Lubis et al. (2025) Projected changes in cross-equatorial northerly surges and their hydrological impacts in the near future

Identification

• Journal: npj Climate and Atmospheric Science
• Year: 2025
• Date: 2025-11-20
• Authors: Sandro W. Lubis, Chuan‐Chieh Chang, Samson Hagos, Ming Zhao, Ziming Chen, Karthik Balaguru, L. Ruby Leung
• DOI: 10.1038/s41612-025-01239-x

Research Groups

• Pacific Northwest National Laboratory (Richland, WA, USA)
• NOAA/Geophysical Fluid Dynamics Laboratory (Princeton, NJ, USA)

Short Summary

This study projects a significant increase in the regional hydrological impacts of cross-equatorial northerly surges (CENS) on extreme precipitation in Southeast Asia and northwestern Australia by 2030–2050, despite no change in CENS characteristics. This intensification is attributed to enhanced moistening efficiency and moist static instability in a warmer, more humid environment, leading to more intense CENS convection.

Objective

• To investigate the near-future (2030–2050) response of cross-equatorial northerly surge (CENS) events and their associated hydrological impacts, specifically on precipitation intensity and extreme rainfall frequency, under a high-emission greenhouse gas scenario (SSP5-8.5).

Study Configuration

• Spatial Scale: Western Maritime Continent, Southeast Asia (e.g., southern Indonesia, Java, Bali, Timor Leste), and northwestern Australia (e.g., Darwin, Kimberley region). Model horizontal resolutions range from approximately 20 km to 55 km.
• Temporal Scale: Near-future period (2030–2050) compared to a historical baseline (1979–1999).

Methodology and Data

• Models used: Ensemble of seven high-resolution atmospheric general circulation models (AGCMs) from CMIP6 HighResMIP (CNRM-CM6-1-HR, EC-Earth3P-HR, GFDL-CM4C192, HadGEM3-GC31-HM, HiRAM-SIT-HR, MPI-ESM1-2-XR, MRI-AGCM3-2-S) with prescribed sea surface temperatures (SSTs).
• Data sources:
  • Reanalysis: ERA5, NCEP-DOE Reanalysis II (NCEP-2), Japanese 55-year Reanalysis (JRA-55), and Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) for historical validation.
  • Observational SST/Sea Ice: HadISST2.2.0 dataset for historical forcing (1979–2014).
  • Future SST Projections: Ensemble mean of CMIP5 future climate simulations under the RCP8.5 scenario for future forcing (2015–2050).
  • Emissions Scenario: Shared Socioeconomic Pathway 5-85 (SSP5-85) for future climate simulations.

Main Results

• The regional impacts of CENS on the intensity and frequency of extreme precipitation events are projected to significantly increase in the near future (2030–2050), particularly over coastal regions of southern Indonesia and northwestern Australia.
• The risk of CENS-related extreme precipitation is projected to increase by up to 39 ± 1.2% relative to the seasonal probability.
• Despite a significant decrease in cold surge (CS) frequency, there are no statistically significant trends in the frequency, duration, or intensity of CENS events by 2050.
• The stronger hydrological impact is attributed to enhanced moistening efficiency and moist static instability due to a more humid and warmer environment, leading to more intense CENS convection.
• CENS-associated precipitation anomalies are projected to intensify by 2–4 mm day⁻¹ (8–40% relative increase) over large parts of the southern Maritime Continent and northern Australia.
• The probability of daily rainfall exceeding the 95th percentile during CENS events increases by approximately 10–39% across the southern Maritime Continent and adjacent coastal regions.
• This intensification is driven by increased vertical moisture advection (upward motion and moisture flux) in a more humid and stratified atmosphere.
• Tropical intraseasonal variability (MJO, CCEWs) contributes only partially (less than 10% of total OLR anomaly, mainly from Equatorial Rossby waves) to the projected intensification of CENS-induced convection. Equatorial Rossby wave activity also strengthens low-level northerly flows, potentially offsetting the decline in CS frequency.

Contributions

• This is the first study to investigate the near-future response of CENS events and their associated hydrological impacts under a high-emission scenario (SSP5-85) using an ensemble of high-resolution climate models.
• It reveals that CENS impacts on precipitation will intensify significantly even without changes in CENS frequency or intensity, highlighting the critical role of thermodynamic and dynamic environmental changes (enhanced moistening efficiency, moist static instability, vertical moisture advection).
• The study underscores the improved capability of HighResMIP models in simulating regional precipitation over Southeast Asia, providing more reliable future projections.
• The findings emphasize the urgent need for enhanced CENS forecasting capabilities and effective disaster management strategies in high-risk regions.

Funding

• U.S. Department of Energy Office of Science Biological and Environmental Research (Global and Regional Model Analysis program area)
• U.S. Department of Energy (Contract DE-AC05-76RL01830 to Pacific Northwest National Laboratory)
• National Energy Research Scientific Computing Center (NERSC), U.S. Department of Energy Office of Science User Facility (Contract No. DE‐AC02-05CH11231)

Citation

@article{Lubis2025Projected,
  author = {Lubis, Sandro W. and Chang, Chuan‐Chieh and Hagos, Samson and Zhao, Ming and Chen, Ziming and Balaguru, Karthik and Leung, L. Ruby},
  title = {Projected changes in cross-equatorial northerly surges and their hydrological impacts in the near future},
  journal = {npj Climate and Atmospheric Science},
  year = {2025},
  doi = {10.1038/s41612-025-01239-x},
  url = {https://doi.org/10.1038/s41612-025-01239-x}
}