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Abstract The study presents (a) a 44‐year wintertime climatology of resolved gravity wave (GW) fluxes and forcing in the extratropical stratosphere using ERA5, and (b) their composite evolution around gradual (final warming) and abrupt (sudden warming) transitions in the wintertime circulation, focusing on lateral fluxes. The transformed Eulerian mean equations are leveraged to provide a glimpse of the importance of GW lateral propagation (i.e., horizontal propagation) toward driving the wintertime stratospheric circulation by analyzing the relative contribution of the vertical versus meridional flux dissipation. The relative contribution from lateral propagation is found to be notable, especially in the Austral winter stratosphere where lateral (vertical) momentum flux convergence provides a peak climatological forcing of up to −0.5 (−3.5) m/s/day around 60°S at 40–45 km altitude. Prominent lateral propagation in the wintertime midlatitudes also contributes to the formation of belts of GW activity in both hemispheres. 

Abstract Thermodynamical and dynamical aspects of the climate system response to an-thropogenic forcing are often considered in two distinct frameworks: The former in the context of the forcing-feedback framework; the latter in the context of eddy-mean flow feedbacks and large-scale thermodynamic constraints. Here we use experiments with the dynamical core of a general circulation model (GCM) to provide insights into the relationships between these two frameworks. We first demonstrate that the climate feedbacks and climate sensitivity in a dynamical core model are determined by its prescribed thermal relaxation timescales. We then perform two experiments: One that explores the relationships between the thermal relaxation timescale and the climatological circulation; and a second that explores the relationships between the thermal relaxation timescale and the circulation response to a global warming-like forcing perturbation. The results indicate that shorter relaxation timescales (i.e., lower climate sensitivities in the context of a dynamical core model) are associated with 1) a more vigorous large-scale circulation, including increased thermal diffusivity and stronger and more poleward storm tracks and eddy-driven jets and 2) a weaker poleward displacement of the storm tracks and eddy-driven jets in response to the global warming-like forcing perturbation. Interestingly, the circulation response to the forcing perturbation effectively disappears when the thermal relaxation timescales are spatially uniform, suggesting that the circulation response to homogeneous forcing requires spatial inhomogeneities in the local feedback parameter. Implications for anticipating the circulation response to global warming and thermodynamic constraints on the circulation are discussed. 

A combination of 240 years of output from a state-of-the-art chemistry–climate model and a twentieth-century reanalysis product is used to investigate to what extent sudden stratospheric warmings are preceded by anomalous tropospheric wave activity. To this end we study the fate of lower tropospheric wave events (LTWEs) and their interaction with the stratospheric mean flow. These LTWEs are contrasted with sudden stratospheric deceleration events (SSDs), which are similar to sudden stratospheric warmings but place more emphasis on the explosive dynamical nature of such events. Reanalysis and model output provide very similar statistics: Around one-third of the identified SSDs are preceded by wave events in the lower troposphere, while two-thirds of the SSDs are not preceded by a tropospheric wave event. In addition, only 20% of all anomalous tropospheric wave events are followed by an SSD in the stratosphere. This constitutes statistically robust evidence that the anomalous amplification of wave activity in the stratosphere that drives SSDs is not necessarily due to an anomalous amplification of the waves in the source region (i.e., the lower troposphere). The results suggest that the dynamics in the lowermost stratosphere and the vortex geometry are essential, and should be carefully analyzed in the search for precursors of SSDs. 

Abstract The Whole Atmosphere Community Climate Model, version 4 (WACCM4), is used to investigate the influence of stratospheric conditions on the development of sudden stratospheric warmings (SSWs). To this end, targeted experiments are performed on selected modeled SSW events. Specifically, the model is reinitialized three weeks before a given SSW, relaxing the surface fluxes, winds, and temperature below 10 km to the corresponding fields from the free-running simulation. Hence, the tropospheric wave evolution is unaltered across the targeted experiments, but the stratosphere itself can evolve freely. The stratospheric zonal-mean state is then altered 21 days prior to the selected SSWs and rerun with an ensemble of different initial conditions. It is found that a given tropospheric evolution concomitant with the development of an SSW does not uniquely determine the occurrence of an event and that the stratospheric conditions are relevant to the subsequent evolution of the stratospheric flow toward an SSW, even for a fixed tropospheric evolution. It is also shown that interpreting the meridional heat flux at 100 hPa as a proxy of the tropospheric injection of wave activity into the stratosphere should be regarded with caution and that stratospheric dynamics critically influence the heat flux at that altitude. 

Abstract Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability. 

Abstract Variations in the width and strength of the Hadley cells are associated with many radiative, thermodynamic, and dynamical forcings. The physical mechanisms driving these responses remain unclear, in part because of the interactive nature of eddy‐mean flow adjustment. Here, a modeling framework is developed which separates the mean flow and time‐mean eddy flow in a gray radiation general circulation model with simple representations of ocean heat transport and ozone. In the absence of eddies, with moist convection and weak numerical damping, the Hadley cell is confined to the upper troposphere and has a vanishingly small poleward momentum flux. Eddies allow the cell to extend down to the surface, double its heat transport, and flux momentum poleward, the latter two being basic consequences of a deepening of the circulation. Because of convection and damping—which mimics, in part, the effect of eddy stresses—previous work may have underestimated the impact of eddies on earth's circulation. Quasigeostrophic eddy fluxes are sufficient to produce Hadley and Ferrel cells, but with a substantially greater Hadley cell strength than when all eddy impacts are considered, including eddy fluxes of moisture, mass, and momentum and eddy impacts on surface fluxes and clouds. 

Abstract Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (∼10–50 km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary‐scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affect tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (nonionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modeling, chemistry, and impact on other atmospheric layers.