Alban Lazar

Sorbonne Université | UPMC · Laboratoire d'océanographie et du climat : expérimentations et approches numériques (LOCEAN) - UMR 7159

We investigate the atmospheric response to seasonal variations in sea surface temperature (SST) in the eastern tropical Atlantic during the boreal summer, using the Weather Research and Forecasting (WRF) regional atmospheric model. Three ensembles of 11 simulations each are produced with different SST forcings: the control ensemble (CTL) uses the o...

In addition to their well-known seasonal cycle, Eastern Boundary Upwelling Systems (EBUS) undergo modulation on shorter synoptic to intraseasonal time scales. Energetic intensifications and relaxations of upwelling favorable winds with 5-10 days typical time scales can impact the EBUS dynamics and biogeochemical functioning. In this work the dynami...

Sea surface temperature (SST) variability in the North Eastern Tropical Atlantic has its center of action in the Senegalese–Mauritanian upwelling system, where its drivers are wind-induced ocean dynamics and air–sea thermodynamic processes. Thus, a better understanding of the local wind variations, together with their predictability, contributes to...

The Global South is a term largely referring to the regions of Latin America, Asia, Africa, and Oceania. Today, the Exclusive Economic Zones of the Global South (GSEEZ) are the most populated and touristic and inhabited marine regions, with much of the world’s population depending on its resources and wealth. Throughout the GSEEZ, megadiverse and p...

The observation station “Melax” was deployed in 2015 on the wide and shallow south Senegalese shelf to study the ocean dynamics, air‐sea interactions, and dissolved oxygen (DO) cycle. Data from February 2015 to August 2016 were used to study the main physical processes affecting the variability of DO in the bottom layer (∼30 m depth) on time scales...

This study investigates the physical processes controlling the mixed layer buoyancy using a regional configuration of an ocean general circulation model. Processes are quantified by using a linearized equation of state, a mixed-layer heat, and a salt budget. Model results correctly reproduce the observed seasonal near-surface density tendencies. Th...

A recent study has brought to light the co-existence of two distinct Atlantic Equatorial Modes during negative phases of the Atlantic Multidecadal Variability: the Atlantic Niño and Horse-Shoe (HS) mode. Nevertheless, the associated air-sea interactions for HS mode have not been explored so far and the prevailing dynamic view of the Atlantic Niño h...

Ocean boundary current systems are key components of the climate system, are home to highly productive ecosystems, and have numerous societal impacts. Establishment of a global network of boundary current observing systems is a critical part of ongoing development of the Global Ocean Observing System. The characteristics of boundary current systems...

The Equatorial Mode (EM) governs the tropical Atlantic inter-annual variability during boreal summer. It has profound impacts on the climate of adjacent and remote areas. However, predicting the EM is one of the most challenging and intriguing issues for the scientific community. Recent studies have suggested a possible connection between the borea...

Natural variability and change of the Earth’s climate have significant global societal impacts. With its large heat and carbon capacity and relatively slow dynamics, the ocean plays an integral role in climate, and provides an important source of predictability at seasonal and longer timescales. In addition, the ocean provides the slowly evolving l...

The tropical Atlantic is home to multiple coupled climate variations covering a wide range of timescales and impacting societally relevant phenomena such as continental rainfall, Atlantic hurricane activity, oceanic biological productivity, and atmospheric circulation in the equatorial Pacific. The tropical Atlantic also connects the southern and n...

Plain language summary Oxygen is a key requirement for respiration by marine living organisms. Warming of the atmosphere and the ocean surface reduces the oxygenation of offshore waters. Similarly, the extra load of nutrients from agriculture or waste waters modifies algal production, particularly in coastal regions, often resulting in oxygen‐deple...

As climate model uncertainties remain very large for future rainfall in the Sahel, a multi-centennial perspective is required to assess the situation of current Sahel climate in the context of global warming. We present here the first record of hydroclimatic variability over the past 1600 years in Senegal, obtained from stable oxygen isotope analys...

Glider measurements acquired along four transects between Cap-Vert Peninsula and the Cape Verde archipelago in the eastern tropical North Atlantic during March–April 2014 were used to investigate fine-scale stirring in an anticyclonic eddy. The anticyclone was formed near 12°N off the continental shelf and propagated northwest toward the Cape Verde...

This article presents a climatological study of the tropical Atlantic surface wind convergence, one of the main drivers of the marine intertropical convergence zone (ITCZ) precipitations, including coastal northeastern Brazilian and West African rainfalls. Climatological monthly mean surface wind convergence budget, as well as that of their month-t...

The Atlantic Multidecadal Oscillation (AMO) is the leading mode of Atlantic Sea Surface Temperature (SST) variability at multidecadal time-scales. Previous studies have denoted that AMO could modulate El Niño-Southern Oscillation (ENSO) variance. However, the role played by AMO in the Tropical Atlantic Variability (TAV) is still uncertain. Here, it...

