Rapid adjustments after volcanic eruptions - A stepwise approach towards a better understanding of short-term adjustments in climate models

"Radiative" or "rapid" adjustments refer to the climate system's responses to an instantaneous radiative forcing, which are independent of surface temperature changes. They occur on timescales from hours to months or even longer, making it difficult to distinguish them from feedbacks. Despite variations in definitions, understanding these processes is essential for advancing climate modeling.

Volcanic eruptions, which introduce scattering aerosol to the stratosphere, serve as natural experiments for studying short-term adjustments. However, the gradual global spread of aerosols during a volcanic eruption complicates analysis. To address this, we took a stepwise approach, starting with idealized model simulations, gradually increasing complexity and finally comparing model results with satellite measurements of the Mt. Pinatubo eruption in 1991. We analyzed data of the abrupt-solm4p experiment from the Cloud Feedback Model Intercomparison Project (CFMIP) within the 6th Coupled Model Intercomparison Project (CMIP6). This experiment simulates a 4% reduction in the solar constant. Additionally, we analyzed an MPI-ESM 1.2 model experiment with both absorbing and non-absorbing stratospheric aerosol layers, using fixed and fully coupled sea surface temperatures. Moreover, results from the volc-pinatubo-full experiment of the CMIP6 Volcanic MIP (VolMIP) were considered, simulating a Pinatubo like eruption, but initializing it from different years of the control run to account for climate variability. Finally, model results were compared to ERA5 reanalysis data of the Pinatubo eruption in 1991.

This study focused on changes in climate variables, cloud properties, and radiative fluxes during the first year after the onset of forcing. All model experiments were initialized on January 1st as the start of forcing, while ERA5 data was used from January 1st, 1992, onwards, since volcanic forcing from the Mt. Pinatubo eruption was strongest at that point.

Results show rapid cooling in the troposphere, especially over Antarctica and the Southern Hemisphere. In contrast, the stratosphere warms significantly when absorbing aerosol is present in the stratopshere. These temperature changes affect the jet streams, as well as the polar night jet, leading to a disruption of the polar vortex and consequently increased surface temperature in the Arctic. Within the first month, the troposphere cools faster than the ocean surface, reducing vertical stability and increasing relative humidity over the ocean. Conversely, over land in the tropics, the opposite effect occurs, influencing land-sea circulation.