Northern Gulf Of Guinea Research Articles

Provenance of Beach Sands in the Northern Gulf of Guinea, SW Cameroon

Abstract Petrographical, mineralogical and geochemical studies were carried out on the beach sands of Limbe (South-West Cameroon) to highlight their provenance, source rocks alteration and the tectonic framework. The Limbe beach sediments were classified as sandy litharenite type. The sediments were derived from mafic and andesite source rocks. The sediments are composed of quartz, olivine, augite, amphibole, pyroxene, plagioclase, potassium feldspar, muscovite and biotite, indicating the primary phases of magmatic source rocks; kaolinite and illite in the secondary phase; and rutile in their residual composition. The presence of ferromagnesian minerals indicates that they were derived from the mafic rocks, which are the basalts of the Cameroon Volcanic Line (CVL). However, the presence of quartz associated with plagioclase and feldspars suggests that alkaline granites played a minor role in the sedimentation of the Limbe beaches. Total Quartz - Total Feldspar - Total Lithic Fragments (Qt-Ft-Lt) mineralogical relationships demonstrate that these beach sediments would be formed in an undissected arc tectonic context, typical of volcanic sources with high relief producing black sands or sands enriched in iron oxides. Their microscopic characteristics of angular to subangular shapes suggest that the magmatic sources are proximal to the shoreline deposits. The characteristics of microporosity, iron oxide inclusions, conchoidal fracture, heavy mineral grains, and opaque minerals support this proximal source. The conchoidal fractures defining the angularity of these beach sediments would suggest its derivation from crystalline rocks. The predominance of heavy minerals suggests that the rocks proximal to the beach have been altered. The abundance of primary minerals associated with these secondary heavy minerals justifies the existence of unweathered lithic grains in the source rocks and an alteration that occurred under a tropical monsoon climate that ranged from semi-humid to humid. The use of the multi-element tectonic discrimination diagram implies an arc framework that is compatible with the geology of the study area.

The surface Guinea Current variability from satellite data

The advent of satellite measurements over the recent decades has made a great contribution to the understanding of near-surface circulation and particularly to the understanding of the surface Guinea Current (GC) along the northern coast of Gulf of Guinea (GG). This study focuses on the GC variability and its relationship with the coastal upwelling from 19 years (2000-2018) of GEKCO (Geostrophic and Ekman Current Observatory) satellite data. These data reproduce the major patterns of GC along the northern GG. The GC flows eastward with a strong spatial and temporal variability with maximum intensity during boreal summer in the east of both Cape Palmas and Cape Three Points. This study showed that the geostrophic current is the dominant component of surface circulation. The Ekman current being lower component contributes also to the intensification of GC in summer. Our study showed that the intensification of GC in summer results from the concomitant acceleration of the geostrophic and wind-driven Ekman currents. Our work highlights the main patterns of GC and its role in the coastal upwelling as limiting it in the east.

Impact of Stratospheric Geoengineering on Sea Surface Temperature in the Northern Gulf of Guinea

Among techniques proposed to limit global warming, there is Stratospheric Aerosol Geoengineering (SAG) which is aiming to increase Earth-atmosphere albedo by injecting sulfur dioxide into the stratosphere in order to reduce the solar radiation that reaches the earth. This study aims to assess the potential impact of SAG on Sea Surface Temperature (SST) in the Northern Gulf of Guinea and its causes using GLENS (Geoengineering Large Ensemble) simulations performed under a high anthropogenic emission scenario (RCP8.5). Here, we focus on two dynamically different regions: Sassandra Upwelling in Côte d’Ivoire (SUC, located east of Cape Palmas) and Takoradi Upwelling in Ghana (TUG, located east of Cape Three Points). Results show that in the SUC region, under climate change, there is an increase in SST (referred to as the current climate) all year long (by 1.52 °C on average) mainly due to an increase in net heat flux (lead by the decrease in longwave radiation) and also in weak vertical mixing (caused by strong stratification which dominates the vertical shear). Under SAG, SST decreases all the seasonal cycle with its maximum in December (−0.4 °C) due to a reduction in the net heat flux (caused by a diminution of solar radiation) and an increase in vertical advection (due to an increase in vertical temperature gradient and vertical velocity). In the TUG region, under climate change, SST warming is a little more intense than in the SUC region and SST changes are driven by an increase in the net heat flux and strong stratification. The cooling of the SST in TUG is similar to the SUC region, but contrary to this region, the cooling under SAG is not only explained by a decrease in the net heat flux but also by the remote forcing of wind changes at the western equatorial Atlantic.

