Ventilation of Oxygen to Oxygen Minimum Zone Due to Anticyclonic Eddies in the Bay of Bengal

Abstract

Intense oxygen minimum zone (OMZ) occurs in the middepth of the Eastern Tropical Pacific (ETP), Arabian Sea (AS), and Bay of Bengal (BoB). However, the occurrence of anammox/denitrification was reported only in the ETP and AS and its absence in the BoB is attributed to presence of traces of dissolved oxygen (DO). Anticyclonic eddies (ACEs) supply high-nutrient, organic-rich, and oxygen-poor waters from the coastal upwelling regions leading to strengthening of OMZ in the offshore of AS and ETP. In the absence of western boundary upwelling, we hypothesize that the ACEs supply DO-rich water leading to weakening of OMZ in the BoB. Six ACEs were sampled by bio-argo floats and measured in situ hydrography and DO. All sampled eddies were formed in the eastern BoB, close to Andaman Sea, and moved toward west. Warm and DO-rich waters were observed in the core of OMZ (150-300 m), influenced by ACEs, by 0.5-1.46°C and 3.2-6.5 μM, respectively, than no eddy region in the BoB. Based on the mean lifetime of ACE, the rate of ventilation in the 100-300 m was estimated to be 0.07 ± 0.02 μmol/L/day, which is 3 to 4 times higher than bacterial respiration rates (0.02 μmol/L/day). Concerns of OMZ intensification leading to denitrification/anammox conditions in the BoB have been expressed, but with several ACEs forming in time and space in the BoB the denitrification occurrence is unlikely.SARMA AND UDAYA BHASKAR 2145

Oxygen Variability Controls Denitrification in the Bay of Bengal Oxygen Minimum Zone

“…However, external processes regularly inject substantial (~5 to 10 μmol/kg) amounts of oxygen into this OMZ. This observation is consistent with the findings of Sarma and Bhaskar (). The high oxygen in these events inhibits denitrification.…”

“…They found higher mean oxygen levels than seen by Bristow et al (), which were generated by periodic anticyclonic eddies passing through the BoB. However, the profiling float examined by Sarma and Bhaskar () was located further south than the region sampled by Bristow et al ().…”

“…This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. profiling float examined by Sarma and Bhaskar (2018) was located further south than the region sampled by Bristow et al (2016).…”

Organic Nutrients Support High Primary Production in the Bay of Bengal

“…Higher DOP concentrations in the upper 100 m of water column were observed within ACE N and ACE S , and it was lower in the CE and NE regions (Figure 2h). It has been reported that convergence of oxygen-rich waters in ACE regions led to weakening of OMZ in the BoB (Sarma and Udaya Bhaskar, 2018;. It has been reported that convergence of oxygen-rich waters in ACE regions led to weakening of OMZ in the BoB (Sarma and Udaya Bhaskar, 2018;.…”

“…High concentrations of dissolved organic carbon (DOC, Fernandes et al, 2008;Shah, et al, 2018) and dissolved organic nitrogen (DON, Letscher, et al, 2013) are found in the BoB and attributed to supply through river discharge. Recently, it was observed that anticyclonic eddies (ACEs) transport oxygen-rich waters to oxygen minimum zone (OMZ) due to convergence leading to weakening of OMZ (Sarma and Udaya Bhaskar, 2018;. Recently, it was observed that anticyclonic eddies (ACEs) transport oxygen-rich waters to oxygen minimum zone (OMZ) due to convergence leading to weakening of OMZ (Sarma and Udaya Bhaskar, 2018;.…”