Upstream Surface Roughness and Terrain are Strong Drivers of Contrast in Tornado Potential Between North and South America
North and South America both possess severe thunderstorm hotspots due to their similar geographic setup—a mountain range to the west and an equatorward source of warm, moist air—yet only North America is a hotspot for tornadoes because the equatorward ocean surface is smooth and flat. This work shows that a smooth, flat ocean-like upstream surface may substantially increase the potential for tornadoes downstream.
This work adds a critical missing ingredient to our conceptual understanding of the geographic controls of tornado hotspots. A smooth upstream surface permits stronger low-level winds to flow into the continental interior. The low-level jet helps generate strong near-ground environmental vertical wind shear necessary for tornadoes.
Numerical experiments using CESM2 are conducted to examine the sensitivity of severe storm environments to the upstream surface. Smoothing the northern part of South America or roughening the Gulf of Mexico are both shown to modify the severe storm hotspots of the respective continents. The impact is a modification of low-level winds that impact the storm's relative helicity, which is a key ingredient for tornadic activity. Idealized experiments based on the CAM6 aquaplanet that add simple representations of landmasses are used to generalize the findings. Both realistic and idealized experiments successfully capture the direct impacts of the large-scale upstream surface roughness on the geography of tornado potential over both continents. This analysis indicates 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.