Understanding climate
for the benefit of society

Photo series of Briksdalsbreen, a western outlet glacier from Jostedalsbreen, the largest glacier on the European mainland (487 km2), from 1993 to 2015. During the 1990s the glacier experienced frontal advance due to higher-than-normal winter precipitation (positive NAO index, see Figure 2). After 1997, however, the front of Briksdalsbreen has retreated more than half a kilometre, mainly due to higher-than-normal summer temperatures.  Photo: Sigbjørn Myklebust (1993-1997), Ove Brynestad (2001-2003), Kurt Erik  Nesje (2004), Atle Nesje (2005-2015).

Glacier mass balance and climate 

After 1997, the front of Briksdalsbreen has retreated more than half a kilometre, mainly due to higher-than-normal summer temperatures.  

The mass balance of glaciers is mainly influenced by winter precipitation and summer temperature. In a recent study, Mathias Trachsel (Dept. of Biology, University of Bergen) and Atle Nesje (Dept. of Earth Science, University of Bergen, Bjerknes Centre for Climate Research and Uni Research Climate) used simple statistical models to assess and quantify the relative importance of summer temperature and winter precipitation for annual mass balances of eight Scandinavian glaciers (for location, see Figure 1)

Kart breer
Figure 1. Map of glaciers and summer temperature and winter precipitation. Glaciers: Ålfotbreen (ALF), Rembesdalskåka (REM), Nigardsbreen (NIG), Storbreen (STO), Hellstugubreen (HEL), Gråsubreen (GR), Engabreen (ENG) and Storglaciären (STORGL). Meteorological stations Bergen, Glomfjord and Bodø are indicated. Inset maps show 1961–1990 normal summer (MJJAS) temperature and winter (ONDJFMA) precipitation (data available at http://met.no/Klima/Klimastatistikk and processed in R). From Trachsel and Nesje (2015).

 


The study demonstrates that winter precipitation is more important for maritime glaciers, whereas summer temperature is more important for annual balances of continental glaciers.

An important finding is that the relative importance of summer temperature and winter precipitation was not stable through the observation period. Winter precipitation, for example, was more important than summer temperature for the study glaciers in the 25-yr period 1972–1996. The relative importance of summer temperature for the annual mass balance has been increasing towards the present.

The Atlantic Multidecadal Oscillation (AMO) index was consistently negative (cooler than normal summers) between 1963 and 1996 and the North Atlantic Oscillation (NAO) Index was consistently positive (mild and humid winters) between 1987 and 1995, both such weather modes being favourable for glacier growth. Apparently, winter precipitation was more important than summer temperature for annual glacier balance when only considering subsets of years with high NAO index and negative AMO index. 

The importance of summer temperature was increased when analysing subsets of years with low NAO index and positive AMO index.

As a result, the relative importance of winter precipitation and summer temperature for glacier mass balance on Scandinavian glaciers was probably influenced by the state of the AMO and the NAO, as these two indices are associated with changes in summer temperature (AMO) and winter precipitation (NAO).

Figure 2 shows that the annual (net) mass balance on the maritime Ålfotbreen in western Norway is strongly associated with the NAO weather mode (NAO index). 
 

Ålfotbreen figur
Figure 2. Mass balance on the maritime Ålfotbreen in western Norway (data: nve.no/bre and NAO-index) .


 

 

Reference

Trachsel, M. og Nesje, A. 2015: Modelling annual mass balances of eight Scandinavian glaciers. The Cryosphere 9, 1401–1414.  doi:10.5194/tc-9-1401-2015