Impact of Submesoscale Dynamics and Turbulent Mixing on the Senegalo-Mauritanian Upwelling System

Based on a combination of over 20 years of satellite data with extensive in situ measurements from previous research expeditions, an initial step is taken to differentiate the impact of submesoscale processes and turbulent mixing on the Eastern Boundary Upwelling System (EBUS) off Senegal and Mauritania. EBUS are an essential part of the global carbon cycle and are of central importance for the sustainability of economic and food resources. In the tropical Senegalo-Mauritanian EBUS, sea surface temperatures and net primary production exhibits a pronounced seasonal cycle. It is characterized by coastal upwelling in late boreal winter and an abrupt end in late boreal spring with the onset and strengthening of the poleward Mauritania Current. At first glance, the temporal and spatial development follows the annual cycle of the wind stress curl. However, a closer look reveals a more complex picture with a pronounced spatiotemporal heterogeneity, characterized by the influence of (sub)mesoscale eddies and (non-linear) internal tides.

Integrated cross-shelf tidal energy fluxes towards the coast are locally estimated from observations of multiple short-term moorings. Such fluxes should result in increased mixing near the coast, which is in fact supported by assessment of over 800 microstructure turbulence observations. (Internal) tides are known to drive much of the mixing and vertical exchange on a rather narrow coastal strip. Besides, lateral density gradients which were induced by upwelling are regularly subject to conditions favorable for frontogenesis. The associated secondary circulations can induce strong vertical motions and instabilities and export chlorophyll offshore through frontal jets. A snapshot of ship-based measurements of turbulent kinetic energy dissipation rates indicates an order of magnitude larger dissipation on the cold, dense side of the front, whereby surface heat flux is known to play a crucial role. Spatially high-resolution measurements of sea level deflections from the SWOT satellite show considerable variability on scales smaller than 20 km, but the applicability for balanced motions is hampered by the regular occurrence of solitary waves and topographic effects. Given the significance of these observed small-scale processes for the redistribution and alteration in net primary production and expected general changes of submesoscale processes (e.g. due to changing mixed layer depths in the context of global warming), a more precise quantification of their net impact is essential.

Outlook: An interdisciplinary expedition in spring 2025 will supplement the existing data and will use an adaptive sampling strategy, e.g. to tackle the mutual interaction of tides and internal waves with density fronts. Observed tidal fluxes will be interpreted in the context of a high-resolution 3D baroclinic tidal model. These, as well as the work presented, serve as preparatory work for the unprecedented year-round “FUTURO” campaign, which aims to provide a detailed picture of the annual cycle for this important EBUS.