The Flash Drought of 1936
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1 The Flash Drought of 1936 Eric D. Hunt1, Jordan I. Christian2, Jeffrey B Basara2,3, Lauren Lowman4, Jason A. Otkin5, Jesse Bell6, Karla Jarecke7, Ryann A. Wakefield2, Robb M. Randall8 (Submitted to Journal of Applied and Service Climatology on 19 September 2020) ABSTRACT An exceptional flash drought during the spring and summer of 1936 led to extreme heat waves, large losses of human life and significant reductions of crop production. An analysis of historic precipitation and temperature records shows that the flash drought originated over the southeastern United States (U.S.) in April 1936. The flash drought then spread north and westward through the early summer of 1936 and possibly merged with a flash drought that had developed in the spring over the northern Plains. The timing of the flash drought was particu- larly ill-timed as most locations were at or entering their climatological peak for precipitation at the onset of flash drought, thus maximizing the deficits of precipitation. Thus, by early July most locations in the central and eastern U.S. were either in drought or rapidly cascading toward drought. The weeks that followed the 1st of July were some of the hottest on record in the U.S., with two major heat waves: first over the Midwest and eastern U.S. in the first half of July and then across the south-central U.S in the month of August. The combination of the flash drought and heat wave led to an agricultural disaster in the north central U.S. and one of the deadliest events in U.S. history. 1. Introduction with above-average air temperatures, wind speeds, solar radiation, and lower humidity (Otkin et al. 2018). Flash Drought is a recurring feature of the natural climate sys- droughts are more likely to occur in regions of transition tem that has traditionally been classified as one of four, from humid to semi-arid climate regimes, where precipita- often overlapping categories (Wilhite and Glantz 1985): tion is more variable, and in regions where the landscape is meteorological (lack of precipitation and often coupled dominated by row crop agriculture (Christian et al. 2019). with above-average temperatures), agricultural (depleted Flash droughts (Hoell et al. 2020) can cause dispropor- soil moisture and stressed crops), hydrological (depleted tionate amounts of damage to ecosystems, infrastructure, groundwater and below average streamflow), and socio- and increase levels of mortality if coupled with a heat wave. economic (economic losses and human health impacts). In For example, flash droughts and associated heat waves have recent years, the term flash drought has been proposed for occurred on several occasions over the past four decades. droughts that develop more rapidly than normal (Svoboda Two of the worst from a widespread, impact-based stand- et al. 2002; Otkin et al. 2013; Hunt et al. 2014; Ford et al. point occurred in 1988 and 2012. The 1988 flash drought 2015; McEvoy et al. 2016; Otkin et al. 2016; Otkin et al. developed very quickly in the spring over a large portion 2019) and is defined by a rapid onset and/or intensification of the central and eastern United States (U.S.; Basara et al. of drought caused by a lack of precipitation in combination 1 Atmospheric and Environmental Research, Inc., Lexington, MA 2 School of Meteorology, University of Oklahoma, Norman, OK 3 School of Civil Engineering and Environmental Science, University of Oklahoma, Norman, OK 4 Wake Forest University, Department of Engineering, Winston-Salem, NC 5 Space Science and Engineering Center, Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison, Madison, WI 6 University of Nebraska Medical Center, Omaha, NE 7 Oregon State University, Department of Forest Ecosystems and Society, Corvallis, OR 8 CCDC Army Research Laboratory, White Sands Missile Range, NM Corresponding Author: Eric D. Hunt. Atmospheric and Environmental Research, Inc. 2021 Transformation Dr. Suite 1250. Lincoln, NE 68508. Phone: 402-262-0111. Email: [email protected] Journal of Applied and Service Climatology, Volume 2020, Issue 004, DOI: doi.org/10.46275/JOASC.2020.11.001 JoASC, Volume 2020, Issue 004 2 2020) and led to significantly-reduced production of crops days) of drought where precipitation during the period is at like corn (Zea mays L) over the Midwest. The 2012 flash the 20th percentile or lower and coupled with above-aver- drought developed very rapidly in early summer, affected age temperatures lasting for several weeks to a few months almost all of the central U.S. through the summer and early (Otkin et al. 2018). For this study, a flash drought was deter- fall, and was associated with temperatures that were well mined to have occurred if both of the following conditions above average (Otkin et al. 2016; Basara et al. 2019). are met: There have been significant flash droughts in recent years outside