Amitai et al. (2025) Projected cooling in a subtropical lake despite climate warming

Identification

• Journal: Climatic Change
• Year: 2025
• Date: 2025-12-01
• Authors: Yael Amitai, Yoav Levi, Edoardo Bucchignan
• DOI: 10.1007/s10584-025-04068-2

Research Groups

• Kinneret Limnological Institute, Israel Oceanographic and Limnological Research, Migdal, Israel
• Israel Meteorological Service, Bet Dagan, Israel
• CIRA, Italian Aerospace Research Center, Capua, Italy

Short Summary

This study uses a 3D hydrodynamic model driven by regional and global climate projections to reveal an unexpected abrupt cooling of subtropical Lake Kinneret around 2065, despite regional atmospheric warming, attributed to enhanced evaporative cooling and subsequent stratification degradation.

Objective

• To review and analyze long-term (50-year) observational records of air and water temperature.
• To identify past and ongoing changes in Lake Kinneret connected to climatic variations.
• To project and explain future changes in lake temperature and stratification using a 3D hydrodynamic model driven by regional (COSMO-CLM) and global (MIROC5) climate projections.

Study Configuration

• Spatial Scale: Lake Kinneret (average surface area ~167 km², mean depth 25.6 m, maximum depth 41.7 m). Hydrodynamic model with 400 m × 400 m horizontal resolution and 47 vertical layers (0.25 m at the uppermost 2 m, then 1 m down to 43 m depth). Atmospheric models: COSMO-CLM (~8 km resolution over Israel), MIROC5 (140 km to 200 km resolution covering the Kinneret region).
• Temporal Scale:
  • Observational records: 1970–2020 (water temperature), 1970–2017 (air temperature).
  • COSMO-CLM projections: 1980–2070 (RCP 4.5 scenario).
  • MIROC5 projections: 2006–2096 (RCP 4.5 and RCP 8.5 scenarios).
  • Numerical integration time step: 300 s.

Methodology and Data

• Models used:
  • Hydrodynamic model: MIT general ocean circulation model (MITgcm) in 3D and 1D configurations.
  • Atmospheric models: COSMO-CLM (Regional Climate Model), MIROC5 (Global Climate Model, CMIP5).
• Data sources:
  • Lake observations: Weekly depth profiles of water temperature and chloride from the Lake Kinneret Monitoring Program (1970–2020).
  • Air observations: Daily minimum and maximum air temperatures from Israel Meteorological Service stations (Zemah and Bet Zayda, 1970–2017).
  • Model forcing: Daily air temperature, relative humidity, precipitation, and wind fields from COSMO-CLM and MIROC5. Yearly climatologies of downward shortwave and longwave radiation.
  • Model parameterizations: Surface albedo based on MODIS data, NASA-GISS Aerosol Optical Depth distribution.

Main Results

• Observed Trends (1970-2017/2020): Air temperature showed a significant warming trend of approximately 0.4 °C per decade. Lake surface temperature (LST) warmed by approximately 0.3 °C per decade. Lake bottom temperature (LBT) increased at a slower, insignificant rate of 0.07 °C per decade. Stratification stability increased, with an earlier onset of stratification and a thinner epilimnion.
• Projected Climate Changes (2010-2030 to 2050-2070, RCP 4.5): Air temperature is projected to increase by approximately 1–1.5 °C. Relative humidity is projected to decrease by 0.5% (COSMO-CLM) to 2% (MIROC5).
• Unexpected Lake Cooling: Both COSMO-CLM and MIROC5 forced simulations project an abrupt cooling of the lake's water around 2065 (COSMO-CLM ~2063, MIROC5 ~2067), despite atmospheric warming. This cooling is preceded by a weakening of stratification starting around 2055.
• Mechanism: The cooling is attributed to a significant reduction in the Bowen ratio, where latent heat flux becomes predominant over sensible heat flux. Intensified evaporative cooling events lead to enhanced mixing and a warmer hypolimnion, followed by a degradation of stratification stability until an abrupt shift to high mixing potential and a colder water column. This acts as an internal negative feedback restraining the lake's warming.
• Spatial Heterogeneity: The 3D model revealed uneven distribution of latent heat loss across littoral and pelagic regions, with the pelagic zone releasing more latent heat during major loss events, influencing deep-water renewal and the timing of the abrupt cold anomaly.
• Model Robustness: The abrupt cooling phenomenon persisted in sensitivity tests with 1D simulations and different initial conditions, and under the MIROC5 RCP 8.5 scenario, indicating its robustness.

Contributions

• First study to identify and explain an unexpected abrupt cooling in a subtropical lake (Lake Kinneret) around 2065, despite projected atmospheric warming, revealing a non-linear thermal response.
• Demonstrates that enhanced evaporative cooling, leading to a shift in the air-lake heat budget and subsequent degradation of stratification, acts as a critical internal negative feedback mechanism.
• Highlights the importance of 3D hydrodynamic modeling for accurately capturing spatial heterogeneity (e.g., littoral-pelagic heat exchange) and precisely predicting the timing and vertical structure of such complex, non-linear lake responses, which 1D models cannot fully resolve.
• Provides crucial insights into the potential behavior of warm lakes in rapidly warming and drying subtropical and arid climates, suggesting this phenomenon could be indicative of unexpected outcomes globally.
• Integrates extensive long-term observational data (50 years) with high-resolution regional and global climate projections to assess both past variability and future impacts on lake thermal dynamics.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Citation

@article{Amitai2025Projected,
  author = {Amitai, Yael and Levi, Yoav and Bucchignan, Edoardo},
  title = {Projected cooling in a subtropical lake despite climate warming},
  journal = {Climatic Change},
  year = {2025},
  doi = {10.1007/s10584-025-04068-2},
  url = {https://doi.org/10.1007/s10584-025-04068-2}
}