correlations between the SAM and EOF2 of EKE can be found under time filtering. Here we confirm the results of Boljka et al () by showing that the correlations between the SAM, EKE, and heat flux depend strongly on the timescale one considers. Additionally, we demonstrate that the weak correlations between these variables in unfiltered data are partly due to cancellation between low‐ and high‐frequency signals.…”

Planetary geostrophic Boussinesq dynamics: Barotropic flow, baroclinic instability and forced stationary waves

“…In the case of small‐amplitude eddies, Boljka and Shepherd () and Boljka et al . () provide a framework for studying interactions of the mean flow with planetary and synoptic scales. In the case of large‐amplitude eddies, the two‐scale model of Dolaptchiev and Klein () would be the asymptotic consistent extension of the present planetary scale model to the synoptic scale.…”