Sandrine Djakouré - Academia.edu (original) (raw)

Papers by Sandrine Djakouré

Journal of geography, environment and earth science international, Feb 17, 2024

Atmospheric and climate science, Dec 31, 2023

The West African Monsoon (WAM) is characterized by strong decadal and multi-decadal variability a... more The West African Monsoon (WAM) is characterized by strong decadal and multi-decadal variability and the impacts can be catastrophic for the local populations. One of the factors put forward to explain this variability involves the role of atmospheric dynamics, linked in particular to the Saharan Heat Low (SHL). This article addresses this question by comparing the sets of preindustrial control and historical simulation data from climate models carried out in the framework of the CMIP5 project and observations data over the 20 th century. Through multivariate statistical analyses, it was established that decadal modes of ocean variability and decadal variability of Saharan atmospheric dynamics significantly influence decadal variability of monsoon precipitation. These results also suggest the existence of external anthropogenic forcing, which is superimposed on the decadal natural variability inducing an intensification of the signal in the historical simulations compared to preindustrial control simulations. We have also shown that decadal rainfall variability in the Sahel, once the influence of oceanic modes has been eliminated, appears to be driven mainly by the activity of the Arabian Heat Low (AHL) in the central Sahel, and by the structure of the meridional temperature gradient over the inter-tropical Atlantic in the western Sahel.

Frontiers in Marine Science, Dec 11, 2023

Marine heat waves (MHWs) can potentially alter ocean ecosystems with farreaching ecological and s... more Marine heat waves (MHWs) can potentially alter ocean ecosystems with farreaching ecological and socioeconomic consequences. In this study, we characterize the MHWs in the eastern tropical Atlantic Ocean with a focus on the Gulf of Guinea (GG). The Optimum Interpolation Sea Surface Temperature (OISST) data from January 1991 to December 2020 and PIRATA network temperature data, from October 2019 to March 2020, have been used for this purpose. The results show that the eastern tropical Atlantic has experienced an annual average of 2 MHWs events in recent decades. Based on the spatial distribution of the different characteristics of these MHWs, we subdivided the eastern tropical Atlantic Ocean into three zones: the northern coast of the GG, the equatorial zone and the Congo-Gabon coastal region. The trend associated with MHWs events showed an increase in the number of MHWs since 2015 in the different zones. This increase was greater at the northern coast of the GG than at the equator and the Congo-Gabon coast. Long-duration MHWs are more frequent in the equatorial zone. High intensity MHWs were observed in different areas with sea surface temperature anomalies greater than 2°C. These anomalies are more intense at the Congo-Gabon coast. The results also revealed that the MHWs event at the equator from mid-October 2019 to March 2020 initially occurred in the subsurface before it appeared at the surface. These results also suggested that ocean temperatures are a potential predictor of MHW events. KEYWORDS marine heat waves, sea surface temperature, ocean-atmosphere interaction, eastern tropical Atlantic Ocean, Gulf of Guinea, climate change Frontiers in Marine Science frontiersin.org 01

Research Square (Research Square), Oct 24, 2022

Scientific Reports, Jan 27, 2023

Regular and long-term monitoring of coastal areas is a prerequisite to avoiding or mitigating the... more Regular and long-term monitoring of coastal areas is a prerequisite to avoiding or mitigating the impacts of climate and human-driven hazards. In Africa, where populations and infrastructures are particularly exposed to risk, there is an urgent need to establish coastal monitoring, as observations are generally scarce. Measurement campaigns and very high-resolution satellite imagery are costly, while freely available satellite observations have temporal and spatial resolutions that are not suited to capture the event scale. To address the gap, a network of low-cost, multi-variable, shore-based video camera systems has been installed along the African coasts. Here, we present this network and its principle of sharing data, methods, and results obtained, building toward the implementation of a common integrated coastal management policy between countries. Further, we list new contributions to the understanding of still poorly documented African beaches' evolution, waves, and sea level impacts. This network is a solid platform for the development of inter-disciplinary observations for resources and ecology (such as fisheries, and sargassum landing), erosion and flooding, early warning systems during extreme events, and science-based coastal infrastructure management for sustainable future coasts. Coastal areas contain some of the world's most diverse and productive resources that support a variety of economic activities, including fisheries, tourism, recreation, and transportation. They are, nevertheless, extremely vulnerable to human intervention and climate change. Their recreational, ecosystem and protection services and economic values give them high societal attention, especially given that these areas are home to more than a trillion dollars' worth of infrastructure and investments globally 1,2. Many coastal areas have undergone extensive modification and development over the past few decades, greatly increasing their vulnerability to both the anticipated effects of global climate change and the natural coastal dynamics 3,4 .

