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2017’s Ridiculously Resilient Ridge & the North American Winter Dipole

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Daniel Swain December 4 2017 Read full WeatherWest blog here

Origins of the [Ridiculously Resilient Ridge or] “Triple R” and California’s severe drought

In 2013, a curious feature began to emerge on the weather maps: a region of unusually high atmospheric pressure (known as a “ridge” in meteorological circles) was consistently pushing the Pacific jet stream to the north of California, resulting in very dry conditions. At the time, I (somewhat jokingly) termed this anomalous high pressure zone the “Ridiculously Resilient Ridge” due to its implausible longevity, assuming that it would most likely recede by the subsequent blog post. Instead, the “Triple R” held strong straight through the entire winter—and then recurred, in slightly modified form, throughout the winters of 2014-2015 and 2015-2016.

 

The multi-year persistence of this anomalous atmospheric ridge was nothing short of extraordinary. The co-occurrence of record low precipitation and record high temperatures associated with the Triple R ultimately yielded California’s most severe multi-year drought on record…..

Warm West/Cool East” extremes have become more common in recent years

Our recent work (led by Deepti Singh) answers this question affirmatively: there has indeed been an increase in the number of days each winter characterized by simultaneously very warm temperatures across the American West and very cold temperatures across the East….

…this increase in contrasting dipole extremes appears to be caused primarily by the increased rate of warming in the western U.S. relative to the eastern U.S…

Tropical Pacific may offer early warning of “Triple R”-like patterns

We found that there do indeed appear to be strong relationships between Pacific Ocean temperatures and persistent West Coast ridges conducive to dry conditions in California. Especially prominent are the links to western tropical Pacific Ocean warmth. These connections appear several months in advance, which not only suggests a causal linkage but also hints that it may be possible to predict the occurrence of “Triple R”-like ridges several months in advance….

What about “The Blob?” Well, we did find a strong statistical linkage between warm ocean conditions in the North Pacific and West Coast ridging—similar to that which occurred during the recent drought. In this case, though, the “chicken or egg” issue rears its head once again: while a time-lagged relationship between autumn ocean temperature and winter ridging did exist in observations, only an contemporaneous relationship existed in climate model simulations….

Some conclusions, and thoughts about the present winter

Ocean temperatures have been cool in the eastern tropical Pacific and warm in the western tropical Pacific since early autumn. (NOAA via tropicaltidbits.com)

Ultimately, we confirm that unusual ocean temperatures are linked to seasonally-persistent West Coast winter ridging similar to the Triple R. Tropical warmth (in the West Pacific) and coolness (in the East Pacific) are both linked to different patterns of North Pacific winter ridging, and may offer an early warning of seasons with an elevated risk of dry conditions in California.

Interestingly, tropical Pacific Ocean temperatures during autumn 2017 were warm in the west and cool in the east amidst a modest (and ongoing) La Niña event—a combination that suggests a substantially elevated likelihood of West Coast ridging this winter. To date, Southern California has experienced one of its driest starts to the Water Year on record, and strikingly persistent West Coast ridging is now expected to last at least two weeks. It will certainly be interesting to see how this winter plays out in the context of these new research findings.

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