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L. M. Polvani, D. W. Waugh, and R. Alan Plumb

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D. W. Waugh, R. A. Plumb, and L. M. Polvani

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The nonlinear response of a barotropic, nondivergent, spherical flow representative of the upper troposphere (but without a tropical Hadley cell) to localized, extratropical topographic forcing is examined using high-resolution contour surgery calculations. The response is shown to vary greatly with forcing amplitude and can be significantly different from the linear response. At large amplitude, Rossby wave breaking occurs in the tropics irrespective of the direction of the equatorial winds, and leads to small-scale stirring and the formation of a “tropical surf zone,” which inhibits the meridional propagation of the disturbance.

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Olga V. Tweedy, Luke D. Oman, and Darryn W. Waugh

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Seasonal differences in the impact of the Madden–Julian oscillation (MJO) on tropical and extratropical upper troposphere–lower stratosphere (UTLS) temperature, circulation, and trace gases are examined using trace gases (ozone, carbon monoxide, and water vapor) and temperature from measurements from the Microwave Limb Sounder (MLS) and meteorological fields from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). During boreal winter months (November–February), atmospheric fields exhibit a well-known planetary-scale perturbation consistent with the upper-level flow modeled by Gill, with twin high and low pressure extratropical systems associated with a Rossby wave response. However, the circulation anomalies in the UTLS differ during boreal summer months (June–September), when background UTLS circulation north of the equator is dominated by the Asian summer monsoon anticyclone. The twin high and low pressure extratropical systems are much weaker but with a stronger equatorial Kelvin wave front that encircles the globe as the MJO propagates eastward. These differences are explained in terms of seasonal variations in vertically propagating Kelvin waves that strongly depend on the zonal structure of the climatological background winds. The trace gas response to the MJO is strongly coherent with circulation anomalies showing strong seasonal differences. The stronger equatorial Kelvin wave front during the summer produces enhanced upwelling in the tropical tropopause layer, resulting in significant cooling of this region, reduced ozone and water vapor, and enhanced carbon monoxide.

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A. H. Sobel, R. A. Plumb, and D. W. Waugh

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Existing quantitative calculations of material transport across the stratospheric polar vortex edge are difficult to interpret. This is because what is actually calculated has not been clearly shown to be irreversible transport, because of ambiguities inherent in defining the vortex edge, and (relatedly) because the uncertainties in the various sorts of calculations have not been quantified. The authors discuss some of the conceptual and technical difficulties involved in such calculations. These typically use a tracer coordinate, so that an air parcel’s “position” is defined as a function of some tracer that it carries. Also examined is the sensitivity to noise of a method that has been used in several prior studies, which the authors call the “contour crossing” method. When contour crossing is implemented with no explicit threshold to discriminate noise from signal, a realistic amount of noise in the tracer data can cause apparent transports across the vortex edge in the range of ten percent to several tens of percent of the vortex area per month, even if the true transport is zero. Moreover, contour crossing does not discriminate between dynamically driven transport and that due to large-scale nonconservative effects acting upon the tracer used to define the coordinate. The authors introduce a new method, which is called the “local gradient reversal” method, for estimating the dynamically driven component of the transport. This method is conceptually somewhat similar to contour surgery but applies to gridded fields rather than material contours. Like contour crossing, it can thus be used in conjunction with the reverse domain filling advection technique, while contour surgery is used with contour advection or contour dynamics. Local gradient reversal is shown to be less sensitive to noise than contour crossing.

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R. K. Scott, L. M. Polvani, and D. W. Waugh

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This paper considers the effect of time-dependent lower boundary wave forcing on the internal variability found to appear spontaneously in a stratosphere-only model when the forcing is perfectly steady. While the time-dependent forcing is found to modulate the internal variability, leading in some cases to frequency locking of the upper-stratospheric response to the forcing, the temporal and spatial structure of the variability remains similar to the case when the forcing is time independent. Experiments with a time-periodic modulation of the forcing amplitude indicate that the wave flux through the lower boundary is only partially related to the instantaneous forcing, but is more significantly influenced by the condition of the polar vortex itself. In cases of purely random wave forcing with zero time mean, the stratospheric response is similar to that obtained with steady forcing of magnitude equal to the root-mean-square of the time-varying forcing.

