Glacier dynamics across precipitation gradients in the Olympic Mountains of Washington state, USA: Modeled ice elevation feedbacks can overcome orographically forced drying

Margason, Andy A.

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https://hdl.handle.net/2142/115444 Copy

Description

Title

Glacier dynamics across precipitation gradients in the Olympic Mountains of Washington state, USA: Modeled ice elevation feedbacks can overcome orographically forced drying

Author(s)

Margason, Andy A.

Issue Date

2022-04-26

Director of Research (if dissertation) or Advisor (if thesis)

Anders, Alison M

Committee Member(s)

Proistosescu, Cristian

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Rain Shadow
• Glaciers
• Olympic Mountains

Abstract

Glaciers are sensitive indicators of climate change through time. Continental ice sheets respond and change with climate variations over 100,000-year periods, while smaller alpine glaciers respond to climate variability over decades. Not only do glaciers change in time, but they also vary in space. The Olympic Mountains of Washington State, USA, experience a pronounced precipitation gradient from west to east with modern annual precipitation ranging between ∿6.5 meters on the high west-facing slopes to ∿0.5 meters in the north-east lowlands. In the Quinault valley, on the west side of the range, a glacier extended onto the coastal plain reaching a maximum position defined by prominent moraines about ~40 kya. On the other hand, there is no evidence of a large Elwha glacier extending into the lowlands during the most recent glacial episode. We hypothesize that the asymmetry in past glacier extent was driven by spatial variability in precipitation. We consider two research questions: 1) Was the past precipitation gradient like the modern gradient; and, 2) How would spatial variability in precipitation impact glacier extent in the Quinault and Elwha valleys? To constrain past precipitation gradients, we explore variability in observed and modeled precipitation gradients over timescales from 6-hours to ~100 years. Across three data sets, basin-averaged precipitation in the Elwha is 54% of that in the Quinault, with variability of less than 5% at the annual timescale. Specifically, this ratio does not consistently vary with regional climate patterns (i.e. ENSO 3.4, PDO, TNH, etc.). At the 6-hour timescale, cooler temperatures and lower wind speeds are correlated with a flatter or reversed precipitation gradient. Overall, our analysis does not suggest a mechanism for increasing the precipitation gradient, but overwhelmingly indicates spatially coherent variability in precipitation across the peninsula. We conclude that the past precipitation gradient was likely similar to the modern gradient. We use a 1-D glacier flowline model, driven by sea-level summer temperature and annual precipitation to approximate glacier extent in the Quinault and Elwha basins. We find several equilibrium states for the Quinault glacier at the mapped maximum moraine position within paleoclimate constraints for cooling and drying, relative to today. We assume the Elwha remained drier than the Quinault and model Elwha extent at the Quinault equilibria. At the warm end of the paleoclimate constraint (10.5˚C), the Elwha remains a small valley glacier in the high headwaters. Yet, for the cooler end of the allowable paleoclimate (7˚C), the Elwha glacier advances to a narrow notch in the valley. As the ice is forced to flow through a smaller cross-section, it thickens, triggering an ice-elevation feedback. This feedback leads to rapid extension of the Elwha glacier down to elevations only ~100 meters above those reached by the Quinault. While there is uncertainty in the glacial record of the Elwha, it is unlikely that such a large glacier existed during the most recent glaciation. This suggests that spatial variability in precipitation alone may not be sufficient to explain past glacier extent. We focused on spatial differences in accumulation of glacier ice and neglected spatially variable drivers of ablation. For example, differences in cloudiness on the east and west sides of the range could allow for enhanced mass loss in the Elwha relative to the Quinault, contributing to asymmetry in glacier extent. Glacier sensitivity to climate is well-established and used to assess temporal changes in climate. However, our work shows that glacier response to climate change is dependent on valley morphology and can vary tremendously with precipitation gradients within a single mountain range.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Andrew Margason

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