Sea Surface Temperature Impact on Diurnal Cycle and Seasonal Evolution of the Guinea Coast Rainfall in Boreal Spring and Summer

Abstract ERA5 reanalyses and observations of convective clouds and precipitation are used over the northern Gulf of Guinea between 7°W and 3°E to study the influence of ocean surface temperature and the land–sea temperature gradient on Guinea Coast rainfall (GCR) in boreal spring and summer. Seasonal composites are calculated around two dates indexing the onset (Tref) and demise (Tend) of the GCR. The Tref date corresponds to the emergence of the equatorial upwelling in boreal spring, which “pushes” the zonal precipitation belt northward against the Guinea coast. The Tend date characterizes the emergence of the coastal upwelling in July, which is known to coincide with the beginning of the “Little Dry Season” that lasts until September. Along the Guinea Coast, the diurnal cycle of the air–sea temperature gradient controls precipitation through the land–sea breeze, which explains why precipitation reaches its maximum around noon over the ocean, and in the late afternoon over the continent. The emergence of the Guinea Coast upwelling in July induces a weakening of southerlies on a seasonal scale, and a weaker land breeze on a diurnal scale. It induces a decrease in the convergence of humidity transport across the coast and in coastal oceanic precipitation. Therefore, the GCR is seasonally controlled by the latitude of the maximum tropospheric water vapor content and the annual cycle of the West African monsoon, but the ocean surface temperature is responsible for the abruptness of its onset via the intensification of the equatorial upwelling around the end of May, and possibly of its demise as well via the emergence of the coastal upwelling by early July.

Composition, weathering, and provenance of beach sands adjacent to volcanic rocks in the northern Gulf of Guinea, SW Cameroon

The southern coastal portion of the Tertiary Cameroon Volcanic Line (CVL), in the Gulf of Guinea, is bordered by Pan-African granitoids, metamorphic rocks, and Cretaceous sedimentary successions, which are prone to contribute sediments to the nearby Limbe beach. To better constrain the weathering intensity and provenance of the beach sediments, the mineralogy and chemical composition of these sediments were analyzed. The low values of SiO2/Al2O3 ratios and Zircon-Tourmaline-Rutile (ZTR) index, coupled with high Index of Compositional Variability (ICV) indicate compositionally immature sediments. This is further supported by the presence of rock forming minerals such as olivine, pyroxene, and amphibole. The Th/Sc versus Zr/Sc binary and Al2O3–TiO2–Zr ternary diagrams suggest that the studied sediments have not experienced recycling. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), the A–CN–K and (A-K)-C-N triangular diagrams indicate that the Limbe beach sediments were undergone moderate to intense weathering. Several provenance discrimination diagrams based on Al2O3, TiO2, Zr, TiO2, La/Sc, and Th/Co, and the ternary plot of V–Ni–Th*10, and the comparison of chondrite normalized rare earth elements (REE) patterns of the studied sediments with potential source rocks suggest that the source area is composed of mafic rocks of CVL. This indicates that the granitic, metamorphic, and sedimentary rocks close to the volcanic structure do not significantly contribute to the sedimentation of the studied beach. In addition, the results of this study indicate that longshore currents do not influence the chemical composition of the sediments.

Observed monsoon precipitation suppression caused by anomalous interhemispheric aerosol transport

This study uses observations and atmospheric reanalysis products in order to understand the impacts of smoke aerosols advected from the Southern Hemisphere on the dynamics of the West African monsoon. Seasonal biomass burning and resulting aerosol emissions have been well documented to affect regional weather patterns, especially low-level convection. Out of all monsoon months, precipitation shows the most variability over land during August, in which anomalous smoke aerosol values can increase (decrease) by 33% (29%) in the Northern Gulf of Guinea and precipitation can decrease (increase) by up to ~ 2.5 mm day−1 (~ 3 mm day−1) along the West African monsoon region accounting for a 17% (18%) change in precipitation. Smoke aerosols produced by biomass burning occurring near Central Africa are advected towards the Gulf of Guinea at elevations around the 850 hPa level. Satellite observations show an increase (decrease) in cloud fraction and optical depth below (above) the 300-hPa level in the Gulf of Guinea and along the West African coastline along with concurrent decreases (increases) in cloud droplet radius during dirty (clean) aerosol episodes. Additional observations of shortwave radiation quantify changes in cloud coverage and monsoon dynamics. On average, reductions in surface shortwave radiation of ~ 10–15 W m−2 occur over the Gulf of Guinea during increased aerosol transport, with aerosols accounting for ~ 33–50% of that reduction. Reductions in shortwave radiation are associated with decreased convective available potential energy (CAPE). This demonstrates that increased transport of aerosols perturbs surface radiation, convection in the lower troposphere and eventually cloud coverage, potentially leading to the observed monsoon precipitation suppression. In a broader social context, this region houses 200 million people and thus understanding these climate patterns may carry great importance.

