Song et al. (2025) Mediterranean rapid warming drives abrupt runoff decline in South China around 2002

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2025
• Date: 2025-09-25
• Authors: Zichen Song, Cheng Sun, Menghao Dong, Wei Lou, Linfeng Shi
• DOI: 10.1016/j.ejrh.2025.102775

Research Groups

• State Key Laboratory of Remote Sensing and Digital Earth, Faculty of Geographical Science, Beijing Normal University, Beijing, China
• State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China

Short Summary

This study investigates a pronounced decadal abrupt change (DAC) in summer runoff over South China around 2002 and its teleconnection mechanisms. It reveals that rapid warming of Mediterranean Sea surface temperature (SST) triggered an eastward-propagating atmospheric wave train, establishing an anomalous high-pressure system over East Asia, which led to regional moisture divergence, enhanced surface drying, and ultimately an abrupt decline in South China's summer runoff.

Objective

• To investigate a pronounced Decadal Abrupt Change (DAC) in summer runoff over South China around 2002.
• To explore the teleconnection mechanisms linking this hydrological shift with concurrent Mediterranean Sea surface temperature (SST) rapid warming.
• To elucidate the spatiotemporal characteristics of runoff DAC and related variables.
• To identify the underlying mechanisms of these changes, including synergistic effects among multiple factors.
• To quantify the relative contributions of different climate factors in driving runoff DAC using a random forest regression model.
• To explain variations in South China’s runoff and related variables from the perspective of Mediterranean SST changes using numerical model experiments.

Study Configuration

• Spatial Scale: South China (core region 100°E-120°E, 20°N-30°N), East Asia, Mediterranean Sea (specifically 0°E-25°E, 35°N-45°N), Northern Hemisphere, global.
• Temporal Scale:
  • Runoff DAC around 2002.
  • Data periods: ERA5 (1940–2024), GRUN (1901–2014), CNRD (1961–2018), TerraClim (1958–2020), NOAA OISST (1979–2018), HadISST (1870–2024).
  • AMIP simulations (1979–2014).
  • SPEEDY-VEGAS simulation (1960–2018, with 19 years spin-up; ensemble means 1979–2018).
  • Comparison periods for DAC: 1993-2002 (pre-DAC) and 2003-2012 (post-DAC).

Methodology and Data

• Models used:
  • SPEEDY-VEGAS coupled model (Simplified Parameterizations, Primitive Equation Dynamics atmospheric general circulation model coupled with Vegetation-Global-Atmosphere-Soil ecosystem/land surface model).
  • Atmospheric Model Intercomparison Project (AMIP) ensemble (16 models).
  • HTESSEL (Hydrology Tiled ECMWF Scheme for Surface Exchanges over Land) land surface model (used in ERA5).
  • Variable Infiltration Capacity (VIC) hydrological model (used for CNRD).
  • Random Forest regression model.
• Data sources:
  • ERA5 monthly reanalysis dataset (runoff, meridional wind, zonal wind, vertical velocity, geopotential height, shortwave radiation, longwave radiation, moisture divergence and flux).
  • Global Runoff Reconstruction (GRUN) dataset.
  • China Natural Runoff Dataset (CNRD).
  • TerraClim monthly datasets (runoff, soil moisture, precipitation, evaporation, temperature).
  • NOAA Optimum Interpolation SST (OISST) version 2.1 (SST, Outgoing Longwave Radiation).
  • Hadley Centre Sea Surface Temperature dataset version 1.1 (HadISST).
  • ERA5-Land dataset (for soil moisture validation).
  • Observed runoff gauge station data (used for GRUN training).
  • CRU and JRA (inputs for TerraClim).

Main Results

• A significant summer runoff decreased Decadal Abrupt Change (DAC) occurred around 2002 across South China (100°E-120°E, 20°N-30°N).
  • ERA5 showed a decrease of 0.03771 m/month in summer runoff around 2002. Other datasets (TerraClim, GRUN, CNRD) confirmed this with decreases of 0.0339 m/month, 0.01475 m/month, and 0.02516 m/month, respectively.
• Concurrent DACs were observed in local hydroclimatic variables around 2002:
  • Precipitation decreased by 0.03546 m/month.
  • Evaporation increased by 0.00799 m/month.
  • Temperature increased by 0.48 K.
  • Soil moisture decreased by 0.00854 m/month.
• Random Forest analysis identified precipitation changes as the primary driver of runoff DAC, with its exclusion leading to the largest increase in RMSE (0.03331 m/month) compared to other variables.
• Thermal radiation conditions in South China also showed DACs around 2002, with increases in surface shortwave radiation (unit not explicitly stated, but typically W/m²) and outgoing longwave radiation (unit not explicitly stated, but typically W/m²), decreases in surface longwave radiation (unit not explicitly stated, but typically W/m²), and increases in moisture divergence (unit not explicitly stated, but typically kg/(m²·s)).
• An anomalous anticyclonic system was identified in the 200 hPa geopotential height field over South China around 2002, characterized by subsidence and moisture divergence.
• Mediterranean Sea surface temperature (SST) exhibited a warming DAC around 2002, with increases of 0.71 K (HadISST) and 0.77 K (OISSTv2), demonstrating temporal synchronization with South China's runoff DAC.
• The SPEEDY-Vegas coupled model successfully reproduced the observed runoff DAC and associated atmospheric circulation patterns when forced with realistic Mediterranean SST warming.
• Both model and observational results revealed the physical mechanism: Mediterranean warming triggers an eastward-propagating atmospheric Rossby wave train, establishing an anomalous high-pressure system over East Asia, inducing regional moisture divergence and enhanced surface drying, ultimately driving the reduction in regional moisture convergence and runoff decline.
• 16 AMIP models further confirmed this teleconnection pathway, showing a negative correlation between Mediterranean SST and South China precipitation (observed: -0.67, p < 0.05; best models: approximately -0.4, p < 0.05).

Contributions

• Identified and characterized a pronounced decadal abrupt decline in summer runoff over South China around 2002.
• Established a novel teleconnection pathway linking rapid Mediterranean SST warming to South China's hydroclimate via an eastward-propagating atmospheric wave train.
• Quantified the dominant role of precipitation in driving the runoff DAC using a Random Forest model.
• Provided comprehensive observational analysis, numerical model experiments (SPEEDY-Vegas), and AMIP model validations to robustly demonstrate the physical mechanism of this teleconnection.
• Highlighted the synergistic effects of local hydro-thermal system DACs and remote SST forcing on regional runoff changes.

Funding

• National Natural Science Foundation of China (42375025 and 41975082)
• Visiting Fellowship Program of the State Key Laboratory of Marine Environmental Science (Xiamen University)

Citation

@article{Song2025Mediterranean,
  author = {Song, Zichen and Sun, Cheng and Dong, Menghao and Lou, Wei and Shi, Linfeng},
  title = {Mediterranean rapid warming drives abrupt runoff decline in South China around 2002},
  journal = {Journal of Hydrology Regional Studies},
  year = {2025},
  doi = {10.1016/j.ejrh.2025.102775},
  url = {https://doi.org/10.1016/j.ejrh.2025.102775}
}