The Northern Hemisphere wintertime circulation response to volcanic eruptions has been explored extensively using general circulation models. In observations and some models, the response is characterized by an enhanced stratospheric polar vortex (SPV), a positive mode of the North Atlantic Oscillation (NAO), and warm surface temperatures during the winter over North America and Eurasia. A weak surface air temperature signal in previous studies has led to conflicting conclusions on the robustness of the response. Here, we use simulations with the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM) of six nuclear war scenarios to present a new perspective on the connection between stratospheric aerosol heating, the SPV, and the surface temperature response. We show that stratospheric aerosol heating by soot is the primary contributor to the SPV response in nuclear war simulations, which is coupled to the troposphere and projects as a positive mode of the NAO at the surface. Winter warming is observed across northern Eurasia, albeit poleward of the response after volcanic eruptions. We compare the results to simulations of volcanic eruptions and find that observed Eurasian warming in the first winter after the 1963 Agung, 1982 El Chichón, and 1991 Pinatubo volcanic eruptions is simulated with the NCAR CAM5 climate model only when tropical sea surface temperatures, including the observed El Niño, are specified along with the volcanic aerosols. This suggests an undiagnosed tropospheric mechanism connecting the tropics and high latitudes, as without specifying sea surface temperatures, internal variability dominates the simulated winter warming response after historical volcanic eruptions.
Proxy data and observations suggest that large tropical volcanic eruptions induce a poleward shift of the North Atlantic jet stream in boreal winter. However, there is far from universal agreement in models on this effect and its mechanism, and the possibilities of a corresponding jet shift in the Southern Hemisphere or the summer season have received little attention. Using a hierarchy of simplified atmospheric models, this study examines the impact of stratospheric aerosol on the extratropical circulation over the annual cycle. In particular, the models allow the separation of the dominant shortwave (surface cooling) and longwave (stratospheric warming) impacts of volcanic aerosol. It is found that stratospheric warming shifts the jet poleward in both the summer and winter hemispheres. The experiments cannot definitively rule out the role of surface cooling, but they provide no evidence that it shifts the jet poleward. Further study with simplified models demonstrates that the response to stratospheric warming is remarkably generic and does not depend critically on the boundary conditions (e.g., the planetary wave forcing) or the atmospheric physics (e.g., the treatment of radiative transfer and moist processes). It does, however, fundamentally involve both zonal-mean and eddy circulation feedbacks. The time scales, seasonality, and structure of the response provide further insight into the mechanism, as well as its connection to modes of intrinsic natural variability. These findings have implications for the interpretation of comprehensive model studies and for postvolcanic prediction.
more » « less- NSF-PAR ID:
- 10083843
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 32
- Issue:
- 4
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- p. 1101-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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