A Multiscale Perspective on Convective Storms in South America: From Tornadic Environments to Intraseasonal Modulation and Future Changes
October 29, 2025
Daniel Veloso
Committee: Eric Maloney (Advisor); Kristen Rasmussen (Co-advisor); James Hurrell; Delphine Farmer (Chemistry)
Abstract
Convective storms are a key component of the Earth’s climate system, producing a significant fraction of global precipitation, and are often associated with severe weather, including flash floods, large hail, and strong winds. While convection is a global phenomenon, its nature is strongly modulated by geographical characteristics. South America is a natural laboratory for studying a wide spectrum of convective storms, including some of the planet's most intense convective storms. The uniqueness of this region is defined by its vast meridional extension (12°N–55°S), which fosters a broad range of climatic regimes, and by its complex geography, dominated by features such as the Amazon rainforest and the Andes mountains.This dissertation provides a comprehensive, multi-scale investigation into the convective storms of South America, examining their behavior in the present-day climate, their modulation by subseasonal variability, and their projected response to global warming. By integrating satellite observations, reanalysis products, and convection-permitting climate modeling, this work addresses critical gaps in the understanding of high-impact weather in a vulnerable and understudied region.
In the first chapter, an analysis of the environment supporting tornadoes in southeast South America (SESA) is presented. Based on a self-constructed database and ERA5 reanalysis, this work identifies the key ingredients for tornadogenesis at multiple timescales. The proximity environment before and during tornado occurrence depicts enhanced instability, deep-layer wind shear, storm-relative helicity, reduced convective inhibition, and a lowered lifting condensation level. At the synoptic scale, tornado events are linked to a strong anomalous trough crossing the Andes, and the subsequent strengthening of the South American low-level jet that increases moisture advection supporting deep convection in the subtropics. At planetary scales, the environment is preconditioned by Rossby wave trains originated a few weeks prior to tornado occurrence in the western Pacific, and preferred Madden-Julian Oscillation (MJO) phase 3. This chapter reveals a complex interplay of processes that foster severe weather as well as potential for predictability a few weeks in advance.
In the second chapter, the influence of the MJO on austral summer convective storms is explored in South America. In tropical South America, MJO wind anomalies circumnavigating the tropics directly modulate the large-scale circulation and thermodynamics over the continental land, enhancing column moisture and precipitation during MJO phases 8 and 1, and bringing opposite conditions during phases 3 to 5. This modulation translates to distinct changes in individual storm structures in central and eastern tropical South America, with MJO phases 8 and 1 favoring wider and shallower storms that produce more rain, while phases 3 to 5 favor deeper and narrower storms. This connection establishes a critical link between subseasonal variability and storm-scale behavior, suggesting a potential pathway for improved predictability.
In the third and last chapter, this dissertation investigates future changes in austral summer precipitation and storm characteristics using a long-term, high-resolution, convection-permitting climate simulation over South America. Using a pseudo-global warming approach, the model projects a continental shift toward less frequent but more intense precipitation, predominantly over tropical South America. This change is driven by an increase in the occurrence and total rainfall of storms containing intense deep convective cores. This transition toward more deep convective cores at the expense of moderate wide convective systems is consistent with a projected increase in atmospheric instability (CAPE) across the continent.
Collectively, this dissertation advances the fundamental understanding of the atmospheric processes governing convective storms in South America. By characterizing the present-day severe weather environment, identifying key sources of subseasonal predictability, and projecting future changes in extreme convective storms, this research provides crucial insights into the evolving nature of high-impact weather in the region.