Collaborative Proposal: The Role of Arctic Amplification in Modifying Mid-latitude Atmospheric Circulation and Promoting Extreme Weather Events

Intellectual Merit: recent work has suggested and demonstrated a mechanism connecting Arctic Amplification (AA - enhanced Arctic warming) with an increased probability of extreme weather events in northern hemisphere mid-latitudes, as well as a feedback to arctic wind patterns that drive further ice loss. This study will extend this work using output from reanalyses, model simulations from the CMIP5 archive, and experimental CESM model runs to investigate how and why the ongoing loss of arctic sea ice and high-latitude terrestrial snow cover may cause mid-latitude weather patterns to change. Past and projected patterns in 500 hPa heights will be analyzed using a combination of Self-Organizing Maps (SOMs) 'a neural-network-based technique that reduces large data sets to their representative patterns' a new meridional circulation index, and an extreme weather index to address the hypothesis that Arctic Amplification of past and future global climate change promotes circulation patterns that favor extreme weather events in middle latitudes of the northern hemisphere. The PIs suggest that changes in the energy budget of the arctic surface, because of sea-ice loss and earlier snow melt on high-latitude land, reduce meridional thickness gradients, which induce generally weaker zonal flow aloft and higher amplitude circulation features. They think these upper-level flow patterns slow the eastward progression of large-scale atmospheric Rossby waves, favoring more persistent weather conditions that increase the probability of extreme weather such as cold-air outbreaks, heat waves, droughts, and heavy precipitation. They believe the primary physical mechanism driving the change is an enhanced and seasonally varying arctic heating: in fall/winter it is ocean-based associated with substantial sea ice loss, while in warmer months it is land-based due primarily to earlier snow melt and reduced soil moisture. Higher-amplitude flow trajectories may also exacerbate sea-ice and snow-cover loss, thereby constituting a positive feedback loop. Because extreme weather events, such as temperature and precipitation extremes, are often caused by high-amplitude, slow-moving upper-level circulation patterns, this analysis will focus on detecting and characterizing these patterns, interpreting the weather conditions associated with them, and identifying regions that are likely to be affected. This study attempts to connect arctic change with the rest of the global climate system. Broader Impacts: Changes in extreme weather as a result of greenhouse gas accumulation will directly and dramatically affect millions of people across the globe, thus a better understanding and more accurate future projection of changes in the frequency, location, and severity of droughts, floods, and temperature extremes are urgently needed by decision-makers at all levels of society. Preliminary work has already attracted world-wide attention via articles in prominent newspapers, radio interviews, interviews for the Weather Channel, magazine articles, and numerous online blogs. This will also be an opportunity for a new post-doc. The PIs will prepare a module for the NSF-supported 'Beyond Penguins and Polar Bears' project for K-5 teachers. Both PIs make frequent public and school presentations on climate-change related issues, and because extreme weather is so relevant to society at large, results from the proposed study will figure prominently into these presentations.