Alexis Chaigneau

Institute of Research for Development | IRD · 065 - Laboratory for Studies in Geophysics and Spatial Oceanography (LEGOS)

The state and dynamics of the oceans and seas that surround Africa are changing at an increasing pace due to anthropogenic pressures. The livelihoods of many Africans depend on fishing and ocean- driven monsoon rains, and some African coastlines are eroding rapidly, potentially with catastrophic results to populations and infrastructure. Yet few Af...

This dataset contains mesoscale eddies from the Ocean Eddy Detection and Tracking Algorithms (TOEddies) Atlas colocated with Argo profiling floats. Applied to daily gridded maps of satellite Absolute Dynamic Topography, TOEddies provides information on eddy dynamical characteristics (e.g., size and intensity) over a 30+ year period (1993–2023) and...

This study investigates the effects of tidal cycles on the zooplankton community within the Cotonou Channel, an important waterway connecting the large Nokoué Lagoon to the Atlantic Ocean in Benin. From the determination of zooplankton composition from 25-hour samples collected in July 2020, alpha diversity indices and abundance were assessed, whil...

This study investigates the effect of mesoscale eddies on air–sea heat and fresh water exchange in the tropical Atlantic Ocean (TAO) using 8 years of satellite altimetry data, combined with sea surface temperature (SST), latent and sensible heat fluxes (LHF and SHF), infrared fluxes (IRF) and precipitation (PR) data. Results indicate that approxima...

Submesoscale oceanic structures (<10-20 km) such as eddies and fronts are often difficult to describe given the influence of the mesoscale. In order to characterize the surface signatures of submesoscale structures, we utilize a custom spatial filtering function to separate the meso- and large-scale sea surface height (SSH) signal from the small sc...

Coherent ocean vortices, or eddies, are usually tracked on the surface of the ocean. However, tracking subsurface eddies is important for a complete understanding of deep ocean circulation. In this study, we develop an algorithm designed for the detection of subsurface eddies in the Arabian Sea using Nucleus for European Modelling of the Ocean (NEM...

This study aimed to investigate the seasonal variation of zooplankton diversity and abundance in the Nokoué Lagoon in southern Benin. Through extensive sampling, a total of 109 zooplanktonic taxa were identified and quantified. The average zooplankton abundance was found to be 60 individuals per liter, with copepods and rotifers being the most domi...

The southwestern tropical Atlantic is a region of complex ocean dynamic where originates the strong western boundary current system composed of North Brazil current and North Brazil undercurrent. The region includes a variety of features including the Atoll das Rocas (AR) and Fernando de Noronha (FN) ridge that may favour mesoscale eddy dynamics. H...

The study concerns the water level (WL) evolution in lagoons under the influence of tides and river fluxes. We derive new approximate analytical solutions of the Stigebrandt (1980) equations and apply them to the Nokoué Lagoon (Benin), a large tropical coastal lagoon fed by substantial river input. We show the solutions accurately predict the mean...

This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (West-Africa). The average WL ranges between 1.3 and 2.3 m between the low- and high-water seasons. Seasonal as well as weak interannual variations between 2018 and 2019 are driven by rainfall regime over the catchment and associated river inflow. At sub-m...

Based on 18 years of satellite altimetry data and temperature/salinity (θ/S) profiles from Argo floats, we analyze the isopycnal θ/S anomalies (θ'/S') within newborn eddies in the tropical Atlantic Ocean (TAO). Our results show that on density‐coordinates, both anticyclonic eddies (AEs) and cyclonic eddies (CEs) can exhibit positive, negative, or n...

Nokoué Lagoon in the South of Benin is a large intermittent coastal water body in West Africa, which supports one of the largest inland fisheries of the region. The seasonal and interannual variability of its salinity was studied, based on 3 years of monthly surveys (Dec 2017–Dec 2020). This dataset allows us to identify fine-scale salinity structu...

