Eastern Tropical Atlantic Research Articles

El Niño in 1991–1992 and its manifestations in the tropical Atlantic

A variety of anomalous conditions in the tropical Pacific and Atlantic oceans during the 1991–1992 El Nino event are described. The current El Nino is shown to have specific features, evident in the behaviour of the southern oscillation index and sea surface temperature of the equatorial Pacific Ocean. Also, it was accompanied by warming of the eastern tropical Atlantic. Temperature anomalies of the tropical Atlantic surface water temperature reached their maximum (>3°C) in May–June 1991, i.e. 2–3 months earlier than in the Pacific Ocean.

The internannual temperature and salinity variations in the eastern tropical Atlantic

The internnual variations of the parameters of the temperature and salinity seasonal variability in the ocean active layer are described based on the oceanographic measurements carried out during 1963–1975. Some peculiarities of the seasonal evolution of theT, S indices in the anomalous years have been revealed.

Comparison of Two Satellite-based Rainfall Algorithms Using Pacific Atoll Raingage Data

Abstract Rainfall estimates for two simple satellite-based rainfall algorithms are verified over the tropical Pacific using a new method that incorporates sparsely distributed raingages. The resulting linear regression relationship between monthly areal rainfall and the highly reflective cloud index agrees with earlier results. However, the GOES precipitation index (GPI), which was calibrated using radar rainfall data obtained from the eastern tropical Atlantic, produces biased areas rainfall estimates over most of the tropical Pacific. However, its precision is greater than the highly reflective cloud index, perhaps due to the GPI's larger spatial dimensions. With the incorporation of calibration coefficients determined in this study, the GPI will produce unbiased estimates of areal rainfall for the tropical Pacific region.

Larval age, growth and mortality in the oceanic squid Sthenoteuthis pteropus (Cephalopoda, Ommastrephidae) from the eastern tropical Atlantic

An investigation was carried out on larvae of the oceanic tropical squid Sthenoteuthis pteropus in the equatorial Atlantic (2°30′N–7°S;12°W –8°30′E) The age of the larvae was calculated from the statolith microstructure of 20 larvae; mortality was estimated from the size structure of 1128 larvae. The larval stage lasts 32–38 days. At ages ranging from 14 to 38 days. the daily relative growth rates of mantle length decrease from 7.5 to 2.8% day −1 and from 14–16 to 5.8% of body weight day −1 At age 12–24 days, mortality rates were estimated using both raw catch data and corrected data accounting for net avoidance. The mean value of raw mortality rates was 0.189, the corrected value was 0.158. During the proboscis division (transformation of the larva into juvenile) at age 25–35 days, a sharp decrease in larval growth rates and a simultaneous increase in mortality rates (raw 0.443, corrected 0.379) were observed.

Evidence of forest extension in west Africa since 22,000 BP: A pollen record from the eastern tropical Atlantic

Based on a pollen study of Atlantic deep-sea core KS12 (3°52′06″ N-1°56′16″ W; 544 cm length; 2,955 m water depth) a continental paleoenvironmental reconstruction is proposed for the Gulf of Guinea for the time interval 22,000 BP-6500 BP. Repeated freshwater inputs by nearby coastal rivers were responsible for pollen transport from tropical humid environments during the whole period. Strong northerly trade circulation during the time interval 22,000–10,500 BP, with a maximum intensity at ca. 17,000 BP was responsible for pollen transport from Saharan and Sahelian environments which were located more to the south at that time. Near the coast, Guinean swamp and lowland forests were always present. They were particularly well developed around 22,000 BP, and later around 8,500 BP. After 17,000 BP, the progressive replacement of (semi)-arid elements (Sahelian and Saharan) by more humid Sudanian, Sudano-Guinean and Guinean associations indicates the onset of the modern vegetation zonation in West Africa.

Chlorofluoromethanes in South Atlantic Antartic intermediate water

Distribution of the dissolved atmospheric chlorofluoromethanes (CFMs) F-11 and F-12 in the South Atlantic Ocean are used to study the ventilation and circulation of Antarctic Intermediate Water (AAIW). CFM distributions on an isopycnal surface representative of AAIW are consistent with recently ventilated water entering the subtropical gyre in the southwestern Atlantic and then being advected anticyclonically around this gyre. The westward-flowing northern limb of the gyre apparently divides near the coast of South America with some water flowing southward to recirculate in the gyre, and the balance flowing northward along the coast of Brazil. At the equator this northward current divides again with one branch going eastward along the equator and the other continuing into the Northern Hemisphere. In the eastern tropical Atlantic, the CFM concentrations on this isopycnal surface in the cyclonic gyre are extremely low between the subtropical gyre and the equatorial tongue. Along the prime meridian, the F-11 and F-12 concentrations on the 27.2 θσ isopycnal surface between the mixed layer outcrop and the northern edge of the subtropical gyre are fitted to a one-dimensional advection-diffusion model. This model assumes that the CFMs enter the tubtropical gyre solely by northward diffusion from the mixed layer outcrop to the southern edge of the subtropical gyre, and that their distributions within the gyre are controlled by both advective and diffusive processes. Velocity and eddy diffusion coefficients are calculated from a least-squares fit to the data. These values are then used to calculate a mean oxygen consumption rate which is consistent with rates calculated using models of other time-dependent geochemical tracers.

