Brewer-Dobson Circulation Research Articles

Secular change in atmospheric Ar∕N<sub>2</sub> and its implications for ocean heat uptake and Brewer–Dobson circulation

Abstract. Systematic measurements of the atmospheric Ar∕N2 ratio have been made at ground-based stations in Japan and Antarctica since 2012. Clear seasonal cycles of the Ar∕N2 ratio with summertime maxima were found at middle- to high-latitude stations, with seasonal amplitudes increasing with increasing latitude. Eight years of the observed Ar∕N2 ratio at Tsukuba (TKB) and Hateruma (HAT), Japan, showed interannual variations in phase with the observed variations in the global ocean heat content (OHC). We calculated secularly increasing trends of 0.75 ± 0.30 and 0.89 ± 0.60 per meg per year from the Ar∕N2 ratio observed at TKB and HAT, respectively, although these trend values are influenced by large interannual variations. In order to examine the possibility of the secular trend in the surface Ar∕N2 ratio being modified significantly by the gravitational separation in the stratosphere, two-dimensional model simulations were carried out by arbitrarily modifying the mass stream function in the model to simulate either a weakening or an enhancement of the Brewer–Dobson circulation (BDC). The secular trend of the Ar∕N2 ratio at TKB, corrected for gravitational separation under the assumption of weakening (enhancement) of BDC simulated by the 2-D model, was 0.60 ± 0.30 (0.88 ± 0.30) per meg per year. By using a conversion factor of 3.5 × 10−23 per meg per joule by assuming a one-box ocean with a temperature of 3.5 ∘C, average OHC increase rates of 17.1 ± 8.6 ZJ yr−1 and 25.1 ± 8.6 ZJ yr−1 for the period 2012–2019 were estimated from the corrected secular trends of the Ar∕N2 ratio for the weakened- and enhanced-BDC conditions, respectively. Both OHC increase rates from the uncorrected- and weakened-BDC secular trends of the Ar∕N2 ratio are consistent with 12.2 ± 1.2 ZJ yr−1 reported by ocean temperature measurements, while that from the enhanced-BDC is outside of the range of the uncertainties. Although the effect of the actual atmospheric circulation on the Ar∕N2 ratio is still unclear and longer-term observations are needed to reduce uncertainty of the secular trend of the surface Ar∕N2 ratio, the analytical results obtained in the present study imply that the surface Ar∕N2 ratio is an important tracer for detecting spatiotemporally integrated changes in OHC and BDC.

Different Relationships between Arctic Oscillation and Ozone in the Stratosphere over the Arctic in January and February

We compare the relationship between the Arctic Oscillation (AO) and ozone concentration in the lower stratosphere over the Arctic during 1980–1994 (P1) and 2007–2019 (P2) in January and February using reanalysis datasets. The out-of-phase relationship between the AO and ozone in the lower stratosphere is significant in January during P1 and February during P2, but it is insignificant in January during P2 and February during P1. The variable links between the AO and ozone in the lower stratosphere over the Arctic in January and February are not caused by changes in the spatial pattern of AO but are related to the anomalies in the planetary wave propagation between the troposphere and stratosphere. The upward propagation of the planetary wave in the stratosphere related to the positive phase of AO significantly weakens in January during P1 and in February during P2, which may be related to negative buoyancy frequency anomalies over the Arctic. When the AO is in the positive phase, the anomalies of planetary wave further contribute to the negative ozone anomalies via weakening the Brewer–Dobson circulation and decreasing the temperature in the lower stratosphere over the Arctic in January during P1 and in February during P2.

Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry–climate model with mixed-layer ocean

