relationship between the equatorial and meridional modes of climatic variability in the tropical atlantic the tropical atlantic ocean exhibits two primary modes of interannual climate variability an equatorial mode analogous to but weaker than the pacific el nino phenomenon and a meridional mode that does not have a pacific counterpart the equatorial mode is responsible for warm and cold sea surface temperature sst events mainly in the gulf of guinea and is identifiable by abnormal changes in the equatorial thermocline slope resulting from zonal wind anomalies in the western tropical atlantic the meridional mode is characterized by a north south interhemispheric gradient of sst anomalies here it is shown using observed surface and subsurface oceanic temperatures that the meridional mode is linked to the equatorial mode at both decadal and short interannual 1 2 years time scales both modes involve north south displacements of the itcz as in the annual response

mapping climatological seasonal variations of surface currents in the tropical atlantic using ship drifts the seasonal variability of current velocities in the tropical atlantic was studied by grouping ship drift velocity observations into 2 5 boxes and calculating monthly mean velocity values these values were used to calculate and map the annual mean velocity the seasonal variation about the mean the annual and semiannual harmonics and the first two empirical orthogonal functions eofs the seasonal variation is primarily zonal in the equatorial band and in the north equatorial countercurrent necc and primarily alongshore near the coast of south america maxima of seasonal variation of 23 cm s are centered in the necc near 6 n 42 5 w and in the gulf of guinea near 2 n 7 5 w most 80 of the variance in the necc and along the western boundary of the area studied has an annual period most of the variance along the equator in the mid atlantic has a semiannual period over the whole region 49 of the seasonal variance is explained by the annual harmonic and 69 is explained by a combination of the annual and semiannual harmonics the second eof contains 29 of the variance of the data set and shows a simultaneous speeding slowing of the major equatorial currents the south equatorial current sec the north brazil current the necc and the guinea current along their principal axes of variation with a concurrent slowing speeding of the guyana current and the brazil current the pattern of variation resembles a large equatorial gyre centered along 4 n that flows clockwise from may to september and counterclockwise most of the rest of the year the first eof accounts for 35 of the variance and shows a simultaneous speeding slowing of eastward velocity from 5 s to 10 n except in the western sec the western end of the north brazil current and the guinea current the out of phase fluctuations apparent in the western sec near the equator are explained by the slowing and reversal of the south equatorial current near 2 n 40 w between april and june this slowing of the sec is associated with the start up of the necc which accelerates eastward in may june and july and flows eastward across the atlantic from july through december the sec divides seasonally near the eastern tip of brazil where residual alongshore velocities are northward for half the year peaking during may and june and southward for the other half of the year a second division can be seen north of the amazon where the north brazil current continues up the coast into the guyana current during the first half of the year but partially retroflects into the necc during the last half of the year a fast eastward flow in the necc coincides with below average velocities in the guyana current and above average velocities in the western sections of the north brazil current and the sec north of the equator

