Potential mechanisms responsible for spatial variability in intensity and thickness of oxygen minimum zone in the Bay of Bengal

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 boundary of the OMZ varied from 60 to 200 m and 100 to 800 m respectively with the thickness of 80 to 650m in the BoB. Relatively thicker OMZ is noticed in the northern than southern BoB associated with stratification. The salinity difference between surface and 100 m is higher in the northeastern (NE; >2.5) followed by northwestern (NW; 1.8-2.5) and lower in the southern BoB (0.6 - 1.2) indicating weaker stratification in the latter region associated with weaker OMZ. The oxygen concentrations in the OMZ in the NW was low (<1.5 mM) than NE BoB (2.5 mM) 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 towards offshore from shelf indicating organic matter from the shelf sediments may be supporting bacterial carbon demand in the OMZ. The particle back-scatter signal peaked in the OMZ region with 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 control the intensity of the OMZ in the BoB, as anticyclonic eddies ventilate oxygen, therefore weakens OMZ, whereas cyclonic eddies intensify OMZ through increase in primary production and upwelling of oxygen-poor waters. Therefore, this study suggests that spatial variations in intensity of OMZ in the BoB is governed by stratification, primary production, sinking carbon fluxes and organic matter decomposition within the OMZ and modification of oxygen concentration in the OMZ by eddies.

孟加拉湾（Bay of Bengal, BoB）氧最小层（oxygen minimum zone, OMZ）的边界与厚度空间变异性，基于2013-2019年间搭载于生物地球化学（biogeochemical, BGC）Argo浮标上的传感器所获取的溶解氧实测数据推导得出。孟加拉湾OMZ的上边界和下边界分别介于60-200米与100-800米之间，厚度为80-650米。与分层（stratification）相关，孟加拉湾北部的OMZ相对南部更厚。表层与100米深度之间的盐度差在东北部（NE；>2.5）最高，其次是西北部（NW；1.8-2.5），孟加拉湾南部最低（0.6-1.2），表明后者区域分层较弱，且OMZ也较弱。西北部OMZ的氧浓度较低（<1.5 mM），而东北部为2.5 mM，表明厚且强的OMZ出现在西北部区域，与分层和高初级生产力（primary production）相关。从陆架向远海方向观测到颗粒后向散射信号（particle-back scatter signal）显著降低，表明陆架沉积物中的有机质可能为OMZ中的细菌碳需求提供支持。颗粒后向散射信号在OMZ区域达到峰值，且孟加拉湾北部的信号高于南部，这与前者的低氧浓度一致，表明陆架沉积物中的有机质可能为OMZ中的碳需求提供支持。此外，涡旋（eddies）的出现显著控制孟加拉湾OMZ的强度：反气旋涡旋（anticyclonic eddies）促进氧气交换，从而削弱OMZ；而气旋涡旋（cyclonic eddies）通过增加初级生产力和贫氧水上升流来增强OMZ。因此，本研究表明，孟加拉湾OMZ强度的空间变化由分层、初级生产力、OMZ内的下沉碳通量（sinking carbon fluxes）与有机质分解（organic matter decomposition），以及涡旋对OMZ中氧浓度的调控共同主导。