Impact of intraseasonal wind bursts on sea surface temperature variability in the far eastern tropical Atlantic Ocean during boreal spring 2005 and 2006: focus on the mid-May 2005 event

Abstract. The impact of boreal spring intraseasonal wind bursts on sea surface temperature variability in the eastern tropical Atlantic Ocean in 2005 and 2006 is investigated using numerical simulation and observations. We especially focus on the coastal region east of 5° E and between the Equator and 7° S that has not been studied in detail so far. For both years, the southerly wind anomalies induced cooling episodes through (i) upwelling processes, (ii) vertical mixing due to the vertical shear of the current, and for some particular events (iii) a decrease in incoming surface shortwave radiation. The strength of the cooling episodes was modulated by subsurface conditions affected by the arrival of Kelvin waves from the west influencing the depth of the thermocline. Once impinging the eastern boundary, the Kelvin waves excited westward-propagating Rossby waves, which combined with the effect of enhanced westward surface currents contributed to the westward extension of the cold water. A particularly strong wind event occurred in mid-May 2005 and caused an anomalous strong cooling off Cape Lopez and in the whole eastern tropical Atlantic Ocean. From the analysis of oceanic and atmospheric conditions during this particular event, it appears that anomalously strong boreal spring wind strengthening associated with anomalously strong Hadley cell activity prematurely triggered the onset of coastal rainfall in the northern Gulf of Guinea, making it the earliest over the 1998–2008 period. No similar atmospheric conditions were observed in May over the 1998–2008 period. It is also found that the anomalous oceanic and atmospheric conditions associated with the event exerted a strong influence on rainfall off northeast Brazil. This study highlights the different processes through which the wind power from the South Atlantic is brought to the ocean in the Gulf of Guinea and emphasizes the need to further document and monitor the South Atlantic region.

Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport, and the role of coastal shallow circulations

Abstract. The complex vertical distribution of aerosols over coastal southern West Africa (SWA) is investigated using airborne observations and numerical simulations. Observations were gathered on 2 July 2016 offshore of Ghana and Togo, during the field phase of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa project. This was the only flight conducted over the ocean during which a downward-looking lidar was operational. The aerosol loading in the lower troposphere includes emissions from coastal cities (Accra, Lomé, Cotonou, and Lagos) as well as biomass burning aerosol and dust associated with long-range transport from central Africa and the Sahara, respectively. Our results indicate that the aerosol distribution on this day is impacted by subsidence associated with zonal and meridional regional-scale overturning circulations associated with the land–sea surface temperature contrast and orography over Ghana and Togo, as typically observed on hot, cloud-free summer days such as 2 July 2016. Furthermore, we show that the zonal circulation evidenced on 2 July is a persistent feature over the Gulf of Guinea during July 2016. Numerical tracer release experiments highlight the dominance of aged emissions from Accra on the observed pollution plume loadings over the ocean, in the area of aircraft operation. The contribution of aged emission from Lomé and Cotonou is also evident above the marine boundary layer. Given the general direction of the monsoon flow, the tracer experiments indicate no contribution from Lagos emissions to the atmospheric composition of the area west of Cotonou, where our airborne observations were gathered. The tracer plume does not extend very far south over the ocean (i.e. less than 100 km from Accra), mostly because emissions are transported northeastward near the surface over land and westward above the marine atmospheric boundary layer. The latter is possible due to interactions between the monsoon flow, complex terrain, and land–sea breeze systems, which support the vertical mixing of the urban pollution. This work sheds light on the complex – and to date undocumented – mechanisms by which coastal shallow circulations can distribute atmospheric pollutants over the densely populated SWA region.

