Interannual variations of shallow firn temperature at Greenland summit

Abstract

Firn-temperature profiles are calculated in a thermal model using continuous surface temperatures derived from automatic weather station data and passive-microwave data in the Greenland summit region during the period 1987-99. The results show that significant interannual variations of mean summer (June-August) and annual temperatures occur in the top 15 m, in addition to the normal seasonal cycle of firn temperature. At 5 m depth, the seasonal cycle is damped to 13% of the surface seasonal range, but even at 15 m about 1% or 0.6°C of the seasonal cycle persists. Both summer and mean annual temperature decrease from 1987 to 1992, followed by a general increasing trend. Interannual variability is 5°C at the surface, but is dampened to 3.2°C at 5 m depth and 0.7°C at 15 m depth. Dampening of the interannual variability with depth is slower than dampening of the seasonal cycle, because of the longer time constant of the interannual variation. The warmer spring and summer temperatures experienced in the top 5 m, due to both the seasonal cycle and interannual variations, affect the rate of firn densification, which is nonlinearly dependent on temperature. During the 12 year period 1987-99, the annual mean surface temperature is -29.2°C, and the annual mean 15 m temperature is -30.1°C, which is > 1°C warmer than a 15 m borehole temperature representing the period around 1959 and warmer than the best-fit temperature history by Alley and Koci (1990) back to AD 1500.