The Congo River plume: Impact of the forcing on the far‐field and near‐field dynamics

Abstract

[1] The first numerical simulations of the Congo River plume dynamics are presented in this study. The different forcing mechanisms responsible for the seasonal variations of the plume extend are separately analyzed and the Regional Ocean Modeling System (ROMS) is employed to carry out both a process orientated study-with simple baseline simulations and a sensitivity study-with realistic 1 year runs setup in 2005. The baseline model is forced only by the river flow, in the presence of realistic bathymetry. Tides, wind stress, surface heat flux, and ocean boundary conditions are the forcing added to the realistic model. The typical seasonal orientation of the Congo River plume is found to be northward during most of year except for the February-March (FM) season when the plume has a large westward extension (about 800 km) and its area nearly doubles. The northward extension of the plume is explained by a buoyancy-driven upstream coastal flow-due to the unique geomorphology of the Congo River estuary-and the combined influences of the ambient ocean currents and the wind. During the FM season, the surface ocean circulation is driving both (1) the westward extension of the plume and (2) the southward transport of the Nyanga River fresh waters which feed the Congo River plume. In the near-field region of the plume, the presence of the deep Congo canyon has two main effects: (1) its depth increases the intrusion of sea water into the river mouth and (2) its orientation initiates the formation of the upstream flow.

Sea surface salinity variability in response to the Congo river discharge

“…2, two centers of salinity variability are found. The southern center is located north of the mouth of the Congo River and has a northwestward extension on the order of 1000 km, comparable to the extension found in the observational study of Hopkins et al (2013) and the modeling study of Denamiel et al (2013). The northern center occupies a somewhat larger area in the Gulf of Guinea., As mentioned above, hereafter we focus our attention on the variability to the south of the Equator, which is more likely to be the direct result of Congo river discharge.…”

“…Among the first realistic numerical model simulations of the Congo River plume dynamics, Denamiel et al (2013) also showed that the Congo River plume extends northwestward during most of the year except for the February-March period when the plume has a large westward extension. The northwestward extension of the Congo River plume can be clearly identified in the Aquarius data (Fig.…”

“…In the region of fresher waters south of the Equator the continental shelf is generally relatively broad (around 200 km or so in width) though there is a hint of a deeper region (canyon) near the mouth of the Congo River near 6°S. Denamiel et al (2013) argued that the orientation of this canyon (towards the northwest) is a crucial factor in determining the initial direction of motion of the freshwater plume from the river.…”

“…Eisma and Van Bennekom (1978) were the first to describe the Congo River plume, finding it to be oriented generally towards northwest (see also Braga et al, 2004) rather than towards the south, as might be expected due to the location of the river (6°S) and the influence of the Coriolis effect. In the modeling study of Denamiel et al (2013) it was suggested that the northward extension of the plume can be explained as a buoyancy-driven upstream coastal flow that is influenced by the geomorphology of the Congo River estuary (see Jansen et al, 1984) and the combined influences of the ambient ocean currents and winds. Using Soil Moisture and Ocean Salinity (SMOS) satellite surface salinity and chlorophyll data, Hopkins et al (2013) also found that for most of the year, the main axis of the fresh water plume extends northwest between 400 and 1000 km along the coastline, extending to the Equator and even slightly further north.…”

Impact of data assimilation on ocean current forecasts in the Angola Basin

“…Shchepetkin and McWilliams [49] describes an in-depth review of the numerics and formulation. Previous studies have utilised this model in understanding the Congo River plume dynamics [15] and effects on ocean temperature [56]. The model domain extends between 1 o S − 21 o S and 3.7 o E − 13.8 o E with a 10km resolution and 40 terrain-following verti-5 cal levels.…”