Oxygen budget of the north-western Mediterranean deep- convection region

Abstract

Abstract. The north-western Mediterranean deep convection plays a crucial role in the general circulation and biogeochemical cycles of the Mediterranean Sea. The DEWEX (DEnse Water EXperiment) project aimed to better understand this role through an intensive observation platform combined with a modelling framework. We developed a three-dimensional coupled physical and biogeochemical model to estimate the cycling and budget of dissolved oxygen in the entire north-western Mediterranean deep-convection area over the period September 2012 to September 2013. After showing that the simulated dissolved oxygen concentrations are in a good agreement with the in situ data collected from research cruises and Argo floats, we analyse the seasonal cycle of the air–sea oxygen exchanges, as well as physical and biogeochemical oxygen fluxes, and we estimate an annual oxygen budget. Our study indicates that the annual air-to-sea fluxes in the deep-convection area amounted to 20 molm-2yr-1. A total of 88 % of the annual uptake of atmospheric oxygen, i.e. 18 mol m−2, occurred during the intense vertical mixing period. The model shows that an amount of 27 mol m−2 of oxygen, injected at the sea surface and produced through photosynthesis, was transferred under the euphotic layer, mainly during deep convection. An amount of 20 mol m−2 of oxygen was then gradually exported in the aphotic layers to the south and west of the western basin, notably, through the spreading of dense waters recently formed. The decline in the deep-convection intensity in this region predicted by the end of the century in recent projections may have important consequences on the overall uptake of atmospheric oxygen in the Mediterranean Sea and on the oxygen exchanges with the Atlantic Ocean, which appear necessary to better quantify in the context of the expansion of low-oxygen zones.

Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea

“…This phenomenon, known as "open-ocean deep convection" (see Marshall and Schott [2] for a review), is a key process of the ocean thermohaline circulation. Deep convection is characterized by localized and intense diapycnal mixing events [3] important for the ocean ventilation [4][5][6], subsequent intense phytoplankton spring blooms [7][8][9] and carbon sequestration [10]. In accordance with idealized results [11], the net downwelling induced by deep convection however occurs along the boundary circulation where a large amount of heat is released to the atmosphere [12].…”

Oxygen export to the deep ocean following Labrador Sea Water formation

“…With a global mean apparent oxygen utilization rate of 0.1 µmol kg −1 year −1 at 1500 m depth (Karstensen et al, 2008), an area of 26,159,000 km 2 for the deep Atlantic Ocean between the equator and 50 • N, and a mean layer thickness of 800 m for the isopycnal range 27.68 kg m −3 ≤ σ θ ≤ 27.8 kg m −3 , the annual oxygen consumption for this volume of water is 2.2 Tmol. The rapid southward export of LSW discussed here supplies 71% of the oxygen demand in this layer, with the rest likely provided by slower export of LSW through other routes, as well as uptake in other deep water formation regions such as the Irminger Sea (Palevsky and Nicholson, 2018), Iceland basin (Maze et al, 2012), and the Gulf of Lion in the Mediterranean Sea (Ulses et al, 2021). Thus, despite playing a less significant role than previously thought in setting the strength of the overturning (Lozier et al, 2019), convection in the Labrador Sea does appear to crucially contribute to the supply of oxygen from the subpolar North Atlantic to the deep subtropical and tropical Atlantic Ocean.…”

Impact of Intermittent Convection in the Northwestern Mediterranean Sea on Oxygen Content, Nutrients, and the Carbonate System

“…Furthermore, the MLD calculated by this model at DYFAMED was also used by Heimbürger et al (2013) and Pasqueron de Fommervault, Migon, Dufour, et al (2015). The numerical domain of the model has a mesh size ranging from 0.8 km in the north to 1.4 km in the south of the Mediterranean Sea (Ulses et al, 2021). In the present study, the daily median MLD using the 0.03 kg m −3 criterion over a 20 km area around the DYFAMED location was extracted from the simulation (Supplementary data, Figure S4 in Supporting Information S1) and is used for the Ligurian Sea in Figures 2 and 3.…”

“…The biological processes at that depth depend greatly on the input of DOC (Dissolved Organic Carbon) to heterotrophic organisms, however, this data is lacking. Ulses et al (2021) have shown that in the northwestern Mediterranean Sea and specifically in the Gulf of Lion, the annual O 2 budget is dominated by air-sea exchanges and physical transport during convective years such as the winter 2012-2013 much more than by biogeochemical activity (with respiration and nitrification from heterotrophic organisms under the euphotic zone estimated at an oxygen consumption of 4 mol m −2 year −1 ).…”