Environmental Engineering Reference
In-Depth Information
this decrease in volume is larger than the decrease in sea-ice concentra-
tion, indicating thinning of the sea ice. Substantial sea-ice loss occurs at
all longitudes especially in the west Antarctic. In summer the greatest loss
is east of the peninsula. Ice loss at these levels suggests a reduction in the
seasonal cycle of sea-ice concentration and volume and means a reduction
of brine rejection at the margins of Antarctica. This has implications for the
future formation of Antarctic deep water and therefore for global ocean heat
transport (Sen Gupta et al., 2009).
Snow Cover
Snow cover depends on both temperature and precipitation and is
strongly negatively correlated with air temperature. Due to its high albedo,
much less solar energy is absorbed by snow-covered areas as contrasted
with snow-free areas, and surface temperatures tend to be lower, especially
during periods of snow melt. Snow also plays an important role in storing
moisture that is released when temperatures rise above freezing. It is a sig-
nificant variable in the water balance of snow-covered regions and plays
an important role in water management, especially in the western United
States, where much of the region's runoff originates in snow that accumu-
lates in mountainous areas in winter. Along with snowfall amounts and
snow-covered area, snow water equivalent (SWE) and snow cover duration
(SCD) are variables that can be used to characterize snow cover.
Current State of Snow Cover
In the Northern Hemisphere the snow-cover area (SCA) in March has
exhibited a negative trend since about 1970, and this decrease has been as-
sociated with increased winter temperatures. This correspondence between
March SCA and winter temperature appears to have strengthened toward
the end of the 20th century (McCabe and Wolock, 2010). Since 1950, SWE
has displayed a negative trend in the Pacific Northwest, particularly at lower
elevation (Mote, 2003). A shift in the Pacific Decadal Oscillation (PDO) may
have contributed to this trend but Mote et al. (2005), in an analysis expanded
to include all of the western United States, argued that the PDO alone could
not explain the trend in SWE. These conclusions are supported by Hamlet
et al. (2005) who attributed the downward trend in SWE to increases in
temperature rather than a decrease in precipitation. Figure 4.18 illustrates
the observed trends in SWE as well as the trends simulated by the Variable
Infiltration Capacity (VIC) hydrologic model. Although some positive trends
