Extreme cold events over North America such as the February 2021 cold wave have been suggested to be linked to stratospheric polar vortex stretching. However, it is not resolved how robustly and on which timescales the stratosphere contributes to the surface anomalies. Here we introduce a simple measure of stratospheric wave activity for reanalyses and model outputs. In contrast to the well-known surface influences of sudden stratospheric warmings (SSWs) that increase the intraseasonal persistence of weather regimes, we show that extreme stratospheric wave events are accompanied by intraseasonal fluctuations between warm and cold spells over North America in observations and climate models. Particularly, strong stratospheric wave events are followed by an increased risk of cold extremes over North America 5–25 days later. Idealized simulations in an atmospheric model with a well-resolved stratosphere corroborate that strong stratospheric wave activity precedes North American cold spells through vertical wave coupling. These findings potentially benefit the predictability of high-impact winter cold extremes over North America.
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Previous studies have suggested possible connections between the decreasing Arctic sea‐ice and long‐duration (>5 days, LD) cold weather events in Eurasia and North America. Here we document the occurrences of weather regimes in winter by their durations, based on the empirical orthogonal function analyses of the daily geopotential height fields at 500 hPa (z500) for the months of November–March 1979–2019. Significant changes in the occurrence frequency and persistence of Ural ridge (UR) and weak stratospheric polar vortex (PV) were found between winters following high and low autumn sea‐ice covers (SIC) in the Barents and Kara seas. It is shown that a strengthening of the UR is accompanied with a weakening of the PV, and a weak PV favors Greenland ridge (GR). Cold spells in East Asia persist for 5 more days after an LDUR. Cold spells from Canada to the U.S. occur 2–5 days after an LD Ural trough (UT) and are associated with a z500 anomaly dipole centered over Alaska (+) and Hudson Bay (−). Cold spells in the eastern U.S. occur 1–4 days after an LDGR due to circulations resembling the Pacific‐North America pattern. Increased occurrences of UR in winter are associated with a decreased eastward propagation of synoptic waves from the North Atlantic to Japan and the North Pacific.
more » « less- Award ID(s):
- 2140909
- PAR ID:
- 10520743
- Publisher / Repository:
- Fan et al. JGR 2023
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 129
- Issue:
- 11
- ISSN:
- 2169-897X
- Subject(s) / Keyword(s):
- Arctic sea ice, blocking. cold surges
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Weather regimes defined through cluster analysis concisely categorize the anomalous regional circulation pattern on any given day. Owing to their persistence and low dimensionality, regimes are increasingly used in subseasonal-to-seasonal prediction and in analysis of climate variability and change. However, a limitation of existing regime classifications for North America is their seasonal dependence, with most existing studies defining regimes for winter only. Here, we normalize the seasonal cycle in daily geopotential height variance and use empirical orthogonal function analysis combined with
k -means clustering to define a new set of year-round North American weather regimes: the Pacific Trough, Pacific Ridge, Alaskan Ridge, and Greenland High regimes. We additionally define a “No Regime” state to represent conditions close to climatology. To demonstrate the robustness of the classification, a thorough assessment of the sensitivity of the clustering solution to various methodological choices is provided. The median persistence of all four regimes, obtained without imposing a persistence criterion, is found to be one week, approximately 3 times longer than the median persistence of the No Regime state. Regime-associated temperature and precipitation anomalies are reported, together with the relationship between the regimes and modes of climate variability. We also quantify historical trends in the frequency of the regimes since 1979, finding a decrease in the annual frequency of the Pacific Trough regime and an increase in the summertime frequency of the Greenland High regime. This study serves as a foundation for the future use of these regimes in a variety of weather and climate applications.Significance Statement Weather regimes provide a simple way of classifying daily large-scale regional weather patterns into a few predefined types. Existing methods usually define regimes for a specific season (typically winter), which limits their use, or provides only a minimal assessment of their robustness. In this study, we objectively quantify four weather regimes for use year-round over North America, while we classify near-normal conditions as No Regime. The four regimes represent persistent large-scale weather types that last for about a week and occasionally much longer. Our new classification can be applied to subseasonal-to-seasonal forecasts and climate model output to diagnose recurrent weather types across the North American continent.
