Variability of bottom dissolved oxygen on the southern Senegalese shelf at intraseasonal to interannual time scales using a modelling approach

Abstract

Recent in-situ measurements on the west African continental shelf have shown that the bottom waters undergo episodic hypoxia and even anoxia. In this study, we investigate the variability of bottom dissolved oxygen (DO) over the southern Senegalese shelf at seasonal, intraseasonal and interannual time scales over the period 2015–2019 using a high-resolution (2 km) regional coupled dynamical-biogeochemical model. The model is forced by daily winds to represent synoptic variability during the upwelling-favourable cold season (November-May). Near-bottom DO evaluation using data from the Melax mooring located over the mid shelf indicates that the model represents well the seasonal and intraseasonal variability. A bottom DO budget shows that advection of deep, low-oxygenated waters over the shelf edge during the upwelling season generates a DO loss, which is overcompensated by ventilation of the entire shelf due to vertical mixing. Biogeochemical processes compensate this DO supply: DO is consumed by organic matter and zooplankton respiration, slightly compensated by regenerated and new primary production. Because of a sluggish shelf circulation during the summer season (July–September, JAS), oxygen concentration declines in parts of the outer shelf bottom layer, in spite of a weaker surface primary production and organic matter export than during the heart of the cold season (February–April, FMA). Analysis of the synoptic variability reveals the complexity of the DO response to the duration and intensity of upwelling and relaxation events, with the largest DO decrease of $\sim 110\mu mol{O}_2{L}^{-1}$ associated with a strong upwelling event (cumulative upwelling index $\sim 2{\mathrm{Nm}}^{-2}day$) obtained in 2016. Year to year variability indicates two periods of enhanced hypoxia in FMA 2017 and JAS 2016 characterized by a high retention of bottom waters over the shelf.