Impacts of the El Niño/Southern Oscillation
on the Pacific Northwest
George H. Taylor, State Climatologist
Tye Parzybok, Research Scientist
October, 1997
I. Introduction
The El Niño-Southern Oscillation (ENSO) exerts a profound influence on global weather and climate patterns. A great deal of time and effort have been spent investigating the phenomenon, with good success. Increasingly, ENSO predictions and assessments are being used for decision-making, with benefits in terms of economics, public safety, and the environment. Oregon and the Pacific Northwest are strongly influenced by ENSO, and as ENSO information has improved and become more publicized it is being used more frequently in both the public and private sector for everyday and long-term decisions. Below is a brief overview of ENSO's effects in this region, followed by several examples of how such information is influencing decisions.
II. Effects of ENSO in the Pacific Northwest
The earliest systematic study of ENSO in the
Northwest was Redmond and Koch (1991). They concluded that there are "a few
dominant modes which account for most of the temporal variation in the surface
climate." They determined that the Southern Oscillation Index (SOI) can be
used as a predictor for climate, especially during winter; greatest correlations
between SOI and winter climate patterns occurred with about a 4-month time lag,
with summer average SOI correlating well with conditions in the Northwest during
the following winter (SOI values less than zero represent El Niño
conditions, near zero values are "normal" or average, and positive
values represent La Niñas). The results were sufficiently strong that the
authors suggested a cause-effect relationship.
In recent years, Oregon Climate Service (OCS) has
studied various aspects of this SOI-climate relationship in the Northwest. We
have also investigated use of other indices and correlations involving averages
other than the summer SOI. Some general results are listed below.
1. Precipitation
For the most part, El Niño or warm events
correlate with below-average precipitation the following winter in the
Northwest. In southern Oregon, the correlation is fairly low, but north of about
Roseburg (and extending into British Columbia) the correlation is fairly strong.
The winter of 1982-83 was a notable exception (it was a very wet winter
throughout the Northwest), but the intensity and timing of that event was
unprecedented, at least in the last 75 years.
Figure 1 is a plot showing the summer average SOI versus precipitation the following winter in Portland; the line shows the best-fit linear correlation. El Niño years (negative SOI values) are associated with lower winter precipitation, while La Niña conditions (positive SOIs) are likely to produce wetter than average winters.
Figure 1.
Portland Winter Precipitation vs. Previous Summer SOI
Figure 2 shows Water Year (Oct.-Sep.) precipitation for the Oregon Coast
climate division compared with the previous summer's SOI (by categories),
showing a similar relationship.
Figure 2. Water Year precipitation, Oregon Coast division, vs. previous summer SOI
2. Temperature
Winter temperatures correlate well with SOI values. In general, negative SOI (El Niño) conditions are associated with mild winter temperatures, while positive SOIs (La Niñas) have a greater likelihood of colder than average winter temperatures. These correlations apply for both long-term (monthly and seasonal averages) and short-term (individual days) periods.
Figure 3 below shows mean monthly February temperatures at Astoria, Oregon,
compared with the previous Oct.-Dec. average SOI. El Niño years (negative
values) generally result in mild conditions during late winter, while La Niñas
are associated with colder temperatures. Figure 4 shows extreme low temperatures
in Salem, Oregon in February, compared with the same Oct.-Dec. SOI average.
Extreme cold events occur almost exclusively during La Niña years.
Figure 3. Mean January-March temperature, Astoria, vs. Oct.-Dec. average SOI
Figure 4. Extreme low temperature, Salem, February, vs. Oct.-Dec. average SOI
3. Snowfall
A consistent correlation throughout Oregon exists between SOI and total snowfall. At either end of the SOI distribution (strong El Niños or strong La Niñas), total snowfall in valley locations is relatively low; this is true both east and west of the Cascades. Although years with moderate (near-zero) SOI values may also have low snowfall totals, the years with greatest snowfall occur in conjunction with these moderate values. Figure 5 shows a plot of total snowfall compared with the previous summer SOI for Hood River, Oregon.
Figure 5. Total winter snowfall, Hood River, vs. previous summer SOI
4. Growing season
El Niño conditions generally correspond with longer growing seasons, primarily because of milder late winter-early spring temperatures. The Hood River example below is a typical situation.
The Hood River Valley, just north of Mt. Hood (Oregon's highest point) is one of Oregon's most important agricultural area. Each year the 40 square mile valley produces over $70 million in agricultural products, mostly fruit (apples, cherries, peaches, and pears) and nuts. The most significant problem for Hood River growers is cold temperatures in the spring. For example, a late occurrence of a hard freeze (24°F) could virtually decimate a year's fruit crop, especially if trees had already set fruit. To counter such problems, many growers have installed large wind machines to improve mixing on cold, calm nights; some have actually hired helicopters to fly over their orchards during such periods.
Several years ago, OCS undertook a brief research project to examine the occurrence of low temperatures. We found a significant correlation between summer SOI and the date of the last occurrence of 24° the following spring. Figure 6 shows the correlation. During El Niño years, late freezes seldom occur, but during La Niña conditions they are frequent.
Figure 6.
Julian day of last 24 degree temperature, Hood River, vs. previous summer SOI
The information presented, although rather preliminary, has been used as a planning tool by Hood River growers. Further refinement of this analysis may yield much more useful information, but this cursory study has shown the potential of using ENSO data for even extreme-event forecasting in an important economic area.
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