Atlantic SST variance changes in warmer climates: Atmospheric and oceanic mechanisms

Atlantic sea-surface temperature (SST) variability is a primary source of predictability in climate over the surrounding continents at seasonal to decadal timescales. However, there is little agreement across models on how it will change in the future. Here, we investigate changes in Atlantic SST variance by the end of the century under RCP8.5 forcing, using six individual-model initial condition large ensembles with 20-100 ensemble members each. By looking across ensemble members that differ only in their phasing of internal variability, we are able to accumulate sufficient statistics for time periods as short as a decade. Most but not all models show reduced amplitude SST variability in the warmer end-of-century climate, especially at decadal and longer timescales. We discuss the reasons for these changes and their differences across models, focusing on three main mechanisms of SST variance change: (i) changes in ocean mixed-layer depth, (ii) reduced forcing from atmospheric wind-stress and buoyancy forcing (which is a robust response to global warming in CMIP-class models), and (iii) changes in the mean state and variability of the Atlantic Meridional Overturning Circulation. We also discuss these Atlantic SST variance changes in the context of global SST variance changes, and in particular the relationship with changes in ENSO. We use a simple thermodynamic model of the upper ocean to quantify the relative importance of these different mechanisms.