Controls on mass balance sensitivity of maritime glaciers in the Southern Alps, New Zealand: The role of debris cover

The "mass balance sensitivity" of a glacier provides a means for assessing its response to future warming and contribution to sea level rise. Many studies have concluded that the first-order control on mass balance sensitivity is climatic, where higher-precipitation (and less continental) glaciers are most sensitive while lower-precipitation (and more continental) glaciers are least sensitive. The Southern Alps in New Zealand experience a limited range of continentality (9-13 K) but strong gradients in precipitation (2.5-11 m a^-1). Using an energy balance model applied on a regional scale we find that the central Southern Alps glaciers are very sensitive to temperature change (1.9 m w.e. a^-1 K^-1, with a range of -1.1 to -4.0 m w.e. a^-1 K^-1) and that an 82% increase in precipitation is required to offset a 1 K warming. Spatial variations in mass balance sensitivity cannot be simply explained as a function of precipitation. Topographic effects are important, and we find that debris cover reduces mass balance sensitivity. Mass balance amplitude, which takes into account debris cover, hypsometry and other topographic characteristics, is a better predictor of mass balance sensitivity than precipitation. The mass balance gradient is almost as good a predictor indicating that hypsometry is not a necessary component of sensitivity calculations. Estimating mass balance sensitivity as a function of mass balance gradient allows for parameterizations of mass balance sensitivity based on glacier inventory data. This provides a simple and robust way to assess glacier mass balance sensitivity on a global scale, which may refine future predictions of valley glacier melt and its contribution to sea level rise.