Adjustments to an abrupt solar forcing in the CMIP6 abrupt-solm4p experiment

Abstract. The concept of "radiative" or "rapid" adjustments refers to the response of the climate system to an instantaneous radiative forcing, independent of surface temperature changes. These adjustments can occur over time scales from hours (e.g. aerosol-cloud-interactions) to months (e.g. stratospheric temperature changes) or even longer, making it challenging to distinguish adjustments from feedbacks over longer time scales. Despite variations in definitions, understanding these processes is essential for advancing climate modeling.

Strong volcanic eruptions, which produce scattering aerosol layers in the stratosphere, offer natural experiments to study short-term adjustments. However, the gradual spread of aerosols over the globe over months complicates analysis, requiring more controlled experiments. The abrupt-solm4p experiment within the Cloud Feedback Model Intercomparison Project (CFMIP) as part of the 6th Coupled Model Intercomparison Project (CMIP6) simulates an instantaneous 4 % reduction in the solar constant, starting from a pre-industrial run on 1 January 1850. This study analyzed changes in climate variables, cloud properties, and radiative fluxes over different time scales (hours, days, months and up to 150 years) to understand adjustment processes.

Four models were evaluated, showing initial rapid cooling, particularly over Antarctica and the southern hemisphere, slowing down the polar night jet, disrupting the polar vortex and increasing Arctic cloud cover. During the first month, the troposphere cools down faster than the ocean surface, decreasing vertical stability and increasing cloud cover over ocean, while the opposite effect happens in the tropics over land. This in turn affects land-sea-circulation. On longer time scales we find robust changes of cloudiness.