Research Highlights

The Dominant Role of the Summer Hemisphere in Subtropical Lower Stratospheric Wave Drag Trends

Abstract - 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.

Reference: Abalos, M., Randel, W. J., & Garcia, R. R. (2024). The dominant role of the summer hemisphere in subtropical lower stratospheric wave drag trends. Geophysical Research Letters, 51, e2023GL105827. doi:10.1029/2023GL105827

Potential Non-Linearities in the High Latitude Circulation and Ozone Response to Stratospheric Aerosol Injection

Abstract - The impacts of Stratospheric Aerosol Injection (SAI) on the atmosphere and surface climate depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. We use a set of CESM2(WACCM6) SAI simulations achieving three different levels of global mean surface cooling and demonstrate that unlike some direct surface climate impacts driven by the reflection of solar radiation by sulfate aerosols, the SAI-induced changes in the high latitude circulation and ozone are more complex and could be non-linear. This manifests in our simulations by disproportionally larger Antarctic springtime ozone loss, significantly larger intra-ensemble spread of the Arctic stratospheric jet and ozone responses, and non-linear impacts on the extratropical modes of surface climate variability under the strongest-cooling SAI scenario compared to the weakest one. These potential non-linearities may add to uncertainties in projections of regional surface impacts under SAI.

Reference - Bednarz, E. M., Visioni, D., Butler, A. H., Kravitz, B., MacMartin, D. G., & Tilmes, S. (2023). Potential non-linearities in the high latitude circulation and ozone response to stratospheric aerosol injection. Geophysical Research Letters, 50, e2023GL104726. doi:10.1029/2023GL104726

Climate, Variability, and Climate Sensitivity of “Middle Atmosphere” Chemistry Configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6))

Abstract - Simulating whole atmosphere dynamics, chemistry, and physics is computationally expensive. It can require high vertical resolution throughout the middle and upper atmosphere, as well as a comprehensive chemistry and aerosol scheme coupled to radiation physics. An unintentional outcome of the development of one of the most sophisticated and hence computationally expensive model configurations is that it often excludes a broad community of users with limited computational resources. Here, we analyze two configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6)) with simplified “middle atmosphere” chemistry at nominal 1 and 2° horizontal resolutions. Using observations, a reanalysis, and direct model comparisons, we find that these configurations generally reproduce the climate, variability, and climate sensitivity of the 1° nominal horizontal resolution configuration with comprehensive chemistry. While the background stratospheric aerosol optical depth is elevated in the middle atmosphere configurations as compared to the comprehensive chemistry configuration, it is comparable among all configurations during volcanic eruptions. For any purposes other than those needing an accurate representation of tropospheric organic chemistry and secondary organic aerosols, these simplified chemistry configurations deliver reliable simulations of the whole atmosphere that require 35% and 86% fewer computational resources at nominal 1 and 2° horizontal resolution, respectively.

Reference: Davis, N. A., Visioni, D., Garcia, R. R., Kinnison, D. E., Marsh, D. R., Mills, M., et al. (2023). Climate, variability, and climate sensitivity of “Middle Atmosphere” chemistry configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6)). Journal of Advances in Modeling Earth Systems, 15, e2022MS003579. doi:10.1029/2022MS003579

Modeling the Development of an Equatorial Plasma Bubble During a Midnight Temperature Maximum With SAMI3/WACCM-X

Abstract - We report results from a self-consistent global simulation model in which a large-scale equatorial plasma bubble (EPB) forms during a midnight temperature maximum (MTM). The global model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM-X. We consider solar minimum conditions for the month of August. We show that an EPB forms during an MTM in the Pacific sector and is caused by equatorward neutral wind flows. Although this is consistent with the theoretical result that a meridional neutral wind (V) with a negative gradient (∂V/∂θ < 0) is a destabilizing influence [Huba & Krall, 2013, https://doi.org/10.1002/grl.50292] (where a northward meridional neutral wind V is positive and θ is the latitude and increases in the northward direction), we find that the primary cause of the EPB is the large decrease in the Pedersen conductance caused by the equatorward winds.

Reference - Huba, J. D., Liu, H.-L., & McInerney, J. (2023). Modeling the development of an equatorial plasma bubble during a midnight temperature maximum with SAMI3/WACCM-X. Geophysical Research Letters, 50, e2023GL104388. doi:10.1029/2023GL104388

Stratospheric Climate Anomalies and Ozone Loss Caused by the Hunga Tonga-Hunga Ha'apai Volcanic Eruption

Abstract - The Hunga Tonga-Hunga Ha'apai (HTHH) volcanic eruption in January 2022 injected unprecedented amounts of water vapor (H₂O) and a moderate amount of the aerosol precursor sulfur dioxide (SO₂) into the Southern Hemisphere (SH) tropical stratosphere. The H₂O and aerosol perturbations have persisted during 2022 and early 2023 and dispersed throughout the atmosphere. Observations show large-scale SH stratospheric cooling, equatorward shift of the Antarctic polar vortex and slowing of the Brewer-Dobson circulation. Satellite observations show substantial ozone reductions over SH winter midlatitudes that coincide with the largest circulation anomalies. Chemistry-climate model simulations forced by realistic HTHH inputs of H₂O and SO₂ qualitatively reproduce the observed evolution of the H₂O and aerosol plumes over the first year, and the model exhibits stratospheric cooling, circulation changes and ozone effects similar to observed behavior. The agreement demonstrates that the observed stratospheric changes are caused by the HTHH volcanic influences.

Reference - Wang, X., Randel, W., Zhu, Y., Tilmes, S., Starr, J., Yu, W., et al. (2023). Stratospheric climate anomalies and ozone loss caused by the Hunga Tonga-Hunga Ha'apai volcanic eruption. Journal of Geophysical Research: Atmospheres, 128, e2023JD039480. doi:10.1029/2023JD039480

Community Earth System Model