Atmospheric Rivers and Historic California Floods: A Look in the Past and a Peek in the Future

The extent to which Atmospheric Rivers (ARs) have been responsible for historic floods in California is examined using a new catalog of ARs that made landfall at the west coast of North America during 1851-2012. This catalog based on the NOAA 20th Century Reanalysis V2c (SIO-N2) provides a large array of variables that can be used to examine AR cases and their climate-scale variability in exceptional detail. The new record of AR activity provides a perspective on the seasonal - inter-decadal variability of AR activity affecting the hydroclimate of the North American West. AR links to extreme precipitation and flooding are demonstrated using high-resolution observational precipitation data and an extensive record of historic floods compiled by the California Department of Water Resources and the US Army Corps of Engineers. We will discuss the climatic links of AR activity and flooding as well as the detailed role of ARs in driving specific historical floods. Peeking into the future, we apply our automated AR Detection Scheme to choice CMIP5 GCMs and study modeled ARs in conjunction with statistically downscaled precipitation, we validate AR activity diagnosed in GCMs against that derived from Reanalyses as follows. We assess the realism of the seasonal march of AR landfalls from the Gulf of Alaska coastline in late summer-early fall to California by the late fall-early winter as well as the contribution of AR-related precipitation to seasonal totals and especially extremes. We also validate linkages between seasonal AR activity and climate variability expressed in Pacific sea surface temperatures. Using the most successful GCMs, we will assess projected trends in land-falling AR behavior along with their contribution to projected extreme precipitation trends over California. In California's Mediterranean climate, the precipitation regime is projected to progressively favor extreme precipitation events at the expense of the moderate events. Meanwhile, the atmospheric branch of the hydrologic cycle is projected to intensify. We verify that Atmospheric Rivers (ARs) provide the essential link connecting these trends, signifying increasing propensity for extreme precipitation and flooding against a progressively drier regional hydroclimate.