Ocean Science

In this paper, we review observational and modelling results on the upwelling in the tropical Atl... more In this paper, we review observational and modelling results on the upwelling in the tropical Atlantic between 10 • N and 20 • S. We focus on the physical processes that drive the seasonal variability of surface cooling and the upward nutrient flux required to explain the seasonality of biological productivity. We separately consider the equatorial upwelling system, the coastal upwelling system of the Gulf of Guinea and the tropical Angolan upwelling system. All three tropical Atlantic upwelling systems have in common a strong seasonal cycle, with peak biological productivity during boreal summer. However, the physical processes driving the upwelling vary between the three systems. For the equatorial regime, we discuss the wind forcing of upwelling velocity and turbulent mixing, as well as the underlying dynamics responsible for thermocline movements and current structure. The coastal upwelling system in the Gulf of Guinea is located along its northern boundary and is driven by both local and remote forcing. Particular emphasis is placed on the Guinea Current, its separation from the coast and the shape of the coastline. For the tropical Angolan upwelling, we show that this system is not driven by local winds but instead results from the combined effect of coastally trapped waves, surface heat and freshwater fluxes, and turbulent mixing. Finally, we review recent changes in the upwelling systems associated with climate variability and global warming and address possible responses of upwelling systems in future scenarios. Dedication. In memory of our dear colleague and friend Prof. Mathieu Rouault whose passion for upwelling dynamics was limitless.

This study investigates the importance of convective systems for extreme rainfall along the north... more This study investigates the importance of convective systems for extreme rainfall along the northern coast of the Gulf of Guinea (GG) and their relationship with atmospheric and oceanic conditions. Convective system data (MCSs), daily precipitation, sea surface temperature (SST) and moisture flux anomalies are used from June to September 2007-2016. The results show that 2/3 of MCSs crossing Abidjan are produced in June, which is the core of the major rainy season. Likewise, 2/3 of MCSs originate from continental areas, while 1/3 comes from the ocean. Oceanic MCSs are mostly initiated close to the coast, which also corresponds to the Marine Heat Waves region. Continental MCSs are mostly initiated inland. The results also highlight the moisture flux contribution of three zones which have an impact on the onset and the sustaining of MCSs: (i) the seasonal migration of ITCZ, (ii) the GG across the northern coastline, (iii) and finally the continent. These contributions of moisture fluxe...

Journal of Coastal Research

Climate

Ocean conditions influence the economies and climate of West Africa. Based on the 30-year daily O... more 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...

In this paper, we review observational and modelling results on the upwelling in the inner tropic... more In this paper, we review observational and modelling results on the upwelling in the inner tropical Atlantic. We focus on the physical processes that drive the seasonal variability of surface cooling and upward nutrient flux required to explain the seasonality of primary productivity. We separately consider the equatorial upwelling system, the northern coastal upwelling system of the Gulf of Guinea and the tropical Angolan upwelling system. For the equatorial regime, we discuss the forcing of upwelling velocity and turbulent mixing as well as the underlying dynamics responsible for thermocline movements and current structure. The coastal upwelling system in the Gulf of Guinea is concentrated along northern boundary and is driven by both, local and remote forcing. The particular role of the Guinea Current, nonlinearity and the shape of the coastline are emphasized. For the tropical Angolan upwelling, we show that this system is not wind-driven, but instead results from the combined effect of coastally trapped waves, surface heat and freshwater fluxes, and turbulent mixing. Finally, we review recent changes in the upwelling systems associated with climate variability and global warming and address possible responses of upwelling systems in future scenarios. Short summary Tropical upwelling systems are among the most productive ecosystems globally. The tropical Atlantic upwelling undergoes a strong seasonal cycle that is forced by the seasonal cycle of the zonal wind along the equator and the near-coastal wind field off Africa. Besides the wind forcing that lead to an up-and downward movement of the nitracline, turbulent diffusion results in upward mixing of nutrients. Here, we review the different physical processes responsible for upward nutrient supply.

Frontiers in Marine Science, 2021

Wind-driven coastal upwelling can be compensated by onshore geostrophic flow, and river plumes ar... more Wind-driven coastal upwelling can be compensated by onshore geostrophic flow, and river plumes are associated with such flow. We investigate possible limitation of the northeast Gulf of Guinea upwelling by the Niger River plume, using regional ocean model simulations with or without river and dynamical upwelling indices. Here, the upwelling is weakened by 50% due to an onshore geostrophic flow equally controlled by alongshore thermosteric and halosteric sea-level changes. The river contributes to only 20% of this flow, as its plume is shallow while upwelling affects coastal temperature and salinity over a larger depth. Moreover, the river-induced mixed-layer thinning compensates the current increase, with no net effect on upwelling. The geostrophic compensation is due to an abrupt change in coastline orientation that creates the upwelling cross-shore front. The river nonetheless warms the upwelling tongue by 1°C, probably due to induced changes in horizontal advection and/or stratif...

Atmospheric and Climate Sciences, 2021

Rain flooding during June on the West Africa coastal area is analyzed by using the 95th and 75th ... more Rain flooding during June on the West Africa coastal area is analyzed by using the 95th and 75th percentiles, which represent extreme and intense rainfall events respectively. Thus, the contribution of these events that reaches around 50% shows their impact on the rainfall in June. Atmospheric and oceanic factors influence the rain flooding. Indeed, the extreme events are associated with easterly waves propagating from 20˚E, while those of intense events are initiated around 5˚E. The impact of oceanic conditions exhibits the warming of the equatorial rail and the Atlantic cold tongue, the warming of the whole ocean basin and a north-south dipole of SST anomalies. The West African monsoon that reaches Abidjan corresponds to a low-level atmospheric flow, whose upward motion extends in latitude from the ocean to the continent. An increase of disturbance contributes to enhancing these events. This is confirmed by the inflow on to the continent of oceanic moisture coming from the ventilation by evaporation of warm water. In addition, the coupled ocean-atmosphere simulations are one of the best candidates that could help to better explain these dramatic events. This study is useful because of showing solutions that could help in adoption of policies for the risks management related to these events.

