Industrial-Age Doubling of Snow Accumulation in the Alaska Range Linked to Tropical Ocean Warming

Future precipitation changes in a warming climate depend regionally upon the response of natural climate modes to anthropogenic forcing. North Pacific hydroclimate is dominated by the Aleutian Low, a semi-permanent wintertime feature characterized by frequent low-pressure conditions that is influenced by tropical Pacific Ocean temperatures through the Pacific-North American (PNA) teleconnection pattern. Instrumental records show a recent increase in coastal Alaskan precipitation and Aleutian Low intensification, but are of insufficient length to accurately assess low frequency trends and forcing mechanisms. Here we present a 1200-year seasonally- to annually-resolved ice core record of snow accumulation from Mt. Hunter in the Alaska Range developed using annual layer counting and four ice-flow thinning models. Under a wide range of glacier flow conditions and layer counting uncertainty, our record shows a doubling of precipitation since ~1840 CE, with recent values exceeding the variability observed over the past millennium. The precipitation increase is nearly synchronous with the warming of western tropical Pacific and Indian Ocean sea surface temperatures. While regional 20th Century warming may account for a portion of the observed precipitation increase on Mt. Hunter, the magnitude and seasonality of the precipitation change indicate a long-term strengthening of the Aleutian Low

EXTENDED COLUMN TEST RESULTS DURING SURFACE WARMING: DOES SURFACE WARMING AFFECT FRACTURE PROPAGATION POTENTIAL?

ABSTRACT: Dry slab avalanche stability typically increases over time in the absence of active loading from new snow or wind. However, field observations suggest that occasionally slopes showing no signs of instability in the morning avalanche later in the day when the snow surface is warmed by the sun. In this paper we present evidence that dry snowpack fracture propagation propensity may increase during sunny days as the snow surface warms up and becomes wet. During four warm, sunny days in the winters of 06/07 and 07/08, we tracked changes in results for both Extended Column and Propagation Saw tests. Our data suggest that snow surface temperature affects fracture propagation propensity on inclined slopes, with fractures more likely to propagate when the snow surface is wet. We support our test results with two case studies where explosives and ski cuts produced no avalanches when the snow surface was cold and dry, but when those same slopes were re-tested after the snow surface warmed to zero degrees they avalanched. In both cases the weak layer was dry and had a temperature below zero. We hypothesize that fracture propagation propensity may increase due to increased surface creep or due to changes in the mechanical properties of the slab

Slope scale spatial variability across time and space: Comparison of results from continental and maritime climates

ABSTRACT: Understanding the spatial variability of the snowpack is a crucial step to improve accuracy in field data collection and avalanche forecasting. While there has already been a large volume of literature assessing the spatial variability of the snowpack, inconsistent sampling designs make comparing results difficult. This work uses an overlapping 10 by 10 m grid to collect Extended Column (ECT), Compression (CT) and Stuffblock (SB) test data at the slope scale across a range of environmental settings and climatic regimes in Montana and New Zealand. The overlapping grid methodology standardizes data collection between our sites, as well as allowing for repeat data collection on the same slope, thereby providing a new method for attempting to assess changes in spatial variability over time. Preliminary results suggest that the spatial variability of fracture propagation and fracture initiation may increase over time, and that the spatial variability of the fracture propagation propensity may be related to the processes causing the instability. As we collect more data, these results will provide further insight into the problem of snow pit location and representivity, both in terms of space and time

Quantifying changes in weak layer microstructure associated with loading events

ABSTRACT: Researchers and practitioners have long utilized a variety of penetrometers to investigate the snowpack. Identifying definitive relationships between penetrometer-derived microstructural information and stability has been challenging. The purpose of this study is two-fold: 1. We propose a simple field test that can be implemented by the scientific community to establish relationships between load and penetrometer-derived microstructural strength, 2. Utilizing the SnowMicroPen (SMP) data, we quantify changes in weak layer residual strength and structural dimensions associated with a loading event. Our dataset is from Moonlight Basin, Montana and includes three modified loaded-column tests, each paired with 5 SMP profiles. Depth hoar comprised the targeted weak layer. Results indicate that loading events cause the residual bond strength and bond frequency in large-grained weak layers to decrease significantly. Much like a compression test at a micro-scale, the force required for the SMP to rupture individual bonds as well as the micro-strength decrease significantly when the slab stress is increased by artificially adding blocks of snow. A decrease in observed bond frequency within the weak layer (or an increase in the distance between bonds) also occurs after a loading event, probably because some bonds within the weak layer have already failed or are so close to failing that the penetrometer cannot detect their rupture. Artificial removal of slab stress resulted in greater rupture forces and distances between bonds, likely due to elastic rebound. This indicates that long after a natural loading event has occurred, elastic deformation still exists within the weak layer