Characteristics, Evolution, and Formation of Cold Air Outbreaks in the Great Plains of the United States

Abstract

Wintertime cold air outbreaks (CAOs) in the Great Plains of the United States (US) have significant socioeconomic, environmental, and infrastructural impacts; the events of December 1983 and February 2021 are key examples of this. Previous studies have investigated CAOs in other parts of North America, particularly the eastern US, but the development of CAOs in the Great Plains and their potential sub-seasonal to seasonal (S2S) predictability have yet to be assessed. This study firstly identifies 37 large-scale CAOs in the Great Plains between 1950 and 2021, before examining their characteristics, evolution, and driving mechanisms. These events occur under two dominant weather regimes at event onset: one set associated with anomalous ridging over Alaska and the other set associated with anomalous pan-Arctic ridging. Alaskan ridge CAOs evolve quickly (i.e., on synoptic timescales) and involve stratospheric wave reflection. Conversely, Arctic high CAOs are preceded by weak stratospheric polar vortex conditions several weeks prior to the event. Both categories of CAOs feature anomalous upward wave activity flux from Siberia, with downward wave activity flux over Canada seen only in the Alaskan ridge CAOs. The rapid development of the Alaskan ridge CAOs, also linked with a North Pacific wave train and anomalous wave activity flux from the central Pacific, suggests that these events could be forced by tropical modes of variability. These findings present evidence that different forcing mechanisms, with contrasting timescales, may produce distinct sources of predictability for these CAOs on the S2S timescale.

Diverse surface signatures of stratospheric polar vortex anomalies

“…The regional differences are dependent on exactly where the anomalous ridge sets up. Notably, the strong vortex cluster is linked to strong ridging over the western US, which has previously been linked on the sub-seasonal scale to wave reflection and associated cold air outbreaks over North America (Messori et al, 2022;Millin et al, 2022).…”

The Role of Wave Breaking in the Development and Subseasonal Forecasts of the February 2021 Great Plains Cold Air Outbreak

“…However, there is a shift toward the ArH regime, which features anomalous ridging across northern Canada/Greenland, for the 28 January initialization. This weather regime is a preferential pattern for extreme cold in the US and specifically the Great Plains (Lee, Furtado, & Charlton‐Perez, 2019; Millin et al., 2022). This regime shift begins on the same initialization that the extreme event probability first shows an increase, indicating that the 28 January 2021 initialization is a key lead time to study the simulation of tropospheric precursors to the CAO.…”

“…The extreme event threshold is calculated by taking the 10th percentile of the T2M anomaly from the climatology of hindcast initializations, which yields a lead-dependent extreme event threshold for a given model initialization and grid point, similar to Kautz et al (2020). The choice of the 10th percentile is to remain in accordance with the threshold used to define Great Plains CAOs in Millin et al (2022). To complement the extreme event probability analysis, we apply empirical orthogonal function and k-means clustering (Pedregosa et al, 2011) analyses, as in Lee, , to the 500 hPa GPH anomalies over the North American domain (180°E-330°E, 19.5°N-79.5°N) for November through March between 1950 and 2021, but regridded to 1.5° horizontal resolution to match the S2S models.…”

“…The stratospheric polar vortex, which is the boreal wintertime cyclonic circulation 10–50 km high in the atmosphere near the North Pole, has links to Northern Hemisphere CAO development. If vertically propagating planetary‐scale waves break in the stratosphere, the stratospheric polar vortex will weaken and warm; if the westerly circulation reverses then a major sudden stratospheric warming occurs (Baldwin & Dunkerton, 2001; Charlton & Polvani, 2007; Nakagawa & Yamazaki, 2006), which can shift the eddy‐driven jet equatorward while enhancing the likelihood of Northern Hemisphere CAOs (e.g., Kolstad et al., 2010; Limpasuvan et al., 2004; Maycock et al., 2020; Millin et al., 2022). Significant attention has been given to the role of the stratospheric polar vortex in the February 2021 CAO, given that a major sudden stratospheric warming occurred around 1 month prior (Lee, 2021).…”

“…One tropospheric feature that is often associated with extreme cold is strong, anomalous mid‐to‐high latitude ridging, termed “blocking” when quasi‐stationary and persistent (Nakamura & Huang, 2018; Schwierz et al., 2004). Anomalous pan‐Arctic or Alaskan ridging are examples of such anticyclonic blocking patterns (e.g., Cellitti et al., 2006; Konrad & Colucci, 1989; Lee, Furtado, & Charlton‐Perez, 2019; Millin et al., 2022; Stan & Straus, 2007). Early papers found that the development of atmospheric blocking dominantly initiates in the Atlantic and Pacific regions, has a stable impact on temperature and precipitation patterns, and is linked with a persistent, quasi‐stationary disruption of the zonal flow (Rex, 1950a, 1950b, 1951).…”