Review of winter 2010 in North America
Summary. The winter of 2010 was anomalously cold in the contiguous United States. Mean seasonal temperatures in many southern and southeastern states were coldest since the late 1970s. At the same time, Canada experienced its warmest and driest winter on record. This temperature anomaly pattern is consistent with a combined effect of a strong El Niño event and extremely strong negative phase of the North Atlantic Oscillation. As usual during El Niño events frequent storms traveled along the southern United States, and many of those states experienced above normal precipitation. Major snowstorms were pounding the East Coast, particularly in February. In several eastern cities, February was the snowiest month on record.
Discussion
The distribution of SAT anomalies during the winter of 2010 (Fig. 1) resembled a typical response to El Niño events, which tend to produce a positive phase of the Pacific/North American (PNA) circulation pattern. The average seasonal (DJF) PNA index for this winter was 0.72. The positive phase of the PNA is characterized by above average temperatures in northwestern North America and below normal temperatures in the southeastern United States and Gulf states. What was remarkable, however, was the magnitude of negative temperature anomalies in the latter region. Mean seasonal temperatures in Texas, Oklahoma, Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida and South Carolina were coldest since the late 1970s, and were among the coldest 10 on record since 1895.
 a) Forecast.
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 b) Observed.
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| Fig. 1. Forecast (a) and observed (b) temperature anomalies in the winter (DJF) of 2010. | |
The spatial extent of negative temperature anomalies was also noteworthy. During El Niño winters they are usually confined to the southeastern corner of the United States. This winter, however, they covered nearly two-thirds of the contiguous states, extending northwestward as far as the Northern Plains. As a result, the average winter temperature for the contiguous United States was the coldest since 1984 and ranked the 18th coldest for the entire record, 1895-2010 (Fig. 2). On the decadal time scale, it’s been steadily declining since its peak in the late 1990s.
These cold temperatures were a result of frequent outbreaks of frigid polar air, which can be attributed to a combined effect of a strong El Niño event and extremely strong negative phases of the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). This winter the NAO index was the lowest on record (see more discussion about the NAO in the review of winter 2010 in Europe).
Another consequence of the El Niño/negative NAO combination was a very mild winter in Canada. Across Canada, the winter of 2010 was 4°C warmer than normal, the warmest winter since Environment Canada started compiling nationwide statistics in 1948 (see also a look back at the winter that was ... or wasn't ... in Canada).
As usual during El Niño events frequent storms traveled along the southern United States, and many of those states experienced above normal precipitation (map). The season-long wet spell was notable for the Southeast. Major snowstorms, with the nicknames such as 'Snowmageddon', 'Snoverkill' and 'Snowicane', were pounding the East Coast, particularly in February. In several eastern cities, February was the snowiest month on record.
Although the seasonal temperature anomaly pattern was generally consistent with what one can expect during El Niño years, there was considerable variability in that pattern from month to month (Fig. 3) as discussed below.
 a) November 2009 - forecast.
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 b) November 2009 - observed.
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 c) December 2009 - forecast.
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 d) December 2009 - observed.
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 e) January 2010 - forecast.
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 f) January 2010 - observed.
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 g) February 2010 - forecast.
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 h). February 2010 - observed.
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| Fig. 3. Forecast(left column) and observed (right column) monthly temperature anomalies from November 2009 through February 2010. | |
November
November was very warm over much of the continent, in a sharp contrast with anomalously cold October. In Canada mean monthly temperatures exceeded +5°C and were in the upper 90th percentile of occurrences. According to the Environment Canada, the Canadian province Ontario experienced a record-breaking month, with many locations breaking records that go as far back as the 1940s. For the contiguous US it was the 3rd warmest November on record since 1895. Only in November 1999 and 2001, average national temperatures were higher. The overall pattern of atmospheric circulation was characterized by nearly a complete disappearance of the mean Hudson Bay trough and predominance of an anomalously zonal flow across the continent. The forecast correctly predicted the signs of temperature anomalies for the large portion of the continent, although negative temperature anomalies in the southeast occupied a smaller area compared to the forecast.
December
The forecast for December was a tough call, because it was almost opposite to what normally occurs during El Niño years. Indeed, December experienced frequent outbreaks of cold Canadian air directed to the central and western United States, which was more typical for La Niña years. The cold air, however, penetrated much farther south than expected. This was largely a result of a strong negative phase of the NAO. The NAO index in December was -1.9, the lowest value (tied with 1963) since 1950. For that particular reason temperatures in the northeastern Canada were much above average. For the contiguous United States the December 2009 temperature was 30.2°F, which is 3.2°F below the long-term average, making it the coldest December since 2000. The eastern US, with temperatures near or moderately below normal, was not was cold as the western US.
January
At the beginning of January, a severe freeze (the worst since 1989 by some accounts) hit the Florida citrus. It was a classical set up for an “advective” freeze associated with a strong cold anticyclone, sliding southeastward from northwest Canada through the Great Plains to the Gulf States (Rogers and Rohli, 1991). By the second decade of the month, however, the temperatures moderated and became close to normal. A strong upper atmospheric ridge was formed over northwestern North America and temperature quickly warmed up to above normal values in Canada, western and northern United States. For the rest of the month, the situation resembled the one typical for strong El Niño events, such as the 1982-83 and 1997-98 events.
February
Cold air in the wake of several reinforcing Arctic air masses dominated much of the United States during February, creating temperatures that were much-below average in the Deep South and below average in the Plains and mid-Atlantic states. Both the South and Southeast climate regions experienced their seventh coldest February on record. Meanwhile, warmer-than-average temperatures dominated the Northwest and Northeast climate regions.
Unseasonably warm weather and rain caused problems during the Olympic Games in Vancouver, disrupting the Alpine skiing schedule. It was the warmest February in Vancouver since 1958, which was also an El Niño year.
Overall, the winter 2010 forecast was quite satisfactory. Even some major aspects of intra-seasonal variability, such as the anti-El Niño temperature anomaly pattern in December, were predicted correctly. However, the magnitude of negative anomalies in the southeastern US and positive anomalies in northeastern Canada in December and January were underestimated. Those anomalies were the result of an extremely negative phase of the NAO that was not expected. February forecast had the highest confidence factors compared to other months and scored the best.