Roughly one-third of sudden stratospheric warming (SSW) events lack a strong canonical surface impact, and this can lead to a forecast bust if a strong impact had been predicted. Hence, it is important to predict before the SSW onset if an event will propagate downward. The predictability of the downward impact of SSWs is considered in 7 subseasonal-to-seasonal forecast models for 16 major SSWs between 1998 and 2022, a larger sample size than considered by previous works. The models successfully predict which SSWs have a stronger downward impact to 100hPa, however they struggle to predict which have a stronger tropospheric impact. The downward impact is stronger if the deceleration of the 10hPa winds is better predicted. Downward impact is stronger for split and for absorbing SSWs, and is better predicted in high-top models. In contrast, there is little relationship between SSWs with above-average predictability and the subsequent downward impact.

The term “polar vortex” remained largely a technical term until early January 2014 when the United States media used it to describe an historical cold air outbreak in eastern North America. Since then, “polar vortex” has been used more frequently by the media and public, often conflating circulation features and temperatures near the surface with only partially related features at the tropopause and in the stratosphere. The polar vortex in its most common scientific usage refers to a hemispheric-scale stratospheric circulation over the Arctic that is present during the Northern Hermisphere cold season. Reversal of the zonal-mean zonal winds circumnavigating the stratospheric polar vortex (SPV), termed major sudden stratospheric warmings (SSWs) can be linked to mid-latitude cold air outbreaks. However, this mechanism does not explain the cold US winter of 2013/14. This study revisits the winter of 2013/14 to understand how SPV variability may still have played a role in the severe winter weather. Observations indicate that anomalously strong vertical wave propagation occurred throughout the winter and disrupted, but did not fully break, the SPV. Instead, vertically propagating waves were reflected back downward, building a blocking high near Alaska and downstream troughing across central North America, a classic signature for extreme cold air outbreaks across central and eastern North America. Thus, the association of the term “polar vortex” with winter 2013/14, while not justified by the most common usage of the term, serves as a case study of the wave-reflection mechanism of stratospheric polar vortex influence on mid-latitude weather.

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. 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 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 4 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.

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. 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 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 4 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 those during recent decades, with consistent increases (decreases) in WWEs originating in patterns with an anomalously warm (cold) Arctic.