3.5.3 Atmospheric circulation indices
We obtained northern hemisphere monthly atmospheric circulation indices from the NOAA Climate Prediction Centre, which represent the leading patterns extracted from rotated empirical orthogonal functions (Barnston and Livezey, 1987), applied to 500 hPa height anomalies in the analysis region 20 – 90° N. In turn, these reflect large-scale changes in atmospheric waves and jet stream patterns influencing temperature, precipitation and storm tracks as well as the position and intensity of the jet stream (NOAA, 2012). We included the indices that have been shown to influence the climate of Eurasia, specifically the NAO, the East Atlantic pattern (EA), the East Atlantic / Western Russia pattern (EAWR), the Scandinavia pattern (SCAND) and the Polar-Eurasian pattern (POL).
The NAO is the most prominent atmospheric mode during winter months, consisting in a north-south dipole of anomalies, one centred over Greenland and one over the North Atlantic between 35 and 40°N. It is associated with the intensity and location of the North Atlantic jet stream and storm track, with the positive (negative) phase leading to above- (below-) average temperature anomalies over Europe, extending over Siberia in prolonged phases (NOAA, 2012). The NAO has been widely used to analyse snow cover variability over Europe (Bednorz, 2004; Bojariu and Gimeno, 2003). The EA is the second most prominent mode, and consists of a north-south dipole of anomaly centres spanning the North Atlantic from east to west (NOAA, 2012). The positive phase of the EA is associated with above-average temperature in Europe and positive precipitation trends over Scandinavia for all months. The EA-WR pattern has four main anomaly centres: Europe, Northern China, north Atlantic and north of the Caspian Sea. It is a primary teleconnection affecting Eurasia throughout the year and in its positive phase it is commonly associated with below-average temperature anomalies for large portions of western Russia. (NOAA, 2012). The SCAND describes a primary circulation centre over Scandinavia and weaker centres over western Europe and western Mongolia. Its positive phase is characterized by anticyclonic anomalies over Scandinavia, giving rise to below-average temperatures over central Eurasia (Bueh and Nakamura, 2007). The POL is most commonly associated with above-average temperatures of eastern Siberia, reflecting an enhanced (weakened) polar vortex during a positive (negative) phase (NOAA, 2012).
We also included the Arctic Oscillation (AO), or Northern Hemisphere annular mode, which was obtained from NOAA CPC, based on 1000 hPa height anomalies. The AO is a planetary scale pattern of climate variability related to the zonal flow between 35 and 55°N, and has been widely linked to snow cover variability in Central Asia (Bamzai, 2003; Clark et al., 1999; Ye and Wu, 2017). Finally, we calculated the Siberian high index (SH), which expresses the strength of the Siberian high, a semi-permanent and quasi-stationary anticyclone stationed over Siberia throughout the winter and early spring months, associated with the coldest air masses of the northern hemisphere (Panagiotopoulos et al., 2005). The SH was calculated by averaging daily values of MSLP obtained from reanalysis data into monthly composites for the area 40-65° N, 80-120° E, as outlined by Panagiotopoulos et al. (2005) and standardized based on the 1990-2017 mean and standard deviation of MSLP for each month. For all indices, we produced winter and spring mean values by averaging DJF and MAM data.