Tornadoes are one of the most devastating natural hazards on Earth, causing significant property losses and casualties in a few localized regions (1–7). Tornadoes tend to form from severe thunderstorms (8). In general, severe thunderstorms occur within environments characterized by (i) high conditional instability, which provides a source of buoyancy to support strong updrafts; and (ii) strong lower-tropospheric vertical wind shear, which supports long-lived and well-organized convection (9–18). Tornadoes typically require a third ingredient: the potential for near-ground rotation defined by intense storm-relative helicity within the lowest 1 km above the surface (SRH01), which represents horizontal vorticity that can be stretched as air flows into the storm to generate tornado rotation (19–25).
These tornadic environments are most prevalent over central North America (CNA; denoted by the box in Fig. 1a), particularly the Great Plains (a.k.a. Tornado Alley) (10, 26), where over 1200 tornadoes and over 10 extreme tornado outbreaks occur annually (27, 28), an order of magnitude greater than in any other region (29–39). This behavior is commonly ascribed to elevated terrain to the west (Rocky Mountains and Mexico’s Sierra Madre mountains) and equatorward oceans to the south (Gulf of Mexico and Caribbean Sea) (40–42). The north-south oriented mountain range drives the strong climatological southerly low-level flow over the Great Plains (43–45). The easterly trade winds turn northward when they encounter the east slope of the mountains and increase in speed as the flow develops anticyclonic shear vorticity (43, 44), though seasonally varying heating of the sloping terrain may also be important (45). Embedded within these southerly winds is a particularly strong flow feature known as a low-level jet (LLJ), which in this region is commonly referred to as the Great Plains low-level jet (GPLLJ; 46, 108). The GPLLJ frequently is discernible throughout the day, though it is nocturnally enhanced (46, 108). This daily cycle is associated with diurnal oscillations of near-surface processes including boundary layer frictional stress (47), heating of the sloping terrain (48), and their combination (49–51). In addition, southerly winds associated with the warm sector of synoptically driven troughs are also found to enhance the southerly mean flow and LLJ over the Great Plains (40, 42, 115). This low-level southerly flow transports warm, moist near-surface air from upstream oceans into CNA (52, 53, 112, 113) beneath a well-mixed eastward flow aloft (54). These air currents conspire to create environments favorable for severe thunderstorms and tornadoes (40, 55, 56).
A similar geographic setup is also found in South America, where the Andes Mountains stretch from north to south similar to the Rockies and Sierra Madre mountains, and the Amazon basin is as warm and moist as tropical oceans (colors in Extended Data Fig. 1a–b; 57, 58). The Andes drive the climatological northerly low-level flow, within which is embedded a particularly strong flow feature commonly referred to as the South American low-level jet (SALLJ; 60, 61, 77). Similar to the Great Plains, strong northerly low-level flow is present east of the Andes as well; this feature is also enhanced synoptically by the passage of troughs (e.g., the northwestern Argentinean low and the Chaco low; 79, 116, 117). These northerly low-level winds transport warm and moist near-surface air from the upstream Amazon basin and tropical Atlantic Ocean into central South America (CSA; denoted by the box in Fig. 1a; 59–62, 64, 109) to produce conducive environments for severe convection (63, 70, 103–106). Indeed, spaceborne radar and other remote sensing observations have shown that severe thunderstorms are more frequent (Fig. 1b) and extend to greater heights in CSA compared to CNA (65–67). However, tornado potential is substantially lower in CSA (10, 26), and relatively few major tornadoes have been reported in CSA, in stark contrast to CNA where there is a long history of devastating events (Fig. 1a; 29–33, 68). While storm reporting in CNA is mature and represents a relatively robust database, storm reports in CSA are not routinely available in a centralized database and thus precise comparisons in tornado frequency between these two regions should be interpreted with caution (31, 32, 68, 83). Despite these large disparities in observational practices, the contrast between CNA and CSA is so stark that it is unlikely to be due to differences in reporting practices alone.
A recent South American severe thunderstorm field campaign (RELAMPAGO-CACTI) demonstrated that environments along the eastern edge of the Andes in CSA are not favorable for tornadoes owing to relatively weak near-ground (0–1 km) vertical wind shear (69–71), corroborating recent work using reanalysis data (56, 72). In both CNA and CSA, a dominant source of low-level vertical wind shear is the poleward LLJ over each continent (7, 59, 73, 74, 94, 95, 110, 114). The vertical structure (e.g., peak height) differs between GPLLJ and SALLJ, as the Andes are in general taller and steeper than the Rockies such that slope effects are minimized while deeper lee cyclones develop near the Andes that help drive the SALLJ (59, 63, 71, 74, 75, 77, 78).
