Unraveling Year-Round Extreme Heat in the Southern Great Plains: Definition Development, Weather Regime Analysis, and the December 2021 Warm Spell Case Study

Abstract

Extreme heat events have detrimental impacts across various socio-economic sectors.More specifically, summer extreme heat (i.e., heat waves) has greater impacts on human health. In contrast, winter extreme heat (i.e., winter warm spells) affects agriculture more significantly, due to increased pest populations during certain crop’s growing season. However, few studies have investigated winter warm spells, and none have specifically examined their impact on the central United States, a key agricultural region. Therefore, exploring aspects of year-round extreme heat events in the Southern Great Plains (SGP) is crucial. Previously established extreme heat definitions have lacked foundational support forparameters selected to compose the definition, particularly for one that can be utilized year-round. Thus, a statistical analysis of daily maximum and minimum temperatures in the SGP is conducted to identify key definition parameters supporting a singular year- round extreme heat definition. Based on the shape and spread of daily maximum and minimum temperature distributions in the SGP, a relative threshold is proposed as the most supported threshold type for the definition. Furthermore, sensitivity testing using various percentile thresholds and temperature variables revealed that the 90th percentile and daily maximum temperatures were the key parameters, supporting previous studies on changes in the frequency of heat wave events in the central United States. These findings led to the development of a year-round extreme heat definition supported by statistical analyses of all aspects of the definition. A climatology of year-round extreme heat events was developed using the establishedyear-round extreme heat definition, with further investigation into the atmospheric and sur- face characteristics associated with winter warm spell events in the SGP. One methodology for examining atmospheric patterns associated with extreme heat events across the SGP is analyzing weather regimes. Weather regimes characterize recurrent and persistent upper- level atmospheric patterns. Additionally, investigating the transitions between one weather regime to one that is commonly associated with extreme heat events in the SGP provides insights into how well these regimes could serve as potential predictors. The Aleutian High and East Pacific Ridge summer weather regimes are most strongly associated with heat wave events with different summer weather regimes transitioning into the leading regimes characterized by SGP heat wave events on varying timescales. Whereas, the Arctic Low, Pacific Trough, and Alaskan Ridge winter weather regimes are most frequently characterized with winter warm spells. Most commonly, all three of these winter weather regimes transitioned from a West Coast Ridge winter weather regime. However, this study demonstrates that these dominant weather regimes do not solely occur during extreme heat events in the SGP. Therefore, the December 2021 winter warm spell case study indicates that warm air advection plays a role in initiating the events, while strong surface heat feedbacks (e.g., surface sensible heat flux) further sustain and promote extreme surface temperatures during these winter warm spell events. These results represent initial steps in understanding extreme heat events year-round in the SGP; however, further investigation is needed to better diagnose these impactful extreme heat events, particularly winter warm spell events.