Abstract: 

Climate model simulations are known to be highly sensitive to parameter choices in the sub-grid-scale representation of deep convection, as deep convection plays a critical role in the transport of heat and momentum globally. Over the years, it has also become evident that the intermodel spread in climate sensitivity is largely driven by uncertainty in the magnitude of the cloud feedback in the tropics, specifically the low cloud feedback. This begs the question: how do cloud feedbacks and climate sensitivity relate to deep convection? In this talk, I will present evidence that changes to tropical deep convection resulting in a reduction of high cloudiness and enhanced free-tropospheric drying (negative longwave feedback) can lead to a net positive cloud feedback in CMIP6 models by further reducing low cloudiness (positive shortwave feedback). I will discuss two physical mechanisms, related to changes in subsidence rate, that may explain this response. First, a reduction in tropical ascent area and an increased frequency of heavy precipitation result in high cloud reduction and free-tropospheric drying, which increases longwave cooling and reduces subsidence weakening, reducing low cloudiness (Radiation-Subsidence Pathway). Second, increased longwave cooling decreases tropospheric stability, which also reduces subsidence weakening and low cloudiness (Stability-Subsidence Pathway). Results from mechanism denial and perturbed physics ensemble experiments will also be discussed.