It was a winter Charles Dickens would write an opening line about. Parts of the Midwest and Eastern US experienced periods of bitterly cold weather delivered from the Arctic by the jet stream. The West Coast, meanwhile, received very little precipitation—producing a historic drought in California that looms large over the coming summer—as a result of the jet stream zagging in the opposite direction.

The detours of the jet stream were large, and they were persistent. The northward-bending “ridge” shielded the West Coast from moisture-bearing weather that would normally water the Californian landscape and restock the supply of mountain snow that provides meltwater over the dry summer.

Many wondered if climate change could be partly responsible—a question that gets asked about every extreme now. It is, as always, a difficult question to answer, considering the inherent and substantial variability of weather. However, it’s also plainly true that average atmospheric conditions have changed over the past century. The hard part is teasing out the contribution of those changing conditions to specific weather events.

Searching for correlations

Two of the usual suspects for weather variability on the West Coast are the El Niño Southern Oscillation and the Pacific Decadal Oscillation. This winter, however, both were pretty much neutral. To look for links between the jet stream and other events, Utah State University’s Simon Wang and his collaborators created an index measuring the difference in the latitude of the jet stream over the West Coast and the Eastern US. The only oscillation that showed any hint of correlation with this index was the Arctic Oscillation—a see-saw of atmospheric pressure that periodically allows Arctic air to spill southward—but that didn’t explain much.

They did, however, find a decent correlation with something else interesting. Winters just before an El Niño (in which warm water pools at the surface in the eastern equatorial Pacific) tended to produce this year’s pattern. As it happens, forecast models have been converging on the development of an El Niño later this year.

Unless El Niños can travel through time, something else must have set up that jet stream ridge off the West Coast. The researchers identified conditions in the vicinity of the Philippine Sea in the late summer and fall that appear to have been responsible. Those same conditions in the western side of the North Pacific have previously been suggested to help trigger the development of an El Niño. That strengthens the case that the correlation between West Coast ridges and winters and upcoming El Niños is meaningful.

Finding larger trends

But does climate change have anything to do with it? The researchers’ index of jet stream detours over North America showed that this winter pattern has become more variable over time—although a calmer period occurred in the 1970s.

Many studies testing the contribution of climate change to an extreme event run large numbers of simulations with and without anthropogenic greenhouse gas emissions, comparing the probability of that same extreme event in each. If it’s considerably more common in the anthropogenic simulations, you can conclude that climate change made the extreme event more likely, much as steroid use in baseball made home runs more likely.

In this case, however, the researchers were looking at trends in variability to see whether they would give us a reason to think climate change contributed to the extreme nature of last winter’s drought. They compared two climate model simulations to this data: one that only took natural climate forcings (like solar activity and volcanic eruptions) into account and one that took only changing greenhouse gas forcings into account.

The “greenhouse gas only” simulation produced behavior remarkably similar to the observed trend—with variability increasing in recent years. The variability in the “natural forcings only” simulation, on the other hand, decreased from 1970 to the present.

Now, that still means this potential “El Niño precursor” gave rise to the ridge and added to the cold invasion of the Eastern US, which probably also got a boost from the Arctic Oscillation. But climate change may have amplified that behavior. The researchers write, “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 predictable.” That is, the western North Pacific could tell us when the US is in for this type of winter weather pattern, which is projected to generally become more intense in the future.

While interesting, this study is a first-pass look at something that will undoubtedly be explored in more detail. Researchers will want to investigate how it is, for example, that climate change could cause this increase in variability. That will feed into the ongoing focus on the subtle ways that climate change might be affecting the jet stream, and thus mid-latitude weather extremes.

Geophysical Research Letters, 2014. DOI: 10.1002/2014GL059748 (About DOIs).