spatial and temporal patterns of benthic macrofaunal communities on the deep continental margin in the gulf of guinea abstract density taxonomic composition at higher taxon level and vertical distribution of benthic macrofaunal communities and sediment characteristics pore water nitrogen organic carbon sulfur c n ratio n alcohol biomarkers were examined at three deep sites on the congo gabon continental margin this study was part of the multidisciplinary biozaire project that aimed at studying the deep benthic ecosystems in the gulf of guinea sampling of macrofaunal communities and of sediment was conducted during three cruises january 2001 december 2001 and december 2003 at two downslope sites 4000 m depth one located near the congo submarine channel 15 km in the south and the other one far from the channel 150 km in the south the third area located 8 km north of the congo channel in the surroundings of a giant pockmark at 3160 m depth was sampled during one cruise in december 2003 at these three locations the macrofaunal communities presented relatively high densities 327 987 ind 0 25 m 2 compared with macrofaunal communities at similar depths that is due to high levels of food input related to the congo river and submarine system activities that affect the whole study area the communities were different from each other in terms of taxonomic composition at higher taxon level phylum class order for all the groups except for the polychaetes classified into families the polychaetes dominated the communities and were responsible for the increase in densities observed at both deep sites 4000 m between january 2001 and december 2003 whereas the tanaidaceans the isopods and the bivalves were the other most abundant taxa responsible for the spatial differences between these sites the community at 3150 m differed from the two deep communities by higher abundances in bivalves nemerteans and holothuroids the composition of the polychaete community also differed among sites in the vicinity of the congo channel the expected positive effect of the additional organic matter transported through the turbiditic currents on to the surrounding benthic communities was not observed as the increase in densities during the study period was higher at the site located away from the congo channel than near the channel 80 vs 30 that may be due to the low food value of the organic matter of terrestrial origin carried through the turbidites and or to the disturbance caused by these turbidites conversely far from the channel the macrofaunal communities benefit from organic matter of higher energetic value originating mainly from marine sources but also from continental sources carried by the congo plume or by near bed currents across or along the continental slope spatial and temporal variability in trophic and physical characteristics of the sediment habitat at both deep sites also affected the vertical distribution of the macrofaunal communities the activities of the very active congo system structure the deep macrofaunal communities on a large area in terms of densities composition and vertical distribution the food input is enhanced at regional scale as well as the heterogeneity of the sediment characteristics mainly in terms of organic matter quality marine vs terrigenous in turn the densities are enhanced as well as the regional diversity of the macrofaunal communities in terms of taxonomic composition and distribution

oceanic kelvin waves and tropical atlantic intraseasonal variability 1 kelvin wave characterization 1 oceanic kelvin waves from the equator to the west african coast are investigated in the framework of tropical atlantic intraseasonal variability in order to better highlight the wave propagations a 25 95 day band pass filter was applied to the sea surface height ssh product derived from the topex poseidon altimeter and an ocean general circulation model simulation for the 1993 2000 period in addition to equatorial eastward propagations our analysis reveals recurrent and continuous propagations distinguishable over thousands of kilometers poleward along the coasts as far as about 10 15 latitude a novel result with altimeter data the variance of the filtered ssh signal goes from 1 cm at the equator to 2 cm at the african coast estimates of the phase speed range from 1 5 to 2 1 m s along the equator and the west african coastline such values are very close to those of equatorial kelvin wave propagations likely dominated by the first two baroclinic modes supporting the fact that the coastal propagations are coastally trapped kelvin waves in order to simplify the description of these kelvin waves we present an intraseasonal climatology which reveals regular boreal autumn winter equator to coast propagations an improved description is achieved thanks to the computation of an extended empirical orthogonal function for the boreal autumn winter propagations lag correlation of ssh signals allows for a twofold quantification the phase speed and the importance of remote forcing along the coast the remote forcing effect of intraseasonal kelvin waves is clearly evidenced over coastal upwelling regions as far as 10 15 latitude the physical mechanism associated with the forcing of the kelvin waves and its impacts will be investigated in a future paper

