While the Arabian Sea’s Oxygen Minimum Zone (OMZ), one of the most severe in the world, threatens the equilibrium of many marine ecosystems, the processes that oxygenate this region are still poorly understood. Recently, researchers at the University of Gothenburg, in collaboration with Sultan Qaboos University and the University of East Anglia , have shed light on the role of the water in the Persian Gulf in providing oxygen at certain critical depths, thanks to high-resolution observations by underwater gliders. This is a contribution to a better understanding of these mechanisms, which is crucial for improving climate forecasts and anticipating the state of health of the oceans in the context of climate change.
By Laurie Henry
The ventilation of the oxygen minimum zone (OMZ) in the Arabian Sea is a key issue in the current context of climate change. This zone, characterised by low oxygen concentrations, plays a central role in the biogeochemical dynamics of the oceans and has an impact on the regional marine ecosystem. But climate models, because of their inability to capture small-scale processes, such as dense flows from marginal seas, have difficulty in accurately reproducing the extent and intensity of this MPA.
A scientific challenge to track minimum oxygen levels
The Arabian Sea OMZ is one of the world’s most hypoxic ocean areas. Located at a depth of between 150 and 1,250 metres, this sub-oceanic zone is home to severe hypoxic conditions that compromise marine biodiversity and disrupt crucial biogeochemical processes such as the nitrogen cycle.
Oxygen flows in this area are strongly influenced by the monsoon cycle, which regulates surface water inputs and vertical exchanges. However, these inputs are insufficient to stabilise the fragile balance between biological oxygen consumption and its renewal. Deep waters, where oxygen is becoming increasingly scarce, are becoming veritable biogeochemical traps, exacerbating the formation of greenhouse gases such as nitrous oxide. There is therefore an urgent need to shed light on how these dynamics work in the face of accelerating climate change.
Among the potential ventilation mechanisms, the water in the Persian Gulf plays a decisive but still largely unknown role. This dense, salt-laden, well-oxygenated water mass is formed by intense evaporation in the Persian Gulf before flowing through the Strait of Hormuz towards the Arabian Sea. Along the way, it carries oxygen into the intermediate layers where it penetrates, contributing to the ventilation of the OMZ. However, the complexity of its interactions with mesoscale eddies and coastal topography make it difficult to accurately assess its impact on oxygenation of the zone. The seasonal and interannual variability of oxygen fluxes linked to these processes remains poorly understood, limiting the ability of models to predict the evolution of the OMZ under different climate scenarios.
High-resolution observations to unravel oceanographic processes
To gain a better understanding of the contribution of Persian Gulf Water (PGW) to the ventilation of the OMZ, a team of researchers led by Estel Font from the University of Gothenburg deployed four autonomous underwater gliders in the Gulf of Oman for 18 months, covering several seasons and capturing interannual variations.
These gliders collected high-resolution data on temperature, salinity and dissolved oxygen at various depths. The gliders’ routes, spread over transects of up to 80 km, were used to map the spatial and temporal variability of the PGW and to observe its path from the Strait of Hormuz to the Arabian Sea.
A glider in the Gulf of Oman. © University of East Anglia
The data collected was then integrated into geostrophic models (simulating ocean currents by assuming a balance between the pressure gradient and the Coriolis force, to predict large-scale circulations) to estimate the speeds of currents at depth, taking into account the complex interactions with medium-scale eddies responsible for the entrainment and lateral transport of water masses.
Glider campaigns in the Gulf of Oman. The contours show the coastline (black solid line), the 100 m isobath (black dotted line) and the break in the plateau at the 300 m isobath (red dotted line). The two black lines crossing the plateau indicate the location of the transect in 2015-16 (dotted line) and 2021-22 (dashed line).© E. Font et al., 2024
In addition, the researchers used the Turner angle. This is used to assess the stability of the water column and identify areas of differential mixing of heat and salt. Two regimes can be identified: salt fingering, which occurs when layers of warm, salty water rest on colder, less salty layers, and diffusive convection. This phenomenon favours the vertical exchange of oxygen by allowing the efficient transfer of heat, salt and oxygen through the water column, playing a crucial role in the ventilation of the oxygen minimum zone.
The researchers also calculated the Richardson number to identify the zones where shear instability can occur. This parameter is used to measure the balance between the stability of the water column and the shear forces caused by currents. When this number is low, the differences in speed between the layers of water encourage turbulence that mixes the water masses. This mechanism helps to push oxygen up into the deeper zones, improving ventilation in the oxygen-depleted zone.
Using modelling, the researchers then determined the precise oxygen contribution of this dense water at different times of the year. The results, published in the Journal of Geophysical Research: Oceans, show increased ventilation during periods when the PGW is confined along the continental slope and instabilities favour mixing.
The dynamic contribution of Persian Gulf water to the ventilation of the OMZ and implications for climate modelling
The study estimated that water from the Persian Gulf contributes an average of 1.3 Tmol (Teramoles) of oxygen per year to the Arabian Sea OMZ. That’s 41.6 million tonnes of oxygen.
However, this contribution varies significantly from season to season, with fluctuations of up to ±1.6 Tmol/year, or 51.2 million tonnes of oxygen per year. This variability is mainly attributed to the complex interactions between mesoscale currents and submarine topography.
Time series of glider observations. (a) Time series of conservative temperature, (b) absolute salinity and (c) dissolved oxygen concentration as observed by gliders in 202122.. © E. Font et al., 2024
For example, periods of high eddy kinetic energy result in more efficient dispersion of oxygen-rich water towards the interior of the OMZ, allowing for better ventilation. On the other hand, during phases of low eddy activity, the PGW remains largely trapped along the coastal slopes, limiting its oxygen supply to restricted areas.
The study also demonstrated the importance of mixing mechanisms such as double diffusive mixing and shear instabilities. When combined, these processes favour the vertical integration of oxygen in the OMZ. For example, the study found that favourable mechanical shear conditions occur about 14% of the time at critical depths, where oxygen gradients are steepest. These conditions create ‘windows’ of opportunity for oxygen to be efficiently incorporated into the intermediate layers, helping to reduce hypoxia over short periods.
Oxygen transport by surface water. Annual cycle of oxygen transport by surface water along a 35 km transect from the 300 m isobath.. © E. Font et al., 2024
These new data offer concrete prospects for climate modelling. By integrating these fine dynamics into numerical models, it will be possible to better simulate oxygen flows in the Arabian Sea and other similar regions.
The results also highlight the importance of continuous, high-resolution monitoring to capture the small-scale variations that influence these processes. In practical terms, these advances will make it possible to project more accurately the evolution of the MPA in the face of climate change, taking into account future scenarios of warming and changes in ocean currents.
Source : Font, E., Swart, S., Bruss, G., Sheehan, P. M. F., Heywood, K. J., & Queste, B. Y. (2024). “Ventilation of the Arabian Sea oxygen minimum zone by Persian Gulf water”. Journal of Geophysical Research – Oceans, 129(5), [e2023JC020668]. https://doi.org/10.1029/2023JC020668