BIOGEOGRAPHY OF CLADOPHOROPSIS MEMBRANACEA (CHLOROPHYTA) BASED ON COMPARISONS OF NUCLEAR rDNA ITS SEQUENCES1

ABSTRACTNucleotides were compared at 988 sites, spanning both internal transcribed spacers (ITS1 and ITS2) of the nuclear ribosomal DNA, among 17 isolates of the green alga Cladophoropsis membranacea (Hofman Bang ex C. Agardh) Boergesen and two isolates of Struvea anastomosans (Harvey) Piccone and Grunow. Collections were made from Bonaire, Curaçao, St. Croix, the Canary Islands, the Cape Verde Islands, Mauritania, Syria, the Red Sea, Okinawa, and Hawaii. Two nucleotide substitutions were found between the sequenced coding regions of C. membranacea and S. anastomosans. Of the 720 nucleotides compared in ITS1 and ITS2, an average of 6.7% sequence divergence was found within C. membranacea and 17.4% between C. membranacea and S. anastomosans. Sequences were analyzed using maximum parsimony. Phylogenetic hypotheses were compared with the biogeographic hypothesis of an east‐west Tethyan vicariance. Results of the study allow the identification of widely dispersed biogeographic populations, the identification of an underlying Tethyan imprint, and support for the hypothesis that dispersal has occurred between the Caribbean and the tropical eastern Atlantic. These results demonstrate the usefulness of ITS sequences at the sub‐specific level in C. membranacea.

Intra and interspecific association of large pelagic fishes inferred from catch data of surface longline

This paper analyses the distribution of longline catches of swordfish, Xiphias gladius, and six species of pelagic elasmobranchs, during a fishing voyage in the tropical eastern Atlantic. Seven scales of sampling units, ranging from about 0.72 to 7.2 km were used. Observed frequencies of appearance of each species in the basic sampling unit, were compared with theoretical frequencies obtained from the Poisson and negative binomial distributions. For the study of the interspecific relationships, contingency tables were used and the degree of association was estimated using the χ2test and the coefficient v of Kendall. The catch distribution of the swordfish and the devil ray (Mobula sp.) can be explained by both randomness and a slight trend to association. For the pelagic sharks, a clear trend towards intraspecific clustering was found. Catches of two species of the genus Carcharhinus showed the most positive trends to interspecific association, especially in short ranges, probably due to attraction to previously hooked fish. The possible causes of these distributions are discussed.

Community structures of cold and low-nutrient adapted heterotrophic sediment bacteria from the deep eastern tropical Atlantic

Community structures of cold and low-nutrient adapted heterotrophic sediment bacteria from the deep eastern tropical Atlantic

Chronostratigraphy of Plio-Pleistocene Sequences, Offshore Louisiana

Plio-Pleistocene cool stages correlate with sequence boundaries in the Gulf of Mexico, offshore Louisiana. The cool/warm stages are defined by planktonic foram assemblages in the Vema 16-205 core in the eastern tropical Atlantic. The mean duration of each climatic stage was 230,000 years. These climatic stages were incorporated into a composite standard section with oxygen isotope events, planktonic foraminiferal and nannofossil datums, and magnetostratigraphic events from oceanic core holes: DSDP 502 in the Columbia basin, Caribbean; DSDP 552 on the Rockall Plateau, northeast Atlantic; DSDP 572C in the equatorial Pacific; Vema 28 core 219 on the Solomon Rise, west Pacific; and the Eureka 57 core 135 in DeSoto Canyon, offshore Florida. The oxygen isotope events and the paleontologic datums were identified in two exploration wells in the Green Canyon block, offshore Louisiana, and correlated with seismic sequence boundaries. Iterative graphic correlation with the composite standard refines the positions of the tops of the isotope events where data noise makes the pick equivocal providing a precise, high-resolution correlation tool. Each oxygen isotope event represents approximately 31,000 years. The sequences in the Green Canyon area are from 300 m to 800 m thick, and were deposited at rates from 800 to 1500more » cm/ka. The duration of the hiatuses at the sequence boundaries range from 27,000 to 200,000 years. These seismic/depositional sequences were deposited during the alter parts of the cool stage and the warm climates, which indicate rising sea level to highstand. The ages of the sequence boundaries are approximately 500,000 years, 900,000 years, 1.5 Ma, 2.0 Ma, 2.7 Ma, 3.0 Ma, and 3.75 Ma.« less

