Injection of Oxygenated Persian Gulf Water Into the Southern Bay of Bengal

Peter M F Sheehan,Benjamin G M Webber,Adrian J Matthews,Alejandra Sanchez‐Franks,P N Vinayachandran,Karen J Heywood

doi:10.1029/2020gl087773

Abstract

AbstractPersian Gulf Water (PGW) is an oxygenated, high‐salinity water mass that has recently been detected in the Bay of Bengal (BoB). However, little is known about the transport pathways of PGW into the BoB. Ocean glider observations presented here demonstrate the presence of PGW in the southwestern BoB. Output from an ocean reanalysis product shows that this PGW signal is associated with a northward‐flowing filament of high‐salinity water. Particle tracking experiments reveal two pathways: one in the eastern Arabian Sea that takes a minimum of 2 years and another in the western Arabian Sea that takes a minimum of 3 years. The western pathway connects to the BoB via equatorial currents. The greatest influx of PGW occurs between 82° and 87°E during the southwest monsoon. We propose that injection of PGW to the BoB oxygen minimum zone (OMZ) contributes to keeping oxygen concentrations in the BoB above the level at which denitrification occurs.

Highlights

Persian Gulf Water (PGW) is identifiable in the Arabian Sea as a salinity maximum with a density of between 26.2 and 26.8 g kg−1 (Jain et al, 2017; Schott & McCreary, 2001) that is distinct from the other high‐salinity water masses of the northern Indian Ocean, that is, Arabian Sea High‐Salinity Water (22.8–24 kg m−3) and Red Sea Water (27–27.4 kg m−3; Jain et al, 2017; Prasanna Kumar & Prasad, 1999)
The flow occurs between the 26.2 and 26.8 kg m−3 isopycnals and exhibits the elevated salinity and oxygen concentrations that are characteristic of PGW (Bower et al, 2000; McCreary et al, 2013; Prasad et al, 2001; Queste et al, 2018)
Other water masses between these isopycnals lack the elevated oxygen concentrations of PGW (You, 1997), suggesting that PGW is likely the only means by which the Bay of Bengal (BoB) oxygen minimum zone (OMZ) may be ventilated between these isopycnals

Summary

Introduction

Persian Gulf Water (PGW) is an oxygenated, high‐salinity water mass that forms in the shallow waters of the Persian Gulf (Figure 1a; Bower et al, 2000; Prasad et al, 2001), its high salinity a consequence of the very high evaporation in that region (Prasad et al, 2001; Yao & Johns, 2010). Sridevi and Sarma (2020) report that cyclonic and anticyclonic eddies can increase and decrease subsurface oxygen concentrations, respectively Taken together, these results point to a complex relationship between physics and biogeochemistry in the BoB, with oxygen concentrations depending on multiple processes that often operate over small spatial scales. Most recently, Jain et al (2017) identified PGW in a dozen temperature, salinity, and oxygen profiles from across the BoB; they propose that the Southwest Monsoon Current (SMC; Figure 1; Vinayachandran et al, 1999; Webber et al, 2018), a relatively strong northeastward flow that occurs during the southwest monsoon (June to September), is the primary conduit for PGW entering the BoB, but pathways from the Persian Gulf to the BoB, and the timescales of transport have not been identified. We present a case study of PGW injection into the BoB, from which we determine transport pathways and timescales from the northern Arabian Sea to the BoB.

Glider Observations of PGW in the BoB

Pathways and Timescales of PGW Transport to the BoB

Influence of PGW on the BoB OMZ

Findings

Conclusions