4.2.1 Atmospheric circulation indices and temperature anomalies
To identify the most suitable indices for snow cover analysis, we first
looked at their correlation with temperatures, by considering aggregated
winter (DJF) and spring (MAM) indices. The AO and SH emerge as the
strongest indices: both the winter AO and SH are correlated with
temperature anomalies from all three stations and across basins in the
reanalysis data, although the AO is non-significantly (p >
0.05) correlated at some grid points (Fig. 6a). Specifically, the winter
AO appears positively correlated with winter temperatures across Siberia
exerting its influence north to the Barents Sea and south to the Altay
Mountains (see Fig. 6a). Conversely, the Siberian high exerts a more
local impact in winter, influencing temperatures north of the Tibetan
plateau, across Northern China, East Kazakhstan and Mongolia, with its
southernmost tip over southeast China (Fig. 6c).
Spring SH and winter AO are also strongly correlated with spring
temperature anomalies at all stations and grid cells in the Upper Irtysh
(see Table 2 and Fig. 6b,d). The correlation between the winter AO and
ERA-Interim spring temperature anomalies is significant
(p<0.05) across the Altay Mountains, between eastern
Kazakhstan, northern Mongolia and southern Russia, including the study
area (see Fig.7b), with patterns of significant correlation with
temperatures displaced to the east compared to the winter. The
correlation between spring SH and spring temperature anomalies from
ERA-Interim shows a similar pattern to the correlation between winter
temperature and winter SH, although the extent of the influence is
limited to a latitudinal band between 70° E and 120 ° E (see Fig. 6b,d).
Other indices show moderate correlation at either individual stations or
grid cells, e.g. the winter NAO is also correlated with winter and
spring temperatures, although the strength of the relationship is
generally lower than for the AO, except for the winter index - winter
temperature anomalies at ERA grid cells. The winter SCAND and EA also
correlate with winter temperatures. The first correlation however is
only significant at weather stations, while the latter at two stations
and limited grid cells (see Table 2).