4.2.2 Atmospheric circulation indices and snow cover
variability
The relationship between the two strongest indices (i.e. winter AO and
spring SH), spring temperature anomalies from reanalysis data and Altay
weather station and DPSCD in the Upper Irtysh are plotted in Fig. 7. All
variables show significant moderate to strong cross-correlation,
particularly temperature anomalies from ERA-Interim and Altay stations
(0.94). Additionally, high interannual variability exists for the AO and
temperatures, while the SH is mostly negative from 2000 to 2017. The
largest anomalies in all series occur in 2010, with negative values for
the AO and temperatures and positive ones for the SH and DPSCD. In
contrast, positive anomalies in temperatures in 2008 are synchronous
with a positive AO and negative SH and DPSCD. In spite of the general
agreement between the variables, some exceptions occur, such as positive
temperature anomalies in 2004 followed by a late DPSCD and the negative
winter AO in 2013 with above average temperatures the following spring.
Among the two indices, the winter AO shows the strongest (negative)
correlation with DPSCD in all basins, while Spring SH is significantly
positively correlated with DSPCD only in Kara Erys and Narym basins.
Both indices also correlate with Upper Irtysh basin as a whole (see Fig.
8 and Table 3). Across different elevation ranges, winter AO shows
moderate to strong correlation with DPSCD up to 1500 m a.s.l. in Kara
Ertis, Bukhtarma and Narym basins, with generally higher values in the
Kara Ertis basin (max |r| 0.73) compared to the others
(see Fig.8a,c). As regards spring SH, the highest (positive)
correlations are again seen in Kara Ertis basin up to 2000 m a.s.l.. In
the other basins, correlation is either not significant or significant
in fewer elevation ranges compared to the AO (see Fig. 8b,d).
The per-pixel correlation between the AO and SH and DSCD shows stronger
correlations compared to DPSCD across multiple elevation ranges (see
Fig. 9). Spatially, the correlation with the winter AO shows elevation
dependence, with the strongest values below 2000 m a.s.l. south of the
Altay Mountains in most basins. The main exception is an area of lower
correlation in the south-eastern part of Kara Ertis, on the Mongolian
side, corresponding to the dry steppes of the Gobi Desert (see Fig. 9a).
Across elevation ranges, moderate to strong significant negative
correlation with winter AO is seen up to 2000 m a.s.l. (3500 m a.s.l. in
Kara Ertis basin), with the strongest values all between 1000 and 1500 m
a.s.l. (see Fig. 9c). The spring SH shows generally lower correlation
values, mainly limited to the Kara Ertis basin, while very few pixels
correlate in the smaller basins, limited to the lower elevations. The
highest correlations are seen in the Gobi Desert, the same area that
showed lower correlation with the winter AO, and west of lake Ulungur in
the southern Kara Ertis basin (see Fig. 9b). Moderate significant
positive correlation with DSCD is seen up to 2000 m a.s.l. in this
basin. Significant correlation is also seen up to 1000 m a.s.l. in Narym
and 500 m a.s.l. in Bukhtarma and Uba basins (see Fig. 9d).
Both snow cover variables show significant correlations with spring
temperature anomalies from weather stations in the relevant elevation
band, i.e. 500-1000 m a.s.l. for Altay and Fuyun and 1000-1500 m a.s.l.
for Baitag. For DPSCD, the strongest correlation is seen in Kara Ertis
basin with a max. |r| of 0.79 at Altay; this basin has
the highest correlations at all three stations, while no significant
correlation with temperatures from any weather station is seen in Uba
basin (see Table 4). For DSCD, all correlations with temperatures from
weather stations are statistically significant: the strongest
correlation is between DSCD in Kara Ertis basin and Baitag spring
temperature anomalies, with high values also in Narym and Uba basins
(see Table 4).