ABSTRACT: Identifying controls on the stable water isotope composition of precipitation in the southwestern Yukon using GCMs.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

The goal of our work is to better understand what controls the stable water isotope (SWI) composition of precipitation in the southwestern Yukon, and in particular, to better-interpret the SWI signal from the Mount Logan ice core. To this end, we are conducting experiments with the GISS ModelE general circulation model, which is equipped with SWI diagnostics. One feature of interest in the Mt. Logan ice core record is a significant drop in d18O in the 1850's towards more depleted values. The current explanation for this shift is a transition in the North Pacific circulation towards a deeper Aleutian Low, with the stronger meridional flow bringing moisture from more southerly sources. Because of their greater arrival times, these air masses would have undergone a greater isotopic depletion than moisture from closer, colder sources under a more zonal flow regime. Although physically plausible, it is possible that the d18O drop caused by this proposed shift in circulation might be offset by warmer source evaporation conditions and integrated air mass trajectories, both of which would be associated with less depleted precipitation. To test the physical plausibility of the meridional hypothesis, we conducted numerical experiments with the NASA GISS ModelE isotopically-equipped general circulation model. In the Yukon, SWI variability is influenced, via the regional temperature, by the Pacific North America pattern and ENSO. We found that positive d18O anomalies in the SW Yukon region were in fact associated with a deeper Aleutian Low; it would appear that the effect of a longer transit time is offset by a warmer moisture transport pathway, in disagreement with the current moisture shift explanation. Our results are in agreement, however, with recent tree-ring reconstructions of the North Pacific Index, which suggest an 1850's shift towards a weaker Aleutian Low. We also found that the degree of Pacific control on the SW Yukon isotope signal is highly dependant on seasonality. During the summer months, both the local temperature control and underlying circulation controls on d18O are largely absent. During the winter months, when most precipitation in the SW Yukon occurs, the circulation features become much more enhanced. This suggests that in addition to the annual resolution at which ice-core reconstructions are carried out, much information can potentially be gained by separating the winter from summer signals. To determine the relative importance of moisture source conditions and transport pathway characteristics, we also conducted a sensitivity test with the GCM to isolate the effects of isotopic fractionation at the ocean and land surface from that in the atmosphere. The absence of isotopic fractionation during surface evaporation resulted in a near-constant 12 permil enrichment in precipitation in the SW Yukon. The absence of fractionation during atmospheric transport, however, had a significant impact on the d18O variability in the SW Yukon region.

Bibliography of Canadian Geomorphology