Seasonal variability and oceanic drivers of compound climate extremes in the coastal Ganges Delta (1971-2021)

Deltaic regions, such as the coastal Ganges Delta, are highly vulnerable to compound climate extremes—defined here as the simultaneous co-occurrence of precipitation and temperature anomalies (e.g., concurrent heavy rainfall and extreme heat) within the same month—, yet the long-term behavior and climatic drivers of such events remain underexplored. Understanding these compound risks is urgently needed to strengthen early warning systems and climate-informed risk management in this highly vulnerable region. Using 50 years (1971–2021) of 0.5° Climatic Research Unit (CRU) climate data, this study investigates the seasonal frequency, spatial extent, and ocean–atmospheric influences on four compound climate states—dry/hot, dry/cold, wet/hot, and wet/cold. Compound events are identified where precipitation and temperature exceed the 25th or 75th percentile thresholds simultaneously. Correlation and cross-wavelet coherence analyses link these events with eight large-scale climate indices. Results reveal a pronounced intensification of wet/hot extremes, increasing by +0.09 dec −1 during JJAS and + 0.03 dec −1 during MAM, now covering ~40% of the delta in monsoon months. In contrast, cold-related compound extremes are rare and declining. The Atlantic Multidecadal Oscillation (AMO) is positively associated with pre-monsoon wet/hot events ( r = 0.33, p < 0.05), while the North Atlantic Oscillation (NAO) negatively correlates with monsoon hot events ( r = −0.50, p ≤ 0.01). The El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Indian Ocean Dipole (IOD) show seasonally variable but weaker impacts.These findings highlight increasing compound hot-wet risks, underscore North Atlantic influence on regional hydrothermal extremes, and support the need for improved early warning systems in South Asia's most climate-sensitive delta.