ENSO Effects on Pacific Northwest Winters
By now most Pacific Northwest residents, or at least those who are weather aware, have heard enough about El Niño and La Niña to know that these phenomena affect our weather significantly. We often hear that El Niño brings us mild, dry winters and La Niña brings us cold, wet winters. Together these phenomena are called ENSO, or El Niño/Southern Oscillation, a term that describes changing sea surface temperatures in the eastern Pacific Ocean off the coast of Peru. During El Niño events, this area of the ocean warms; and during La Niña events, it cools; having a profound impact on the weather in much of the world, including the United States, Central America, and Australia. These warm or cool periods may last 9 - 18 months, or, sometimes, several years; and they may be weak, moderate or strong (an event deviating significantly from normal is considered a strong event). As far as scientists can tell, ENSO has been occurring throughout history, but it is only recently that its effects have been studied to any great extent. In recent years, some exceptionally strong El Niños have received significant media attention and disrupted the economy of Peru (by dumping rain on normally desert lands) and other places. The purpose of this article will not be to discuss specifically the mechanisms by which ENSO effects our winters; rather, it will hopefully draw some meaningful correlation between ENSO and winter severity, with implications for gardeners who may be interested in knowing how to care for their plants through the winter.
El Niño generally diverts the jet stream, and thus winter storms, into California, leaving the Pacific Northwest high and dry (comparatively). These winters often have low precipitation and mild winters in the Pacific Northwest, sometimes resulting in a poor mountain snowpack. La Niña usually aims the jet stream right at the Pacific Northwest, from the north Pacific, so that cold moist air and vigorous storms come right at us, bringing plenty of mountain snow. With La Niña in place, severe fluctuations in the jet stream are also likely to occur, opening the door for modified arctic air to enter the region from time to time. So, while it has been established that El Niño generally brings us milder than average winters, and La Niña brings us cool, wet winters, some questions important to gardeners remain unanswered. For gardeners, it is not the average temperatures, but the extreme events that determine what plants will survive the winter. For example, a winter can occur that is colder than average, but as long as storms are continually blowing in off the Pacific no modified arctic air may enter the region. So I wondered: is El Niño likely to actually block modified arctic air from entering the region? Are La Niña winters that much more likely to bring severe cold events? What happens during ENSO-neutral winters, when the waters in the Pacific are neither hot nor cold?
So I conducted a simple study not based on average temperatures, but based on just one parameter: the single coldest temperature recorded in the entire winter, which I shall refer to as the seasonal minimum temperature or SMT. I used the period 1949-50 to 1995-96 for my analysis, since those were the data I could get. I looked at four locations - Olympia Airport, Seattle City Office, Bellingham Airport, and Sea-Tac Airport - and averaged the SMT's of these four locations together to get a general feel for how cold the extreme event that winter was for western Washington in general. I divided each winter into one of five categories: strong La Niña, moderate La Niña, ENSO-neutral, moderate El Niño, or strong El Niño, based directly on the deviation from normal of sea surface temperatures in the eastern Pacific (to qualify as 'strong' the sea surface temperature had to deviate at least 0.5°C from average). I looked at two separate samples, one comparing ENSO conditions during the winter itself to that winter's SMT, and one comparing ENSO conditions during the preceding autumn to the SMT. Comparing ENSO conditions in the autumn to the severity of the following winter is obviously very useful in predicting just how cold that winter will be.
My conclusions are as follows:
Most obviously, the correlation between ENSO and the SMT is strong (more so even than I expected). La Niña winters tend to be cold, with the intrusion of modified arctic air more likely to occur, and El Niño winters tend to be mild, with modified arctic air intrusion much less likely. The stronger the ENSO event, the more influence it seems to exert on the ability of modified arctic air to enter our region.
If one compares winter ENSO conditions with the SMT, one notices that ENSO neutral winters were, on average, just about the same as the mean, and there is a huge difference between a moderate El Niño and a strong El Niño, the latter being on average milder by about 4.5°F.
If one compares the SMT with ENSO conditions the preceding autumn, one notices a correlation that is equally strong. Thus, ENSO conditions in the autumn CAN be used to predict the severity of the following winter with some accuracy! Interestingly, one sees little difference between the SMT in a moderate autumn El Niño vs. a strong one. However, a strong La Niña in the autumn makes a severe winter very likely. Based on this small sample, one can expect a strong La Niña in the autumn to be followed by a colder than average winter 80-90% of the time.
A few notable exceptions were observed. A strong El Niño was present during the winter of 1972-73, yet December 1972 brought a landmark freeze to the Northwest, especially Oregon and Northern California, where many all-time lows for the month of December still stand from 1972. The winter of 1975-76 was not exceptionally severe and can be called milder than average, despite the influence of a strong La Niña. So nothing is guaranteed.
It should also be noted that ENSO-neutral winters were all over the place. Some were very mild, some were of "average" severity, and others quite severe. The very cold, snowy winter of 1968-69, during which many all-time low temperatures were set in the region (especially east of the Cascades) and snow remained on the ground in much of the Puget Sound area through most of January, followed an ENSO-neutral autumn, which tended towards a moderate El Niño by winter. However, the extremely mild winter of 1966-67 was also ENSO-neutral.
Also, I think it's fair to say that a moderate El Niño or La Niña has less of a bearing on our winter severity than a strong one. So, just because you hear about El Niño doesn't mean you can plan on a mild winter for your garden. Modified arctic air can easily enter the region during a moderate El Niño event. And, if you hear about La Niña, don't get too flustered because, if it is not a strong La Niña, you may luck out!
This study has a couple of shortfalls. First, using the SMT alone is rather simplistic. A more thorough study might take into account the number and duration of severe cold events rather than using only the SMT. Criteria could be set for the severity and duration of what can be called a "significant arctic blast" and these could be studied on an individual basis. Also, this study does not take into account the years 1996-2006, since those data were not readily available to me.
So it's true: El Niño brings mild winters for our gardens, and La Niña brings cold ones. But these are generalizations, not guarantees! As evidenced above, the risks are significant. And forget about making plans for winter under ENSO-neutral conditions - just batten down the hatches and hope for the best!