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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.”    

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.