ocean climate of the south east atlantic observed from satellite data and wind models the near coastal south east atlantic ocean off africa is a unique and highly dynamic environment comprising the cool benguela current warm angola current and warm agulhas current strong coastal upwelling and the congo river strongly influence primary production much of the present knowledge of the south east atlantic has been derived from ship borne measurements and in situ sensors which cannot generally provide extensive spatial and temporal coverage similarly previous satellite studies of the region have often focused on small spatial areas and limited time periods this paper provides an improved understanding of seasonal and interannual variability in ocean dynamics along the south east atlantic coast of africa using time series of satellite and model derived data products eighteen years of satellite sea surface temperature data are complimented by 7 years of sea level data three years of chlorophyll a data illustrate the seasonal biological response but the time series is not of sufficient length for investigating interannual variability in chlorophyll biomass modelled wind fields are used to describe atmospheric forcing of the surface ocean this is the first synoptic scale description of the south east atlantic from a suite of large spatial coverage long time series products previous studies of seasonal and interannual variability in the region are reviewed and used to interpret key oceanographic features and processes identified in the satellite data key findings of this study are 1 descriptions of seasonal and interannual variability from these data show climate forcing of the south east atlantic coast of africa from both the northern and southern boundaries bimodal seasonal signals of equatorial origin propagate poleward along the angolan coast while the trade winds and events in the agulhas region dominate the agulhas bank and southern benguela the northern benguela is a mixed regime under the influence of forcing from both directions 2 the benguela nino years of 1984 and 1995 are clearly observed in sea surface temperature and sea level anomalies and correspond to anomalously weak southerly winds at the equator these conditions were also observed in 1999 suggesting this too may have been a benguela nino year 3 consideration of putative nino type events in the equatorial atlantic from this and other studies suggests that the frequency of these events is much higher than previously estimated and may be similar to the frequency of el nino southern oscillation enso events in the pacific ocean furthermore years of anomalously strong southerly winds at the equator occur during pacific enso years c 2003 elsevier ltd all rights reserved

isopycnal and diapycnal circulation of the upper equatorial atlantic ocean in 1983 1984 isopycnal circulation and diapycnal processes in the equatorial atlantic ocean are investigated using eight cruises from november 1982 to july 1984 an eastward decrease of the transport of the equatorial undercurrent is observed averaging 18 2 106 m3 s 1 at 35 w to 10 2 106 m3 s 1 at 4 w there is also a meridional convergence at the undercurrent level and a larger meridional divergence in the surface layer of 15 106 m3 s 1 between 35 w and 4 w in the eastern equatorial atlantic the undercurrent core density as well as the salinity above the undercurrent experience a large seasonal cycle during the season where the eastern atlantic thermocline is closest to the surface and surface waters are coldest the current peaks in denser waters and subsurface salinity maximum are weaker an analysis of the salinity on isopycnal surfaces indicates that there is significant diapycnal mixing in the upper thermocline in the eastern atlantic the upper thermocline vertical heat diffusivity coefficient scaled over 1 of latitude varies between 3 cm2 s 1 during the upwelling season and nearly 0 early in the year this seasonal mixing is an important element of undercurrent dynamics the turbulent heat flux at the base of the surface layer averages 50 w m 2 between 1 5 n and 1 5 s the surface layer heat budget implies an average oceanic gain of 60 w m 2 between 1 5 n and 1 5 s which could result from exchanges with the atmosphere the heat flux and fresh water seasonal cycles needed to close the budgets are not realistic however the eastern equatorial atlantic thermocline seasonal upwelling is associated with a large inflow into the surface layer over the two years this flow averaging 11 12 106 m3 s 1 originates from the upper thermocline and diapycnal transports near the core of the undercurrent or below are found to be small below the core of the undercurrent we also find that mixing is not intense for instance at 26 5 in the upper part of the thermostad vertical heat diffusivity is only 0 6 cm2 s 1 assuming that mixing takes place over 1 of latitude the uncertainties on these budgets are however large and assumptions on the dynamics that were used could not be checked