Abstract. In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH4) mixing ratios have been analysed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation-driven temperature feedback. Therefore, we perform sensitivity simulations with doubled and quintupled present-day (year 2010) CH4 mixing ratios with the chemistry–climate model EMAC (European Centre for Medium-Range Weather Forecasts, Hamburg version – Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry) and include in a novel set-up a mixed-layer ocean model to account for tropospheric warming. Strong increases in CH4 lead to a reduction in the hydroxyl radical in the troposphere, thereby extending the CH4 lifetime. Slow climate feedbacks counteract this reduction in the hydroxyl radical through increases in tropospheric water vapour and ozone, thereby dampening the extension of CH4 lifetime in comparison with the quasi-instantaneous response. Changes in the stratospheric circulation evolve clearly with the warming of the troposphere. The Brewer–Dobson circulation strengthens, affecting the response of trace gases, such as ozone, water vapour and CH4 in the stratosphere, and also causing stratospheric temperature changes. In the middle and upper stratosphere, the increase in stratospheric water vapour is reduced with respect to the quasi-instantaneous response. We find that this difference cannot be explained by the response of the cold point and the associated water vapour entry values but by a weaker strengthening of the in situ source of water vapour through CH4 oxidation. However, in the lower stratosphere water vapour increases more strongly when tropospheric warming is accounted for, enlarging its overall radiative impact. The response of the stratosphere adjusted temperatures driven by slow climate feedbacks is dominated by these increases in stratospheric water vapour as well as strongly decreased ozone mixing ratios above the tropical tropopause, which result from enhanced tropical upwelling. While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH4 radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH4 changes in this chemistry–climate model set-up is not significantly different from the climate sensitivity in carbon-dioxide-driven simulations, provided that the CH4 effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes.

Using Machine Learning to Make Computationally Inexpensive Projections of 21st Century Stratospheric Column Ozone Changes in the Tropics

Stratospheric ozone projections in the tropics, modeled using the UKESM1 Earth system model, are explored under different Shared Socioeconomic Pathways (SSPs). Consistent with other studies, it is found that tropical stratospheric column ozone does not return to 1980s values by the end of the 21st century under any SSP scenario as increased ozone mixing ratios in the tropical upper stratosphere are offset by continued ozone decreases in the tropical lower stratosphere. Stratospheric column ozone is projected to be largest under SSP scenarios with the smallest change in radiative forcing, and smallest for SSP scenarios with larger radiative forcing, consistent with a faster Brewer-Dobson circulation at high greenhouse gas loadings. This study explores the use of machine learning (ML) techniques to make accurate, computationally inexpensive projections of tropical stratospheric column ozone. Four ML techniques are investigated: Ridge regression, Lasso regression, Random Forests and Extra Trees. All four techniques investigated here are able to make projections of future tropical stratospheric column ozone which agree well with those made by the UKESM1 Earth system model, often falling within the ensemble spread of UKESM1 simulations for a broad range of SSPs. However, all techniques struggle to make accurate projects for the final decades of the SSP5-8.5 scenario. Accurate projections can only be achieved when the ML methods are trained on sufficient data, including both historical and future simulations. When trained only on historical data, the projections made using models based on ML techniques fail to accurately predict tropical stratospheric ozone changes. Results presented here indicate that, when sufficiently trained, ML models have the potential to make accurate, computationally inexpensive projections of tropical stratospheric column ozone. Further development of these models may reduce the computational burden placed on fully coupled chemistry-climate and Earth system models and enable the exploration of tropical stratospheric column ozone recovery under a much broader range of future emissions scenarios.

The stratospheric Brewer–Dobson circulation inferred from age of air in the ERA5 reanalysis

The stratospheric Brewer–Dobson circulation inferred from age of air in the ERA5 reanalysis

Co-varying temperatures at 200 hPa over the Earth’s three poles

The Earth’s three poles, the North Pole, South Pole, and Third Pole (i.e., the Tibetan Plateau and its surroundings), hold the largest amount of fresh water on Earth as glaciers, sea ice, and snow. They are sensitive to climate change. However, the linkages between climate variations of the three poles, particularly between the South Pole and Third Pole, remain largely unknown. The temperatures at 200 hPa over the three poles are the highest in the summer and are less affected by surface conditions, which could reflect large-scale dynamic linkages. Temperatures at 200 hPa peak the three poles during their respective hemispheric summer and exhibit in-phase variations on interdecadal timescales (10–100 years). The 200 hPa temperatures over the North Pole and South Pole were significantly correlated with the Brewer-Dobson circulation (BDC), which transports stratospheric ozone poleward, heating the air at 200 hPa. Tropopause warming over the Third Pole was found to enhance the poleward BDC, particularly to the South Pole, linking the Third Pole’s climate to the other two poles. Additionally, the Interdecadal Pacific Oscillation (IPO) also exhibits links with the 200 hPa temperatures of the three poles.

