Mouth Of Amazon Research Articles

The novel mangrove environment and composition of the Amazon Delta

Both freshwater floodplain (várzeas and igapós) forests and brackish-saline mangroves are abundant and well-described ecosystems in Brazil.1 However, an interesting and unique wetland forest exists in the Amazon Delta where extensive mangroves occur in essentially freshwater tidal environments. Unlike the floodplain forests found upriver, the hydrology of these ecosystems is driven largely by large macro-tides of 4-8m coupled with the significant freshwater discharge from the Amazon River. We explored these mangroves on the Amazon Delta (00°52' N to 01°41' N) and found surface water salinity to be consistently <5; soil pore water salinity in these mangrove forests ranged from 0 nearest the Amazon mouth to only 5-11 at the coastal margins to the north (01°41' N, 49°55' W). We also recorded a unique mix of mangrove-obligate (Avicennia sp., Rhizophora mangle) and facultative-wetland species (Mauritia flexuosa, Pterocarpus sp.) dominating these forests. This unique mix of plant species and soil porewater chemistry exists even along the coastal strands and active coastlines of the Atlantic Ocean. Part of these unique mangroves have escaped current global satellite mapping efforts, and we estimate that they may add over 180km2 (20% increase in mangrove area) within the Amazon Delta. Despite having a unique structure and function, these freshwater-brackish ecosystems likely provide similar ecosystem services to most mangroves worldwide, such as sequestering large quantities of organic carbon, protection of shoreline ecosystems from erosion, and habitats to many terrestrial and aquatic species (monkeys, birds, crabs, and fish).

Eustatic and tectonic change effects in the reversion of the transcontinental Amazon River drainage system

ABSTRACT: The development of the transcontinental Amazon River System involved geological events in the Andes Chain; Vaupés, Purus and Gurupá arches; sedimentary basins of the region and sea level changes. The origin and age of this river have been discussed for decades, and many ideas have been proposed, including those pertaining to it having originated in the Holocene, Pleistocene, Pliocene, Late Miocene, or even earlier times. Under this context, the geology of the sedimentary basins of northern Brazil has been analyzed from the Mesozoic time on, and some clarifications are placed on its stratigraphy. Vaupés Arch, in Colombia, was uplifted together with the Andean Mountains in the Middle Miocene time. In the Cenozoic Era, the Purus Arch has not blocked this drainage system westward to marine basins of Western South America or eastward to the Atlantic Ocean. Also the Gurupá Arch remained high up to the end of Middle Miocene, directing this drainage system westward. With the late subsidence and breaching of the Gurupá Arch and a major fall in sea level, at the beginning of the Late Miocene, the Amazon River quickly opened its pathway to the west, from the Marajó Basin, through deep headward erosion, capturing a vast drainage network from cratonic and Andean areas, which had previously been diverted towards the Caribbean Sea. During this time, the large siliciclastic influx to the Amazon Mouth (Foz do Amazonas) Basin and its fan increased, due to erosion of large tracts of South America, linking the Amazon drainage network to that of the Marajó Basin. This extensive exposure originated the Late Miocene (Tortonian) unconformity, which marks the onset of the transcontinental Amazon River flowing into the Atlantic Ocean.

An extensive reef system at the Amazon River mouth.

Large rivers create major gaps in reef distribution along tropical shelves. The Amazon River represents 20% of the global riverine discharge to the ocean, generating up to a 1.3 × 10(6)-km(2) plume, and extensive muddy bottoms in the equatorial margin of South America. As a result, a wide area of the tropical North Atlantic is heavily affected in terms of salinity, pH, light penetration, and sedimentation. Such unfavorable conditions were thought to imprint a major gap in Western Atlantic reefs. We present an extensive carbonate system off the Amazon mouth, underneath the river plume. Significant carbonate sedimentation occurred during lowstand sea level, and still occurs in the outer shelf, resulting in complex hard-bottom topography. A permanent near-bottom wedge of ocean water, together with the seasonal nature of the plume's eastward retroflection, conditions the existence of this extensive (~9500 km(2)) hard-bottom mosaic. The Amazon reefs transition from accretive to erosional structures and encompass extensive rhodolith beds. Carbonate structures function as a connectivity corridor for wide depth-ranging reef-associated species, being heavily colonized by large sponges and other structure-forming filter feeders that dwell under low light and high levels of particulates. The oxycline between the plume and subplume is associated with chemoautotrophic and anaerobic microbial metabolisms. The system described here provides several insights about the responses of tropical reefs to suboptimal and marginal reef-building conditions, which are accelerating worldwide due to global changes.

