Will California get more precipitation in future winters?

Featured image of a road in Death Valley in California by jplenio on Pixabay

Paper: Winter Precipitation Changes in California Under Global Warming: Contributions of CO2, Uniform SST Warming, and SST Change Patterns
Authors: L. Dong and L. R. Leung

As with any job tasked with predicting the future, climate scientists have a tough but important responsibility: understand how the climate will be different at the end of the century. Predicting future climate is especially critical in areas with large, vulnerable populations and that grow a large part of the food supply. California, for example, has a population of over 39 million and is a source of two-thirds of the fruits and one-third of the vegetables grown in the US. Changes to its climate will impact not only its own residents but also the population and economy of the whole country.

People are drawn to California for its Mediterranean climate: temperate and wet winters, warm and dry summers, and overall fairly dry conditions. Many other places in the world–such as the Mediterranean basin and western Australia–have similar climates, but there’s a key difference: while climate models predict that most Mediterranean climate regions will experience drier winters due to climate change, they predict that California will get wetter winters. A new study by Dong and Leung published in Geophysical Research Letters uses climate models to explore this discrepancy and understand why Californians should expect more winter precipitation in the future.

The answer lies in a feature of the atmosphere known as the jet stream. The jet stream is an area of narrow, strong winds about 7 miles up in the atmosphere that generally blow from west to east. The jet stream can be thought of as a highway that winter storms travel on, so the position of the jet stream controls what areas get hit by these storms. Dong and Leung’s model simulations suggest that in the future, the jet stream will extend over California more often in the winter, thus bringing more precipitation-producing storms.

But why do the models project this shift in the location of the jet stream in the first place? By simulating different scenarios, they were able to break down the jet stream changes into contributions from carbon dioxide increases in the atmosphere, sea surface temperature warming, and changes in the spatial pattern of how sea surface temperature warms. They found that carbon dioxide increases have almost no impact on the jet stream location, while the spatial pattern of sea surface temperature warming had a slight effect. Sea surface temperature warming had the greatest impact by far.

The study indicates that California will likely see more precipitation during future winters. Ultimately this is caused by climate change warming the ocean, which in turn shifts the winter jet stream to extend over California and drives more winter storms into the region. While these results do agree with other studies, there is still some uncertainty due to the relatively low spatial resolution (about 100 km) of the climate models. Despite the uncertainty, predicting how climate change will affect California’s precipitation is important for the state’s greatest challenge: the planning of water resource allocation.

Will California get more precipitation in future winters? by Alyssa Stansfield is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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