instability waves in the equatorial atlantic ocean abstract evidence is presented for the generation of planetary waves by barotropic instability within the cyclonic shear region of the atlantic ocean s south equatorial current sec immediately following the springtime intensification of the southeast trade wind which accelerates the sec westward a packet of waves with central periodicity of around 25 days is observed lasting for about three cycles independent wavenumber analyses on 1983 and 1984 data give newly identical zonal wavelengths and phase speed estimates of around 1100 km and 50 cm s 1 the waves are anisotropic and spatially inhomogeneous with generation confined primarily to the mixed layer an energetics analysis using 1983 data centered upon the equator at 28 w shows a rapid increase in total perturbation energy tpe reaching values of 2000 erg cm 3 within two weeks the subsequent decrease in tpe at this location is due primarily to meridional pressure work divergence baroclinic instability is negligible because both the meridional

direct velocity measurements in the malvinas current we present a set of current meter measurements collected in the southwest atlantic from december 1993 to june 1995 four moorings were set out to monitor the malvinas falkland current at 40 41 s and one mooring was located within the brazil malvinas front area at 38 s the mean flow of the malvinas current has an equivalent barotropic structure whereas the vertical structure of the variable part of the flow is dominated by a barotropic like empirical mode for regions shallower than 2500 m surface intensified which appears as a linear combination of barotropic and baroclinic modes suggesting mode coupling due to the steep topography this mode appears to gradually become nearly exactly barotropic toward shallower depths higher empirical modes bottom intensified and associated with higher frequencies suggest waves trapped by the conjugated effects of topography and stratification the time variability is discussed in terms of along and cross isobath flows the along shelf flow features substantial energy in the 50 to 70 day band there is no evidence of an annual cycle nor of a distinct semiannual cycle but there is a suggestion of significant energy at periods a little shorter 135 days the cross shelf variability is important with a quite clear annual cycle associated with excursions of the subantarctic front its phase coincides with that of the brazil current transport suggesting that annual migrations of the front are predominately determined by the strength of the latter whereas the intensity of the malvinas current would only have marginal impact a conclusion that may not hold at other timescales

improved global surface currents from the merging of altimetry and sea surface temperature data abstract six years of surface velocities have been calculated over the global ocean by combining altimeter derived geostrophic velocities and sea surface temperature sst data the method is based on the inversion for the velocity of the heat conservation equation using the altimeter velocities as background by accurately prescribing the error both on the background velocities and on the forcing term heat fluxes the altimeter background velocities are successfully improved in areas characterized by strong sst gradients and remain unchanged in low sst gradient regions where by construction no additional information may be brought by the sst field this allows us to provide for the first time a global field of surface velocities from the merging of altimetry and sst both the spatial and temporal resolution of the altimeter derived velocities is enhanced by the sst information validation is performed through comparison to in situ drifting buoy velocities major improvements 10 20 globally up to 35 locally are obtained on the meridional component of the velocity and in the equatorial band due to the large uncertainty on the heat fluxes and the spatio temporal resolution of the input datasets the high frequency ageostrophic components of the circulation is hardly resolved by the method and outside the equatorial band improvements are mainly on the geostrophic component it is shown that the level of accuracy obtained by combining altimeter velocities based on a two satellite configuration and microwave sst data is equivalent or higher to the one from a four altimeter constellation in western boundary currents this opens the perspective for a systematic use of the method to improve the altimeter derived surface velocities over the 1993 2001 period for which only 2 altimeters have been flying simultaneously finally over the recent years the use of higher resolution sst products based on both microwave and infrared measurements leads to further enhancement of the accuracy of the altimeter velocities on both the zonal and the meridional component in strong sst gradients areas

thermostads and circulation in the upper layer of the atlantic ocean abstract there are three major permanent thermostads with roughly the same potential densities in the upper layer of the atlantic ocean one is the thermostad of the 13 c water in the equatorial atlantic the original type of the 13 c water is formed in the thermocline in the eastern sector of the south atlantic subtropical gyre by vertical mixing of dense low salinity water from the winter outcrop farther south and overlying less dense high salinity water there might also be a lateral contribution of relatively high salinity water from the indian ocean the original 13 c water thus formed is transported northwestward along the northern edge of the subtropical gyre and fed into the north brazilian current which flows equatorward along the coast of brazil in the region of the equator the equatorial undercurrent and the subsurface north and south equatorial countercurrents branch off from the north brazilian current and carry the 13 c water eastward to the thermostad region vertical mixing does not explain the development of the thermostad but is found to be essential in determining the ultimate characteristics of the 13 c water the other two thermostads are those of the 18 c water in the sargasso sea and the subantarctic mode water in the western south atlantic unlike the 13 c water both of these mode waters are formed as thermostads in the surface layer by winter convection but vertical mixing in the subtropical gyres may play a role in determining their characteristics all the three thermostads appear to be required to balance the system of flows in opposing directions

