Differences in the quasi-biennial oscillation response to stratospheric aerosol modification depending on injection strategy and species

Abstract

Abstract. A known adverse side effect of stratospheric aerosol modification (SAM) is the alteration of the quasi-biennial oscillation (QBO), which is caused by the stratospheric heating associated with an artificial aerosol layer. Multiple studies found the QBO to slow down or even completely vanish for point-like injections of SO2 at the Equator. The cause for this was found to be a modification of the thermal wind balance and a stronger tropical upwelling. For other injection strategies, different responses of the QBO have been observed. A theory which is able to explain those differences in a comprehensive manner has not yet been presented. This is further complicated by the fact that the simulated QBO response is highly sensitive to the used model even under identical boundary conditions. Therefore, within this study we investigate the response of the QBO to SAM for three different injection strategies (point-like injection at the Equator, point-like injection at 30∘ N and 30∘ S simultaneously, and areal injection into a 60∘ wide belt along the Equator). Our simulations confirm that the QBO response significantly depends on the injection location. Based on the thermal wind balance, we demonstrate that this dependency is explained by differences in the meridional structure of the aerosol-induced stratospheric warming, i.e., the location and meridional extension of the maximum warming. Additionally, we also tested two different injection species (SO2 and H2SO4). The QBO response is qualitatively similar for both investigated injection species. Comparing the results to corresponding results of a second model, we further demonstrate the generality of our theory as well as the importance of an interactive treatment of stratospheric ozone for the simulated QBO response.

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

“…Similar inter-model spreads have been reported in the recent literature for CMIP6 models for both effective equilibrium climate sensitivity (ECS; the equilibrium warming for a doubling of CO 2 ; see Zelinka et al, 2020) and transient climate response (TCR; the temperature warming with a doubling of CO 2 in a scenario with a 1 % per year CO 2 increase; see Meehl et al, 2020). Some models (amongst them CESM2(WACCM) and UKESM1-0-LL, also present in this study) have been found to have values well above previously established likely ranges for both ECS and TCR (Gettelman et al, 2019;Sherwood et al, 2020). Some of the relationships between the variables reported in Table 2 are explored in Fig.…”

A Model Intercomparison of Stratospheric Solar Geoengineering by Accumulation-Mode Sulfate Aerosols

“…The MAECHAM5-HAM (Niemeier et al, 2020;Niemeier and Timmreck, 2015;Stier et al, 2005) and SOCOL-AER 5 models (Feinberg et al, 2019;Stenke et al, 2013) share the same dynamical core from MAECHAM5 (Roeckner et al, 2003;Roeckner et al, 2006) and used the same horizontal resolution (T42 or 2.8°x2.8° in longitudes and latitudes) and model top (0.01 hPa or approximately 80 km). However, the MAECHAM5-HAM model uses 90 vertical levels and internally generates a quasi-biennial oscillation (QBO), which has been found in several studies to modify the effects of geoengineering injections (Aquila et al, 2014;Richter et al, 2017;Niemeier et al, 2020, Franke et al, 2021. The SOCOL-…”

“…The AM-H 2 SO 4 scenarios were designed to minimize the growth in average particle size as a function of injection rate, since sulfur is added to each grid box as particles of approximately 0.1 µm radius which grow mainly by coagulation. In addition, 10aerosol heating in the tropical lower stratosphere increases the strength of the Brewer-Dobson circulation, resulting in greater lofting in the tropical stratosphere which, for accumulation mode and smaller particles, (Niemeier et al, 2020), can prolong the residence time for 25 Tg(S) yr -1 injections relative to 5Tg(S) yr -1 injections though the details of this process are model dependent and include changes in the QBO for the CESM2 and MAECHAM5-HAM models (Franke et al, 2021).…”

“…Yet, stratospheric solar 15 geoengineering (SSG) scenarios differ from volcanic aerosol injections in that with SSG sulfur will presumably be injected into aircraft plumes, producing high local concentrations and strong gradients, and because emissions will be continuous in time, factors which will yield different microphysical behaviour (Heckendorn et al, 2009). Studies of SSG by injection of gas phase SO 2 have found limitations including: (1) reduced efficacy at higher loading due to unfavorable aerosol size distributions with possible limitations on achievable radiative forcing (Heckendorn et al, 20 2009;English et al, 2012;Niemeier and Timmreck, 2015), (2) depletion of stratospheric ozone (Tilmes et al 2009;Pitari et al 2014), (3) stratospheric heating which also perturbs stratospheric circulation and water vapor (Ferraro et al, 2011;Aquila et al, 2014;Richter et al, 2017;Niemeier amd Schmidt, 2017;Franke et al, 2021), (4) enhanced diffuse light at the Earth' surface (Kravitz et al, 2012), and (5) impacts on upper tropospheric ice clouds (Kuebbeler et al, 2012;Visioni et al, 2018a). These limitations might be addressed through use of various solid aerosol particles for SSG (e.g.…”

“…The modal models (CESM2 and MAECHAM5-HAM) input the AM-H 2 SO 4 particles into their accumulation-mode: for the CESM2 model, the input size distribution has a dry mode radius of 0.1 µm and wet mode radius of 0.12 µm with σ=1. set of model calculations (excluding SOCOL-AER due to lack of an internal QBO) has been analysed for changes in the QBO by Franke et al (2021). 10 Tg(S) yr -1 (optional)…”