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 prolonged, frigid cold spell to anomalous warmth or from drought to heavy precipitation. These events are often highly disruptive to agriculture, ecosystems, and daily activities. In this study, we propose and demonstrate 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 continental-scale circulation regime to another distinctly different pattern, as determined using an objective pattern clustering analysis called self-organizing maps. 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 from 72 years of daily fields. We analyze the occurrence of WWEs originating with long-duration events (LDEs) (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 2 days following a LDE is substantially different, measured using internal matrix distances and thresholds. Changes in WWE frequency are assessed objectively based on reanalysis and historical climate model simulations, and for the future using climate model projections. Temporal changes in the future under representative concentration pathway 8.5 forcing are more robust than those in recent decades. We find consistent increases in WWEs originating in patterns with an anomalously warm Arctic and decreases in cold-Arctic patterns.