Atmospheric River frequency and strength in the Miocene over the North American west coast

Abstract

Atmospheric rivers (ARs) are essential to many communities, providing a seasonal source of rainfall, however, ARs are also hazardous through flooding and landslides. The double-sided nature of ARs mandates that we study them within the context of a warming climate. The Miocene Climatic Optimum (MCO; ~15Ma) provides a potential portrayal of a future warmer climate with CO₂ levels ranging from 400-1000 ppm and temperatures 7-10°C warmer than today. To understand changes in AR characteristics we compare AR frequency and strength between Preindustrial and a suite of MCO simulations with varying CO2 concentration (400 and 560 ppm) using the Community Earth System Model (CESM1.3). To track ARs we used the Image-Processing based Atmospheric River Tracking (IPART) algorithms. Specific focus was given to ARs around the western coast of North America during November, as ARs are highly prevalent and make up a majority source of precipitation for the western North American winter. Over a ten-year average we find an increase of ~31% in the number of AR events from Preindustrial to MCO conditions. We also find a ~10.4% increase in integrated water vapor transport (IVT) strength, indicating significant potential increases in AR-associated precipitation. This study shows how ARs behave under much warmer climate systems and provides insights into future AR trends.