Robust winter warming over Eurasia under stratospheric sulfate geoengineering – the role of stratospheric dynamics

Abstract

Abstract. It has been suggested that increased stratospheric sulfate aerosol loadings following large, low latitude volcanic eruptions can lead to wintertime warming over Eurasia through dynamical stratosphere–troposphere coupling. We here investigate the proposed connection in the context of hypothetical future stratospheric sulfate geoengineering in the Geoengineering Large Ensemble simulations. In those geoengineering simulations, we find that stratospheric circulation anomalies that resemble the positive phase of the Northern Annular Mode in winter are a distinguishing climate response which is absent when increasing greenhouse gases alone are prescribed. This stratospheric dynamical response projects onto the positive phase of the North Atlantic Oscillation, leading to associated side effects of this climate intervention strategy, such as continental Eurasian warming and precipitation changes. Seasonality is a key signature of the dynamically driven surface response. We find an opposite response of the North Atlantic Oscillation in summer, when no dynamical role of the stratosphere is expected. The robustness of the wintertime forced response stands in contrast to previously proposed volcanic responses.

The impact of stratospheric aerosol intervention on the North Atlantic and Quasi-Biennial Oscillations in the Geoengineering Model Intercomparison Project (GeoMIP) G6sulfur experiment

“…The issue of wintertime precipitation changes driven by changes in the phase of the NAO and the associated changes in the strength and position of the North Atlantic storm-track are unlikely to be solved by injecting SO2 using more sophisticated geographic injection strategies owing to the relatively long lifetime of the sulphate aerosols in the stratosphere during which they may be transported (and induce stratospheric heating) far from the injection site. Simpson et al (2018) and Banerjee et al (2021) both performed simulations using the Geoengineering Large Ensemble (Tilmes et al, 2018b) where the injections were performed at latitudes of 30° N, 15° N, 15° S and 30° S and both studies found a very similar forced positive phase of the DJF NAO to that found here in G6sulfur using equatorial injection. Simpson et al (2018) performed further experiments where the stratospheric heating was enhanced and found a stronger impact on the positive phase of the DJF NAO and the associated precipitation patterns, suggesting that the absorption of solar radiation at wavelengths greater than ~1.3 µm by stratospheric sulphate (Dykema et al, 2016) is the root cause of this response.…”

“…While varying in degree, all models show a clear warming over northern Eurasia consistent with the positive NAO anomaly (Hurrell, 1995;Shindell et al, 2004), although the warming is still less than in ssp585. They also show cooling over the Labrador Sea and warming over the eastern USA, again as expected from a long term positive shift in the NAO (Scaife et al, 2005), although the picture is less consistent for differences over North America (Banerjee et al, 2021;Jones et al, 2021). The corresponding differences in Northern Hemisphere wintertime precipitation are shown in Fig.…”

“…Although there are differences between volcanic eruptions and SAI, the most obvious of which is that explosive volcanic eruptions are sporadic while SAI is most frequently modelled using continuous emissions, there are obvious similarities between them and so two recent studies have examined the possible effects of SAI on the NAO. Jones et al (2021) used ensembles of three simulations from two Earth-system models while Banerjee et al (2021) used a 20member ensemble from a single model; both concluded that SAI could induce a significant positive anomaly in the NAO.…”

Volcanic stratospheric injections up to 160 Tg(S) yield a Eurasian winter warming indistinguishable from internal variability

“…At this point then, one is inevitably led to ask: if Pinatubo and Krakatau are not large enough, how large does an eruption need to be to cause wintertime surface warming at Northern mid-latitudes? At the upper bound, in a geoengineering context, it has recently been shown that large and sustained stratospheric sulfate injections do indeed produce wintertime warming over Eurasia (Kravitz et al, 2017, see their Figure 8, bottom right panel), and this surface warming (which is absent in the summer months) has been linked to stratosphere-troposphere dynamical coupling affecting the NAO (Banerjee et al, 2020). DallaSanta et al (2019) also found a robust impact of sustained lower stratospheric tropical warming on the NAO, using an idealized model.…”

Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations

“…As mentioned before, the presence of aerosols in the stratosphere also produces a perturbation of stratospheric dynamics (Richter et al, 2017;Visioni et al, 2020a) that, in turn, might affect precipitation (Simpson et al, 2019) and temperature (Jiang et al, 2019) at the surface. The response is driven by the absorption of infrared radiation by the aerosols resulting in the heating of the stratospheric air and is thus dependent on the overall burden and the size of the particles (Pitari et al, 2016) but also on interactions with the chemical cycles in the stratosphere (Visioni et al, 2017b;Richter et al, 2017) and the incursion of water vapor from the troposphere due to the warming of the tropopause layer (Visioni et al, 2017b;Tilmes et al, 2018b;Boucher et al, 2017). In Fig.…”