On the Causes of the Minor Dry Season over the Coastal Region of the Northern Gulf of Guinea

Along the littoral shelf of northern coast of the Gulf of Guinea (GG), a minor dry season of the rainfall regime is concomitantly observed with the occurrence of a major coastal upwelling in July-August-September (JAS). It was then supposed that this upwelling drives that minor dry season. But no previous studies have tried to understand this minor dry season and, this study is the first focusing on this question. The investigations undertaken to explain this dry season on the Ivorian littoral shelf with the ERA-Interim data from the European Centre for Medium Range Weather Forecasts over the 1980-2016 period have shown that the minor dry season is driven by the Northward migration of the Inter Tropical Convergence Zone (ITCZ) during this period and, enhanced by the occurrence of the major coastal upwelling of the northern GG at the same time. These two phenomena interact as follow: i) the ITCZ is located in JAS far in the north cutting off convective processes along the coast, ii) the air on the coastal region is poor in humidity, iii) the air temperature on the bordering region of the GG is cooled by the coastal upwelling to value less than 26°C and not favorable for providing convection.

Influence of the Coastal Upwelling along the Northern Gulf of Guinea on the West Monsoon Flux

This study focused on the influence of the seasonal coastal upwelling north of the Gulf of Guinea (GG) on the West Africa Monsoon (WAM) fluxes during the 1980-2016 period based on the ERA-Interim data from the European Centre for Medium Range Weather Forecasts (ECMWF). The results show that the WAM is responsible of most of the rainfall over West Africa but convective precipitation remains the major component of this rainfall. In coastal area, the convective processes are active during April-June when advection of high moisture quantity from the ocean to the land is established. In July-August, the dry season of the bordering region of the GG coincides with the maximum coastal upwelling, which cools SST and the continental air temperature to values less than 26oC. The presence of this cold air temperature combined with the impoverishment of humidity and reduction of wind counteract the convection and then lead to the decrease of the convective rainfall. This action could be the mechanism by which the north coastal upwelling of the GG influences the flux of the West African Monsoon.

Respective Roles of the Guinea Current and Local Winds on the Coastal Upwelling in the Northern Gulf of Guinea

AbstractThe northern Gulf of Guinea is a part of the eastern tropical Atlantic where oceanic conditions due to the presence of coastal upwelling may influence the regional climate and fisheries. The dynamics of this coastal upwelling is still poorly understood. A sensitivity experiment based on the Regional Oceanic Modeling System (ROMS) is carried out to assess the role of the detachment of the Guinea Current as a potential mechanism for coastal upwelling. This idealized experiment is performed by canceling the inertia terms responsible for the advection of momentum in the equations and comparing with a realistic experiment. The results exhibit two major differences. First, the Guinea Current is found to be highly sensitive to inertia, as it is no longer detached from the coast in the idealized experiment. The Guinea Current adjusts on an inertial boundary layer, the inertial terms defining its lateral extension. Second, the upwelling east of Cape Palmas disappears in absence of the Guinea Current detachment. This is in contrast with the upwelling east of Cape Three Points, which is still present. The results suggest that two different generation processes of the coastal upwelling need to be considered: the upwelling east of Cape Palmas (which is due to inertia, topographic variations, and advective terms effects resulting in important vertical pumping) and the upwelling east of Cape Three Points (which is principally induced by local winds). In addition to recent work ruling out the role of eddies, this study clarifies the processes responsible for this coastal upwelling.

A biophysical model of S. aurita early life history in the northern Gulf of Guinea

S. aurita is the most abundant small pelagic fish in the northern Gulf of Guinea. Its reproduction and recruitment depend crucially on environmental conditions. We developed a biophysical model of S. aurita early life history by coupling offline an individual-based model with the regional oceanic modeling system (ROMS). We used this model to investigate the main factors driving variability in eggs and larval dispersal and survival in the northern Gulf of Guinea. Precisely, individuals were released from different spawning areas along the coast and tracked for a period of 28days corresponding to their planktonic phase. Individuals that remained in the coastal recruitment areas at an age more than 7days, at which they can supposedly actively retain themselves in a favorable area, were considered as recruited. Simulation results show the importance of the spawning areas around Cape Palmas and Cape Three Points where cyclonic eddies trap eggs and larvae along the coast, preventing their advection offshore by the Guinea Current. The spawning period also plays a key role in the recruitment success, with highest coastal retention obtained during the major upwelling period (July–September). We find that a second retention peak can occur during the minor upwelling period (February–March) when larval mortality due to temperature is included in the model. These results are in general agreement with knowledge of S. aurita reproduction in the northern Gulf of Guinea.

