Large parts of the ice sheet experience surface melt in summer. The presence of surface melt can be assessed using satellite passive microwave brightness temperatures (Abdalati and Steffen, 1995). Figure 8.3 illustrates the portion of the ice sheet that underwent surface melt during the summer of 1999, considered by Abdalati and Steffen (2001) to be a fairly representative year. The melt area shows a general association with
Temperatura (° C) Temperature C)
latitude and elevation - melt occurs in the southern and coastal regions of the ice sheet but not in the highest and hence coldest parts. However, melt extent varies considerably from year to year. For the ice sheet considered as a whole, the area undergoing surface melt correlates quite strongly with surface air temperature anomalies. There appears to have been a general increase in melt area since the advent of passive microwave records in 1979 (Abdalati and Steffen, 2001).
The presence of melt inferred from passive microwave data does not imply that runoff is actually occurring. In higher regions where melt is observed, it may only be occurring in a near-surface layer. At lower elevations, meltwater that is formed will percolate to lower depths and re-freeze. It is only near the coast that actual runoff is observed. In the southern part of the ice sheet, the area experiencing melt extends far inland from the estimated equilibrium line (the line along which the net mass balance is zero).
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