Measuring “weather whiplash” events in North America: a new large-scale regime approach

The term “weather whiplash” was recently coined to describe abrupt swings in weather conditions from one extreme to another, such as from a frigid cold spell to anomalous warmth or from drought to prolonged precipitation. These events are often highly disruptive to agriculture, ecosystems, and daily activities. AER Principal Scientist Judah Cohen, in collaboration with colleagues at Woodwell and the University of Wisconsin, developed a novel metric to identify weather whiplash events (WWEs) and track their frequency over time. We define a WWE as a transition from one persistent large-scale circulation regime to another distinctly different one, as determined using an objective pattern cluster analysis called self-organizing maps (SOMs). We focus on the domain spanning North America and the eastern N. Pacific Ocean. A matrix of representative atmospheric patterns in 500-hPa geopotential height anomalies is created. We analyze the occurrence of WWEs originating with long-duration events (defined as lasting four or more days) in each pattern, as well as the associated extremes in temperature and precipitation. A WWE is detected when the pattern two days following a long-duration event is substantially different, measured using internal matrix distances and thresholds. Changes in WWE frequency are assessed objectively based on reanalysis and climate model output, and in the future with climate model projections. Temporal changes in the future under RCP 8.5 forcing are more robust than in recent decades, with consistent increases (decreases) in WWEs originating in patterns with an anomalously warm (cold) Arctic.


 Figure WWE_post 080922.png

Figure caption. Top: Representative patterns in daily 500-hPa geopotential height anomalies [m, shaded] for all months from 1948-2019 calculated using Self-Organizing Maps. The domain covers 30oN-80oN and 180-60oW (midlatitude North America/NE Pacific). Percentages indicate the frequency of occurrence of each node during winter (JFM) and summer (JAS; in parentheses). Numbers to left of each node are for reference purposes. Bottom: Projected changes in the total number of WWEs from 2006-2030 to 2076-2100 in (a) winter and (b) summer based on ten simulations by CESM assuming RCP 8.5 forcing. The Xs indicate statistical significance > 95% based on a student’s t-test. Note difference in scales between annual and seasonal counts. In general patterns related to a warm Arctic are predicted to increase while patterns related to a cold Arctic are predicted to decrease.

Citation and Web link:  Measuring “weather whiplash” events in North America: a new large-scale regime approach

A. Francis, N. Skific, S. J. Vavrus, J. Cohen

Accepted to Journal of Geophysical Research - Atmospheres

5 August 2022

Paper #2022JD036717R

Principal Scientist and Director of Seasonal Forecasting