This study delineates (using GIS overlay, thresholding, and data compositing methodologies) and characterizes long-term seasonal, decadal, and monthly heatwave patterns in US climate regions.
3.1 Heat wave extent per climate region
This study found that generally, longer heat wave events (TOP25) covered vastly more of the climate regions compared to shortest events (BOTTOM25). TOP25 heat wave regions constituted as much as 57% of the land area in Northwest, 51% of the Southwest and 47% of the area in Eastern North Central (refer to Figure 4). Top three climate regions by area include the Southwest (>557,000 km2), South (>524,000 km2) and Northwest (>368,000 km2). Proportions of area with BOTTOM25 events, on the other hand, ranged from 11% in Central US to 41% in the Southwest. Based on land area, Southwest, South, and Western North Central regions were top ranked for BOTTOM25 events, with shortest-duration heat waves extending over >452,000 km2, 262,000 km2, and >229,000 km2, respectively. Regions affected by BOTTOM25 and TOP25 events were similar (in size and location) for Western North Central (19% vs 22%), Western (32% vs 39%), and the Southwest (41% vs 51%).
Different patterns were observed in heat waves over the decade scale (Figure 5). In the 1980s, TOP25 events were commonest in the Northeast (>73% of land area), Northwest (>64% of area) and Central US (>53%). In the 1990s, the Northwest topped with over 70% of its area experiencing TOP25 events, followed by the Southwest (>65%) and the Northeast (>55%). Different patterns were observed in 2000s, in which the West region was ranked second (having TOP25 events in over 70% of land area) after the Northwest (>72%). The Southeast was ranked third in this decade with over 67% of its land area experiencing these extreme heat events. Northwest, West, and Northeast regions had the highest proportions of area under TOP25 heat waves (approx. 67%, 62% and 59%, respectively). Proportions of area under BOTTOM25 events were always lower than those for TOP25 events, except for West North Central (in 1980s), Southwest (1980s), Central (in 1990s), East North Central (in 1990s and 2010s), and South (in 2000s). Proportions of land under heat waves generally increased from a decade to the next in Northwest and Southeast, except for the 2000s to 2010s. Consistent expansion of area under heat waves was observed in the West North Central region.
The study found that, for some climate regions, heat waves extended over more land in the months when extremely high temperatures were expected to subside (Figures 6A and B). For instance, highest proportions of area under TOP25 heat waves were observed in the month of May for the Southwest, Southeast, Northeast, and South (84%, 78%, 71% and 60%, respectively, see Figure 6A). Similar patterns were observed in Northwest and Central regions for the month of September (Figures 6A and B). Here, heat waves were respectively identified in approx. 96% and 81% of the land area (Figure 6A). This indicates that the severe heat wave period is expanding for these climate regions. Proportions of land under TOP25 heat waves generally exceeded proportions of land under BOTTOM25 events, except for Northwest in May (0% vs 22%) and West North Central in August (16% vs 17%).
3.2 Long-term heatwave characteristics
Heat wave characteristics generally showed similar patterns among shortest- and longest-duration events. While area of heat waves was significantly higher for the TOP25 group compared to BOTTOM25 (US averages of 606,000 vs 481,000 km2, respectively), there were similar spatial patterns in distribution (Figure 7A). In both groups, heat wave events were highly extensive in the South and Southwest regions. Events in these regions respectively had average areas of 887,000 km2 and 758,000 km2 (TOP25) and 597,000 km2 and 667,000 km2 (BOTTOM25).
The least expansive heat wave events of all were found in Northeast in both TOP25 and BOTTOM25 groups (average area of 312,000 km2 and 254,000 km2, respectively) and East North Central (265,000 km2) in the BOTTOM25 group. Average temperatures ranged between 28 and 38 °C for shortest-duration events (US average of 32.7 °C) and 30 to 38 °C for longest-duration events (US average of 34.6 °C). BOTTOM25 events in the Northwest, and Northeast regions experienced comparatively lower temperatures (Figure 7B). Temperatures averaged 28.4 and 30.1 °C in these regions, respectively.
