Quadro et al. (2025) The Atmospheric Water Cycle over South America as Seen in the New Generation of Global Reanalyses

Identification

• Journal: Hydrology
• Year: 2025
• Date: 2025-11-29
• Authors: Mário Francisco Leal de Quadro, Dirceu Luís Herdies, Ernesto Hugo Berbery, Caroline Bresciani, Fabrício Daniel dos Santos Silva, Michel Nobre Muza, Cássio Aurélio Suski, Diego Portalanza
• DOI: 10.3390/hydrology12120316

Research Groups

• Federal Institute of Santa Catarina (IFSC), Brazil
• National Institute for Space Research (INPE), Brazil
• Cooperative Institute for Climate & Satellites (CICS), Earth System Science Interdisciplinary Center (ESSIC), University of Maryland, USA
• Institute of Atmospheric Sciences, Federal University of Alagoas (UFAL), Brazil
• Facultad de Ciencias Agrarias, Universidad Agraria del Ecuador (UAE), Ecuador
• Center of Natural and Exact Sciences, Department of Physics, Federal University of Santa Maria, Brazil
• Instituto de Investigación, Universidad Agraria del Ecuador (UAE), Ecuador
• Instituto Superior Tecnológico Argos, Ecuador

Short Summary

This study evaluates precipitation and key atmospheric water-cycle terms over South America using three modern global reanalyses (MERRA-2, ERA5, CFSR/CFSv2) against two observation-based datasets (CPC Unified Gauge, MSWEP-V2) from 1980–2021. It finds MERRA-2 generally exhibits the smallest precipitation biases and highest correlations, along with better moisture-budget closure, making it the most reliable for basin-scale water-budget analyses in the region.

Objective

• To evaluate how three modern reanalyses (MERRA-2, ERA5, and CFSR/CFSv2) represent South American precipitation and key water-cycle terms (precipitation (P), evaporation (E), vertically integrated moisture-flux convergence (MFC), and runoff (R)), with particular focus on the Tropical and Subtropical South Atlantic Convergence Zone (TSACZ, SSACZ) and southeastern South America (SESA).
• To assess how well these reanalyses reproduce the observed spatial patterns, seasonal cycles, and multidecadal changes in precipitation relative to two complementary observation-based references (CPC Unified Gauge and MSWEP-V2).
• To determine the consistency of the associated water cycle terms and the magnitudes and regional patterns of the atmospheric moisture-budget residual as a measure of closure quality.
• To identify which reanalysis provides the most coherent depiction of the South American water cycle for hydroclimate and water-budget applications given the observed differences.

Study Configuration

• Spatial Scale: South America (88° W–35° W, 55° S–12° N), with specific regional analyses for the Tropical South Atlantic Convergence Zone (TSACZ), Subtropical South Atlantic Convergence Zone (SSACZ), and southeastern South America (SESA). All fields mapped to a common 0.5° × 0.5° latitude–longitude grid.
• Temporal Scale: 1980–2021 (42 years) for monthly means, with seasonal composites (DJF, MAM, JJA, SON).

Methodology and Data

• Models used:
  • MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, version 2): NASA GMAO, uses Goddard Earth Observing System (GEOS, version 5.12.4) atmospheric model and Gridded Statistical Interpolation System (GSI) analysis system. Horizontal resolution: approximately 0.5° × 0.625°, 72 hybrid-eta vertical levels.
  • ERA5: ECMWF, uses T639 spherical-harmonic (approximately 31 km) model, 137 vertical levels, and a 12-hourly four-dimensional Variational Analysis Ensemble. Horizontal resolution: approximately 0.3° × 0.3°.
  • CFSR/CFSv2 (Climate Forecast System Reanalysis / Climate Forecast System version 2): NCEP, uses Global Forecast System (GFS) model at T382 spectral resolution (approximately 35 km), 64 sigma-pressure hybrid vertical levels. Includes a fully coupled ocean model (GFDL MOM version 4) and Noah land surface model. Horizontal resolution: approximately 0.5° × 0.5°.
• Data sources:
  • Observation-based precipitation products (references):
    • CPC Unified Global Daily Gauge Analysis (gauge-only, 0.5° resolution).
    • Multi-Source Weighted-Ensemble Precipitation—version 2 (MSWEP-V2) (merges 76,747 gauges, satellite, and reanalysis data, 0.1° spatial, 3-hourly temporal resolution).
  • Reanalysis products: MERRA-2, ERA5, CFSR/CFSv2.
  • Derived variables: Precipitation (P), Evaporation (E), Vertically Integrated Moisture Flux Convergence (MFC), Runoff (R), and atmospheric moisture-budget residual (ε).

