Abstract
Abstract. 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. 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.
Highlights
As the leading climate variability pattern affecting winter climate over Europe, the North Atlantic Oscillation (NAO) has been extensively studied over the last decades (Wanner et al, 2001; Hurrell and Deser, 2010; Moore and Renfrew, 2012; Deser et al, 2017)
Due to the aforementioned statistical impact for the winter NAO evolution, we focus on the November Eurasian snow dipole index as a predictor for the following NAO state (Gastineau et al, 2017; Han and Sun, 2018; Santolaria-Otín et al, 2021)
We used a set of centennial ensemble seasonal hindcasts (ASF-20C) and a complementary set with perturbed land initial conditions (ASF-20C-EXP) to address some of the open questions regarding the relationship between Eurasian autumn snow cover and the state of the NAO in the following winter
Summary
As the leading climate variability pattern affecting winter climate over Europe, the North Atlantic Oscillation (NAO) has been extensively studied over the last decades (Wanner et al, 2001; Hurrell and Deser, 2010; Moore and Renfrew, 2012; Deser et al, 2017). The NAO state strongly impacts the hydroclimate as well as the ecological and socioeconomic conditions over major population clusters of Europe and North America. The NAO projects onto strong pressure gradients over the North Atlantic, strong westerly winds and mild but wet conditions for central Europe. A negative winter NAO is connected to a southwardly displaced Atlantic jet stream, weaker westerlies, and cold, dry conditions for central Europe. M. Wegmann et al.: Impact of Eurasian autumn snow on the winter North Atlantic Oscillation
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