Using current, hydrographic and satellite observations collected off Northeast Brazil around the Fernando de Noronha Archipelago and Rocas Atoll during two oceanographic cruises (spring 2015 and fall 2017), we investigated the general oceanic circulation and its modifications induced by the islands. In spring 2015, the area was characterized by low...

In the tropical Atlantic Ocean, the North Brazil Undercurrent (NBUC) and the North Brazil Current (NBC) play important roles as near-surface northward paths for the Atlantic meridional overturning circulation. To investigate the meridional evolution of the near-surface western boundary circulation in that region, two surveys were carried out off No...

Mesoscale eddy characteristics are studied in the Tropical Atlantic Ocean (30°N-30°S), using 23 years of daily altimetry sea level anomalies. From the analysis of more than 32000 coherent eddy trajectories, we found that these vortices are mainly generated both in the eastern part of the Atlantic ocean, especially in the Eastern Boundary Upwelling...

In this paper, we analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation),...

The erosion of the subsurface salinity maximum, signature of the Caribbean Subtropical UnderWater (SUW), within the Loop Current Eddy (LCE) Poseidon (August 2016 to July 2017) in the Gulf of Mexico (GoM) and the formation of the Gulf Common Water (GCW) during its journey westward, was observed using glider data. Most of the dilution of the SUW high...

Mediterranean Sea eddies are often generated in fixed geographical locations linked to bathymetric or coastline changes, instabilities around islands or from orographic wind forcing. Because of that, the mean circulation exhibits features with the same special scales as the mesoscale field. We provide here a first comparison of eddies detected and...

In Cotonou, as in many expanding West African cities, major population growth and infrastructural development has not kept up with informal settlement development onto floodable plains and marshes. The population of the slum, which makes up about 60% of the city’s inhabitants, is highly disadvantaged and vulnerable to rising sea levels, flooding, s...

The Gulf of Mexico exhibits strong circulation due to the Loop Current System, along with high mesoscale eddy activity. In order to investigate eddy characteristics, we apply an automatic eddy‐tracking algorithm to absolute dynamic topography maps and analyze the spatial distribution and evolution of eddy attributes including amplitude, radius, and...

We study mesoscale eddy characteristics in the tropical Atlantic Ocean, using 23 years of daily altimetry sea level anomalies. Eddies are mainly generated in the eastern boundary upwelling systems and in the Brazil Current region. Their westward propagation speed reaches 20 cm/s in equatorial areas, decreasing with latitudes. They present typical a...

Plain Language Summary Mesoscale eddies in the ocean, analogous to their atmospheric cyclone and anticyclone counterparts, play a major role on the transport and redistribution of oceanic properties like heat or salt. They are highly variable and transient, so to fully understand their behavior, eddy tracking algorithms are typically used to identi...

The signature of westward propagating mesoscale eddies in sea surface salinity (SSS) is analyzed for the tropical Pacific by collocating 7 years (2010–2016) of Soil Moisture and Ocean Salinity SSS satellite data with coherent mesoscale eddies automatically identified and tracked from altimetry‐derived sea level anomalies. First, the main characteri...

La investigación busca evaluar el impacto de caminos preferidos de remolinos de mesoescala (distinguiendo entre estacionarios y transientes) sobre varios trazadores físicos (temperatura, salinidad), biogeoquímicos (oxígeno, carbón, nutrientes) y otros trazadores en el Pacífico Oriental frente a las costas de California y Chile. Mientras estudios pr...

The Indo-Atlantic interocean exchanges achieved by Agulhas Rings are tightly linked to global ocean circulation and climate. Yet they are still poorly understood because they are difficult to identify and follow. We propose here an original assessment on Agulhas Rings, achieved by TOEddies, a new eddy identification and tracking algorithm that we a...

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...

Marine debris are a growing global concern, with huge amounts of plastic accumulating inside the five subtropical gyres in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Oceans. Plastic items of various sizes are swallowed by marine life and have been reported to penetrate the food chain all the way up to human bodies,...

The northwestern Indian Ocean exhibits a relatively highly energetic eddy field during the southwest monsoon season between June and September. This study analyzes the seasonal and interannual variability of the eddy characteristics and their trajectories in the northwestern Indian Ocean using altimetric sea surface height observations from 1993 to...

