Have you noticed the lack of big snow in the U.S. this winter? We've been fortunate to see some drenching rains over the past few months and even a handful of small tornadoes. But beside last week's big Denver snow there hasn't been a lot of snow pummeling the U.S. compared to the past few years. Remember last year's ice storm scare here in Houston and the snow that paralyzed the Super Bowl in Dallas in 2011? How about the "Snowmageddon" that shut down many of the major cities along the East Coast last year and in 2010? The answer to winter's cold may lie in a climate signature that can be measured but is difficult to predict, called the North American Oscillation. That's the blue graph above.
Basically, cold air bounces back and forth across the North Pole much the way a ping pong ball crosses the both sides of the net. Sometimes that cold air spends a lot of time in North American and sometimes its over Europe. Scientists measure this oscillation and assign a numeric value corresponding to the strength of the cold air called the NAO index. The graph above shows that for 2012, the last blue bar on the right, the NAO index has been over 2. This means that Europe will be very cold. That has indeed been the case; whole rivers have frozen over in Turkey, Ukraine and Bosnia. Compare that to the NAO index in the two bars before that, which are 2010 and 2011. The value is strongly negative, which corresponds to our cold winters the past two years in the U.S.
So, will the NAO bounce back toward a negative value, bringing brutal winter weather back to North America this year? Hard to tell, because climate scientists don't have much skill in predicting the NAO. Current models show that it may briefly dip toward 0 and perhaps briefly go negative over the next few weeks, so we may yet need to bundle up even more.
To read more about this check out this NASA link: earthobservatory.nasa.gov/IOTD/view.php