Geoscience Reference
In-Depth Information
6.5.7
Trends in Discharge
Annual discharge from Eurasian rivers draining into the Arctic Ocean is increasing.
This was first documented by Peterson et al. (
2002
), who found that the annual dis-
charge aggregated for the six largest Eurasian rivers draining into the Arctic Ocean
increased by about 7 percent over the 1936-1999 period. Aggregate discharge was
computed to be 128 km
3
greater than it was when routine measurements began. To
put this change into perspective, the annual mean discharge from the Yenisey River
is about 630 km
3
yr
−1
. Pointing to controls by atmospheric circulation patterns, they
showed that annual Eurasian discharge was correlated with the index of the NAO.
Yang et al. (
2002
) examined changes in discharge at the mouth of the Lena Basin
for the 1935-1999 period. They noted significant increases (35-90 percent) in dis-
charge during the cold season, decreases in river ice thickness and a hydrologic
shift toward more discharge in May, and lower daily maximum discharge in June.
Hydrologic changes in summer were less apparent. Serreze et al. (
2003a
) noted sim-
ilar, albeit larger winter discharge increases in the Yenisey as well as shifts in peak
discharge to earlier in the season.
J. McClelland et al. (
2004
) argued that while both the annual trend in discharge
and the shift in peak discharge to earlier in the season represent true climate sig-
nals, the winter increases in discharge are largely a result of dam operations, with
this effect most pronounced in the Yenisey. They interpreted the upward trend in
annual discharge as primarily a result of increasing precipitation, with changes in
permafrost conditions (melting ground ice, effects of altered active layer depth on
near-surface groundwater flow) as well as increases in forest fire frequency possi-
bly playing contributing roles. Regarding the latter, fire scar chronologies in some
forests of central Siberia and the Russian far east point to substantial increases in
fire frequency (Arbatskaya and Vaganov,
1997
; Cushman and Wallin,
2002
). With
loss or damage of vegetation, evapotranspiration decreases, so more of the precipi-
tation becomes runoff. In a subsequent paper focusing on the 1964-2000 period,
McClelland et al. (
2006
) highlighted that while discharge from Arctic-draining riv-
ers in Eurasia was increasing, discharge from rivers in North America had exhib-
ited a small decrease (Dery and Wood,
2005
). Interest in the changing hydrology
of northern high latitudes continued with the observation of a record-high aggre-
gate annual discharge from the six largest Arctic-draining Eurasian rivers in 2007
(Rawlins et al.,
2009
).
Based on evaluation of atmospheric reanalysis data, Zhang et al. (
2012
) found
that the increasing discharge of Eurasian rivers can be explained as a consequence
on increasing atmospheric vapor flux convergence. The record discharge observed
in 2007 fits this picture; this event was a reflection of strong positive anomalies in
late winter snow water equivalent across much of northern Eurasia and positive net
precipitation anomalies through summer. Results from most global climate models
point to increases in atmospheric vapor flux convergence and net precipitation (hence
river runoff) over northern high latitudes through the twenty-first century. This is
related to the larger vapor-holding capacity of a warmer atmosphere. Whether we
