Journal Article
Atmospheric Chemistry and Physics, vol. 23, iss. 1, pp. 687-709, 2023
Authors
Ewa M. Bednarz, Daniele Visioni, Ben Kravitz, Andy Jones, James M. Haywood, Jadwiga Richter, Douglas G. MacMartin, Peter Braesicke
Abstract
Abstract. The paper constitutes Part 2 of a study performing a first systematic
inter-model comparison of the atmospheric responses to stratospheric aerosol
injection (SAI) at various single latitudes in the tropics, as simulated by
three state-of-the-art Earth system models – CESM2-WACCM6, UKESM1.0, and
GISS-E2.1-G. Building on Part 1 (Visioni et al., 2023) we demonstrate
the role of biases in the climatological circulation and specific aspects of
the model microphysics in driving the inter-model differences in the
simulated sulfate distributions. We then characterize the simulated changes
in stratospheric and free-tropospheric temperatures, ozone, water vapor, and
large-scale circulation, elucidating the role of the above aspects in
the surface SAI responses discussed in Part 1. We show that the differences in the aerosol spatial distribution can be
explained by the significantly faster shallow branches of the Brewer–Dobson
circulation in CESM2, a relatively isolated tropical pipe and older tropical
age of air in UKESM, and smaller aerosol sizes and relatively stronger
horizontal mixing (thus very young stratospheric age of air) in the two GISS
versions used. We also find a large spread in the magnitudes of the tropical
lower-stratospheric warming amongst the models, driven by microphysical,
chemical, and dynamical differences. These lead to large differences in
stratospheric water vapor responses, with significant increases in
stratospheric water vapor under SAI in CESM2 and GISS that were largely not
reproduced in UKESM. For ozone, good agreement was found in the tropical
stratosphere amongst the models with more complex microphysics, with lower
stratospheric ozone changes consistent with the SAI-induced modulation of
the large-scale circulation and the resulting changes in transport. In
contrast, we find a large inter-model spread in the Antarctic ozone
responses that can largely be explained by the differences in the simulated
latitudinal distributions of aerosols as well as the degree of
implementation of heterogeneous halogen chemistry on sulfate in the models. The use of GISS runs with bulk microphysics demonstrates the importance of
more detailed treatment of aerosol processes, with contrastingly different
stratospheric SAI responses to the models using the two-moment aerosol
treatment; however, some problems in halogen chemistry in GISS are also
identified that require further attention. Overall, our results contribute
to an increased understanding of the underlying physical mechanisms as well
as identifying and narrowing the uncertainty in model projections of climate
impacts from SAI.