In the highly productive Northern Humboldt Current System, 3 seabird species, the Guanay cormorant Phalacrocorax bougainvillii, the Peruvian booby Sula variegata and the Peruvian pelican Pelecanus thagus, commence breeding in austral spring, coinciding with the lowest availability of their prey, the Peruvian anchovy Engraulis ringens. This strategy...

Since the 1960s, the Instituto del Mar del Perú (IMARPE) collected tens of thousands of in-situ temperature and salinity profiles in the Northern Humboldt Current System (NHCS). In this study, we blend this unique database with the historical in-situ profiles available from the World Ocean Database for the period 1960–2014 and apply a four-dimensio...

In the Peruvian eastern boundary upwelling system hydrogen sulfide outbreaks and their coastal surface sulfur plumes influence the coastal ecosystem, local fisheries, aquaculture, and the tourism due to their toxic properties. Hydrogen sulfide is produced by organic matter degradation in oxygen-limited conditions and may cause mass mortality of mar...

In the Peruvian eastern boundary upwelling system hydrogen sulfide outbreaks and their coastal sulfur plumes influence the marine ecosystem, the local fisheries and the tourism due to their toxic properties. They may cause mass mortalities of marine organisms and represent a major limitation for coastal aquaculture. In the past, the local inhabitan...

In the Southwest Pacific Ocean, waters transit from the subtropical gyre before being redistributed equatorward and poleward. While the mean pathways are known, the contribution to the mixing and transport of the water from mesoscale eddies has not been comprehensively investigated. In this research, satellite altimetry data, combined with an eddy...

In the Peruvian eastern boundary upwelling system hydrogen sulfide outbreaks and their coastal sulfur plumes influence the marine ecosystem, the local fisheries and the tourism due to their toxic properties. They may cause mass mortalities of marine organisms and represent a major limitation for coastal aquaculture. In the past, the local inhabitan...

Mesoscale activity is an important component of the Solomon Sea circulation that interacts with the energetic low-latitude western boundary currents of the South Tropical Pacific Ocean carrying waters of subtropical origin before joining the equatorial Pacific. Mixing associated with mesoscale activity could explain water mass transformation observ...

In marine ecosystems top predator populations are shaped by environmental factors affecting their prey abundance. Coupling top predators’ population studies with independent records of prey abundance suggests that prey fluctuations affect fecundity parameters and abundance of their predators. However, prey may be abundant but inaccessible to their...

Physical forcing of the surface ocean includes a variety of energetic processes, ranging from internal wave (IW) to submesoscale and mesoscale. Recent works based on acoustic data showed that, off Peru, the vertical displacements of the oxycline depth provide a robust proxy of isopycnals displacements over a wide range of horizontal scales. These s...

In recent years, persistent quasi-zonal jets or striations have been ubiquitously detected in the world ocean using satellite and in situ data as well as numerical models. This study aims at determining the role of mesoscale eddies in the generation and persistence of striations off Chile in the eastern South Pacific. A 50-year climatological integ...

Informe Instituto del Mar del Peru (ISSN 0378-7702)

In this study, we first show that it is difficult to reconstruct the vertical structure of vortices using only surface observations. In particular we show that the recent SQG and ISQG methods systematically lead to surface intensified vortices and those subsurface intensified vortices are thus not correctly modelled. We then investigate the possibi...

In fluctuating environments, matching breeding timing to periods of high resource availability is crucial for the fitness of many vertebrate species, and may have major consequences on population health. Yet, our understanding of the proximate environmental cues driving seasonal breeding is limited. This is particularly the case in marine ecosystem...

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...

In the four major Eastern Boundary Upwelling Systems (EBUS), mesoscale eddies are known to modulate the biological productivity and transport near-coastal seawater properties toward the offshore ocean, however little is known about their main characteristics and vertical structure. This study combines 10 years of satellite-altimetry data and Argo f...

