Characterization of the Boreal Summer Upwelling at the Northern Coast of the Gulf of Guinea Based on the PROPAO In Situ Measurements Network and Satellite Data (original) (raw)
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Regional Studies in Marine Science, 2020
The aim of this study is to characterize the coastal upwelling variability at seasonal and inter-annual time scales in the northern Gulf of Guinea (NGoG) using Sea Surface Temperature (SST) collected with autonomous "ONSET" thermometers. Results show that the ONSET SST data are suitable for numerical model evaluation, and provide relevant information in addition to satellite and reanalysis data at seasonal cycle. The minor and major coastal upwellings are present in all the products. The inter-annual SST variability is more pronounced in the western part of the region (Côte d'Ivoire and Ghana) than in the eastern part (Benin and Nigeria). The pattern differences between the west and the east of the region highlight a large spatial variability of the SST in the NGoG. Indeed, the signal of the minor upwelling season is visible only in the west of the basin, namely between Cape Palmas and Cape Three Points. We also observe a well-established thermal gradient between the western and eastern parts of the basin. This gradient is increasing from west to east during the major upwelling season, and decreasing from east to west during the rest of the year. The coastal ONSET data allow to evidence higher SST anomalies than those deduced from satellite and reanalysis products. Although the cold or warm events observed in 2008, 2010 and 2012 are well detected by all products, only the ONSET data set reveal the strong negative SST anomaly observed in 2009 along the coast of Ghana and Côte d'Ivoire.
Climate
Ocean conditions influence the economies and climate of West Africa. Based on the 30-year daily Optimum Interpolation Sea Surface Temperature (OISST) dataset during May–October, upwelling surface variability and marine heatwaves (MHWs) at the northern coast of the Gulf of Guinea are investigated. The cooling surface decreases more rapidly around Cape Palmas than around Cape Three Points and extends eastward. MHWs variability exhibits a frequent occurrence of such events since 2015 that is consistent with the observed oceanic warming and the decrease in upwelling surface. The empirical orthogonal functions performed on the annual cumulated intensity of MHWs show four variability modes that include the whole northern coast, an east–west dipole between the two capes, a contrast between the northern coast at the two capes and the meridional section east of 5° E, and a north–south opposition. These patterns show 3-year, 6-year, and 8-year trends, and are related to coastal upwelling at t...
Seasonal Variability of the Buoyancy Flux along the Northern Coast of the Gulf of Guinea
Applied Categorical Structures, 2021
The buoyancy flux Bo at the air/sea interface is very useful to understand the variability of the stratification of the mixed layer, the oceanic mixing, the phytoplankton dynamics and then the coastal upwelling. The atmospheric reanalysis ERA5 and the oceanic reanalysis ORAP5 data have been used in this study to describe the sea surface Bo and, its influence on the variability of the mixing in the mixed layer and consequently on the coastal upwelling along the northern coast of the Gulf of Guinea. The Bo is negative along the coast and, is characterized by a seasonal variability dominated by the thermal buoyancy flux. This study has also shown that the mixing layer is very shallow along the coast and deeper offshore. The negative value of the Bo increases the stratification of the mixed layer and reduces the mixing. This could explain why the mixed layer is shallow in this region. This work suggests that an increasing trend of the global warming could have dramatic impact in this area by increasing the stratification in the mixed layer and would contribute to reducing the coastal upwelling intensity.
