| Summary: | During the 2012–2013 winter, the negative phase of the North Atlantic Oscillation (NAO) predominated, resulting in a cold winter over Europe and northern Asia punctuated by episodes of frigid weather. This climate anomaly is part of a recent trend towards negative values of the NAO index that has occurred over recent winters. The negative trend of the NAO may be related to atmospheric internal variability butit may also be partly forced by slowly varying components of the climate system. In the present study, we investigate the influence of surface conditions on the atmospheric circulation for the 2012–2013 winter using an atmospheric global climate model. In particular, the role of low Arctic sea ice concentration, warm tropical/North Atlantic sea surface temperature and positive Siberian snow cover anomalies are isolated by prescribing them in a set of different numerical experiments. Our simulations suggest that each of these surface forcings favored a negative NAO during the 2012–2013 winter. In our model, the combined NAO response to tropical/North Atlantic SST, Arctic sea ice and Siberian snow anomalies accounts for about 30% of the observed NAO anomaly. Different physical mechanisms are explored to elucidate the atmospheric responses and are shown to involve both tropical and extratropical processes.
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