Inadequate in situ observations make the northern Bay of Bengal a blind spot for models

The northern Bay of Bengal (NBoB) is one of the most underexplored and data-scarce regions of the global ocean, where strong freshwater input, monsoonal winds, and seasonal forcing interact to shape shallow upper-ocean structures. In this study, CTD profiles (collected in February and March in 2020) and gridded Argo data were combined with model output to evaluate mixed-layer depth (MLD), thermal inversion (TI), and differences between model and in situ observations across the coastal–offshore continuum. Coastal observations revealed extremely shallow mixed layers (4–12 m), strong haline stratification, and frequent wintertime temperature inversions that were often absent or muted in the model. Station-wise comparison showed a significant mismatch between CTD and model estimates, where the model generally overestimates MLD and smooths out fine-scale features such as abrupt temperature changes and multi-layer inversions. Offshore profiles highlighted strong near-surface stratification and well-developed thermocline structures that the model reproduced only below 80–100 m, and collocated Argo–model analyses showed warm, fresh, and low-density biases in the upper 50 m. These discrepancies likely reflect the combined effects of strong regional stratification, model vertical resolution, and limitations in representing fine-scale shelf processes. The results emphasize that continuous and expanded in situ observations remain important for characterizing subsurface structures and for improving evaluation of regional reanalysis products in the highly dynamic NBoB.