Characteristics of southern California atmospheric rivers

Abstract

Atmospheric rivers (ARs) are channels of high water vapor flux that transport moisture from low to higher latitudes on synoptic timescales. In areas of topographical variability, ARs may lead to high-intensity precipitation due to orographic forcing. ARs landfalling along North America’s west coast are linked to extreme events including those leading to flooding and landslides. In southern California (SCA), proper AR forecasting is important for regional water resources as well as hazard mitigation and as the area’s annual precipitation totals occur from relatively few storms per season, any changes to storm frequency and/or intensity may have dramatic consequences. Yet, as most regional AR studies focus on the Pacific Northwest, there is little information about SCA ARs. We develop an algorithm to identify ARs landfalling on North America’s west coast between 1979 and 2013 within total precipitable water reanalysis fields. ARs are then categorized according to landfall region. To determine and differentiate the characteristics and spatial distributions of ARs affecting these areas, we examine lag composites of various atmospheric variables for each landfall region. SCA ARs differ from ARs landfalling farther north in the days prior to landfall with the position and amplitude of a trough offshore from the Asian continent and ridge over Alaska, as well as the displacement and eastward extension of the jet core that potentially guides AR moisture southwards. The relationships between AR landfalls and the El Nino/Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), and the Pacific/North American Teleconnection Pattern (PNA) are also investigated.