pathways and variability of the off equatorial undercurrents in the atlantic ocean the cold upwelling waters of the eastern tropical oceans not only interact with the atmospheric circulation via changing the sea surface temperatures but also influence the biological activity via affecting the nutrient and oxygen contents of the upwelling waters while the sources of the equatorial upwelling associated with the equatorial undercurrent euc have been studied extensively the relevance of the northern and southern off equatorial undercurrents neuc seuc for the off equatorial upwelling regions has remained unclear in this study we use output from a high resolution 1 12 model flame to investigate the mean pathways and variability of the off equatorial undercurrents oeucs in the atlantic in particular a calculation of lagrangian trajectories helps to gain insight into the source waters of the oeucs and their connection to the upwelling regions in the eastern tropical atlantic in the model solution the sources of both oeucs belong almost exclusively to the southern hemisphere the pathways of the source waters are found to be governed by strong recirculations between the different eastward and westward zonal currents because of intense eddy motions associated with the tropical instability wave activity whereas the seuc is predominantly fed through the recirculation in the ocean interior the neuc is also fed by a weak inflow from the western boundary current investigation of the fate of the neuc shows only a weak direct supply to the upwelling in the guinea dome and along the african coast but a significant contribution to the equatorial upwelling

tropical instability wave energetics observations from the tropical instability wave experiment ocean velocity data from an array of subsurface moorings deployed from may 1990 to june 1991 during the tropical instability wave experiment are used to study the energetics of planetary waves in the vicinity of the equator at 1408w such waves observed from august to december 1990 were initiated by barotropic instability arising primarily from the cyclonic shear region of the south equatorial current and equatorial undercurrent just north of the equator subsequently local barotropic production continued to maintain and modulate these tropical instability waves through a combination of cyclonic shear and meridional velocity component divergence just north of the equator the end of the wave season coincided with the propagation past the array of a large intraseasonal kelvin wave

estimating advective near surface currents from ocean color satellite images abstract improved maximum cross correlation mcc techniques are used to retrieve ocean surface currents from the sequential ocean color imagery provided by multiple newer generations of satellite sensors on hourly scales in the yellow sea and the u s east and gulf coasts the mcc calculation is validated in a series of bio optical forecasting biocast experiments with predetermined synthetic velocities and its products are evaluated by examining the errors and biases with respect to the high frequency radar hfradar measurements the root mean square rms errors in our best current products derived from the overlap of satellite sensor swath between the viirs sequential orbits are less than 0 17 m s 1 in the evaluation area outside of the chesapeake bay the most accurate current products are those derived from the imagery data of r rs 551 b b 551 and c 551 while the image sequences of b b 551 and z eu lee are identified as the most suited products for the retrieval of currents because of their best production capacities of valid velocity vectors mechanisms between the advective processes and the dynamic changes of bio optical properties are discussed regarding the performances of various color products on the retrieval of currents similarities of velocity distribution in the retrieved vector arrays are collected across different mcc products derived from ocean color datasets that are of different types and derived from different spectral channels of satellite overpasses the inter product similarities themselves can be used to characterize the near surface advection as well and usually have smaller errors than each of the individual mcc currents moreover efforts are also under way to improve the ocean color derived currents by merging several of the mcc products with similarities to increase the total spatial coverage this study not only seeks the image derived products best representing the sea surface current structures in coastal areas but also exploits how these currents can be improved or optimized to support the ocean forecasts

