It’s official! This was one of the warmest winters ever in the United States. According to NOAA, this meteorological winter (December-February) was the fourth warmest on record in the lower forty-eight. Registering an average temperature of 36.8°F, the country was 4°F above its long-term average.
Scientists say that a number of factors played a part in producing this unseasonable warmth. To begin with, the Jet Stream, the boundary between warm southern air and cold arctic air, stayed well to the north this winter. As a result, mild conditions dominated the season and most of the country received below average snowfall. In fact, it was our third smallest winter snow cover in forty-six years of satellite record keeping. Without snow to reflect the sun’s rays, the exposed ground absorbed solar energy, helping to perpetuate the warm conditions.
Two other significant factors in this winter’s story are La Nina and the North Atlantic Oscillation (NAO). These powerful climate systems influence the shape and path of the Jet Stream. La Nina, part of the larger El Nino Southern Oscillation (ENSO), brings cooler than normal water to the eastern Pacific Ocean. As a result, high pressure builds over the cool water and pushes the Jet Stream northward. NAO, the southern branch of the larger Arctic Oscillation, runs in positive and negative phases, depending on the pressure differences between Iceland and the Azores. A positive phase, like the one that dominated this winter, promotes a fairly straight path for the Jet Stream. It also brings warmer than normal conditions to the eastern two-thirds of the U.S.
Given these constraints, the Jet Stream only managed to dip south a few times this season. When it did, we were abruptly reminded that it was, in fact, still winter. Those few cold blasts, however, never lasted very long. On the whole, it felt like a year without a winter.