Seasonal cycle of the salinity barrier layer revealed in the northeastern Gulf of Guinea

Abstract

The region located in the far northeast of the Gulf of Guinea (NEGG), eastern tropical Atlantic, remains poorly documented due to a lack of available in situ ocean data. Heavy rainfall and intense river discharges observed in this region induce a strong salinity stratification that may have a significant impact on the mixed layer depth and on sea surface temperatures, through the so-called barrier-layer effect. By using recent in situ data and climatological outputs from a numerical simulation, we reveal the existence of a barrier layer in the NEGG and describe its seasonal occurrence. In the NEGG, the barrier layer limits the mixed layer depth. From January to March, significant values for the barrier-layer thickness are observed mostly due to the horizontal advection of fresh water. From April, vertical mixing along with vertical advection increase the sea surface salinity; hence, the barrier-layer thickness decreases and reaches its minimum in July. During the rest of the year, values for the barrier-layer thickness are again high, mostly under the influence of the Niger River discharge and precipitation, with the highest values recorded in October, when the river discharge and precipitation are at a maximum.

Coastal Upwelling Limitation by Onshore Geostrophic Flow in the Gulf of Guinea Around the Niger River Plume

“…The resulting advection, acting like a delayed horizontal geostrophic compensation, could explain the warming. The stratification strengthening, associated with a barrier layer often present in river plumes and in particular here, could also contribute by trapping solar heat and inhibiting vertical mixing near the surface (Mignot et al, 2007;Foltz and McPhaden, 2009;Dossa et al, 2019). A heat budget is necessary to quantify both processes and their relative spatiotemporal contribution.…”

“…Upward anomalies are also found above the MLD between 2°E and 4°E, and with a larger amplitude and vertical extension west of 8°W and east of 4°E, just in front of Niger outflows. A clear river-induced effect is the deepening of isohalines all along the coast east of 6°W, and a strong thinning (up to 50%) of the MLD east of 3°E by salinity-induced stratification (Dossa et al, 2019).…”

Seasonal Variability of Freshwater Plumes in the Eastern Gulf of Guinea as Inferred From Satellite Measurements

“…Rivers freshwater inputs also modify the upper ocean thermohaline properties (salinity and temperature), and thus the buoyancy and vertical stratification of the surface layers. Strong vertical salinity gradients associated with fresh pools are generally found in the upper layer of the tropical oceans (e.g., Alory et al., 2012; Cole et al., 2015; de Boyer Montégut et al., 2014; Dossa et al., 2019; Durand et al., 2019; Kang et al., 2013; Maes & O'Kane, 2014; Materia et al., 2012; Pailler et al., 1999; Reul, Quilfen, et al., 2014). Rivers runoffs can shallow the mixed layer depth (MLD), by limiting or preventing vertical mixing between the upper warm layer and the cold ocean interior, through the so‐called barrier layer mechanism.…”

Global Analysis of Coastal Gradients of Sea Surface Salinity

“…Second, there are also differences in vertical levels as satellites estimate salinity in the upper few centimeters, while TSG data are acquired at 5-10 m depth [17,34]. However, given the dominance of coastal freshening (Figure 11), probably due to river discharges or coastal rains, which are associated with strong stratification [54,[90][91][92], the different depth of acquisition would induce a negative bias in satellite SSS at the coast. The fact that we observed the opposite (Figure 4) suggests a small influence of the depth difference.…”