Preprints
https://doi.org/10.5194/wcd-2021-52
https://doi.org/10.5194/wcd-2021-52

  04 Aug 2021

04 Aug 2021

Review status: this preprint is currently under review for the journal WCD.

Impact of Eurasian autumn snow on the winter North Atlantic Oscillation in seasonal forecasts of the 20th century

Martin Wegmann1,2,3, Yvan Orsolini4, Antje Weisheimer5,6, Bart van den Hurk6,7, and Gerrit Lohmann3 Martin Wegmann et al.
  • 1Institute of Geography, University of Bern, Bern, Switzerland
  • 2Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 3Alfred-Wegener-Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany
  • 4NILU–Norwegian Institute for Air Research, Kjeller, Norway
  • 5National Centre for Atmospheric Science, Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, United Kingdom
  • 6European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom
  • 7Deltares, Delft, The Netherlands

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 is 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 cover 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.

Martin Wegmann et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2021-52', Anonymous Referee #1, 19 Aug 2021
  • RC2: 'Comment on wcd-2021-52', Anonymous Referee #2, 31 Aug 2021
  • RC3: 'Comment on wcd-2021-52', Anonymous Referee #3, 04 Sep 2021

Martin Wegmann et al.

Martin Wegmann et al.

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Short summary
Northern Hemisphere winter weather is influenced by the strength of westerly winds 30 kilometres above the surface, the so-called polar vortex. Eurasian autumn snow cover is thought to modulate the polar vortex. So far however, the modelled influence of snow on the polar vortex did not fit the observed influence. By analysing a model experiment for the timespan of 110 years, we could show that the causality of this impact is indeed sound and snow cover can weaken the polar vortex.