Interplay between decreasing subsidence and stratocumulus-cumulus transitions

Stratocumulus (Sc) clouds have high impact on the climate system because of their strong cooling effect (albedo).
In the subtropical oceans they naturally break up into cumulus (Cu) as environmental conditions change, a phenomenon called Stratocumulus--Cumulus Transition (SCT). This results in significant loss cloudiness and thereby cooling.
To better understand the physical processes and environmental conditions influencing most this transition we develop a conceptual mixed layer model with dynamic cloudiness and sea surface temperature (SST).
In this model Sc and Cu states are two alternative attractors of the dynamics.
We tested various different parameterizations, and in most configurations the model broadly reproduces observed variability in cloud fraction, SST, heat fluxes, and moisture.
In this isolated system we robustly show that transitions between Sc and Cu are most sensitive to changes in circulation.
In climate change scenarios we show that SCT is enhanced due to direct radiative effects of increasing CO2 and due to weakening subtropical subsidence deepening the boundary layer. An accelerated SCT via weakening subisdence is a high strength positive feedback in the climate system based at its core on cloud-circulation coupling.
We close the talk by (1) highlighting the differences between linear and nonlinear response of clouds to weakening subsidence and (2) motivating the community to embrance high magnitude but low likely hood events as part of standard analyses of models.