Spatial and temporal seasonal trends in coastal upwelling off Northwest Africa, 1981–2012
Deep Sea Research Part I: Oceanographic Research Papers, 2014
Seasonal coastal upwelling was analyzed along the NW African coastline (11-351N) from 1981 to 2012. Upwelling magnitudes are calculated by wind speed indices, sea-surface temperature indices and inferred from meteorological station, sea-surface height and vertical water column transport data. A permanent annual upwelling regime is documented across 21-351N and a seasonal regime across 12-191N, in accordance with the climatology of previous studies. Upwelling regions were split into three zones: (1) the Mauritania-Senegalese upwelling zone (12-191N), (2) the strong permanent annual upwelling zone (21-261N) and (3) the weak permanent upwelling zone (26-351N). We find compelling evidence in our various indices for the Bakun upwelling intensification hypothesis due to a significant coastal summer wind speed increase, resulting in an increase in upwelling-favorable wind speeds north of 201N and an increase in downwelling-favorable winds south of 201N. The North Atlantic Oscillation plays a leading role in modifying interannual variability during the other seasons (autumn-spring), with its influence dominating in winter. The East Atlantic pattern shows a strong correlation with upwelling during spring, while El Niño Southern Oscillation and Atlantic Multi-decadal Oscillation teleconnections were not found. A disagreement between observationally-based wind speed products and reanalysisderived data is explored. A modification to the Bakun upwelling intensification hypothesis for NW Africa is presented, which accounts for the latitudinal divide in summer wind regimes. Fig. 3. The monthly climatology (1981-2010) for the Mauritania-Senegalese upwelling zone (12-191N, blue line), permanent upwelling zone (21-261N, red line) and the weak permanent upwelling zone (26-351N, green line) for the seven UI W (units: m 3 s À 1 100 m À 1 ) and three UI ΔSST (units: 1C) indices. Error bars on the individual months highlight the (1s) monthly variability. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
SEASONAL SEA LEVEL VARIABILITY IN THE GULF OF GUINEA FROM ALTIMETRY AND TIDE GAUGE
Sea level from the Topex/Poseïdon (T/P) radar altimetry measurements and tide gauges records are used to perform an intercomparison at Sao Tomé, Pointe Noire and San Pedro sites. A Gaussian low-filter is applied to both data sets which suppresses high frequency fluctuations on periods of less than 2 months. The results obtained show the capability of the altimeter data to reproduce the sea level time series with an rms difference of 3.4 cm at Pointe Noire and San Pedro, and 2.3 cm at Sao Tomé.
Journal of Physical Oceanography, 2017
The 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...
Frontiers in Physics, 2015
The climatological seasonal cycle of the sea surface temperature (SST) in the northeastern tropical Atlantic (7-25 • N, 26-12 • W) is studied using a mixed layer heat budget in a regional ocean general circulation model. The region, which experiences one of the larger SST cycle in the tropics, forms the main part of the Guinea Gyre. It is characterized by a seasonally varying open ocean and coastal upwelling system, driven by the movements of the intertropical convergence zone (ITCZ). The model annual mean heat budget has two regimes schematically. South of roughly 12 • N, advection of equatorial waters, mostly warm, and warming by vertical mixing, is balanced by net air-sea flux. In the rest of the domain, a cooling by vertical mixing, reinforced by advection at the coast, is balanced by the air-sea fluxes. Regarding the seasonal cycle, within a narrow continental band, in zonal mean, the SST early decrease (from September, depending on latitude, until December) is driven by upwelling dynamics off Senegal and Mauritania (15-20 • N), and instead by air-sea fluxes north and south of these latitudes. Paradoxically, the later peaks of upwelling intensity (from March to July, with increasing latitude) essentially damp the warming phase, driven by air-sea fluxes. The open ocean cycle to the west, is entirely driven by the seasonal net air-sea fluxes. The oceanic processes significantly oppose it, but for winter north of ∼18 • N. Vertical mixing in summer-autumn tends to cool (warm) the surface north (south) of the ITCZ, and advective cooling or warming by the geostrophic Guinea Gyre currents and the Ekman drift. This analysis supports previous findings on the importance of air-sea fluxes offshore. It mainly offers quantitative elements on the modulation of the SST seasonal cycle by the ocean circulation, and particularly by the upwelling dynamics.
Phenology and mechanisms of the early upwelling formation in the southern coast of Senegal
African Journal of Environmental Science and Technology, 2021
Mechanisms of the interannual variability of upwelling onset dates on the Petite-Cote of Senegal are investigated using daily NEMO model data outputs and NOAA ocean surface temperature observations (OISST). We first determined the phenology (onset dates, end dates, duration and occupied area of the upwelling) over the Petite-Côte located in south of the Senegalese coast. Our results show that upwelling in the Petite-Côte starts on average on 03-December (with a standard deviation of 13 days) and ends on average on 13-June (with a standard deviation of 11 days). Upwelling lasts, on average, 6 months in the region. The maximum upwelling intensity is noted on 15-May and the maximum occupied area exhibits a February-May plateau. We then performed a composite analysis based on the earliness of the upwelling setup dates to understand the mechanisms involved. Our results show that for the earliest upwelling years, we note a strengthening of the vertical velocities 3 weeks before the onset of the upwelling but we note especially an abnormal intensification of the coastal jet. The latter seems to be the initiator of the early onset of the upwelling in the region. These reinforcements are associated with a significant decrease in surface temperature (SST) and an intensification of northerly trade winds.
