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| Titel |
Why can't climate models capture the observed connection between seasonal snow cover and the Northern Annular Mode? |
| VerfasserIn |
Karen L. Smith, Paul J. Kushner, Judah Cohen, Christopher G. Fletcher |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057386
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| Zusammenfassung |
| The suite of general circulation models (GCMs) in the Coupled Model Intercomparison
Project (CMIP3) fail to reproduce the observed negative lag correlation between interannual
variations in October Eurasian snow cover anomalies and the tropospheric Northern Annular
Mode (NAM) in the subsequent winter. This deficiency is directly related to the fact that
models fail to reproduce the observed positive lag correlation between October Eurasian
snow cover anomalies and wintertime Rossby wave activity fluxes from the troposphere to
the stratosphere. This issue is analyzed here in the context of the following hypothesis about
the snow-circulation connection: in order to achieve amplification of the wave activity, the
vertically propagating Rossby wave train associated with the snow cover anomaly must
reinforce the climatological stationary wave, which corresponds to constructive linear
interference between the anomalous wave and the climatological wave. It is shown that the
lag in peak wave activity flux arises because the Rossby wave train associated with the
snow cover is in quadrature or out of phase with the climatological stationary wave
from October to mid-November. Beginning in mid-November the associated wave
anomaly migrates into a position that is in phase with the climatological wave,
leading to constructive interference and anomalously positive upward wave activity
fluxes until mid-January. (This linear interference effect is not only associated with
stratospheric warming due to Eurasian snow cover anomalies but is a general feature
of Northern Hemisphere troposphere-stratosphere interactions, and in particular
dominated the negative NAM events of the fall-winter of 2009-2010.) By contrast, the
CMIP3 GCMs typically show a negative correlation between October Eurasian snow
cover and December wave activity flux, which is related to destructive interference
between the wave train associated with the snow and the background stationary
wave. This work clarifies why it might be so challenging for models to simulate
connections between the NAM and snow cover or other tropospheric sources of NAM
variability: in order to capture these connections, both the structure of the forced wave
anomalies and the climatological stationary waves must be accurately captured. |
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