A New Climatology of Depth of Nitracline in the Bay of Bengal for Improving Model Simulations

The dissolved nitrate is one of the major essential nutrients for primary production in the tropical ocean and it is brought to the surface though mixing. The depth of nitracline determines how much of nitrate enters to the upper ocean through mixing. The depth of nitracline is traditionally estimated using nitrate concentrations measured at standard depths that introduces significant error due to interpolation of data. Based nitrate profiles measured at 5 m interval using nitrate sensors onboard Argo, the exact depth of nitracline was derived in the Bay of Bengal that displayed a significant linear relationship with depth of 26°C isotherm (D26). Based on climatological D26, the temporal and spatial variations in the depth of nitracline was estimated for the entire Bay of Bengal. The depth of nitracline varied between 5 and 80 m with large spatial and temporal variability in the Bay of Bengal and it is 5–20 m deeper than simulations of numerical models. The relationship between the depth of nitracline and photic zone integrated primary production indicates that 7.5 ± 3 mgC m⁻² d⁻¹ of primary production increases due to shallowing of 1 m of depth of nitracline. Therefore, models seem to be over estimating the photic zone integrated primary production by 5%–25% in the Bay of Bengal. The numerical models may improve the simulation of primary production and carbon cycling by accounting the accurate estimation of depth of nitracline in the model initialization.