Under Pressure: Cetaceans and Fisheries Co-occurrence off the Coasts of Ghana and Côte d'Ivoire (Gulf of Guinea)

Within the Gulf of Guinea high levels of fisheries-related cetacean mortality (bycatch and direct-capture) has been documented. For locally rare species such removals could potentially lead to significant population level effects. However, information on the cetacean abundance and distribution is scarce. Similarly, it remains largely unreported where fishing fleets operate offshore. A cetacean survey took place during geophysical surveys (2013-2014) along the coasts of Ghana and Cote d’Ivoire. This provided a unique opportunity to study both offshore cetacean and fishing communities. Due to large group-sizes, melon-headed whales were the most abundant (0.34 animals km-1) followed by Fraser’s dolphins and short-finned pilot whales. Range state records were confirmed for melon-headed whale and Fraser’s dolphin in Ivoirian waters and ten further species represented first at-sea sightings. The artisanal fishing canoe was most abundant (92% of all vessels) and recorded up to 99.5 km from the Ghanaian coast. Asian trawlers operated over shelf areas and tuna purse-seine vessels in deep oceanic and slope waters. Fraser’s dolphins, melon-headed whales, pantropical spotted dolphins, bottlenose dolphins and pilot whales were recorded in areas with the highest fishing densities. Melon-headed whales, pilot whales and rough-toothed dolphins were observed in vicinity of trawlers; bottlenose dolphins, pantropical spotted dolphins and pilot whales in vicinity of canoes. Some notable differences were found in the species composition between the present surveys and port-based surveys of landed cetaceans (bycatch/direct-captures). These may be explained by (1) feeding strategies (nocturnal vs. diurnal; surface vs. deep water); (2) different attractions to vessels/fishing gear; (3) variable body sizes; and (4) difficulty to positively identify species. Despite these differences, both cetaceans and fishing vessels predominantly occurred in shelf and slope waters (< 1,000 m depth contour), making fishery-related mortality likely. The poor knowledge on population trends of cetaceans in this unique upwelling region, together with a high demand for cetacean products for human consumption (as ‘marine bushmeat’) may lead to a potential decline of some species that may go unnoticed. These new insights can provide a foundation for the urgently required risk assessments of cetacean mortality in fisheries within the northern Gulf of Guinea.

Impact of land cover characterization on regional climate modeling over West Africa

The impact of high resolution modern vegetation cover on the West African climate is examined using the International Centre for Theoretical Physics Regional Climate Model implementing the NCAR Community Land Model. Two high resolution 25 km long-term simulations driven by the output from a coarser 50-km resolution simulation are performed for the period 1998–2010. One high resolution simulation uses an earlier and coarser-resolution version of plant functional type distribution and leaf area index, while the other uses a more recent, higher-quality, and finer-resolution version of the data. The results indicate that the new land cover distribution substantially alters the distribution of temperature with warming in Central Nigeria, northern Gulf of Guinea and part of the Sahel due to the replacement of C4 grass with corn; and cooling along the coastlines of the Gulf of Guinea and in Central Africa due to the replacement of C4 grass with tropical broadleaf evergreen trees. Changes in latent heat flux appear to be largely responsible for these temperature changes with a net decrease (increase) in regions of warming (cooling). The improved land cover distribution also results in a wetter monsoon season. The presence of corn tends to favor larger precipitation amounts via more intense events, while the presence of tropical broadleaf evergreen trees tends to favor the occurrence of both more intense and more frequent events. The wetter conditions appear to be sustained via (1) an enhanced soil moisture feedback; and (2) elevated moisture transport due to increased low-level convergence in regions south of 10N where the most substantial land cover differences are present. Overall the changes induced by the improved vegetation cover improve, to some extent, the performance of the high resolution regional climate model in simulating the main West African summer monsoon features.