As climate model uncertainties remain very large for future rainfall in the Sahel, a multi-centennial perspective is required to assess the relationship between global warming and the Sahel hydroclimate. We present here a new record of hydrologic conditions over the past 1600 years in Senegal, obtained from stable oxygen isotope analyses (δ18O) in...

The Joint International Laboratory ECLAIRS set up an oceanographic and meteorological buoy, dedicated to monitoring and analysis of the short and long-term changes in climate, atmosphere and marine environment within the Senegal coastal upwelling. The buoy "MELAX" was deployed early 2015 in the heart of the Senegalese upwelling by 30 m-depth at (14...

Analysis of the tropical Atlantic variability modes under different AMO phases

Analysis of the role of the oceanic processes in the onset, development and decay of the Meridional Mode through sensitivity experiments with NEMO model in the tropical Atlantic basin

Upwelling off southern Senegal and Gambia takes place over a wide shelf with a large area where depths are shallower than 20 m. This results in typical upwelling patterns that are distinct (e.g., more persistent in time and aligned alongshore) from those of other better known systems, including Oregon and Peru where inner shelves are comparatively...

In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from...

In the second half of the 20th century, a strong multidecadal drought occurred in the Sahel that had catastrophic social and economic consequences. This event made it urgent to better understand the forcing driving precipitation changes in the Sahel and potential impact of anthropogenic global warming. Here, we reconstruct past changes of the preci...

The variability of the tropical Atlantic is strongly linked to changes in the location and intensity of the ITCZ. The air-sea interactions control the tropical Atlantic variability, given rise to different variability modes. At inter-annual time scales, two modes emerge in the tropical Atlantic basin, the Equatorial Mode (EM) and the Meridional Mod...

Atlantic Niño is the leading mode of inter-annual variability of the tropical Atlantic basin at inter-annual time scales. A recent study has put forward that two different Atlantic Niño patterns co-exist in the tropical Atlantic basin during negative phases of the Atlantic Multidecadal Oscillation. The leading mode, Basin-Wide (BW) Atlantic Niño is...

The tropical Atlantic climate is dominated by two modes of variability at inter-annual time scales, the Equatorial Mode (EM) and the Meridional Mode (MM). They are characterized by specific Sea Surface Temperature anomalies (SSTA) distributions, respectively with a maximum over the central-eastern equator during boreal summer, and with an inter-hem...

An air-sea coupled mode of inter-annual variability akin to ENSO emerges in the tropical Atlantic basin, named as Atlantic Niño. The teleconnections of the Atlantic Niño phenomenon have changed during recent decades, coinciding with an alteration of its spatial configuration. Previous studies have suggested that the background state could favour pa...

abstract

The tropical variability has a great importance on the climate of adjacent and remote areas. Two leading modes of inter-annual variability emerge in the tropical Atlantic basin: the Atlantic Niño and the Meridional Mode, which peak in boreal summer and spring respectively. The impacts of both modes on the precipitation regime of the Brazilian Norde...

Satellite data (images of sea surface temperature and chlorophyll-a), ocean surface wind products, Lagrangian observations (surface drifters) and other ancillary data (upwelling index) are used to describe the upwelling seasons off NW Africa during 2009-2013, with particular focus on the coasts of Senegal and Mauritania. The impact of the upwelling...

The upper Eastern Tropical North Atlantic (ETNA) is characterized by the Dome of Guinea centered around 10°N-23°W. This region is of importance for climate study because of strong air-sea coupling. This coupling is believed to be active during boreal winter and spring and to control the northward excursion of the Inter-Tropical Convergence Zone dur...

The climatological seasonal cycle of the sea surface temperature (SST) in the north-eastern 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 characterize...

Tropical Atlantic Sea Surface Temperatures (SSTs) maximum Intra-Seasonal Variability (ISV) and their interaction with local surface winds are investigated applying statistical analysis to observations and to a recent coupled reanalysis over the 2000–2009 decade. Five cores of strong ISV emerge, with standard deviation reaching about 1°C in frontal...

The Equatorial Mode or Atlantic Niño is the leading mode of the tropical Atlantic interannual variability. The positive phase of this mode is characterized by anomalous positive SST anomalies in the eastern equatorial Atlantic during summer months, associated with a weakening of the trades in the western side of the basin. Recent studies have demon...

Tropical Atlantic variability is strongly biased in coupled General Circulation Models (GCM). Most of the models present a mean Sea Surface Temperature (SST) bias pattern that resembles the leading mode of inter-annual SST variability. Thus, understanding the causes of the main mode of variability of the system is crucial. A GCM control simulation...

A paleoreconstruction of the length and intensity of the rainy season over western Africa has been recently proposed, using analysis of fossil mollusk shells from the Saloum delta region, in western Senegal. In order to evaluate the significance of local long-term reconstructions of precipitations from paleoclimate proxies, and to better characteri...

