Driving mechanisms of the dissolved oxygen budget in the Levantine Sea: a coupled physical-biogeochemical modelling approach

Abstract. The Levantine Basin is an ultra-oligotrophic region and the formation site of the Levantine Intermediate Waters. For the first time, a high-resolution 3D coupled hydrodynamic-biogeochemical model, SYMPHONIE-Eco3MS, was used to investigate the seasonal and interannual variability of dissolved oxygen (O₂) in the Levantine Basin and estimate its basin-wide budget for the period 2013–2020. Our results show that the simulated O₂ concentrations align well with in situ data from research cruises and Argo floats. During winter, the surface layer is undersaturated in oxygen by up to 2 % across the entire basin, leading to atmospheric oxygen absorption. The model shows that on an annual scale, the basin acts as a net sink for atmospheric oxygen, with the Rhodes Gyre exhibiting uptake rates twice as high as the rest of the Levantine Basin. The surface layer also serves as a source of dissolved oxygen for intermediate depths, with 4.2 ± 1.1 mol m^-2 year^-1 of dissolved oxygen vertically transported. Oxygen is transported laterally into the basin from the Ionian Sea and exported towards the Aegean Sea, with winter heat loss intensity enhancing this lateral export at both surface and intermediate layers. The Levantine Basin alternates between autotrophic and heterotrophic states, depending on the intensity of winter surface heat loss. Spatially, the Rhodes Gyre emerges as a significant oxygen pump, contributing 41 % of the total oxygen production in the surface layer in the Levantine basin. This study highlights the need for further modeling studies on pluri-annual and multi-decadal scales to explore the interannual variability and evolution of the annual oxygen budget across the entire Eastern Basin, particularly in the context of climate change.