Tropical Atlantic upper-ocean variability during the past two millennia

• Ano de defesa: 2025
• Autor(a) principal: Verona, Laura Sobral
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Atlantic Meridional Mode; Atlantic Subtropical Cells; Atlantic Zonal Mode; Células Subtropicais do Atlântico; CMIP6; Last Millennium; Modo Meridional do Atlântico; Modo Zonal do Atlântico; Precipitação; Precipitation; Último Milênio; Volcanism; Vulcanismo
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/21/21135/tde-02092025-135259/

The tropical Atlantic presents a complex interplay between sea surface temperature (SST) variability, upper-ocean circulation, and regional hydroclimate, driven by land-atmosphere-ocean coupling and subsurface shallow circulation from interannual to multidecadal scales. These interactions are potentially modulated by both natural external forcing, particularly volcanic eruptions, and anthropogenic warming. The evolution of these interconnected dynamics is investigated over the past two millennia using a combination of CMIP6-endorsed simulations, focusing on three key aspects: (1) 20th-century changes in the Atlantic Zonal Mode (AZM), Atlantic Meridional Mode (AMM), and associated precipitation in response to anthropogenic forcing; (2) the role of volcanic forcing in driving tropical Atlantic variability anomalies during the Last Millennium (LM, 850-1850 C.E.); and (3) long-term mechanisms that link tropical Atlantic variability with the Atlantic Subtropical Cells (STCs) and the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). Our results indicate a post-1970 weakening of tropical Atlantic variability, characterized by a reduction in SST-driven feedbacks controlling AZM and AMM behavior. This weakening is linked to anthropogenic warming by the relaxation of trade winds, which leads to thermocline deepening and suppression of AZM variability. In addition, a persistent interhemispheric warming trend and a background negative Atlantic Multidecadal Variability have contributed to a weaker AMM. The decline of both climate modes is associated with shifts in the Intertropical Convergence Zone (ITCZ), which alter regional precipitation patterns. During the LM, volcanic forcing emerges as a significant external modulator of tropical Atlantic variability. Large eruptions trigger a delayed Atlantic Niño cooling following an initial Pacific El Niño-like response, driven by basin-wide equatorial atmospheric wave interactions. A sea level pressure dipole across ocean basins is sustained by differential cooling across the tropics, highlighting the role of volcanic forcing in modulating Atlantic Niño intrinsic dynamics, beyond variability driven by the El Niño-Southern Oscillation. Extending the analysis over the past two millennia (500-2015 C.E.), we demonstrate that the STCs act as a positive feedback mechanism for AMM variability, via equatorial upwelling and asymmetrical Ekman transports. In contrast, AZM variability is primarily influenced by the Equatorial Undercurrent and eastern equatorial upwelling. However, ongoing anthropogenic warming has disrupted traditional STC pathways, introducing asymmetrical changes in surface STC intensity across hemispheres due to shifting wind patterns. Within the thermocline, modern AMOC weakening and its superposition with the STCs drive a more complex zonal reorganization, leading to thermocline adjustments that buffer the equatorial upwelling response despite large-scale circulation changes. These findings provide important insights into how external forcings shape tropical Atlantic climate variability across multiple timescales, connecting surface and subsurface processes. By identifying key ocean-atmosphere interactions over the past 2,000 years, this study improves our understanding of climate system responses to different external forcings, contrasting them with the internal climate variability. This offers a more comprehensive view of tropical Atlantic ocean-atmosphere dynamics and their implications for present and future climate change.