California Drought/Polar Vortex Jet Stream Pattern Linked to Global Warming

By Dr. Jeff Masters
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Published: 6:56 PM GMT on April 16, 2014

From November 2013 - January 2014, a remarkably extreme jet stream pattern set up over North America, bringing the infamous "Polar Vortex" of cold air to the Midwest and Eastern U.S., and a "Ridiculously Resilient Ridge" of high pressure over California, which brought the worst winter drought conditions ever recorded to that state. A new study published this week in Geophysical Research Letters, led by Utah State scientist S.-Y. Simon Wang, found that this jet stream pattern was the most extreme on record, and likely could not have grown so extreme without the influence of human-caused global warming. The study concluded, “there is a traceable anthropogenic warming footprint in the enormous intensity of the anomalous ridge during winter 2013-14, the associated drought and its intensity."


Figure 1. An extreme jet stream patten observed at 00 UTC on January 16, 2014. Color-coded wind speeds at a pressure of 300 mb (roughly 9,000 meters or 30,000 feet) show the axis of the jet stream over North America, with a large upside-down "U"-shaped ridge of high pressure over the West Coast. California is outlined in orange. The strongest winds of the jet stream (orange colors, 160 mph) were observed over the Northeast United States, where a strong "U"-shaped trough of low pressure was anchored. Image generated from the 00 UTC January 16, 2014 run of the GFS model, and plotted using our wundermap.

Using observations and a climate model to diagnose the human contribution to the jet stream pattern
The researchers studied the historical pressure patterns for November - January over North America during the period 1960 - 2014, and found that a strong "dipole" pattern of high pressure over Western North America and low pressure over Eastern North America, such as occurred during the winter of 2013 - 2014, tended to occur naturally during the winter immediately preceding an El Niño event. Since NOAA is giving a greater than 50% of an El Niño event occurring later in 2014, this past winter's dipole pattern may have been a natural expression of the evolving progression towards El Niño. The study also found that the dipole pattern could be intensified by two other natural resonances in the climate system: the Arctic Oscillation, and a variation of ocean temperatures and winds in the Western North Pacific called the Western North Pacific (WNP) pattern. But the dipole of high pressure over California combined with the "Polar Vortex" low pressure trough over Eastern North America during November 2013 - January 2014 was of unprecedented intensity, and extremes in this dipole pattern--both in the positive and negative sense--have been increasing since 2000 (the peak negative value occurred during the winter of 2009 - 2010.) The researchers used a climate model to look at whether human-caused climate change might be interfering with the natural pattern to cause this unusual behavior. They ran their climate model both with and without the human-caused change to the base state of the climate included, and found that they could not reproduce the increase in amplitude of the dipole pattern unless human-caused global warming was included. They concluded, "It is important to note that the dipole is projected to intensify, which implies that the periodic and inevitable droughts California will experience will exhibit more severity. The inference from this study is that the abnormal intensity of the winter ridge is traceable to human-induced warming but, more importantly, its development is potentially predicable." In an email to me, the lead author of the study, Simon Wang, emphasized that the opposite sign of the dipole--an extreme trough of low pressure over Western North American, combined with an extreme ridge of high pressure over Eastern North America--is also expected to be more intense when it occurs, leading to an increase in extremely wet winters in California.

Dr. Joe Romm's post on the study, "Bombshell: Study Ties Epic California Drought, ‘Frigid East’ To Manmade Climate Change", has this quote by climate scientist Michael Mann on the new research:

We know that human-caused climate change has played a hand in the increases in many types of extreme weather impacting the U.S., including the more pronounced heat waves and droughts of recent summers, more devastating hurricanes and superstorms, and more widespread and intense wildfires.

This latest paper adds to the weight of evidence that climate change may be impacting weather in the U.S. in a more subtle way, altering the configuration of the jet stream in a way that disrupts patterns of rainfall and drought, in this case creating an unusually strong atmospheric “ridge” that pushed the jet stream to the north this winter along the west coast, yielding record drought in California, flooding in Washington State, and abnormal warmth in Alaska. The recent IPCC assessment downplays these sorts of connections, making it very conservative in its assessment of risk, and reminding us that uncertainty in the science seems to be cutting against us, not for us. It is a reason for action rather than inaction.