One geographic contrast between CNA and CSA that has not yet been considered is that the tropical South American landmass is very rough due to land cover (e.g., Amazon rainforest roughness length z0m ~ 1 m) and variable terrain (e.g., the Eastern Highlands), compared to the smooth and flat tropical ocean surface equatorward of North America (the Gulf of Mexico and Caribbean Sea; z0m ~ 10− 4 m;79, 80). Small-scale land surface roughness has been shown to reduce tornado formation locally (81). A modest reduction of the Amazon surface roughness by deforestation has been shown to intensify near-surface poleward winds and moisture transport along the climatological path of SALLJ (82). However, the role of large-scale upstream surface roughness, due to land cover and variable terrain, in modulating tornado activity downstream is unknown.
We hypothesize that the rough tropical South American landmass, compared to the smooth, flat tropical ocean surface equatorward of eastern North America, suppresses tornado potential downstream over CSA. A rough upstream land surface is expected to weaken poleward low-level winds, including the LLJ, that flow downstream to help produce tornadic environments. We first test this hypothesis in two ways to demonstrate this effect using global climate model (GCM) experiments: (i) with the tropical South American land surface smoothed and flattened to be ocean-like; and (ii) with the Gulf of Mexico and Caribbean Sea surface roughened to values comparable to a forested flat land, respectively. We then show that this result holds for an arbitrary midlatitude landmass using aquaplanet (i.e., an ocean-covered Earth) GCM experiments with an idealized continent and mountain range with a smooth versus rough equatorward ocean surface.
We define tornado potential, Ntor, as the mean frequency of tornadic environments per unit area and per year or per season (83). As a tornado typically forms from a pre-existing severe thunderstorm as described above, we decompose tornado potential as the product of the severe thunderstorm potential (mean frequency of severe thunderstorm environments per unit area and per year or per season), Nsts, and a conditional probability of strong near-ground rotation potential (SRH01) given the existence of a severe thunderstorm environment, Protation|sts (83):
Ntor = Nsts Protation|sts. (1)
Therefore, relative changes in Ntor are explained by the sum of relative changes in Nsts and Protation|sts and their interaction term:
\(\delta\) Ntor = \(\delta\)Nsts + \(\delta\)Protation|sts + \(\delta\)Nsts\(\delta\)Protation|sts. (2)
Using ERA5 historical reanalysis data for 1985–2014 (83, 84), we first confirm that tornado potential is substantially lower in CSA as compared to CNA (Fig. 1c), consistent with observed tornado activity (Fig. 1a; 83). This contrast is not driven by severe thunderstorm potential, as annual Nsts is actually slightly larger in CSA than in CNA (Fig. 1d), similar to observed lightning activity (Fig. 1b; 83). Instead, this contrast is driven by the near-ground rotation potential, with an annual domain-mean Protation|sts that is twice as large in CNA as in CSA (Fig. 1e).
We conduct GCM experiments using the CESM2.1.1 (83, 85) to test our hypothesis that the contrast in tornado potential between CSA and CNA is caused by the strong roughness of the land surface upstream of CSA relative to the smooth, flat ocean surface upstream of CNA. The control simulation (denoted as ctrl) is a historical global climate simulation during 1985–2014 (83). In general, ctrl quantitatively reproduces the lower magnitude of Ntor in CSA than in CNA (Fig. 1f). This difference is driven principally by a much smaller Protation|sts (Fig. 1h) rather than a difference in Nsts (Fig. 1g). The ctrl also produces a realistic distribution of the low-level winds as compared to ERA5, including the seasonal cycle of 850-hPa winds (vectors in Extended Data Fig. 1a–b) and the diurnal cycle of the frequency and intensity of LLJ within CSA and CNA (Extended Data Fig. 1c–d; 83).
We first show that our hypothesis is supported by comparing ctrl against an experiment in which the tropical South American landmass is smoothed and flattened to be ocean-like (denoted as smooth; Extended Data Fig. 2; 83). The change to an ocean-like surface upstream substantially increases tornado potential downstream over CSA (Fig. 2a–b). Relative to ctrl, annual Ntor in smooth increases by up to + 203% particularly over northeastern Argentina (Fig. 2c). Experiments deconstructing contributions from upstream land cover, resolved terrain, and parameterized elevation roughness demonstrate that the responses are driven approximately equally by the land cover (Extended Data Fig. 5a) and resolved terrain (Extended Data Fig. 5b), and to a lesser extent by the parameterized elevation roughness (Extended Data Fig. 5c).