Climatological impact of the Brewer–Dobson circulation on the N<sub>2</sub>O budget in WACCM, a chemical reanalysis and a CTM driven by four dynamical reanalyses

Abstract. The Brewer–Dobson circulation (BDC) is a stratospheric circulation characterized by upwelling of tropospheric air in the tropics, poleward flow in the stratosphere, and downwelling at mid and high latitudes, with important implications for chemical tracer distributions, stratospheric heat and momentum budgets, and mass exchange with the troposphere. As the photochemical losses of nitrous oxide (N2O) are well known, model differences in its rate of change are due to transport processes that can be separated into the mean residual advection and the isentropic mixing terms in the transformed Eulerian mean (TEM) framework. Here, the climatological impact of the stratospheric BDC on the long-lived tracer N2O is evaluated through a comparison of its TEM budget in the Whole Atmosphere Community Climate Model (WACCM), in a chemical reanalysis of the Aura Microwave Limb Sounder version 2 (BRAM2) and in a chemistry transport model (CTM) driven by four modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim reanalysis (ERA-Interim; Dee et al., 2011), the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al., 2015), and the Modern-Era Retrospective analysis for Research and Applications version 1 (MERRA; Rienecker et al., 2011) and version 2 (MERRA-2; Gelaro et al., 2017). The effects of stratospheric transport on the N2O rate of change, as depicted in this study, have not been compared before across this variety of datasets and have never been investigated in a modern chemical reanalysis. We focus on the seasonal means and climatological annual cycles of the two main contributions to the N2O TEM budget: the vertical residual advection and the horizontal mixing terms. The N2O mixing ratio in the CTM experiments has a spread of approximately ∼20 % in the middle stratosphere, reflecting the large diversity in the mean age of air obtained with the same CTM experiments in a previous study. In all datasets, the TEM budget is closed well; the agreement between the vertical advection terms is qualitatively very good in the Northern Hemisphere, and it is good in the Southern Hemisphere except above the Antarctic region. The datasets do not agree as well with respect to the horizontal mixing term, especially in the Northern Hemisphere where horizontal mixing has a smaller contribution in WACCM than in the reanalyses. WACCM is investigated through three model realizations and a sensitivity test using the previous version of the gravity wave parameterization. The internal variability of the horizontal mixing in WACCM is large in the polar regions and is comparable to the differences between the dynamical reanalyses. The sensitivity test has a relatively small impact on the horizontal mixing term, but it significantly changes the vertical advection term and produces a less realistic N2O annual cycle above the Antarctic. In this region, all reanalyses show a large wintertime N2O decrease, which is mainly due to horizontal mixing. This is not seen with WACCM, where the horizontal mixing term barely contributes to the TEM budget. While we must use caution in the interpretation of the differences in this region (where the reanalyses show large residuals of the TEM budget), they could be due to the fact that the polar jet is stronger and is not tilted equatorward in WACCM compared with the reanalyses. We also compare the interannual variability in the horizontal mixing and the vertical advection terms between the different datasets. As expected, the horizontal mixing term presents a large variability during austral fall and boreal winter in the polar regions. In the tropics, the interannual variability of the vertical advection term is much smaller in WACCM and JRA-55 than in the other experiments. The large residual in the reanalyses and the disagreement between WACCM and the reanalyses in the Antarctic region highlight the need for further investigations on the modeling of transport in this region of the stratosphere.

