Introduction"Since carbon dioxide (CO 2 ) plays a significant role in the heat budget of the atmosphere, it is reasonable to suppose that continued increases would affect climate." With these words, the late Verner Suomi eloquently stated the hypothesis of climate change science in 1979, in his foreword to the seminal "Charney Report" (National Research Council, 1979) on the role of CO 2 and climate. The remarkable prescience of the Charney Report was noted upon its 40th anniversary (Nicholls, 2019). Specifically, the scientists who developed the report under the direction of the late Jule Charney predicted that, for a doubling of CO 2 in Earth's atmosphere, the Earth's surface air temperature would warm by approximately 3 C, with a probable error of 1.5 C. Prior to the Charney report, Sawyer (1972) made the case for a warming troposphere due to increasing CO 2 and presciently predicted 0.6 K warming by the year 2000. Remarkably, the most recent assessment of Earth's equilibrium climate sensitivity (ECS) (i.e., the average global surface air temperature change after a doubling CO 2 ) (Sherwood et al., 2020) sets its bounds between 2.3 and 4.5 C, nearly identical to the Charney report range. Verner Suomi's hypothesis statement, "carbon dioxide plays a significant role in the heat budget of the atmosphere" is true throughout the entire perceptible atmosphere from Earth's surface to the edge of space. It has long

The rationale behind the experimental design and choice of diagnostics is presented. To facilitate scientific interpretation of the results in other planned QBOi studies, consistent descriptions of the models performing each experiment set are given, with those aspects particularly relevant for simulating the QBO tabulated for easy comparison.

Abstract. Stratospheric age of air (AoA) is a useful measure of the overall capabilities of a general circulation model (GCM) to simulate stratospheric transport. Previous studies have reported a large spread in the simulation of AoA by GCMs and coupled chemistry-climate models (CCMs). Compared to observational estimates simulated AoA is mostly too low. Here we attempt to untangle the processes that lead to the AoA differences between the models and between models and observations. AoA is influenced by both, mean transport along the residual circulation and two-way mixing; we quantify the effects of these 5 processes using data from the CCM inter-comparison projects CCMVal-2 and CCMI-1. Transport along the residual circulation is measured by the residual circulation transit time (RCTT). We interpret the difference between AoA and RCTT as additional aging by mixing. Aging by mixing thus includes mixing on both the resolved and subgrid scale. We find that the spread inAoA between the models is primarily caused by differences in the effects of mixing, and only to some extent by differences in residual circulation strength. These effects are quantified by the mixing efficiency, a measure of the relative increase of AoA by only controlled by horizontal mixing, but by vertical mixing and vertical diffusion as well. Possible causes for the differences in the models' mixing efficiencies are discussed. Differences in subgrid scale mixing (including differences in advection schemes and model resolutions) likely contribute to the differences in mixing efficiency. However, differences in the relative contribution of resolved versus parametrized wave forcing do not appear to be related to differences in mixing efficiency or AoA.

