An insight into the severe 2019-2021 drought over Southeast South America from a daily to decadal and regional to large-scale variability perspective

An acceleration in the global water cycle with severe rainfall and drought episodes is expected to occur in many regions due to climate change¹. Spatial and temporal disturbances in the atmospheric water budget encompassing changes in precipitation and evaporation rates, soil moisture levels, groundwater recharge and water available for runoff are foreseen, posing great challenges to the global freshwater availability² , food security³ and the sustainability of natural ecosystems⁴ . Thus, the assessment of hydro-meteorological extremes is crucial, particularly in regions such as South America (SA) that are extremely vulnerable to climate change and lack a comprehensive assessment of this extremes. Moreover, SA has two main watersheds (Amazon and La Plata basin) essential for the regional hydroclimate and local and remote precipitation in a global scale, through moisture recycling, transport and convergence.

Regions of Southeast SA, particularly over the La Plata basin and the Pantanal, have witnessed severe drought conditions in recent years. Pronounced soil dryness started to be recorded during mid-2018 over Southeast Brazil, but rapidly spread to areas in Paraguay, Bolivia, and northern Argentina. This abnormal situation lasted until 2021, leading to huge agricultural losses. Extremely low streamflow levels in the Paraná and Paraguay rivers caused serious constraints in the hydropower generation and water supply, and led to disruptions in the waterways that are fundamental for the fluvial transport and economy of these countries. Moreover, the Pantanal biome was also dramatically affected, particularly during 2020, when pronounced soil dry-out conditions concurred with several heatwaves, leading to devastating fires that resulted in catastrophic burned area levels⁷.

This study presents a detailed analysis of the 2019–2021 drought episode over Southeast SA from a climate change and variability context aiming: to (1) evaluate the exceptionality of the soil dry-out conditions within a historical record of 70 years; (2) provide a detailed spatiotemporal evolution (from daily to decadal and regional to large-scale) of soil moisture anomalies across the southeast SA; and (3) assess the large-scale tropical and subtropical atmospheric mechanisms that were responsible for pronounced disturbances in the normal processes of moisture transport and convergence, and that, ultimately, explained the observed soil moisture deficits over southeast SA.

References

• [1] Chagas, V. B. P., Chaffe, P. L. B. & Blöschl, G. Climate and land management accelerate the Brazilian water cycle. Nat. Commun. 13, 5136 (2022).
• [2] Konapala, G., Mishra, A. K., Wada, Y. & Mann, M. E. Climate change will affect global water availability through compounding changes in seasonal precipitation and evaporation. Nat. Commun. 11, 1–10 (2020).
• [3] Lesk, C., Rowhani, P. & Ramankutty, N. Influence of extreme weather disasters on global crop production. Nature 529, 84–87 (2016).
• [4] Seddon, A. W. R., Macias-Fauria, M., Long, P. R., Benz, D. & Willis, K. J. Sensitivity of global terrestrial ecosystems to climate variability. Nature 531, 229–232 (2016).

Acknowledgments:

JG, AR, RT are grateful to Fundação para a Ciência e a Tecnologia for the PhD Grant 2020.05198.BD, I.P./MCTES for the national funding (PIDDAC) – UIDB/50019/2020 and for Dhefeus (2022. 09185.PTDC). RL is grateful to CNPq (Grant 311487/2021-1) and FAPERJ (Grant E26/202.714/2019).