Why has this winter been so warm?
January 21st, 2012 at 10:13 am (Weather)
This question has come up a few times recently. Even here in L.A. we’ve noticed that the winter has seemed, well, not very wintry. (During the first week in January, temperatures here were reaching the high 80’s!) Friends in states with true winter seasons have commented on the lack of snow and mild temperatures. What’s going on?
Bill Patzert, a climatologist at JPL, explains some of the mechanisms in “What Happened to All The Snow?” He cites two factors: the La Niña effect, which results in drier weather for the U.S., and something called arctic oscillation. This oscillation is a changing relationship between atmospheric pressures in the Arctic and those further south in the mid-latitudes.
Right now we are in a “strong positive” phase (corresponding to the left side of the graphic), so storms tend to be driven further north and to miss the continental U.S. So Alaskans are still getting plenty of snow!
This difference is not just anecdotal; there are quantitative measurements that we can study. Below are plots of the Arctic Oscillation (AO) Index from last winter and this winter showing how it has gone from mostly negative in 2010 to mostly positive in 2011.
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| Winter 2010 | Winter 2011 |
And here is a longer-term view of the AO Index from 1950 to 2002. It’s clear why this phenomenon is referred to as an “oscillation,” although the 90’s appear to have stayed positive for quite a while.
