Introduction

The term “weather whiplash” has appeared frequently in recent media reports describing abrupt shifts from one type of weather extreme to another. These shifts typically consist of a severe cold spell being replaced by a period of above-normal temperatures, a prolonged drought followed by intense precipitation, or the reverse sequence in either case. A striking example of a whiplash event occurred in early September 2020 when a prolonged record-breaking heatwave over a large region of the central Rocky Mountains in the U.S. abruptly ended with a temperature drop exceeding 60oF in some areas along with several inches of snow. In 2018 a six-week cold spell in eastern North America flipped to a February heatwave that broke high temperature records from northern Maine to New Orleans and sent Bostonians flocking to the beaches (https://www.wunderground.com/cat6/summer-february-80-massachusetts-78-nyc). The disruptive “false spring” in March 2012 that struck the Midwest and lasted for several weeks was followed by killing freezes in April that wreaked havoc on fruit farmers, whose crops blossomed too early in March and were then damaged by the anomalous cold spell (https://medium.com/dose/whats-a-false-spring-b64cb977d59). Only a handful of studies has investigated these types of disruptive weather shifts, and as yet there is no consistent definition of a weather whiplash event, as distinct from the passage of fronts associated with progressive synoptic weather systems.
Metrics of variability are one approach used to assess whiplash events. An investigation of atmospheric temperatures during recent (1988/89 – 2014/15) winters (DJF) in the northern hemisphere by Cohen (2016) found that, consistent with previous studies (e.g., Screen 2014), daily winter near-surface temperature variability decreased in high latitudes (60oN-90oN), as would be expected with a weaker poleward temperature gradient owing to amplified Arctic warming. In contrast to most studies, Cohen (2016) also found that variability in creased in low- to mid-latitudes (0-50oN), which was interpreted as a possible indication of increased weather whiplash. Larger variability in daily temperatures, however, suggests a timescale associated with fast-moving synoptic weather systems, such as frontal passages and changes in wind direction, rather than a shift from one persistent regime to another.
Loecke et al. (2017) also focused on extreme precipitation events to define a weather whiplash index associated with drought-to-flood transitions. Focusing on the Upper Mississippi River basin, they calculated the index using the total precipitation from January to June of each year minus the total from July to December of the previous year divided by the total over both periods. They applied the index to identify events in projections by 30 of the models that participated in the climate model intercomparison project, version 5 (CMIP5) forced with representative concentration pathway (RCP) 8.5. They found that 19 of the models exhibited a robust positive trend in their index while the others had no significant trend.
The study by Swain et al. (2018) focused on regional precipitation extremes in California’s rainy season (Nov.-Mar.), coining the term “precipitation whiplash” as a transition from anomalously dry to anomalously wet seasons in consecutive years. They analyzed a large number of climate simulations (forced with RCP8.5 conditions) created with the National Center for Atmospheric Research (NCAR) Community Earth System Model, CESM1. A whiplash event was identified when one rainy season with precipitation totals below the 20thpercentile during the pre-industrial period was followed by a season with totals exceeding the 80th percentile. They found a significant increase in events over most of southern California, Mexico, and parts of northern California through the 21st century, which implies increasing challenges for agencies that manage freshwater resources in that region.
He and Sheffield (2020) also focused on precipitation to develop a metric of whiplash, defining a drought-pluvial seesaw similar to Swain et al. (2018), in which dry spells were followed by wet spells (pluvials). They analyzed data from the past nearly seven decades (1950-2016) on a global scale for the spring/summer (April-September) season and for fall/winter (October-March). Unlike Swain et al. (2018) who focused on consecutive wet seasons, this study investigated three-month lags in the drought-pluvial seesaw. They calculated the ratio of the frequency of events during the past 30 years (1987-2016) to that in the first 30-year period (1950-1979). Over North America, they found increased ratios over more than half of the area during the warm season along with a three-fold increase in frequency over about one-fifth of the region during the cold season, mainly in the central U.S. The authors note challenges, however, in interpreting the results owing to differing consistency of regional metrics for droughts and pluvials as well as disentangling the roles of natural variability and climate change.
We build on this growing body of research into disruptive and abrupt shifts in weather extremes, with a focus on a domain spanning North America and the eastern North Pacific Ocean. We demonstrate a novel approach to assess weather whiplash based on abrupt shifts in the large-scale circulation regime following the persistent dominance of one pattern. This method does not rely on measurements or simulations of precipitation or temperature, thereby avoiding uncertainties introduced by instrument error, local heterogeneity, and model physics associated with precipitation processes. For this study, we adopt the following definition of a weather-whiplash event (WWE): a long-lived (4 or more consecutive days), continental-scale pattern in the upper-level circulation that shifts abruptly (over 1-2 days) to a substantially distinct pattern, bringing a stark end to persistent weather conditions throughout the region. This definition eliminates confusion in the possible misidentification of WWEs caused by sharp, localized weather changes owing to synoptic features such as fronts, discreet disturbances (e.g., squall lines, tropical storms), and shifts in low-level winds from differing surface types (e.g., from onshore to offshore, downslope to upslope, or forest to grasslands). We submit that a WWE should be identified when one persistent and anomalous circulation pattern is replaced abruptly by a very different one, as distinct from the passage of fronts on synoptic time scales.