More Like this

1. 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.

   Upward 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.

    

   more » « less
2. Injection strategy – a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering

   https://doi.org/10.5194/acp-23-13665-2023  

   Bednarz, Ewa M. ; Butler, Amy H. ; Visioni, Daniele ; Zhang, Yan ; Kravitz, Ben ; MacMartin, Douglas G. ( January 2023 , Atmospheric Chemistry and Physics)

   Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that stratospheric aerosol injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far, most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections, namely an equatorial injection, an annual injection of equal amounts of SO2 at 15∘ N and 15∘ S, an annual injection of equal amounts of SO2 at 30∘ N and 30∘ S, and a polar strategy injecting SO2 at 60∘ N and 60∘ S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres, and changes in the speed of the residual circulation. These impacts tend to maximise under the equatorial injection strategy and become smaller as the aerosols are injected away from the Equator into the subtropics and higher latitudes. In conjunction with the differences in direct radiative impacts at the surface, these different stratospheric changes drive different impacts on the extratropical modes of variability (Northern and Southern Annular modes), including important consequences on the northern winter surface climate, and on the intensity of tropical tropospheric Walker and Hadley circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical, and chemical processes driving the atmospheric circulation and ozone response to SAI and lay the foundation for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.

    

   more » « less
3. Changes in Stratosphere‐Troposphere Exchange of Air Mass and Ozone Concentration in CCMI Models From 1960 to 2099

   https://doi.org/10.1029/2023JD038487  

   Wang, Mingcheng ; Fu, Qiang ( July 2023 , Journal of Geophysical Research: Atmospheres)

   This study investigates changes in stratosphere‐troposphere exchange (STE) of air masses and ozone concentrations from 1960 to 2099 using multiple model simulations from Chemistry Climate Model Initiative (CCMI) under climate change scenario RCP6.0. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to the tropical tropopause as the upper boundary of lowermost stratosphere. The multi‐model mean (MMM) trends of air mass STEs are all small over all regions, which are within 0.3 (0.1) % decade⁻¹for 1960–2000 (2000–2099). The MMM trends of ozone STE for 1960–2000 are 0.3%, −2.7%, 3.4%, −0.9%, and −2.7% decade⁻¹over the Northern hemisphere (NH) extratropics, Southern hemisphere (SH) extratropics, tropics, extratropics, and globe, respectively. The corresponding ozone STE trends for 2000–2099 are 3.0%, 4.3%, 0.8%, 3.5%, and 4.7% decade⁻¹. Changes in ozone STEs are dominated by ozone concentration changes, driven by climate‐induced changes and ozone‐depleting substance (ODS) changes. For 1960–2000, small changes in ozone STEs in the NH extratropics are due to a cancellation between effects of climate‐induced changes and ODS increases, while the ODS effect dominates in the SH extratropics, leading to a large ozone STE magnitude decrease. Increased ozone transport from tropical troposphere to stratosphere for 1960–2000 is due to increased tropospheric ozone. A decreased global ozone STE magnitude for 1960–2000 was largely caused by ODS‐induced ozone loss that is partly compensated by climate‐induced ozone changes. For 2000–2099, about two‐thirds of global ozone STE magnitude increases are caused by ozone increases in the extratropical lower stratosphere due to climate‐induced changes. The remaining one‐third is caused by ozone recovery due to the phaseout of ODS.

    

   more » « less
4. Using climate model simulations to constrain observations

   Santer, B. D. ; Po-Chedley, S. ; Mears, C. ; Fyfe, J. C. ; Gillett, N. ; Fu, Q. ; Painter, J. F. ; Solomon, S. ; Steiner, A. K. ; Wentz, F.J. ; et al ( January 2021 , Journal of climate)

   null (Ed.)

   We compare atmospheric temperature changes in satellite data and in older and newer multi-model and single-model ensembles performed under phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). In the lower stratosphere, multi-decadal stratospheric cooling during the period of strong ozone depletion is smaller in newer CMIP6 simulations than in CMIP5 or satellite data. In the troposphere, however, despite differences in the forcings and climate sensitivity of the CMIP5 and CMIP6 ensembles, their ensemble-average global warming over the satellite era is remarkably similar. We also examine four well-understood properties of tropical behavior governed by basic physical processes. The first three properties are ratios between trends in water vapor (WV) and trends in sea surface temperature (SST), the temperature of the lower troposphere (TLT), and the temperature of the mid- to upper troposphere (TMT). The fourth property is the ratio between TMT and SST trends. All four trend ratios are tightly constrained in CMIP simulations. Observed ratios diverge markedly when calculated with SST, TLT, and TMT trends produced by different groups. Observed data sets with larger warming of the tropical ocean surface and tropical troposphere yield atmospheric moistening that is closer to model results. For the TMT/SST ratio, model-data consistency depends on the selected combination of observed data sets used to estimate TMT and SST trends. If model expectations of these four covariance relationships are realistic, one interpretation of our findings is that they reflect a systematic low bias in satellite tropospheric temperature trends. Alternately, the observed atmospheric moistening signal may be overestimated. Given the large structural uncertainties in observed tropical TMT and SST trends, and because satellite WV data are available from one group only, it is difficult to determine which interpretation is more credible. Nevertheless, our analysis illustrates the diagnostic power of simultaneously considering multiple complementary variables and points towards possible problems with certain observed data sets. 

   more » « less
5. East Asian summer monsoon delivers large abundances of very short-lived organic chlorine substances to the lower stratosphere

   https://doi.org/10.1073/pnas.2318716121  

   Pan, Laura L. ; Atlas, Elliot L. ; Honomichl, Shawn B. ; Smith, Warren P. ; Kinnison, Douglas E. ; Solomon, Susan ; Santee, Michelle L. ; Saiz-Lopez, Alfonso ; Laube, Johannes C. ; Wang, Bin ; et al ( March 2024 , Proceedings of the National Academy of Sciences)

   Deep convection in the Asian summer monsoon is a significant transport process for lifting pollutants from the planetary boundary layer to the tropopause level. This process enables efficient injection into the stratosphere of reactive species such as chlorinated very short-lived substances (Cl-VSLSs) that deplete ozone. Past studies of convective transport associated with the Asian summer monsoon have focused mostly on the south Asian summer monsoon. Airborne observations reported in this work identify the East Asian summer monsoon convection as an effective transport pathway that carried record-breaking levels of ozone-depleting Cl-VSLSs (mean organic chlorine from these VSLSs ~500 ppt) to the base of the stratosphere. These unique observations show total organic chlorine from VSLSs in the lower stratosphere over the Asian monsoon tropopause to be more than twice that previously reported over the tropical tropopause. Considering the recently observed increase in Cl-VSLS emissions and the ongoing strengthening of the East Asian summer monsoon under global warming, our results highlight that a reevaluation of the contribution of Cl-VSLS injection via the Asian monsoon to the total stratospheric chlorine budget is warranted.

    

   more » « less