Abstract
The recent advance of some fast-responding glaciers in New Zealand demonstrates that regional variability in atmospheric circulation can counteract the effects of global warming. Until recently, a key challenge in quantifying how weather systems influence glaciers has been the lack of observational data from high-elevation sites in the Southern Alps. Using high-quality meteorological and glaciological observations from Brewster Glacier over four summers and two winters, a physically based mass balance model is used to resolve daily ablation and solid precipitation. Composite analysis confirms ablation is largely controlled by changes in meridional airflow, with high ablation associated with north to northwest airflow, while low ablation is associated with south to southwest airflow. The largest snowfall events in the cool season also occur during northwesterly airflow, highlighting the importance of seasonality (i.e., whether an event occurs in the ablation or accumulation period) for mass balance. Application of a synoptic weather typing approach commonly used for the New Zealand region, known broadly as Kidson weather types, revealed that three established regimes (trough, zonal, and blocking) are less effective in resolving variability in ablation and snowfall than a simple procedure that groups weather types using geostrophic flow direction. The frequency of four weather types can explain approximately 40% of the variance in mass balance over a 42-year period. This highlights that future regional variability or trends in large-scale circulation and their seasonality, superimposed on background warming, will be important in shaping glacier mass balance in the coming decades.