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1. The Spillover of Tropospheric Ozone Increases Has Hidden the Extent of Stratospheric Ozone Depletion by Halogens

   https://doi.org/10.1029/2023AV001154  

   Prather, Michael J (June 2024, AGU Advances)

   Abstract Stratospheric ozone depletion from halocarbons is partly countered by pollution‐driven increases in tropospheric ozone, with transport connecting the two. While recognizing this connection, the ozone assessment's evaluation of observations and processes have often split the chapters at the tropopause boundary. Using a chemistry‐transport model we find that air‐pollution ozone enhancements in the troposphere spill over into the stratosphere at significant rates, that is, 13%–34% of the excess tropospheric burden appears in the lowermost extra‐tropical stratosphere. As we track the anticipated recovery of the observed ozone depletion, we should recognize that two tenths of that recovery may come from the transport of increasing tropospheric ozone into the stratosphere. 

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2. Calibrating the Tropospheric Air and Ozone Mass

   https://doi.org/10.1029/2025AV001651  

   Prather, Michael J (June 2025, AGU Advances)

   Abstract We divide the atmosphere into distinct spheres based on their physical, chemical, and dynamical traits. In deriving chemical budgets and climate trends, which differ across spheres, we need clearly defined boundaries. Our primary spheres are the troposphere and stratosphere (∼99.9% by mass), and the boundary between them is the tropopause. Every global climate‐weather model has one or more methods to calculate the lapse rate tropopause, but these involve subjective choices and are known to fail near the sub‐tropical jets and polar regions. Age‐of‐air tracers clock the effective time‐distance from the tropopause, allowing unambiguous separation of stratosphere from troposphere in the chaotic jet regions. We apply a global model with synthetic tracer e90 (90‐day e‐folding), focusing on ozone and temperature structures about the tropopause using ozone sonde and satellite observations. We calibrate an observation‐consistent tropopause for e90 using tropics‐plus‐midlatitudes and then apply it globally to calculate total tropospheric air‐mass and tropopause ozone values. The tropopause mixing barrier for the current UCI CTM is identified by a transition in the vertical transport gradient to stratospheric values of 15 days km⁻¹, corresponding to an e90 tropopause at 81 ± 2 ppb with a global tropospheric air mass of 82.2 ± 0.3%. The best e90 tropopause based on sonde pressures is 70–80 ppb; but that for ozone is 80–90 ppb, implying that the CTM tropopause ozone values are too large. This approach of calibrating an age‐of‐air tropopause can be readily applied to other models and possibly used with observed age‐of‐air tracers like sulfur hexafluoride. 

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3. Tropospheric Thermal Forcing of the Stratosphere through Quasi-Balanced Dynamics

   https://doi.org/10.1175/JAS-D-23-0081.1  

   Lin, Jonathan; Emanuel, Kerry (March 2024, Journal of the Atmospheric Sciences)

   The steady response of the stratosphere to tropospheric thermal forcing via an SST perturbation is considered in two separate theoretical models. It is first shown that an SST anomaly imposes a geopotential anomaly at the tropopause. Solutions to the linearized quasigeostrophic potential vorticity equations are then used to show that the vertical length scale of a tropopause geopotential anomaly is initially shallow, but significantly increased by diabatic heating from radiative relaxation. This process is a quasi-balanced response of the stratosphere to tropospheric forcing. A previously developed, coupled troposphere–stratosphere model is then introduced and modified. Solutions under steady, zonally symmetric SST forcing in the linearβ-plane model show that the upward stratospheric penetration of the corresponding tropopause geopotential anomaly is controlled by two nondimensional parameters: 1) a dynamical aspect ratio and 2) a ratio between tropospheric and stratospheric drag. The meridional scale of the SST anomaly, radiative relaxation rate, and wave drag all significantly modulate these nondimensional parameters. Under Earthlike estimates of the nondimensional parameters, the theoretical model predicts stratospheric temperature anomalies 2–3 larger in magnitude than that in the boundary layer, approximately in line with observational data. Using reanalysis data, the spatial variability of temperature anomalies in the troposphere is shown to have remarkable coherence with that of the lower stratosphere, which further supports the existence of a quasi-balanced response of the stratosphere to SST forcing. These findings suggest that besides mechanical and radiative forcing, there is a third way the stratosphere can be forced—through the tropopause via tropospheric thermal forcing. Significance StatementUpward motion in the tropical stratosphere, the layer of atmosphere above where most weather occurs, is thought to be controlled by weather disturbances that propagate upward and dissipate in the stratosphere. The strength of this upward motion is important since it sets the global distribution of ozone. We formulate and use simple mathematical models to show the vertical motion in the stratosphere can also depend on the warming in the troposphere, the layer of atmosphere where humans live. We use the theory as an explanation for our observations of inverse correlations between the ocean temperature and the stratosphere temperature. These findings suggest that local stratospheric cooling may be coupled to local tropospheric warming. 

