Quantifying differences in Pacific Northwest storm tracks using oxygen isotopes and back-trajectory modeling

Authors: Karly Schmidt-Simard*, University of North Carolina - Chapel Hill, Erika K. Wise, University of North Carolina - Chapel Hill, Adam Csank, University of Nevada, Reno, Stephanie McAfee, University of Nevada, Reno
Topics: Water Resources and Hydrology, Climatology and Meteorology, Physical Geography
Keywords: stable isotope, δ18O, storm track, HYSPLIT, hydroclimate, Pacific Northwest, precipitation
Session Type: Poster
Day: 4/4/2019
Start / End Time: 8:00 AM / 9:40 AM
Room: Lincoln 2, Marriott, Exhibition Level
Presentation File: No File Uploaded

Storm tracks exert considerable control on the timing and amount of precipitation in the Pacific Northwest. Interpreting precipitation patterns in paleoclimate proxies requires quantifying the relationship between these tracks and the stable isotope composition of the rain they produce. Here, we isolate the top and bottom deciles of stable oxygen isotope ratios of weekly precipitation samples from four sites in Washington and Oregon for each season. We then use NOAA’s HYSPLIT back-trajectory model to simulate the storm tracks that produced this precipitation, and we analyze and quantify the differences between these tracks to improve our understanding of the relationship between storm tracks and the isotopic ratios of the precipitation they produce. We use Hotelling's T2 test statistic to determine whether the top-decile and bottom-decile seasonal cluster mean trajectories at each site result from different storm-track trajectories. We also compute the number of hours over land (HOL) for each cluster mean trajectory to infer the degree of rainout. Our analyses confirm significantly different seasonal cluster mean trajectories for the top and bottom deciles of δ18O values for all but three of the twenty combinations of sites and seasons (α ≤ 0.05). We also find that HOL ranges from two to thirty-four hours, is generally highest during the summer and fall months, and is frequently higher for bottom-decile trajectories. This quantification suggests that δ18O values of precipitation in this region can be used to differentiate between storm tracks and provides important context for the interpretation of paleoclimate proxies such as tree rings.

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