Seasonal variability of the circulation in the Arabian Sea at intermediate depth and its link to the Oxygen Minimum Zone

Abstract. Oxygen minimum zones (OMZs) in the open ocean occur below the surface in regions of weak ventilation and high biological productivity. Very low levels of dissolved oxygen affect marine life and alter biogeochemical cycles. One of the most intense but least understood OMZs in the world ocean is located in the Arabian Sea in a depth range between 300 to 1000 m. An improved understanding of the physical processes that have an impact on the OMZ in the Arabian Sea is necessary for a reliable assessment of its current state and future development. This study uses a combination of observational data as well as reanalysis velocity fields from the ocean model HYCOM (Hybrid Coordinate Ocean Model) to investigate the advective pathways of Lagrangian particles into the Arabian Sea OMZ at intermediate depths. In the eastern basin, the OMZ is strongest during winter monsoon with a core thickness of 1000 m depth and oxygen values of less than 5 μmol/kg. The minimum of oxygen concentration might be favored by a maximum advection of Lagrangian particles that follows the main advective pathway along the perimeter of the basin into the eastern basin of the Arabian Sea during winter monsoon. These Lagrangian particles pass regions of high primary production and respiration contributing to a transport of low oxygenated water into the eastern part of the OMZ. The maximum of oxygen concentration in the western basin of the Arabian Sea in May coincides with a maximum southward advection of particles along the western boundary during spring intermonsoon supplying the western core of the OMZ with higher oxygenated water. The maximum of oxygen concentration in the eastern basin of the Arabian Sea in May might be associated with the northward inflow of Lagrangian particles across 10° N into the Arabian Sea which is highest during spring intermonsoon. The Red Sea outflow of advective particles into the western and eastern basin starts during the summer monsoon associated with the northeastward current during the summer monsoon. Whereas particles from the Persian Gulf advect over the whole year. As the weak seasonal cycle of oxygen concentration in the eastern and western basin can be explained by seasonal changing advective pathways at intermediate depths into the ASOMZ, the simplified backward trajectory approach seems to be a good method for prediction of the seasonality of advective pathways of Lagrangian particles into the ASOMZ.