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

seasonal fluctuations of the surface north equatorial countercurrent necc across the pacific basin 1 based on multisatellite derived sea surface current sea surface height anomaly and geostrophic current data the annual variability of the surface necc across the pacific basin is investigated inferred from the surface current data the necc axis east and west of the dateline moves northward along its pathway in the latter half of a year and the first half of a year respectively the sea surface height anomaly in the necc region and the meridional movement of necc are well reproduced by a linear wind driven first mode baroclinic rossby wave model model experiments by setting the phase speed of baroclinic rossby wave or the ekman pumping to zero in different regions are carried out to clarify the influence of rossby wave and local ekman pumping on the movement of necc east of the dateline both ekman pumping and rossby wave are important in determining the northward movement of the necc axis however the necc axis west of the dateline moves northward primarily due to the westward transmission of positive sea surface height anomalies or downwelling rossby waves that originate in the east of the dateline the local ekman pumping plays a minor role in controlling the migration of the necc axis west of the dateline

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

decadal variability of east australian current transport inferred from repeated high density xbt transects a ctd survey and satellite altimetry a time series of the net geostrophic transport through the tasman sea representing the flow of the east australian current eac extension is determined from a full depth ctd section 15 years of high density xbt transects and satellite altimetry data a section between sydney and wellington px34 has been occupied four times per year since 1991 with high resolution xbt sampling two methods to infer baroclinic transport from proxy data along the section are presented the first uses shallow xbt transects to derive geostrophic transport relative to a deep 2000 m reference level in the second approach synts the subsurface temperature and salinity structure are inferred from satellite surface height and temperature fields using a model developed from historical in situ observations the baroclinic transport is then computed in the usual manner the methods are validated using both a full depth ctd occupation of the px34 section and further transects crossing the eac in the northern tasman sea there is close agreement between the 49 xbt and synts px34 transport estimates obtained between 1992 and 2006 the time series of transport through the sydney wellington section shows a range of temporal signals from eddyscale seasonal interannnual to decadal in particular we note that the net eac flow ranges from 5 sv in 1995 to a maximum of 16 sv in 2000 2001 this decadal variation confirms the eac response to a spin up of the south pacific circulation forced by changes in the basin wide winds and matches the changes in oceanic properties observed in the tasman sea

south equatorial undercurrent in the western to central tropical atlantic 1 the south equatorial undercurrent seuc in the western to central tropical atlantic is investigated by a combination of shallow floats with a few acoustically tracked shipboard current measurements and hydrography float trajectories show a well confined seuc revealing large standing meanders near its western origin transports determined from 31 sections across the seuc increase from 5 6 sv at 35 w near the western boundary to 10 2 sv 800 km farther east internal recirculations north and south of the seuc were indicated by the float trajectories and a weak transport reduction farther along its eastward progression is observed the deep part of the south equatorial current carries on both sides of the seuc interior water masses westward and supplies almost 5 sv to the seuc between 35 w and 28 w or about half of the seuc transport in the interior tropical atlantic

diagnosing the annual cycle of the equatorial undercurrent in the atlantic ocean from a general circulation model abstract ten year long output series from a general circulation model forced by daily realistic winds are used to analyze the annual cycle of the equatorial undercurrent euc in the atlantic ocean two well defined transport maxima are found one present during boreal summer and autumn in the central part of the basin is generally recognized and regarded as a near equilibrium response to the equatorial easterly trades that culminate in this period another one most pronounced near the western boundary occurs in april may when the trades relax this second maximum is less patent in the observations but concomitant signals in previously published analyses of the north brazil current and surface velocity seasonal variations might be indirect manifestations of its reality because this intensification appears at periods when the boundary between the tropical and equatorial gyres nears the equator the authors relate its existence to wind stress curl variations at subequatorial latitudes a link between

changes in the ventilation of the oxygen minimum zone of the tropical north atlantic changes in the ventilation of the oxygen minimum zone omz of the tropical north atlantic are studied using oceanographic data from 18 research cruises carried out between 28 5 and 23 w during 1999 2008 as well as historical data referring to the period 1972 85 in the core of the omz at about 400 m depth a highly significant oxygen decrease of about 15 mol kg 1 is found between the two periods during the same time interval the salinity at the oxygen minimum increased by about 0 1 above the core of the omz within the central water layer oxygen decreased too but salinity changed only slightly or even decreased the scatter in the local oxygen salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and or zonal jets acting on the background gradients here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the omz contribute to the ventilation of the omz a conceptual model of the ventilation of the omz is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the omz according to the model a weakening of zonal jets which is in general agreement with observed hydrographic evidences is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the north atlantic omz

