Start
March 9, 2022 - 3:00 pm
End
March 9, 2022 - 4:00 pm
Categories
Weather and Climate SystemsWeather and Climate Systems
The Stratospheric Processes Influencing Cold Air Outbreaks in the Great Plains of the United States
Oliver T Millin
Wednesday, March 9
03:00 PM
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Wintertime cold air outbreaks (CAOs) are extreme cold events involving the displacement of cold air into the midlatitudes from a polar origin. 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. However, most studies on CAOs in the US have focused on the characteristics and precursors of CAOs in the eastern half of the US, with Great Plains CAOs yet to be investigated. This work firstly identifies 37 large-scale CAOs in the Great Plains between 1950 and 2021, before examining their characteristics, evolution, and driving mechanisms. By using a k-means clustering methodology to define five North American weather regimes, the CAOs are split into two dominant categories based on the weather regime at event onset: one set associated with anomalous Alaskan ridging and the other set associated with anomalous pan-Arctic ridging. Alaskan ridge CAOs evolve on synoptic timescales and feature stratospheric wave reflection while being preceded by anomalously strong stratospheric polar vortex (SPV) conditions. Conversely, Arctic high CAOs are preceded by weak SPV conditions several weeks prior to the event. Both categories of CAOs show upward wave activity flux from Siberia, with downward wave activity flux only shown over Canada for Alaskan ridge CAOs. The rapid development of Alaskan ridge CAOs is also accompanied by a North Pacific wave train and anomalous wave activity flux originating from the central Pacific, yielding the possibility of tropical remote forcing. As such, these findings demonstrate that different forcing mechanisms, with contrasting timescales, may produce distinct sources of predictability on the sub-seasonal to seasonal (S2S) timescale. Finally, the results of this analysis are placed in the context of the February 2021 Great Plains CAO by showing the characteristics and evolution of this event within ECMWF S2S real-time forecasts.