A test of the cosmogenic10Be(meteoric)/9Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin

We present an extensive investigation of a new erosion and weathering proxy derived from the 10Be(meteoric)/9Be(stable) ratio in the Amazon River basin. This new proxy combines a radioactive atmospheric flux tracer, meteoric cosmogenic 10Be, with 9Be, a trace metal released by weathering. Results show that meteoric 10Be concentrations ([10Be]) and 10Be/9Be ratios increase by >30% from the Andes to the lowlands. We can calculate floodplain transfer times of 2–30 kyr from this increase. Intriguingly however, the riverine exported flux of meteoric 10Be shows a deficit with respect to the atmospheric depositional 10Be flux. Most likely, the actual area from which the 10Be flux is being delivered into the mainstream is smaller than the basin-wide one. Despite this imbalance, denudation rates calculated from 10Be/9Be ratios from bed load, suspended sediment, and water samples from Amazon Rivers agree within a factor of 2 with published in situ 10Be denudation rates. Erosion rates calculated from meteoric [10Be], measured from depth-integrated suspended sediment samples, agree with denudation rates, suggesting that grain size-induced variations in [10Be] are minimized when using such sampling material instead of bed load. In addition, the agreement between erosion and denudation rates implies minor chemical weathering intensity in most Amazon tributaries. Indeed, the Be-specific weathering intensity, calculated from mobilized 9Be comprising reactive and dissolved fractions that are released during weathering, is constant at approximately 40% of the total denudation from the Andes across the lowlands to the Amazon mouth. Therefore, weathering in the Amazon floodplain is not detected.

Weathering Intensity in Lowland River Basins: From the Andes to the Amazon Mouth

Actively eroding mountains supply un-weathered material into large river basins. It is still not known whether these un-weathered minerals undergo significant chemical weathering during storage in continental alluvial deposits within the surrounding lowland areas. Here we use previously reported weathering and erosion fluxes of rivers from the Amazon Basin to assess this effect. We show that the fraction of total denudation (weathering plus erosion) occurring as dissolved export experiences only a slight increase during transfer through the lowlands. The overall low weathering intensity of Andean sediments throughout the Amazon basin is attributed to the fact that source rocks are recycled meta-sedimentary rocks.

A New Cryptic Species of South American Freshwater Pufferfish of the Genus Colomesus (Tetraodontidae), Based on Both Morphology and DNA Data

The Tetraodontidae are an Acantomorpha fish family with circumglobal distribution composed of 189 species grouped in 19 genera, occurring in seas, estuaries, and rivers between the tropical and temperate regions. Of these, the genus Colomesus is confined to South America, with what have been up to now considered only two species. C. asellus is spread over the entire Amazon, Tocantins-Araguaia drainages, and coastal environments from the Amazon mouth to Venezuela, and is the only freshwater puffers on that continent. C. psittacus is found in coastal marine and brackish water environments from Cuba to the northern coast of South America as far south as to Sergipe in Brazil. In the present contribution we used morphological data along with molecular systematics techniques to investigate the phylogeny and phylogeography of the freshwater pufferfishes of the genus Colomesus. The molecular part is based on a cytochrome C oxidase subunit I dataset constructed from both previously published and newly determined sequences, obtained from specimens collected from three distinct localities in South America. Our results from both molecular and morphological approaches enable us to identify and describe a new Colomesus species from the Tocantins River. We also discuss aspects of the historical biogeography and phylogeography of the South American freshwater pufferfishes, suggesting that it could be more recent than previously expected.

Erratum for “Physical and Numerical Modeling of the Entrainment by a High‐Concentration Mud Suspension” by A. W. Bruens, J. C. Winterwerp, and C. Kranenburg