a global optimal solution with higher order continuity for the estimation of surface velocity from infrared images a global optimal solution gos provides surface velocities from advanced very high resolution radiometer avhrr remote image sequences using bilinear interpolation algorithms although an accurate velocity field can be estimated by gos from a sequence of infrared images the field has only first order continuity because an actual coastal ocean has a complex irregular coastland and some ocean studies need vorticity and divergence analysis which must be extracted from the velocity field the development of generic gos algorithms with higher order continuity and smoothed cutouts around these edges is very important this paper addresses the issues of higher order continuity and smoothed cutouts around coastland edges for using gos to estimate surface velocities gos bilinear polynomials previously applied to square tiles with first order continuity are replaced by surface b spline functions the new gos algorithms can be applied to avhrr images containing complicated coastal land boundaries even clouds to yield smooth velocity fields next to land and higher order continuity velocity fields the velocity fields obtained through the applications of the first and higher order gos techniques to a sequence of two national oceanic atmospheric administration avhrr images which were taken from the new york bight fields are compared with those measured with the codar array the retrieved velocity fields are used directly to calculate the surface divergence and vorticity it is found that the angular and magnitude errors of the velocity by the first and third order goss are quite close for both numerical model data and avhrr image sequences and the velocity field estimated by the third order gos is globally smoothed

do tropical cells ventilate the indo pacific equatorial thermocline source waters of the indo pacific equatorial thermocline are studied with a high resolution ocean model in the annual mean fields tropical and subtropical overturning cells are found that upwell at the equator and downwell at 5 degrees and 20 degrees poleward of the equator respectively tropical cells are common in ocean models but their role in ventilating the equatorial thermocline is obscure because the downwelled water is too warm to match the subsurface equatorial waters the tropical cells are much weaker when the overturning is considered in density coordinates when high frequency mass fluxes are included tropical cells are compensated by an eddy induced overturning seasonal variations and tropical instability waves are responsible for the compensation it follows that only subtropical cells transfer surface water to the equatorial thermocline strong tropical cells are shown to be an artefact

river velocities from sequential multispectral remote sensing images 1 we address the problem of extracting surface velocities from a pair of multispectral remote sensing images over rivers using a new nonlinear multiple tracer form of the global optimal solution gos the derived velocity field is a valid solution across the image domain to the nonlinear system of equations obtained by minimizing a cost function inferred from the conservation constraint equations for multiple tracers this is done by deriving an iteration equation for the velocity based on the multiple tracer displaced frame difference equations and a local approximation to the velocity field the number of velocity equations is greater than the number of velocity components and thus overly constrain the solution the iterative technique uses gauss newton and levenberg marquardt methods and our own algorithm of the progressive relaxation of the over constraint we demonstrate the nonlinear multiple tracer gos technique with sequential multispectral landsat and aster images over a portion of the potomac river in md va and derive a dense field of accurate velocity vectors we compare the gos river velocities with those from over 12 years of data at four noaa reference stations and find good agreement we discuss how to find the appropriate spatial and temporal resolutions to allow optimization of the technique for specific rivers

sea surface velocities from sea surface temperature image sequences 1 method and validation using primitive equation model output an inverse variational finite element model is developed for the purpose of estimating ocean surface velocity fields from sequences of temperature fields the cross isotherm component of the velocity is controlled by a mixed layer integrated formulation of the heat balance the aperture problem imposes additional constraints on the flow field we control the divergence and the vorticity of the horizontal velocity the method is then applied to sequences of sea surface temperature sst fields from a fine mesh numerical simulation over the brazil malvinas confluence region the difference between the actual velocity and the sst inverted velocity is 11 rms in magnitude and 17 rms in direction these differences are analyzed divergence and vorticity are computed the hypothesis of neglecting the source terms made in the formulation of the mixed layer integrated heat balance is verified the sensitivity of the solution to the influence of the constraints is examined perturbations are performed yielding fields of ellipses of covariance we obtain on average 15 uncertainties in magnitude and 25 in direction of the velocity differences between the actual and sst inverted velocities fall into those covariance ellipses except over regions where temperature does not change from one image to the other

oceanic fluxes on the woce a11 section abstract the most southerly woce one time section in the south atlantic designated a11 was occupied in january 1993 the cruise track lay across the cool subantarctic zone of the circumpolar current in the west and the warm subtropical gyre in the east in this paper estimates of the flux of heat salt oxygen and other tracers across the section are presented a brief description of the distribution of physical and chemical properties is followed by a determination of the flux in the surface ekman layer direct measurements of current shear made from a shipborne adcp and estimates from cruise wind data and climatological data all yield a flux of 5 1 sv sv 106 m3 s 1 equatorward when combined with geostrophic estimates relative to a level near 3500 dbar or the bottom where shallower an initial guess for the flow field is derived this initial guess is combined with absolute currents derived from the adcp when the ship is underway unrealistic aspects of the circulation are found and we conclu

