Çelik et al. (2026) North Atlantic Oscillation modulation of drought and vegetation activity in Türkiye

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2026
• Date: 2026-01-10
• Authors: Mehmet Ali Çelik, Adem Kıraç, Célia M. Gouveia
• DOI: 10.1007/s00704-025-05971-5

Research Groups

• Geography Department, Faculty of Arts and Sciences, Igdir University, Igdir, Türkiye: Mehmet Ali Çelik, Adem Kıraç
• Nakhchivan State University, Department of Geography, Nakhchivan, Azerbaijan: Mehmet Ali Çelik
• Instituto Português do Mar e da Atmosfera, I.P, Lisboa, Portugal: Célia M. Gouveia
• Faculdade de Ciências, Instituto Dom Luiz, Universidade de Lisboa, Lisboa, Portugal: Célia M. Gouveia

Short Summary

This study investigates the relationships between the North Atlantic Oscillation (NAO), drought, and vegetation activity in Türkiye, revealing a statistically significant positive NAO trend over the last 40 years linked to increased drought, with vegetation responses varying significantly between natural ecosystems and irrigated agricultural areas.

Objective

• To analyze climate variability, as represented by the North Atlantic Oscillation (NAO), and drought variability, using standardized precipitation (SP).
• To evaluate the link between NAO and drought events on vegetation dynamics, as assessed by the Normalised Difference Vegetation Index (NDVI), to understand the role of large-scale teleconnections across different regions of Türkiye.

Study Configuration

• Spatial Scale: Türkiye, divided into seven precipitation regime regions: Black Sea (BLS), Northwest Turkey (NWTR), Southern Aegean and Western Mediterranean (SAEG-WMED), Mediterranean (MED), West Continental Central Anatolia (WCCAN), East Continental Central Anatolia (ECCAN), and Continental Eastern and Southeastern Anatolia (CEAN-CSEAN).
• Temporal Scale: 41 years, from 1982 to 2022.

Methodology and Data

• Models used: Standardised Precipitation (SP) derived from ERA5 precipitation data. Statistical methods included Pearson correlation, linear trend analysis (Mann-Kendall test), Principal Component Analysis (PCA), and Hierarchical Cluster Analysis.
• Data sources:
  • North Atlantic Oscillation (NAO) monthly index: NOAA Climate Prediction Center (CPC), based on Rotated Principal Component Analysis (RPCA) of 500-hPa geopotential height anomalies.
  • Precipitation data: ERA5 reanalysis, with a spatial resolution of 0.25° x 0.25°.
  • Vegetation data: Global Inventory Modeling and Mapping Studies (GIMMS) Normalised Difference Vegetation Index (NDVI) dataset from NOAA AVHRR satellite, with a spatial resolution of 8 km.

Main Results

• A statistically significant positive NAO trend was observed over the last 40 years, particularly in winter and early spring (January, March, April, November), with Z-scores exceeding +1.96 (p-values < 0.05).
• Positive NAO phases are associated with decreased winter precipitation and increased drought frequency in western, northern, and central Türkiye.
• Conversely, negative NAO phases often coincide with notably wetter conditions across all precipitation regions, with increased frequency in June and August.
• Long-term precipitation trends (SP) were generally weak and statistically insignificant across most months and regions.
• Vegetation activity (NDVI) showed significant regional and seasonal trends:
  • Significant declines in NDVI from 1982 to 2022 were observed in western (NWTR, SAEG) and northern (BLS) regions, particularly in natural vegetation and forest zones, attributed to prolonged water stress from recurrent positive NAO phases.
  • Statistically significant increases in NDVI were found in irrigated agricultural zones and pasturelands of central, southeastern, and eastern Türkiye (e.g., CEAN, ECCAN), where large-scale irrigation projects (e.g., GAP, KOP) mitigated drought effects.
• PCA and Cluster analyses identified three clusters of regions based on NDVI trends, demonstrating distinct responses related to land use, with forested areas generally experiencing vegetation decline and irrigated farmland showing steady greening due to human intervention.

Contributions

• This study is the first to comprehensively examine the interconnected relationships between NAO, precipitation, and vegetation dynamics at a national level across Türkiye, addressing a significant gap in existing literature that often focused on localized or single-factor analyses.
• It provides a spatially and temporally heterogeneous assessment of NAO's influence on precipitation and vegetation across Türkiye's seven distinct precipitation zones, offering a more nuanced understanding of regional climate-vegetation responses.
• The research highlights the contrasting resilience of natural ecosystems (vulnerable to NAO-driven drought) versus human-modified agricultural landscapes (resilient due to irrigation), underscoring the critical need to consider both climatic drivers and anthropogenic factors in future land-climate studies and drought management planning.

Funding

This research received no specific grant from any funding agency.

Citation

@article{Çelik2026North,
  author = {Çelik, Mehmet Ali and Kıraç, Adem and Gouveia, Célia M.},
  title = {North Atlantic Oscillation modulation of drought and vegetation activity in Türkiye},
  journal = {Theoretical and Applied Climatology},
  year = {2026},
  doi = {10.1007/s00704-025-05971-5},
  url = {https://doi.org/10.1007/s00704-025-05971-5}
}