by MICHAEL PIDWIRNY
During the winter months of December, January, and February the climate of the northwest USA and southern British Columbia is defined by cooling temperatures and an increase in precipitation. Temperatures cool mainly because of the reduction in heat energy supplied by the Sun. During the winter months, the intensity of the solar radiant energy declines because of lower Sun angles and a shortening of day length. Figure 1 describes the average near-surface temperature (2 meters above ground level) for the winter season. Temperatures along the west coast are moderated by the stored heat energy found in the surface waters of the Pacific Ocean.
Figure 2 describes the typical patterns of winter precipitation for North America. High amounts of precipitation occur along much of the west coast because of mid-latitude cyclones and orographic uplift. During winter, mid-latitude cyclones often originate in the northeastern Pacific Ocean and then move in an easterly direction. Orographic enhancement of the quantity of precipitation falling occurs because of the presence of mountains found running from Alaska to California. Central North America is relatively dry compared to the west coast because most of the precipitable water held in the clouds of the mid-latitude cyclones previously precipitated out and the cold continental air masses found here hold little moisture.
Variation in year-to-year winter weather of the northwest USA and southern British Columbia is strongly influenced by several large-scale cyclic climate phenomena which modify large scale atmospheric circulation and sea surface temperatures in the Pacific Ocean. Further, these climate factors tend to have a significant impact on temperature and precipitation trends during the winter months over southern British Columbia and Washington state. El Niño – usually brings warmer than average winters and below-average precipitation to this area of the Pacific Northwest. La Niña – is often associated with cold winters with average to above-normal precipitation. Generally, the effects of significant El Niño and La Niña are limited to one or maybe two consecutive winter seasons. Operating on a much longer timescale of one to three decades there is another cyclic climate factor of importance known as the Pacific Decadal Oscillation. The Pacific Decadal Oscillation seesaws between a warm or a cold state and these phases seem to amplify the climatic effects of co-occurring El Niño and La Niña events, respectively.
El Niño is the name given to the cyclical development of warm ocean surface waters on the east side of Pacific Ocean at the equator. This climate event usually occurs around Christmas and usually lasts for a few weeks (weak) to a few months (strong). El Niño is created by a reduction in the speed of the Trade Winds right along the equator which results in a shift in atmospheric circulation and pressure patterns in this region of the planet (Figure 3). Sometimes an extremely warm El Niño can develop and last for more than a year. Since 1935, significant El Niño events have developed in 1958, 1966, 1978, 1983, 1987, 1990, 1992, 1993, 1998, 2005, 2010 and 2016. Figure 4 shows the general global patterns of winter surface temperature warming and cooling associated with El Niño.
La Niña is the name given to the cyclical development of cold ocean surface waters on the east side of Pacific Ocean at the equator. Similar to El Niño , a La Niña climate event usually occurs around Christmas and normally lasts for a few weeks (weak) to a few months (strong). La Niña is created by an increase in the speed of the Trade Winds along the equator which results in a shift in atmospheric circulation and pressure patterns in this region of the planet (Figure 5). Sometimes an extremely cold La Niña can develop and last for more than a year. Since 1935, significant La Niñas have occurred in 1950, 1956, 1967, 1971, 1974, 1976, 1999, 2008, and 2011. Figure 6 shows the general global patterns of winter surface temperature cooling and warming associated with La Niña.
Figure 7 shows the relative strength of El Niño and La Niña events from 1930 to 2019 accord to the Southern Oscillation Index (SOI). On this figure, negative values indicate El Niño conditions with lower values suggesting stronger events. High positive values indicate significant La Niña events.
The Pacific Decadal Oscillation
The Pacific Decadal Oscillation (PDO) is a cyclical pattern of ocean-atmosphere climate variability that occurs in the North Pacific Ocean. The PDO is detected as a change in sea surface temperatures over the Pacific Ocean from 20 to 60° North latitude. There are two phases that can last many years to several decades as shown in Figure 8. During the warm or positive phase, sea surface temperatures in the western North Pacific Ocean become cooler, while the eastern side of this ocean warms (Figure 9). The warm phase results in a zone of warm seawater hugging the west coast of North America from Alaska down to the Baja Peninsula. During the cold or negative phase, sea surface temperatures in the western North Pacific Ocean becomes warmer, while the eastern of this ocean cools down (Figure 10). Significant reversals in the prevailing phase of the PDO have occurred around 1957, 1961, 1977, 1998, and 2014 (Figure 8).
Forecast Winter 2020/21
La Niña conditions are now being observed over the equatorial Pacific (Figure 11). Computer models suggest there is an 85% chance that La Niña will continue in December, January, and February.
The Pacific Decadal Oscillation (PDO) index from January 2018 to September 2020 is shown in Figure 12 (and see web page Monthly PDO Index). From the summer of 2018 until fall 2019 the monthly PDO index rose from near zero to around +1.0. A sudden decline into negative territory occurred in October 2019, then a rebound to higher values in November and December, and mainly negative values from January to September 2020.
In conclusion, current patterns associated with La Niña and the Pacific Decadal Oscillation suggest that the climate of the winter of 2020/21 will be colder than normal with higher than normal precipitation for southern British Columbia and western Alberta.
Climate Prediction Center – North American Multi-Model Ensemble Long-Range Monthly Forecasts – December 2020
There is one more important piece of information that can provide us with some insight as to what the winter season will be like in Pacific Northwest USA and southern British Columbia in 2020/21. National Oceanic and Atmospheric Administration’s Climate Prediction Center creates long-range seasonal forecasts based on the average of seven different General Circulation Model simulations. Figure 13 describes the December surface mean temperature forecast for North America released in November 2020. This forecast suggests temperatures will be normal for southern British Columbia and Alberta, and 0.25 to 2.0°C above-normal for Washington state, eastern Oregon, Idaho, Montana, Utah, and Colorado.
Figure 14 describes the December precipitation forecast for North America from the Climate Prediction Center released in November 2020. This forecast suggests well above-normal precipitation for much of British Columbia, Alberta Rocky Mountains, Washington state, Oregon, northern Idaho, Montana, and northern California.
Climate Prediction Center – North American Multi-Model Ensemble Long-Range Seasonal Forecasts – January 2021 and February 2021.
Figures 15 and 16 describe respective January and February surface mean temperature forecasts for North America released in December 2020. The January forecast suggests temperatures will be slightly below-normal for British Columbia and Alberta. Mainly normal for Washington State, Oregon, Montana, and northern Idaho (Figure 15). While much of California, Idaho, Wyoming, Utah, and Colorado will see above-normal temperatures.
The February forecast suggests temperatures will be below-normal for British Columbia, Alberta, Montana, Oregon, and Washington state (Figure 16). While California, Idaho, Wyoming, Utah, and Colorado will see normal to above-normal temperatures.
Figures 17 and 18 describe respective January and February precipitation rate forecasts for North America released in December 2020. The January forecast suggests precipitation will be above-normal for most of British Columbia, Alberta, Washington state, Oregon, Idaho, western Wyoming, western Colorado, and Montana (Figure 17). Below-normal precipitation will occur in California. Elsewhere precipitation conditions will be near normal.
The February forecast suggests precipitation will be above-normal in Washington state, northern Idaho, Oregon, western Montana, and southern British Columbia, and western Alberta (Figure 18). Near normal for much of Utah and Colorado. California, New Mexico, and Arizona will see below-normal precipitation.