Abstract
The Bay of Bengal Large Marine Ecosystem (BOBLME; 4–23°N, 79–101°E) supports tens of millions of fisheries dependent livelihoods and is critically sensitive to Indian Ocean thermodynamics and South Asian Monsoon variability. Yet a systematic, basin scale characterization of long-term sea surface temperature (SST) trends and marine heatwave (MHW) dynamics across the BOBLME has been absent. We analyzed 43 years (1982–2024) of NOAA OISST v2.1 daily data (0.25°; n = 15,706), validated against CMEMS OSTIA SST (R = 0.900, RMSE = 0.529°C, N = 8.28 million observations) and CMEMS CORA in-situ temperature (R = 0.920, RMSE = 0.411°C), confirming climate quality suitability. SST trends were quantified using the Theil-Sen slope estimator with Mann-Kendall testing and Benjamini-Hochberg FDR correction (α = 0.05). MHWs were detected following Hobday et al. (2016) at the 90th-percentile threshold relative to a unified 1982–2011 climatological baseline. The Bay of Bengal warmed at +0.186°C decade^-1 across the study period independently confirmed via basin averaged and per grid cell approaches with statistically significant positive trends across all 4,624 valid ocean grid cells (100%) and peak warming of +0.27°C decade^-1 in the northeastern BoB near the Myanmar-Bangladesh coast. Over 686,000 MHW event-cell occurrences (n = 686,466) were recorded. MHW frequency increased at +0.11 events yr^-2 (92.5% of cells) and duration at +0.30 days yr^-1 (98.0% of cells), while peak intensity showed no statistically significant trend at any grid cell indicating that BoB MHW intensification is driven primarily by increasing frequency, duration, and cumulative heat exposure rather than by increasing peak severity per event. The year 2024 recorded the highest basin averaged frequency (7.44 events yr^-1) and cumulative intensity (11.79°C·days) in the 43-year record. ENSO teleconnections dominate inter-annual variability, though the multi-decadal warming trend appears largely independent of ENSO phasing, consistent with anthropogenic forcing. The pre-monsoon SST maximum of 29.76°C, within a basin warming at +0.186°C decade^-1, is approaching the upper thermal tolerance limits of commercially critical species including Indian mackerel, anchovies, and yellowfin tuna, while the near basin wide, statistically significant increase in MHW duration reduces recovery time for temperature sensitive fisheries. These results call for climate resilient adaptive fisheries management and spatially explicit planning frameworks across the BOBLME.

Keywords: Sea surface temperature; marine heatwaves; Bay of Bengal Large Marine Ecosystem; adaptive management; climate resilient fisheries.