Importance and variability of the Northern Omani Dead Zone
An autonomous underwater glider was deployed along a 80 km transect extending from Muscat out into the Gulf from March to June 2015 as part of a project funded by ONR Global and the UK NERC. The glider surveyed the top 1000m across the continental shelf, a very steep continental slope, and the open ocean while measuring temperature, salinity, oxygen, chlorophyll a fluorescence, optical backscatter, photosyntheticall active radiation and provided estimates of depth-averaged currents and up/downwelling.
We observe high variability in three dynamic features defining the vertical oxygen profile. The oxycline depth varies by over 50m throughout the season, impacting on available habitat space and DCM dynamics. Between 150-350m, the data show high variability on the order of weeks with the boundary of the suboxic zone (< 6 umol.kg-1) varying by up to 200m. This upper boundary is determined by the volume of the Persian Gulf Water (PGW) outflow which travels along the shelf edge. Below 400m, the oxygen concentrations reached levels below the detection limit of the Aanderaa 4430F sensors. This is the first sustained observation of anoxia within the Gulf of Oman. Observations of anoxic water are put into context using historical databases and the impact on the wider Arabian Sea OMZ are discussed. Overall, variability of oxygen concentrations was tightly linked to outflow from the Strait of Hormuz and position of two semi-permanent eddies within the Gulf.