The increase in CSA tornado potential in smooth is driven principally by the increase of up to + 115% in Protation|sts (Fig. 2e), while Nsts (Fig. 2d) and the interaction term (Fig. 2f) increase modestly (+ 42% and + 46%). These responses are associated with enhanced easterly winds at 850 hPa over the upstream surface due to the reduced upstream surface resistance to the flow in smooth. Through the mechanical forcing (43, 44) of the Andes (60, 61, 77), the stronger easterly trade winds lead to stronger northerly mean winds flowing downstream along the eastern slope of the central Andes into CSA (vectors in Fig. 2c). This induces an anticyclonic anomaly at 850 hPa that enhances the South American Subtropical High. Such a strong enhancement of Ntor and northerly low-level flow within CSA is found in all seasons (Extended Data Fig. 3). Associated with this flow response, the low-level vertical wind shear from the surface to 1 km above increases, which is the principal driver of the increase in SRH01 (Extended Data Fig. 4a) that produces environments more favorable to tornadoes within CSA in smooth. The modest increase in severe thunderstorm potential is driven by an increase in low-level moisture below 4 km (Extended Data Fig. 4b), likely associated with enhanced moisture transport from the South Atlantic Ocean by the intensified easterlies upstream and northerly flow downstream that generates thermodynamic environments more favorable to convection. These results are consistent with a strong increase in the frequency of SALLJ within CSA, especially during the afternoon (Extended Data Fig. 4c), as well as the enhanced intensity of SALLJ throughout the day (Extended Data Fig. 4d). These results provide evidence that the rough equatorward land surface of South America due to land cover and terrain, in contrast to a smooth and flat ocean surface, strongly suppresses central South American tornado potential primarily by reducing near-ground rotation potential, and secondarily by reducing severe thunderstorm potential.
While we focus on the upstream surface impact on the downstream region in this work, we note a contrasting response locally over the upstream surface: the stronger inland intrusion of the easterlies from the subtropical South Atlantic Ocean in smooth, deflected by the central Andes, induces a weakened northerly mean flow over northern Bolivia and Peru (vectors in Fig. 2c). This is primarily due to the flattened terrain (vectors in Extended Data Fig. 5b), though the reduced land-cover roughness itself favors stronger easterlies over the Amazon basin and hence a stronger northerly flow over the northern Bolivia (vectors in Extended Data Fig. 5a; 82). This results in transient responses locally over Bolivia-Paraguay with Nsts increases (Fig. 2d) but Protation|sts decreases (Fig. 2e). How the tropical land-cover roughness and the tropical terrain resistance, and their interaction, might modulate severe weather activity locally over northern South America is a topic worthy of future work.
Additional analyses and GCM experiments (83) add robustness to the result in smooth. The response of Ntor within CSA in smooth is consistent for different definitions of severe thunderstorm and tornadic environments, including the addition of a low convective inhibition (Extended Data Fig. 5d) or using higher thresholds of CAPE and S06 (Extended Data Fig. 5e). Indeed, a robust increase of tornado potential is detected in smooth across a wide range of lower-bound thresholds for CAPE (150 J kg− 1 < CAPE < 3000 J kg− 1) and S06 (10 m s− 1 < S06 < 30 m s− 1) (Extended Data Fig. 11a–c). While the land cover determines both the surface momentum roughness (z0m) and enthalpy roughness (z0k), an experiment with z0k fixed produces only a slightly stronger response of Ntor than smooth, showing that z0m is the dominant driver as compared to z0k (Extended Data Fig. 5f). Smoothing and flattening a smaller area of the upstream surface also produces a strong response in Ntor, but it is smaller than in smooth (Extended Data Fig. 5g). This indicates that the magnitude of this response depends on the area of the upstream surface, though the upstream surface closer to CSA appears to have a larger effect than the upstream surface far away from CSA.
We next show that our hypothesis is also supported by comparing ctrl against an experiment in which the upstream equatorward ocean surface of North America is roughened to be as rough as a rainforest-covered flat land surface (denoted as rough; 83). For simplicity, flow resistance due to variable terrain is not considered. The change to a rough ocean surface upstream substantially suppresses the tornado potential downstream over CNA (Fig. 3a–b). Relative to ctrl, annual Ntor in rough decreases by up to -41% (i.e. an increase of + 70% in ctrl relative to rough), particularly over the Great Plains and the Southeast U.S. (Fig. 3c).
