Gary McManus
Editor’s Note: Gary McManus is an associate state climatologist for the Oklahoma Climatological Survey
Two significant ice storms and several heavy snows, including a record-setting 13.5 inches in Oklahoma City during the historic Christmas Eve blizzard, have left Oklahomans wondering where the warm winters of the past 20 years have gone.
While Vancouver has had to truck snow in for their Olympic Games, Oklahoma and points even farther south have gotten a crash course in Midwestern-style winters. Oklahomans have no doubt enjoyed that string of warmer winters over the last couple of decades. In fact, 14 of the previous 20 winters in Oklahoma have been above the long-term average in temperature, including the all-time warmest in 1991-92 and the second warmest on record in 1999-00. That is not to say that every winter has been or should be warmer than the last, of course. Natural variability still plays a part in Oklahoma’s weather conditions and always will. Even during this string of warmer winters, cold winters will still materialize, such as the current winter and that of 2000-01. Natural variability at times leaves Oklahoma at the mercy of weather patterns from far-off parts of the world. That is exactly what has occurred this season as sea-surface temperatures in the equatorial Pacific Ocean and higher-than-normal pressure over the North Pole has left us chilled in a seemingly perpetual gloom.
The 2009-10 El Niño event, previously forecast to be of moderate intensity, turned out to be the strongest since the 1997-98 “super” El Niño. During a strong El Niño, the storm track shifts to the south during winter and can bring more storm systems across the state. Along with those storm systems comes more cloudiness and precipitation. Those two trends are evident in data from the Oklahoma Mesonet, the state’s weather network. The statewide average precipitation total since the beginning of climatological winter Dec. 1 through Feb. 22 stands at 5.26 inches, a third of an inch above normal and the 29th wettest winter since 1921.
The totals from southern Oklahoma are much higher than normal, with southwestern Oklahoma experiencing its 11th wettest winter thus far. Similar totals exist for south central and southeastern Oklahoma. This is exactly the type of pattern expected with El Niño and the southerly shifted storm track.
A casualty of all that cloudiness and precipitation is sunshine, and data from the Oklahoma Mesonet exemplifies that once again. The Mesonet’s instruments that measure solar radiation have received a mere 46.2 percent of possible sunshine this winter, the second-lowest total since the Mesonet began in 1994. Only the winter of 1997-98, the second wettest on record and the “super” El Niño winter, was gloomier.
The impact on Oklahoma’s temperatures has been to suppress the daytime high temperatures much more than the lows. The average high temperature from the Oklahoma Mesonet this winter has been nearly 6 degrees below normal while low temperatures are less than 2 degrees below normal. The cloudiness helps block solar radiation during the day while trapping heat during the overnight hours. For the winter thus far, the statewide average temperature was 34.6 degrees. Oklahoma City’s lowest daily temperature this season has been 6 degrees, which is certainly cold, but nowhere near the sub-zero temperatures seen in its past. The coldest daily temperature reported during Oklahoma City’s relatively warm winter last year was also 6 degrees.
Another culprit responsible for our cold winter is a phenomenon known as the Arctic Oscillation (AO), which can shift between negative and positive phases during the span of a couple of weeks. When it is in the negative phase, as it has been for much of the winter, high pressure over the North Pole deflects the bitterly frigid air from that region farther to the south into the interior of the United States.
Temperatures in the Arctic region rise tremendously and those farther to the south plummet. The values for the AO Index, which measures the relative strength of the AO, during December were the most negative for that month since record keeping began in 1950, and have dropped to that level again during February. Combine a strongly negative AO with a strong El Niño and you get the historic snows that much of the eastern half of the United States has seen this winter, as well as those in Oklahoma and Texas.
Even as the vagaries of Mother Nature conspire to give us a winter more reminiscent of our past than our present, it is important to remember that a single season or year does not make a trend. Until the weather pattern changes, Oklahomans might see their Midwestern-style winter continue.
