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

Alory, Gaël; Da‐Allada, C. Y.; Djakouré, Sandrine; Dadou, Isabelle; Jouanno, Julien; Loemba, Dorelle Prudence

doi:10.3389/fmars.2020.607216

Cited by 25 publications

(20 citation statements)

References 64 publications

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“…For the TUG region, only changes in the meridional advection due to changes in both onshore geostrophic and Ekman meridional currents were responsible for the warm SST event in 2010 (Figures 11 and 12). Changes in the onshore meridional geostrophic current are caused by the alongshore sea surface height anomalies (Alory et al, 2021) while the changes in meridional Ekman current are due to the weakening of the zonal wind stress (not shown). All these findings highlight the major role played by wind stress changes in the 2010 SST warming and are in agreement with the suggestions of Lefèvre et al (2013).…”

Section: Summary and Discussionmentioning

confidence: 99%

“…The MLD is defined as the depth where the density increase compared to density at 10 m equals 0.03 kg m −3 . The density criterion used to calculate MLD is the one recommended by de Boyer Montégut et al (2004) and previously used in several studies in the region (e.g., Alory et al, 2021;Da-Allada et al, 2014;Jouanno et al, 2011Jouanno et al, , 2017. However, this criterion does not take into account the (rare) cases where the MLD is shallower than 10 m. The meridional current V is decomposed into geostrophic and Ekman components.…”

Section: Model Mixed-layer Heat Budgetmentioning

confidence: 99%

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Causes of the Northern Gulf of Guinea Cold Event in 2012

Da‐Allada

Agada²,

Baloïtcha³

et al. 2021

JGR Oceans

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Particularly cool sea surface temperatures (SSTs) were observed in 2012 along the Northern Gulf of Guinea coast. This strong cooling event was seen from February to June and reached maxima in the coastal upwelling areas: SST anomalies of −1°C were observed in Sassandra Upwelling area in Côte d'Ivoire (SUC, situated east of Cape Palmas) and SST anomalies of −0.5°C were observed in Takoradi Upwelling area in Ghana (TUG, located east of Cape Three Points). In SUC and TUG regions, the 2012 decrease in SST was the coldest event recorded over the 1990–2018 period (29 years). From the analysis of regional simulations, we show that the mechanisms behind this SST decrease differ in the two regions. In the SUC region, we identify changes in both zonal advection (related to zonal SST gradient changes) and increased vertical mixing as the main drivers of the anomalous cooling. The anomalous vertical mixing is linked to increased vertical shear of the zonal current in response to the Guinea Current strengthening. In the TUG region, acceleration of the southward advection of the surface water, due to the intensification of the meridional Ekman current generated by the strengthening of the zonal wind stress, was identified as the major cause of the SST anomalous cooling.

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Section: Summary and Discussionmentioning

confidence: 99%

Section: Model Mixed-layer Heat Budgetmentioning

confidence: 99%

Causes of the Northern Gulf of Guinea Cold Event in 2012

Da‐Allada

Agada²,

Baloïtcha³

et al. 2021

JGR Oceans

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“…Except for probably spurious local freshening at a few points along the coast in SMAP RSS, SMOS BEC and SMAP RSS largely agree with GLORYS on the large-scale meridional gradient between 40°S and 32°S. Note that, while some upwelling regions are associated with coastal SSS increase [75,76], others are associated with coastal freshening (e.g., [77]). The mean vertical salinity gradients are indeed different and can be of opposite sign in the four major EBUS [78].…”

Section: Great Australian Bightmentioning

confidence: 59%

“…Note that, while some upwelling regions are associated with coastal SSS increase [75,76], others are associated with coastal freshening (e.g., [77]). The mean vertical salinity…”

