Stable isotopes of water (δ18O-δ2H) in intra-events: deciphering the history of rainfall in the central portion of the state of São Paulo

Author(s)

Vinicius dos Santos

• Document type: Doctoral Thesis
• Press: Rio Claro. 2023-01-09.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Geociências e Ciências Exatas. Rio Claro
• Defense date: 2022-12-15
• Advisor: Didier Gastmans; Ana María Durán Quesada

Abstract

Stable isotopes of water, hydrogen (1H/2H) and oxygen (16O/17O/18O) are powerful tracers of water movement in the hydrological cycle. Due to the necessity to understand how the hydrological cycle has the answer to climate change, this study's purpose is to understand the isotopic rainfall formation and evolution in rainfall types, mainly convective and stratiform, that has been a topic of interest in isotopes community. The 312 isotope samples in 18 intra-events (collected in 5-10-30 minutes intervals) were analyzed between September/2019 to February/2021. The Micro Rain Radar, GOES-16 satellite imagery and compact weather station were used to classify the rainfall types into convective, stratiform, mixed and no-classified. These rainfall types were formed by the interaction between humidity (from the Atlantic Ocean, with 3 pathways arriving in the São Paulo state, the Atlantic Ocean, South of Brazil and the Amazon Forest) and varied atmospheric systems (e.g., Cold fronts, South America Convergence Zone, thermal instability). The 3 humidity pathways generated a vapor that characterized the past rainfall (regional influence) before the local formation, with depletion in heavy isotopes during these pathways. Due to distinct mechanisms, this depletion was amplified and modified during local rainfall formation. Thus, convective rainfall reveals the diurnal difference in convective activity, generating more negative (less negative) δ18O values during day (night), while stratiform rainfall reflected the life cycle, of developing phases (with enriched δ18O values), the mature phase (δ18O moderate), and the dissipating phase (negative δ18O values). In the convective and stratiform rainfall, the negative δ18O values (-8 ~ -16‰) represented the heavy depletion in isotopes, amplified the regional processes, that they were modified by local evaporative processes, during the falling raindrops, resulting in enriched δ18O values (>-7 ~ 3‰) and lower d-excess (<10‰). This interpretation was confirmed by good (r > 0,50) and significative (p < 0,05) linear regression models. Our findings generated an excellent observational dataset that could be used in climatic models and help to answer the gap of interpretations about the climatic control in the isotopic composition of rainfall in inland tropical areas. (AU)

• FAPESP's process: 19/03467-3 - Variability of precipitation isotopic composition in central portion of São Paulo State based on intra-events evaluation
• Grantee: Vinícius dos Santos
• Support Opportunities: Scholarships in Brazil - Doctorate