Myself and other climatologists have received numerous inquiries lately about the hand of cards we’ve been dealt this winter so far. How unusual has this winter been? Where did all the snow go? Is it El Nino? Is it global warming?
The quick and dirty answers are:
1) very, but not unprecedented; 2) to New England; 3) not really; 4) sort of.
1) How unusual has this winter been?
From the WestWide Drought Tracker are a couple of maps that show temperature and precipitation departures from 1981-2010 normals. The headline story is how warm it has been. Much of western Oregon/Washington and California had their warmest Dec-Feb and Oct-Feb in 120-years, whereas for the region as a whole this will likely be the second warmest such period of record behind the epic 1933-1934 period.
The root cause for the unusual warmth across western North American this winter is a persistent upper-level ridge (part of a larger wave pattern) that has brought air from a more southwesterly direction into the area. The business end or downstream end of the planetary wave has unkindly returned the favor by advecting Arctic and continental polar air southward and eastward into the eastern half of the US. This pattern was particularly acute in February, but actually shows up in an animation of 1-to-12 month temperature anomalies across the lower 48.
Conversely, precipitation has been pretty close to normal for the water year to date. Most of the NW has been within 10% of normal. The main exception being east of the Bitterroot Range into western Montana.
2) Where did all the snow go?
“Winter has seemed to have completely forgotten about us out here,” Kathie Dello, deputy director of the Oregon Climate Service. In fact, during a NOAA call last month the humiliating factoid of more snow being on the ground in Boston than at Crater Lake Oregon was dropped. The snowpack situation is abysmal in the Cascades and northern Idaho panhandle, although perhaps not as bad as the situation in the Sierra Nevada.
Warmer temperatures have coincided with much higher than normal snow levels and a pitiful ratio of precipitation falling as snow in the warmer (lower elevation/more maritime) mountains across the region. Additionally, many of the larger precipitation events this winter have been delivered via atmospheric rivers. These systems bring a slug of juicy and very warm air (e.g., well above normal snow levels) into the region and have further contributed to the snow drought.
The higher elevation mountains of central Idaho, Yellowstone and western Montana have fared a bit better due to temperatures being climatologically much colder in these areas, and the above normal precipitation in western Montana.
3) Is this El Nino?
El Nino winters are typically associated with well above normal temperatures and below normal precipitation across the northwestern US with opposing anomalies in the southwestern US. While it has been warm, the rest of the teleconnection with El Nino has not materialized this winter until recently, and even that is shaky.
Signals of a strong El Nino represented by ocean surface and sub-surface temperature across the Pacific were present last summer. Unfortunately, the atmosphere was an unwilling dance partner in the “El Nino – Southern Oscillation” samba that defines an El Nino. Yesterday, the Climate Prediction Center declared a wimpy and rather awkward El Nino. It is unclear what the emergence of an El Nino in spring means in terms of the Pacific Northwest. Some statistical evidence suggests a higher likelihood of very warm springs (Mar-May) during an El Nino for the region.
4) Is it global warming?
The earth and the region have certainly warmed over the past 50-100 years. In fact, global mean temperatures from Oct-Jan were the warmest they have been since 1880 according to NOAA. However, the whole globe isn’t equally warm as is evident from the cold temperatures across the eastern US and Canada. Likewise, there have been areas globally that have experienced temperature anomalies beyond what we have experienced. Below is a map of Land Surface Temperature anomalies from MODIS from Oct-Feb highlighting the areas that have been warmer and cooler than normal.
The central cause of the warmth across the western US has been a persistent ridge (also dubbed the Ridiculously Resilient Ridge), that has delivered more southwesterly flow into the region (vs. westerly or northwesterly). Conversely, the downstream side of this wave has brought cold Arctic and continental polar air into the eastern half of the country. It is not clear whether such patterns are a response to climate change. Rather, it is likely that this is natural variability possibly arising from unusual ocean surface temperatures that ingrain memory into the climate system and are responsible for persistent regimes.
That said, anthropogenic climate change should make temperatures in the NW during such ridging events warmer than they would be in the absence of such warming. Feedbacks due to the lack of snow-cover could amplify just how much warmer things have been, although affixing numbers to just how much warmer is a bit more challenging.
Another way of looking at this winter is to compare it in the context of projected changes in the average temperature and precipitation for the region for future climate periods. A group of scientists at Oregon State University and the University of Idaho including David Rupp, Philip Mote, Katherine Hegewisch and myself have evaluated these models and provided summaries of such data to various groups. The figure below shows projected changes in precipitation and temperature for the NW from 20 different models for the 2040-2069 period vs. 1971-2000 (in model world) for a future pathway with some level of climate policy. Note that all of the models show warming, with an average warming of around 2.2C. There is a tendency for models to project negligible increases in Oct-Feb precipitation, but nothing to get too excited about.
The smaller dots show observations from 1896-2015, color coded by decades. The two warmest years on record for the Oct-Feb time period are Oct 1933-Feb 1934 and Oct 2014-Feb 2015. The impact of anthropogenic forcing during 1933-1934 was obviously much lower than this year, which speaks to the range of variability inherent in the climate system. Interestingly, 1933-1934 was significantly cold outside of the northern tier of the US and clearly an extremely regional signal. Thus by many accounts, the climate of NW this winter serves as a preview of what might be more “normal” by the middle part of the century.