a linear stratified ocean model of the equatorial undercurrent a linear stratified ocean model is used to study the wind driven response of the equatorial ocean the model is an extension of the lighthill 1969 model that allows the diffusion of heat and momentum into the deeper ocean and so can develop non trivial steady solutions to retain the ability to expand solutions into sums of vertical normal modes mixing coefficients must be inversely proportional to the square of the back ground vaisala frequency the model is also similar to the earlier homogeneous ocean model of stommel i960 he extended ekman dynamics to the equator by allowing his model to generate a barotropic pressure field the present model differs in that the presence of stratification allows the generation of a baroclinic pressure field as well the most important result of this paper is that linear theory can produce a realistic equatorial current structure the model undercurrent has a reasonable width and depth scale there is westward flow both above and below the undercurrent the meridional circulation conforms to the classical picture suggested by cromwell 1953 unlike the stommel solution the response here is less sensitive to variations of parameters ocean boundaries are not necessary for the existence of the undercurrent but are necessary for the existence of the deeper equatorial intermediate current the radiation of equatorially trapped rossby and kelvin waves is essential to the development of a realistic undercurrent because the system supports the existence of these waves low order vertical modes can very nearly adjust to sverdrup balance defined below which in a bounded ocean and for winds without curl is a state of rest as a result higher order vertical modes are much more visible in the total solution this property accounts for the surface trapping and narrow width scale of the equatorial currents the high order modes tend to be in yoshida balance defined below and generate the characteristic meridional circulation pattern associated with equatorial ekman pumping

a numerical study of the southwestern atlantic shelf circulation stratified ocean response to local and offshore forcing 1 this article discusses the results of a suite of numerical simulations of the oceanic circulation in the southwestern atlantic shelf region that are aimed to characterize its mean circulation and seasonal variability and to determine the dynamical mechanisms controlling them our experiments indicate that south of 40s the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf which is controlled by the discharges from the magellan straits tidal mixing wind forcing and the offshore influence of the malvinas current farther north the region from 40 st o 33s presents the highest seasonal variability with intrusions of cold sub antarctic waters

response of the northern north atlantic and arctic oceans to a sudden change of the north atlantic oscillation 1 the influence of a switch from a long time high north atlantic oscillation nao state to an enduring nao situation is investigated with a coupled sea ice ocean model we compare the response to a sustained shift to nao conditions with that to a short lived transition to nao and back to nao forcing observed changes in sea ice and oceanic variables between high and low nao states are well captured in the model response the ocean circulation adjustment includes a fast barotropic anomaly accompanied by an enhancement of the meridional overturning and the northward heat transport at 48 n the slow response contains a substantial decrease of the northward heat transport which is caused by a reduction of the transport of subpolar and subtropical gyres after a delay of around 2 years the oceanic heat transport reacts to counteract the meridional temperature gradient imposed by the surface forcing the propagation time of baroclinic rossby waves is irrelevant for the delay between the forcing and the oceanic response rather the strength of the subpolar gyres is determined by the rapid spin up of a topographic sverdrup circulation and the subsequent slower changes through the jebar joint effect of baroclinicity and relief in an experiment where only the wind stress is changed to nao conditions the adjustment is dampened strongly by the nao thermohaline forcing the gyres decrease slower than in the nao case comparison with an experiment where only the wind stress is changed to nao conditions indicates that the thermohaline forcing is most important the heat fluxes counteract the wind stress forcing such that the meridional overturning and the northward heat transport decrease only slightly compared to the control run

on the dynamics of equatorial subsurface countercurrents abstract the equatorial subsurface countercurrents sscc are strong steady geostrophically balanced eastward flows situated below the high speed core of the equatorial undercurrent euc at 3 5 n and s the dynamics of these currents are explored using a continuously stratified vertically diffusive linear steady state ocean model forced by zonal winds with effectively no wind stress curl model results agree favorably with observations in that both euc and sscc like structures are generated a diagnosis of the model momentum vorticity and continuity balances at various depths and latitudes reveals that the sscc lie outside a vertically diffusive equatorial momentum boundary layer so that both components of velocity are geostrophically balanced they are however located at the poleward of a broader diffusive equatorial vorticity boundary layer within this boundary layer cyclonic vorticity associated with the euc diffuses to the level of the sscc where it is balanced by poleward advection of pla