Effects of elevated humidity on stratospheric ozone content in the tropics

This study examines the effects of elevated humidity on stratospheric ozone content in the tropics, a case study of Nigeria. The specific stations in Nigeria studied with their co-ordinate include Makurdi (7.7°N 8.5°E), Maiduguri (11.9°N 13.2°E), Kano (12.0°N 8.5°E), Port-Harcourt (4.9°N 7.0°E) and Lagos (6.5°N 3.4°E). Using monthly mean stratospheric ozone data from Earth Probe Total Ozone Mass Spectroscopy (EPTOMS) together with humidity and temperature data from the Nigerian Meteorological Agency (NIMET), studies were carried out over the five stations for a period of fifteen years (1998-2012). The results show that elevated humidity cools the stratosphere, slowing down the rate of chemical reactions that destroy ozone, thereby resulting to more ozone amount. Ozone variation is latitude dependent with more variations in station observed in the north. Using Spearman’s Rank Correlation Coefficient analysis, it was equally observed that ozone and temperature are strongly anti-correlated in Port Harcourt, Makurdi and Lagos stations (r = -0.8531, -0.7832, and -0.8759, respectively); and positively correlated in Maiduguri and Kano stations (r = 0.3776 and 0.4965, respectively). The positive correlation may be attributed to the high dehydration of water vapor in those stations. Results also revealed that ozone and humidity are positively correlated (r = 0.9073, 0.9021, 0.7133, 0.7552, and 0.7692), for Port Harcourt, Makurdi, Maiduguri, Lagos, and Kano. Both ozone and humidity attain maximum during wet season and minimum during the dry season in Nigeria. The implication of more humidity in the wet season is due to the evaporation of raindrops and surface puddles. The implication of more ozone concentration in the wet season may be attributed to the transportation of ozone from the influence of Brewer-Dobson Circulation (BDC). Again it was found out that temperature and humidity are negatively correlated. Consequently, our findings show that, at elevated humidity, the surface temperature is low with more stratospheric ozone content. Key words: Humidity, stratospheric ozone, temperature, rainfall.

Numerical impacts on tracer transport: A proposed intercomparison test of Atmospheric General Circulation Models

AbstractThe transport of trace gases by the atmospheric circulation plays an important role in the climate system and its response to external forcing. Transport presents a challenge for Atmospheric General Circulation Models (AGCMs), as errors in both the resolved circulation and the numerical representation of transport processes can bias their abundance. In this study, two tests are proposed to assess transport by the dynamical core of an AGCM. To separate transport from chemistry, the tests focus on the age‐of‐air, an estimate of the mean transport time by the circulation. The tests assess the coupled stratosphere–troposphere system, focusing on transport by the overturning circulation and isentropic mixing in the stratosphere, or Brewer–Dobson Circulation, where transport time‐scales on the order of months to years provide a challenging test of model numerics. Four dynamical cores employing different numerical schemes (finite‐volume, pseudo‐spectral, and spectral‐element) and discretizations (cubed sphere versus latitude–longitude) are compared across a range of resolutions. The subtle momentum balance of the tropical stratosphere is sensitive to model numerics, and the first intercomparison reveals stark differences in tropical stratospheric winds, particularly at high vertical resolution: some cores develop westerly jets and others easterly jets. This leads to substantial spread in transport, biasing the age‐of‐air by up to 25% relative to its climatological mean, making it difficult to assess the impact of the numerical representation of transport processes. This uncertainty is removed by constraining the tropical winds in the second intercomparison test, in a manner akin to specifying the Quasi‐Biennial Oscillation in an AGCM. The dynamical cores exhibit qualitative agreement on the structure of atmospheric transport in the second test, with evidence of convergence as the horizontal and vertical resolution is increased in a given model. Significant quantitative differences remain, however, particularly between models employing spectral versus finite‐volume numerics, even in state‐of‐the‐art cores.

Overestimated acceleration of the advective Brewer–Dobson circulation due to stratospheric cooling

AbstractTropospheric warming and stratospheric cooling influence the vertical structure of the atmosphere. Numerous studies have analysed the thermal expansion of the troposphere, however, stratospheric cooling reverses the sign of this shift in the middle stratosphere, causing a downward shift in the upper stratosphere and mesosphere. This is a robust feature in transient climate model simulations, but its impact is commonly unappreciated. Here, we quantify the trend difference of the residual mean vertical velocity (), a proxy for diagnosing the advective Brewer–Dobson circulation (BDC) strength, which arises from implicit neglect of the shrinking distance between stratospheric pressure levels in the CCMI‐1 (Chemistry‐Climate Model Initiative part 1) data request. There, a log‐pressure formula with constant scale height is recommended to compute . However, stratospheric cooling in transient climate simulations causes a reduction of the geometrical distance between pressure levels and thereby also the scale height significantly decreases over time. Using the general scale height definition for the transformation, the trends are therefore smaller. In both cases, the units are m·s−1, but in the latter case it is the constant measure of length geopotential metres and not log‐pressure metres. We quantify that, due to the temperature dependence of log‐pressure metres, past studies that based trend analyses on log‐pressure overestimated the advective BDC acceleration by ∼ 20%. This result is consistent among the CCMI‐1 projections over the 1960–2100 period. We highlight that other diagnostics can also be affected by the neglect of the declining stratospheric pressure level distance. A detailed description of the diagnostics is necessary for consistent assessments of trends. Data processing tools should generally not include the constant scale height assumption if the data are used for trend analyses.

