Abstract
The NCAR CAM3 GCM with prescribed, satellite‐based snow albedo (SA) is used to investigate the remote effects of snow cover on Northern Hemisphere (NH) winter climate. A pair of 100 ensemble member experiments are integrated through the autumn‐winter season, with prescribed high and low SA over Eurasia (EA). Similar to other non‐CAM GCM studies using prescribed snow mass, anomalous EA snow albedo produces a wave activity pulse that propagates into the stratosphere, culminating in a negative phase Arctic Oscillation– (AO‐) like surface response. This occurs for idealized but representative SA anomalies, as well as for more realistic SA anomalies. Similar experiments over North America (NA) and the entire NH are also performed. Unlike prior studies, anomalous NA snow yields a significant AO signal. Here, the local NA surface cooling elicits a transient eddy response, which propagates downstream to Eurasia, resulting in significant but short‐lived cooling and upward propagating wave activity over Siberia. A negative AO‐like response develops, primarily confined to the stratosphere/upper troposphere, which eventually gives way to a tropospheric AO‐like response of the opposite phase, due to equatorward wave refraction and wave divergence. Reanalysis data support this NA snow‐positive AO response. Snow forcing experiments for the whole NH, however, yield a weakened AO signal. This is due to a muted wavenumber‐1 and ‐2 response caused by destructive interference between the background stationary wave and the corresponding Rossby wave response, which results in negligible wave activity reaching the stratosphere.
Highlights
[2] The dominant mode of extratropical Northern Hemisphere (NH) wintertime atmospheric circulation variability is referred to as the Arctic Oscillation (AO) [Thompson and Wallace, 1998] or the Northern Annular Mode (NAM) [Thompson and Wallace, 2001]
[10] In this paper, we present snow‐AO experiments with the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM3), focusing on the remote effects of EA, North America (NA), and NH snow cover
Similar to prior General Circulation Model (GCM) results, we show that EA snow cover is able to affect climate remotely and modulate the dominant mode of NH wintertime atmospheric circulation variability
Summary
[2] The dominant mode of extratropical Northern Hemisphere (NH) wintertime atmospheric circulation variability is referred to as the Arctic Oscillation (AO) [Thompson and Wallace, 1998] or the Northern Annular Mode (NAM) [Thompson and Wallace, 2001]. [6] Observations support a snow‐AO mechanism, whereby anomalously high EA snow cover results in an increase in upward stationary Rossby waves, which deposit their easterly momentum in the stratosphere, slowing the polar vortex and increasing polar cap geopotential heights, resulting in a negative stratospheric AO This anomaly propagates downward through the troposphere [Haynes et al, 1991; Baldwin and Dunkerton, 1999, 2001], resulting in a negative AO‐like response at the surface [Saito et al, 2001; Cohen et al, 2007; Hardiman et al, 2008]. Enhanced equatorward wave activity in the troposphere, associated with a poleward momentum flux, resulted in a weak, positive AO‐like surface response This is consistent with Klingaman et al [2008] who showed snow cover over the northern Great Plains of the United States forced a positive NAO, but not until late winter (January–February).
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