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4. Revisiting the Stratosphere–Troposphere Exchange of Air Mass and Ozone Based on Reanalyses and Observations

   https://doi.org/10.3390/atmos16091050  

   Hall, Anna; Fu, Qiang; Dong, Cong (September 2025, Atmosphere)

   Our previous study examined the stratosphere-troposphere exchange (STE) of air mass and ozone using ERA5 and MERRA2 reanalysis data and observations for 2007–2010. Their analysis applied a lower stratosphere mass budget approach, with the 380 K isentropic surface serving as the upper boundary of the lowermost stratosphere. This study employs a dynamic isentropic surface fitted to the tropical tropopause, providing an update to the results using the static 380 K boundary. Additionally, we improve the numerical scheme for deriving the mass of the lowermost stratosphere. Under this new framework, the air mass upward flux at the isentropic surface in the tropics increases from 19.3 × 109, 19.3 × 109, and 22.0 × 109 kg s−1 in our previous study to 21.9 × 109, 20.9 × 109, and 26.3 × 109 kg s−1 in the present study for ERA5, MERRA2, and observations, respectively. The global ozone fluxes across the fitted isentrope become −347.6, −362.5 and −368.4 Tg yr−1 as compared to −345.7, −359.5 and −335.6 Tg yr−1 at the 380 K level from our previous study for ERA5, MERRA2 and observations, respectively. The corresponding extratropical ozone fluxes are −539.3, −541.3 and −565.5 Tg yr−1 versus previous estimates of −538.1, −542.5 and −527.8 Tg yr−1. The increased role of tropical cirrus clouds near the tropopause is also highlighted under the updated framework in observations. The contribution of cloud heating to tropical air mass flux increases from 2.0% in our previous study to 8.2% in the present analysis, while for ozone, the corresponding contribution increases from 1.8% to 8.1%. We further show that the improved estimate of the change rate of mass in the lowermost stratosphere has an impact on seasonal ozone STE results from chemistry climate models presented in another of our previous studies. These findings provide new insights into the processes governing stratosphere-troposphere exchange. 

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5. Characterizing variability and vertical structure of water vapor in the extratropical lower stratosphere

   https://doi.org/10.5194/egusphere-2026-412  

   Tinney, Emily Nicole; Randel, William J (February 2026, EGUsphere)

   Abstract. Stratospheric water vapor strongly affects Earth’s radiative balance, especially in the extratropical lower stratosphere, yet large uncertainties remain regarding its variability, long-term trends and relationships to tropopause behavior. Here we characterize the seasonality, vertical structure, and variability of water vapor near the extratropical tropopause and the extratropical transition layer (ExTL). We analyze 17 years (2005–2021) of satellite observations from the Aura Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS), with simulations from the Whole Atmosphere Community Climate Model (WACCM) using specified dynamics calculations. Analyses are performed in geometric and tropopause-relative vertical coordinates to assess the influence of coordinate on variability. Tropopause-relative coordinates substantially reduce variance in ExTL water vapor, but artificially enhance variability in the tropical upper troposphere. Tropopause-relative coordinates reveal a clear distinction between variability in the ExTL and the stratospheric overworld approximately 2.5 km above the tropopause. Above this level, there are hemispherically coherent anomalies linked to transport of air originating in the tropics. In contrast, ExTL variability lacks hemispheric coherence and likely reflects smaller-scale stratosphere-troposphere exchange. Interannual changes in ExTL water vapor show a statistically significant negative trend in both hemispheres in MLS data, but that behavior is not reproduced in ACE-FTS measurements or WACCM simulation. Despite this, the datasets agree well on overworld variability and regression responses to climate oscillations. The uncertainty in ExTL trends highlights the need for continued evaluation of long-term satellite records and improved model representation in this climatically sensitive layer. 

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