Cascading Drought‐Heat Dynamics During the 2021 Southwest United States Heatwave

Abstract

In 2021 the contiguous United States (US) experienced its hottest summer on record. This included an extended high temperature period over the western United States, with distinct maxima in the Southwest in mid-June and northwest in late June, raging wildfires and widespread exceptional drought conditions. The overlapping occurrence of these multiple record-setting climate extremes marks the western United States summer of 2021 as a case of record shattering and compound climate hazards-phenomena that are increasing in frequency under climate change (AghaKouchak et al., 2020;Fischer et al., 2021;Zscheischler et al., 2018). Specifically, the colocation of an extreme drought present at the beginning of the summer and the first wave of record shattering temperatures in mid-June (Figure 1) raise the possibility that the two events were physically linked cascading hazards (Tilloy et al., 2019): Drought, through its influence on surface fluxes, might have triggered or intensified the heat wave, which in turn could exert a positive feedback on drought through increased evaporative demand or decreased precipitation.The question of whether the Southwest drought and heat extreme were cascading extremes or simply an instance of a multivariate compound extreme event (Zscheischler et al., 2020), in which drought and heat had no direct physical influence on each other, is relevant to understanding how hot and dry extremes might interact in a changing climate. There is now robust evidence that compound dry-hot extreme events have increased due to anthropogenic warming in recent decades (Alizadeh et al., 2020;Sarhadi et al., 2018). Dryness and heat are often products of a common synoptic circulation, such that they can occur together without a direct physical link between the two. There is also evidence that land-atmosphere interactions during drought have led to hotter droughts in recent decades (Chiang et al., 2018), potentially increasing the intensity of heat extremes in a manner consistent with cascading hazards. Indeed, increases in the strength of negative correlation between temperature and precipitation over much of the globe point to a potential role of land-atmosphere feedbacks under climate change contributing to increased frequency of compound dry-hot extremes (Zscheischler & Seneviratne, 2017).

Typicality of the 2021 Western North America summer heatwave

“…The record-breaking temperatures were the result of preconditioning combined with an anomalous atmospheric circulation. Prior to the heatwave, the southern portion of the region was anomalously dry [18,21] and it is known that long-term drought increases the probability of occurrence of heatwaves, as a result of the drying of the soil [22][23][24], possibly leading to the onset of a cycle of heat, drought, and fire [25,26]. This was compounded by the presence of an intense atmospheric Ω-block, like the one associated with the 2010 Russian heatwave [27][28][29].…”

Interaction between dry and hot extremes at a global scale using a cascade modeling framework

“…These results suggest that the confounding influence of daily climate variables can have a spatially disproportionate effect on the dry-to-hot and hot-to-dry event cascade across the terrestrial surface. Such spatial heterogeneity may arise from variation in surface energy partitioning 40 , which is mainly controlled by the background aridity of a region 5 , 44 .…”

“…During strong heatwaves governed by anticyclones in the midlatitudes, and sufficiently far from the center of the anticyclone where advection is weak, there can be advection of hot and dry upwind which will promote (bare-soil) evaporation and hence enable even faster soil desiccation 53 . Moreover, in transitional regimes, sometimes due to limited albedo effect from vegetation dieback and exposed soil from existing drought, the energy partitioning effect becomes a more dominant factor than net radiation in influencing drought-heatwave cascade 40 .…”

“…This suggests that the same increase in PET may result in a higher increase in the likelihood (indicated by a greater odd ratio) of such events in this regime. This is possible in arid climates, where scarce vegetation cover during high atmospheric moisture stress driven by a reduction in precipitation leads to an increased albedo from the exposed soil surface, thereby limiting the net radiation during an ongoing drought-heatwave-drought cascade 40 . Thus, a similar increase in VPD and Rn and a decrease in Pr in the arid regime can lead to a relatively higher increase in the likelihood of a dry-to-hot cascade event compared to that in other climate regimes.…”

Modulation of North American Heat Waves by the Tropical Atlantic Warm Pool

“…In addition to internal atmospheric processes, land‐atmosphere coupling has been shown to modulate heat wave occurrence through soil moisture deficit (Donat et al., 2016; Jia et al., 2016; Osman et al., 2022; Schumacher et al., 2022). This is particularly important in the Great Plains, a region where soil moisture and surface air temperature are strongly correlated through longwave radiation, sensible, and latent heating (Koster et al., 2006).…”