North Pacific atmospheric rivers in a warming climate : rising temperatures, distinct flavors, and surface impacts
Keywords: Earth System Science.

Atmospheric rivers (ARs) are long, filamentary plumes of water vapor in motion. Colloquially known as "rivers in the sky", ARs live up to their name, as they transport more water than major global terrestrial rivers such as the Mississippi. ARs provide a large proportion of total precipitation to the West Coast states (Washington, Oregon, California), and an even larger proportion of extreme precipitation. Depending on its storm characteristics, (as well as its proximity in time to other ARs) an AR event may induce multiple compounding hazards—such as floods or landslides—and/or result in extensive water resource gains. Both the potential benefits and hazards motivate targeted research that bridges knowledge about AR processes and societally relevant surface impacts. This dissertation uses a combination of atmospheric reanalyses and station observations to characterize ARs landfalling along the West Coast of North America and their surface impacts. In doing so, it advances understanding of AR processes and characteristics in the observational era, as well as changes that have emerged over the past four decades. The first chapter provides the first quantification of AR temperature climatology and AR temperature trends. The chapter documents that West Coast ARs have warmed as much as 1.7°C in some regions. The rates of AR warming are attributed to a combination of trends in background land temperatures and pre-landfall AR track temperatures. Because ARs make up 30-50% of annual West Coast precipitation, increasing AR temperatures have implications for the proportion of rain vs. snow, which in turn has important implications for water availability, floods, and rain-on-snow hazards. The second chapter identifies different "flavors" of ARs based on the characterization of AR moisture transport as either moisture- or wind-dominated. The chapter documents that these flavors of ARs induce different magnitudes of surface winds and precipitation. For example, wind-dominated ARs are generally associated with greater precipitation than moisture-dominated events, which is particularly apparent in high IVT events and over mountainous regions. These differences in surface impacts are linked to differences in the large-scale atmospheric environment associated with the flavors, such as large-scale geopotential height patterns at the time of landfall. Finally, the chapter documents that annual average AR moisture dominance has significantly increased in the Pacific Northwest region over the 1980-2016 study period. The final chapter builds upon Chapter 1 and probes more deeply into AR temperature, quantifying temperature evolution from origin to landfall for the full suite of ARs making landfall along the Pacific coast of North America during the satellite-era (1980-2017). This work quantifies the role of various origin conditions such as temperature, precipitable water, integrated moisture transport and origin location in shaping AR temperature evolution. Of these, origin location and origin temperature are particularly influential in determining an AR's temperature evolution profile. Chapter 3 also investigates events associated with an extratropically-transitioning tropical cyclone ("ET-ARs"), which have been previously identified as a potentially hydroclimatically important subset of ARs. The chapter documents that although ET-ARs do not exhibit a different model of AR temperature evolution (i.e., origin location and temperature are equally influential in ET-ARs as the rest of the population), the distribution does exhibit significantly warmer landfall temperatures for ARs making landfall in British Columbia and Alaska. This dissertation identifies processes and characteristics that have implications for AR temperature and associated surface impacts (such as extreme winds and precipitation). The findings reveal the inherent diversity of ARs, including their characteristics and impacts at landfall (Chapters 1, 2), and the origin conditions and trajectory pathways that lead to these outcomes (Chapters 1, 3). In identifying pertinent AR characteristics and whether there have been changes during the historical era, these findings also provide a foundation for further research and future directions for bridging AR research to societal application