New evidence for a South Atlantic stock of humpback whales wintering on the Northwest African continental shelf

Seventeen confirmed and four probable sightings of humpback whales, Megaptera novaeangliae, were documented from platform-of-opportunity Research Vessel (R/V) Dr. Fridtjof Nansen in a passing-mode visual survey of continental shelf waters between Conakry and Cap Vert Peninsula (Dakar), 21 October -5 November 2011. None were encountered in the northern stratum Dakar-Agadir, 6 November -15 December 2011. Total visual survey effort was 468 h, 01 min, covering 5335 km. Depending on the exclusion or inclusion of probable records, the sum of group sizes totalled 33 or 43 individuals, respectively. Humpback whale encounter rates between Conakry and Dakar then amounted to 1.74 or 2.27 whales/ 100 km, respectively. Group sizes ranged from 1-6 individuals (mean = 1.94, S.D. = 1.20, n =17; median = 2). Minimally five of 17 groups (29.4%) consisted of adult-calf pairs, with a minimum crude birth rate ranging from 0.060-0.152. All sightings occurred in shallow water, 22-60 m (mean = 35.0 m, S.D. = 10.13, n = 17) but survey effort in deeper, off-shore water was negligible. Sea-surface temperature at sighting locations ranged from 25.5-29.0°C (mean = 27.34, S.D. = 0.96, n = 17). No humpback whales were sighted during a second survey covering Conakry-Tangier-Las Palmas, from 9 May to 22 July 2012 (519 h, 13 min; 6278 km). The Cape Verde Islands comprised, to date, the only known wintering ground located in the North-East Atlantic. This study showed that Senegal, The Gambia, Guinea-Bissau and Guinea are normal range states for M. novaeangliae. A temporal signature, six to seven months out-of-phase with mid-season in the Cape Verde Islands, and neonate presence strongly suggest that the wide Conakry-Dakar continental shelf serves as both wintering and nursery grounds for a South Atlantic stock, in agreement with Bamy et al. (2010). This stock may comprise the northwesternmost component of the large humpback whale assemblage migrating in and out of the northern Gulf of Guinea in austral winter/spring. Further research is required to consolidate insights linking temporal and spatial distribution off western Africa with hemispheric stock identity and migration paths.

New Evidence for a South Atlantic Stock of Humpback Whales Wintering on the Northwest African Continental Shelf

Seventeen confirmed and four probable sightings of humpback whales, Megaptera novaeangliae, were documented from platform-of-opportunity Research Vessel (R/V) Dr. Fridtjof Nansen in a passing-mode visual survey of continental shelf waters between Conakry and Cap Vert Peninsula (Dakar), 21 October -5 November 2011. None were encountered in the northern stratum Dakar-Agadir, 6 November -15 December 2011. Total visual survey effort was 468 h, 01 min, covering 5335 km. Depending on the exclusion or inclusion of probable records, the sum of group sizes totalled 33 or 43 individuals, respectively. Humpback whale encounter rates between Conakry and Dakar then amounted to 1.74 or 2.27 whales/ 100 km, respectively. Group sizes ranged from 1–6 individuals (mean = 1.94, S.D. = 1.20, n = 17; median = 2). Minimally five of 17 groups (29.4%) consisted of adult-calf pairs, with a minimum crude birth rate ranging from 0.060–0.152. All sightings occurred in shallow water, 22–60 m (mean = 35.0 m, S.D. = 10.13, n = 17) but survey effort in deeper, offshore water was negligible. Sea-surface temperature at sighting locations ranged from 25.5–29.0°C (mean = 27.34, S.D. = 0.96, n = 17). No humpback whales were sighted during a second survey covering Conakry-Tangier-Las Palmas, from 9 May to 22 July 2012 (519 h, 13 min; 6278 km). The Cape Verde Islands comprised, to date, the only known wintering ground located in the North-East Atlantic. This study showed that Senegal, The Gambia, Guinea-Bissau and Guinea are normal range states for M. novaeangliae. A temporal signature, six to seven months out-of-phase with mid-season in the Cape Verde Islands, and neonate presence strongly suggest that the wide Conakry-Dakar continental shelf serves as both wintering and nursery grounds for a South Atlantic stock, in agreement with Bamy et al. (2010). This stock may comprise the northwesternmost component of the large humpback whale assemblage migrating in and out of the northern Gulf of Guinea in austral winter/spring. Further research is required to consolidate insights linking temporal and spatial distribution off western Africa with hemispheric stock identity and migration paths.

