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In contrast to the general stratospheric ozone recovery following international agreements, recent observations show an ongoing net ozone depletion in the tropical lower stratosphere (LS). This depletion is thought to be driven by dynamical transport accelerated by global warming, while chemical processes have been considered to be unimportant. Here we use a chemistry–climate model to demonstrate that halogenated ozone-depleting very short-lived substances (VSLS) chemistry may account for around a quarter of the observed tropical LS negative ozone trend in 1998–2018. VSLS sources include both natural and anthropogenic emissions. Future projections show the persistence of the currently unaccounted for contribution of VSLS to ozone loss throughout the twenty-first century in the tropical LS, the only region of the global stratosphere not projecting an ozone recovery by 2100. Our results show the need for mitigation strategies of anthropogenic VSLS emissions to preserve the present and future ozone layer in low latitudes.

It is well established that the shallow branch of the Brewer‐Dobson circulation accelerates in a warming climate due to enhanced wave drag in the subtropical lower stratosphere. This has been linked to the strengthening of the upper flanks of the subtropical jets. However, the seasonality of the zonal wind trends, peaking in the winter hemisphere, is opposite to that of the Eliassen‐Palm flux convergence trends, peaking in summer. We investigate the seasonality in the wave drag trends and find a different behavior for each hemisphere. The Shepherd and McLandress (2011, https://doi.org/10.1175/2010jas3608.1) mechanism, involving transient wave dissipation at higher levels following the rise of the critical lines, is found to maximize in austral summer. On the other hand, in the Northern Hemisphere the wave drag increase peaks in summer primarily due to the changes in the stationary planetary waves (monsoonal circulations) associated with enhanced deep convection. The Brewer‐Dobson circulation, responsible for mass, heat and constituents global transport in the stratosphere, is projected to accelerate in a warming climate. This circulation is driven by the momentum transferred by dissipating waves. We explore the seasonality of trends in wave dissipation in the subtropical lower stratosphere. First, we show that the largest changes in the wave dissipation take place in the summer hemisphere, opposite to the largest changes in the zonal wind, which is known to control wave dissipation conditions. We investigate this apparent contradiction and find that (a) the conditions are particularly favorable for the waves to be affected by the changing wind in summer, due to their spectral characteristics and the structure of the background zonal wind, in particular the proximity of the zero wind line; and (b) in the Northern Hemisphere the changes are primarily associated with stationary waves triggered by enhanced deep convection in a warmer climate. Future subtropical trends in lower stratospheric wave drag are strongest in the summer hemisphere, whereas zonal wind trends peak in winterThe largest changes in transient wave drag due to critical line shift are found in the Southern Hemisphere summerThe Northern Hemisphere summer trends are mainly due to changes in stationary wave drag linked to stronger and higher deep convection Future subtropical trends in lower stratospheric wave drag are strongest in the summer hemisphere, whereas zonal wind trends peak in winter The largest changes in transient wave drag due to critical line shift are found in the Southern Hemisphere summer The Northern Hemisphere summer trends are mainly due to changes in stationary wave drag linked to stronger and higher deep convection

It has recently been recognized that, in addition to greenhouse gases, anthropogenic emissions of ozone‐depleting substances (ODS) can induce long‐term trends in the Brewer‐Dobson circulation (BDC). Several studies have shown that a substantial fraction of the residual circulation acceleration over the last decades of the twentieth century can be attributed to increasing ODS. Here the mechanisms of this influence are examined, comparing model runs to reanalysis data and evaluating separately the residual circulation and mixing contributions to the mean age of air trends. The effects of ozone depletion in the Antarctic lower stratosphere are found to dominate the ODS impact on the BDC, while the direct radiative impact of these substances is negligible over the period of study. We find qualitative agreement in austral summer BDC trends between model and reanalysis data and show that ODS are the main driver of both residual circulation and isentropic mixing trends over the last decades of the twentieth century. Moreover, aging by isentropic mixing is shown to play a key role on ODS‐driven age of air trends. Concentrations of human‐emitted ozone‐depleting substances in the stratosphere have increased substantially during the last decades of the twentieth century, which are found in the Southern Hemisphere. These substances have caused stratospheric ozone depletion, which not only has important direct impacts on human health but also leads to important changes in the atmospheric circulation and climate. Here the impact of these substances on the stratospheric mean transport circulation is examined using chemistry‐climate model simulations and reanalysis data. In addition to destroying ozone, these substances act as greenhouse gases. In this paper, we find that the radiative forcing impacts of human‐emitted ozone‐depleting substances on the stratospheric circulation over the last decades of the twentieth century are negligible. In contrast, the Antarctic ozone hole has contributed substantially to the acceleration of stratospheric tracer transport not only in the polar region but globally. The impacts, largest in the austral summer stratosphere, are examined in detail separating the overturning circulation and irreversible mixing. It is shown that trends in the model compare qualitatively well with estimates from two different reanalyses. This paper contributes to advance our understanding of the impact of human emissions on the stratospheric circulation and to untangle the forcings of recent trends. Increasing ODS have played a key role in observed BDC trends over the last decades of the twentieth centuryOzone depletion dominates the ODS impact on BDC trends, while the radiative effect of these substances is negligibleIncluding changes in mixing in addition to the residual circulation is crucial to interpreting past mean age trends