Main Results

• All reanalysis products reproduce the large-scale spatial and seasonal patterns of precipitation over South America, but significant disagreements persist over the Andes and parts of the central/northern Amazon, regions characterized by sparse gauge coverage and complex orography.
• Relative to the observation-based products (CPC/MSWEP-V2), MERRA-2 generally exhibits the smallest precipitation biases and the highest correlations (between 0.8 and 0.9), followed by ERA5 (correlations close to 0.8). CFSR/CFSv2 shows a consistent warm-season wet bias and the lowest correlations across the analyzed regions.
• The atmospheric moisture-budget residuals (a proxy for non-closure) are smallest in MERRA-2, moderate in ERA5, and largest in CFSR/CFSv2, with clear regional and seasonal dependencies.
• CFSR/CFSv2 produces higher precipitation over TSACZ/SSACZ, lower runoff, and a large positive residual, indicating poorer atmospheric moisture-budget closure compared to MERRA-2 and ERA5.
• The observed regional patterns of moisture transport and convergence are physically consistent with known hydroclimate mechanisms, such as the South American Low-Level Jet (SALLJ) influence on the La Plata Basin.

Contributions

• Provides a comprehensive, multi-decadal evaluation of the new generation of global reanalyses (MERRA-2, ERA5, CFSR/CFSv2) for the South American atmospheric water cycle, explicitly accounting for observational uncertainty using two complementary reference datasets.
• Offers practical guidance for hydroclimate and water-budget applications in South America, recommending MERRA-2 as the most reliable for basin-scale water-budget analyses due to its internal consistency, ERA5 as a strong alternative for higher spatial/temporal resolution, and advising caution with CFSR/CFSv2 for warm-season totals and closure-sensitive diagnostics.
• Documents improvements in the new generation of reanalyses while highlighting persistent challenges in data-sparse and complex-orography regions, emphasizing that reanalyses are not direct observations.
• Confirms the physical consistency of reanalysis-based moisture transport patterns with known hydroclimate mechanisms, such as the South American Low-Level Jet.

Funding

• Agrarian University of Ecuador (UAE) (project: "Evaluation of CMIP6 Simulations for Observational Outlook, Modeling, Future Scenarios, Climate Impact Drivers, and Extreme Indices in Ecuador.", Resolution No. 324-2024)
• Inter-American Institute for Global Change Research (IAI) Cooperative Research Network 2094 (CRN-2094)
• Coordenação de Aper-feiçoamento de Pessoal de Nível Superior pela Coordenação de Programas de Qualificação de Quadro de Docentes (CAPES) scholarship
• NASA grant NNX08AE50G (Terrestrial Hydrology Program)
• National Council for Scientific and Technological Development (CNPq, Brazil)
• Federal Institute of Santa Catarina (IFSC)
• Santa Catarina Research and Innovation Support Foundation (FAPESC)

Citation

@article{Quadro2025Atmospheric,
  author = {Quadro, Mário Francisco Leal de and Herdies, Dirceu Luís and Berbery, Ernesto Hugo and Bresciani, Caroline and Silva, Fabrício Daniel dos Santos and Gomes, Helber Barros and Muza, Michel Nobre and Suski, Cássio Aurélio and Portalanza, Diego},
  title = {The Atmospheric Water Cycle over South America as Seen in the New Generation of Global Reanalyses},
  journal = {Hydrology},
  year = {2025},
  doi = {10.3390/hydrology12120316},
  url = {https://doi.org/10.3390/hydrology12120316}
}