Chrysanti et al. (2026) Large-scale drivers and ocean-land feedbacks contributing to extreme precipitation during the January 2021 South Kalimantan flood, Indonesia

Identification

• Journal: Weather and Climate Extremes
• Year: 2026
• Date: 2026-03-25
• Authors: Asrini Chrysanti, Sangyoung Son
• DOI: 10.1016/j.wace.2026.100894

Research Groups

• School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, South Korea
• Faculty of Civil and Environmental Engineering, Bandung Institute of Technology, Bandung, Indonesia

Short Summary

This study investigated the meteorological drivers and ocean-land feedbacks contributing to the January 2021 South Kalimantan flood, finding that active cold surges and cross-equatorial northerly surges, modulated by the Madden-Julian Oscillation and Kelvin waves, were primary drivers, with land-ocean feedbacks playing a secondary, amplifying role. The research utilized both standalone and coupled atmospheric-hydrological models to delineate synoptic and mesoscale interactions.

Objective

• To analyze extreme weather events from a hydro-meteorological perspective, focusing on delineating synoptic and mesoscale interactions and identifying the principal drivers of the significant flood event in South Kalimantan.
• To advance scientific understanding of flood dynamics and evaluate the potential of coupled atmospheric-hydrological models for improved forecasting of such events.

Study Configuration

• Spatial Scale: South Kalimantan and Barito Basin, Indonesia; encompassing the Indian Ocean, Java Sea, Pacific Ocean, and South China Sea. Model domains included 9 km, 3 km, and 1 km for atmospheric simulations, with a 200 m routing grid for hydrology.
• Temporal Scale: The extreme rainfall event occurred from 8 to 15 January 2021. Simulations were conducted for 8 days, from 10 to 18 January 2021, with spin-up periods ranging from 1 to 5 months for soil moisture equilibrium and 1 to 3 days for ensemble initialization. Data covered hourly, three-hourly, and daily resolutions.

Methodology and Data

• Models used:
  • Weather Research and Forecasting (WRF) model (version 4.3)
  • WRF-Hydro model (version 5.1.1)
  • Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (version 5.1)
• Data sources:
  • ERA5 reanalysis data (hourly, 0.25° spatial resolution)
  • Daily rainfall records from Syamsyudin Noor rain gauge (Indonesian Agency for Meteorology, Climatology, and Geophysics - BMKG)
  • Hourly water level measurements from Muara Teweh station (Research & Development Center for Water Resources - PUSAIR)
  • Three-hourly satellite-based rainfall estimates from Global Satellite Mapping of Precipitation (GSMaP)
  • Daily sea surface temperature anomalies (SSTA) from Group for High-Resolution Sea Surface Temperature (GHRSST) Level 4 Ultra High Resolution product (0.01° spatial resolution)
  • Outgoing Longwave Radiation (OLR) anomaly data from ERA5 for wave filtering

Main Results

• South Kalimantan experienced extreme rainfall from 8 to 15 January 2021, with a record daily intensity of 250 mm on 14 January. Two peak events were identified: 10 January (125 mm) and 14–15 January (380 mm).
• The event was primarily driven by active cold surges (CS) and cross-equatorial northerly surge (CENS) systems, which generated low-level convergence zones.
• These convergence zones were modulated by the Madden-Julian Oscillation (MJO) and convectively coupled Kelvin waves (CCKWs).
• The first rainfall peak was associated with CS, CENS, and the Borneo Vortex (BV), while the second peak was influenced by weakening BV, strengthening CS and CENS, and significant contributions from CCKWs and MJO.
• Mesoscale analysis revealed convective systems propagating from oceanic to inland regions. The first peak showed a diurnal propagation pattern, while the second peak's propagation was driven more by strong background winds associated with MJO and Kelvin waves.
• The WRF-coupled model demonstrated improved simulation of rainfall spatial distribution and intensity, particularly in propagating rainfall peaks inland, compared to the standalone WRF model.
• Positive soil moisture anomalies prior to the second peak contributed to the flooding mechanism and enhanced convective activity by facilitating the advection of high equivalent potential temperature (θe) inland.
• The dominant moisture source for the extreme rainfall originated from the South China Sea, transported by northeasterly flow associated with CENS.

Contributions

• This study represents the first assessment of floods and extreme events in Indonesia using a fully coupled atmospheric-hydrological (WRF-Hydro) modeling approach.
• It advances the scientific understanding of flood dynamics and evaluates the potential of coupled models for improved forecasting of such events in the Maritime Continent.
• The research highlights the central role of cross-equatorial northerly surges (CENS) in preconditioning the environment for extreme rainfall, which is subsequently amplified by intraseasonal disturbances like the MJO and embedded Kelvin waves.
• It provides quantitative support for the hypothesis that land-atmosphere feedbacks act as co-evolving components of the rainfall system, with soil-moisture heterogeneity modulating boundary-layer moist static energy and convergence.
• The findings underscore the necessity for further sensitivity studies to elucidate the specific roles of ocean and land surface interactions in convective initiation, organization, and intensity during extreme rainfall events.

Funding

• National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00356663)
• Research project (RS-2023-00218873) of Korea Environment Industry & Technology Institute (KEITI) through R&D Program for Innovative Flood Protection Technologies against Climate Crisis Program, funded by Korea Ministry of Environment (MOE)
• Korea Meteorological Administration Research and Development Program under Grant (RS-2025-02220421)

Citation

@article{Chrysanti2026Largescale,
  author = {Chrysanti, Asrini and Son, Sangyoung},
  title = {Large-scale drivers and ocean-land feedbacks contributing to extreme precipitation during the January 2021 South Kalimantan flood, Indonesia},
  journal = {Weather and Climate Extremes},
  year = {2026},
  doi = {10.1016/j.wace.2026.100894},
  url = {https://doi.org/10.1016/j.wace.2026.100894}
}