tropical instability waves at 0 n 23 w in the atlantic a case study using pilot research moored array in the tropical atlantic pirata mooring data temperature salinity velocity and wind from a mooring at 0 n 23 w are used along with satellite data for sea surface temperature and sea level to examine the contribution of tropical instability waves tiws to the energy and heat balance of the equatorial atlantic mixed layer the tiws appear as periodic 20 30 day fluctuations of currents temperature and salinity which intensify beginning in june and peak in late boreal summer the intensification occurs in phase with strengthening of the southeasterly trade winds and the seasonal appearance of the equatorial tongue of cold mixed layer temperatures in 2002 these waves which warm the mixed layer by 0 35 c during summer months are maintained by both barotropic and baroclinic conversions that are of comparable size salinity fluctuations previously neglected increase the magnitude of baroclinic energy conversion

lagrangian perspectives of deep water export from the subpolar north atlantic 1 direct observations at the grand banks have raised a quandary concerning the pathways of the lower branch of the meridional overturning circulation in contrast to moored current meters that depict an intense narrow deep western boundary current dwbc observations using different float types failed to show this continuous export path here this issue is addressed by a lagrangian analysis of synthetic particles in an eddy resolving circulation model due to intense eddy activity around the grand banks about 40 of the deep water in the dwbc is diverted into the interior spreading southward along the western flank of the mid atlantic ridge or with the eddying flow field in the basin interior imposing constraints on the vertical displacements of particles similar to those experienced by observational floats further reduces their adherence to the dwbc particularly near the southern tip of the grand banks

sea surface velocities from sea surface temperature image sequences 2 application to the brazil malvinas confluence area an inverse variational model is applied to four exceptionally cloud free sea surface temperature image sequences of the brazil malvinas confluence area for the purpose of estimating sea surface velocity fields the velocity fields are constructed by minimizing a mixed layer integrated form of the heat balance equation while the horizontal divergence and vorticity of the flow field are controlled the source terms involved in the heat balance are determined by fitting a polynomial to diurnal temperature variations as a function of temperature and longitude a sensitivity study of the solution to perturbations of the weights imposed to the constraints on divergence and vorticity as well as of the representation of the source terms yields error fields associated with the velocity a comparison of the estimated velocity fields with available concomitant in situ velocity measurements shows that the fields are realistic errors are of the order of 15 20 in magnitude and 20 25 in direction these errors fall within the error bars of 25 30 in magnitude and of 30 35 in direction except for the summer image series surface velocities of 0 5 0 6 m s 1 are obtained across the malvinas current they are 1 m s 1 in the frontal region in the brazil current overshoot and in warm and cold core rings and vary between 0 3 and 0 8 m s 1 in the brazil current with some hypotheses on the vertical shear of the horizontal flow the transport of the malvinas current is estimated to be 25 5 sv between 41 and 40 s

ocean surface currents from avhrr imagery comparison with land based hf radar measurements we focus on inverting the surface temperature or heat equation to obtain the surface velocity field in the coastal ocean and compare the results with those from the maximum cross correlation mcc technique and with the in situ velocity fields measured by the rutgers university coastal ocean dynamics radar codar when compared with codar fields velocities from the heat equation and mcc have comparable accuracies but the heat equation technique better resolves the finer scale flow features we use the results to directly calculate the surface divergence and vorticity this is possible because we convert the traditionally underdetermined heat inversion problem to an overdetermined one without constraining the velocity field with divergence vorticity or energy statements because no a priori assumptions are made about the vorticity it can be calculated directly from the velocity results the derived vorticity field has typical open ocean magnitudes 5 times 10 5 s and exhibits several structures a warm core ring gulf stream filament and a diverging flow consistent with the types of flows required to kinematically deform the sea surface temperature patterns into the observed configurations

