Glacier dynamics across precipitation gradients: Can ice-elevation feedbacks overcome the rain shadow in the Olympic Mountains, USA?

Abstract. Glacier extent is known to be sensitive to climate variability through time. The impact of spatial variability in climate on glaciers has been much less studied. The Olympic Mountains of Washington State, USA, experience a pronounced precipitation gradient with modern annual precipitation ranging between ~6.5 meters on the high west-facing slopes to ~0.5 meters in the northeast lowlands. In the Quinault valley, on the west side of the range, a glacier extended onto the coastal plain reaching a maximum position during the early Wisconsin Episode glaciation. There is no evidence of a large Elwha glacier extending into the northeast lowlands at that time. We hypothesize that the asymmetry in past glacier extent was driven by spatial variability in precipitation. To evaluate this hypothesis, we first constrain the past precipitation gradient, and then model glacier extent. We explore variability in observed and modelled 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 15 % at the annual timescale. Specifically, this ratio does not consistently vary with regional climate patterns. On average, modelled 6-hour accumulated precipitation in the Elwha is 78 % of that in the Quinault during a winter season, with a few low-precipitation time periods exhibiting a flatter or even 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 one-dimensional 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 position within paleoclimate constraints for cooling and drying, relative to today. We assume the Elwha remained drier than the Quinault, and model Elwha extent for the climates of 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 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. Therefore, we suggest that the last glacial maximum climate was more likely to have been within the warm end of the paleoclimate range. Alternatively, spatially variable drivers of ablation including differences in cloudiness could have contributed to asymmetry in glacier extent. Future research to constrain past precipitation gradients and evaluate their impact on glacier dynamics is needed to better interpret the climatic significance of past glaciation and to predict future response of glaciers to climate change.