PNNL

The Science

California has a Mediterranean-type climate with temperate wet winter and warm dry summer, featuring a pronounced wet season from October to April. In the warmer future, a distinctive wetter winter but drier spring and fall signify a sharpened seasonal cycle in mean precipitation, marked by a stronger but shorter wet season over California. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory led a study to explore the contributions of extreme and non-extreme precipitation in the sharpened seasonal cycle over California. Although both extreme and non-extreme precipitation display a sharpened seasonal cycle, increased extreme precipitation due to enhanced intensity and frequency dominates the wetter winter, while decreased non-extreme precipitation due to fewer wet days induces drier spring and fall in a warmer climate. Consistent with the seasonality change in extreme precipitation, atmospheric river frequency also shows a sharpened seasonal cycle under climate warming, suggesting potential causal relationship that deserves further investigation.

The Impact

Extreme precipitation has substantial socioeconomic consequences through its impacts on flash flooding, landslides, strong winds, and agriculture and food production. California has experienced more extreme precipitation events in recent decades so there is an urgent need for improving understanding of how extreme precipitation may change under warming. Previous studies highlighted the large uncertainty in projecting precipitation changes in California. However, by focusing on the seasonal cycle, this study identified robust changes in a multi-model ensemble of future projections, revealing a sharpened seasonal cycle of both extreme and non-extreme precipitation that has significant implications for flooding, drought, and wildfires in California.

Summary

In this study, researchers identified the seasonality changes in extreme precipitation over California under warming. They focused on the relative contributions of extreme and non-extreme precipitation, in terms of both intensity and frequency, to the sharpened seasonal cycle by analyzing historical and future climate simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive. The team found that increases in both extreme and non-extreme precipitation contribute to the wetter winter, while decreases in non-extreme precipitation dominate the drier spring and fall. In particular, the increased extreme precipitation in winter comes from both enhanced intensity and frequency, while the decreased non-extreme precipitation in spring and fall mainly comes from fewer wet days.

Scientists also investigated the underlying physical mechanisms that lead to the seasonality changes in extreme precipitation, with the help of a moisture budget decomposition framework. Results indicated that thermodynamic effect from increased moisture dominates the extreme precipitation increase in winter, while dynamic effect from weakened circulation offsets the moisture increase, resulting in no changes in spring and fall. Therefore, both thermodynamic and dynamic effects contribute to the sharpened wet season for extreme precipitation, which is the result of changes in seasonality of extreme days rather than seasonality changes of circulation or moisture intensity during extreme precipitation days. In addition, the seasonality changes in extreme precipitation via dynamic and thermodynamic effects are consistent with those in atmospheric river (AR) days, confirming the close relationship between extreme precipitation and landfalling ARs along the West Coast.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov

Funding

The U.S. Department of Energy Office of Science, Biological and Environmental Research supported this research as part of the Regional and Global Climate Modeling and Multi-Sector Dynamics program areas.