Summary: Forcing of the Arctic Oscillation by Eurasian Snow Cover
ROBERT J. ALLEN* AND CHARLES S. ZENDER
Department of Earth System Science, University of California, Irvine, Irvine, California
(Manuscript received 22 October 2010, in final form 9 May 2011)
ABSTRACT
Throughout much of the latter half of the twentieth century, the dominant mode of Northern Hemisphere
(NH) extratropical wintertime circulation variability--the Arctic Oscillation (AO)--exhibited a positive
trend, with decreasing high-latitude sea level pressure (SLP) and increasing midlatitude SLP. General cir-
culation models (GCMs) show that this trend is related to several factors, including North Atlantic SSTs,
greenhouse gas/ozone-induced stratospheric cooling, and warming of the Indo-Pacific warm pool. Over the
last approximately two decades, however, the AO has been decreasing, with 2009/10 featuring the most
negative AO since 1900. Observational and idealized modeling studies suggest that snow cover, particularly
over Eurasia, may be important. An observed snow­AO mechanism also exists, involving the vertical
propagation of a Rossby wave train into the stratosphere, which induces a negative AO response that couples
to the troposphere. Similar to other GCMs, the authors show that transient simulations with the Community
Atmosphere Model, version 3 (CAM3) yield a snow­AO relationship inconsistent with observations and
dissimilar AO trends. However, Eurasian snow cover and its interannual variability are significantly under-
estimated. When the albedo effects of snow cover are prescribed in CAM3 (CAM PS) using satellite-based
snow cover fraction data, a snow­AO relationship similar to observations develops. Furthermore, the late-
twentieth-century increase in the AO, and particularly the recent decrease, is reproduced by CAM PS. The

Source: Allen, Robert J. - Department of Earth Sciences, University of California at Riverside

Collections: Geosciences; Environmental Sciences and Ecology