The Northern Bay of Bengal (NBoB) is one of the most productive yet dynamically complex marginal seas in the Indo-Pacific region, where intense monsoonal forcing, massive freshwater input from the Ganges-Brahmaputra-Meghna (GBM) river system, and strong bathymetric gradients collectively generate sharp oceanic fronts that regulate biological productivity and ecosystem structure. Despite this importance, a systematic long-term analysis of both physical and biological frontal variability and their driving mechanisms in the NBoB has been absent. Here we present the first 22-year (2003 to 2024) satellite-based climatology of sea surface temperature (SST) and chlorophyll-a (CHL-a) frontal probability (FP) across the NBoB (15 to 22°N, 80 to 98°E), derived from MODIS-Aqua Level-3 daily observations using the Canny Edge Detection algorithm. CHL-a and SST FP exhibit strikingly opposite seasonal cycles: CHL-a FP peaks in May (3.97%) and during SON (3.70%), while SST FP is highest in DJF (January: 2.67%) and reaches a minimum in JJA (July: 0.43%), governed by the monsoon wind reversal and the seasonal collapse of temperature gradients across the continental shelf. Frontal activity is strongly constrained by water depth (Spearman ρ = -0.559 for CHL-a and -0.767 for SST; both p < 0.001), with shelf FP up to 4 to 5 times higher than the deep basin, and the sharpest gradient at the 200 m isobath. CHL-a and SST fronts show the strongest spatial coincidence during SON and DJF, reflecting coupled physical and biological responses to the productivity rebound following the monsoon retreat and winter thermal intensification. Mann-Kendall trend analysis reveals a statistically significant 22-year increase in CHL-a FP (τ = 0.385, p = 0.013; Sen slope = +0.0146% per year), while SST FP shows no significant trend (p = 0.367), indicating that biological frontal activity is gradually intensifying in a manner decoupled from the thermal front structure. Lagged cross-correlation analysis identifies ENSO as a significant driver of both CHL-a FP (ρ = +0.160 at +4 months) and SST FP (ρ = +0.287 at +7 months), while the Indian Ocean Dipole exerts its strongest control on SST FP (ρ = +0.329 at +11 months) with negligible effect on CHL-a FP north of 15°N. GBM river discharge correlates significantly with CHL-a FP only during JJA (ρ = +0.43, p = 0.048), while wind stress governs the seasonal FP cycle but not interannual variability. These results offer the first systematic multi-driver attribution of frontal variability in the NBoB and provide new quantitative insight into how ocean productivity boundaries in this climatically sensitive region respond to monsoon dynamics, riverine forcing, and basin-scale climate modes.
 

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