the U.S. as well. Perhaps the most climatologically • Total precipitation over a 60-day period was less extreme example is the flash drought and subsequent heat- than 50 percent of average at a location that normally wave that occurred in Russia in 2010 (Barriopedro et al. receives an average of no less than 100 mm of pre- 2011; Miralles et al. 2014; Christian et al. 2020). The flash cipitation over the 60-day period of record. drought began in late May and was followed by the worst • Average temperature over a 60-day period that was heat wave on record in Russia by early August. This event more than 0.5°C greater than normal at a given lo- led to exceptional loss in wheat production, increased forest cation that also has an average temperature > 10°C fires, and caused thousands of deaths from the heat and poor during that 60-day period. air quality. During the 1930s, much of the continental U.S. was af- The temperature and precipitation thresholds were set to fected by some of the most intense droughts on record. In ensure that a minimal amount of substantive precipitation the southern High Plains, the droughts contributed to the would ordinarily occur and that temperatures are typically formation of the Dust Bowl period and were further pro- warm enough to support modest evaporative demand. We longed by the dust’s impact on drought (Cook et al. 2008; used average temperature over the 60-day period to incor- Cook et al. 2009). While the 1930’s had several particularly porate minimum temperatures, which can be very important harsh drought years (e.g., 1934), the drought of 1936 was for human health in a heat wave (Kaiser et al., 2007). While particularly extreme. As will be shown in Section 4, the heat a flash drought can occur in less than two months based waves associated with this event are among the worst on on criteria set forth in Otkin et al. (2018), in this study it is record for many places in the central U.S. important to demonstrate that this was of sufficient enough Heim (2017) demonstrated that most of the U.S. from duration to be called an extreme event across a large section the Rocky Mountains to the eastern states were in se- of the U.S. While significant impacts to agriculture certain- vere to extreme drought in July of 1936 according to the ly verified for this event (as shown later), the methodology Palmer Drought Severity Index (PDSI; Palmer 1965) and described above is specific to determining agricultural flash various other reports (Cronin and Beers 1937; Sutch 2009) drought. discuss the tremendous impact of drought to crops in the The evolution of the event was analyzed in the following north central U.S. The Extended Reconstructed Sea Surface manner. A total of 90 locations between the front range of Temperature (ERSST; Huang et al. 2014) shows that the the Rocky Mountains and East Coast of the United States Pacific Decadal Oscillation (PDO; Newman et al. 2016) was were selected for analysis of precipitation and temperature. strongly positive in the spring and summer of 1936, a phe- The period of analysis for all stations began on 15 April nomenon which is thought to have contributed heavily to 1936 and ended 121 days later on 13 August 1936 and data the prevalence of U.S. drought in the 1930’s (McCabe et al. from that period of analysis were compared against the en- 2004). However, the overall understanding of how the 1936 tire period of record for that location. The dates (15 April drought evolved across large swaths of the U.S. is limited. to 13 August) were chosen to capture the main part of the Therefore, the objective of this paper is to further quantify growing season and the climatological peak of precipita- the rapid spatial evolution of the 1936 flash drought event tion. In addition to computing the total precipitation, we using historic precipitation and temperature data. also determined the percent of normal precipitation (com- pared to the 1981-2010 climate normals) and the accumu- lated precipitation deficit over a moving 60-day window. 2. Data and Methods Of the moving 60-day windows, the one with the greatest The temperature and precipitation data used for this study precipitation deficit was noted and the last date of that 60- are obtained from the Applied Climate Information System day window was marked as the peak date. The accumulated (ACIS; Hubbard et al. 2004). Observation sites for monitor- precipitation deficit is calculated by comparing the 60-day ing the evolution of the drought were required to have no precipitation that occured in 1936 to the climatological missing daily precipitation reports within the period from average . The temperature anomalies were also computed 1 April to 31 August of 1936 and have continuous records by taking the average of the daily temperature departures from that city for at least 100 years to be eligible for this over the same 60-day moving window to better indicate study.