observation of the marine atmospheric boundary layer in the gulf of guinea during the 2006 boreal spring in situ satellite and model analyses data in april july 2006 are used to investigate the links between sea surface temperature sst and the marine atmospheric boundary layer mabl in the gulf of guinea the study region between 10 w and 6 e is divided into three areas with different characteristics the north area 4 5 n to 1 n with the wettest atmosphere and the warmest sst the upwelling area 1 n to 4 s with the strongest sst decrease and the south area 4 s to 8 s with a drier atmosphere and a more slowly decreasing sst than in the upwelling area the key zone of the air sea interactions in this region seems to be the sst front between north and upwelling areas on the one hand the study of the mabl on either side of the front shows a well mixed layer between the surface and about 500 m high sensitive to surface variations which gets shallower deeper when the sst decreases increases the mabl height about 1500 m follows the same variations but is not exactly collocated with the sst variations on the other hand the observation of the mabl across the sst front shows a strengthening of southeasterlies in the south area coinciding with a strong sst decrease in the upwelling area in the latter the wind weakens above the colder sst besides in the north area the wind strengthens above the warmer sst however the wind acceleration spans from the equator to 2 n in april and as far as 4 n in june a convergence zone is observed in the vicinity of 2 n in april suggesting a convection activity there favoured by the sst front copyright 2011 royal meteorological society

seasonal modes of surface cooling in the gulf of guinea a numerical simulation of the tropical atlantic ocean indicates that surface cooling in upwelling zones of the gulf of guinea is mostly due to vertical mixing at the seasonal scale the spatial structure and the time variability of the northern and southern branches of the south equatorial current sec and of the guinea current are correlated with the timing and distribution of turbulent heat fluxes in the gulf of guinea through modulation of the velocity shear at the subsurface these surface currents control the vertical turbulent exchanges bringing cold and nutrient rich waters to the surface this mechanism explains the seasonality and spatial distribution of surface chlorophyll concentrations better than the generally accepted hypothesis that thermocline movements control the nutrient flux the position of the southern sec explains why the cold tongue and high chlorophyll concentrations extend from the equator to 4 s in the southeastern part of the basin

temperature variations in the equatorial atlantic as revealed by drifting buoys from may through september 1983 surface buoys deployed as a component of the focal experiment programme francais ocean et climat dans l atlantique equatorial drifted in the cold waters of the south equatorial current in the equatorial atlantic ocean the surface temperature sst is analysed along the buoy tracks in late may the strong temperature drop near 4 w and 3 s is associated with vertical mixing as cold patches appear near the equator horizontal advection is important in june and july as the cold water tongue expands southwards and westwards it is also important later in the western atlantic south of the equator where the cooling by advection is balanced primarily by heating from the atmosphere

main processes of the atlantic cold tongue interannual variability the interannual variability of the atlantic cold tongue act is studied by means of a mixed layer heat budget analysis a method to classify extreme cold and warm act events is proposed and applied to ten various analysis and reanalysis products this classification allows 5 cold and 5 warm act events to be selected over the period 1982 2007 cold warm act events are defined by the presence of negative positive sea surface temperature sst anomalies at the center of the equatorial atlantic in late boreal spring preceded by negative positive zonal wind stress anomalies in the western equatorial atlantic an ocean general circulation model capable of reconstructing the interannual variability of the act correctly is used to demonstrate that cold act events develop rapidly from may to june mainly due to intense cooling by vertical mixing and horizontal advection the simulated cooling at the center of the basin is the result of the combined effects of non local and local processes the non local process is an upwelling associated with an eastward propagating kelvin wave which makes the mixed layer more shallow and preconditions the upper layers to be cooled by an intense heat loss at the base of the mixed layer which is amplified by a stronger local injection of energy from the atmosphere the early cooling by vertical mixing in march is also shown to be a good predictor of june cooling in july horizontal advection starts to warm the mixed layer abnormally and damps sst anomalies the advection anomalies which result from changes in the horizontal temperature gradient are associated in some cases with the propagation of rossby waves along the equator during warm act events processes are reversed generating positive sst anomalies a downwelling kelvin wave triggers stratification anomalies and mixed layer depth anomalies amplified by a weaker injection of energy from the atmosphere in may june in july warm act events are abnormally cooled due to negative horizontal advection anomalies resulting from processes similar to those that occur during cold act events this additional cooling process extends the period of cooling of the act reducing sst anomalies

