Quantifying the Contributions of the Stratospheric and Tropospheric Pathways toward the Acceleration of the Stratospheric Polar Vortex after Nuclear War

Nuclear war simulations using the Community Earth System Model with the Whole Atmosphere Community Climate Model version 4 (CESM-WACCM4) produce nuclear winter-like conditions following the injection of 150 Tg of black carbon into the upper troposphere and lower stratosphere over the United States and Russia. Significant global cooling occurs for five India-Pakistan nuclear war scenarios with black carbon amounts of 5 Tg, 16 Tg, 27 Tg, 36 Tg, 46 Tg as well. Contrary to the global cooling pattern, warming in Eurasia is observed in all the simulations during the first or second winter as a result of a strongly positive Arctic Oscillation (AO) during the winter months of all the simulations. Similar to the effect of large volcanic eruptions, intense heating of the aerosols in the stratosphere strengthens the pole-to-equator temperature gradient during boreal winter, contributing to an anomalous westerly wind response in the stratosphere. At higher latitudes, this signal propagates down into the troposphere and promotes a positive, amplified AO pattern that advects warm, oceanic air over northern Eurasia. In addition to this stratospheric mechanism, it has been theorized for large tropical volcanic eruptions that a tropospheric pathway could play a role in strengthening the stratospheric polar vortex through a reduction in planetary wave activity from a reduction in baroclinic instability. Observational evidence of this mechanism after volcanic eruptions has been elusive, as planetary wave activity increased after the three largest volcanic eruptions of the 20th century. The results from modeling experiments of volcanic eruptions have also been contradictory. Because it is still unclear if the tropospheric pathway for stratospheric polar vortex strengthening plays any role in smaller stratospheric aerosol injections, we amplify the signal of this potential mechanism by using nuclear war simulations with a very large forcing. With a high-top model that can resolve stratospheric processes well, we apply the radiative forcing in the nuclear war simulations already conducted into new runs with aerosol heating turned off, allowing the surface to cool without heating the stratosphere, isolating the tropospheric pathway. Then, we analyze changes to the stratospheric polar vortex and Eliassen-Palm flux under these surface-cooling-only cases and compare it to the cases with aerosol heating. The difference between the two will reveal the contribution, if any, of the tropospheric pathway to the strengthening of the stratospheric polar vortex for different negative radiative forcings, and will provide insight into interactions between the two pathways. This allows us to assess its importance relative to the stratospheric pathway, which can be broadly applicable to smaller aerosol loadings and can inform our expectations of the changes in Northern Hemisphere wintertime circulation after the next large volcanic eruption.