Understanding rapid adjustments to shortwave forcing: from idealized solar perturbations to model and observational analysis of the 1991 Pinatubo eruption

Rapid adjustments are a key component of effective radiative forcing, influencing both short- and long-term climate responses and prediction uncertainty. Volcanic eruptions act as “natural laboratories” for studying these adjustments, providing insights into the atmospheric and surface mechanisms that occur in response to sudden stratospheric aerosol perturbations.

To disentangle these responses from internal variability and anthropogenic trends, we adopt a stepwise approach, analysing six model and observational datasets that capture rapid adjustments to imposed negative shortwave forcing. These include idealized reduced-solar-constant datasets (abrupt-solm4p from CFMIP), idealized stratospheric aerosol layer simulations with non-absorbing and absorbing aerosols (provided by Moritz Günther), and fixed as well as fully coupled sea surface temperatures. Furthermore, the volc-pinatubo-full simulations from VolMIP, CMIP6 historical simulations, ERA5 reanalysis, and CLARA satellite observations were analysed.

Across these datasets, we identify characteristic adjustment patterns of radiative fluxes, temperature, circulation, and cloud properties on timescales of months to a year after peak forcing. In volcanic eruptions, stratospheric temperature and dynamical adjustments play a key role and are often closely coupled to tropospheric responses. Comparing idealized solar and aerosol forcing with realistic Pinatubo simulations and observations allows us to assess the extent to which simplified experiments capture essential adjustment patterns typical for volcanic eruptions.

Results reveal consistent vertical and regional adjustment fingerprints across datasets, while also highlighting model limitations. For example, due to low stratospheric resolution and simplified QBO parametrizations, models fail to reproduce the full stratospheric temperature response observed in ERA5, whereas observations are more strongly influenced by internal variability than ensemble-mean model results.

These findings demonstrate the value of volcanic eruptions as a useful tool for constraining rapid adjustments to shortwave forcing and for improving their representation in climate models.