Low-frequency modes in internal variability of surface solar radiation

We investigate potential reasons for decadal-scale internal variability of surface solar radiation (SSR) using model data from the Coupled Model Intercomparison Project - Phase 6. We compare unforced coupled atmosphere-ocean (piControl) to atmosphere-only (piClim) simulations with prescribed climatological sea surface temperatures (SSTs) to access the relevance of SSTs for unforced SSR inter-annual variability. Further, the connection between SSTs and known climate modes of variability is exploited. We focus on coupled and ocean-only modes of variability such as El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Using the climate indices which describe these modes, we relate the SST field to SSR trends in different regions and we find a relationship between periods with strongly changing SSTs and decadal SSR depending on the regions. Unforced clear-sky SSR trends appear to mimic the SST trend pattern, while all-sky trends show a complex spatial structure with trends opposite in sign in different regions. These results are based only on pre-industrial control simulations (CMIP6 piControl) and can be used to infer in which direction internal variability has affected SSR in the historical period and whether it has enhanced or suppressed the anthropogenic signal from aerosols.