January 19, 2020 Update
by MICHAEL PIDWIRNY
During the winter months of December, January and February the climate of northwest USA and southern British Columbia is defined by cooling temperatures and an increase in precipitation. Temperatures cool mainly because of 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 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 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 northwest USA and southern British Columbia is strongly influence 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 2019/20
Neither El Niño or La Nina (neutral) conditions have been observed over equatorial Pacific Ocean from mid-November 2019 to mid-January 2020. (Figures 11 and 12). National Oceanic and Atmospheric Administration’s Climate Prediction Center computer models are predicting a 70% chance that neutral conditions will remain for the next few months.
The Pacific Decadal Oscillation (PDO) index from January 2018 to December 2019 is shown in Figure 13 (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 and then a rebound to higher values in November and December. Satellite sea surface temperature measurements in Figure 12 show a cooling trend in the above-average sea surface temperatures along the west coast from California to Alaska.
In conclusion, current patterns associated with El Niño / La Niña and the Pacific Decadal Oscillation do not provide any concrete insight for forecasting the climate of the winter of 2019/20.
Climate Prediction Center – North American Multi-Model Ensemble Long-Range Seasonal Forecasts – December 2019
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 2019/20. 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 2019. This forecast suggests temperatures will be about 1.0 to 2.0°C above-normal for British Columbia, Alberta, Washington state, eastern Oregon, Idaho, Montana, Utah and Colorado.
Figure 14 describes what actually occurred December 2019 in terms of surface mean temperature. The Climate Prediction Center did a pretty good job forecasting temperatures along the west coast of North America. The forecast model failed to forecast the below-normal temperature in eastern North America.
Figure 15 describes the December precipitation forecast for North America from the Climate Prediction Center released in November 2019. This forecast suggests above-normal precipitation for much of British Columbia, Alberta Rocky Mountains, Washington state, Oregon, northern Idaho, and Colorado.
Figure 16 describes what actually occurred December 2019 in terms of precipitation. The Climate Prediction Center did a pretty good job forecasting above average precipitation in southern British Columbia. The November forecast also seems to have accurately predicted below-average precipitation conditions in eastern North America.
Climate Prediction Center – North American Multi-Model Ensemble Long-Range Seasonal Forecasts – January 2020 and February 2020.
Figures 17 and 18 describe respective January and February surface mean temperature forecasts for North America released in December 2019. The January forecast suggests temperatures will be slightly above-normal for British Columbia, Alberta, Montana, and Washington state (Figure 17). While Oregon, California, Idaho, Wyoming, Utah, and Colorado will see near normal temperatures.
The February forecast suggests temperatures will be near normal for British Columbia, Alberta, Montana, Oregon, and Washington state (Figure 17). While California, Idaho, Wyoming, Utah, and Colorado will see slightly above-normal temperatures.
Figures 19 and 20 describe respective January and February precipitation rate forecasts for North America released in December 2019. The January forecast suggests precipitation will be above-normal for British Columbia, Alberta Rockies, parts of Colorado, and northern Washington state (Figure 19). Below-normal precipitation will occur in California. Else where precipitation conditions will be near normal.
The February forecast suggests precipitation will be above-normal in Washington state, Idaho, Oregon, western Montana, and southern British Columbia and Alberta (Figure 20). California, New Mexico, and the southwestern corner of British Columbia will see below-normal precipitation.