Jan 04, 2012
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With the arrival of the New Year, many people east of the Mississippi River have been wondering what has happened to winter. Most areas of the Eastern US have seen well below normal snowfall and unseasonably warm weather. This is especially attention grabbing following the previous two winters, when much of the East saw well above normal snowfall and below normal temperatures. The reasons accounting for winter’s mildness thus far is the continuation of La Nina along with positive North Atlantic (NAO) and Arctic Oscillations (AO). These features drastically influence longer term global weather patterns and their state.
Most have heard of La Nina (cooler than normal equatorial waters in the Pacific), or it’s opposite El Nino (warmer than normal) and their influences on winter weather patterns (Fig.1); but unless you are in the field of meteorology, you are likely not aware of the NAO and AO, two principal mechanisms driving winter weather in the East.
The NAO is an oscillation of the pressure difference between an area of low pressure near Iceland and high pressure near the Azores in the subtropical Atlantic. When the NAO is positive, there is a significant difference in pressure between the two with a strong Azores High. When the NAO is negative, the difference in pressure weakens and the aforementioned Azores High is not as strong (Fig. 2). The AO is similar to the NAO, except it represents oscillations in atmospheric pressure in the polar region and the central Atlantic. The AO takes into account the difference between low pressure near the North Pole and high pressure over the central Atlantic. When the AO is positive, the pressure difference is greater with the central Atlantic High being strong. When the AO is negative, the opposite is true with a small difference and weaker area of high pressure in the mid latitudes (Fig. 3).
So, just how have the North Atlantic and the Arctic Oscillation led to a mild winter and a lack of snow in the Eastern US thus far? As previously stated, the positive phase of both oscillations results in strong high pressure in the mid and subtropical latitudes of the Atlantic. This is exactly what has been happening this winter with the western periphery of strong high pressure dominating the Eastern US. As a result, the higher pressures have kept the Arctic air from progressing southward. This has also resulted in a very unfavorable storm track for snowfall in the East with winter storms tracking well west and north of the Eastern US
It is lower pressures in the central Atlantic associated with a negative North Atlantic Oscillation and negative Arctic Oscillation that are more favorable for setting up weather patterns conducive for cold and snow in the Eastern US. So far the winter of 2011-2012 has yet to see this occur. With that in mind, the oscillation patterns could change as the current state of the science does not allow for an accurate prediction of the oscillation patterns more than a just few weeks in advance. Stay tuned.
Images courtesy: NOAA, State Climate Office of North Carolina.
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