Title: A high-latitude convective cloud feedback
Description: Available data suggest that during the late Cretaceous and early Paleogene (∼100 to ∼35 million years ago) Earth had an "equable" climate: the equator to pole surface temperature difference and seasonal cycle in high-latitude surface temperature were both much smaller than they are today. The combination of much warmer high latitudes, particularly during winter, and only somewhat warmer tropics has traditionally been difficult to model in global climate models and to explain physically. In this thesis a positive feedback on high-latitude surface temperatures based on the onset of convection and convective clouds is proposed and investigated using a hierarchy of climate models. This feedback mechanism is based on an initial warming leading to destabilization of the high-latitude atmosphere to convection, causing convection, which results in convective clouds and increased atmospheric moisture, both of which trap outgoing longwave radiation and lead to further warming. It is also shown that this convective cloud feedback could be active in a future atmosphere with increased greenhouse gasses, increasing high-latitude climate uncertainty under global warming scenarios.

A variety of climate models are used to understand the convective cloudfeedback in this thesis. Simple, analytical models are used in two chapters to understand more complex models and their underlying physics. A zonally-averaged, two-level model of the atmosphere without a seasonal cycle, but containing a hydrological cycle and parameterizations of convection, precipitation, and clouds, and a longwave radiation scheme that explicitly depends on CO 2 , water vapor, and cloud fraction is constructed and used as an initial test of the feedback mechanism. The National Center for Atmospheric Research (NCAR) single column model (SCAM), which contains state-of-the-art atmospheric physics parameterizations, high vertical resolution, a full seasonal cycle, a thermodynamic sea ice model, and a mixed layer ocean is used in two chapters. Finally, it is shown that the feedback is active at high carbon dioxide levels in some of the fully-coupled state-of-the-art ocean-atmosphere global climate models used in the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report.