Figure 2. One of the key water supply reservoirs for Central California, Lake Oroville, as seen on January 20, 2014. Thanks to an unusually intense ridge of high pressure over Western North America, California endured its driest November - January period on record this past winter, resulting in the worst winter drought on record. Image credit: California Department of Water Resources.

Other research connecting extreme circulation patterns to human causes
This week's paper by Dr. Wang is the second he has authored which has found a human fingerprint on extreme atmospheric circulation patterns. His 2013 paper, "Identification of extreme precipitation threat across midlatitude regions based on short-wave circulations," discussed how there's been a trend during the period 1979 - 2010 towards a pronounced circulation shift involving the low-level jet stream (LLJ), which is capable of bringing more extreme precipitation events (and droughts) to the mid-latitudes. Using four different climate models, the study found that the circulation shift only occurs when one runs climate models with the effects of human-caused emissions of greenhouse gases like carbon dioxide included; "control" runs of these models using only natural changes to the climate could not reproduce the observed increase in this more extreme circulation pattern. The paper concluded that several recent extreme precipitation events, including those leading to the 2008 Midwest flood in U.S., the 2011 tornado outbreaks in southeastern U.S., the 2010 Queensland flood in northeastern Australia, and to the opposite sense, the 2012 central U.S. drought, could have been influenced by human-caused changes to the atmospheric circulation. The fact that his research helps us understand how human-caused climate change is contributing to higher amplitude jet stream patterns should make them more predictable, potentially saving lives and money.

References
Wang, S, Davies, R.E., and R.R. Gillies, 2013, "Identification of extreme precipitation threat across midlatitude regions based on short-wave circulations," J. Geophys. Res. Atmos., 118, 11,059–11,074, doi:10.1002/jgrd.50841.

Wang, S., Hipps, L., Gillies, R.R., and J.-H. Yoon, 2014, "Probable causes of the abnormal ridge accompanying the 2013-14 California drought: ENSO precursor and anthropogenic warming footprint", Geophysical Research Letters, DOI: 10.1002/2014GL059748. News Release.

Related information
There is an growing body of research exploring connections between human-caused climate change and the increase in unusual jet stream patterns we've seen in recent years. Most of this research focuses on potential linkages between Arctic warming and atmospheric circulation patterns. Below are links to wunderground blog posts on the subject, and to the original research studies and associated press releases.

"The Changing Face of Mother Nature", wunderground guest post by Dr. Jennifer Francis of Rutgers, April 22, 2013.

Extreme jet stream causing record warmth in the east, record cold in the west (January 2013)

Arctic sea ice loss tied to unusual jet stream patterns (April 2012)

Our extreme weather: Arctic changes to blame? (December 2011)

Florida shivers; Hot Arctic-Cold Continents pattern is back (December 2010)

Jet stream moved northwards 270 miles in 22 years; climate change to blame? (June 2008)

Dr. Ricky Rood has done a whole series of posts on climate change and the Arctic Oscillation, including:

Cold Weather in Denver: Climate Change and Arctic Oscillation (8)

Are the changes in the Arctic messing with our weather? The Future of Blocking

Papers linking Arctic warming to an increase in negative AO/NAO conditions
Deser, C., R. Tomas, M. Alexander, and D. Lawrence, 2010, "The seasonal atmospheric response to projected Arctic sea ice loss in the late 21st century," J. Clim., 23, 333–351, doi:10.1175/2009JCLI3053.1.

Francis, J.A., and S.J. Vavrus (2012), "Evidence linking Arctic amplification to extreme weather in mid-latitudes," Geophysical Research Letters, 21 February, 2012. Accompanying article at the Yale Forum on Climate Change, Linking Weird Weather to Rapid Warming of the Arctic.

Francis, J. A., W. Chan, D. J. Leathers, J. R. Miller, and D. E. Veron, 2009, "Winter northern hemisphere weather patterns remember summer Arctic sea-ice extent," Geophys. Res. Lett., 36, L07503, doi:10.1029/2009GL037274.

Honda, M., J. Inoue, and S. Yamane, 2009, "Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters," Geophys. Res. Lett., 36, L08707, doi:10.1029/2008GL037079.

