Forcing of the Arctic Oscillation by Eurasian snow cover

1 Throughout much of the latter half of the 20th century, the dominant mode of North2 ern Hemisphere (NH) extratropical wintertime circulation variability−the Arctic Oscillation 3 (AO)−exhibited a positive trend, with decreasing high-latitude sea-level pressure (SLP) and 4 increasing mid-latitude SLP. General circulation models (GCMs) show this trend is related 5 to several factors, including North Atlantic SSTs, greenhouse gas/ozone induced strato6 spheric cooling, and warming of the Indo-Pacific warm pool. Over the last ∼2 decades, 7 however, the AO has been decreasing, with 2009-2010 featuring the most negative AO since 8 1900. Observational and idealized modeling studies suggest that snow cover, partic9 ularly over Eurasia (EA), may be important. An observed snow-AO mechanism also exists, 10 involving the vertical propagation of a Rossby wavetrain into the stratosphere, which induces 11 a negative AO response that couples to the troposphere. Similar to other GCMs, we 12 show that transient simulations with the Community Atmosphere Model version 3 (CAM3) 13 yield a snow-AO relationship inconsistent with observations, and dissimilar AO trends. How14 ever, EA snow cover and its interannual variability are significantly underestimated. When 15 the albedo effects of snow cover are prescribed in CAM3 (CAM PS) using satellite-based 16 snow cover fraction data, a snow-AO relationship similar to observations develops. Fur17 thermore, the late 20th century increase in the AO, and particularly the recent decrease, is 18 reproduced by CAM PS. We therefore conclude that snow cover has helped force observed 19 AO trends, and may play an important role in future AO trends. 20