International Journal of …, 2008
This study examined the sea surface temperature (SST) patterns in the Gulf of Guinea (GOG) and discussed their implications for the spatio-temporal variability of precipitation in West Africa. The SST data spanning over 49 years at 2°× 2°resolutions were sourced from the archive of the International Research Institute for climate prediction. The specific locations where data were collected to represent the GOG are longitudes 10°W, 8°W, 6°W, 4°W, 2°W, 0°, 2°E, 4°E, 6°E, and 8°E, and latitudes 5°N, 3°N, 1°N, 1°S, 3°S, and 5°S. The results obtained show that the SST in the GOG generally decreases westward and southward, while the reverse of the case holds for its variability values. Detailed observations show that the SST is generally below the latitudinal average and its variability values, above the latitudinal average mainly between longitudes 8°W and 2°E. The results also show that during the period of July-August-September, the SST is anomalously colder between longitudes 8°W and 2°E and the West African coastal border and latitude 3°N. This area was observed as constituting the area of the coastal upwelling. The intra-annual distribution of the SST, as evident along latitudes 3°N and 5°N, shows two main regimes and two transitional periods in between the regimes. The first regime is November to May, and the second, July to September. The first regime transits to the second during the month of June and the second back to the first, during the month of October. The comparative analysis of the first and second half of the period of study indicates that the SST of the GOG has undergone some warming over time. The comparison also shows that although the SST has generally risen, an area of relatively cool SST near the Guinea coast has expanded from longitudes 7°W-0°W to 8°W-3°E.
In situ wind, current and temperature measurements carried out on and off the continental shelf of the Ivory Coast during the FOCAL (Français Océan Climat de la zone équatoriale AtLantique) program in 1983 and 1984, are described. The thermal structure at the coast mainly depends on both the intensity and the meridional extension of the Guinea current. In 1983, except for mid-January and mid-November, the thermocline is closed to the surface due to the yearlong presence of the Guinea current both onshore and offshore. In 1984, on the contrary, from the end of January through at least mid-May, the Guinea current is present on the shelf but i's weak and narrow south of the shelf break; the thermocline at that time, compared to 1983, is some 15 m deeper. The eastward flow on the shelf alone is therefore unable to maintain a shallow thermocline. In summer (July-August) 1983 and 1984, the Guinea current is present on and offshore. Substantial changes are not, however, observed in the speed of the Guinea current at the coast in spring-summer, in comparison with the autumn-winter season, which could justify the larger amplitude of the surface cooling in summer than in winter. The upward displacement of the thermocline in spring-summer is amplified by the increase of the wind velocity component parallel to the coast. The mean upwelling rate induced by local wind forcing at the coast, in summer, through a simple linear model, is 1.12m/day in 1983 and 0.83m/day in 1984. In autumn-winter the secondary temperature minimum, which appears mainly subsurface, is linked to the second intensification of the Guinea current, both on and offshore. Horizontal advection does not explain the main surface cooling observed on the shelf. Oceanol. Acta, 1988, 11, 2, 125-138. RÉSUMÉ Étude des refroidissements saisonniers des eaux du plateau continental de Côte d'Ivoire pendant le programme FOCAL Parallèlement aux observations hauturières effectuées dans le cadre du programme FOCAL (Français Océan Climat de la zone équatoriale AtLantique) en 1983 et 1984, il a été développé un programme côtier qui avait pour objet l'étude des refroidissements saisonniers qui affectent les eaux du plateau continental de la Côte d'Ivoire. Les mesures effectuées au-dessus et au large du plateau continental permettent d'affirmer que la variabilité du champ thermique à la côte est étroitement liée à l'intensité et à l'extension en latitude du courant de Guinée. En 1983, les observations montrent que la thermocline est proche de la surface toute l'année, conséquence d'une présence quasi-permanente du courant de Guinée sur le plateau, mais surtout au large de ce plateau continental. De janvier à mi-mai 1984, le courant de Guinée est présent sur le plateau, mais quasi-inexistant au large, surtout en avril-mai, et la thermocline se trouve, par rapport à la situation observée en 1983, à une immersion supérieure d'une quinzaine de mètres environ; le courant de Guinée, circonscrit uniquement au plateau continental, ne peut donc pas assurer seul un important déplacement vertical ascendant de la thermocline. En été (1983 et 1984), le courant de Guinée est à nouveau présent au large, et la thermocline se retrouve près de la surface, à la côte. Toutefois, par rapport à la situation observée en automne-hiver, aucun changement important, tant