Aftab et al. (2026) The Standardized Soil Moisture-Temperature Compound Index (SSTCI): daily scale global data set and event catalogue for compound dry-hot extremes (1961–2023)

Identification

• Journal: Earth system science data
• Year: 2026
• Date: 2026-11-03
• Authors: Rukhshinda Aftab, Daniel Fiifi Tawia Hagan, Guojie Wang, Baoying Shan, Syed Husnain Shah, Zhou Chensi, Xikun Wei, Fareeha Siddique, Ali Hasan Jaffry, Emmanuel Yeboah
• DOI: 10.5194/essd-18-7981-2026

Research Groups

• School of Remote Sensing and Geomatics Engineering, Nanjing University of Information Science and Technology, Nanjing, Jiangsu, China
• Hydro-Climate Extremes Lab, Ghent University, Ghent, Belgium
• Department of Civil and Environmental Engineering, Politecnico di Milano, Milano, Italy
• Key Laboratory for Climate Risk and Urban-Rural Smart Governance, School of Geography, Jiangsu Second Normal University, Nanjing, China
• Faculty of Geographical Sciences and Engineering, Henan University, Kaifeng, Henan, China

Short Summary

This study provides a daily global compound dry–hot index (SSTCI) and a ready-to-use CDHE (Compound Dry-Hot Extreme) catalogue for 1961–2023 at 0.1° resolution. Built on soil moisture memory and surface air temperature, the findings indicate that CDHEs have become more frequent and longer lasting in many regions in recent decades.

Objective

• To develop and provide a global, daily-scale Standardized Soil Moisture–Temperature Compound Index (SSTCI) and a corresponding event catalogue for compound dry-hot extremes (CDHEs) from 1961 to 2023, explicitly incorporating grid-specific soil moisture memory.

Study Configuration

• Spatial Scale: Global land (excluding Antarctica) at 0.1° resolution.
• Temporal Scale: Daily resolution, covering the period 1961–2023.

Methodology and Data

• Models used:
  • Standardized Soil Moisture–Temperature Compound Index (SSTCI)
  • Standardized Antecedent Soil Moisture Index (SASMI)
  • Standardized Temperature Index (STI)
  • Frank copula (for integrating SASMI and STI)
  • Objective event-refinement procedure (Shan et al., 2024) for CDHE catalogue generation.
• Data sources:
  • ERA5-Land reanalysis: Volumetric soil water content in the uppermost soil layer (0–0.07 m; swvl1) at 0.1° resolution.
  • ERA5 reanalysis: Hourly maximum 2 m air temperature (mx2t) at 0.25° resolution (bilinearly interpolated to 0.1°).
  • Evaluation datasets: Daily, gap-free Normalized Difference Vegetation Index (NDVI) product (Li et al., 2024), daily Standardized Precipitation–Evapotranspiration Index (SPEI; Liu et al., 2024), daily Evapotranspiration Deficit Index (DEDI; Zhang et al., 2023b), and a global reproduction of the Standardized Compound Dry-hot Index (SCDHI; Zhou et al., 2025).

Main Results

• The Standardized Antecedent Soil Moisture Index (SASMI) shows broad agreement with established daily drought metrics (DEDI and SPEI) in terms of Spearman rank correlation and extreme-day agreement across most global land areas.
• The Standardized Soil Moisture–Temperature Compound Index (SSTCI) exhibits strong temporal co-variability and consistent extreme-day agreement with a global reproduction of the Standardized Compound Dry-hot Index (SCDHI).
• Event-scale analyses demonstrate that SSTCI anomalies are physically consistent with independent vegetation stress (NDVI reductions) during documented CDHEs (e.g., the 2006 Sichuan–Chongqing event).
• SSTCI reliably captures the timing and evolution of well-documented CDHEs across multiple regions and timescales, including the 2010 Russian Mega event and the 2022 Yangtze River Basin event, resolving early soil-moisture preconditioning and subsequent heat amplification.
• The Global CDHE Event Catalogue reveals a widespread increase in CDHE occurrence frequency across hotspot regions (e.g., Southern Great Plains, Yangtze River Basin, Northeast Australia, South Africa) from the historical baseline (1961–1999) to the recent warming period (2000–2023), with annualized rates increasing by factors up to 3.6.
• A clear shift towards longer-lasting CDHEs is observed across large parts of the global land surface since 2000, particularly in northern Eurasia, high-latitude North America, and tropical South America.
• The catalogue successfully reconstructs the spatiotemporal footprint of large-scale events, such as the 2022 Northern Hemisphere summer CDHEs, revealing hemispheric-scale synchronization patterns.

Contributions

• Development of the Standardized Soil Moisture–Temperature Compound Index (SSTCI), a novel daily-scale global index for compound dry-hot extremes that explicitly incorporates grid-specific soil moisture memory.
• Creation of the first global, daily-scale, event-based catalogue of compound dry-hot extremes (CDHEs) for 1961–2023, providing discrete, statistically independent events with explicit timing and magnitude attributes.
• Addressing limitations of previous compound indices by using direct soil moisture (rather than precipitation proxies) and accounting for antecedent soil moisture memory and regional heterogeneity of dry-down rates.
• Providing openly accessible datasets and source code, facilitating direct application for risk and impact assessment, process attribution, and early warning for compound dry-hot hazards.
• Demonstrating the utility of the index and catalogue for diagnosing event mechanisms, tracking daily evolution, and analyzing long-term changes in CDHE frequency and duration globally.

Funding

• National Key Research and Development Program of China (grant no. 2026YFE0100700)
• National Natural Science Foundation of China (Regular Project: Causal Inference of Vegetation–Climate Interactions, Project no. 42275028)
• Research Foundation Flanders under the FWO–NSFC bilateral research project funding (CausalHeat, G0AOO25N)

Citation

@article{Aftab2026Standardized,
  author = {Aftab, Rukhshinda and Hagan, Daniel Fiifi Tawia and Wang, Guojie and Shan, Baoying and Shah, Syed Husnain and Chensi, Zhou and Wei, Xikun and Siddique, Fareeha and Jaffry, Ali Hasan and Yeboah, Emmanuel},
  title = {The Standardized Soil Moisture-Temperature Compound Index (SSTCI): daily scale global data set and event catalogue for compound dry-hot extremes (1961–2023)},
  journal = {Earth system science data},
  year = {2026},
  doi = {10.5194/essd-18-7981-2026},
  url = {https://doi.org/10.5194/essd-18-7981-2026}
}