Monts sous-marins et thons dans l'Atlantique tropical est

It is well known by fishermen that large biomasses of tuna are sometimes concentrated on sea mounts. Many sea mounts are known in the eastern tropical Atlantic. This phenomenon has surprisin- gly never been studied by scientists. The present paper makes a review of tuna catches by the international tuna fleets during recent years (1980 to 1987) in the immediate vicinity of each of the sea mounts known in'' the area. Several thousand tons of tuna can be taken yearly on some sea mounts. Other sea mounts are apparently always poor in tuna, even when they are located in regular fishing areas. The major biological characteristics of the association between sea mount and tuna are analyzed three species - yellowfin (Tlzunnus albacares), skipjack (Katsuwonus pelamis) and bigeye tuna (Paratlzunnus obesus) - are usually taken, in similar proportions on the sea mounts. Those tuna are smaller on the sea mount compared to adjacent areas. Some hypotheses are developed to explain the causes of this concentration phenomenon and the dynamics of this aggregation, using the daily catch and effort statistics of fisheries, and the results of regular and sonic tagging. The prospects for future research on this topic of tuna aggregation on sea mounts, and the rational exploitation of those structures are discussed. As the exact locations and depth of the sea mounts are presently easily known by satellite imagery, it should be possible, in the immediate future, to know the exact positions of all the sea mounts in the area, which could help the fishermen to fish more efficiently.

Cyclonic gyre in the tropical South Atlantic

A cyclonic gyre within the eastern tropical South Atlantic is resolved by an extensive oceanographic station array obtained in 1983 and 1984. The gyre is centered near 13°S and 5°E with a sea surface relief relative to 1500 decibars (db) of 8 dyn cm. The 500 db surface relative to 1500 db reveals a much diminished cyclonic circulation, shifted slightly to the south. The weak baroclinic expression of the cyclonic gyre is confined for the most part to the upper 300 db, with a surface characteristic speed of only 3 cm s-1 . A transport of 5 × 106 m3 s-1 across a line from the gyre center to the African continental margin, including the Angola Current, may best depict the gyre 0-300 db transport (relative to 1500 db). Ship drift data fvr the region do not show the presence of the cyclonic gyre because the wind-induced Ekman layer masks the gyre. The thermocline of the cyclonic gyre is significantly saltier and lower in oxygen than the suspected source water: the main thermocline of the South Atlantic subtropical gyre. A strong front near 18°S, the Angola-Benguela Front, separates the cyclonic gyre regime from that of the subtropical gyre. Using the regional freshwater balance, gyre thermocline residence time is determined to be between 4.4 and 8.5 years, implying an oxygen utilization of 0.3-0.5 mll-1 y-1 . Below the thermocline there is some evidence for southward Plow of North Atlantic Deep Water along the eastern boundary. It is inferred that this flow is fed by eastward spreading of North Atlantic Deep Water along the equatorial belt.

Evidence for a change in the periodicity of tropical climate cycles at 2.4 Myr from whole-core magnetic susceptibility measurements

THE Earth's orbit about the Sun is influenced by gravitational interactions with the Moon and other planets. The resulting orbital perturbations give rise to cyclical variations in orbital eccentricity, obliquity and precession with periods of 100, 41, 19 and 23 kyr, respectively1. These variations are climatically important because they affect the seasonal and latitudinal distribution of solar radiation. Magnetic-susceptibility measurements in deep-sea sediment cores can be used as a sensitive indicator of temporal variations in the concentration of terrigenous material supplied to the sea bed. Here we use this approach to study the evolution of cycles of terrigenous sedimentation at Ocean Drilling Program sites in the Arabian Sea and the eastern tropical Atlantic. We show, from spectral analyses of the two susceptibility time series spanning the past 3.2–3.5 Myr, that the records are driven strongly by orbital forcing. Before 2.4 Myr before present (BP), both records are strongly influenced by the 23- and 19-kyr periodicities; but after 2.4 Myr, they are both significantly influenced by the 41-kyr periodicity. This shift coincides with the beginning of major Northern Hemisphere glaciation, which indicates, for the Indian Ocean site, that the supply of terrigenous sediment (carried by monsoonal winds) has responded to the rapid increase in ice cover at this time. We cannot, however, exclude possible effects of changes in bottom-water circulation on the susceptibility record of the Atlantic site.

