Dynamics of and Precursors to California Megafloods, Present and Future

The coastal Pacific Southwest, including California, is characterized by a temperate “mediterranean” climate with a well-defined wet winter/dry summer seasonal cycle of precipitation. Because this region exists within a climatic transition zone between stable subtropical and active mid-latitude regimes, California is subject to high precipitation variability over a wide range of timescale linked to shifts in the jet stream and associated Pacific storm track. Recent evidence suggests that rapid swings between extreme wet and dry in California will occur with even greater frequency due greenhouse forcing, with a majority of this increase stemming from a substantial shift in the frequency of extreme wet events on sub-seasonal timescales. Indeed, global climate model ensemble simulations suggest a multi-fold increase in the risk of extremely severe multi-week storm sequences analogous to that which produced California’s “Great Flood of 1862,” a modern day repeat of which would likely have catastrophic consequences. Yet relatively little is known regarding the actual meteorology of such an event beyond what has been pieced together from historical accounts, as there are no instances of such an extreme (and, therefore, rare) event in the direct instrumental record. In this work, we build upon previous work using a large ensemble of climate model simulations to explore the large-scale dynamics of and hemispheric precursors to California megafloods, including how these characteristics may change in a warming climate. We also discuss initial findings regarding a targeted dynamical downscaling experiment aimed at better understanding the mesoscale characteristics of extremely prolonged precipitation events in this region.