Years of No Winter: R3 and the Blob - MRI Blog

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Years of No Winter: R3 and the Blob

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Years of No Winter: R3 and the Blob by Connie Millar:

CIRMOUNT’s mountain climate conference (MtnClim) is modeled after the popular Pacific Climate Workshop (PACLIM), which alternates biennially with MtnClim. PACLIM 2015 convened at Asilomar State Conference Center on the Monterey coast of California, March 8-11. Although PACLIM historically emphasizes Pacific rim and oceanic climate issues, current themes are highly relevant to mountain regions. I report highlights from PACLIM 2015, which had a focus this year on the ongoing drought of western North America (WNA).



PACLIM 2015 Conference on the Monterey Coast, California. Image courtesy of Scotty Strachan. 

Unlike much of central Europe, where hot summer droughts such as that of 2003 impart severe effects, much of WNA typically has a summer-dry climate. Even where summer monsoon occurs, as in the Southwest, winter precipitation as snow in the mountains provides a large proportion of annual water storage. Thus, warm, dry winters have pervasive impacts. Winter temperatures in the past 3-4 years have been especially high, as Kelly Redmond reported. Many parts of WNA have achieved record calendar-year average highs, and winter temperatures deviate from long-term records in some regions by + 4-10 °C. Water-year precipitation for much of the West, particularly the south- and central West, were only 5-25% of the long-term averages, with snowpack in the Sierra for 2015 through Feb lower than the record dry year (1977). Much of the precipitation that did occur fell as rain rather than snow; at Lake Tahoe, CA, the freezing level in 2014 was 600 m higher than the 30-year average.


Lake Tahoe and surroundings on 23 February in 2012, 2013 and 2014. Image from USGS/NASA.

Much of this drought is attributable to the “ridiculously resilient ridge” (R3), as Dan Swain reported, a persistent high-pressure ridge that has centered over the Gulf of Alaska and northeastern Pacific during winters for the past 3 years, bringing extraordinarily dry conditions to California. While the spatial structure of the ridge is not particularly anomalous, its ability to re-build in the same region and persist for so long is atypical. This persistent, “blocking ridge” deflects zonal (west to east) flows that historically brought storms to WNA in the cool season, sending them instead far to the north, missing much of WNA.

In addition to maintaining extreme drought over WNA, the R3 has contributed to the formation of a “gargantuan mass” of unusually warm water in the northeastern Pacific Ocean, with ocean temperatures 1-4 °C warmer than usual, persisting for as long as 18 months, and leading to migration of tropical marine mammals into northern waters. “The Blob”, nearly 2000 km wide and 100 m deep, is maintained by the same lack of winds stirring up cold deep water that also failed to bring winter storms over WNA.  Between the R3 and the Blob, drought and warm conditions on land have contributed to wide-ranging impacts on terrestrial ecosystems, including record low snow, lake, and river levels, heightened wildfires, high climatic water deficits that stress and kill plants, and cascading impacts to local communities and distant urban areas. Climate simulations that included and excluded greenhouse gas emissions suggest that the risk of the R3 is much higher under emission scenarios, suggesting yet another potential anthropogenic climate impact.

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