Effects of mesoscale eddies on suboxia and hypoxia in the Arabian Sea

Hosting perennial and pervasive suboxic conditions, the World’s thickest oxygen minimum zone (OMZ) in the Arabian Sea (AS) is a major hotspot of oceanic denitrification that plays a critical role in the global marine nitrogen cycle. Although mesoscale eddies have been suggested to play an important role in the O₂ budget of the AS, their effects on the volume and the distribution of suboxic waters have not been studied. Here we explore the effects of mesoscale eddies on suboxia in the AS, and elucidate the implications this has for denitrification, primary production (PP) and the habitat size of O₂-sensitive species. To this end, we conducted a series of four coupled physical-biogeochemical simulations of the AS at horizontal resolutions ranging from 1/3^o to 1/24^o using the ROMS model coupled to an NPZD-type ecosystem model. We show that increased mesoscale eddy activity enhances the mixing between the well ventilated waters in the southern AS and the O₂-depleted waters in the north, thus halving the volume of suboxia (O₂< 10μmol) and lowering denitrification rates by up to 35%. Because of reduced denitrification and increased vertical mixing in the surface layer, enhanced eddy activity also increases PP, thereby intensifying remineralization and O₂ consumption at depth by up to 40% in the northeastern AS. This secondary effect attenuates the eddy-enhanced ventilation in the north while further expanding hypoxia (O₂< 60μmol) in the south by up to 20%. Our results indicate that eddies contribute to the oxygenation of the OMZ while spreading the low-O₂ conditions farther away from the OMZ core. While the eddy-driven changes enhance the planktonic production, they are potentially detrimental to low-O₂ intolerant species around the OMZ as their habitats get compressed. Overall, our study stresses the need for properly accounting for mesoscale eddies in general circulation models for a realistic representation of the size and intensity of OMZs.