Climate change intensifies the global water cycle, altering the frequency and magnitude of dry and wet spells. This poses significant threats to freshwater resources, food security, and ecosystem sustainability. Increasing temperatures lead to higher lower-tropospheric water vapor, impacting the evaporation-precipitation balance and soil moisture. Long-term drying trends can further limit evaporation and reduce moisture recycling, influencing atmospheric water budgets, especially in drylands. Reduced soil moisture might enhance atmospheric moisture convergence, increasing extreme precipitation events from convective storms. Conversely, under high evaporative demand, soil moisture imbalance can constrain surface latent heat fluxes, increasing sensible heat and escalating temperatures. The interplay between soil moisture, atmospheric water budget, and temperature forms a complex feedback system modulated by climate change (increasing temperatures, altered vegetation, shifting atmospheric dynamics). Relatively little research has concurrently assessed temperature, precipitation, and soil moisture trends over South America (SA). The IPCC reports warming trends for most of SA, with the tropics experiencing the largest increases. Precipitation projections for SA show a dipole pattern: drier conditions in the Amazon and wetter conditions in the central-eastern regions (La Plata Basin). Evaporation from the Amazon and La Plata basins is crucial for regional hydroclimate and precipitation. External moisture sources, such as the Atlantic trade winds feeding the Amazon, also contribute significantly. These hydrological processes are modulated by large-scale oceanic-atmospheric modes (ENSO, MJO, AMM, and AZM), influencing permanent and transient systems that play a crucial role in SA's precipitation and evaporation (ITCZ, SACZ, SALLJ, cold fronts, cyclones, anticyclones). A severe drought began in CESA in mid-2018, affecting Southeast Brazil, Paraguay, Bolivia, and Argentina, causing significant agricultural losses and water supply issues. Flash droughts, characterized by rapid intensification of already dry conditions, exacerbated the impacts, particularly in the Pantanal wetland, leading to devastating fires and economic losses. This study aims to characterize this unprecedented drought in CESA, exploring its historical context, assessing the exceptionality of soil moisture anomalies, and describing the atmospheric mechanisms involved at various timescales.

Existing literature highlights the strengthening of the global water cycle due to climate change, leading to more frequent and intense dry and wet spells. Studies have shown the impact of this intensification on global freshwater availability, food security, and the sustainability of natural ecosystems. The role of increasing temperatures in raising lower-tropospheric water vapor and its influence on the evaporation-precipitation balance has been extensively documented. Research also emphasizes the feedback mechanisms between soil moisture, atmospheric water budget, and temperature, particularly in drylands. The influence of large-scale climate patterns like ENSO, MJO, and Atlantic modes on South American hydroclimate has been discussed in previous studies, highlighting their impact on precipitation and evaporation. However, less attention has been given to the combined assessment of temperature, precipitation, and soil moisture trends in South America, especially the concurrent impact of large-scale climate modes on the region. The previous research on drought in the La Plata Basin has focused on the individual impact of climate modes on precipitation and has not fully considered the combined influence of the land surface dynamics with the large-scale atmospheric forcing mechanisms which led to the record-breaking drought.

