Climatic controls on glacier mass balance in Glacier Bay National Park and Preserve, Alaska

Hunter, L; Powell, R

The importance of sediment dynamics on controlling the behavior of tidewater termini is becoming clear. However, climatic forcing on glaciers in Glacier Bay National Park and Preserve needs to be addressed to evaluate its relative importance. Little climatologic data exist for the region, so a preliminary assessment of climate forcing has been performed using data collected by the Icefield Ranges Research Project (IRRP), meteorologic observations from near sea level weather stations, and a reconnaissance mass balance study using snowlines determined from aerial photographs of Glacier Bay. The emphasis is on the climatic regimes that influence glaciers flowing into Tarr and Muir Inlets. Grand Pacific Glacier is fed by high elevation n6ves in the Fairwcather Range and has been advancing into Tarr Inlet since the 1920s. Its upper neves arc located, in part, west of the Fairwcather divide where they should receive heavy coastal maritime precipitation; however, accumulation area ratio (AAR) and balance ratio (BR) calculations indicate that Grand Pacific Glacier is too poorly nourished to maintain a tidewater terminus. Muir Glacicr is fed by low elevation neves in thc Takhinsha Mountains and has the lowest equilibrium line altitude (ELA) of all glaciers studied. Despite a low ELA, it has only recently established quasi-stability following more than 100 years of retreat. Precipitation in Glacier Bay is introduced directly from the Gulf of Alaska Heavy precipitation on the coastal flanks of the Fairweather Range and a precipitation shadow extending across Glacier Bay and the Takhinsha Mountains arc expected; but IRRP observations, local glacier mass balance investigations, and regional wind patterns indicate that this concept is oversimplified. High peaks along the crest of the Fairweather Range are an effective barrier to storms moving NE from thc Gulf of Alaska that cause storms to bc deflected and enter Glacier Bay from the south. Asynchronous response of glacier behavior to climatic forcing observed by meteorologic trends and reconnaissance glacier mass balance investigations indicate that tidewater termini in Glacier Bay are insensitive to climatic forcing. Morainal bank sediment dynamics at Grand Pactfic and Muir Glaciers appear to modulate calving speeds and termini behavior by controlling grounding-line water depth, which at least over short periods of time is more important than climatic forcing.