New species of Corcyrogobius, Thorogobius and Wheelerigobius from West Africa (Teleostei: Gobiidae)

Three new West African species of gobiid fishes are described from the Gulf of Guinea in the tropical eastern Atlantic. These are Corcyrogobius lubbocki sp. nov., Thorogobius rofeni sp. nov. and Wh...

A reassessment of Northern Atlantic seaweed biogeography

Abstract On the basis of 25 seaweed floras of both the North Atlantic Ocean and the Mediterranean Sea mutual similarity dendrograms have been obtained for whole floras as well as for major taxonomic groups. Nine biogeographical regions were recognized: Tropical Western Atlantic Region, Warm Western Atlantic Region, Cold Western Atlantic Region, Arctic Atlantic Region, Cold Eastern Atlantic Region, Warm Eastern Atlantic Region, Subtropical Eastern Atlantic Region, Tropical Eastern Atlantic Region, Mediterranean Region. With some minor differences the pattern in the various taxonomic groups is very similar. Statistical relationships have been sought between species numbers and several geographical features (latitude, longitude, average February seawater temperature, average August seawater temperature, tidal height and coastal length). For the whole flora (including the Rhodophyta), tidal height appears to be inversely related to species numbers, and average seawater temperature in August and February is di...

Towards the prediction of Sahelian rainfall from sea surface temperatures in the Gulf of Guinea

The possible link between sea surface temperature (SST) in specific areas of the Gulf of Guinea and summer Sahelian rainfall is investigated for the purpose of forecasting Sahelian rainfall. Analyses of data series of rainfall in the Sahel and SST in the Eastern Tropical Atlantic reveal that summer Sahelian rainfall is dependent on SST distribution, according to the period (dry or wet) and/or the specific oceanic area (coastal, equatorial or intermediate zone) under consideration. The occurrence of warm surface waters in the Gulf of Guinea is coincident with reduction of summer rainfall in the Sahel, mainly during dry periods. The same relationship does not hold during wet periods. It is suggested that summer Sahelian rainfall could be predicted: (a) from SST in the intermediate warm waters area and the equatorial upwelling zone 1–2 months earlier; (b) from previous quarterly rainfall data in the Sahel. DOI: 10.1111/j.1600-0870.1987.tb00287.x

Two new anthiine fishes from the eastern tropical Atlantic

Two anthiine fishes from the eastern tropical Atlantic are described as newHolanthias cyprinoides andAnthias helenensis. The former is distinguished from the other Atlantic species ofHolanthias in having the forked caudal fin with rounded lobes and from the Indo-Pacific species in having no elongated dorsal spines or soft rays. The latter is closely related toAnthias asperilinguis Gunther (South America, Atlantic coast), but differs from it in having more pectoral fin rays and more gill rakers. The present investigation onAnthias suggests that AtlanticAnthias is a genus distinct fromPseudanthias of the Indo-Pacific.

Single and multispecies systems: The the Eastern Tropical Atlantic

The commercial tuna fishery in the Eastern Tropical Atlantic is based on harvests from three species of tuna: Yellowfin, Skipjack, and Bigeye. Two models are developed to examine the bioeconomics of this fishery. In the first model, species are presumed ecologically independent and selectively harvested. In the second model, two of the species (Yellowfin and Skipjack) are assumed to be interspecific competitors and jointly harvested independent of the third species (Bigeye). Data on landings, effort, cost, and revenue are assembled allowing estimates of the bioeconomic parameters for the model where all species are independent and selectively harvested. Open access and bioeconomic equilibria are identified for a combination of cost and discount rates. The pristine, open access, and bioeconomic equilibria are also determined for the multispecies model based on parameters from the single species (independent) models and plausible values for interaction and joint production coefficients. Equilibria are compared, and management policies from a single- and multispecies perspective are explored.

Deep-sea sediments in the eastern equatorial Atlantic off the African coast and meteorological flow patterns over the Sahel

Deep-sea mineral dust deposits are a powerful tool allowing to deduct atmospheric flow patterns. Here deep-sea sediments from the eastern tropical Atlantic were used to derive climatic conditions over the African continent where in summer the dust is lifted in the transition zone of the Sahara and the Sahel. The meteorological process causing this aeolian dust injection are the squall lines, the rain bearing disturbances of the Sahel. They produce a high surface wind velocity area in their northern vicinity. The deep-sea deposits of sediments and their spatial patterns allowed to conclude that the dust raising mechanism connected with the squall lines were similar for present day conditions and for the time of the last glacial maximum. The Saharan trades and the African monsoon had about the same extension in either time slice coupled with an identical travel belt of the squall lines. However, the intensity and the frequency of the squall lines were lower in glacial times.

Two New Anthiine Fishes from the Eastern Tropical Atlantic

Two New Anthiine Fishes from the Eastern Tropical Atlantic