the shallow and deep western boundary circulation of the south atlantic at 5 11 s repeated shipboard observation sections across the boundary flow off northeastern brazil as well as acoustic doppler current profiler adcp and current meter records from a moored boundary array deployed during 2000 04 near 11 s are analyzed here for both the northward warm water flow by the north brazil undercurrent nbuc above approximately 1100 m and the southward flow of north atlantic deep water nadw underneath at 5 s the mean from nine sections yields an nbuc transport of 26 5 3 7 sv sv 106 m3 s 1 along the boundary at 11 s the mean nbuc transport from five sections is 25 4 7 4 sv confirming that the nbuc is already well developed at 11 s at both latitudes a persistent offshore southward recirculation between 200 and 1100 m depth reduces the net northward warm water flow through the 5 s section west of 31 5 w to 22 1 5 3 sv and through the 11 s section to 21 7 4 1 sv west of 32 0 w the 4 yr long nbuc transport time series from 11 s yields a seasonal cycle of 2 5 sv amplitude with its northward maximum in july interannual nbuc transport variations are small varying only by 1 2 sv during the four years with no detectable trend the southward flow of nadw within the deep western boundary current at 5 s is 25 5 8 3 sv with an offshore northward recirculation yielding a nine section mean of 20 3 10 1 sv west of 31 5 w for antarctic bottom water a net northward flow of 4 4 3 0 sv is determined at 5 s for the 11 s section the moored array data show a pronounced energy maximum at 60 70 day period in the nadw depth range which was identified in related work as deep eddies translating southward along the boundary based on a kinematic eddy model fit to the first half of the moored time series the mean nadw transfer by the deep eddies at 11 s was estimated to be about 17 sv given the large interannual variability of the deep near boundary transport time series which ranged from 14 to 24 sv the 11 s mean was considered to be not distinguishable from the mean at 5 s

hydrographic conditions in the brazil malvinas confluence during austral summer 1990 the confluence 3 cruise during february 1990 provides the first hydrographic survey of the brazil malvinas confluence region ever obtained with such a refined spatial resolution over the continental slope the closely spaced stations on the slope permit the observation of the branching of the malvinas current near 40 s the eastern branch returns south while the eastern one is squeezed along the continental slope managing to continue northward as far north as 36 3 s the brazil malvinas b m front has a north south orientation on the slope and an east west direction offshore the b m front is marked by very strong gradients in temperature and salinity fine scale vertical structures and intrusions are numerous in the vicinity of the front and cover a wide range of scales the largest intrusion observed is a 50 km wide 350 m thick subsurface lense of thermocline water present on the malvinas side of the front the fine scale vertical structure results in very high temperature and salinity fluxes across the front of the order of 2 10 2 c m 1 s 1 and 10 3 psu m 1 s 1 two types of mode waters are identified as follows subtropical mode waters north of the front and subantarctic mode waters on both sides of the front the subantarctic mode waters samw appear to recirculate near the western boundary a young variety of samw lighter very oxygenated propagates northward along the continental slope with the malvinas current it detaches from the coast near 37 s and is strongly altered in the frontal region it is then entrained southward with the malvinas return flow and brazil current extension the confluence 3 hydrographic survey provides a good description of the north atlantic deep water nadw and circumpolar deep water cdw convergence near the western boundary strong fine scale vertical structures are observed in the vicinity of the nadw cdw front and the associated temperature and salinity fluxes induced are strong 5 10 3 cm s and 2 10 3 psu m s respectively

seasonal and interannual variations of oceanic conditions in the angola dome using outputs from a high resolution ogcm seasonal and interannual variations of the angola dome ad are revisited although the ad was previously considered to be one large cold tongue extending from the west african coast it is shown that two cold domes exist these two domes have remarkably different mechanisms for their seasonal variation the weak dome whose center is located at 6 s 1 e develops from may to september owing to the divergence of heat transport associated with upwelling the strong dome on the other hand extends from the west coast of africa between 20 and 15 s and develops from april to august by the surface heat flux the interannual variation of the weak dome is strongly influenced by the atlantic nino an unusual relaxation of easterly wind stress in the central equatorial atlantic ocean associated with the atlantic nino triggers second baroclinic downwelling equatorial kelvin waves which propagate eastward along the equator and poleward along the coast after reaching the african coast as coastal kelvin waves then downwelling rossby waves radiate away from the coast and cause significant warming in the weak dome region the interannual variation of the south equatorial undercurrent may be associated with that of the ad its transport decreases by 0 6 sv and its core shifts equatorward by 0 2 when the ad is anomalously weak