equatorial currents transport changes for extreme warm and cold events in the atlantic ocean 1 in this work the equatorial atlantic ocean circulation dynamics for warm and cold composite events are analyzed warm and cold typical years were composited from the respective five warmest and the five coldest years of the available record equatorial currents velocity and transport are calculated during a typical warm event the circulation decreases in the upper ocean while during the typical cold event there is intensification the strengthening weakening of the circulation during cold warm events increases decreases the stress between the equatorial undercurrent euc and the south equatorial current sec associated with an increase decrease in entrainment of cold waters into the surface and their westward transport the euc showed striking differences between both composited events for the cold warm event the euc shows a greater reduced transport and core velocity and a deeper shallower structure

oxygen minimum zone in the north atlantic south and east of the cape verde islands the open ocean oxygen minimum zone omz south and east of the cape verde islands is studied from ctd hydrography adcp velocities argo float trajectories and historical data with a focus on the zonal supply and drainage paths the strongest oxygen minimum is located north of the north equatorial countercurrent necc at about 400 to 500 m depth just above the boundary between central water and antarctic intermediate water aaiw it is shown that the necc the north equatorial undercurrent at 4 to 6 n and a northern branch of the necc at 8 to 10 n are the sources for oxygen rich water supplied to the omz in summer and fall a weak eastward necc at 200 m depth also exists in winter and spring as derived from argo floats drifting at shallow levels historical oxygen data from 200 m depth confirm this seasonality showing high low oxygen content in summer and fall spring within the supply paths compared to the strong oxygen supply at 150 to 300 m depth the ventilation of the omz at 300 to 600 m depth is weaker westward drainage of oxygen poor water takes place north of the guinea dome i e north of 10 n most pronounced at 400 to 600 m depth in july 2006 the total eastward transport of both necc bands above 27 1 kg m 3 at 23 w was about 13 sv 1 sv 106 m3 s 1 about half of this water volume circulates within the guinea dome or recirculates westward north of the guinea dome

interaction between the atlantic meridional and nino modes 1 interaction between the tropical atlantic meridional and nino modes is investigated using observations and a quasi analytical linear equatorial wave model it is found that equatorial zonal wind stress anomalies associated with the boreal spring meridional mode generate eastward propagating equatorial kelvin waves in the central and eastern atlantic ocean where variability associated with the nino mode is strongest these same wind stress anomalies force westward propagating equatorial rossby waves that reflect at the western boundary into eastward propagating kelvin waves during boreal spring and summer the boundary generated kelvin waves are of the opposite sign to those directly forced by the winds earlier in the spring so they tend to damp the nino mode during boreal summer the interaction between the boreal spring meridional mode and the summer nino is therefore mediated by directly wind forced equatorial kelvin waves and the delayed negative feedback from western boundary reflections of wind forced rossby waves

a census of north brazil current rings observed from topex poseidon altimetry 1992 1998 six years of topex poseidon altimeter data are used to investigate the formation of rings and eddies shed by the north brazil current upper layer thickness maps were used to identify 34 of these features formed in the north brazil current retroflection region an average of more than 5 rings and eddies per year the ensemble of ring trajectories closely parallels the 500 m isobath and one out of six rings penetrate into the caribbean sea through the southern lesser antilles the rest of the rings and eddies follow a northern trajectory past barbados once they reach 58w their estimated mean translation speed is 14 km day and their mean length scale is approximately 100 km our results suggest that the formation rate of nbc rings and eddies is nearly twice that previously thought and that they may account for more than 1 3 of the interhemispheric transport within the atlantic meridional overturning cell

variations of sea surface temperature wind stress and rainfall over the tropical atlantic and south america abstract empirical orthogonal functions e0fs and composite analyses are used to investigate the development of sea surface temperature sst anomaly patterns over the tropical atlantic the evolution of large scale rainfall anomaly patterns over the equatorial atlantic and south america are also investigated the eof analyses revealed that a pattern of anomalous sst and wind stress asymmetric relative to the equator is the dominant mode of interannual and longer variability over the tropical atlantic the most important findings of this study are as follows atmospheric circulation anomalies precede the development of basinwide anomalous sst patterns over the tropical atlantic anomalous sst originate off the african coast simultaneously with atmospheric circulation anomalies and expand westward afterward the time lag between wind stress relaxation strengthening and maximum sst warming cooling is about two months anomalous atmospheric circulation patterns over northern tropical atlantic are phase