Jaiser, R., K. Dethloff, D. Handorf, A. Rinke, J. Cohen (2012), "Impact of sea ice cover changes on the Northern Hemisphere atmospheric winter circulation", Tellus A 2012, 64, 11595, DOI: 10.3402/tellusa.v64i0.11595

Liu et al. (2012), "Impact of declining Arctic sea ice on winter snowfall", Proc. Natl. Academy of Sciences, Published online before print February 27, 2012, doi: 10.1073/pnas.1114910109. Accompanying press release. My blog post.

Overland, J. E., and M. Wang, 2010, "Large-scale atmospheric circulation changes associated with the recent loss of Arctic sea ice," Tellus, 62A, 1.9.

Petoukhov, V., and V. Semenov, 2010, "A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents," J. Geophys. Res.-Atmos., ISSN 0148-0227.

Seager, R., Y. Kushnir, J. Nakamura, M. Ting, and N. Naik (2010), "Northern Hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10," Geophys. Res. Lett., 37, L14703, doi:10.1029/2010GL043830.

Seierstad, I. A., and J. Bader (2009), "Impact of a projected future Arctic Sea Ice reduction on extratropical storminess and the NAO," Clim. Dyn., 33, 937-943, doi:10.1007/s00382-008-0463-x.

Tang et al., "Cold winter extremes in northern continents linked to Arctic sea ice loss," Environ. Res. Lett. 8 014036 doi:10.1088/1748-9326/8/1/014036. My April 2013 blog post.

Papers linking Arctic warming to Western U.S. drought
Sewall, Jacob O., 2005, Precipitation Shifts over Western North America as a Result of Declining Arctic Sea Ice Cover: The Coupled System Response, Earth Interact., 9, 1–23. doi: http://dx.doi.org/10.1175/EI171.1

Sewall, J.O., and L.C. Sloan, 2004, Disappearing Arctic sea ice reduces available water in the American west, Geophys. Res. Lett., 31, L06209, doi:10.1029/2003GL019133. Accompanying news release.

Papers linking Arctic sea ice loss to changes in summer rainfall
Li Y, LR Leung, Z Xiao, M Wei, and Q Li. 2013, Interdecadal Connection between Arctic Temperature and Summer Precipitation over the Yangtze River Valley in the CMIP5 Historical Simulations, Journal of Climate 26(19):7464-7488. DOI: 10.1175/JCLI-D-12-00776.1 Accompanying press release.

Li, Y, and L.R. Leung, 2013, "Potential Impacts of the Arctic on Interannual and Interdecadal Summer Precipitation in China", Journal of Climate 26(3):899-917. DOI: 101175/JCLI-D-12-00075.1

Screen, J.A., 2013, "Influence of Arctic sea ice on European summer precipitation", Environ. Res. Lett. 8 044015 doi:10.1088/1748-9326/8/4/044015. Accompanying press release.

Wu, B., Zhang R, D’Arrigo. R., and J. Su, 2013, "On the relationship between winter sea ice and summer atmospheric circulation over Eurasia," J. Clim. 26 5523–36

Papers exploring the link between Arctic warming to changes in large-scale atmospheric circulation
Cassano, E.N., Cassano, J.J, Higgins, M.E., and M.C. Serreze, 2013, "Atmospheric impacts of an Arctic sea ice minimum as seen in the Community Atmosphere Model", Int. J. Climatol., in press. (doi:10.1002/joc.3723)

Overland, J.E., Francis, J.A., Hanna, E., and M. Wang, 2012, "The recent shift in early summer Arctic atmospheric circulation," Geophys. Res. Lett. 39 L19804, DOI: 10.1029/2012GL053268

Petoukhov, V., Rahmstorf, S., Petri, S., Schellnhuber, H. J. (2013), "Quasi-resonant amplification of planetary waves and recent Northern Hemisphere weather extremes" Proceedings of the National Academy of Sciences, (Early Edition) [doi:10.1073/pnas.1222000110]. Easy-to-read description of the paper by the authors, published at http://theconversation.edu.au. Accompanying press release. My March 2013 blog post.

Screen, J.A., and I. Simmonds, 2013, "Exploring links between Arctic amplification and mid-latitude weather", Geophys. Res. Lett. 40 959–64.

Jeff Masters

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About The Author
Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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