Reale et al. (2025) Response of Early Winter Precipitation and Storm Activity in the North Atlantic–European–Mediterranean Region to Indian Ocean SST Variability

Identification

• Journal: Geophysical Research Letters
• Year: 2025
• Date: 2025-10-16
• Authors: Marco Reale, Alessandro Raganato, Fabio D’Andrea, Muhammad Adnan Abid, Assaf Hochman, Nazimur Rashid Chowdhury, Stefano Salon, Fred Kucharski
• DOI: 10.1029/2025gl116732

Research Groups

• National Institute of Oceanography and Applied Geophysics—OGS, Trieste, Italy
• School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
• Laboratoire de Météorologie Dynamique, IPSL, ENS, PSL Research University, École Polytechnique, Institute Polytechnique de Paris, Sorbonne Université, CNRS, Paris, France
• Department of Physics, Atmospheric, Oceanic and Planetary Physics (AOPP), University of Oxford, Oxford, UK
• National Centre for Atmospheric Science (NCAS), Oxford, UK
• Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem (HUJI), Jerusalem, Israel
• Department of Mathematics, Informatics and Geosciences, University of Trieste, Trieste, Italy
• Earth System Physics, The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy

Short Summary

This study investigates the influence of Indian Ocean Dipole (IOD) variability on early winter precipitation and storm activity in the North Atlantic–European–Mediterranean (NAEM) region, revealing a significant December-specific teleconnection characterized by a positive NAO-like pattern, altered precipitation, and reduced cyclone activity driven by changes in baroclinicity.

Objective

• To investigate the long-term response of early winter precipitation and storm activity in the North Atlantic–European–Mediterranean (NAEM) region to Indian Ocean Dipole (IOD) variability, identifying the underlying physical mechanisms, including the role of environmental baroclinicity, and isolating IOD effects from ENSO.

Study Configuration

• Spatial Scale: North Atlantic–European–Mediterranean (NAEM) region, specifically 40°W to 55°E and 20°N to 70°N. Cyclone activity quantified in 2° grid cells.
• Temporal Scale: January 1979 to February 2025 (boreal winter: December–February). Analysis of hourly and monthly fields.

Methodology and Data

• Models used:
  • Objective cyclone identification and tracking procedure (Lionello et al., 2002; Reale and Lionello, 2013).
  • Eady Growth Rate (EGR) as a diagnostic of environmental baroclinicity (Hoskins and Valdes, 1990).
  • Linear regression approach to quantify the impact of the Indian Ocean Dipole (IOD), with ENSO influences removed.
• Data sources:
  • ERA5 reanalysis (Hersbach et al., 2020, 2023) at 0.25° resolution for hourly precipitation, monthly Mean Sea Level Pressure (MSLP), surface air and sea surface temperature, zonal and meridional wind speed, and potential temperature at 500 and 850 hectopascals.
  • Climdex project indices for precipitation extremes: monthly total precipitation on wet days (prectot), number of wet days per month (nwet), Simple Daily Intensity Index (SDII), number of heavy precipitation days per month (R10mm), and Consecutive Wet Day index (CWD).
  • Dipole Mode Index (DMI) for IOD (Saji et al., 1999).
  • NINO-3.4 Sea Surface Temperature (SST) index for ENSO (Capotondi et al., 2015; Kug et al., 2009).

Main Results

• A statistically significant positive North Atlantic Oscillation (NAO)-like pattern is observed over the NAEM region in response to IOD variability, strongest in December and shifting eastward through February.
• IOD modulates precipitation, with increased total precipitation, event frequency, and wet spell duration between 50°N–60°N and 40°W–0°W in December, and opposite signals over the Atlantic (30°N–40°N) and Western-Central Mediterranean.
• A significant reduction in cyclone activity is observed in December over the East Atlantic and Western Mediterranean (between 30°N and 45°N), while a modest, non-significant increase occurs between Greenland and Iceland.
• Changes in cyclone track density are primarily driven by variations in the Eady Growth Rate (EGR), which is linked to enhanced vertical wind shear associated with a strengthened meridional temperature gradient over the NAEM.
• The observed precipitation increase in some areas is associated with a significantly increased moisture convergence flux, even in the absence of significant changes in cyclone track density.

Contributions

• Provides a comprehensive, long-term analysis (1979-2025) of the Indian Ocean Dipole's influence on early winter hydro-climate and storm activity in the North Atlantic–European–Mediterranean region, explicitly disentangling its effects from ENSO.
• Highlights the critical role of December as the month with the most significant IOD teleconnection, characterized by a robust NAO-like pattern and specific impacts on precipitation characteristics and cyclone activity.
• Identifies the Eady Growth Rate, vertical wind shear, and meridional temperature gradient as key physical drivers explaining the observed changes in storm activity.
• Underscores the importance of understanding tropical-extratropical teleconnections for improving sub-seasonal to seasonal prediction capabilities in the NAEM region, offering valuable insights for enhancing preparedness for weather-related risks.

Funding

• OGS and CINECA under the High‐Performance Computing Training and Research for Earth Sciences (HPC‐TRES) program founded by PRACE‐Italy national research infrastructure.
• National Recovery and Resilience Plan project TeRABIT (Terabit network for Research and Academic Big data in Italy—IR0000022—PNRR Missione 4, Componente 2, Investimento 3.1 CUP I53C21000370006) in the frame of the European Union—NextGenerationEU funding (M. Reale).
• CINECA award under the ISCRA initiative.
• Supercomputing resources and support from ICSC—Centro Nazionale di Ricerca in High Performance Computing, Big Data and Quantum Computing—Spoke 4 Earth and Climate and hosting entity, funded by European Union—NextGenerationEU.
• U.K. Research and Innovation (UKRI) under the U.K. government's Horizon Europe project (ASPECT) Grant [101081460] (M. Adnan Abid).
• Israel Science Foundation (Grant 978/23) (A.H.).
• Ministry of Science, Innovation, and Technology of Israel (Grant 4749) (A.H.).
• Federal Ministry of Education and Research (BMBF), Germany, and the Ministry of Innovation Science and Technology of Israel within the GRaCCE project (A.H.).
• Planning and Budgeting Committee of the Israeli Council for Higher Education under the “MedWORLD” Consortium (A.H.).

Citation

@article{Reale2025Response,
  author = {Reale, Marco and Raganato, Alessandro and D’Andrea, Fabio and Abid, Muhammad Adnan and Hochman, Assaf and Chowdhury, Nazimur Rashid and Salon, Stefano and Kucharski, Fred},
  title = {Response of Early Winter Precipitation and Storm Activity in the North Atlantic–European–Mediterranean Region to Indian Ocean SST Variability},
  journal = {Geophysical Research Letters},
  year = {2025},
  doi = {10.1029/2025gl116732},
  url = {https://doi.org/10.1029/2025gl116732}
}