tropical instability vortices in the atlantic ocean 1 this paper examines the physical characteristics and dynamics of atlantic tropical instability vortices based on observational data from 1997 to 2000 and a high resolution numerical model simulation most prominent during boreal summer the vortices are characterized by 400 km spatial scales westward translation at 0 40 cm s 1 anticyclonic circumferential velocities of 30 110 cm s 1 and sstanomalies that decrease from 2 ci n midbasin to 0 5 c or less near the western boundary in contrast the sea level anomalies grow as the vortices progress westward reaching maximum amplitudes of 14 cm near 40 w the large circumferential velocities leading to relative vorticities of 4 10 6 s 1 and rossby numbers of 0 5 indicate that centrifugal effects may play an important role in the vortices vorticity balance we address the vortices vertical structure and vorticity dynamics by examination of a high resolution numerical model there is a reasonably good agreement between the model and observations simulated vortices are confined mostly to the mixed layer which increases westward in depth from 30 to 100 m near the western boundary in the eastern basin potential vorticity within the vortices is not conserved but decreases because of wind stress forcing in contrast in the western basin decreases in potential vorticity are due mostly to the vortices acquisition of southern hemisphere water we estimate an annualized cross equatorial transport of up to 1 2 sv associated with the vortices which is eventually contributed to the north brazil current system index terms 4231 oceanography general equatorial oceanography 4520 oceanography physical eddies and mesoscale processes 4528 oceanography physical fronts and jets keywords tropics instability atlantic

comparison of velocity estimates from advanced very high resolution radiometer in the coastal transition zone two methods of estimating surface velocity vectors from advanced very high resolution radiometer avhrr data were applied to the same set of images and the results were compared with in situ and altimeter measurements the first method used an automated feature tracking algorithm and the second method used an inversion of the heat equation the 11 images were from 3 days in july 1988 during the coastal transition zone field program and the in situ data included acoustic doppler current profiler adcp vectors and velocities from near surface drifters the two methods were comparable in their degree of agreement with the in situ data yielding velocity magnitudes that were 30 50 less than drifter and adcp velocities measured at 15 20 m depth with rms directional differences of about 60 these differences compared favorably with a baseline difference estimate between adcp vectors interpolated to drifter locations within a well sampled region high correlations between the avhrr estimates and the coincident geosat geostrophic velocity profiles suggested that the avhrr methods adequately resolved the important flow features the flow field was determined to consist primarily of a meandering southward flowing current interacting with several eddies including a strong anticyclonic eddy to the north of the jet incorporation of sparse altimeter data into the avhrr estimates gave a modest improvement in comparisons with in situ data

alkenone temperature anomalies in the brazil malvinas confluence area caused by lateral advection of suspended particulate material alkenone temperatures derived from suspended particulate organic material which was collected in austral summer 2001 from surface waters 5 m south of the brazil malvinas confluence deviate from measured temperatures by 4 to 7 c when uk 37 ratios are converted to temperature using the muller et al 1998 calibration and up to 3 c when using the calibration of conte et al 2006 in contrast alkenone temperatures determined from particulate material sampled north of the confluence reveal close correspondence to in situ temperatures or show slightly higher values we suggest that the southern samples are biased by suspended organic detritus originating from the cold subpolar waters of the northward flowing malvinas current whereas the northern samples carry an uk 37 signal of tropical subtropical origin transported southward with the brazil current on the basis of surface ocean transport pathways and velocities simulated with the large scale geostrophic lsg ocean general circulation model we identify areas of the world ocean where alkenone temperatures are potentially biased to higher or lower values due to long particle residence times and lateral advection by surface currents

three dimensional dynamics of the subsurface countercurrents and equatorial thermostad part i formulation of the problem and generic properties a fully three dimensional primitive equation simulation is performed to reunite the local equatorial dynamics of the subsurface countercurrents sccs and thermostad with the large scale tropical ventilated ocean dynamics it captures i the main characteristics of the equatorial thermostad the sccs location and their eastward evolution and the potential vorticity budget with its equatorial homogenization to zero values and ii the largescale meridional shoaling of the thermocline equatorward it supports the idea that the two dimensional hadley cell mechanism proposed by marin et al is a candidate able to operate in a fully three dimensional ocean the main difference between the 2d hadley cell mechanism and the oceanic 3d case is that for the 3d case the large scale meridional velocity at zeroth order is geostrophic while the cell mechanism is a next order smallscale mechanism a detailed budget of the zonal momentum equation is provided for the ageostrophic dynamics at work in the sccs the mean meridional advection and the coriolis term dominate discounting the possibility that lateral eddies play a major role for the sccs creation a 3 layer idealized ventilation model calibrated to the three dimensional simulation parameters is able not only to capture the tropical density structure but also to isolate the main controlling factors leading to the triggering of the equatorial secondary cells with its associated jet and thermostad namely the shoaling of the equatorial thermocline because of low potential vorticity injection at distant subduction latitudes it is also shown that equatorial recirculation gyres play a quantitative role that may be of the same order of magnitude as ventilation from higher latitudes