By collocating 10 years (1999–2009) of remotely sensed surface turbulent heat fluxes with satellite altimetry data, we investigate the impact of oceanic mesoscale eddies on the latent and sensible heat fluxes in the South Atlantic Ocean. In strongly energetic regions, such as the Brazil–Malvinas confluence and the Agulhas Current retroflection, edd...

In the present study we investigated the biogeography of macrozooplankton and fish biomass in the Bay of Biscay. In this region, we defined six different landscapes based on the hydrogeographical characteristics observed in spring 2009. We then related landscape's characteristics and environmental parameters such as light attenuation depth and chlo...

In marine ecosystems, like most natural systems, patchiness is the rule. A characteristic of pelagic ecosystems is that their 'substrate' consists of constantly moving water masses, where ocean surface turbulence creates ephemeral oases. Identifying where and when hotspots occur and how predators manage those vagaries in their preyscape is challeng...

Ocean surface turbulence creates oases for marine life at varying scales. Understanding this patchiness is critical as it strongly influences inter-specific interactions. However, wide-ranging observations are lacking, particularly at scales <10 km. Here we use acoustic and GPS tracking data across a range of scales (100 m-100 km) to quantify the c...

In the Humboldt Current system, large colonies of boobies and cormorants breed in sympatryand feed on anchovy. Their breeding success depends largely on prey availability around the colonies, which may be compensated to some extent through variation in their foraging effort. Anchovy availabilityin this ecosystem may depend on natural shifts in its...

The development of animal tracking technologies (including for instance GPS and ARGOS satellite systems) and the increasing resolution of remote sensing observations call for tools extracting and describing the geometric patterns along a track or within an image over a wide range of spatial scales. Whereas shape analysis has largely been addressed...

This study aims to develop an index that allows distinguishing between surface and subsurface intensified eddies from surface data only, in particular using the sea surface height and the sea surface temperature available from satellite observations. To do this, we propose the use of a simple index based on the ratio of the sea surface tem- peratur...

Geostrophic alongshore velocity data from a glider repetitive section off the coast of Peru (14°S) are used to study the cross-shore structure and temporal variability of the Peru current system during a five-week period in April-May 2010. Besides providing substantial information on the surface frontal jet associated with the Peru Coastal Current...

Bifrequency acoustic data, hydrological measurements and satellite data were used to study the vertical distribution of macrozooplankton in the Bay of Biscay in relation to the hydrological conditions and fish distribution during spring 2009. The most noticeable result was the observation of a 'biocline' during the day i.e., the interface where zoo...

[1] In November 2005 off Cabo Corrientes, a cyclonic eddy was sampled from (i) remotely sensed data of sea level anomaly, near-surface wind, chlorophyll-a concentration and sea surface temperature and (ii) direct observations from a lowered Acoustic Doppler profiler, and a conductivity, temperature, and depth sensor. The sea level anomaly data set...

The near-coastal circulation of the Northern Humboldt Current System is described analyzing ˜8700 velocity profiles acquired by a shipboard acoustic Doppler current profiler (SADCP) during 21 surveys realized between 2008 and 2012 along the Peruvian coast. This data set permits observation of (i) part of the Peru Coastal Current and the Peru Oceani...

Here we show how routine acoustic data, primarily collected for stock assessment, allows for a 3D vision of the abiotic and biotic components of marine ecosystem and to study their interactions. Data, which can be obtained from any vessel geared with multi-frequency echosounders, provide relevant information to study the impact of physical processe...

In marine ecosystems, like most natural systems, patchiness is the rule. A characteristic of pelagic ecosystems is that their 'substrate' consists of constantly moving water masses, where ocean surface turbulence creates ephemeral oases. Identifying where and when hot-spots occur and how predators manage those vagaries in their preyscape is challen...

The Humboldt Current System (HCS) sustains the world's largest small pelagic fishery. While a cooling of this system has been observed during recent decades, there is debate about the potential impacts of rising atmospheric CO2 concentrations on upwelling dynamics and productivity. Recent studies suggest that under increased atmospheric CO2 scenari...