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Trent W. Ford and Justin T. Schoof

Abstract

Extreme heat events have been connected with antecedent soil moisture in many global regions, such that dry soils increase sensible heat content of the near-surface atmosphere and impede precipitation through boundary layer growth. However, negative soil moisture–temperature feedbacks (dry soils = higher temperatures) are founded on investigations of maximum temperature that neglect the potentially important latent heating component provided by soil moisture. In this study, the association of spring soil moisture and subsequent summer oppressive heat events is quantified, defined by equivalent temperature. The advantage of equivalent temperature over maximum temperature is that it accounts for both the temperature and moisture components of atmospheric heat content. Quantile regression and composite analysis are used to determine the association between spring soil moisture and summer oppressive heat events using a 25-yr station observation record in Illinois. A consistent response of summer oppressive heat events to antecedent 5-cm soil moisture anomalies is found at all four stations. The frequency of oppressive summer equivalent temperature events is significantly increased following spring seasons with wetter-than-normal soils compared with spring seasons with dry soils. This provides evidence of a possible positive soil moisture–temperature feedback. Further, it is found that oppressive heat events correspond with the combination of wetter-than-normal spring soils and persistent summertime upper-level ridging to the northeast of the region, thereby leading to the conclusion that abundant-to-surplus spring soil moisture is necessary but not sufficient for the occurrence of oppressive heat in Illinois.

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Trent W. Ford, Liang Chen, and Justin T. Schoof

Abstract

Monthly to seasonal precipitation extremes, both flood and drought, are important components of regional climates worldwide, and are the subjects of numerous investigations. However, variability in and transition between precipitation extremes, and associated impacts are the subject of far fewer studies. Recent such events in the Midwest region of the United States, such as the 2011–12 flood to drought transition in the upper Mississippi River basin and the flood to drought transition experienced in parts of Kentucky, Ohio, Indiana, and Illinois in 2019, have sparked concerns of increased variability and rapid transitions between precipitation extremes and compounded economic and environmental impacts. In response to these concerns, this study focuses on characterizing variability and change in Midwest precipitation extremes and transitions between extremes over the last 70 years. Overall we find that the Midwest as a region has gotten wetter over the last seven decades, and that in general the annual maximum and median wetness, defined using the standardized precipitation index (SPI), have increased at a larger magnitude than the annual minimum. We find large areas of the southern Midwest have experienced a significant increase in the annual SPI range and associated magnitude of transition between annual maximum and minimum SPI. We additionally find wet to dry transitions between extremes have largely increased in speed (i.e., less time between extremes), while long-term changes in transition frequency are more regional within the Midwest.

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