Section: California Upwelling Systemmentioning

confidence: 99%

Global Analysis of Coastal Gradients of Sea Surface Salinity

Dossa

Alory

Silva³

et al. 2021

Remote Sensing

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Sea surface salinity (SSS) is a key variable for ocean–atmosphere interactions and the water cycle. Due to its climatic importance, increasing efforts have been made for its global in situ observation, and dedicated satellite missions have been launched more recently to allow homogeneous coverage at higher resolution. Cross-shore SSS gradients can bear the signature of different coastal processes such as river plumes, upwelling or boundary currents, as we illustrate in a few regions. However, satellites performances are questionable in coastal regions. Here, we assess the skill of four gridded products derived from the Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellites and the GLORYS global model reanalysis at capturing cross-shore SSS gradients in coastal bands up to 300 km wide. These products are compared with thermosalinography (TSG) measurements, which provide continuous data from the open ocean to the coast along ship tracks. The comparison shows various skills from one product to the other, decreasing as the coast gets closer. The bias in reproducing coastal SSS gradients is unrelated to how the SSS biases evolve with the distance to the coast. Despite limited skill, satellite products generally agree better with collocated TSG data than a global reanalysis and show a large range of coastal SSS gradients with different signs. Moreover, satellites reveal a global dominance of coastal freshening, primarily related to river runoff over shelves. This work shows a great potential of SSS remote sensing to monitor coastal processes, which would, however, require a jump in the resolution of future SSS satellite missions to be fully exploited.

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“…Inside the river plume, the circulation is mainly in geostrophic balance as observed (Mazzini et al, 2019;Alory et al, 2021) and modeled in other buoyant plumes (Nof and Pichevin, 2001;Ou et al, 2009;Schiller et al, 2011). Locally, the circulation in the river plume is ageostrophic [R o ∼ O(1)].…”

Section: River Plume Dynamicsmentioning

confidence: 97%

Non-Linear Processes in the Gironde River Plume (North-East Atlantic): Instabilities and Mixing

et al. 2021

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Instability and mixing are ubiquitous processes in river plumes but their small spatial and temporal scales often limit their observation and analysis. We investigate flow instability and mixing processes in the Gironde river plume (Bay of Biscay, North-East Atlantic ocean) in response to air-sea fluxes, tidal currents, and winds. High-resolution numerical simulations are conducted in March (average river discharge) and in August (low discharge) to explore such processes. Two areas of the Gironde river plume (the bulge and the coastal current) experience different instabilities: barotropic, baroclinic, symmetric, and/or vertical shear instabilities. Energy conversion terms reveal the coexistence of barotropic and baroclinic instabilities in the bulge and in the coastal current during both months. These instabilities are intensified over the whole domain in August and over the inner-shelf in March. The Hoskins criterion indicates that symmetric instability exists in most parts of the plume during both periods. The evolution of the Gironde plume with the summer stratification, tidal currents and winds favors its development. During both seasons, ageostrophic flow and large Rossby numbers characterize rapidly-growing and small-scale frontal baroclinic and symmetric instabilities. The transition between these instabilities is investigated with an EKE decomposition on the modes of instability. In the frontal region of the plume, during both months, symmetric instabilities grow first followed by baroclinic and mixed ones, during wind bursts and/or high discharge events. In contrast, when the wind is weak or relaxing, baroclinic instabilities grow first followed by symmetric and then mixed ones. Their growth periods range from a few hours to a few days. Mixing at the ocean surface is analyzed via Potential Vorticity (PV) fluxes. The net injection of PV at the ocean surface occurs at submesoscale buoyant fronts of the Gironde plume during both months. Vertical mixing at these fronts has similar magnitude as the wind-driven and surface buoyancy fluxes. During both months, the frontal region of the plume is restratified during wind relaxation events and/or high river discharge events through frontogenetic processes. Conversely, wind bursts destratify the frontal plume interior through non-conservative PV fluxes.

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

Cited by 25 publications

References 64 publications

Causes of the Northern Gulf of Guinea Cold Event in 2012

Causes of the Northern Gulf of Guinea Cold Event in 2012

Global Analysis of Coastal Gradients of Sea Surface Salinity

Non-Linear Processes in the Gironde River Plume (North-East Atlantic): Instabilities and Mixing

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