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.
The Arctic stratospheric polar vortex is an important driver of winter weather and climate variability and predictability in North America and Eurasia, with a downward influence that on average projects onto the North Atlantic Oscillation (NAO). While tropospheric circulation anomalies accompanying anomalous vortex states display substantial case‐by‐case variability, understanding the full diversity of the surface signatures requires larger sample sizes than those available from reanalyses. Here, we first show that a large ensemble of seasonal hindcasts realistically reproduces the observed average surface signatures for weak and strong vortex winters and produces sufficient spread for single ensemble members to be considered as alternative realizations. We then use the ensemble to analyze the diversity of surface signatures during weak and strong vortex winters. Over Eurasia, relatively few weak vortex winters are associated with large‐scale cold conditions, suggesting that the strength of the observed cold signature could be inflated due to insufficient sampling. For both weak and strong vortex winters, the canonical temperature pattern in Eurasia only clearly arises when North Atlantic sea surface temperatures are in phase with the NAO. Over North America, while the main driver of interannual winter temperature variability is the El Niño–Southern Oscillation (ENSO), the stratosphere can modulate ENSO teleconnections, affecting temperature and circulation anomalies over North America and downstream. These findings confirm that anomalous vortex states are associated with a broad spectrum of surface climate anomalies on the seasonal scale, which may not be fully captured by the small observational sample size.more » « less
- NSF-PAR ID:
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Medium: X
- Sponsoring Org:
- National Science Foundation
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