Over the last few decades, the Arctic has warmed at a rate 3-4 times the global average, referred to as Arctic Amplification. The rate of amplified warming has been attributed to complex interactions amongst feedback processes, making it difficult to understand and isolate the leading causes. Due to this uncertainty, there is a large divide between model simulated changes in the Arctic and those in observations, undermining confidence in our ability to project future polar warming and its impacts. Thus, this research seeks to bridge the gap between climate models and observations by imposing essential observed conditions (e.g., circulation changes, aerosol emissions, etc.) in climate models to understand and quantify their roles in shaping various aspects of climate variability in the Arctic, including the warming rates of some key fields determining cryosphere conditions, extreme weather, energy budget, and climate states. Multiple versions of two climate models (CESM and E3SM) are used in my research, enabling me to investigate these topics with less impacts due to the sensitivity of a specific model to imposed forcing. According to this overarching goal, my overall effort is equally allocated to address the following issues, which are detailed in the subsequent chapters: Chapter 1 details the key motivations driving the research questions explored in the following chapters. The representation of internal variability in climate models, manifested as large-scale circulation, is a leading factor causing biases relative to observations. Chapter 2 addresses this effect of circulation on summer sea ice, employing an atmospheric wind nudging approach. This chapter characterizes the optimal large-scale wind pattern contributing to enhanced sea ice decline: a quasi-barotropic anticyclonic pattern with high pressure over the Arctic and Greenland that adiabatically warms the lower troposphere and increases downwelling longwave radiation. This pattern is found to coincide with periods of enhanced sea ice decline in preindustrial simulations and paleoclimate products, suggesting it is likely of internal origin. In Chapter 3, I examine the role of large-scale circulation in driving moisture transport into the Arctic and their radiative impacts during Northern Hemisphere summer. Using a combined nudging and moisture tagging approach in the iCESM1, it is found that the large-scale circulation drives an increase in atmospheric rivers which dominate high latitude moistening and the water vapor feedback. Two thirds of the of poleward transport passes through the high latitude land masses via a land capacitor effect, first originating from the tropical Atlantic and Mediterranean Sea. Chapter 4 investigates the response of extreme black carbon transport to large-scale circulation during the MOSAiC field campaign. Using statistical techniques and wind nudging in the E3SMv2 it is found that an Arctic Oscillation pattern is the leading determinant of poleward black carbon transport. Simulations with constrained circulation show improvements in mean poleward black carbon transport and occurrence of extreme events, but still underestimate the magnitude of transport. Altogether, this research addresses knowledge gaps pertaining to the uncertainties in the leading drivers of warming and sea ice loss between models and observations, identifying that many of the discrepancies are likely associated with the representation of large-scale circulation changes.