Sensitivity of age of air trends to the derivation method for non-linear increasing inert SF<sub>6</sub>

Abstract. Mean age of air (AoA) is a diagnostic of transport along the stratospheric Brewer–Dobson circulation. While models consistently show negative trends, long-term time series (1975–2016) of AoA derived from observations show non-significant positive trends in mean AoA in the Northern Hemisphere. This discrepancy between observed and modelled mean AoA trends is still not resolved. There are uncertainties and assumptions required when deriving AoA from trace gas observations. At the same time, AoA from climate models is subject to uncertainties, too. In this paper, we focus on the uncertainties due to the parameter selection in the method that is used to derive mean AoA from SF6 measurements in Engel et al. (2009, 2017). To correct for the non-linear increase in SF6 concentrations, a quadratic fit to the time series at the reference location, i.e. the tropical surface, is used. For this derivation, the width of the AoA distribution (age spectrum) has to be assumed. In addition, to choose the number of years the quadratic fit is performed for, the fraction of the age spectrum to be considered has to be assumed. Even though the uncertainty range due to all different aspects has already been taken into account for the total errors in the AoA values, the systematic influence of the parameter selection on AoA trends is described for the first time in the present study. For this, we use the EMAC (ECHAM MESSy Atmospheric Chemistry) climate model as a test bed, where AoA derived from a linear tracer is available as a reference and modelled age spectra exist to diagnose the actual spatial age spectra widths. The comparison of mean AoA from the linear tracer with mean AoA from a SF6 tracer shows systematic deviations specifically in the trends due to the selection of the parameters. However, for an appropriate parameter selection, good agreement for both mean AoA and its trend can be found, with deviations of about 1 % in mean AoA and 12 % in AoA trend. In addition, a method to derive mean AoA is evaluated that applies a convolution to the reference time series. The resulting mean AoA and its trend only depend on an assumption about the ratio of moments. Also in that case, it is found that the larger the ratio of moments, the more the AoA trend gravitates towards the negative. The linear tracer and SF6 AoA are found to agree within 0.3 % in the mean and 6 % in the trend. The different methods and parameter selections were then applied to the balloon-borne SF6 and CO2 observations. We found the same systematic changes in mean AoA trend dependent on the specific selection. When applying a parameter choice that is suggested by the model results, the AoA trend is reduced from 0.15 to 0.07 years per decade. It illustrates that correctly constraining those parameters is crucial for correct mean AoA and trend estimates and still remains a challenge in the real atmosphere.

Impact of the 2018 Ambae Eruption on the Global Stratospheric Aerosol Layer and Climate

AbstractDuring an extended volcanic unrest starting in 2017, two main moderate stratospheric eruptions occurred at the Ambae volcano (15°S and 167°E), Vanuatu, in April and July 2018. Observations from a geostationary orbit show that the April and July eruptions injected a volcanic plume into the lower stratosphere. While aerosol enhancements from the April eruption have only had an impact on the Southern Hemisphere, the plume from the July eruption was distributed within the lower branch of the Brewer–Dobson circulation to both hemispheres. Satellite, ground‐based and in situ observations show that the background aerosol is enhanced throughout the year after the July eruption on a global scale. A volcanic‐induced perturbation of the global stratospheric aerosol optical depth up to 0.011 is found, in the ultraviolet/visible spectral range. This perturbation is comparable to that of recent moderate stratospheric eruptions like from Kasatochi, Sarychev, and Nabro. Top of the atmosphere radiative forcing values are estimated between −0.45 and −0.6 W/m2 for this event, showing that the Ambae eruption had the strongest climatic impact of the year 2018. Thus, the Ambae eruption in 2018 has to be taken into account when studying the decadal lower stratospheric aerosol budget and in climate studies.

