Effects of PDO regime on summer melt of glaciers in the Cascades

Phelps, Carly
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University of Delaware
When interpreting changes in glacier mass balance, it is useful to know which forcings are contributing to the observed changes. One important climate forcing in the Pacific Northwest is the Pacific Decadal Oscillation (PDO). In this study, we investigated whether August PDO phase has an effect on the summer melt of Cascades glaciers using the Weather Research and Forecasting (WRF-ARW) model. We created two ensembles of model runs, one consisting of ten Augusts during which the PDO index was positive, the other consisting of ten Augusts during which the PDO index was negative. All WRF model runs were for the entire month of August. The results indicate that positive PDO summers produce warmer conditions to the west of the Cascades, and cooler, cloudier conditions to the east. Areas east of the Cascades frequently exhibited an opposite response to PDO phase compared to areas west of the Cascades. The magnitudes of the changes due to PDO phase, as well as the magnitudes of the differences in repose to PDO phase across the Cascades, are both of a similar magnitude to that expected from modern greenhouse gas forcing. Results also indicate that PDO phase is unlikely to have an effect on the mass balance of Cascades glaciers.