A simple steady state firn-densification model is modified to account for short-term time variations of accumulation rate and surface temperature. The temporal surface-elevation- and mass changes at two sites in the percolation zone of an ice sheet in response to various climate histories are determined. It is shown that a straight-forward translation of observed short-term ice-sheet surface-elevation variations into mass changes may be completely misleading, particularly for the percolation zone of the ice sheet, where temperature driven variations of melting/re-freezing rates have a strong impact on near surface density. In the lower percolation zone, the mass change associated with a temperature anomaly in respect to the mean climate may for example amount to as little as 10 percent of the observed, simultaneous surface elevation change. Moreover, significant surface elevation change may occur even in periods of constant surface climate, and consequently unchanged mass balance, as a delayed response to previous changes of the local surface climate. Forcing the model with cyclic temperature variations mimicking fluctuations of West Greenland instrumental temperature records during the past hundred years, shows that surface elevation may increase by as much as 1 m in 5 years in periods with no mass change at all. In the lower percolation zone, temperature induced density changes rather than fluctuations of accumulation rate are the dominant cause of short-term surface elevation changes in contrast to the situation in the higher percolation zone, where short-term surface-elevation change is dominated by fluctuations of accumulation rate.
- mass/volume change
- Ice sheet