Nature Communications: Future Arctic Sea Ice Loss Could Drive Substantial Decrease In CA Rainfall

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.

There is considerable uncertainty in model projections of twenty-first century precipitation changes over California1, 2. These uncertainties are largely a consequence of the sensitivity of Californian rainfall to both tropical and mid-latitude atmospheric circulation changes, such as variability in the El Niño/Southern Oscillation (ENSO) and shifts in the Pacific jet stream2,3,4. California’s winter precipitation has decreased over the past two decades, and between 2012 and 2016 California has entered into one of the most severe droughts on record5,6,7,8,9. The impacts of reduced rainfall have been intensified by anomalously high temperatures that have enhanced potential evapotranspiration5. Several studies posit that California’s drought has an anthropogenic component arising from increased temperatures, with the likelihood of such warming-enhanced droughts expected to increase in the future5, 10, 11.

The exceptionally dry conditions during the winters of 2012–2015 were accompanied by a prominent dynamical feature: a persistent geopotential ridge located in the North Pacific. This ridge pushed storm tracks further north, resulting in wetter than normal conditions over the northwest and substantial drying over the southwest of the United States6. A La Niña event in 2011/12 and anomalously warm sea-surface temperatures (SST) over the west tropical Pacific in 2012/13 and 2013/14 may have helped sustain the North Pacific geopotential ridge6, 8. Climate model simulations forced with observed SSTs alone were unable to capture all the important features of the recent precipitation decline, suggesting that California’s recent precipitation deficit may in part be attributable to other factors, such as internal atmospheric variability8.

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While the temperature impacts of sea-ice loss are most pronounced in the northern high- to mid-latitudes (Fig. 2a), precipitation anomalies show a more global response (Fig. 2b, c). We focus our analysis on the December–February season, because these months yield the largest impact of sea-ice changes on Californian precipitation in our model simulations. The most striking feature of the precipitation response to Arctic sea-ice loss is the reorganization of tropical rainfall and an apparent northward precipitation shift. The Arctic sea-ice decline also results in significantly less precipitation over California—a consequence of a geopotential ridge in the North Pacific that steers the wet winter air masses northward into Alaska and Canada, away from California (Fig. 2e).



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https://www.nature.com/articles/s41467-017-01907-4