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Abstract. Accurate simulation of atmospheric circulation,particularly in the lower stratosphere, is challenging due to unresolvedwave–mean flow interactions and limited high-resolution observations for validation. Gravity-induced pressure gradients lead to a small butmeasurable separation of heavy and light gases by molecular diffusion in thestratosphere. Because the relative abundance of Ar to N2 is exclusivelycontrolled by physical transport, the argon-to-nitrogen ratio (Ar∕N2)provides an additional constraint on circulation and the age of air (AoA),i.e., the time elapsed since entry of an air parcel into the stratosphere. Here we use airborne measurements of N2O and Ar∕N2 from ninecampaigns with global coverage spanning 2008–2018 to calculate AoA and toquantify gravitational separation in the lowermost stratosphere. To thisend, we develop a new N2O–AoA relationship using a Markov chain Monte Carlo algorithm. We observe that gravitational separation increases systematically with increasing AoA for samples with AoA between 0 and 3 years. These observations are compared to a simulation of the TOMCAT/SLIMCAT 3-D chemical transport model, which has been updated to includegravitational fractionation of gases. We demonstrate that although AoA atold ages is slightly underestimated in the model, the relationship betweenAr∕N2 and AoA is robust and agrees with the observations. Thishighlights the potential of Ar∕N2 to become a new AoA tracer that is subject only to physical transport phenomena and can supplement the suite of available AoA indicators.

Abstract. Accurate simulation of atmospheric circulation, particularly in the lower stratosphere, is challenging due to unresolved wave-mean flow interactions and limited high-resolution observations for validation. Gravity-induced pressure gradients lead to a small but measurable separation of heavy and light gases by molecular diffusion in the stratosphere. Because the relative abundance of Ar to N2 is exclusively controlled by physical transport, the argon-to-nitrogen ratio (Ar/N2) provides an additional constraint on circulation and the age of air (AoA), i.e. the time elapsed since entry of an air parcel into the stratosphere. Here we use airborne measurements of N2O and Ar/N2 from nine campaigns with global coverage spanning 2008–2018 to calculate AoA and to quantify gravitational separation in the lowermost stratosphere. To this end, we develop a new N2O-AoA relationship using a Markov Chain Monte Carlo algorithm. We observe that gravitational separation increases systematically with increasing AoA for samples with AoA between 0 to 3 years. These observations are compared to a simulation of the TOMCAT/SLIMCAT 3-D chemical transport model, which has been updated to include gravitational fractionation of gases. We demonstrate that although AoA at old ages is slightly underestimated in the model, the relationship between Ar/N2 and AoA is robust and agrees with the observations. This highlights the potential of Ar/N2 to become a new AoA tracer that is subject only to physical transport phenomena and can supplement the suite of available AoA indicators.

We present improved estimates of air–sea CO2 exchange over three latitude bands of the Southern Ocean using atmospheric CO2 measurements from global airborne campaigns and an atmospheric 4-box inverse model based on a mass-indexed isentropic coordinate (Mθe). These flux estimates show two features not clearly resolved in previous estimates based on inverting surface CO2 measurements: a weak winter-time outgassing in the polar region and a sharp phase transition of the seasonal flux cycles between polar/subpolar and subtropical regions. The estimates suggest much stronger summer-time uptake in the polar/subpolar regions than estimates derived through neural-network interpolation of pCO2 data obtained with profiling floats but somewhat weaker uptake than a recent study by Long et al. [Science 374, 1275–1280 (2021)], who used the same airborne data and multiple atmospheric transport models (ATMs) to constrain surface fluxes. Our study also uses moist static energy (MSE) budgets from reanalyses to show that most ATMs tend to have excessive diabatic mixing (transport across moist isentrope, θe, or Mθe surfaces) at high southern latitudes in the austral summer, which leads to biases in estimates of air–sea CO2 exchange. Furthermore, we show that the MSE-based constraint is consistent with an independent constraint on atmospheric mixing based on combining airborne and surface CO2 observations. [ABSTRACT FROM AUTHOR]

What we imagine, like what we remember, represents a good part of what we are and a good part of what we will become. --Tim O'Brien (1) There is Indiana [...]