Ozone layer state and level of ultraviolet irradiance over the territory of Ukraine in 2019

The paper presents main results of total ozone column (TOC) and levels of ultraviolet irradiance daily monitoring conducted for the Ukrainian territory in 2019. Monitoring and analysis were performed using TOC data measured by the satellite Ozone Monitoring Instrument (the OMI) and estimated values of ultraviolet index. The research was made on the regular grid with 1° spatial resolution and separately for geographical coordinates of centers of Ukrainian administrative regions. In 2019 the TOC ranged within a multiyear average standard with and its average value was equal to 260–360 D.u. During winter ozone concentration reached its highest values due to intensification of Brewer-Dobson circulation. Weakened polar vortex and ozone-rich air masses advection at the upper atmospheric levels which transport high ozone concentrations, resulted in the TOC positive anomalies emergence over Ukraine. The air masses penetrated Ukrainian territory from the north-west and this led to emergence anomalously high TOC values during a number of periods: February 23–24, March 2, March 12–14, March 26. In these periods 450–500 D.u. was a typical ozone content. Seasonal TOC decrease, observed from April to June, was caused by a weakened Brewer-Dobson circulation and intensification of photochemical ozone loss. Nevertheless, TOC deviations from average standard were negligible and this resulted in absence of very high UV-index values. The highest UV-index values did not reach 7 units. In July the TOC was slightly higher than usual concentrations, which was probably caused by dynamic factors. Such TOC values influenced the level of ultraviolet irradiance and the UV-index varied within 3–4 units' range which is typical for Ukraine in April–May. Such ozone state in 2019 made it impossible for UV-index to reach extremely high levels, especially during the summer period. Further TOC changes were characterized by typical seasonal ozone decrease with minimum values observed in October resulted from formation and intensification of polar vortex. Average minimum values in October amounted to 260–270 D.u., however, it is a typical TOC content for this period with a relative deviation from the standard constituting about -0.3σ. Seasonal ozone increase in 2019 started in mid-November. At the end of December the first high positive deviations happened. The highest of them reached close to anomalous 2–2.4σ values.

Stratospheric water vapor feedback and its climate impacts in the coupled atmosphere–ocean Goddard Earth Observing System Chemistry-Climate Model

The stratospheric water vapor feedbacks on climate for abrupt CO2 quadrupling are investigated with the coupled atmosphere–ocean Goddard Earth Observing System Chemistry-Climate model. A feedback suppression method is used to quantify the stratospheric water vapor climate feedback parameter and the impacts of stratospheric water vapor increases on temperature and circulation. It is found that increases in stratospheric water vapor change the model’s net climate feedback parameter by 0.11 W m−2 K−1, contributing to 0.5 K, or 10%, of the global-mean surface warming under abrupt CO2 quadrupling. Stratospheric water vapor increases lead to significant impacts on stratospheric temperature and dynamics. The increases induce stratospheric dynamical changes that strongly modify stratospheric cooling patterns. About 30% of the acceleration of the stratospheric Brewer-Dobson circulation under 4 × CO2 is attributed to the stratospheric water vapor increases. In the troposphere, the stratospheric water vapor feedback plays a role in Arctic amplification and is responsible for 14% of the Arctic warming. It also affects tropospheric circulation, causing greater poleward shift of the northern hemisphere tropospheric midlatitude jet.

Future trends in stratosphere-to-troposphere transport in CCMI models

Abstract. One of the key questions in the air quality and climate sciences is how tropospheric ozone concentrations will change in the future. This will depend on two factors: changes in stratosphere-to-troposphere transport (STT) and changes in tropospheric chemistry. Here we aim to identify robust changes in STT using simulations from the Chemistry Climate Model Initiative (CCMI) under a common climate change scenario (RCP6.0). We use two idealized stratospheric tracers to isolate changes in transport: stratospheric ozone (O3S), which is exactly like ozone but has no chemical sources in the troposphere, and st80, a passive tracer with fixed volume mixing ratio in the stratosphere. We find a robust increase in the tropospheric columns of these two tracers across the models. In particular, stratospheric ozone in the troposphere is projected to increase 10 %–16 % by the end of the 21st century in the RCP6.0 scenario. Future STT is enhanced in the subtropics due to the strengthening of the shallow branch of the Brewer–Dobson circulation (BDC) in the lower stratosphere and of the upper part of the Hadley cell in the upper troposphere. The acceleration of the deep branch of the BDC in the Northern Hemisphere (NH) and changes in eddy transport contribute to increased STT at high latitudes. These STT trends are caused by greenhouse gas (GHG) increases, while phasing out of ozone-depleting substances (ODS) does not lead to robust transport changes. Nevertheless, the decline of ODS increases the reservoir of ozone in the lower stratosphere, which results in enhanced STT of O3S at middle and high latitudes. A higher emission scenario (RCP8.5) produces stronger STT trends, with increases in tropospheric column O3S more than 3 times larger than those in the RCP6.0 scenario by the end of the 21st century.