This paper presents the results of a physical and numerical model study on the entrainment by a high-concentration, cohesive sediment suspension. This turbulent, near-bed suspension is referred to as a concentrated benthic suspension (CBS). In the current configuration, the CBS entrains low-concentrated overlying water. The results of the physical and numerical experiments are in line with the results of similar experiments, published in literature, in which, in contrast, the upper, turbulent layer entrains water and sediment from the lower layer. The entrainment rate for these experiments is almost inversely proportional to the bulk Richardson-number E∝Ri*-n, with n≈1, indicating a profound effect of the side wall of the experimental facility. We anticipate that the conditions studied in this paper are characteristic for the behavior of many high-concentration systems, such as the mouth of the Amazon River, and in the Loire Estuary and Ems River. A sequence of entrainment during high flow velocities, and settling/consolidation during low flow velocities, driven by tidally induced horizontal pressure gradients, keeps the sediment fluid and mobile; this process can be referred to as tidal pumping. The experimental results could be reproduced to a fair degree by a 1DV point model. This, however, does not necessarily imply that state-of-the-art numerical engineering models are readily applicable for simulations of the hydro-sedimentological conditions in complicated systems, such as the Amazon mouth and Loire Estuary. In these systems, one expects a gradual transition from a fully turbulent upper part of the CBS layer, to laminar conditions deeper down within this layer, with increasing values of viscosity and yield strength; such transition is not accounted for in state-of-the-art turbulence models. Yet, the current work provides a framework for a better understanding of these systems.

Time-variations of equivalent water heights'from Grace Mission and in-situ river stages in the Amazon basin

Gravity Recovery and Climate Experiment (GRACE) mission is dedicated to measuring temporal variations of the Earth's gravity field. In this study, the Stokes coefficients made available by Groupe de Recherche en Géodésie Spatiale (GRGS) at a 10-day interval were converted into equivalent water height (EWH) for a ~4-year period in the Amazon basin (from July-2002 to May-2006). The seasonal amplitudes of EWH signal are the largest on the surface of Earth and reach ~ 1250mm at that basin's center. Error budget represents ~130 mm of EWH, including formal errors on Stokes coefficient, leakage errors (12 ~ 21 mm) and spectrum truncation (10 ~ 15 mm). Comparison between in situ river level time series measured at 233 ground-based hydrometric stations (HS) in the Amazon basin and vertically-integrated EWH derived from GRACE is carried out in this paper. Although EWH and HS measure different water bodies, in most of the cases a high correlation (up to ~80%) is detected between the HS series and EWH series at the same site. This correlation allows adjusting linear relationships between in situ and GRACE-based series for the major tributaries of the Amazon river. The regression coefficients decrease from up to down stream along the rivers reaching the theoretical value 1 at the Amazon's mouth in the Atlantic Ocean. The variation of the regression coefficients versus the distance from estuary is analysed for the largest rivers in the basin. In a second step, a classification of the proportionality between in situ and GRACE time-series is proposed.

Multiple remote sensing techniques as a tool for reconstructing late Quaternary drainage in the Amazon lowland

AbstractMorphologies related to paleochannels are widespread through large areas of the Brazilian Amazonia. Despite the relevance for reconstructing the evolution of the largest fluvial system on Earth, detailed mapping of most of these features is still unavailable. The lack of works emphasizing Amazonian paleochannels represents an enormous bias toward the understanding of both migrating dynamics of Amazonian rivers through time and processes (e.g. tectonics, climate, sea‐level fluctuation) that might have promoted channel abandonment. A few previous studies have demonstrated the successful application of digital elevation model (DEM) collected during the Shuttle Radar Topography Mission (SRTM) for mapping morphologies related to paleochannels still preserved under forested lowland Amazonian areas. The present study applies different remote sensing techniques, including JERS‐1 90m, SAR/SIPAM‐6m, Landsat, and ASTER, to successfully unravel a myriad of paleochannels in Marajó Island (Amazon mouth), which were previously undetected using solely SRTM data. The results presented in this work are promising for mapping paleodrainage in other forested areas of the Amazonas basin. Considering the several problems involved on data acquisition aiming to approach the evolution of Amazonian rivers in space and time (e.g. large dimension, lack of natural geological exposures, difficult access, dense forest cover, and high cloud volume), a proper mapping of Amazonas paleodrainage using different remote sensing techniques is crucial to visualize potential paleochannel morphologies and patterns that are of high relevance for reconstructing the Amazonian fluvial dynamics through time. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The impact of Quaternary sea level changes on the evolution of the Amazonian lowland

