As a former New Englander living in what should be a four-season region (and with due deference to Kent’s S.A.D.), I expect winter to be beautiful, messy and miserably inconvenient. Because I expect it to snow in the winter. So far Central Park is showing goose eggs but the fine folks in the Pacific Northwest are getting clobbered.
What’s up with that?
I know this post is going to spark a global warming discussion, but what I’m really interested in is the actual meteorological mechanism that’s at work. As it happens, NASA has just published a succinct little explanation of what’s guiding the weather so far this season. The short answer: Positive Arctic Oscillation and La Niña.
Arctic Oscillation (AO) has to do with the air moving around the North Pole. A positive AO is associated with low pressure over the North Pole, which keeps the cold air up at the higher latitudes. Negative AO is associated with higher pressure over the North Pole, so the cold air moves farther south and we in the northeast get those nose-hair freezing blasts from across Canada.
The NASA explanation shows us this graph of the pressure for this year’s positive AO as compared with this graph of the pressure for last year’s negative AO.
La Niña is about colder surface water temperatures in the Pacific, which is frankly hard to see here, but I’ll take their word for it.
The NASA explanation says that it has the effect of “push(ing) the jet stream and the cold arctic air northward.” But at the NOAA FAQ page I find some more specific information.
For example, this is a map of average temperature ranks during La Niña events from November to January. That does indeed look familiar.
And here’s the map of the average for precipitation for those same months.
A recipe for snow in Seattle (and not in New York… yet).
(The deeper I dig on this, the more disagreement I find that these two conditions are necessarily predictive. I welcome any deeper insights you can share on the matter.)
