Fazel‐Rastgar et al. (2026) Stratosphere–troposphere interactions and teleconnections associated with Iran’s winter warming in January 2024

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2026
• Date: 2026-02-13
• Authors: Farahnaz Fazel‐Rastgar, Sakineh Khansalari, S. H. Mthembu
• DOI: 10.1007/s00704-026-06080-7

Research Groups

• School of Science, University of KwaZulu-Natal, South Africa
• Research Institute of Meteorology and Atmospheric Science (RIMAS), Tehran, Iran

Short Summary

This study investigates the extreme winter warming and dryness in Iran during January 2024, linking it to a weakened Siberian High, a mid-tropospheric ridge, and a split-type Sudden Stratospheric Warming event, while also identifying a long-term warming trend of 0.057 °C per year since 1980.

Objective

• To examine the unusual warm conditions in Iran during January 2024, focusing on the weakening Siberian High and its interactions with Sudden Stratospheric Warming (SSW) events.
• To investigate the atmospheric processes that typically lead to cold spells in Iran.
• To analyze the broader influence of large-scale climate variability (Arctic Oscillation (AO), El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Siberian High Index (SHI)) on winter temperatures.
• To provide a comprehensive explanation for the observed January 2024 warm anomaly by integrating synoptic diagnostics with statistical analyses over 1980–2024.

Study Configuration

• Spatial Scale: Iran (25°–40° N, 44°–64° E), Middle East, Eurasia, Northern Hemisphere. Siberian High Index (SHI) calculated over 40°–60° N and 80°–120° E.
• Temporal Scale: 1980–2024 for long-term trends and statistical analysis; January 2024 for the case study of the warm anomaly; 1991–2020 for climatological baseline.

Methodology and Data

• Models used:
  • MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2)
  • ERA5 reanalysis (European Centre for Medium-Range Weather Forecasts)
  • NCEP–NCAR Reanalysis
  • GLDAS Noah land surface model (for MERRA-2 precipitation)
• Data sources:
  • Reanalysis datasets (MERRA-2, ERA5, NCEP-NCAR) for daily maximum/minimum 2-meter air temperature, mean sea-level pressure (MSLP), 850 hPa relative humidity, precipitation rate, 500 hPa and 10 hPa geopotential height, and 200 hPa and 10 hPa wind vectors.
  • Arctic Oscillation (AO) monthly index from NOAA Climate Prediction Center.
  • El Niño–Southern Oscillation (ENSO) (Niño 3.4 sea surface temperature anomaly) from NOAA Physical Sciences Laboratory.
  • Indian Ocean Dipole (IOD) (Dipole Mode Index) from Tokyo Climate Center, Japan Meteorological Agency.
  • Siberian High Index (SHI) derived from ERA5 mean sea-level pressure.
  • Statistical analyses: Pearson’s correlation coefficient, Spearman’s rank correlation, linear least-squares fit for detrending, standardization, performed using Python libraries (pandas, NumPy, Matplotlib).

Main Results

• Iran experienced a statistically significant long-term warming trend in January mean temperatures of approximately +0.057 °C per year, accumulating to about +2.6 °C over the 45-year period (1980–2024).
• January 2024 was identified as the warmest January on record, with surface temperature anomalies reaching up to +4 °C over northern and central Iran, accompanied by reduced mid-tropospheric humidity and suppressed precipitation, indicating widespread drought-like conditions.
• Synoptic analysis for January 2024 revealed a weakened and eastward-displaced Siberian High, negative sea-level pressure anomalies (up to -6 hPa) over northern Eurasia, a pronounced mid-tropospheric ridge over Iran (positive 500 hPa geopotential height anomalies up to +50 geopotential meters), and an intensified/eastward-displaced subtropical jet stream (up to 60 m s⁻¹ at 200 hPa).
• A major split-type Sudden Stratospheric Warming (SSW) occurred in mid-January 2024, confirmed by a reversal of 10 hPa zonal winds (from westerlies to easterlies) and a polar vortex split. This SSW promoted large-scale subsidence and reduced cold-air advection over Iran, leading to adiabatic warming and suppressed precipitation.
• The SSW event led to enhanced warmth during its early phase (14–15 January, temperatures 2–3 °C above the climatological mean), followed by cooling later in the month due to the southward movement of cold Arctic air. Nighttime minimum temperatures were more strongly influenced by the SSW than daytime maximum temperatures.
• Correlation analysis (1980–2024) showed the Indian Ocean Dipole (IOD) had the strongest statistically significant inverse relationship with Iran’s January temperature (Pearson r = -0.60, Spearman ρ = -0.69). The Arctic Oscillation (AO) and El Niño–Southern Oscillation (ENSO) showed weak negative correlations, while the Siberian High Index (SHI) showed a weak positive correlation.

Contributions

• Provides a comprehensive explanation for an extreme warm-side surface response to a Sudden Stratospheric Warming (SSW) event over West Asia (Iran), which is an underexplored component of winter climate variability.
• Demonstrates that warm-side surface responses following SSWs can amplify ongoing regional warming and produce contrasting winter anomalies across Eurasia (e.g., cold in Europe/East Asia, warm in Iran).
• Highlights the combined influence of long-term regional warming, large-scale atmospheric circulation changes (weakened Siberian High, intensified subtropical jet, mid-tropospheric ridging), and episodic stratospheric disturbances (SSW) on Iran's winter climate.
• Integrates synoptic diagnostics with statistical analyses of key climate indices to explain a specific extreme event within a long-term context.

Funding

Open access funding provided by University of KwaZulu-Natal.

Citation

@article{FazelRastgar2026Stratospheretroposphere,
  author = {Fazel‐Rastgar, Farahnaz and Khansalari, Sakineh and Mthembu, S. H.},
  title = {Stratosphere–troposphere interactions and teleconnections associated with Iran’s winter warming in January 2024},
  journal = {Theoretical and Applied Climatology},
  year = {2026},
  doi = {10.1007/s00704-026-06080-7},
  url = {https://doi.org/10.1007/s00704-026-06080-7}
}