<p><strong>Abstract.</strong> About 66 million years ago an asteroid about 10 km in diameter struck the Yucatan Peninsula creating the Chicxulub crater. The crater has been dated and found to be coincident with the Cretaceous-Paleogene (K-Pg) mass extinction event, one of 6 great mass extinctions in the last 600 million years. This event precipitated one of the largest episodes of rapid climate change in Earth history, yet no modern three-dimensional climate calculations have simulated the event. Similarly, while there is an on-going effort to detect asteroids that might hit Earth and to develop methods to stop them, there have been no modern calculations of the sizes of asteroids whose impacts on land would cause devastating effects on Earth. Here we provide the information needed to initialize such calculations for the K-Pg impactor and for a 1 km diameter impactor. <br><br> There is considerable controversy about the details of the events that followed the Chicxulub impact. We proceed through the data record in the order of confidence that a climatically important material was present in the atmosphere. The climatic importance is roughly proportional to the optical depth of the material. Several hundred-micron diameter spherules are found globally in an abundance that would have produced an atmospheric layer with an optical depth around 20, yet their large sizes would only allow them to stay airborne for a few days. They were likely important for triggering global wildfires. Soot, probably from global or near-global wildfires, is found globally in an abundance that would have produced an optical depth near 100, which would effectively prevent sunlight from reaching the surface. Nanometer sized iron particles are also present globally. Theory suggests these particles might be remnants of the vaporized asteroid and target that initially remained as vapor rather than condensing on the hundred-micron spherules when they entered the atmosphere. If present in the abundance suggested by theory, their optical depth would have exceeded 1000. Clastics may be present globally, but only the quartz fraction can be quantified since shock features can identify it. However, it is very difficult to determine the total abundance of clastics. We reconcile previous widely disparate estimates and suggest the clastics may have had an optical depth near 100. Sulfur is predicted to originate about equally from the impactor and from the Yucatan surface materials. By mass, sulfur is less than 10 percent of the mass of the spheres and nano-particles. Since the sulfur probably reacted on the surfaces of the soot, nano-particles, clastics and spheres, it is likely a minor component of the climate forcing; however, detailed studies of the conversion of sulfur gases to particles are needed to determine if sulfuric acid aerosols dominated in late stages of the evolution of the atmospheric debris. Numerous gases, including CO<sub>2</sub>, SO<sub>2</sub> (or SO<sub>3</sub>), H<sub>2</sub>O, CO<sub>2</sub>, Cl, Br, and I, were likely injected into the upper atmosphere by the impact or the immediate effects of the impact such as fires across the planet. Their abundance might have increased relative to current ambient values by a significant fraction of current values for CO<sub>2</sub>, and by factors of 100 to 1000 for the other gases. <br><br> For the 1 km impactor, nano-particles might have had an optical depth of 1.5 if the impact occurred on land. If the impactor struck a densely forested region, soot from the forest fires might have had an optical depth of 0.1. Only S and I would be expected to be perturbed significantly relative to ambient gas phase values. 1 km asteroids impacting the ocean may inject seawater into the stratosphere as well as halogens that are dissolved in the seawater. <br><br> For each of the materials mentioned we provide initial abundances and injection altitudes. For particles we suggest initial size distributions and optical constants. We also suggest new observations that could be made to narrow the uncertainties about the particles and gases generated by large impacts.</p>

<p><strong>Abstract.</strong> We perform the first multi-model comparison of the impact of nudged meteorology on the stratospheric residual circulation using hindcast simulations from the Chemistry Climate Model Initiative (CCMI). We examine simulations over the period 1980–2009 from 5 models in which the meteorological fields are nudged towards reanalysis data and compare with equivalent free-running simulations from 9 models. We show that nudging meteorology does not constrain the mean strength of the stratospheric residual circulation and that the inter-model spread is similar, or even larger, than in the free-running simulations. The nudged simulations also simulate stronger upwelling in the tropical lower stratosphere compared to the residual circulation estimated directly from the reanalyses they are nudged towards. Downward control calculations reveal substantial differences between the mean lower stratospheric tropical upward mass flux (TUMF) computed from the modeled wave forcing and that calculated directly from the residual circulation. Although the mean circulation is poorly constrained, the nudged simulations show a high degree of consistency in the interannual variability of the TUMF in the lower stratosphere, which is related to the contribution to variability from the resolved wave forcing. We apply a multiple linear regression (MLR) model to separate the drivers of interannual and long-term variations in the simulated TUMF. The MLR model explains up to ~ 75 % of the variance in TUMF in the nudged simulations and reveals a statistically significant positive trend for most models in TUMF over the period 1980–2009. Overall, nudging meteorological fields leads to increased inter-model spread for most of the measures of the mean climatological stratospheric residual circulation assessed in this study. Our findings show that while nudged simulations by construction produce accurate temperatures and realistic representations of fast horizontal transport, this is not necessarily the case for the slower zonal mean vertical transport. Consequently, caution is required when using nudged simulations to interpret long-lived stratospheric tracers that are controlled by the residual circulation.</p>