AbstractCoring and acoustic surveying (3·5 kHz) in both rivers and lakes in Central Amazonia provide additional insights into the Late Quaternary hydrological and sedimentological development of the Amazon River and its tributaries. Erosion and accumulation phases were found to be linked to Quaternary sea level changes. The low sea level phase during the last glacial maximum caused deep incision of the Amazon River and erosion in major tributaries such as the Rio Negro, 1500 km upstream from the Amazon mouth. A 3·5‐kHz profiling suggests a lowering of the water level by at least 30 m at Manaus. During that phase, the slope of the Amazon valley must have increased, resulting in an increase in bed load transport capacity. The subsequent sea level rise caused a backwater effect far upstream, with silting up of the Amazon valley and the tributary inflows. Former river systems changed into Ria lakes. The floodplains of the Amazon River, the várzea, were formed approximately 5000 years ago when the sea level approached its present‐day level. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Circulation of the thermocline salinity maximum waters off the Northern Brazil as inferred from in situ measurements and numerical results

Abstract. High resolution hydrographic observations of temperature and salinity are used to analyse the subsurface circulation along the coast of North Brazil, off the Amazon mouth, between 2° S and 6° N. Observations are presented from four cruises carried out in different periods of the year (March–May 1995, May–June 1999, July–August 2001 and October–November 1997). Numerical model outputs complement the results of the shipboard measurements, and are used to complete the descriptions of mesoscale circulation. The Salinity Maximum Waters are here analyzed, principally in order to describe the penetration of waters originating in the Southern Hemisphere toward the Northern Hemisphere through the North Brazil Current (NBC)/North Brazil Undercurrent (NBUC). Our results show that, if the Equatorial Undercurrent (EUC) is fed by Northern Atlantic Waters, this contribution may only occur in the ocean interior, east of the western boundary around 100 m depth. Modeling results indicate a southward penetration of the Western Boundary Undercurrent (WBUC) below the thermocline, along the North Brazilian coast into the EUC or the North Equatorial Undercurrent (NEUC) (around 48° W–3° N). The WBUC in the region does not flow more south than 3° N. The northern waters are diverted eastward either by the NBC retroflection or by the northern edge of the associated clockwise rings. The existence of subsurface mesoscale rings associated to the NBC retroflection is evidenced, without any signature in the surface layer, so confirming earlier numerical model outputs. These subsurface anticyclones, linked to the NBC/NBUC retroflection into the North Equatorial Undercurrent and the EUC, contribute to the transport of South Atlantic high salinity water into the Northern Hemisphere.

Impact of sea-level and climatic changes on the Amazon coastal wetlands during the late Holocene

Wetland dynamics in northern Brazil during the Holocene were studied by pollen analysis and AMS radiocarbon dating of three cores. Near the Amazon mouth region, covered mainly by primary Amazon coastal forest and herbaceous vegetation, the pollen record indicates the dominance of mangroves between 4800 and 1100 cal yr b.p. A contraction of the mangrove area and an expansion of herbaceous and fern vegetation occurred between 1100 and 750 cal yr b.p. The period between 750 and 200 cal yr b.p. is characterized by an expansion of mangrove and a decrease in herbaceous and fern vegetation. This trend continued until the present. On Atalaia Island, the sediment core indicates a period with poor pollen preservation between 830 and 630 cal yr b.p. Between 630 and 330 cal yr b.p., mangroves expanded. Later, up to 45 cal yr b.p., the mangrove area decreased and the herbaceous vegetation expanded. During the last hundred years, the relative sea-level rise most probably favored the mangrove expansion as far as the topographically highest sector on this island, while the herbaceous vegetation decreased. The pollen data from Agua Preta Lake indicate dry conditions, as reflected by the poor pollen preservation between 390 and 240 cal yr b.p. Between 240 and 60 cal yr b.p., restinga and Amazon coastal forest with palms dominated this region. For the last 120 years, the record indicates an expansion of the mangrove area. However, recent confinement of mangrove development to the topographically highest area, and the loss of mangrove areas on the lowest surfaces have led to a net loss of mangrove coverage during the last decades.