West region events in both groups were found to have the highest mean magnitude (Figure 7F) – averaging 1.83 for BOTTOM25 events and 1.94 for TOP25 events. TOP25 events in this region were also the most persistent of all (Figure 7E). Heat wave persistence averaged 2,000 for this region compared to a US average of 475. Least persistent heat wave events were found mainly in the East North Central, Central, and Northeast regions (mean persistence <183). Further, heat wave events experienced in the eastern half of the US were more cohesive than the western ones as shown by the fragmentation statistic (Figure 7D). Events in the East North Central were most cohesive of all with an average fragmentation statistic of 0.69 and 0.76 for shortest-duration and longest-duration events, respectively. The Southwest had the most fragmented events, with the fragmentation statistic for shortest-duration and longest-duration events averaging 0.18 and 0.2, respectively. The study also found that shortest-duration events were generally less patchy (ranging from 2,200 to 12,000 patches) compared to longest-duration events which ranged from 4,200 to over 19,700 patches (Figure 7G). Longest-duration events in the South, East North Central, and Central had the greatest number of patches (>15,000). TOP25 events exhibited significantly different durations compared to BOTTOM25 events. The former events lasted longer (14-99 days) than the latter (2.8-9.6 days) on average (Figure 7C). However, this results from the initial use of this variable in defining BOTTOM25 and TOP25 heat wave events. Clear inter-region differences were observable in this variable especially in the TOP25 events. The West and South events lasted the longest (more than 98 and 71 days, respectively).
3.3 Decadal heat wave characteristics
Spatial patterns observed in the decadal distribution of heat wave characteristics were similar to those found in 1981-2018. For example, heat wave events in both groups were most cohesive in East North Central and generally most fragmented in Southwest (compare Figures 8 and 7D). However, the study found that the higher-than-average fragmentation statistic in East North Central was influenced greatly by events in 1980s and 2000s (average fragmentation statistic was 0.77 and 0.73 respectively). Further, highest magnitude (for both groups) and most persistent events (for the longest-duration group) were consistently observed in the West region. In this region, mean magnitude was >1.79 overall while mean persistence ranged 399-620 (for longest-duration events). However, by splitting the data into individual decades, the study found that while shortest-duration events occurred in each decade for all climate regions, longest-duration events were absent in East North Central (in the 1990s and 2010s) and Central region (in the 1990s). Furthermore, number of individual patches in TOP25 heat waves varied significantly – highest in the South in 1990s and 2010s and only average in 1980s and 2000s. The high number of patches in East North Central in the 1981-2018 period are attributed to the highly patchy events in 1980s.
In addition, the study quantified changes in average heatwave characteristics per climate region. Results indicate that generally, average heat wave characteristics varied more for the longest-duration events compared to shortest-duration events. Among the characteristics, number of patches and heat wave persistence showed greater variability across the decades (Figure 9). Highest increases in average heat wave persistence occurred in TOP25 events in West North Central (+377% in 1990s-2000s), Southwest (+306% in 1990s-2000s), Southeast (+279% in 1980s-1990s), and South (+251% in 1980s-1990s and +125% in 2000s-2010s). Greatest changes in number of patches were found in West North Central (+318% in 1990s-2000s), and South (+128% and +176% in 1980s-1990s and 2000s-2010s, respectively). Other TOP25 heat wave characteristics that saw major increases include mean area (+75% in South in 2000s-2010s, +71% in West North Central in 1990s-2000s); average fragmentation (increase of 67% in West North Central in 1990s-2000s); and mean magnitude (58% increase in South in 2000s-2010s). Greatest changes in BOTTOM25 events were found in number of patches in the Southwest, West (respective increases of 96% and 70% in 1990s-2000s), and Central (+72% in 2000s-2010s) regions. Average temperatures showed the least variability across decades for both event groups. Rates of change for most of the variables in the South were lower for 1990s-2000s compared to the other two periods. Contrasting patterns were seen for most heat wave characteristics in West North Central, as well as some elsewhere (e.g., number of patches and persistence in the Northwest, Southwest, and West). These patterns are more detectable in TOP25 events.
3.4 Monthly heat wave characteristics
The temporal results all fall in line with seasonal temperature patterns common in the summer months. Across this period heatwave persistence, for both the top and bottom 25% events, shows a distinct pattern that varies both temporally and spatially (Figure 10). Highest persistence is seen in events in July and August (for the TOP25 group) and in May and September (for the BOTTOM25 group). For the top 25%, in May, the average persistence across the US was 20. However, the South and Southeast regions were above this average with 41 and 25, respectively. In June the average US-wide persistence for the top 25% was 71, 255% higher than May. This average was greatly influenced by persistence in events occurring in West, Southwest, and South which respectively had persistence values of 333 (+1981% from May), 78 (+457% from May), and 71 (+73% from May). In July the West persistence values further increased by 77% compared to June. Overall, in July the US-wide persistence averaged 168 but the West, Southwest and parts of the Northwest were above this average. August had a shockingly similar spatial patten to the July persistence with a US wide average of 170. Lastly, in September the persistence dropped to 58 across the US. All regions exhibited a sharp decline in the persistence values at this time. Overall, in September the US pattern of persistence was homogenous, except for events in the West (average persistence of 339).
