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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.

ENSO CONDITIONS AND SEASONAL MINIMUM TEMPERATURES AT FOUR LOCATIONS 1949-1996
Olympia Airport, Seattle City Office, Bellingham Airport, and Sea-Tac Airport
Temperatures in degrees Fahrenheit
 
Winter All Year* Nov-March* Sept-Nov* OlympiaSeattleB'hamSea-TacAvg**
1949-50strong La Niña strong La Niña (n/a) -1 12 -2 0 2.25
1950-51moderate La Niñastrong La Niña strong La Niña 9 20 10 11 12.5
1951-52moderate El Niñomoderate El Niñomoderate El Niño8 19 9 10 11.5
1952-53ENSO neutral ENSO neutral ENSO neutral 15 26 16 17 18.5
1953-54ENSO neutral ENSO neutral ENSO neutral 7 22 10 16 13.75
1954-55strong La Niña strong La Niña strong La Niña 13 21 11 10 13.75
1955-56strong La Niña strong La Niña strong La Niña -1 13 3 6 5.25
1956-57moderate La Niñamoderate La Niñastrong La Niña 0 15 8 8 7.75
1957-58strong El Niño strong El Niño strong El Niño 25 33 22 27 26.75
1958-59ENSO neutral ENSO neutral ENSO neutral 7 18 9 13 11.75
1959-60ENSO neutral ENSO neutral ENSO neutral 14 29 13 24 20
1960-61ENSO neutral ENSO neutral ENSO neutral 18 30 22 23 23.25
1961-62moderate La Niñamoderate La Niñamoderate La Niña4 20 13 13 12.5
1962-63moderate La Niñamoderate La Niñamoderate La Niña5 16 9 12 10.5
1963-64moderate El Niñomoderate El Niñomoderate El Niño21 31 22 25 24.75
1964-65moderate La Niñamoderate La Niñastrong La Niña 3 11 -1 10 5.75
1965-66strong El Niño strong El Niño strong El Niño 21 28 13 25 21.75
1966-67ENSO neutral ENSO neutral ENSO neutral 23 34 22 28 26.75
1967-68moderate La Niñamoderate La NiñaENSO neutral 12 27 18 24 20.25
1968-69moderate El Niñomoderate El NiñoENSO neutral 3 10 0 6 4.75
1969-70moderate El Niñomoderate El Niñomoderate El Niño17 29 17 29 23
1970-71strong La Niña strong La Niña moderate La Niña12 26 13 23 18.5
1971-72moderate La Niñamoderate La Niñastrong La Niña -7 16 8 12 7.25
1972-73strong El Niño strong El Niño strong El Niño -3 (n/a) 7 13 (8.5)
1973-74strong La Niña strong La Niña strong La Niña 6 20 14 16 14
1974-75moderate La Niñamoderate La Niñamoderate La Niña19 28 17 25 22.25
1975-76strong La Niña strong La Niña strong La Niña 13 24 18 22 19.25
1976-77moderate El Niñomoderate El Niñomoderate El Niño12 25 19 24 20
1977-78moderate El Niñomoderate El Niñomoderate El Niño10 20 17 21 17
1978-79ENSO neutral ENSO neutral moderate La Niña-8 13 9 12 6.5
1979-80ENSO neutral moderate El NiñoENSO neutral 0 16 9 15 10
1980-81ENSO neutral ENSO neutral ENSO neutral 15 24 8 21 17
1981-82ENSO neutral ENSO neutral ENSO neutral 5 17 1 14 9.25
1982-83strong El Niño strong El Niño strong El Niño 19 26 20 24 22.25
1983-84moderate La Niñamoderate La Niñamoderate La Niña-7 12 12 8 6.25
1984-85moderate La Niñamoderate La Niñamoderate La Niña11 22 12 19 16
1985-86ENSO neutral ENSO neutral ENSO neutral 0 12 3 10 6.25
1986-87strong El Niño strong El Niño moderate El Niño19 27 23 23 23
1987-88strong El Niño strong El Niño strong El Niño 16 24 17 22 19.75
1988-89strong La Niña strong La Niña strong La Niña 2 11 5 7 6.25
1989-90ENSO neutral ENSO neutral ENSO neutral 10 22 16 19 16.75
1990-91moderate El NiñoENSO neutral ENSO neutral 0 (n/a) 5 12 (7.5)
1991-92strong El Niño strong El Niño moderate El Niño22 26 23 25 24
1992-93ENSO neutral ENSO neutral ENSO neutral 10 21 12 23 15.75
1993-94ENSO neutral ENSO neutral ENSO neutral 10 21 13 19 15.75
1994-95strong El Niño strong El Niño moderate El Niño6 23 17 20 16.5
1995-96moderate La Niñamoderate La Niñamoderate La Niña10 17 10 14 12.75
47-winter average 9.04 21.27 12.17 17.02 14.79

AVERAGE SMT** FOR EACH ENSO CATEGORY
Based on Nov-MarchBased on Sept-Nov
Strong La Niña 11.47 (8 winters) 10.19 (9 winters)
Moderate La Niña12.13 (10 winters) 13.16 (8 winters)
ENSO neutral 14.91 (14 winters) 14.83 (16 winters)
Moderate El Niño15.86 (7 winters) 19.96 (8 winters)
Strong El Niño 20.31 (8 winters) 19.8 (5 winters)

* Represents the time span evaluated to determine ENSO phase. "All year" is calculated from July through June.
** Average of all four locations for each winter.

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!

More information:
ENSO at Wikipedia
Cascade Snowfall and Snowdepth During El Niño and La Niña seasons

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