Air–sea interaction in the Gulf of Guinea at intraseasonal time‐scales: wind bursts and coastal precipitation in boreal spring

AbstractThe role of air–sea interaction in the boreal spring precipitation of the West African monsoon is explored through the wind variability in the Gulf of Guinea. Linear regressions are performed in May–June during the decade 2000–2009 to investigate the origin and effect of the surface wind strengthening north of the Equator. It appears that the equatorial sea‐surface temperature cooling intensifies a surface‐wind equatorial divergence/coastal convergence circulation, and generates a cross‐equatorial pressure gradient, which both strengthen the southerlies north of the Equator. This increases subsidence above the ocean and convection in the northern Gulf of Guinea. In addition, an abrupt change is observed in the surface wind pattern in the eastern equatorial Atlantic (EEA) between April and July. To investigate the transition mechanisms, a reference date is defined as the date when the surface wind north of the Equator becomes and remains stronger than south of the Equator. Thus the maintenance of strong southerlies north of the Equator is linked to a coincident installation of a deep circulation on the whole troposphere and a northward shift of the low atmospheric local circulation. The resulting sharp seasonal transition coincides each year with a southeasterly wind burst, suggesting that the equatorial SST cooling plays a role in the precipitation along the African coast during boreal spring. Copyright © 2012 Royal Meteorological Society

Biostratigraphical Analysis and Palaeoenvironmental Reconstruction of Cenomanian-Turonian Boundary in Côte d'Ivoire Onshore Sedimentary Basin, West Africa

The microfauna studied in this work was provided by the interval 1350-1590m of an onshore oil well Tiemi&eacute; 1, located about 70 km West of Abidjan,in northern Gulf of Guinea. The residues of 23 samples of drill cuttings washed with soapy water, dried and sorted, revealed fourteen of mainlyplanktonic foraminiferal species dominated by the Whiteinellids,Hedbergellids and Heterohelicids. The dominance of microplankton in thelower part of the interval (1390-1590 m) , combined with the total absence ofbenthic foraminifera, reflects an environment of external platform with athick water column probably responsible of reduced oxygenation over theseafloor. At the top (1350-1390 m), absence of any microfauna suggests ashallow shelf and on which a turbulent marine winnowing phenomenonwould have probably prohibited their accumulation. This assemblagecharacterizing Turonian-Cenomanian transition in many geographicalprovinces bordering Atlantic Ocean, was discussed at bothbiostratigraphical and palaeoenvironmental and palaeogeographic plans.

Species occurrence of cetaceans in Guinea, including humpback whales with southern hemisphere seasonality

An initial inventory of the dolphins and whales occurring in Guinea's coastal waters is documented primarily from specimens and photographic evidence obtained from strandings and by-catches. Seven species are fully validated, four odontocetes, Tursiops truncatus, Sousa teuszii, Stenella frontalis, Kogia breviceps and three balaenopterid whales: Balaenoptera brydei, Balaenoptera acutorostrata and Megaptera novaeangliae. Another three reported species (Globicephala macrorhynchus, Steno bredanensis and Delphinus delphis) are insufficiently supported but thought to be valid. Small cetaceans landed as by-catch and a stranded whale were used for human consumption, but no evidence of substantial takes, directed or by-catch, was found. However, concern is raised about even minimal takes of the vulnerable Atlantic humpback dolphin. The seasonal presence of three confirmed humpback whales, two strandings (July and September) and a sighting (October), is synchronous with the species' southern hemisphere wintering/breeding season in low latitudes. We hypothesize that these whales may comprise the north-westernmost range of the population that breeds/overwinters in coastal waters of the Bight of Benin, northern Gulf of Guinea.

Synthesis of the crustal structure of the transform continental margin off Ghana, northern Gulf of Guinea

Results of a detailed geophysical transect across the transform continental margin off Ghana, at the eastern end of the Romanche Fracture Zone in the Equatorial Atlantic, are presented. Seismic refraction, single-channel seismic reflection, gravity, and magnetic data were collected, and seismic, gravity, and magnetic models along the transect are shown. The 6- to 11-km-wide ocean–continent transition (OCT) is characterized by a high-velocity, high-density, high-magnetization crustal zone. The models show no evidence for any underplating of the continental crust adjacent to the margin but minor melting and intrusion of the continental crust may have occurred in the vicinity of the OCT.