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Chengji Liu and Elizabeth A. Barnes

Abstract

Isentropic mixing is an important process for the distribution of chemical constituents in the mid- to high latitudes. A modified Lagrangian framework is applied to quantify the mixing associated with two distinct types of Rossby wave breaking (i.e., cyclonic and anticyclonic). In idealized numerical simulations, cyclonic wave breaking (CWB) exhibits either comparable or stronger mixing than anticyclonic wave breaking (AWB). Although the frequencies of AWB and CWB both have robust relationships with the jet position, this asymmetry leads to CWB dominating mixing variability related to the jet shifting. In particular, when the jet shifts poleward the mixing strength decreases in areas of the midlatitude troposphere and also decreases on the poleward side of the jet. This is due to decreasing CWB occurrence with a poleward shift of the jet. Across the tropopause, equatorward of the jet, where AWB mostly occurs and CWB rarely occurs, the mixing strength increases as AWB occurs more frequently with a poleward shift of the jet. The dynamical relationship above is expected to be relevant both for internal climate variability, such as the El Niño–Southern Oscillation (ENSO) and the annular modes, and for future climate change that may drive changes in the jet position.

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Elizabeth A. Barnes and Chaim I. Garfinkel

Abstract

As the surface drag is increased in a comprehensive general circulation model (GCM), the upper-level zonal winds decrease and eddy momentum flux convergence into the jet core increases. Globally averaged eddy kinetic energy decreases, a response that is inconsistent with the conventional barotropic governor mechanism whereby decreased barotropic shears encourage baroclinic wave growth. As the conventional barotropic governor appears insufficient to explain the entire response in the comprehensive GCM, the nondivergent barotropic model on the sphere is used to demonstrate an additional mechanism for the effect of surface drag on eddy momentum fluxes and eddy kinetic energy. Analysis of the pseudomomentum budget shows that increased drag modifies the background meridional vorticity gradient, which allows for enhanced eddy momentum flux convergence and decreased eddy kinetic energy in the presence of a constant eddy source. This additional feedback may explain the changes in eddy momentum fluxes observed in the comprehensive GCM and was likely present in previous work on the barotropic governor.

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Elizabeth A. Barnes and Dennis L. Hartmann

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The persistence of the southern annular mode (SAM) is studied during austral winter (June–September) and summer (December–March) using observations of the three-dimensional vorticity budget. Analysis of the relative vorticity tendency equation shows that convergence of eddy vorticity flux in the upper troposphere, coupled with a secondary circulation, constitutes a positive eddy feedback that acts to sustain the vorticity anomaly associated with the jet shift against drag. The feedback exhibits a strong seasonality, with summer months revealing a positive feedback through much of the hemisphere and winter months showing a positive feedback over the Indian Ocean but not over the western Pacific. Results suggest that the lack of wintertime feedback over the western Pacific is due to the weakness of the eddy-driven midlatitude jet in that region.

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Elizabeth A. Barnes and Dennis L. Hartmann

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The eddy-driven jet is located in the midlatitudes, bounded on one side by the pole and often bounded on the opposite side by a strong Hadley-driven jet. This work explores how the eddy-driven jet and its variability persist within these limits. It is demonstrated in a barotropic model that as the jet is located at higher latitudes, the eddy length scale increases as predicted by spherical Rossby wave theory, and the leading mode of variability of the jet changes from a meridional shift to a pulse. Looking equatorward, a similar change in eddy-driven jet variability is observed when it is moved equatorward toward a constant subtropical jet. In both the poleward and equatorward limits, the change in variability from a shift to a pulse is due to the modulation of eddy propagation and momentum flux. Near the pole, the small value of beta (the meridional gradient of absolute vorticity) and subsequent lack of wave breaking near the pole account for the change in variability, whereas on the equatorward side of the jet the strong subtropical winds can affect eddy propagation and restrict the movement of the eddy-driven jet or cause bimodal behavior of the jet latitude. Barotropic quasilinear theory thus suggests that the leading mode of zonal-wind variability will transition from a shift to a pulse as the eddy-driven jets move poleward with climate change, and that the eddy length scale will increase as the jet moves poleward.

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Elizabeth A. Barnes and David W. J. Thompson

Abstract

Do barotropic or baroclinic eddy feedbacks dominate the atmospheric circulation response to mechanical forcing?

To address this question, barotropic torques are imposed over a range of latitudes in both an idealized general circulation model (GCM) and a barotropic model. The GCM includes both baroclinic and barotropic feedbacks. The barotropic model is run in two configurations: 1) only barotropic feedbacks are present and 2) a baroclinic-like feedback is added by allowing the stirring region to move with the jet. The relationship between the latitude of the forcing and the response is examined by systematically shifting the torques between the tropics and the pole. The importance of the mean state is investigated by varying the position of the control jet.

