Abstract
Wind-driven coastal upwelling can be compensated by onshore geostrophic flow, and river plumes are associated with such flow. We investigate possible limitation of the northeast Gulf of Guinea upwelling by the Niger River plume, using regional ocean model simulations with or without river and dynamical upwelling indices. Here, the upwelling is weakened by 50% due to an onshore geostrophic flow equally controlled by alongshore thermosteric and halosteric sea-level changes. The river contributes to only 20% of this flow, as its plume is shallow while upwelling affects coastal temperature and salinity over a larger depth. Moreover, the river-induced mixed-layer thinning compensates the current increase, with no net effect on upwelling. The geostrophic compensation is due to an abrupt change in coastline orientation that creates the upwelling cross-shore front. The river nonetheless warms the upwelling tongue by 1°C, probably due to induced changes in horizontal advection and/or stratification.
Highlights
Coastal upwelling is influenced by numerous processes, beyond the off-shore Ekman transport due to alongshore wind that drives the Eastern Boundary Upwelling Systems (EBUS)
We investigate possible limitation of the northeast Gulf of Guinea upwelling by the Niger River plume, using regional ocean model simulations with or without river and dynamical upwelling indices
The Sea surface temperature (SST) pattern is largely consistent between the model and satellite observations, simulated SST gradients are stronger near the coasts and compare better alongshore with the in situ stations (Figure 1B)
Summary
Coastal upwelling is influenced by numerous processes, beyond the off-shore Ekman transport due to alongshore wind that drives the Eastern Boundary Upwelling Systems (EBUS). Though it has been successively attributed to several of the above processes (Ingham, 1970; Picaut, 1983; Philander and Pacanowski, 1986; Binet, 1997), most recent studies show it is primarily controlled by detachment of a coastal current behind a cape and Ekman transport in its western and eastern parts, respectively (Djakouré et al, 2014, 2017) It extends over 1,000 km along the coast of West Africa, from Côte d’Ivoire to Nigeria where the Niger River, the largest in the region with 5,600 m3.s−1, creates a large low-salinity plume. This configuration is excellent to investigate possible coastal upwelling limitation by salinitydriven onshore geostrophic flow, using a realistic high-resolution regional ocean model simulation. We interpolated these data on the model grid and averaged them to compare with the model summer mean current
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