variations in the malvinas current volume transport since october 1992 1 we present a set of new current meter measurements collected to monitor the malvinas current mc near its merger with the brazil current at 40 41 s from december 2001 to february 2003 below a jason 1 altimeter track these measurements are compared to former measurements obtained 8 years earlier at the same location they also provide new information on the core of the mc on the continental slope above the 1000 m isobath where a mooring had been previously lost there most of the velocity variation is along isobath 80 of the variance and shows a significant annual cycle the two data sets provide coherent means and statistical parameters on the vertical structure of the flow a 14 year long time series of mc volume transport is derived using satellite altimetry the good correlation between the altimetry derived transport and the transport estimated from the current meter data persists in time over 0 7 for each measurement period a spectacular shift in the spectral composition of transport variations was observed from 1992 until the end of 1997 transport variations occurred at rather short periods 50 90 days and to some degree around 180 days whereas after year 2000 longer periods including a seasonal cycle predominated altimetry derived anomalies of surface geostrophic velocities along the core of the mc show a similar shift in spectral composition suggesting a remote forcing origin

reply to comment on comparison of velocity estimates from advanced very high resolution radiometer in the coastal transition zone by k a kelly and p t strub

investigation of brazil current rings in the confluence region 1 topex poseidon derived along track sha climatological temperature and salinity fields were used within a two layer scheme to estimate the depth of the 8 c isotherm in the southwestern atlantic these fields were used to monitor the formation and characteristics of the brazil current warm core anticyclonic rings shed by the first meander trough after poleward excursions of the brazil current bc results reveal that 40 warm core rings were shed by the bc between january 1993 and october 1998 the observed lifetime ranges between 1 and 4 months with a mean value of approximately 2 months at any given time two to three anticyclonic rings coexisted in the brazil malvinas confluence region most of the rings drifted southward without coalescing with their parent current only four rings were identified as being reabsorbed by the bc front after they were shed no evidence of propagation or absorption of these anticyclones into the eastern limb of the subtropical gyre was observed these rings have a mean horizontal length scale of 55 km mean upper layer thickness of 260 m and mean translation speed of 10 km d 1 volume anomaly and available potential energy computations showed a mean value of 3 6 1012 m3 and 2 5 1015 j respectively the upper layer transport of the bc was also computed and a relationship between variations in the southward transport and ring shedding activity was examined computation of the heat flux anomaly of the bc rings is estimated to be approximately 0 045 pw per annum compilation of these results indicates that warm core rings created by meandering boundary current extensions in different regions are generally similar

water masses and currents in the upper tropical northeast atlantic off northwest africa recent current measurements in the tropical eastern north atlantic reproduce the components of the large scale flow field however the observations as well as the 1 12 flame model computations indicate that a lot of eddy scale variability is superimposed on the mean flow field despite of the disturbance by variability the signature of the guinea dome is well present in november 2002 the guinea dome transport from direct observations was about 2 8 sv above 25 8 kg m3 and 4 sv between 25 8 and 27 1 kg m3 the oxygen minimum in the shadow zone comprises the central water and the antarctic intermediate water aaiw layers and is located between the equatorial current system and the north equatorial current the north equatorial counter and undercurrents at 3 to 6 n are major oxygen sources for the central water layer of the low oxygen regions in the northeastern tropical atlantic a second northern north equatorial countercurrent nnecc band exists at 8 to 10 n the nnecc carries oxygen rich water from the southern hemisphere eastward but with an admixture of water from the northern hemisphere a float at 200 m depth was spreading eastward in the north equatorial undercurrent neuc at 28 w it shifted northward into the nnecc and then was trapped in the guinea dome region for more than 3 years the model indicates the region 22 to 32 w as the area of exchange between the necc neuc and the nnecc bands in the aaiw layer the northern intermediate countercurrent acts as oxygen source for the oxygen minimum zone