Improved Simulation of the Antarctic Stratospheric Final Warming by Modifying the Orographic Gravity Wave Parameterization in the Beijing Climate Center Atmospheric General Circulation Model

The Antarctic stratospheric final warming (SFW) is usually simulated with a substantial delay in climate models, and the corresponding temperatures in austral spring are lower than observations, implying insufficient stratospheric wave drag. To investigate the role of orographic gravity wave drag (GWD) in modeling the Antarctic SFW, in this study the orographic GWD parameterization scheme is modified in the middle-atmosphere version of the Beijing Climate Center Atmospheric General Circulation Model. A pair of simulations are conducted to compare two orographic GWD schemes in simulating the breakdown of the stratospheric polar vortex over Antarctica. The control simulation with the default orographic GWD scheme exhibits delayed vortex breakdown and the cold-pole bias seen in most climate models. In the simulation with modified orographic GWD scheme, the simulated vortex breaks down earlier by 8 days, and the associated cold-pole bias is reduced by more than 2 K. The modified scheme provides stronger orographic GWD in the lower stratosphere, which drives an accelerated polar downwelling branch of the Brewer–Dobson circulation and, in turn, produces adiabatic warming. Our study suggests that modifying orographic GWD parameterizations in climate models would be a valid way of improving the SFW simulation over Antarctica.

Calculation of the Vertical Velocity in the Asian Summer Monsoon Anticyclone Region Using the Thermodynamic Method With in situ and Satellite Data

The correct calculation of the vertical velocity of the Asian Summer Monsoon anticyclone (ASMA) region is helpful to know more accurately the ozone stratosphere-troposphere exchange, so as to explore the variation of ozone in the ASMA region. Therefore, the vertical velocity over the ASMA in June, July, August and September 2012 and 2016 is calculated by the thermodynamic method, which may avoid the deviations from the kinematics method using the mass continuity equation. In order to improve the accuracy, we used high-resolution heating rate datasets obtained via the Beijing Climate Center Radiation Transfer Model based on in situ observations and revised satellite data from MLS/AIRS. The vertical velocity calculated by the thermodynamic method (VT) is then compared with the data from ERA-Interim (VERA-I). In the daytime, values of VT were similar to VERA-I, and were dominated by ascending motion, although VT showed descending motion at the western edge of the ASMA below 100 hPa. The intensity of VT was slightly smaller than that of VERA-I at lower levels (200 hPa –100 hPa) over the ASMA region and significantly weaker above 100 hPa. In the night-time, situation was more complex. Both VT and VERA-I showed the convergence of vertical wind at 150 hPa, and the divergence at 80 hPa, but VT had a smaller standard deviation. Besides, VT showed descending in the western and northern ASMA, but VERA-I only descended in the west. The descending motion in the west, seen in both VT and VERA-I, is produced by the heating difference between Qinghai-Tibet Plateau and Iranian Plateau. The difference of the two vertical velocities in the northern ASMA may indicate the different understandings of the local Hadley Circulation and local Brewer-Dobson Circulation.

Decomposing the response of the stratospheric Brewer–Dobson circulation to an abrupt quadrupling in CO<sub>2</sub>