This study utilized daily data from the European Centre for Medium-Range Weather Forecasts (ERA5) and ERA5-Land reanalysis datasets (1959-2022 and 1951-2022, respectively). Parameters included precipitation, temperature, specific humidity, wind components, geopotential, vertical velocity, and soil moisture (0-7 cm depth). The methodology involved several key steps: 1. **Analysis of Long-Term Trends:** Spatial and temporal analyses were conducted to assess long-term drying trends in SA, focusing on CESA. Trends were determined using the Mann-Kendall test. Bivariate Gaussian distributions were used to characterize temperature-precipitation relationships for two sub-periods (1959–1989 and 1990–2022) to identify shifts in climate characteristics. 2. **Moisture Budget Analysis:** The vertically integrated water vapor transport (IVT) was calculated using equations integrating specific humidity, zonal and meridional wind components over the vertical atmospheric column. The vertically integrated moisture divergence was computed using finite differences. The moisture budget over CESA was analyzed using a line integral approach that incorporates IVT across CESA's borders to quantify net moisture convergence (VIMC). Equations were used to separate precipitation contributions from moisture convergence and local moisture recycling. The influence of moisture inflow from the Amazon was assessed via correlations between IVT at CESA borders and VIMC. 3. **Drought Characterization:** A modified R-index was developed to rank extreme and widespread drought events using soil moisture anomalies. Daily standardized soil moisture anomalies (relative to the 1981–2010 climatology) were computed and filtered with a 31-day running mean. The R-index multiplies the percentage of CESA experiencing anomalies below two standard deviations ("Spatial Extent") by the mean soil moisture anomaly in those areas ("Mean Anomaly"). 4. **Large-Scale Forcing Analysis:** Spatial correlations were calculated between mean annual SSTs and mean annual IVT at CESA's northern border to identify links with ENSO and PDO. Time series of ENSO indicators (SOI, ONI), PDO, and AZM were analyzed, along with correlations between these indices and IVT. Spatial anomaly composites were generated for the nine most extreme R-index peaks, considering soil moisture, geopotential height, temperature, moisture flux divergence, wind fields, and velocity potential. The Rossby Wave Source (RWS) was calculated using the barotropic vorticity equation in pressure coordinates to analyze the extratropical influence on the drought.

The 2019–2022 drought in CESA was unprecedented in terms of intensity and duration, exceeding any previous drought in the historical record. A long-term drying trend in CESA since the 1990s, driven by both natural decadal precipitation variability and increasing temperatures, predisposed the region to this extreme event. The study revealed a strong link between the drought and a combination of large-scale tropical and subtropical atmospheric forcing. Key findings include: 1. **Long-Term Drying Trend:** CESA experienced a statistically significant drying trend over the period 1990–2022, with the Pantanal, Southeast Brazil, central Paraguay, and northern Argentina showing the steepest declines in soil moisture. A shift towards warmer and drier conditions was observed since the 1990s in CESA. 2. **Decadal Variability in Moisture:** Interannual variability in precipitation and vertically integrated moisture convergence (VIMC) in CESA showed clear decadal oscillations, with drier periods in the first and last decades of the analyzed period (1959-2022). Moisture recycling played a lesser role compared to moisture convergence in explaining precipitation variations. The drought years (2019-2020) experienced the lowest VIMC levels ever recorded, while 2021-2022 saw the lowest contributions of moisture recycling. 3. **Moisture Inflow and Outflow:** The moisture inflow from the Amazon, primarily through CESA's northern and western borders, played a significant role in moisture convergence and precipitation in CESA. Outflow from the eastern and southern borders was less influential. The 2019–2022 drought was defined by reduced moisture inflow from the Amazon, contrasting with the much greater inflow during wet years. Anomalous southeast-northwest orientation of the IVT during the drought, contrasting with the expected northwest-southeast pattern during wetter years, demonstrated the weakening of moisture transport from the Amazon towards CESA, which is usually supported by the South American Low-Level Jet (SALLJ). 4. **Atmospheric Convergence and Divergence:** Precipitation anomalies were influenced by both moisture availability and atmospheric convergence. During the drought, CESA experienced lower-than-normal moisture convergence, evidenced by positive anomalies of the vertical integral of divergence of moisture flux. Anomalous low-tropospheric divergence and air spread from CESA resulted in precipitation deficits and positive anomalies in evaporation minus precipitation (E-P) balance. In contrast, during wet years, moisture convergence and negative E-P anomalies prevailed in CESA. This suggests that the decrease in moisture inflow from the Amazon Basin, and the atmospheric divergence were responsible for the precipitation deficits in CESA. 5. **Exceptional Drought Severity and Spatial Extent:** The R-index revealed the exceptional severity and spatial extent of the 2019–2022 drought. The time series of the R-index showed high variability due to fluctuations in drought intensity and spatial coverage. April 2020 saw the most severe conditions, with over 30% of CESA experiencing soil moisture anomalies exceeding two standard deviations. The drought's spatial footprint varied throughout 2020, affecting Pantanal and surrounding areas at different times. The drought extended into 2021 and 2022, with multiple flash droughts occurring. 6. **Large-Scale Tropical-Subtropical Forcing:** The study showed a strong influence of large-scale climate modes on the drought. Decadal variability in CESA precipitation was largely driven by ENSO, with La Niña events associated with precipitation deficits. A lead-lag relationship existed between ENSO and CESA precipitation. PDO and AZM also showed associations with CESA precipitation, particularly at decadal scales. During the 2019–2022 drought, cold SSTs in the central and southeast tropical Pacific and enhanced low-tropospheric divergence were observed. An eastward-shifted Walker cell with an anomalous descending branch over northwest SA and an ascending branch over the equatorial Atlantic was identified. This configuration amplified the meridional Hadley cell, causing subsidence and clear-sky conditions over CESA. This intensified the E-P imbalance. 7. **Synoptic-Scale Influence:** During flash drought episodes, positive 500hPa geopotential height anomalies, warm low-tropospheric conditions, and enhanced moisture flux divergence were observed. These indicated subsidence and clear-sky conditions, leading to increased evaporation. Anomalous meridional wind fields at 200 hPa indicated a Rossby wave train extending from the west-central south Pacific to the south Atlantic. This Rossby wave was originating in the west-central Pacific, associated with strong convection and warm SSTs in the Indo-Pacific. A zonal expansion of the subtropical high-pressure systems in the south Atlantic and south Pacific, along with the poleward shift of the subtropical jet stream, contributed to reduced cyclogenesis and frontal systems reaching CESA.

