Multivariate Metrics to Quantify Co-occurring Extremes Resulting from the Dipole Circulation Pattern

Tuesday, December 11, 2018 - 13:40
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California, USA, experienced an extreme and unprecedented drought from 2012 - 2015, the immediate cause of which was abnormally strong and persistent ridging over the West coast. Wang et. al. 2014 quantify the strength of the ridging using a relative index by comparing the height of the ridging in the West to the depth of troughing in the East. This ‘dipole index’ is thus a measure of the large-scale circulation pattern across the U.S. We use the dipole index to quantify the role this circulation pattern plays in driving co-occurring negative precipitation anomalies coupled with positive temperature anomalies at the seasonal timescale, the simultaneous occurrence of which, greatly exacerbates drought conditions. Further, we employ the GHCN dataset to assess the role the dipole circulation pattern plays in affecting the characteristics of wintertime storms by quantifying changes in storm intensity, duration, and occurrence as a function of the dipole index. Among all large-scale indices we consider, including El Niño-Southern Oscillation, Pacific Decadal Oscillation, and Atlantic Multi-decadal Oscillation the dipole circulation pattern has the largest role in driving rainfall variability in California, and it is the only index which has both a negative relationship with precipitation and a positive relationship with temperature. The dipole index has become more positive over the record indicating this circulation pattern is becoming more prevalent. If this continues, it suggests that extreme drought years like 2014 will become all the more common. Thus seasonal-to-decadal projections of the dipole index present an opportunity to better manage California reservoirs for greater resilience to drought conditions and statewide water shortages.

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