Starting in the fall of 2016, the drought that consumed the Northwest and California since the fall of 2012 started to disappear. It’s back as of this writing. For more on that and this summer’s extreme weather check out Northwest Climate Currents in this month’s CIRCulator. As anyone from the region can tell you, the reprieve from the drought came at a cost: lots of precipitation.
An exceptionally large—often record-breaking—amount of precipitation fell last winter and fall in California and the Northwest. In the Northwest, it showed up as downpours and school-and-city-closing snowstorms dubbed “snowpocalypses” by the local media. While in California, the torrent of precipitation led to substantial flooding. This included the especially dramatic case of the Lake Oroville dam, California’s second largest reservoir, which for the first time in its history had water pouring over its emergency spillway, leading to the evacuation of 188,000 area residents.
So, how did we go from drought to deluge? How did we go from extreme to extreme?
This is the question posed in an opinion piece published in July under the provocative title “California from Drought to Deluge” in Nature Climate Change. The piece’s authors, led by Simon Wang of Utah State University, write the sudden switch from drought to deluge has to do with a third d-word: dipole.
The dipole in question—there are several—is the North American winter dipole. The North American winter dipole—let’s just call it the dipole for short—is a common atmospheric pattern in the Northern Hemisphere’s winter in which a ridge—an area of high atmospheric pressure—sits above the West Coast and a trough—an area of low atmospheric pressure—sits above the East Coast. This dipole effectively divides North America into a warm west and a cool east. The pattern is called a dipole because, much like a magnetic dipole, it is a pattern of simultaneous polar opposites, but instead of magnetic polarization it is a polarization of pressure and temperature.
Placed on a map of North America, a particularly strong example of the dipole shows up as a blob of red in the western United States and Canada representing warm temperatures that bump up against another large blob, this one blue and encompassing the eastern United States and Canada, and representing cool temperatures.
Consider the extreme winters of 2013 to 2014 and 2014 to 2015. During these two winters the dipole was working full-tilt to divide North America by temperature. The west experienced anomalously warm temperatures while the east experienced anomalously cool temperatures. The west saw drought; the east saw school-and-city-closing snowstorms. The winters of 2013 to 2015 represent an amplification of the typical pattern of the dipole. This past winter, on the other hand, represents not an amplification of the typical pattern but a complete reversal.
This switch from one extreme to another is part of a recent trend toward increasing amplification of the North American winter dipole, note Wang and colleagues.
This reversal of the dipole, as illustrated in the Nature Climate Change paper, looks almost exactly opposite to the illustration of the dipole pattern of the 2013 to 2015 winters. Instead of a blob of warm red in the west, we see a blob of cool blue. Instead of a blob of cool blue in the east, we see a blob of warm red. The east was anomalously warm; the west was anomalously cool. The east was especially dry, with the American south experiencing drought conditions, while the west was pummeled by snow and rain, going from drought to deluge.
The reason for the switch, note Wang and colleagues, has something to do with what’s happening in the atmosphere.
This past winter, the high pressure ridge showed up on the East Coast, not the West Coast. While at the same time, the lower pressure trough, showed up on the West Coast, not the East Coast. In fact, on the West Coast, the low pressure trough essentially parked itself in the same location that the high-pressure ridge once called home. This brought the deluge. The low-pressure trough, note Wang and colleagues, effectively funneled atmospheric rivers and their accompanying precipitation from the atmosphere above the Pacific Ocean to California and the Northwest. As Wang and colleagues put it, this really is “the other side of the coin.”
So, what made the coin flip? That answer, write the authors, is hard to come by. Multiple studies have connected extremes in the dipole to La Niña and El Niño events, extreme phases of the El Niño Southern Oscillation (ENSO). The warming Arctic is also being looked at as possibly influencing the amplification in the dipole. Still other studies have found remote connections between warming in other areas of the Pacific and Indian Oceans and amplified drought-producing ridges off the West Coast.
That said, according to Wang and colleagues, one thing seems very likely: the North American winter dipole and its extremes have been undergoing an amplification. This means that the dipole pattern of warm in the west and cool in the east could intensify, but also that the dipole pattern could sometimes flip like a coin as ridges become troughs and the other way around. What all this means, write the authors, is more extreme precipitation events, running the gamut from drought to deluge.
Study: Nature Climate Change
Citation: Wang, S-Y. Simon, Jin-Ho Yoon, Emily Becker, and Robert Gillies. “California from drought to deluge.” Nature Climate Change 7 (2017): 465-468. https://doi.org/10.1038/nclimate3330.
Photo: Oroville Dam Spillway, February 14, 2017. (Photo Credit: California Governor’s Office of Emergency Services, some rights reserved. )
Nathan Gilles is the managing editor of The Climate Circulator, and oversees CIRC’s social media accounts and website. When he’s not writing for CIRC, Nathan works as a freelance science writer. Other Posts by this Author.