5.3 Atmospheric circulation indices and snow cover depletion
Our correlation analysis shows the AO and SH to be the circulation
patterns most influential to patterns of snow cover depletion in the
Upper Irtysh basin in winter and spring. The correlations between winter
AO anomalies and DPSCD are significant below 1500-2000 m a.s.l.:
notably, DPSCD shows higher correlation with the winter AO at higher
elevation, while the opposite is true for the SH, and a similar inverse
pattern is seen for DSCD.
Two winters stand out as highly anomalous in both the AO and SH record
during the period of observation: 2007-2008 (positive) and 2009-2010
(negative), with matching winter temperature anomalies and patterns of
subsequent spring snow cover depletion (Fig. 7). Winter 2009-2010 was in
fact remarkably cold across Europe and central Asia. According to Cohen
et al. (2010), the highly negative AO was initiated by the rapid advance
of Siberian snow cover in October, followed by the strengthening of the
SH, increased upward wave activity flux, stratospheric warming and
downward propagation of height and wind anomalies to the surface. The
link between autumn snow cover and phases of the AO has been reported in
several studies, with feedback mechanisms including soil moisture and
the albedo (Bojariu & Gimeno, 2003, Cohen & Entekhabi, 1999, Saito &
Cohen, 2003); patterns of decline in Arctic sea ice have been linked to
the AO (Liu et al., 2012). Compared to 2009-2010, the winter of
2007-2008 received little attention, although unusually mild
temperatures were reported across Eurasia, linked with the positive
phase of the NAO/AO (Ilkka et al., 2012), in addition to a La Niña event
(Stevenson, 2016).
In our study, the winter AO AO is strongly negatively correlated with
DPSCD and appears to influence spring snow cover retreat through the
modulation of spring temperature anomalies. The lagged influence between
the AO and spring snow cover has in fact been previously reported in the
literature. Bamzai (2003) observed a correlation of 0.46 between JFM AO
and AM snow cover melt date over Eurasia. Foster et al. (2013) found
that the winter AO is negatively correlated with spring snowmelt and
DSCD at latitudes between 50º and 60º N, while the opposite is true
between 60º and 70º N. In Saito & Cohen (2003), evidence for reciprocal
lagged effects was found, i.e. late winter/spring AO leading
spring/summer snow and spring/ summer snow leading the following
fall/early winter AO. In a subsequent study, Saito et al. (2004), found
a consistent relationship between fall Eurasian snow cover and the
winter AO between 1971 and 2000. During the 1980s, an association
between winter AO and spring snow cover also emerged, as well as
autocorrelation between winter and spring snow cover and winter/spring
AO. This latter correlation was also reported by Foster et al. (2013)
but found no confirmation in our study. While our analysis did not
include fall snow cover, its possible direct influence on winter AO and
spring snow cover retreat is appealing for long-range forecasts and
should be discussed in future research, both in the Upper Irtysh and
other Eurasian catchments.
Compared to the NAO, the winter AO exhibits a stronger connection with
both winter/spring temperature anomalies and DPSCR/DSCD. While the
similarity/difference between the AO and the NAO and the actual physical
meaning of the AO are still a matter of debate (Báez et al., 2013), our
study confirms the hypothesis of a weaker (Saito & Cohen, 2003) and
more local effect of the NAO on snow cover variability, restricted to
Europe and western Asia (Clark et al., 1999). The observed association
between the spring SH, temperature anomalies and DPSCD, while confirming
that the Siberian high is indeed a relevant atmospheric influence on the
climate of Kazakhstan (Panagiotopoulos et al., 2005), also show that it
holds a limited potential in hydrological forecasting, as no relevant
lagged correlations were found in our study. In spite of evidence that a
positive SH reinforces negative AO anomalies (Cohen & Entekhabi, 1999,
Cohen et al., 2010), the interplay between the AO and the SH is not
entirely understood (Huang et al., 2016), and whether one index can be
predicted from the other still needs to be assessed.