Regional Studies in Marine Science, 2020

The aim of this study is to characterize the coastal upwelling variability at seasonal and inter-... more 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.

Journal of Physical Oceanography, 2017

The northern Gulf of Guinea is a part of the eastern tropical Atlantic where oceanic conditions d... more 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...

Progress in Oceanography, 2017

S. aurita is the most abundant small pelagic fish in the northern Gulf of Guinea. Its reproductio... more 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 28 days corresponding to their planktonic phase. Individuals that remained in the coastal recruitment areas at an age more than 7 days, 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 to September). We find that a second retention peak can occur during the minor upwelling period (February to 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. Highlights ► A biophysical model of S. aurita early life history in the northern Gulf of Guinea is proposed. ► The physical features are the main processes determining larval drift pathways and retention areas. ► The role played by cyclonic and anti-cyclonic eddies in coastal retention is emphasized. ► Two most favorable spawning periods during the minor and the major upwelling are simulated.

International Journal of Oceanography, 2013

The boreal summer upwelling along the northern coast of the Gulf of Guinea (GG) is characterized ... more The boreal summer upwelling along the northern coast of the Gulf of Guinea (GG) is characterized using new in situ sea surface temperature (SST) from onset sensor and satellite TRMM Microwave Imager (TMI) datasets. This study aims to encourage intensive in situ SST measurements at the northern coast of the Gulf of Guinea. It shows good agreement between daily in situ SST and TMI SST and similar coastal upwelling onset date, end date, and durations calculated using both datasets. Interannual evolution of the onset date at four stations along the northern coast of GG indicates that the upwelling can be initiated at one cape or simultaneously at both the cape of palms and the cape of three points. It can be also initiated eastward towards Cotonou or globally off all the northern coasts of GG. Nonsignificant trend is found on upwelling onset date and end date variability. Moreover, this study shows that SST is significantly warm or cold some years. Ocean conditions during these years ar...

Journal of Geophysical Research: Oceans, 2014

Cyclonic eddies generated downstream of Cape Palmas and Cape Three points have been suggested to ... more Cyclonic eddies generated downstream of Cape Palmas and Cape Three points have been suggested to contribute to the coastal upwelling along the northern coast of the Gulf of Guinea. A numerical analysis using a high-resolution model is used to investigate the mesoscale activity and the coastal upwelling generation processes. An eddy detection and tracking tool is applied to altimeter and model data, showing good agreement between these data sets. About two cyclonic eddies per year with an average radius of 60 km were identified downstream of both capes. These cyclonic eddies have an average lifetime of about 60 days during the major coastal upwelling period (boreal summer) and an eastward propagation. These cyclonic eddies are shallow, energetic (their relative vorticity can reach 3 times the earth's rotation), and dimensionless parameters show that they are in an eddy shedding regime. Mean flow interactions and barotropic instabilities associated with capes are their main generation processes. An idealized experiment is conducted in order to analyze the effect of capes on eddy generation and on coastal upwelling. It reveals that these cyclonic eddies generated downstream of capes are not the process responsible for this coastal upwelling. This experiment also suggests that the cyclonic eddies are the cause of the westward and coastal Guinea Counter Current that is associated with a transfer of energy from eddy kinetic to the mean flow.

Biogeosciences Discussions, 2017

Since 2011, unprecedented and repetitive blooms and large mass strandings of the floating brown m... more Since 2011, unprecedented and repetitive blooms and large mass strandings of the floating brown macroalgae, Sargassum natans and Sargassum fluitans have been reported along the West Indies, the Caribbean, the Brazilian and the West Africa coasts. Recent studies have highlighted a new tank of Sargassum: the North Equatorial Recirculation Region of the Atlantic Ocean. This region is located off the northeast of Brazil, approximately between the equator and 10 • N and from 50 • W to 25 • W. The potential causes of these recent blooms and mass strandings are still poorly understood. Observational datasets and modelling outputs involving hydrological parameters and climate events are examined focusing on their potential feedback on the observed blooms and mass strandings. The results show that combined conditions have been in favor of these recent changes. High anomalously unprecedented positive sea surface temperature observed in the tropical Atlantic in 2010-2011 could have induced favorable temperature conditions for Sargassum blooms. These favorable conditions were then fed by additional continental nutrients inputs, principally from the Amazon River. These continental nutrients load are the consequences of deforestation, agroindustrial and urban activities in the Amazonian forest. The results also suggest that subsurface intake of nutrients from the equatorial upwelling could also contribute to the blooms of the Sargassum seaweed in the Atlantic Ocean but further studies are needed to confirm these additional inputs.