The interannual variability of tropical lower stratosphere ozone and its connections to sea surface temperatures in the equatorial Pacific are examined using a combination of chemistry climate model simulations, satellite observations, and reanalyses. The model simulations and observations show large differences in the magnitude of interannual variability in ozone between northern tropic (NT; EQ‐18° N) and southern tropic (EQ‐18° S) during boreal summer but small differences in winter. The interannual variability during boreal summer is highly correlated with summer sea surface temperatures in the eastern and central Pacific Ocean and El Niño–Southern Oscillation (ENSO) events. Larger variability in NT ozone is primarily due to meridional advection, connected to the changes in the onset date and strength of the Asian summer monsoon anticyclone. The Asian summer monsoon anticyclone forms earlier in a season and tends to be stronger during cold (La Niña) events leading to more isentropic transport of ozone from the extratropics into the NT, with the reverse for warm (El Niño) events. Interannual variability in the lower stratospheric ozone is larger in the northern tropic than in the southern tropic during boreal summerOzone interannual variability during boreal summer is highly correlated with boreal summer ENSO eventsBoreal summer ENSO modifies meridional advection of ozone through changes in the strength of the Asian summer monsoon anticyclone

It is well established that increasing greenhouse gases, notably CO2, will cause an acceleration of the stratospheric Brewer‐Dobson circulation (BDC) by the end of this century. We here present compelling new evidence that ozone depleting substances are also key drivers of BDC trends. We do so by analyzing and contrasting small ensembles of “single‐forcing” integrations with a stratosphere resolving atmospheric model with interactive chemistry, coupled to fully interactive ocean, land, and sea ice components. First, confirming previous work, we show that increasing concentrations of ozone depleting substances have contributed a large fraction of the BDC trends in the late twentieth century. Second, we show that the phasing out of ozone depleting substances in coming decades—as a consequence of the Montreal Protocol—will cause a considerable reduction in BDC trends until the ozone hole is completely healed, toward the end of the 21st century. Ozone depleting substances are a major driver of Brewer‐Dobson circulation trendsEmission of these substances has been phased out by the Montreal ProtocolThat phasing out will result in considerably reduced BDC trends in the 21st century