In contrast to CSA, the response in Ntor is driven by the decrease in both Nsts (up to -30%; Fig. 3d) and Protation|sts (up to -27%; Fig. 3e), with a negligible increase from the interaction term (up to + 5%; Fig. 3f). These relative changes vary by region, as the decrease of Protation|sts is a larger driver over the Southeast U.S. while the decrease of Nsts is a slightly larger driver over the Great Plains. Over the Great Plains, the response is again associated with weakened easterly winds at 925 hPa over the roughened ocean surface in rough, driving weakened southerly winds (vectors in Fig. 3c) through the mechanical forcing of the Rockies (43, 44). Such a weakening of Ntor and southerly low-level flow over the Great Plains is found in all seasons (Extended Data Fig. 6). In this case, the low-level vertical wind shear from the surface to 1 km conditioned on severe thunderstorm environments decreases (Extended Data Fig. 4e) and as does the low-level moisture below 2 km in the mean state (Extended Data Fig. 4f). These results are consistent with the reduced frequency of LLJ, especially during the late afternoon (Extended Data Fig. 4g), though the weakening of LLJ intensity is relatively minor (Extended Data Fig. 4h). Meanwhile, over the Southeast U.S., the conditional low-level shear (Extended Data Fig. 4i) and mean-state moisture (Extended Data Fig. 4j) also decrease by similar magnitudes to the Great Plains, reflecting a weakened onshore flow within severe thunderstorm environments. This is consistent with the reduced LLJ intensity within this area (Extended Data Fig. 4l) and the reduced LLJ frequency during the afternoon, though the nocturnal LLJ frequency slightly increases (Extended Data Fig. 4k). In this region the mean-state onshore flow field at 925-hPa is actually slightly enhanced in rough (vectors in Fig. 3c). Hence, the low-level shear response over the Southeast U.S. is transient, occurring within severe thunderstorm environments but not in the mean state.
Additional analyses and GCM experiments (83) also add robustness to the result in rough. Such a strong response of Ntor holds for modified definitions of severe thunderstorm and tornadic environments, including the addition of a low convective inhibition (Extended Data Fig. 7a) or using higher thresholds of CAPE and S06 (Extended Data Fig. 7b). A robust decrease in tornado potential is detected in rough across a wide range of lower-bound thresholds for CAPE (150 J kg− 1 < CAPE < 3000 J kg− 1) and S06 (10 m s− 1 < S06 < 35 m s− 1) (Extended Data Fig. 11d–f).Roughening a smaller area of the upstream surface (i.e. the Gulf of Mexico only) also produces a strong response but is smaller than in rough (Extended Data Fig. 7c), again indicating the dependence of the response on the area of the upstream surface.
These results for North America are qualitatively consistent with the results of South America above, suggesting that the strong contrast in upstream surface roughness and terrain drives the marked contrast in the climatology of tornado activity between the two continents. The spatial pattern and drivers are a bit more complex, with the thermodynamic response playing a more significant role in CNA than CSA. The Great Plains response is similar to CSA via the role of the poleward low-level flow and the LLJ within it, whereas the response in the Southeast U.S. differs in its transient mechanism. This reflects the well-known contrast in the behavior of tornado activity between the two sub-regions (15, 38, 39, 86, 87). The responses in North America are overall smaller in magnitude than in South America above, perhaps because there is no experimental analog in rough for the influence of the upstream highlands, which was shown to be as important as land cover roughness in smooth (Extended Data Fig. 5a–b). Other topographic differences between these two continents, such as the height (61, 63, 75, 76) and slope (45, 48) of the Rockies versus the Andes that drive differences in the mechanisms of LLJ formation, may also modulate regional severe weather activity differently. In addition, the wider extent of the Rockies may favor more elevated mixed layers and drylines that are more favorable to the formation of severe thunderstorm and tornadic environments over North America (56, 88).
Finally, we show that our hypothesis is supported more generally for a midlatitude landmass poleward of a tropical ocean on an Earthlike planet, using idealized aquaplanet GCM experiments (83, 89–91). We design a smooth aquaplanet simulation (denoted as aquasmooth) that contains the minimal geographic ingredients for severe thunderstorm environments (40, 83). The set-up has an idealized rectangular midlatitude continent with a 3-km north-south oriented plateau along its western edge (Fig. 4a). This height is between the height of the Rockies and Andes. For simplicity, the plateau is narrow (7.5 degrees of longitude) and steep (a slope of 90 degrees) and thus is similar to the shape of the Andes. The aquasmooth successfully reproduces a “tornado alley”-like region of significant tornado potential east of the plateau (Fig. 4a) with a strong seasonal cycle that peaks in springtime (Extended Data Fig. 8). To test our hypothesis, we compare aquasmooth with a rough aquaplanet experiment in which the surface roughness of the tropical ocean equatorward of the continent is enhanced to be analogous to the roughness magnitude of the rainforest-covered flat land surface (denoted as aquarough; Fig. 4b; 83).