Five main findings are presented: 1) Barotropic feedbacks alone are capable of producing the structure of the GCM response to mechanical forcing but are not capable of accounting for its full magnitude. 2) Baroclinic processes generally increase the magnitude of the response but do not strongly influence its structure. 3) For a given forcing, the largest response in all model configurations occurs 5°–10° poleward of the forcing latitude. 4) The maximum response occurs when the forcing is located approximately 10° poleward of the control jet. 5) The circulation response weakens as the mean jet is found at higher latitudes in all model configurations.

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Elizabeth A. Barnes and Dennis L. Hartmann

Abstract

The persistence of the North Atlantic Oscillation (NAO) is studied using observations of the three-dimensional vorticity budget in the Atlantic sector. Analysis of the relative vorticity tendency equation shows that convergence of eddy vorticity flux in the upper troposphere counteracts the effect of anomalous large-scale divergence at the upper level. At low levels, the convergence associated with this large-scale vertical circulation cell maintains the relative vorticity anomaly against frictional drag. The eddy vorticity flux convergence thus acts to sustain the vorticity anomaly associated with the NAO against drag and increases the persistence of the NAO vorticity anomaly. The adiabatic cooling associated with the rising motion in the vorticity maximum sustains the thermal structure of the NAO anomaly, enhancing the baroclinicity, and thus eddy generation. This constitutes a positive eddy feedback that helps maintain the NAO. The positive eddy feedback occurs only in the midlatitude region and is strongest during the negative phase of the NAO when the Atlantic jet is displaced toward the equator, with a high pressure anomaly to the north and a low pressure anomaly to the south. The stronger feedback demonstrated during the negative phase is consistent with the greater persistence observed for this phase of the NAO. The positive feedback appears to be associated with anomalous northward eddy propagation away from the jet.

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David W. J. Thompson, Brian R. Crow, and Elizabeth A. Barnes

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Wave activity in the Southern Hemisphere extratropical atmosphere exhibits robust periodicity on time scales of ~20–25 days. Previous studies have demonstrated the robustness of the periodicity in hemispheric averages of various eddy quantities. Here the authors explore the signature of the periodicity on regional spatial scales.

Intraseasonal periodicity in the Southern Hemisphere circulation derives from out-of-phase anomalies in wave activity that form in association with extratropical wave packets as they propagate to the east. In the upper troposphere, the out-of-phase anomalies in wave activity form not along the path of extratropical wave packets, but in their wake. The out-of-phase anomalies in wave activity give rise to periodicity not only on hemispheric scales, but also on synoptic scales when the circulation is sampled along an eastward path between ~5 and 15 m s−1. It is argued that 1) periodicity in extratropical wave activity derives from two-way interactions between the heat fluxes and baroclinicity in the lower troposphere and 2) the unique longitude–time structure of the periodicity in upper-tropospheric wave activity derives from the contrasting eastward speeds of the source of the periodicity in the lower troposphere (~10 m s−1) and wave packets in the upper troposphere (~25 m s−1).

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Chaim I. Garfinkel, Luke D. Oman, Elizabeth A. Barnes, Darryn W. Waugh, Margaret H. Hurwitz, and Andrea M. Molod

Abstract

A robust connection between the drag on surface-layer winds and the stratospheric circulation is demonstrated in NASA's Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM). Specifically, an updated parameterization of roughness at the air–sea interface, in which surface roughness is increased for moderate wind speeds (4–20 m s−1), leads to a decrease in model biases in Southern Hemispheric ozone, polar cap temperature, stationary wave heat flux, and springtime vortex breakup. A dynamical mechanism is proposed whereby increased surface roughness leads to improved stationary waves. Increased surface roughness leads to anomalous eddy momentum flux convergence primarily in the Indian Ocean sector (where eddies are strongest climatologically) in September and October. The localization of the eddy momentum flux convergence anomaly in the Indian Ocean sector leads to a zonally asymmetric reduction in zonal wind and, by geostrophy, to a wavenumber-1 stationary wave pattern. This tropospheric stationary wave pattern leads to enhanced upward wave activity entering the stratosphere. The net effect is an improved Southern Hemisphere vortex: the vortex breaks up earlier in spring (i.e., the spring late-breakup bias is partially ameliorated) yet is no weaker in midwinter. More than half of the stratospheric biases appear to be related to the surface wind speed biases. As many other chemistry–climate models use a similar scheme for their surface-layer momentum exchange and have similar biases in the stratosphere, the authors expect that results from GEOSCCM may be relevant for other climate models.

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