The Brewer‐Dobson Circulation (BDC) determines the distribution of long‐lived tracers in the stratosphere; therefore, their changes can be used to diagnose changes in the BDC. We evaluate decadal (2005–2018) trends of nitrous oxide (N2O) in two versions of the Whole Atmosphere Chemistry‐Climate Model (WACCM) by comparing them with measurements from four Fourier transform infrared (FTIR) ground‐based instruments, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS), and with a chemistry‐transport model (CTM) driven by four different reanalyses. The limited sensitivity of the FTIR instruments can hide negative N2O trends in the mid‐stratosphere because of the large increase in the lowermost stratosphere. When applying ACE‐FTS measurement sampling on model datasets, the reanalyses from the European Center for Medium Range Weather Forecast (ECMWF) compare best with ACE‐FTS, but the N2O trends are consistently exaggerated. The N2O trends obtained with WACCM disagree with those obtained from ACE‐FTS, but the new WACCM version performs better than the previous above the Southern Hemisphere in the stratosphere. Model sensitivity tests show that the decadal N2O trends reflect changes in the stratospheric transport. We further investigate the N2O Transformed Eulerian Mean (TEM) budget in WACCM and in the CTM simulation driven by the latest ECMWF reanalysis. The TEM analysis shows that enhanced advection affects the stratospheric N2O trends in the Tropics. While no ideal observational dataset currently exists, this model study of N2O trends still provides new insights about the BDC and its changes because of the contribution from relevant sensitivity tests and the TEM analysis. The circulation in the stratosphere is characterized by upward motion above the Tropics, followed by poleward and downward motions above the high latitudes. Changes in the pattern of this stratospheric circulation are currently a challenging topic of research. We investigate the decennial changes of this stratospheric circulation using observations and numerical simulations of the long‐lived tracer nitrous oxide. Observations are obtained from ground‐based and satellite instruments. Numerical simulations include complex atmospheric models that reproduce the chemistry and dynamics of the stratosphere. Both observations and models show differences between the hemispheres in the nitrous oxide decennial changes. Unfortunately, the current observations of nitrous oxide are not perfect. The ground‐based instruments cannot correctly measure the changes of nitrous oxide in the northern hemisphere. The satellite does not measure at all times, and it spatially covers more the high latitudes, which negatively affects the measurements of nitrous oxide. On the other hand, model simulations can provide valuable insights into the changes in the stratospheric circulation. They show that changes in the stratospheric circulation cause the differences between hemispheres in the nitrous oxide trends and show that the circulation changes can be associated with different physical processes. Sparse sampling of Atmospheric Chemistry Experiment Fourier Transform Spectrometer exaggerates the stratospheric nitrous oxide trendsTransformed Eulerian Mean analysis shows that the residual mean advection contributes to the positive nitrous oxide trend in the TropicsThe Whole Atmosphere Community‐Climate Model simulates weaker hemispheric asymmetries of the nitrous oxide trends compared to reanalyses Sparse sampling of Atmospheric Chemistry Experiment Fourier Transform Spectrometer exaggerates the stratospheric nitrous oxide trends Transformed Eulerian Mean analysis shows that the residual mean advection contributes to the positive nitrous oxide trend in the Tropics The Whole Atmosphere Community‐Climate Model simulates weaker hemispheric asymmetries of the nitrous oxide trends compared to reanalyses

Most chemistry‐climate models show an intensification of the Brewer‐Dobson circulation (BDC) in the stratosphere associated with increasing greenhouse gas emissions and ozone depletion in the last decades, but this trend remains to be confirmed in observational data. In this work the evolution of the advective BDC for the period 1979–2012 is evaluated and compared in three modern reanalyses (ERA‐Interim, MERRA, and JRA‐55). Three different estimates of the BDC are computed for each reanalysis, one based on the definition of the residual circulation and two indirect estimates derived from momentum and thermodynamic balances. The comparison among the nine estimates shows substantial uncertainty in the mean magnitude (∼40%) but significant common variability. The tropical upwelling series show variability linked to the stratospheric quasi‐biennial oscillation and to El Niño–Southern Oscillation (ENSO) and also reflect extreme events such as major sudden stratospheric warmings and volcanic eruptions. The trend analysis suggests a strengthening of tropical upwelling of around 2–5%/decade throughout the layer 100–10 hPa. The global spatial structure of the BDC trends provides evidence of an overall acceleration of the circulation in both hemispheres, with qualitative agreement among the estimates. The global BDC trends are mainly linked to changes in the boreal winter season and can be tracked to long‐term increases in the resolved wave drag in both hemispheres. Common variability among estimates of the BDC in three modern reanalysesQualitatively consistent BDC strengthening trends for 1979–2012 in reanalyses

This study identifies the fast (i.e., ∼days–weeks) transport pathways that connect the Northern Hemisphere surface to the upper troposphere and lower stratosphere (UTLS) during northern summer by integrating a large (90 member) ensemble of Boundary Impulse Response tracers in the Whole Atmosphere Community Climate Model version 5. We show that there is a fast transport pathway that occurs over the southern slope of the Tibetan Plateau, northern India, the Arabian Sea, and Saudi Arabia; furthermore, we show that during July this pathway connects the Northern Hemisphere surface to the UTLS on a modal time scale of 5–10 days. A less efficient transport pathway is also identified over the western Pacific. A detailed budget analysis reveals that, while convective processes are responsible for transport to 200–300 hPa, the resolved dynamics, specifically the vertical eddy flux, dominate at 100–150 hPa. Transport variations are analyzed on weekly, monthly, and interannual time scales and are largely related to differences in the resolved dynamics in the UTLS. A fast transport pathway into the UTLS is found over the southern slope of the Tibetan Plateau and northern IndiaResolved dynamics, specifically the vertical eddy flux, dominate transport into the UTLSVariability of northern summer tracer transport is analyzed with respect to weekly, monthly, and interannual time scales