small scale heterogeneities in the oceanic lithosphere inferred from guided waves 1 we analyze seismic waveforms from deep focus earthquakes occurring in the subducting slab beneath japan recorded by broadband ocean bottom seismometers bbobss installed on the northwestern pacific ocean seafloor the data reveal waveforms with a low frequency direct p onset followed by large amplitude high frequency long duration po and so waves from the analysis of the bbobs records and a numerical finite difference method simulation of seismic wave propagation we elucidate the generation and propagation processes of such guided waves we demonstrate that the low frequency direct p and s waves propagate in the asthenosphere and that the following high frequency long duration po and so waves are developed by multiple forward scattering of p and s waves the scattering occurs due to laterally elongated heterogeneities in both the subducting and horizontal parts of the oceanic lithosphere with the apparent velocities vp 8 1 km s vs 4 6 km s being close to the velocities of oceanic lithosphere

comparison of mg ca and alkenone based sea surface temperature estimates in the fresh water influenced gulf of guinea eastern equatorial atlantic this study presents a comparison of sea surface temperature sst estimates based on mg ca ratios of globigerinoides ruber and alkenone unsaturation index u37 k in core sediment recovered from the gulf of guinea eastern equatorial atlantic mg ca and u37 k based sst estimates yield fairly comparable results for the time interval 21 000 14 500 years and for the late holocene the early and middle holocene however are largely characterized by a discrepant trend with warm mg ca and cold u37 k based sst estimates this discrepant sst trend is accompanied by low sea surface salinity estimates high riverine runoff and biogenic sediment which is characterized by high biogenic opal content low carbonate content and relatively low alkenone concentration we hypothesize that the discrepancy in the reconstructed ssts during the middle and early holocene presumably suggests a period of elevated riverine input of dissolved silica and dominantly siliceous phytoplankton bloom in a low saline and warm surface water while alkenone producers were likely prevalent in a season of cold sst and low riverine silica input this study suggests that changes in the local hydrography and nutrient input have strong influence on the u37 k based sst estimate that may be unraveled by combining different sst proxies

santonian to eocene palaeogeographic evolution of the rio grande rise south atlantic deduced from clay mineralogical data dsdp legs 3 and 39 abstract the late cretaceous to eocene morphologic evolution of the rio grande rise interpreted from variations in clay mineralogy progressed in four main stages 1 1 santonian early campanian the rio grande rise largely emerged consisted of flat areas and low relief undulant terrain where smectite formed 2 2 late campanian maastrichtian early paleocene the continental morphology did not change on the sao paulo plateau western rio grande rise where illite and mixed layers were abundant was marked by tectonism associated with the deepening of the vema channel eastern rio grande rise subsided leading to temporary submergence of flat areas and the formation of confined sedimentary basins where attapulgite formed 3 3 late paleocene early eocene the tectonic activity stopped and the continental relief was progressively peneplaned while the abundance of smectite increased subsidence continued in the eastern areas where attapulgite remained abundant 4 4 middle eocene the rio grande rise was almost completely submerged

clay mineral associations and structural evolution of the south atlantic jurassic to eocene abstract clay minerals have been used to reconstruct the geological history of the south atlantic six mineralogical stages have been recognized from the middle jurassic to the eocene 1 1 prior to the oxfordian illite irregular mixed layers and kaolinite marked a period of tectonic extension between africa and antarctica 2 2 from the oxfordian to the early albian smectites developed during subsidence induced peneplanation of the relief along the southern margin while chlorite illite irregular mixed layers and kaolinite characterized a distensive regime in the cape and southern angola basins 3 3 from the albian to the early turonian smectites palygorskite and sepiolite linked to subsidence processes occurred from the southern ocean to the southern angola basin while chlorite illite irregular mixed layers and kaolinite characterized tectonic extension from gabon to nigeria 4 4 from the early turonian to the early campanian smectites palygorskite and sepiolite linked to subsidence were widespread in the south atlantic areas 5 5 from the early campanian to early paleocene chlorite illite irregular and subregular mixed layers and kaolinite marked a reactivation of continental reliefs in the northern areas of the south atlantic in relation to compression in nigeria and opening of the equatorial atlantic 6 6 after the early paleocene smectites palygorskite and sepiolite characterized the widespread subsidence in the south atlantic