The findings demonstrate that the severe 2019–2022 drought in CESA resulted from a complex interplay between a long-term drying trend and acute large-scale atmospheric forcing. The long-term drying trend, stemming from a combination of natural decadal variability and rising temperatures, increased the region's vulnerability to drought. The observed atmospheric forcing mechanisms, including ENSO, PDO, AZM and a Rossby wave, explain the anomalous tropical circulation patterns (eastward-shifted Walker cell and amplified meridional Hadley cell). The combined effects of these factors led to moisture convergence in other areas while causing strong subsidence over CESA, which suppressed precipitation and accelerated evaporation, culminating in the unprecedented soil dryness. The study underlines the critical importance of internal climate variability in modulating precipitation and evaporation in SA, with implications for drought prediction. The results suggest that improved climate models are crucial to simulate the complex interplay of tropical and subtropical dynamics involved in this drought, especially at daily-to-multiyear timescales. The findings enhance our understanding of the various factors that contributed to the severity and exceptional nature of this drought, improving drought prediction and risk management strategies.

The 2019–2022 drought in CESA was a historically unprecedented event, resulting from the combined influence of a long-term drying trend and large-scale atmospheric forcing mechanisms. The study highlights the complex interplay between tropical and subtropical dynamics, driven by ENSO, PDO, and AZM, and the crucial role of Rossby waves in exacerbating the drought. These findings stress the need for improved climate models that can accurately simulate daily-to-multiyear timescale atmospheric processes to better predict the occurrence of future extreme dry spells. Further research should investigate the interaction of the identified atmospheric forcing mechanisms with land-atmosphere feedback mechanisms to understand the amplification of drought conditions.

While the ERA5 and ERA5-Land datasets provided valuable long-term data, inherent limitations in reanalysis products should be considered. The study focused on the interplay between atmospheric forcing and soil moisture, and further investigations could explore other factors such as deforestation or land use change. The R-index, although useful for ranking drought events, may not capture all nuances of drought impacts. The lead-lag relationship between ENSO and CESA precipitation requires more investigation to establish consistency across the entire analysis period.