The Strength of Low-Cloud Feedbacks in Response to CO2 Induced Warming and Tropical Climate: A CESM Sensitivity Study

Variability in the strength of low-cloud feedbacks across climate models is the primary contributor to the spread in their estimates of equilibrium climate sensitivity (ECS). This raises the question: What are the regional implications for key feature of tropical climate of globally weak versus strong low-cloud feedbacks in response to greenhouse gas induced warming. To address this question and formalize our understanding of cloud controls on tropical climate, we perform a suite of idealized fully-coupled and slab-ocean climate simulations across which we systematically scale the strength of the low-cloud feedback within a single model thereby isolating the impact of low-cloud feedback strength. The feedback strength is varied by modifying the low, layered and marine stratus cloud fraction so that it is a function of not only local conditions but also global temperature in a series of abrupt 2xCO2 sensitivity experiments. The unperturbed decrease in low cloud cover (LCC) under 2xCO2 is greatest in the mid- and high-latitude oceans, and subtropical eastern Pacific and Atlantic, a pattern that is magnified as the feedback strength is scaled. Consequently, SST increases more in these regions as well as Pacific cold tongue. As the strength of the low-cloud feedback increases this results in not only increased ECS, but also an enhanced reduction of the large-scale zonal and meridional SST gradients (structural climate sensitivity), with implications for the atmospheric Hadley and Walker Circulations as well as the hydrological cycle.