A pesca do tamoatá Hoplosternum littorale (Hancock, 1828) (Siluriformes: Callichthyidae) na ilha de Marajó

Hoplosternum littorale (Hancock, 1828) (Siluriformes, Callichthyidae) é um peixe de médio porte conhecido na Amazônia brasileira como tamoatá. Apresenta respiração acessória, o que permite ocupar as extensas áreas pantanosas da foz dos rios Amazonas e Orinoco. O tamoatá é o principal recurso pesqueiro da ilha de Marajó, sendo capturado principalmente nos campos alagados da bacia do rio Arari, no município de Santa Cruz do Arari (PA). Seu desembarque representa 6% do total no porto do Ver-o-Peso, em Belém (PA), o principal da Amazônia oriental. A pesca do tamoatá é marcadamente sazonal, ocorrendo na estação seca. De julho a agosto, é mais intensa no rio e lago Arari e de outubro a novembro, nos poços das fazendas, que são os últimos a secarem na região. A pesca é feita por pescadores locais, que vendem sua produção para as geleiras. Estas são embarcações de madeira, com urnas de gelo que levam a produção para ser comercializada nos principais portos urbanos, em especial o do Ver-o-Peso. A pesca é aqui descrita com base em observações de campo e entrevistas estruturadas com pescadores comerciais e fazendeiros da região. A frota de geleiras que desembarca o tamoatá em Belém é composta por 415 embarcações com capacidade de urna de até 27 t. As pescarias são realizadas, basicamente, por redes de emalhar de monofilamento e de cerco de multifilamento.

Heterogeneity in intertidal fish fauna assemblages along the world’s longest mangrove area in northern Brazil

The first comparison of the intertidal fish fauna from three estuaries within the longest contiguous mangrove area of the world, in north Brazil, revealed a significant heterogeneity in the species composition and functional and ecological guilds with an increase of Tetraodontidae and Engraulidae and a decrease of Sciaenidae and Mugilidae towards the Amazon mouth. A classification of the mangrove estuaries to identify types of estuaries and coastal sectors is urgently required to provide a tool for regional management systems in order to improve research, monitoring and conservation of this unique mangrove ecosystem.

Pathways of clay mineral transport in the coastal zone of the Brazilian continental shelf from Ceará to the mouth of the Amazon River

The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceara to the Amazon River mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to be, similarly to that for the shelf between Ceara and the Amazon mouth, a mixing of Amazon sediments with suspended material of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally, relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence, stronger sedimentation of Amazon sediments would occur only west of the river mouth.

Tracing Amazon River water into the Caribbean Sea

Monthly Amazon River discharge is correlated to historical monthly sea surface salinity (SSS) in the western tropical Atlantic Ocean and the Caribbean Sea. At Barbados a very high inverse correlation (R 2 0.92) exists if the discharge is lagged by two months, which corresponds to the travel time from the Amazon mouth to Barbados. Due to its proximity to Barbados, a small amount of the correlation can also be attributed to the Orinoco River. Between Barbados and the central Caribbean Sea (75W) the peak correlation of Amazon River discharge and SSS occurs at progressively longer lag times, representing the longer travel time. The correlation is highest at Barbados and diminishes with distance downstream. Downstream of the central Caribbean Sea (75W) no correlation is evident, as the Amazon water becomes too diluted to be clearly defined by available data. The results of the analysis are used to calculate surface current speeds of 0.34 0.09 m/s from the Amazon mouth to Barbados and 0.10 0.02 m/s from Barbados to the central Caribbean Sea (75W). Downstream of Barbados a subsurface maximum in correlation develops. The correlation is eroded more strongly at shallower depths due to more intense surface processes (e.g., evaporation and precipitation).

Spectral remote sensing reflectances of coastal waters in French Guiana under the Amazon influence

The French Guiana coast, located about 500 km north of the Amazon mouth, is characterized by mangroves and large mud banks migrating northwestwards. These banks are composed of fine clay sediments originating from the Amazon River. In November 1998, field work was carried out on the Mahury River to measure the water colour and the concentration of suspended particles in surface water. An increase of the remote sensing reflectance due to high concentration of mineral particles was observed. The Mahury River survey (November 1998), together with an oceanographic survey onboard the ANTEA vessel (PRECHICO cruise, April 1999), enabled us to identify several classes of remote sensing reflectance spectra characterizing the continental shelf between Cayenne and the Brazil frontier: (1) light blue waters above the shelf break; (2) low salinity dark brown waters above the continental shelf, originating from the Amazon River; (3) green waters above the 20-m isobath; and (4) beige waters close to the coast and in the rivers mouths. The normalized water leaving radiances (nLw) computed from these spectra in the spectral bands of SeaWiFS are examined. The standard OC2 performs poorly in retrieving chlorophyll from the reflectance data because of the relatively high suspended particulate matter concentrations.

