The effect of temporal variations in the surface mass balance and temperature-inversion strength on the interpretation of ice-core signals

The proxy for temperature (delta-signal) in ice cores is stored in the snow/ice during precipitation events, hence it reflects the temperature at which precipitation is formed (here approximated by the inversion temperature Ti) weighted with the accumulation. Results from a 14-year integration (1980-1993) with a regional atmospheric model (RACMO, Delta X = 55 km) show that the annual mean accumulation weighted inversion temperature
(Ti,w) and the annual mean Ti are not covariant in time at four out of five deep drilling sites considered, mainly due to year-to-year variations in the seasonality of precipitation. As a consequence, the surface temperature (Ts,core) derived from RACMO-output, using a method analogous to the retrieval of the surface temperature from ice core delta-signals, deviates from the directly modelled surface temperature Ts on interannual time scales. Results from a 5-year sensitivity integration, forced with a 2 K temperature increase, show an 18%
overestimation of the increase in Ts,core relative to the increase in Ts due to a change in the relation between the inversion strength and the surface temperature in a different climate regime. Similar errors may occur in deriving the temperature difference between last glacial maximum and present-day climate from delta-signals in ice cores.