The physical processes controlling the mixed layer salinity (MLS) seasonal budget in the tropical Atlantic Ocean are investigated using a regional configuration of an ocean general circulation model. The analysis reveals that the MLS cycle is generally weak in comparison of individual physical processes entering in the budget because of strong comp...

In the second half of the 20th century, a strong multidecadal drought occurred in the Sahel that had catastrophic social and economic consequences. This event made it urgent to better understand the forcing driving precipitation changes in the Sahel and potential impact of anthropogenic global warming. Here, we reconstruct past changes of the preci...

The Argo dataset is used to study the winter upper-ocean conditions in the northeastern subtropical (NEA) Atlantic during 2006-12. During late winter 2010, the mixed layer depth is abnormally shallow and a negative anomaly of density-compensated salinity, the so-called spiciness, is generated in the permanent pycnocline. This is primarily explained...

Sardinella aurita is the most abundant small pelagic fish in the Senegalese–Mauritanian region. The success of its reproduction crucially depends on the local circulation as this determines whether larvae reach coastal nursery areas favorable to their survival or are dispersed into the open ocean. As a first step towards evaluating sardinella vulne...

abstract

The southern end of the Canary current system comprises of an original upwelling center that has so far received little attention, the Southern Senegal-Gambia Upwelling Center (SSUC). We investigate its dynamical functioning by taking advantage of favorable conditions in terms of limited cloud coverage. Analyses and careful examinations of over 150...

A large number of Atlantic tropical depressions are generated in the eastern basin in relation to the African easterly wave (AEW) and embedded mesoscale convective systems (MCSs) coming from the African continent. In this paper, the structures of strengthening and dissipating MCSs evolving near the West African coast are analyzed, including the rol...

Low-frequency variability of spiciness is observed in the Subtropical South Atlantic over the period 2002-2013 with the Argo gridded product ISAS. Within the pycnocline, spiciness anomalies propagate at a mean speed of 0.04 ± 0.02 m.s-1, the same speed as the gyre mean circulation, from the Agulhas Retroflection region off South Africa (~35°S-20°E)...

The surface-wind response to sea-surface temperature (SST) and SST meridional gradient is investigated in the Gulf of Guinea by using daily observations and re-analyses in the 2000–2009 decade, with a focus on boreal spring and summer months (May to August), where quasi-biweekly fluctuations in the position of the northern front of the equatorial c...

The Atlantic Niño is the leading mode of interannual Tropical Atlantic Sea Surface Temperature (SST) variability during boreal summer. It has been shown that changes in Sta Helena High play a role in the development of this mode, in such a way that anomalous geostrophic winds associated with a weakening of this High pressure System are observed pre...

The sea surface temperature (SST) intraseasonal variability (ISV) and its interaction with the local surface wind in the tropical Atlantic Ocean are investigated using atmospheric observations and reanalyses of the 2000-2009 decade. Largest SST ISV centers are located in frontal areas of the three main tropical upwelling systems: the Eastern equato...

The northeastern Tropical Atlantic sea surface temperature (SST) becomes very warm in boreal summer, north of the seasonal equatorial cold tongue, with a maximum in the vicinity of the InterTropical Convergence Zone (ITCZ). The ITCZ has a significant contribution in the functioning and partitioning of the water cycle over the ocean, but also over W...

Drifter data, satellite maps and time series of SST, upwelling index and ocean surface wind products are used to characterise the North Western Africa upwelling system in the period 2009-2012, with particular focus to the region between Cap Blanc (Mauritania) and Cap Vert (Senegal). This region corresponds to the southern part of the seasonal trans...

The Atlantic Niño is the leading mode of interannual Tropical Atlantic Sea Surface Temperature (SST) variability during boreal summer. It has been shown that changes in Sta Helena High play a role in the development of this mode, in such a way that anomalous geostrophic winds associated with a weakening of this High pressure System are observed pre...

The Atlantic Niño is the leading mode of interannual Tropical Atlantic Sea Surface Temperature (SST) variability during boreal summer. It has been shown that changes in Sta Helena High play a role in the development of this mode, in such a way that anomalous geostrophic winds associated with a weakening of this High pressure System are observed pre...

Book of abstract international conference ICAWA 2014 : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

In Brehmer Patrice (ed.), Diop H. (ed.), Gomez M.M. (collab.), Diogoul N. (collab.), Koutob V. (collab.), Brandt P. (collab.), Sow B.A. (collab.), Lazar A. (collab.), Capet X. (collab.), Fock H. (collab.), Santos C.F. (collab.), Machu Eric (collab.), Diadihou H. (collab.), Jouffre Didier (collab.), Diallo I. (collab.), Schmidt J. (collab.), Gaye A....

The Atlantic Niño is the leading mode of interannual Tropical Atlantic SST variability during boreal summer. It has been shown that changes in Sta Helena High play a role in the development of this mode, in such a way that anomalous winds associated with a weakening of this High pressure system are observed previously to the Atlantic Niño evolution...