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L. M. Polvani, D. W. Waugh, and R. Alan Plumb

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The formation of a subtropical “transport barrier” in the wintertime stratosphere is investigated in the context of a high-resolution shallow-water model in which Rossby waves are topographically forced on a zonally symmetric basic state. Two sets of experiments are performed: in the first “adiabatic” set, no dissipation or forcing of the mean state is imposed; in the second set, the layer thickness is relaxed to an equilibrium state taken to be representative of middle stratospheric radiative equilibrium temperatures. It is found that in the adiabatic case only a very weak subtropical barrier forms for forcing amplitudes that generate realistically steep potential vorticity gradients at the edge of the polar vortex; the vigorous wave breaking in the surf zone generates secondary waves that spread and, in turn, break well into the summer hemisphere. In contrast, the inclusion of relaxation to a realistic thermal equilibrium leads to the formation of a subtropical region of steep PV gradients. The strong subtropical shear induced by die diabatic relaxation is shown to be an important factor for the formation of the subtropical edge of the surf zone. Furthermore, the authors demonstrate that a simple one-layer shallow-water model can capture the full process of the formation of a surf zone with both polar and tropical edges starting from conditions typical of the early fall–that is, with a flow in which the polar vortex is not initially present. Finally, the authors quantify the mixing of polar and subtropical air into the midlatitude surf zone with the help of the contour advection technique. Although the quantitative estimates depend sensitively on how the edges of the surf zone are defined, our results indicate that more tropical than polar air is entrained into the surf zone.

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R. K. Scott, D. G. Dritschel, L. M. Polvani, and D. W. Waugh

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This work investigates the extent to which potential vorticity gradients affect the vertical propagation of planetary-scale Rossby waves on the edge of a stratospheric polar vortex and their eventual nonlinear saturation and breaking. Using two different numerical modeling approaches, it is shown that wave propagation and wave breaking are significantly reduced when the potential vorticity gradients at the vortex edge are anomalously weak. The efficiency of the first model, based on high-resolution contour dynamics, permits a full exploration of the parameter space of wave forcing amplitude and edge steepness. A more realistic primitive equation model in spherical geometry both confirms the contour dynamics results and highlights some direct implications for stratospheric modeling in more comprehensive models. The results suggest that stratospheric models using horizontal resolutions of spectral T42 or less may significantly underestimate the vertical propagation and breaking of planetary waves, and may consequently misrepresent such important stratospheric processes as the mean meridional circulation, sudden warmings, and the mixing of chemically distinct polar and midlatitude air.

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W. J. M. Seviour, F. Codron, E. W. Doddridge, D. Ferreira, A. Gnanadesikan, M. Kelley, Y. Kostov, J. Marshall, L. M. Polvani, J. L. Thomas, and D. W. Waugh

Abstract

The effect of the Antarctic ozone hole extends downward from the stratosphere, with clear signatures in surface weather patterns including a positive trend in the southern annular mode (SAM). Several recent studies have used coupled climate models to investigate the impact of these changes on Southern Ocean sea surface temperature (SST), notably motivated by the observed cooling from the late 1970s. Here we examine the robustness of these model results through comparison of both previously published and new simulations. We focus on the calculation of climate response functions (CRFs), transient responses to an instantaneous step change in ozone concentrations. The CRF for most models consists of a rapid cooling of SST followed by a slower warming trend. However, intermodel comparison reveals large uncertainties, such that even the sign of the impact of ozone depletion on historical SST, when reconstructed from the CRF, remains unconstrained. Comparison of these CRFs with SST responses to a hypothetical step change in the SAM, inferred through lagged linear regression, shows broadly similar results. Causes of uncertainty are explored by examining relationships between model climatologies and their CRFs. The intermodel spread in CRFs can be reproduced by varying a single subgrid-scale mixing parameter within a single model. Antarctic sea ice CRFs are also calculated: these do not generally exhibit the two-time-scale behavior of SST, suggesting a complex relationship between the two. Finally, by constraining model climatology–response relationships with observational values, we conclude that ozone depletion is unlikely to have been the primary driver of the observed SST cooling trend.

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

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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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Clara Orbe, Paul A. Newman, Darryn W. Waugh, Mark Holzer, Luke D. Oman, Feng Li, and Lorenzo M. Polvani

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Future changes in transport from Northern Hemisphere (NH) midlatitudes into the Arctic are examined using rigorously defined air-mass fractions that partition air in the Arctic according to where it last had contact with the planetary boundary layer (PBL). Boreal winter (December–February) and summer (June–August) air-mass fraction climatologies are calculated for the modeled climate of the Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM) forced with the end-of-twenty-first century greenhouse gases and ozone-depleting substances. The modeled projections indicate that the fraction of air in the Arctic that last contacted the PBL over NH midlatitudes (or air of “midlatitude origin”) will increase by about 10% in both winter and summer. The projected increases during winter are largest in the upper and middle Arctic troposphere, where they reflect an upward and poleward shift in the transient eddy meridional wind, a robust dynamical response among comprehensive climate models. The boreal winter response is dominated by (~5%–10%) increases in the air-mass fractions originating over the eastern Pacific and the Atlantic, while the response in boreal summer mainly reflects (~5%) increases in air of Asian and North American origin. The results herein suggest that future changes in transport from midlatitudes may impact the composition—and, hence, radiative budget—in the Arctic, independent of changes in emissions.

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