Five years after the Sharpeville Massacre propelled South Africa and apartheid into international consciousness, Students for a Democratic Society, founded in 1960 at the University of Michigan in Ann Arbor, organized a significant protest in New York City to both commemorate the anniversary of Sharpeville and, more pressing, to raise awareness of the connections between United States corporations—specifically Chase Manhattan Bank—and apartheid. The protest at Chase Manhattan's headquarters in March 1965 was the culminating event in the "Action Against Apartheid Week" activities taking place in the New York City area. This article examines SDS's protest against Chase Manhattan as a critical precursor to the divestment movements that developed across US college and university campuses during the 1970s and 1980s, exploring the influence of students and youth activists on both US foreign policy and the international anti-apartheid struggle. [ABSTRACT FROM AUTHOR]

We have developed in situ and flask sampling systems for airborne measurements of variations in the O2/N2 ratio at the part per million level. We have deployed these instruments on a series of aircraft campaigns to measure the distribution of atmospheric O2 from 0-14km and 87°N to 85°S throughout the seasonal cycle. The NCAR airborne oxygen instrument (AO2) uses a vacuum ultraviolet (VUV) absorption detector for O2 and also includes an infrared CO2 sensor. The VUV detector has a precision in 5 seconds of ±1.25 per meg (1#963;) δ(O2/N2), but thermal fractionation and motion effects increase this to ±2.5-4.0 per meg when sampling ambient air in flight. The NCAR/Scripps airborne flask sampler (Medusa) collects 32 cryogenically dried air samples per flight under actively controlled flow and pressure conditions. For in situ or flask O2 measurements, fractionation and surface effects can be important at the required high levels of relative precision. We describe our sampling and measurement techniques, and efforts to reduce potential biases. We also present a selection of observational results highlighting the individual and combined instrument performance. These include vertical profiles, O2:CO2 correlations, and latitudinal cross sections reflecting the distinct influences of terrestrial photosynthesis, air-sea gas exchange, burning of various fuels, and stratospheric dynamics. When present, we have corrected the flask δ(O2/N2) measurements for fractionation during sampling or analysis, with the use of the concurrent δ(Ar/N2) measurements. We have also corrected the in situ δ(O2/N2) measurements for inlet fractionation and humidity effects by comparison to the corrected flask values. A comparison of Ar/N2-corrected Medusa flask δ(O2/N2) measurements to regional Scripps O2 Network station observations shows no systematic biases over 10 recent campaigns (+0.2±8.2 per meg, mean and standard deviation, n=86). For AO2, after resolving sample drying and inlet fractionation biases previously on the order of 10-100 per meg, independent AO2 δ(O2/N2) measurements over 6 more recent campaigns differ from coincident Medusa flask measurements by -0.3±7.2 per meg (mean and standard deviation, n=1361), with campaign-specific means ranging from -5 to +5 per meg. [ABSTRACT FROM AUTHOR]

We introduce a transformed isentropic coordinate Mθe, defined as the dry air mass under a given equivalent potential temperature surface (θe) within a hemisphere. Like θe, the coordinate Mθe follows the synoptic distortions of the atmosphere, but unlike θe, has a nearly fixed relationship with latitude and altitude over the seasonal cycle. Calculation of Mθe is straightforward from meteorological fields. Using observations from the recent HIPPO and Atom airborne campaigns, we map the CO2 seasonal cycle as a function of pressure and Mθe, where Mθe is thereby effectively used as an alternative to latitude. We show that the CO2 cycles are more constant as a function of pressure using Mθe as the horizontal coordinate compared to latitude. Furthermore, short-term variability of CO2 relative to the mean seasonal cycle is also smaller when the data are organized by Mθe and pressure than when organized by latitude and pressure. We also present a method using Mθe to compute mass-weighted averages of CO2 on a hemispheric scale. Using this method with the same airborne data and applying corrections for limited coverage, we resolve the average CO2 seasonal cycle in the Northern Hemisphere (mass weighted tropospheric climatological average for 2009-2018), yielding an amplitude of 7.8 ± 0.14 ppm and a downward zero-crossing at Julian day 173 ± 6.1 (i.e., late June). Mθe may be similarly useful for mapping the distribution and computing inventories of any long-lived chemical tracer. [ABSTRACT FROM AUTHOR]