lagrangian sources of frontogenesis in the equatorial atlantic front estimating the processes that control the north equatorial sea surface temperature sst front on the northern edge of the cold tongue in the tropical atlantic is a key issue for understanding the dynamics of the oceanic equatorial atlantic and the west african monsoon diagnosis of the frontogenetic forcings on a realistic high resolution simulation was used to identify the processes involved in the formation and evolution of the equatorial sst front the turbulent forcing associated with the mixed layer turbulent heat flux was found to be systematically frontolytic while the dynamic forcing associated with currents was found to be frontogenetic for the equatorial sst front nevertheless the low frequency component of the turbulent forcing was frontogenetic and initiated the sst front which was then amplified and maintained by the leading dynamic forcing this forcing was mainly driven by the meridional convergence of the northern south equatorial current nsec and the guinea current which points out the essential role played by the circulation in the equatorial sst front evolution the quasi biweekly variability of the equatorial sst front and its forcings were found to be more strongly coupled to the wind energy flux wef than to the surface wind stress in fact the wef controlled the convergence divergence of the nsec and guinea current and thus the meridional component of the leading dynamic forcing the wef explains the equatorial sst front development better than the wind does because it is a coupled ocean atmosphere process

benthic foraminifera off west africa 1 n to 32 s do live assemblages from the topmost sediment reliably record environmental variability abstract recent benthic foraminifera 125 m were investigated from multicorer samples on a latitudinal transect of 20 stations between 1 n and 32 s along the upper slope off west africa samples were selected from a narrow water depth interval between 1200 and 1500 m so that changes in water masses are minimized but changes in surface productivity are important and the only significant environmental variable live rose bengal stained benthic foraminifera were counted from the surface sediment down to a maximum of 12 cm dead foraminifera were investigated in the top 5 cm of the sediment only five live and five dead benthic foraminiferal assemblages were identified using q mode principal component analysis matching distinct primary productivity provinces characterized by different systems of seasonal and permanent upwelling differences in seasonality quantity and quality of food supply are the main controlling parameters on species composition and distribution of the benthic foraminiferal faunas to test the sensitivity of foraminiferal studies based on the uppermost centimeter of sediment only a comparative q mode principal component analysis was conducted on live and dead foraminiferal data from the top 1 cm of sediment it has been demonstrated that on the upper slope off west africa most of the environmental signals as recorded by species composition and distribution of the total live and dead assemblages i e including live and dead foraminifera from the surface sediment down to 12 cm and 5 cm respectively can be extracted from the assemblages in the top centimeter of sediment only on the contrary subsurface abundance maxima of live foraminifera and dissolution of empty tests strongly bias quantitative approaches based on the calculation of standing stocks and foraminiferal numbers in the topmost centimeter

variability of the mixed layer heat budget in the eastern equatorial atlantic during 2005 2007 as inferred using argo floats we examine the variability of the sea surface temperatures in the eastern equatorial atlantic during 2005 2007 by using argo profiling floats prediction and research moored array in the atlantic buoys and satellite in situ and atmospheric data sets the eastern equatorial atlantic characterized by shallow mixed layers all yearlong is divided into nine boxes of nearly equal surface area with respect to the dynamics and thermodynamics in this region monthly mixed layer heat budgets are computed in each box from 10 day argo profiles in all the boxes the net surface heat flux is one of the main causes of the seasonal evolution of sea surface temperatures for the 3 studied years the amount of short wave radiation penetrating through the base of the mixed layer and horizontal heat advection may locally contribute to the temperature variability while entrainment has a weaker contribution to balance the heat budget a residual term exists which includes all processes that cannot be calculated with observations as well as the possible errors in the other terms this residual is more intense in the cold tongue and the northern region and exhibits a clear seasonal cycle with minimum negative values in boreal summer and maximum values in winter this residual compares well with available observations of vertical turbulent mixing collected during etude de la circulation oceanique et des echanges ocean atmosphere dans le golfe de guinee campaigns 2005 2007 in the eastern equatorial atlantic when assuming that the residual is mostly associated with vertical turbulent mixing it can be conjectured that turbulent mixing is a significant cooling source in the cold tongue and north of the equator

