Desierto norte de Chile

Wednesday, February 01, 2012

Warm and snow-less

This winter continues its warm and snow-less march to spring. The current temperature here in Annapolis is 67F, and today's high temperatures here and to our west might top out at 70F! For the first day of February, warmth like that is impressive.

What is governing this warmth? While the atmospheric system is incredibly complex, it often can be diagnosed effectively through the use of climate indices: two that are popular are the El NiƱo-Southern Oscillation (ENSO) index, and the North Atlantic Oscillation (NAO). ENSO measures sea surface temperatures in the tropical eastern Pacific ocean and NAO measures sea level pressures between Iceland and the Azores islands. While the causality of both remains debatable, the effects of each are pretty well known: both affect the frequency, amplitude, and translation of planetary waves, which in turn affect cloud cover, precipitation, and temperature.

The NWS Office in Sterling, VA (who forecasts for us here in Annapolis) has put together a great brief on the combined effects of ENSO & NAO on snowfall in Washington, DC. Their presentation is found here (which came from the media agenda here), and at the Washington Post here.  The years with above-normal snowfall (>15.5" in a year) almost always occur during a negative phase of the NAO, and the majority of the snowy years occur when ENSO is either neutral or warm. The combination of those two yields upper-level troughing and below-normal temperatures over the eastern 2/3rds of the U.S.

The current ENSO Index is -0.979, and the NAO index is around +0.25. On the phase-space diagram from the NWS, that puts us in the upper-left quadrant- the one with the least snowy years. Furthermore, and even less encouraging, is the ensemble NAO forecast, which calls for mostly +NAO for the next 2 weeks. By the time the NAO goes negative again, we'll be heading toward spring!


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