Farukh et al. (2025) Climatological assessment of pre-monsoon heatwave days in Bangladesh and their relationship to Indo Pacific circulation anomalies

Identification

• Journal: Natural Hazards
• Year: 2025
• Date: 2025-12-26
• Authors: M. A. Farukh, Partha Pratim Brahma, Shakhawat Hossain, Md. Jahidul Hoque, Saida Islam Sejuti, Ummoy Sumaia Shammy, Khondakar Sumsul Arefin
• DOI: 10.1007/s11069-025-07746-7

Research Groups

• Department of Environmental Science, Bangladesh Agricultural University, Mymensingh, Bangladesh
• Department of Agrometeorology, Bangladesh Agricultural University, Mymensingh, Bangladesh

Short Summary

This study assesses the climatological trends and drivers of pre-monsoon heatwave days (HWDs) in Bangladesh from 1990 to 2024, revealing a dramatic 12-fold increase in HWDs and a breakdown of climate stationarity. It identifies a "thermal dome" regime, characterized by synoptic-scale subsidence and thermal ridging, as the primary atmospheric mechanism driving the intensification and spatial homogenization of heatwaves across the country.

Objective

• To investigate the temporal trends, spatial distribution, and large-scale Indo-Pacific circulation patterns driving pre-monsoon heatwaves in Bangladesh, focusing on the underlying mechanisms and potential drivers of extreme heat events.

Study Configuration

• Spatial Scale: Bangladesh (eight divisional headquarters) and a broader synoptic domain spanning the Equator to 50°N and 60°E to 120°E (covering South and Southeast Asia).
• Temporal Scale:
  • Daily maximum air temperature (Tmax): 1990–2024.
  • Synoptic climatology data: 1990–2020 (climatological baseline).
  • Radiosonde parameters: March–June, 2010–2024.

Methodology and Data

• Models used:
  • Principal Component Analysis (PCA) (S-mode configuration)
  • k-means clustering (non-iterative)
  • NCEP/NCAR Reanalysis 1 (data assimilation and forecast system, used for synoptic climatology)
• Data sources:
  • Daily maximum air temperature (Tmax): Climate Division of the Bangladesh Meteorological Department (BMD) for eight divisional headquarters.
  • Synoptic climatology data (sea-level pressure, 500 hPa geopotential height, air temperature at multiple levels, relative humidity at multiple levels, vector wind at multiple levels, columnar precipitable water, precipitation rate, lifted index, 100 mb omega): NCEP/NCAR Reanalysis 1 project.
  • Radiosonde parameters (Convective Available Potential Energy, Lifted Index, K Index, Convective Inhibition, Showalter Index, Precipitable Water, Severe Weather Threat Index, Lifted Condensation Level Temperature, Lifted Condensation Level Pressure): Dhaka radiosonde station (VGTJ; station ID 41923) via the Department of Atmospheric Science, University of Wyoming web portal.

Main Results

• Heatwave days (HWDs) in Bangladesh increased over 12-fold between 1994 and 2024, with recent maxima exceeding 60 HWDs per year in regions like Rajshahi, Khulna, and Dhaka.
• Five 'mega-heat' years (≥ 200 national HWDs) occurred between 2014 and 2024, reducing the return period from approximately 30 years to less than 2 years, signifying a breakdown in climate stationarity.
• Historically concentrated in the northwest, extreme heat events have spatially homogenized post-2014, with all divisions, including coastal and temperate zones, experiencing more than 30 HWDs in 2024.
• Principal Component Analysis and k-means clustering identified four dominant circulation patterns, with Cluster 1 consistently contributing the highest number of HWDs (1232 days, 25.4% of total).
• Cluster 1 conditions are characterized by vertically stacked positive temperature anomalies (up to +3 °C at 2 m, +2 °C at 850 hPa, +1.5 °C at 700 hPa, and +2 °C at 500 hPa) over the Meghna Basin.
• Cluster 1 also exhibits deep, vertically extensive negative relative humidity anomalies (−9% to −15% at 925 hPa) and negative columnar precipitable water anomalies, indicating extreme atmospheric dryness.
• Synoptic analysis for Cluster 1 reveals strong anomalous northwesterly winds at 925 hPa, a broad anticyclonic anomaly (thermal dome) at 500 hPa, and anomalous subsidence (positive omega anomaly) in the mid-troposphere, which collectively suppress convection and trap heat.
• Thermodynamic parameters show increasing Convective Available Potential Energy (CAPE) and negative Lifted Index during peak HW months (April-May), but strong Convective Inhibition (CIN) and high Showalter Index in March-April prevent convective development, favoring dry heatwaves.

Contributions

• Provides a comprehensive climatological assessment of pre-monsoon heatwave days in Bangladesh, including temporal trends and spatial distribution, over a 35-year period (1990-2024).
• Identifies and characterizes the large-scale Indo-Pacific circulation patterns (using PCA and k-means clustering) that drive heatwave development in Bangladesh, particularly highlighting the "thermal dome" regime.
• Demonstrates a significant breakdown in climate stationarity for heatwave events, with a dramatic increase in frequency and severity, and a reduction in return periods for 'mega-heat' years.
• Reveals the spatial homogenization of heatwave risk across Bangladesh, including historically cooler coastal and northern regions, indicating a loss of traditional thermal buffers.
• Integrates high-resolution climate diagnostics, thermodynamic indicators (e.g., CIN, SWEAT, LCL T), and socio-ecological vulnerability, offering a systems perspective on Bangladesh's heat crisis.
• Proposes a paradigm shift in national heat risk management, emphasizing anticipatory governance, machine learning-based forecasting, and dynamic land-atmosphere modeling for future heat resilience.

Funding

• Bangladesh Agricultural University Research System (BAURES), BAU (Project ID No: 2024/2195/BAU)

Citation

@article{Farukh2025Climatological,
  author = {Farukh, M. A. and Brahma, Partha Pratim and Hossain, Shakhawat and Hoque, Md. Jahidul and Sejuti, Saida Islam and Shammy, Ummoy Sumaia and Arefin, Khondakar Sumsul},
  title = {Climatological assessment of pre-monsoon heatwave days in Bangladesh and their relationship to Indo Pacific circulation anomalies},
  journal = {Natural Hazards},
  year = {2025},
  doi = {10.1007/s11069-025-07746-7},
  url = {https://doi.org/10.1007/s11069-025-07746-7}
}