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The impacts of climate change on tropospheric transport, diagnosed from a carbon monoxide (CO)-like tracer species emitted from global CO sources, are evaluated from an ensemble of four chemistry-climate models (CCMs) contributing to the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). Model time-slice simulations for present-day and end-of-The-21st-century conditions were performed under the Representative Concentrations Pathway (RCP) climate scenario RCP 8.5. All simulations reveal a strong seasonality in transport, especially over the tropics. The highest CO-Tracer mixing ratios aloft occur during boreal winter when strong vertical transport is co-located with biomass burning emission source regions. A consistent and robust decrease in future CO-Tracer mixing ratios throughout most of the troposphere, especially in the tropics, and an increase around the tropopause is found across the four CCMs in both winter and summer. Decreases in CO-Tracer mixing ratios in the tropical troposphere are associated with reduced convective mass fluxes in this region, which in turn may reflect a weaker Hadley cell circulation in the future climate. Increases in CO-Tracer mixing ratios near the tropopause are largely attributable to a rise in tropopause height enabling lofting to higher altitudes, although a poleward shift in the mid-latitude jets may also play a minor role in the extratropical upper troposphere. An increase in CO-Tracer mixing ratios also occurs near the Equator, centred over equatorial and Central Africa, extending from the surface to the mid-Troposphere. This is most likely related to localised decreases in convection in the vicinity of the Intertropical Convergence Zone (ITCZ), resulting in larger CO-Tracer mixing ratios over biomass burning regions and smaller mixing ratios downwind.

Understanding how the transport of gases and aerosols responds to climate change is necessary for policy making and emission controls. There is considerable spread in model projections of tracer transport in climate change simulations, largely because of the substantial uncertainty in projected changes in the large‐scale atmospheric circulation. In particular, a relationship between the response of tropospheric transport into the high latitudes and a shift of the midlatitude jet has been previously established in an idealized modeling study. To test the robustness of this relationship, we analyze the response of a passive tracer of northern midlatitude surface origin to abrupt 2xCO2 and 4xCO2 in a comprehensive climate model (Goddard Institute for Space Studies E2.2‐G). We show that a poleward shift of the northern midlatitude jet and enhanced eddy mixing along isentropes on the poleward flank of the jet result in decreased tracer concentrations over the midlatitudes and increased concentrations over the Arctic. This mechanism is robust in abrupt 2xCO2 and 4xCO2 simulations, the nonlinearity to CO2 forcing, and two versions of the model with different atmospheric chemistry. Preliminary analysis of realistic chemical tracers suggests that the same mechanism can be used to provide insights into the climate change response of anthropogenic pollutants. Plain Language Summary: Pollutants such sulfate aerosols, soot, and carbon monoxide are transported by atmospheric flows from the northern midlatitude surface to higher altitudes and the Arctic. Here we study how this transport responds to climate change by using a passive tracer without chemistry. During northern winter, the westerly jet accelerates and shifts poleward under increased CO2 concentration. This leads to more mixing that brings cleaner air from the subtropical surface to the midlatitude troposphere but also polluted air from the midlatitude surface to the Arctic troposphere. This pattern is robust in tracers with and without chemistry, suggesting that transport changes play an important role in shaping the response of pollutant distributions to climate change. It also suggests that reducing the uncertainty of the midlatitude jet response will facilitate more accurate projection of pollutant transport in a warming climate. Key Points: Response of tropospheric tracer transport from the northern hemisphere (NH) midlatitude surface to increased CO2 depends on the midlatitude jet responseChanges in isentropic eddy mixing associated with the midlatitude jet dominate the response of the transport to NH high latitudesA poleward shift of the NH midlatitude jet associated with Atlantic Meridional Overturning Circulation weakening leads to less tracer in the midlatitudes and more in the Arctic [ABSTRACT FROM AUTHOR]

Long‐term projections of the Quasi‐Biennial Oscillation (QBO) remain uncertain. Using the high‐top "Middle Atmosphere" (MA) version of the NASA Goddard Institute for Space Studies (GISS) ModelE (E2.2‐AP), we show that increased CO2 reduces the QBO period and weakens its amplitude, consistent with results from recent multimodel studies. The amplitude response is asymmetric, with the easterly phase weakening more than the westerly phase. Results from both coupled atmosphere‐ocean and climatologically fixed sea surface temperature simulations link the period response to changes in parameterized convective gravity wave fluxes, whereas the amplitude changes are more closely tied to changes in tropical upwelling. Comparisons of simulations with and without ozone‐dynamical feedbacks reveal that ozone feedbacks increase the QBO amplitude and period in pre‐industrial control simulations, and reduce the response of the amplitude to increased CO2. The response of stratospheric tracers to projected changes in the QBO is also investigated. Similarity between results using fully interactive (trace gas and aerosol) versus linearized ozone chemical mechanisms suggests that the QBO response to CO2 does not depend on additional chemical feedbacks beyond those associated with ozone, which implies that projected QBO changes may be credibly represented using simplified stratospheric ozone schemes. Key Points: In the NASA GISS Model E2.2‐AP, increased CO2 decreases QBO amplitude and periodAmplitude/period changes are qualitatively linked to upwelling/convection changes, respectivelyOzone feedbacks dampen the QBO amplitude response to increased CO2 [ABSTRACT FROM AUTHOR]

Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for stratosphere‐troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO2 forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO2 (4xCO2) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above. In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO2 forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically significant change. As for the near‐surface response to SSWs, the models, however, are in good agreement as to this signal over the North Atlantic: There is no indication of a change under 4xCO2 forcing. Over the Pacific, however, the change is more uncertain, with some indication that there will be a larger mean response. Finally, the models show robust changes to the seasonal cycle in the stratosphere. Specifically, we find a longer duration of the stratospheric polar vortex and thus a longer season of stratosphere‐troposphere coupling. Key Points: The tropospheric signal of sudden stratospheric warming (SSWs) in the North Atlantic does not change under 4xCO2 forcingThere is high uncertainty in changes of SSW frequency under 4xCO2 forcing; single models show the rate to be significantly halved or doubledThe boreal polar vortex will form earlier and disappear later under increased CO2, extending the season of stratosphere‐troposphere coupling [ABSTRACT FROM AUTHOR]

Extreme cold events over North America such as the February 2021 cold wave have been suggested to be linked to stratospheric polar vortex stretching. However, it is not resolved how robustly and on which timescales the stratosphere contributes to the surface anomalies. Here we introduce a simple measure of stratospheric wave activity for reanalyses and model outputs. In contrast to the well-known surface influences of sudden stratospheric warmings (SSWs) that increase the intraseasonal persistence of weather regimes, we show that extreme stratospheric wave events are accompanied by intraseasonal fluctuations between warm and cold spells over North America in observations and climate models. Particularly, strong stratospheric wave events are followed by an increased risk of cold extremes over North America 5-25 days later. Idealized simulations in an atmospheric model with a well-resolved stratosphere corroborate that strong stratospheric wave activity precedes North American cold spells through vertical wave coupling. These findings potentially benefit the predictability of high-impact winter cold extremes over North America., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2023.)

This paper describes the GISS-E2.1 contribution to the Coupled Model Intercomparison Project, Phase 6 (CMIP6). This model version differs from the predecessor model (GISS-E2) chiefly due to parameterization improvements to the atmospheric and ocean model components, while keeping atmospheric resolution the same. Model skill when compared to modern era climatologies is significantly higher than in previous versions. Additionally, updates in forcings have a material impact on the results. In particular, there have been specific improvements in representations of modes of variability (such as the Madden-Julian Oscillation and other modes in the Pacific) and significant improvements in the simulation of the climate of the Southern Oceans, including sea ice. The effective climate sensitivity to 2 × CO 2 is slightly higher than previously at 2.7-3.1°C (depending on version) and is a result of lower CO 2 radiative forcing and stronger positive feedbacks., (©2020. The Authors.)

Previous observational studies have found a persistent maximum in stratospheric water vapor (SWV) in the upper troposphere lower stratosphere (UTLS) confined by the upper-level anticyclone over the Asian summer monsoon region. This study investigates the simulation of SWV in the Whole Atmosphere Community Climate Model (WACCM). WACCM generally tends to simulate a SWV maximum over the central Pacific Ocean, but this bias is largely improved in the high vertical resolution version. The high vertical resolution model with increased vertical layers in the UTLS is found to have a less stratified UTLS over the central Pacific Ocean compared with the low vertical resolution model. It therefore simulates a steepened PV gradient over the central Pacific Ocean that better closes the upper-level anticyclone and confines the SWV within the enhanced transport barrier.

1998-2016 ozone trends in the lower stratosphere (LS) are examined using the Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) and related NASA products. After removing biases resulting from step-changes in the MERRA-2 ozone observations, a discernible negative trend of -1.67±0.54 Dobson units per decade (DU/decade) is found in the 10-km layer above the tropopause between 20°N and 60°N. A weaker but statistically significant trend of -1.17±0.33 DU/decade exists between 50°S and 20°S. In the Tropics, a positive trend is seen in a 5-km layer above the tropopause. Analysis of an idealized tracer in a model simulation constrained by MERRA-2 meteorological fields provides strong evidence that these trends are driven by enhanced isentropic transport between the tropical (20°S-20°N) and extratropical LS in the past two decades. This is the first time that a reanalysis dataset has been used to detect and attribute trends in lower stratospheric ozone.

While the importance of the seasonal migration of the zonally averaged Hadley circulation on interhemispheric transport of trace gases has been recognized, few studies have examined the role of the zonally asymmetric monsoonal circulation. This study investigates the role of monsoon-like zonally asymmetric heating on interhemispheric transport using a dry atmospheric model that is forced by idealized Newtonian relaxation to a prescribed radiative equilibrium temperature. When only the seasonal cycle of zonally symmetric heating is considered, the mean age of air in the Southern Hemisphere since last contact with the Northern Hemisphere midlatitude boundary layer, is much larger than the observations. The introduction of monsoon-like zonally asymmetric heating not only reduces the mean age of tropospheric air to more realistic values, but also produces an upper-tropospheric cross-equatorial transport pathway in boreal summer that resembles the transport pathway simulated in the NASA Global Modeling Initiative (GMI) Chemistry Transport Model driven with MERRA meteorological fields. These results highlight the monsoon-induced eddy circulation plays an important role in the interhemispheric transport of long-lived chemical constituents.