13 May 2014

Probable Causes of the Abnormal Ridge Accompanying the 2013-14 California Drought: ENSO Precursor and Anthropogenic Warming Footprint

Summary

The 2013-14 near-record California drought was accompanied by an anomalous high-amplitude, upper-level ridge system over the Alaskan coast. Using reanalysis data and simulations from the Community Earth System Model (CESM), scientists at Utah State University and a DOE scientist at Pacific Northwest National Laboratory found that the ridge emerged from continual sources of Rossby wave energy in the western North Pacific starting in late summer and subsequently intensified into winter. The ridge generated a surge of wave energy downwind and further deepened the trough forming a dipole over the U.S. northeast. This wavy-pattern is linked to not only California drought but also a streak of frigid weather in the East and Midwest. The researchers showed that the dipole and associated circulation pattern were not directly linked with either the El Niño-Southern Oscillation (ENSO) or Pacific Decadal Oscillation phenomenon; instead, their research found a correlation with a specific type of ENSO precursor. They attributed the connection between the dipole and ENSO precursor, growing stronger since the 1970s, to increased greenhouse gas emissions as simulated by the CESM. They concluded that there is a traceable anthropogenic warming footprint in the enormous intensity of the anomalous ridge system during winter 2013-14, and the associated drought and its intensity. 

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