Midlatitude forcing mechanisms for glacier mass balance investigated usinggeneral circulation models

A process-oriented modeling approach is applied in order to simulate glacie r mass balance for individual glaciers using statistically downscaled gener al circulation models (GCMs). Glacier-specific seasonal sensitivity charact eristics based on a mass balance model of intermediate complexity are used to simulate mass balances of Nigardsbreen (Norway) and Rhonegletscher (Swit zerland). Simulations using reanalyses (ECMWF) for the period 1979-93 are i n good agreement with in situ mass balance measurements for Nigardsbreen. T he method is applied to multicentury integrations of coupled (ECHAM4/OPYC) and mixed-layer (ECHAM4/MLO) GCMs excluding external forcing. A high correl ation between decadal variations in the North Atlantic oscillation (NAO) an d mass balance of the glaciers is found. The dominant factor for this relat ionship is the strong impact of winter precipitation associated with the NA O. A high NAO phase means enhanced (reduced) winter precipitation for Nigar dsbreen (Rhonegletscher), typically leading to a higher (lower) than normal annual mass balance. This mechanism, entirely due to internal variations i n the climate system, can explain observed strong positive mass balances fo r Nigardsbreen and other maritime Norwegian glaciers within the period 1980 -95. It can also partly be responsible for recent strong negative mass bala nces of Alpine glaciers.