some overlooked features of tropical atlantic climate leading to a new nino like phenomenon abstract the atlantic nino an equatorial zonal mode akin to the pacific el nino southern oscillation enso is phase locked to boreal summer when the equatorial easterly winds intensify and the thermocline shoals in the gulf of guinea a suite of satellite and in situ observations reveals a new mode of tropical atlantic variability that displays many characteristics of the zonal mode but instead peaks in november december nd this new mode is found to be statistically independent from both the atlantic nino in the preceding summer and the pacific enso the origin of this nd zonal mode lies in an overlooked aspect of the seasonal cycle in the equatorial atlantic in november the equatorial easterly winds intensify for the second time increasing upwelling and lifting the thermocline in the gulf of guinea an analysis of high resolution climatological data shows that these dynamical changes induce a noticeable sst cooling in the central equatorial atlantic the shoaling thermocline and increased upwelli

turbidity events observed in situ along the congo submarine channel abstract as part of the multidisciplinary programme biozaire devoted to studying deep sea benthic ecosystems in the gulf of guinea particulate input and its relationship with near bottom hydrodynamics were monitored using long term moorings from 2000 to early 2005 particular attention was given to material input through the congo ex zaire submarine channel that extends 760 km from the congo river mouth to the abyssal plain 5100 m near 6 s due to its direct connection to the congo river the congo canyon and channel system are characterised by particularly active recent sediment transport during this first in situ long term monitoring along the channel an energetic turbidity event was observed in january 2004 at three locations along the channel from 3420 to 4790 m in depth this event tilted and displaced the moorings installed at 3420 m site zr and 4070 m site zd and resulted in high sediment deposition at all three mooring sites the event moved at an average velocity of 3 5 m s 1 along the numerous channel meanders between 3420 and 4070 m then at 0 7 m s 1 between 4070 m and the end of the channel at 4790 m the particle cloud rose above the top of the valley at 4070 m site zd but not at 3420 m site zr where the channel was too deep lastly the mooring line broke at site zd in october 2004 probably due to a strong event like that of 2001 previously described by khripounoff et al khripounoff a vangriesheim a babonneau n crassous p denniellou b savoye b 2003 direct observation of intense turbidity activity in the zaire submarine valley at 4000 m water depth marine geology 194 151 158 between these strong events several peaks of high turbidity and particle flux occurred but without noticeable current increases these events were probably due to local sliding of sediment accumulated on the walls or terraces on the side of the channel the area near 4000 m depth and the lobe appear to be the main depocentres of particulate input rich in organic matter derived from the congo river

a numerical model for low frequency equatorial dynamics a f st efficient numerical procedure for modeling the linear low frequency motions on an equatorial beta plane is develo the model is capable of simulating the seasonal and interannual variability in realistically shaped ocean basins forc d by realistic winds the timestep allowed is the order of days rather than hours as allowed by more conventional schemes the numerical method is designed around the special characteristics f low frequency equato al waves a crucial element is the formulation of proper boundary conditions including those for a partial boundary such as the western end of the gulf of guinea the resp nse of an a i ntic shaped basin to a periodic wind is compared with the analytic results of cane and sarachlk for a mendlonally unbounded ocean the response along the equator is essentially the same narrow boundary layer forms along the guinea coast too narrow to be a single coastally trapped wave it is the sum of many modes the near lack of phase variation along the coast and the smallness of the phase difference be ween the equator and the coast are additional contrasts to the results for an initial value probl m studied by moore and others the implication is that the annually recurring upwelling in the gulf of guinea cannot be adequately modeled as the response to an impulsively applied wind stress