The Southern Annular Mode (SAM) is the dominant mode of climate variability in the Southern Ocean, but only a few observational studies have linked variability in SAM to changes in ocean circulation. Atmospheric potential oxygen (APO) combines atmospheric O2/N2 and CO2 data to mask the influence of terrestrial exchanges, yielding a tracer that is sensitive mainly to ocean circulation and biogeochemistry. We show that observed wintertime anomalies of APO are significantly correlated to SAM in 25‐ to 30‐year time series at three Southern Hemisphere sites, while CO2 anomalies are also weakly correlated. We find additional correlations between SAM and O2 air‐sea fluxes in austral winter inferred from both an atmospheric inversion of observed APO and a forced ocean biogeochemistry model simulation. The model results indicate that the correlation with SAM is mechanistically linked to stronger wind speeds and upwelling, which brings oxygen‐depleted deep waters to the surface. Plain Language Summary: The Southern Annular Mode (SAM) is characterized by variability in the strength of the westerly winds that encircle Antarctica. A more positive SAM index is associated with stronger westerly winds over the ocean at about 60°S latitude. Previous studies based mostly on model simulations have suggested that a positive SAM index is also associated with enhanced upwelling of carbon‐rich waters in the Southern Ocean, which influences the uptake and/or release of carbon dioxide to the atmosphere. The same deep waters that have high carbon also have low concentrations of dissolved oxygen, which can cause subtle variations in atmospheric oxygen levels measured at surface stations. We present an analysis of data sets from three Southern Hemisphere stations where air samples have been collected for 25–30 years. We find that during the Southern Hemisphere winter, anomalies in the atmospheric oxygen record are correlated to the SAM index; we find weaker correlations between anomalies in atmospheric carbon dioxide and the SAM index. These results are consistent with variability in air‐sea oxygen fluxes from a model simulation. The model results indicate that the observed relationship between SAM and air‐sea oxygen fluxes is due to both stronger wind speeds and increased upwelling of oxygen‐depleted deep waters. Key Points: Significant correlations are found between winter atmospheric potential oxygen and the Southern Annular Mode at three long‐term observing sitesA similar relationship between winter air‐sea O2 flux and SAM is found in APO inversion products and a hindcast ocean model simulationOcean model simulations indicate that negative winter APO anomalies are linked to stronger wind speeds and ventilation of O2‐depleted deep water [ABSTRACT FROM AUTHOR]

Accurate simulation of atmospheric circulation, particularly in the lower stratosphere, is challenging due to unresolved wave-mean flow interactions and limited high-resolution observations for validation. Gravity-induced pressure gradients lead to a small but measurable separation of heavy and light gases by molecular diffusion in the stratosphere. Because the relative abundance of Ar to N2 is exclusively controlled by physical transport, the argon-to-nitrogen ratio (Ar/N2) provides an additional constraint on circulation and the age of air (AoA), i.e. the time elapsed since entry of an air parcel into the stratosphere. Here we use airborne measurements of N2O and Ar/N2 from nine campaigns with global coverage spanning 2008–2018 to calculate AoA and to quantify gravitational separation in the lowermost stratosphere. To this end, we develop a new N2O-AoA relationship using a Markov Chain Monte Carlo algorithm. We observe that gravitational separation increases systematically with increasing AoA for samples with AoA between 0 to 3 years. These observations are compared to a simulation of the TOMCAT/SLIMCAT 3-D chemical transport model, which has been updated to include gravitational fractionation of gases. We demonstrate that although AoA at old ages is slightly underestimated in the model, the relationship between Ar/N2 and AoA is robust and agrees with the observations. This highlights the potential of Ar/N2 to become a new AoA tracer that is subject only to physical transport phenomena and can supplement the suite of available AoA indicators. [ABSTRACT FROM AUTHOR]

We present airborne observations of the vertical gradient of atmospheric oxygen (δ(O2/N2)) and carbon dioxide (CO2) through the atmospheric boundary layer (BL) over the Drake Passage region of the Southern Ocean, during the O2/N2 Ratio and CO2 Airborne Southern Ocean Study, from 15 January to 29 February 2016. Gradients were predominately anticorrelated, with excesses of δ(O2/N2) and depletions of CO2 found within the boundary layer, relative to a mean reference height of 1.7 km. Through analysis of the molar ratio of the gradients (GR), the behavior of other trace gases measured in situ, and modeling experiments with the Community Earth System Model, we found that the main driver of gradients was air‐sea exchange of O2 and CO2 driven by biological processes, more so than solubility effects. An exception to this was in the eastern Drake Passage, where positive GRs were occasionally observed, likely due to the dominance of thermal forcing on the air‐sea flux of both species. GRs were more spatially consistent than the magnitudes of the gradients, suggesting that GRs can provide integrated process constraints over broad spatial scales. Based on the model simulation within a domain bounded by 45°S, 75°S, 100°W, and 45°W, we show that the sampling density of the campaign was such that the observed mean GR (± standard error), −4.0± 0.8 mol O2 per mol CO2, was a reasonable proxy for both the mean GR and the mean molar ratio of air‐sea fluxes of O2 and CO2 during the O2/N2 Ratio and CO2 Airborne Southern Ocean Study. Plain Language Summary: Using an aircraft, we measured changes in atmospheric oxygen and carbon dioxide over the surface of the Southern Ocean. We show that these changes can be used to estimate the relative amounts of oxygen and carbon dioxide that the Southern Ocean exchanges in summer. Key Points: We present atmospheric boundary layer gradients of oxygen and carbon dioxide over the Southern OceanGradients are driven primarily by air‐sea fluxes due to biological productionThe ratio of these gradients can be used to approximate the flux ratio of the study region [ABSTRACT FROM AUTHOR]