relationship of topex poseidon altimetric height to steric height and circulation in the north pacific topex poseidon altimetric height is compared with 20 transpacific eddy resolving realizations of steric height the latter are calculated from temperature expendable bathythermograph xbt and salinity expendable conductivity and temperature profiler xctd profiles along a precisely repeating ship track over a period of 5 years the overall difference between steric height and altimetric height is 5 2 cm rms on long wavelengths 500 km containing 17 of the steric height variance the 3 0 cm rms difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the xbt section the 2 4 cm rms difference in the basin wide spatial mean appears to be due to fluctuations in bottom pressure differences between steric height and altimetric height increase near the western boundary but data variance increases even more and so the signal to noise ratio is highest in the western quarter of the transect basin wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement the 1 9 104 m2 s 1 rms difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current a linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data errors diminish from 0 8 c at 200 m to 0 3 c at 400 m geostrophic volume transport 0 800 m shows agreement that is similar to surface transport with 4 8 sverdrup sv 106 m3 s 1 rms difference the combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean the continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized this includes salinity measurements which significantly reduce errors in specific volume and steric height

response of the equatorial atlantic ocean to the 1983 1984 wind from the programme francais ocean et climat dans l atlantique equatorial cruise data set variations of the zonal pressure gradient zpg and the equatorial undercurrent euc are described in the equatorial atlantic ocean between 34 30 w and 6 30 e as observed during eight pairs of programme francais ocean et climate dans l atlantique equatorial cruises between october 1982 and august 1984 in 1982 and 1983 the annual cycle of weak trades early in the year followed by comparatively strong and sustained trades were relatively normal in the boreal winter of 1984 the extent of the collapse of the trade winds was exceptional and consequences were observed in the upper layers of the ocean along the equator the 20 c isotherm depth representative of the thermocline depth was unusually deep along the equator in the gulf of guinea a near zero value of the zpg was observed from the greenwich meridian westward an eastward surface current separated from the euc by a layer of reduced velocity was observed over much the same fetch west of 10 w the maximum velocity of the euc showed very little seasonal variation usually between 70 and 80 cm s 1 except in the boreal fall of 1983 when it exceeded 100 cm s 1 during this period in both 1983 and 1984 the euc transport at 23 w reached 25 106 m3 s 1 or twice its annual average which derived not only from the increase in speed but also for a deepening of the current to below 100 m it was also the season when the zpg was strongest and has a similar amplitude at both the surface and 50 dbar relative to 500 dbar the seasonal variations of the euc in the gulf of guinea were even more pronounced during the boreal summer and fall the current weakened while the local zpg reversed direction compared to the west the stronger penetration of the euc into the gulf occurred in the alternate seasons when there was no local reversal in the eastward acting zpg force the large changes in the oceanic and atmospheric circulation during the 1983 1984 period are hypothesized to be related to the suppression of the coastal period are hypothesized to be related to the suppression of the coastal benguela upwelling along the namibia coast this atlantic equivalent to el nino occurred 1 year after the exceptionally strong event in the pacific

local and equatorial forcing of seasonal variations of the north equatorial countercurrent in the atlantic ocean abstract the seasonal variation of the north equatorial countercurrent necc in the tropical atlantic ocean is investigated by using a linear one layer reduced gravity ocean model and by analyzing sea surface height ssh data from ocean topography experiment topex poseidon t p altimeters the t p data indicate that the seasonal variability of the necc geostrophic transport between 3 and 10 n is dominated by ssh changes in the southern flank of the current since the southern boundary of the necc is located partially within the equatorial waveguide the ssh variation there can be influenced considerably by the equatorial dynamics therefore it is hypothesized that the wind stress forcing along the equator is the leading driver for the seasonal cycle of the necc transport the wind stress curl in the necc region is an important but smaller contributor this hypothesis is tested by several sensitivity experiments that are designed to separate the two forcing mechanisms in the first sensitivity ru