propagation of the seasonal upwelling in the eastern equatorial atlantic abstract several mechanisms have been proposed to explain the coastal and equatorial upwelling in the eastern atlantic guinea gulf the most controversial is the mechanism of remote wind forcing in the western equatorial atlantic suggested by moore et al 1978 most of the possible explanations for the upwelling and their relative importance are discussed in view of recent observations detailed analysis of daily sea surface temperature sst collected at 16 coastal stations along the northern coast of the guinea gulf reveals that the upwelling event propagates westward along this coast at a mean speed of 0 7 m s 1 similar analysis of historical monthly mean sst data shows that the coastal upwelling event propagates poleward from 1 s to at least 13 s at the same phase speed furthermore the northern hemisphere and southern hemisphere coastal upwelling signals seem to start at the same time from the equator the same kind of analysis applied to hydrographic data from a station situated 41 km off abid

on the response of the equatorial thermocline in the atlantic ocean to the seasonally varying trade winds measurements made during the seasonal response of the equatorial atlantic experiment in 1983 when the easterly trade winds near the equator ceased and then intensified as part of their annual cycle show a sequence of events leading to an adjusted thermocline both the duration and extent of the downwelling and upwelling associated with the adjustment process were zonally inhomogeneous along the equator an analytical reduced gravity model was driven by a hierarchy of easterly wind stress forcing functions representative of the atlantic to compare the integral nature of the forced long wave responses with the observations qualitative agreement was found with upwelling and downwelling sequences within the forced region increasing to the east as observed apparent model propagation speeds differed significantly from the prescribed model wave speeds within the forced region owing to the growth parameters of the winds and boundary reflections distorted the responses to the east of the forced region the resulting interference patterns tended to separate the region 0 10 w along the equator from the african coast as one of maximum upwelling

on the north brazilian eddy field abstract the region of large eddies occurring off the coast of brazil between approximately 3 n 10 n is revealed in some detail by repeated temperature observations on the along shore sea lane by ships of opportunity augmented by one airborne xbt expendable bathythermograph survey the general pattern of the circulation implied by the temperature structure is similar to that indicated by past hydrographic measurements and drifter tracks in general a large anticyclonic eddy to the north 7 n 10 n as well as a southern eddy like feature 3 n 6 n appear to be permanent patterns of the western north equatorial circulation this pattern somewhat resembles that found in the somali basin during the southwest monsoon

coupling between the atlantic cold tongue and the west african monsoon in boreal spring and summer 1 the formation of the atlantic cold tongue act is the dominant seasonal sea surface temperature signal in the eastern equatorial atlantic eea a comprehensive analysis of variability in its spatial extent temperature and onset is presented then the physical mechanisms which initiate act onset as well as the feedbacks from the act to the maritime boundary layer and how the act influences the onset of the west african monsoon wam are discussed we argue that in the eea the air sea coupling between the act and wam occurs in two phases from march to mid june the act results from the intensification of the southeastern trades associated with the st helena anticyclone steering of surface winds by the basin shape of the eea imparts optimal wind stress for generating the maximum upwelling south of the equator during the second phase mid june august wind speeds north of the equator increase as a result of the northward progression of the intensifying trades and as a result of significant surface heat flux gradients produced by the differential cooling between the act and the tropical waters circulating in the gulf of guinea gg it is anticipated that the atmospheric divergence induced at low levels north of the equator reduces convection over the gg and that increased northward winds shift convection over land correlations between the act and the wam onset dates over the last 26 years 1982 2007 measure as much as 0 8 this suggests that the act plays a key role in the wam onset