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.