Schematics of the teleconnection through which Arctic sea-ice changes drive precipitation decrease over California. Arctic sea-ice loss induced high-latitude changes first propagate into tropics, triggering tropical circulation and convection responses. Decreased convection and decreased upper level divergence in the tropical Pacific then drive a northward propagating Rossby wavetrain, with anticyclonic flow forming in the North Pacific. This ridge is responsible for steering the wet tropical air masses away from California. Graphic: Kathy Seibert / LLNL

5 Dcember 2017 (LLNL) – Arctic sea ice loss of the magnitude expected in the next few decades could impact California’s rainfall and exacerbate future droughts, according to new research led by Lawrence Livermore National Laboratory (LLNL) scientists.The dramatic loss of Arctic sea ice cover observed over the satellite era is expected to continue throughout the 21st century. Over the next few decades, the Arctic Ocean is projected to become ice-free during the summer. A new study by Ivana Cvijanovic and colleagues from LLNL and University of California, Berkeley shows that substantial loss of Arctic sea ice could have significant far-field effects, and is likely to impact the amount of precipitation California receives. The research appears in the 5 December 2017 edition of Nature Communications.The study identifies a new link between Arctic sea ice loss and the development of an atmospheric ridging system in the North Pacific. This atmospheric feature also played a central role in the 2012-2016 California drought and is known for steering precipitation-rich storms northward, into Alaska and Canada, and away from California. The team found that sea ice changes can lead to convection changes over the tropical Pacific. These convection changes can in turn drive the formation of an atmospheric ridge in the North Pacific, resulting in significant drying over California. Schematics of the teleconnection through which Arctic sea-ice changes drive precipitation decrease over California. Arctic sea-ice loss induced high-latitude changes first propagate into tropics, triggering tropical circulation and convection responses. Decreased convection and decreased upper level divergence in the tropical Pacific then drive a northward propagating Rossby wavetrain, with anticyclonic flow forming in the North Pacific. This ridge is responsible for steering the wet tropical air masses away from California. Graphic by Kathy Seibert/LLNL“On average, when considering the 20-year mean, we find a 10-15 percent decrease in California’s rainfall. However, some individual years could become much drier, and others wetter,” Cvijanovic said.The study does not attribute the 2012-2016 drought to Arctic sea ice loss. However, the simulations indicate that the sea-ice driven precipitation changes resemble the global rainfall patterns observed during that drought, leaving the possibility that Arctic sea-ice loss could have played a role in the recent drought.“The recent California drought appears to be a good illustration of what the sea-ice driven precipitation decline could look like,” she explained.California’s winter precipitation has decreased over the last two decades, with the 2012-2016 drought being one of the most severe on record. The impacts of reduced rainfall have been intensified by high temperatures that have enhanced evaporation. Several studies suggest that recent Californian droughts have a manmade component arising from increased temperatures, with the likelihood of such warming-enhanced droughts expected to increase in the future.  “Our study identifies one more pathway by which human activities could affect the occurrence of future droughts over California — through human-induced Arctic sea ice decline,” Cvijanovic said. “While more research should be done, we should be aware that an increasing number of studies, including this one, suggest that the loss of Arctic sea ice cover is not only a problem for remote Arctic communities, but could affect millions of people worldwide. Arctic sea ice loss could affect us, right here in California.”Other co-authors on the study include Benjamin Santer, Celine Bonfils, Donald Lucas and Susan Zimmerman from LLNL and John Chiang from the University of California, Berkeley.The research is funded by Department of Energy (DOE) Office of Science. Cvijanovic and Bonfils were funded by the DOE Early Career Research Program Award and Lucas is funded by the DOE Office of Science through the SciDAC project on Multiscale Methods for Accurate, Efficient and Scale-Aware Models of the Earth System.

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Arctic sea ice loss could dry out CaliforniaAtmospheric impacts of Arctic sea-ice loss. Shown are the ensemble mean differences (“low Arctic ice” minus control) for December–February (DJF) season. Stippling indicates anomalies that are statistically significant at the 90% confidence level. a Surface temperature anomalies. b Precipitation anomalies (absolute). c Precipitation anomalies (relative). d Outgoing longwave radiation (OLR) anomalies (shading) and the high cloud cover anomalies (contours). e 500 hPa geopotential distribution changes. f Stream function changes. Negative cloud cover anomalies in d are indicated with dashed lines. Graphic: Cvijanovic, et al., 2017 / Nature

ABSTRACT: From 2012 to 2016, California experienced one of the worst droughts since the start of observational records. As in previous dry periods, precipitation-inducing winter storms were steered away from California by a persistent atmospheric ridging system in the North Pacific. Here we identify a new link between Arctic sea-ice loss and the North Pacific geopotential ridge development. In a two-step teleconnection, sea-ice changes lead to reorganization of tropical convection that in turn triggers an anticyclonic response over the North Pacific, resulting in significant drying over California. These findings suggest that the ability of climate models to accurately estimate future precipitation changes over California is also linked to the fidelity with which future sea-ice changes are simulated. We conclude that sea-ice loss of the magnitude expected in the next decades could substantially impact California’s precipitation, thus highlighting another mechanism by which human-caused climate change could exacerbate future California droughts.

Future loss of Arctic sea-ice cover could drive a substantial decrease in California’s rainfall