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. Oxygen minimum zones (OMZs) in the open ocean occur belowthe surface in regions of weak ventilation and high biological productivitywith associated sinking organic matter. Very low levels of dissolved oxygenalter biogeochemical cycles and significantly affect marine life. One of themost intense though poorly understood OMZs in the world ocean is located inthe Arabian Sea between 300 and 1000 m of depth. An improved understanding ofthe physical processes that have an impact on the OMZ in the Arabian Sea isexpected to increase the reliability of assessments of its futuredevelopment. This study uses reanalysis velocity fields from the ocean model HYCOM(Hybrid Coordinate Ocean Model), which are verified with observational data,to investigate advective pathways of Lagrangian particles into the ArabianSea OMZ at intermediate depths between 200 and 800 m. In the eastern basin, the vertical expansion of the OMZ is strongest duringthe winter monsoon, revealing a core thickness 1000 m deep and oxygen valuesbelow 5  µ mol  kg−1 . The minimum oxygen concentration might be favouredby a maximum water mass advection that follows the main advective pathway ofLagrangian particles along the perimeter of the basin into the eastern basinof the Arabian Sea during the winter monsoon. These water masses pass regions ofhigh primary production and respiration, contributing to the transport of low-oxygenated water into the eastern part of the OMZ. The maximum oxygen concentration in the western basin of the Arabian Seain May coincides with a maximum southward water mass advection in thewestern basin during the spring intermonsoon, supplying the western core of theOMZ with high-oxygenated water. The maximum oxygen concentration in the eastern basin of the Arabian Seain May might be associated with the northward inflow of water across10 ∘  N into the Arabian Sea, which is highest during the springintermonsoon. The Red Sea outflow of advective particles into the western and easternbasin starts during the summer monsoon associated with the northeastwardcurrent during the summer monsoon. On the other hand, waters from the PersianGulf are advected with little variation on seasonal timescales. As the weak seasonal cycle of oxygen concentration in the eastern andwestern basin can be explained by seasonally changing advection of watermasses at intermediate depths into the Arabian Sea OMZ (ASOMZ), the simplified backward-trajectory approach seems to be a good method for prediction of theseasonality of advective pathways of Lagrangian particles into the ASOMZ.

Seasonal variability of the Arabian Sea intermediate circulation and its impact on seasonal changes of the upper oxygen minimum zone