A higher persistence of heatwave events was seen in the beginning of the season for the bottom 25% events but a switch was observed as summer progressed, and the persistence numbers were higher for the top 25%. In May the average persistence value for the bottom 25% was 144, almost six times greater than during top 25%. Average persistence in June reached 107 for the US which was 49% higher than the top 25%. These values continued dropping to 87 in July. However, in August the average persistence switched to 94, and continued to increase to 128 in September. For the bottom 25%, there was a distinct difference in the persistence between the eastern and western portions of the country. In May, the western regions (West, Southwest, and Northwest) had the highest persistence across the whole country. In June, this pattern was observed in the same western regions and West North Central in addition. It was also observed that bottom 25% events were generally homogenous across the summer months compared to top 25% (which had many extremely persistent events especially in West).
Across the summer months differing numbers of heat wave patches were exhibited across the climate regions (Figure 11). Within the top 25% group the maximum number of patches, generally, was seen in May (US mean of 16,100), June (US mean of 14,100), and September (US mean of 15,000), a result in contrast to the persistence results which exhibited higher values during the months of July and August. Further, for the top 25%, the South region consistently had the highest number of patches compared to all other regions (ranging 12,500-29,600). The West coast regions, Northwest and West, had some of the lowest number of patches, for the top 25%, across May, July, and August when compared with the other regions. In May, for instance, number of patches for heat wave events in the West averaged 5,042 compared to the US average of 16,058. The number of patches for the lower 25% heat waves was significantly lower than the top 25% (e.g., US average of 5,249 and 14,084 patches for bottom 25% and top 25% events, respectively, over the month of June). Additionally, there is not a large difference in number of patches across the summer months for shortest-duration events. Highest numbers were observed in the West North Central region (average range of 5,664-13,075 across the summer months) compared to the rest of study area.
Other major changes include significant increases in mean magnitude (+72%), fragmentation (+57%), and mean area (+47%), in the West North Central during the August-September period. Mean magnitude also increased consistently in the South region from a -26% change in June-July to a +84% in August-September. Events in East North Central also experienced a similar change in mean magnitude with a 30% decrease in June-July and a 51% increase in July-August.
3.5 Combined heat wave characteristics
The CHCI values ranged from 0 to about 0.145 (Figure 12A). The top 25% version of the CHCI exhibited the highest values overall. For this group, CHCI values ranged from 0 to 0.145 and averaged 0.031 across the US. The highest CHCI averages occurred in the East North Central (0.086), South (0.085), and Central (0.063) regions. Ironically, while the three regions with higher CHCI values are adjacent to one another, the variables driving the index pattern differ (Figure 12B). Within the South region the CHCI components were either 3223 or 3233. These index values show that heat waves in this region were of mid (average) magnitude and higher-than-average area and number of patches. What differs across the South region is the fragmentation of the heatwaves. Regions in yellow (Figure 12B) had heatwaves that were significantly more cohesive (higher fragmentation statistic). Within the Central region, all the CHCI component values were the same (2233). This value indicates an average area and magnitude but a significantly above average fragmentation pattern (more cohesive events) and number of patches. Lastly, within the East North Central, the CHCI component values included 1233 and 1333. This indicates that heat wave events around the Great Lakes and into Wisconsin and parts of Minnesota were highly cohesive, had more than average number of patches and lower than average area, while their magnitude varied from average in some locations to greater than average in others. Across the entirety of the region most of the heat waves had a larger than average fragmentation pattern and number of patches.
In terms of the bottom 25% CHCI, a completely different pattern and distribution of CHCI is exhibited (Figure 12A). Overall, CHCI values ranged from 0 to 0.097 across all the US with an average of 0.0124. Regions exhibiting the highest CHCI values include West North Central and South. Within the West North Central, the CHCI values range from 0.005 to 0.097, with the higher values occurring across Montana and Wyoming. Within the South region the values ranged from 0.009 to 0.059 with component scores including 3133, 3233, 2233, and 2133. Such CHCI component values highlight that the fragmentation and number of heat wave patches were significantly high across those areas with significant CHCI. What varies, in terms of heat wave characteristics, across the region is the mean area and magnitude of the heat wave.