The aquaplanet results support our hypothesis, as annual tornado potential within the tornado region in aquarough is strongly suppressed by up to -67% relative to aquasmooth (i.e. an increase of + 203% in aquasmooth relative to aquarough; Fig. 4c). This response is driven principally by the decrease in Protation|sts (~ -60%; Fig. 4e) with a small contribution from the decrease in Nsts (~ -14%; Fig. 4d) and a small increase due to the interaction term (+ 7%; Fig. 4f). Similar to the real-world GCM experiments above, particularly South America, these responses are associated with weakened easterly winds at 850 hPa over the upstream roughened ocean surface in aquarough, which thus weakens the southerly winds flowing downstream into the continental interior (vectors in Fig. 4c). Such a strong weakening of Ntor and southerly low-level flow within the continental interior is found in all seasons (Extended Data Fig. 8). The weakened low-level winds primarily reduce the low-level vertical wind shear from the surface to 1 km above (Extended Data Fig. 4m), and the inland moisture transport in the lower troposphere below 4 km (Extended Data Fig. 4n), and thereby produces environments less favorable to tornadoes. This result is consistent with a decrease in the frequency and intensity of LLJ throughout the day (Extended Data Fig. 4o–p). We note that aquasmooth does not produce an obvious coastal region of elevated tornado potential farther east that would be analogous to the Southeast U.S., a topic worthy of future research. Nonetheless, this outcome again highlights the contrast in storm behavior between the Southeast U.S. and the Great Plains (15, 38, 39, 86, 87).
These strong responses in aquarough are again robust to modified definitions of severe thunderstorm and tornadic environments (Extended Data Fig. 9a–b). A robust decrease in tornado potential is again detected in aquarough across a wide range of lower-bound thresholds for CAPE (150 J kg− 1 < CAPE < 2700 J kg− 1) and S06 (15 m s− 1 < S06 < 45 m s− 1) (Extended Data Fig. 11g–i). Finally, results are not specific to the magnitude of roughening, as the response increases as the upstream surface is more strongly roughened (Extended Data Fig. 9c–d; 83). This is consistent with the finding that the response over CSA in smooth is stronger than that over CNA in rough due to the complex terrain over the tropical South America (e.g., the presence of Eastern Highlands), which causes a rougher tropical South American landmass than the roughened tropical ocean surface in rough (119).
This work adds a critical missing ingredient to our conceptual understanding of the geographic controls of tornado hotspots on Earth: a smooth, flat ocean-like upstream surface. A smoother upstream surface permits stronger easterly trade winds that feed the poleward low-level winds flowing downstream into the continental interior (red arrows in Fig. 4a–b). Despite mechanistic and structural differences between the continents, the poleward low-level winds, including the GPLLJ over central North America and SALLJ over central South America, help generate the strong near-ground environmental vertical wind shear necessary for tornadoes and also enhance inland moisture transport. Our real-Earth and idealized global climate model experiments successfully capture such direct impacts of the large-scale upstream surface roughness on the geography of tornado potential over both continents.
Our analyses indicate that a smooth, flat ocean surface upstream, as compared to a rougher land, is ideal for generating a regional tornado hotspot as found in central North America. The rainforest cover and terrain complexity further enhance the roughness of the tropical South American landmass. If the tropical South American land surface were less rough (i.e., more ocean-like), tornado potential over central South America might be more similar to that found over central North America (Fig. 2b versus Fig. 3a).
Here we have focused on the role of continental-scale roughness upstream of the severe weather regions of North America (ocean) and South America (land). We hope this work will open up new research questions regarding how regional heterogeneity of topography and the land surface (land cover and variable terrain) can modify regional tornado activity, both downstream and locally, as well as the role of higher resolution topographic interactions with the low-level jets of both North and South America that could be tackled in future work. This includes the emergence of the South American Eastern Highlands and the Amazon rainforest basin in paleoclimate records (92), and Amazonian deforestation and regional land cover changes in the eastern half of the U.S. in the modern climate (82, 93). Moreover, an important question is how terrain and land cover may alter the response of tornadoes in the future, as climate change may shift the large-scale atmospheric circulation and the geographic patterns of severe thunderstorm and tornado activity that it produces (87).