what can we learn from global altimetry hydrography comparisons differences and complementarities between altimeter and hydrographic data are studied to describe the sea level and ocean circulation variations in terms of steric and non steric contributions altimeter sea level anomalies sla and hydrographic dynamic height anomalies dha agree remarkably well they also reveal however systematic large scale differences in several areas of the pacific and indian oceans that are related to a barotropic sverdrup like response of the ocean to wind forcing in addition the use of the two data sets provides a good estimation of the partition between barotropic and baroclinic modes needed for data synthesis or assimilation studies

topex poseidon the 2 cm solution the unprecedented accuracy of topex poseidon t p altimeter data warrants a new evaluation of the methods typically used to form time series of sea level change whereas explicit removal of orbit error has always been required as a first step in altimeter data processing the t p analysis presented here is based simply on unadjusted monthly averages this approach has the advantage of retaining the large scale ocean signal which would be distorted by orbit adjustment using 16 months of data we have evaluated the t p monthly means on spatial scales ranging from mesoscale to global in the tropical pacific comparisons with 17 island tide gauge records and dynamic height derived from 36 thermistor moorings show that the altimeter data have an accuracy of approximately 2 cm when averaged over spatial scales of a few hundred kilometers on basin scales in the northern hemisphere similar agreement is found between the t p data and the dynamic height climatology of levitus 1982 these new altimeter observations are thus providing the first reliable view of global sea level changes on seasonal to interannual timescales

deformation associated with gypsum karst in the salt river escarpment northeastern alberta an escarpment formed by the carbonate keg river and evaporitic chinchaga formations of middle devonian age extends southwards from the slave river 30 km west of fort smith northwest territories newly described folds in the bedrock are due to hydration of the anhydrite to gypsum in the chinchaga formation underlying the escarpment local groundwater flow has also dissolved subsurface cavities in the chinchaga formation as these openings grew the carbonates of the keg river formation subsided along stepped normal faults depressions within the fault blocks occurred where subsidence was concentrated along joints

interannual north equatorial countercurrent variability and its relation to tropical atlantic climate modes 1 a synthesis product of the surface geostrophic circulation is used to quantify the interannual variability of the wind driven north equatorial countercurrent necc the first mode of a complex empirical orthogonal function ceof decomposition of zonal geostrophic velocity in the necc region reveals pronounced latitudinal displacements in addition to variations in current strength while north south migrations of the necc are mainly captured by the real pattern the imaginary pattern accounts for variations in its strength associated with these spatial patterns is prevailing propagation toward the northwest that is most pronounced northward of the mean necc position 6 n there is evidence that the zonal propagation characteristics are consistent with long rossby waves forced in the northeastern tropical atlantic the relationship between interannual necc variations and the tropical atlantic climate modes is investigated through regression and composite analyses sea surface temperature and wind stress patterns resembling the meridional and zonal modes are found for the ceof regression composite analysis further shows consistent patterns for warm phases of the meridional mode and cold phases of the zonal mode the response of the necc to a positive meridional and negative zonal mode event may be viewed as a northward shift of its core and a current strengthening respectively these results support a link between the two dominant tropical atlantic climate modes and show that the relation between interannual necc variability and the meridional and zonal modes can primarily be regarded as a response to changes in the wind field

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

origin of magnetic highs at ultramafic hosted hydrothermal systems insights from the yokoniwa site of central indian ridge high resolution vector magnetic measurements were performed on an inactive ultramafic hosted hydrothermal vent field called yokoniwa hydrothermal field yhf using a deep sea manned submersible shinkai6500 and an autonomous underwater vehicle r2d4 the yhf has developed at a non transform offset massif of the central indian ridge dead chimneys were widely observed around the yhf along with a very weak venting of low temperature fluids so that hydrothermal activity of the yhf was almost finished the distribution of crustal magnetization from the magnetic anomaly revealed that the yhf is associated with enhanced magnetization as seen at the ultramafic hosted rainbow and ashadze 1 hydrothermal sites of the mid atlantic ridge the results of rock magnetic analysis on seafloor rock samples including basalt dolerite gabbro serpentinized peridotite and hydrothermal sulfide showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of yokoniwa these observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite comparisons with the rainbow and ashadze 1 suggest that in ultramafic hosted hydrothermal systems strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite after the completion of serpentinization and production of hydrogen pyrrhotites convert into pyrite or nonmagnetic iron sulfides which considerably reduces their levels of magnetization our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic hosted hydrothermal systems furthermore the results highlight the use of near seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems

new cnes cls09 global mean dynamic topography computed from the combination of grace data altimetry and in situ measurements 1 an accurate knowledge of the ocean mean dynamic topography mdt is mandatory for the optimal use of altimetric data including their assimilation into operational ocean forecasting systems a new global 1 4 resolution mdt was computed for the 1993 1999 time period with improved data and methodology compared to the previous rio05 mdt field first a large scale mdt is obtained from the cls01 altimetric mean sea surface and a recent geoid model computed from 4 5 years of grace gravity recovery and climate experiment data altimetric sea level anomalies and in situ measurements are then combined to compute synthetic estimates of the mdt and the corresponding mean currents while the rio05 mdt was based on 10 years of in situ dynamic heights and drifting buoy velocities the new field benefits from an enlarged data set of in situ measurements ranging from 1993 to 2008 and includes all hydrological profiles from the argo array moreover the processing of the in situ data has been updated a new ekman model was developed to extract the geostrophic velocity component from the drifting buoy measurements the handling of hydrologic measurements has also been revisited compared to the previous rio05 solution the new global mdt resolves much stronger gradients in western boundary currents with mean velocities being doubled in some places moreover in comparison to several other recent mdt estimates we find that the new cnes cls09 mdt is in better agreement with independent in situ observations

variability of the atlantic off equatorial eastward currents during 1993 2010 using a synthetic method 1 we have developed validated and applied a synthetic method to monitor the off equatorial eastward currents in the central tropical atlantic this method combines high density expendable bathythermograph xbt temperature data along the ax08 transect with altimetric sea level anomalies slas to estimate dynamic height fields from which the mean properties of the north equatorial countercurrent necc the north equatorial undercurrent neuc and the south equatorial undercurrent seuc and their variability can be estimated on seasonal to interannual timescales on seasonal to interannual timescales the synthetic method is well suited for reconstructions of the necc variability reproduces the variability of the neuc with considerable skill and less efficiently describes variations of the seuc which is located in a region of low sla variability a positive correlation is found between interannual variations of the necc transport and two indices based on an interhemispheric sea surface temperature sst gradient and southeasterly wind stress in the central tropical atlantic the neuc is correlated on interannual timescales with ssts and meridional wind stress in the gulf of guinea and zonal equatorial wind stress this study shows that both altimetry and xbt data can be effectively combined for near real time inference of the dynamic and thermodynamic properties of the tropical atlantic current system

estimates of the zonal slope and seasonal transport of the atlantic north equatorial countercurrent we use data from six inverted echo sounder moorings and the geosat satellite altimeter to examine the seasonal variability of sea surface elevation monthly sea level maps are constructed using a contemporaneous subsurface temperature survey to provide a reference sea level field the maps are then used to describe the origin and structure of the western tropical atlantic north equatorial countercurrent necc during a 2 year period beginning in november 1987 the data reveal a zonal current which is confined between 3 n and 9 n with a typical width of 300 km the necc flows strongly eastward during november and december 1986 and may 1987 through january 1988 the reappearance of the current is then delayed until august but the current flows strongly from august until the end of our record in october 1988 volume transport is estimated for the 2 year period from surface elevation by approximating the vertical structure of the ocean as a two layer fluid we find that the necc has a maximum transport of 40 106 m3 s 1 at 38 w this water has its origin in the retroflection of the north brazil current eastward of 38 w the necc loses strength as water exits the current to join the currents further south

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

observations of near surface currents and temperature in the central and western tropical atlantic ocean eleven satellite tracked drifting buoys were deployed in the central south atlantic ocean during two austral summer and two austral winter cruises between 7 s and 11 s and 23 w and 31 w during austral winter net buoy drift was to the west surface geostrophic flow was to the east between 7 s and 9 s it is proposed that strong southeast trade winds can induce directly driven surface flows to the west that are more intense than the eastward geostrophic flows associated with the south equatorial countercurrent secc a sustained period of eastward drift within the secc was observed during austral summer when the trades are weaker the trajectories indicate surface waters north of 8 s have a mean northward meridional component and those south of 8 s a southward component the buoys which drifted north became entrained into the north brazilian coastal current nbcc and those that drifted south into the brazil current one buoy left the nbcc at about 5 n to drift northeastward in the north equatorial countercurrent necc this trajectory and historical ship drift reports suggest that the necc may extend only to 35 w to 40 w during boreal winter temperature data obtained as the buoys drifted westward and northward suggest that increases in upper layer heat content can be attributed to heat fluxes through the sea surface