| Summary: | <p>As the leading climate mode of wintertime climate variability over Europe,
the North Atlantic Oscillation (NAO) has been extensively studied over the
last decades. Recently, studies highlighted the state of the Eurasian
cryosphere as a possible predictor for the wintertime NAO. However, missing
correlation between snow cover and wintertime NAO in climate model
experiments and strong non-stationarity of this link in reanalysis data are
questioning the causality of this relationship.</p>
<p>Here we use the large ensemble of Atmospheric Seasonal Forecasts of the 20th
Century (ASF-20C) with the European Centre for Medium-Range Weather
Forecasts model, focusing on the winter season. Besides the main 110-year
ensemble of 51 members, we investigate a second, perturbed ensemble of 21
members where initial (November) land conditions over the Northern
Hemisphere are swapped from neighboring years. The Eurasian snow–NAO
linkage is examined in terms of a longitudinal snow depth dipole across
Eurasia. Subsampling the perturbed forecast ensemble and contrasting members
with high and low initial snow dipole conditions, we found that their
composite difference indicates more negative NAO states in the following
winter (DJF) after positive west-to-east snow depth gradients at the
beginning of November. Surface and atmospheric forecast anomalies through
the troposphere and stratosphere associated with the anomalous positive snow
dipole consist of colder early winter surface temperatures over eastern
Eurasia, an enhanced Ural ridge and increased vertical energy fluxes into
the stratosphere, with a subsequent negative NAO-like signature in the
troposphere. We thus confirm the existence of a causal connection between
autumn snow patterns and subsequent winter circulation in the ASF-20C
forecasting system.</p>
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