Potential Mechanisms Responsible for Spatial Variability in Intensity and Thickness of Oxygen Minimum Zone in the Bay of Bengal

Abstract

Spatial variability in boundaries and thickness of oxygen minimum zone (OMZ) is derived based on measured dissolved oxygen data obtained from sensors on board biogeochemical (BGC) Argo floats between 2013 and 2019 in the Bay of Bengal (BoB). Upper and lower boundaries of the OMZ varied from 60 to 200 m and 100 to 800 m respectively with the thickness of 80–650 m in the BoB. Relatively thicker OMZ is noticed in the northern than southern BoB associated with stratification. The oxygen concentrations in the OMZ in the NW was low (<1.5 μM) than NE BoB (2.5 μM) indicating that thick and intense OMZ occurs in the NW region associating with stratification and high primary production. Significant decrease in particle‐back‐scatter signal was observed toward offshore from shelf indicating organic matter from the shelf sediments may be supporting bacterial carbon demand in the OMZ. The particle backscatter signal peaked in the OMZ region with a higher signal in the north than southern BoB and it is consistent with the low oxygen concentration in the former indicating that organic matter from shelf sediments may be supporting carbon needs in the OMZ. In addition to this, the occurrence of eddies significantly controls the intensity of the OMZ in the BoB through mixing at the upper boundary of OMZ. Therefore, this study suggests that spatial variations in intensity of OMZ in the BoB are governed by stratification, primary and export productions, organic matter decomposition, and eddy‐driven mixing.

Dissolved Oxygen Recovery in the Oxygen Minimum Zone of the Arabian Sea in Recent Decade as Observed by BGC‐Argo Floats

“…According to previous studies (Sarma et al., 2020; Udaya Bhaskar et al., 2021), we identified the boundaries of the OMZ based on the DO concentration. Specifically, in each Argo profile, the depth of the upper boundary is considered to be where the DO concentration first falls below 10 μM, and the depth where the DO concentration rises to >10 μM is considered to be the lower boundary of the OMZ.…”

Oxygen, carbon, and pH variability in the Indian Ocean

“…As discussed above, this uncertainty is particularly large in the northern Indian Ocean where global forecast models generally fail to reproduce the current oxygen concentrations and distributions (McCreary et al, 2013;Rixen et al, 2020;Schimdt et al, 2021). This failure, and the uncertainty in future projections, may also be related to the lack of consideration of the role of cross-shelf transport of organic matter in Earth System models, which is a process that is known to contribute to the development of the OMZs in both the Arabian Sea and Bay of Bengal (Sarma et al, 2020;Udaya Bhaskar et al, 2021).…”