Abstract. We perform time-slice experiments using HadGEM3-A to decompose the long-term (years 101–150) response of the Brewer–Dobson circulation (BDC) to an abrupt quadrupling in CO2 (4×CO2) into (1) a rapid atmospheric adjustment, (2) a contribution from the global-average sea surface temperature (SST) change (+3.4 K), and (3) an SST pattern effect. The SST fields are derived from the CMIP5 multi-model ensemble. Two further experiments explore the impact on the BDC of the spread in global-average SST response to 4×CO2 across the CMIP5 models (range 2.1–4.9 K). At 70 hPa (10 hPa) the annual-mean tropical upward mass flux increases by 45 % (35 %) due to the 4×CO2 perturbation. At 70 hPa, around 70 % of the increase is from the global-uniform SST warming, with the remainder coming in similar contributions from the rapid adjustment and SST pattern effect. In contrast, at 10 hPa the increase in mass flux comes mainly from the rapid adjustment (∼40 %) and the uniform SST warming (∼45 %), with a small contribution from the SST pattern. At 10 hPa, the effect of the multi-model spread in global-mean SST is comparable in magnitude to the rapid adjustment. Conversely, at 70 hPa the effect of spread in global-mean SST is substantially larger than both the rapid adjustment and the SST pattern effect. We derive an approximately linear sensitivity of the tropical upward mass flux to global surface air temperature change of 0.62×109 kg s−1 K−1 (9 % K−1) at 70 hPa and 0.10×109 kg s−1 K−1 (6 % K−1) at 10 hPa. The results confirm the most important factor for the acceleration of the BDC in the lower stratosphere under increased CO2 is global SST changes. We also quantify for the first time that the rapid adjustment to CO2 is of similar importance to SSTs for the increased BDC in the upper stratosphere. This demonstrates a potential for a fast and slow timescale of the response of the BDC to greenhouse gas forcing, with the relative prominence of those timescales being height dependent.

Contributions of Convective and Orographic Gravity Waves to the Brewer–Dobson Circulation Estimated from NCEP CFSR

Abstract Contributions of convective gravity waves (CGWs) and orographic gravity waves (OGWs) to the Brewer–Dobson circulation (BDC) are examined and compared to those from resolved waves. OGW drag (OGWD) is provided by NCEP Climate Forecast System Reanalysis (CFSR), while CGW drag (CGWD) is obtained from an offline calculation of a physically based CGW parameterization with convective heating and background data provided by CFSR. CGWD contributes to the shallow branch of the BDC regardless of the season, while OGWD contributes to both the shallow and deep branches except for the summertime, when OGWs hardly propagate into the stratosphere. At 70 hPa, the annual-mean tropical upward mass fluxes from Eliassen–Palm flux divergence (EPD), OGWD, and CGWD are 68%, 7%, and 4% of the total mass flux, respectively. The tropical upward mass flux at 70 hPa shows an increasing trend during the time period from 1979 to 1998, with 28%, 18%, and 6% of the trend driven by EPD, OGWD, and CGWD, respectively. The width of the turnaround latitudes tends to narrow for the streamfunctions induced by OGWD and CGWD but tends to widen for that induced by EPD. The contributions of GWD from MERRA (MERRA-2) to the climatology and long-term trend of the BDC are 7% (7%) and 13% (4%), respectively, somewhat smaller than the contributions of CGWD plus OGWD, which are estimated from CFSR to be 12% and 20%, respectively.

Effects of missing gravity waves on stratospheric dynamics; part 1: climatology

Energy and momentum deposition from planetary-scale Rossby waves as well as from small-scale gravity waves (GWs) largely control stratospheric dynamics. Interactions between these different wave types, however, complicate the quantification of their individual contribution to the overall dynamical state of the middle atmosphere. In state-of-the-art general circulation models (GCMs), the majority of the GW spectrum cannot be resolved and therefore has to be parameterised. This is commonly implemented in two discrete schemes, one for GWs that originate from flow over orographic obstacles and one for all other kinds of GWs (non-orographic GWs). In this study, we attempt to gain a deeper understanding of the interactions of resolved with parameterised wave driving and of their influence on the stratospheric zonal winds and on the Brewer–Dobson circulation (BDC). For this, we set up a GCM time slice experiment with two sensitivity simulations: one without orographic GWs and one without non-orographic GWs. Our findings include an acceleration of the polar vortices, which has historically been one of the main reasons for including explicit GW parameterisations in GCMs. Further, we find inter-hemispheric differences in BDC changes when omitting GWs that can be explained by wave compensation and amplification effects. These are partly evoked through local changes in the refractive properties of the atmosphere caused by the omitted GW drag and a thereby increased planetary wave propagation. However, non-local effects on the flow can act to suppress vertical wave fluxes into the stratosphere for a very strong polar vortex. Moreover, we study mean age of stratospheric air to investigate the impact of missing GWs on tracer transport. On the basis of this analysis, we suggest that the larger ratio of planetary waves to GWs leads to enhanced horizontal mixing, which can have a large impact on stratospheric tracer distributions.