据悉，由于西部落基山脉地势较高和来自赤道海洋的温暖潮湿的空气，北美中部是全球龙卷风的热点地区。这种传统观点认为，安第斯山脉以西，亚马逊盆地以北的南美洲中部，应该有一个至少和北美中部一样活跃的“龙卷风走廊”。中南美洲经常有强烈的雷暴，但龙卷风相对较少。传统观点忽略了龙卷风特有的一个重要因素：即，光滑、平坦海洋般的上游表面。
研究团队通过进行全球气候模拟实验发现，如果南美洲中部的赤道陆地表面被磨平，变得像海洋一样，那么它的龙卷风潜力就会大大增加。同样，研究人员表明，如果北美中部的赤道海洋表面被粗糙化到与林地相当的值，那么其龙卷风的潜力将大大降低。粗糙的上游表面主要通过减弱极地低空风来抑制龙卷风环境的形成，其特征是山脉以东的低空急流减弱。
结果表明，任何使用简化的大陆和山脉的理想化实验，结果显示对于任何中纬度大陆都是稳健的。研究结果表明，大规模的上游表面粗糙度可能是北美和南美之间龙卷风潜力强烈对比的一级驱动因素。
附：英文原文
Title: Upstream surface roughness and terrain are strong drivers of contrast in tornado potential between North and South America
Author: Li, Funing, Chavas, Daniel R., Medeiros, Brian, Reed, Kevin A., Rasmussen, Kristen L.
Issue&Volume: 2024-6-18
Abstract: Central North America is the global hotspot for tornadoes, fueled by elevated terrain of the Rockies to the west and a source of warm, moist air from equatorward oceans. This conventional wisdom argues that central South America, with the Andes to the west and Amazon basin to the north, should have a “tornado alley” at least as active as central North America. Central South America has frequent severe thunderstorms yet relatively few tornadoes. Here, we show that conventional wisdom is missing an important ingredient specific to tornadoes: a smooth, flat ocean-like upstream surface. Using global climate model experiments, we show that central South American tornado potential substantially increases if its equatorward land surface is smoothed and flattened to be ocean-like. Similarly, we show that central North American tornado potential substantially decreases if its equatorward ocean surface is roughened to values comparable to forested land. A rough upstream surface suppresses the formation of tornadic environments principally by weakening the poleward low-level winds, characterized by a weakened low-level jet east of the mountain range. Results are shown to be robust for any midlatitude landmass using idealized experiments with a simplified continent and mountain range. Our findings indicate that large-scale upstream surface roughness is likely a first-order driver of the strong contrast in tornado potential between North and South America.
DOI: 10.1073/pnas.2315425121
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2315425121