Recording changes in ENADW composition over the last 340 ka using high-precision lead isotopes in a Fe–Mn crust

We report high-precision Pb triple spike data and U–Th isotopic ages on a high-resolution depth profile from a Fe–Mn crust (121DK, northeastern Atlantic 24°N, 21°W, 2000 m). Our results bring important information particularly on the origin of the Pb isotopic composition of the northeastern Atlantic deep water and more generally on the use of Pb isotopes as a paleo-proxy to reconstruct weathering inputs to the north Atlantic Ocean. Growth rates determined using ( 230Th) ex and ( 230Th/ 232Th) ex (2.9±0.2 and 3±0.3 mm/Ma respectively) are found to be more reliable than ( 234U ex), sensitive to sea water contamination, and are similar to the 10Be results (3±0.3 mm/Ma). Very small variations in 206Pb/ 204Pb (18.942–18.964), 208Pb/ 204Pb (39.057–39.083) and 207Pb/ 204Pb (15.696–15.701) are resolved, defining linear trends in Pb isotope diagrams. High-precision Pb triple spike analysis of samples from crust BM1969.05, northwestern Atlantic (39°N, 61°W, 1829 m), are found to entirely reproduce the multi-collector plasma source mass spectrometry data recently published. This interlaboratory cross-calibration establishes that the west north Atlantic deep water (WNADW) does not represent a suitable Pb contributor to the northeastern Atlantic and rather suggests that the Pb isotopic composition of WNADW is significantly altered before or during transport toward the northeastern Atlantic basin. The Pb isotopic compositions of both suspended loads from the Amazon mouth and offshore sediment cores are consistent on average with crust 121DK. We therefore propose that the change in Pb isotopic composition of WNADW may be caused by input of suspended particle loads from the Amazon and possibly Orinoco rivers or by interaction with sediments deposited by those rivers. We speculate further that the short-term compositional variations observed in this study may be caused by fluctuations in the relative contributions of Amazon tributaries to the bulk composition supplied to the ocean because these tributaries sample terrains of drastically different ages (Phanerozoic Andes and Precambrian Amazonian shield).

In search of zonal circulations in the equatorial Atlantic sector from the NCEP–NCAR reanalysis

AbstractThe National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 1958–1997 upper‐air dataset has been evaluated for evidence of equatorial zonal circulation cells over the Atlantic and adjacent continents. For January, April, July and October, maps are presented of mid‐tropospheric vertical motion, upper‐tropospheric divergent flow, and zonal–vertical cross‐sections of vertical and divergent zonal motion and total zonal flow. In the boreal winter half‐year, a centre of intense ascending motion and upper‐tropospheric, mainly northward‐directed outflow is located off the mouth of the Amazon. From this centre there is also some outflow into centres of upper‐tropospheric convergence and subsidence over the equatorial eastern Pacific and eastern Atlantic, respectively. From January to April, the near‐equatorial band of ascending motion shifts southward, and the upper‐tropospheric convergence centre is displaced from the Equator into the South Atlantic. In the boreal summer half‐year, the band of strongest ascending motion is displaced northward, and two separate centres of upper‐tropospheric divergent outflow are found over northern hemispheric Africa and the Central American Seas. From these centres, the outflow is directed approximately southward into the southern hemisphere. The analysis points to the existence of an equatorial zonal circulation cell in the Atlantic sector confined to around January. Copyright © 2001 Royal Meteorological Society

Influence of the Amazon River runoff on the tropical atlantic

The Amazon river has the biggest flow in the world, 0.2 Sv (1 Sv = 106 m3/s), and is responsible for a large part of the low Sea Surface Salinity (SSS) in the west tropical Atlantic ocean. Very few Ocean General Circulation Model (OGCM) include runoff which brings a specific contribution to the Ocean physics. Comparison of simulations with constant or monthly runoffs shows that the spread of the Amazon fresh waters offshore of the north Brazilian coast is controlled by the ocean circulation and not by the Amazon flow. Therefore, in the model like in the observations, SSS minimum is observed in summer three months after the Amazon flood. In agreement with observations, a thick (more than 40 m) Barrier Layer (BL) is present every summer north of the Amazon mouth. Because of the strong and shallow salinity gradient associated with the Amazon freshwater, an important part of the solar radiation is trapped in the BL and creates an inversion of the vertical gradient of temperature. However with this forced model, BL does not seem to have a clear impact on SST and so on the air-sea interaction. Freshwater flux is also able to bend the sea surface. The geostrophic part of the North Brazilian Current (NBC) retroflection is then lightly weaker in presence of the Amazon runoff.