The Circulation Response to Volcanic Eruptions: The Key Roles of Stratospheric Warming and Eddy Interactions

Abstract

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.

The Regional Hydroclimate Response to Stratospheric Sulfate Geoengineering and the Role of Stratospheric Heating

“…However, for the poleward shifting of the westerlies, we can draw from previous literature. A poleward shifting of the midlatitude westerlies is a typical response to stratospheric heating and/or strengthening of the polar vortex in idealized modeling studies (DallaSanta et al, 2019;Haigh et al, 2005;Polvani & Kushner, 2002;Simpson et al, 2009), and a localized poleward shift in the North Atlantic westerlies occurs when the stratospheric vortex is anomalously strong in observed, interannual variability (Baldwin & Dunkerton, 2001). Mechanisms have been proposed, involving the response of transient eddies (e.g., Simpson et al, 2009;Wittman et al, 2007) or planetary scale waves (e.g., Song & Robinson, 2004) to the stratospheric forcing in these contexts.…”

Evaluating the simulated radiative forcings, aerosol properties, and stratospheric warmings from the 1963 Mt Agung, 1982 El Chichón, and 1991 Mt Pinatubo volcanic aerosol clouds

“…For the tropics, extinction comparisons are shown at 24, 28, and 32 km, whereas for SH (35-60 • S) and NH (35-60 • N) mid-latitudes the chosen levels are 20, 24, and 28 km to account for the higher tropical tropopause. Simulated extinctions are compared with raw SAGE v7.0 data (Damadeo et al, 2013) as well as the gapfilled extinction from CMIP6-GloSSAC at 525 nm. As discussed previously, extinctions from Pin14 (and to some extent Pin10) show much better agreement with observational data for all three latitude bands.…”

The unidentified volcanic eruption of 1809: why it remains a climatic cold case

“…Significant cooling south of 30 • S appears only in austral spring 1809. except for Low, where warming anomalies are instead found over the polar ocean, and it is most pronounced in High but also quite extensive in nNHP. Such NH winter warming pattern is known to be induced by atmospheric circulation changes (e.g., Wunderlich and Mitchell, 2017;DallaSanta et al, 2019) and can occur in post-eruption winters as a dynamic response to the enhanced stratospheric aerosol layer, when it displays highly variable amplitude of local anomalies (Shindell et al, 2004). Accordingly, in our simulations the Eurasian winter warming pattern consists of one or two areas with positive temperature anomalies centered over various locations between Fennoscandia and the Central Siberian Plateau in the different simulations.…”