average seasonal variation of the atlantic equatorial currents from historical ship drifts abstract surface currents in the tropical atlantic were studied using historical ship drift data these are the only available data capable of resolving the long term seasonal fluctuations of currents over a broad geographical region the north equatorial countercurrent was found to extend as a continuous eastward flowing current across the atlantic in the band 5 8 n from july to december the maximum average eastward velocity was 30 cm s 1 during july september east of 20 w the countercurrent continued throughout the year west of 20 w and from january to june the countercurrent disappeared and westward currents were observed average westward velocity reached 20 cm 1 in april and may centered near 40 w the annual cycle in surface velocity 20 cm s 1 amplitude agrees closely with that of geostrophic velocity inferred from variations in thermocline depth across the counter current jets centered near 2 n and 4 s separated by a minimum near 1 s the northern jet has a large 14 cm s 1 semiannual v

two views of the cold filament abstract two views of the cold filament first described by weatherly and kelley are presented the first is a local view near 40 n 62 w there its upslope edge is found to be a front which by benthic standards is large its downslope edge was not sampled what distinguishes this benthic front from others is that it is a permanent feature in the abyssal ocean above the cold filament relatively murky detached bottom layers were observed and tracked to where they separated from the bottom at the benthic front apparently these detached layers entrain overlying water a density jump at their base apparently restricts entrainment of underlying water primarily during the detachment process with comparably less entrainment thereafter the second view a regional one comes from examining historical hydrographic sections these indicate that the cold filament extends from the newfoundland ridge westward then southward to 24 n and possibly to 20 n along the base of the continental rise the cold filament i

the tritium krypton 85 age of denmark strait overflow water and gibbs fracture zone water just south of denmark strait the opposite time trends of the input of tritium and 85kr to the surface ocean produce a tritium 85kr ratio for surface water that is a strong function of time and this ratio was used to determine the age of denmark strait overflow water dsow and gibbs fracture zone water gfzw just south of denmark strait dsow and gfzw were identified by their temperature salinity oxygen and silica characteristics in a section of stations 460 km south of denmark strait taken during the transient tracers in the ocean north atlantic study expedition in 1981 dsow was the densest water observed in the section and there were two types a low salinity type and a slightly higher salinity more dense type both types originated from arctic intermediate water aiw behind the greenland iceland ridge the tritium and 85kr data reveal that the low salinity type resided behind the greenland iceland ridge for about 1 year before flowing into the irminger sea compared to about 15 years for the higher salinity type the volume transport of the low salinity type of dsow was estimated to have a lower limit of 0 8 sv gfzw forms in the northeastern atlantic from a mixture of water flowing out of the norwegian sea at about 900 m depth and the northeastern atlantic water into which it flows about 70 of the tritium and 85kr burden of gfzw comes from northeastern atlantic water and 30 from norwegian sea water the age of gfzw just south of denmark strait relative to its formation in the northeastern atlantic is 7 5 4 6 5 years which corresponds to a mean current speed of 1 6 10 1 0 6 cm s

evidence for large scale eddy driven gyres in the north atlantic analysis of a recent climatological database for the north atlantic has detailed an extensive large scale recirculation system in the intermediate waters of the north atlantic basin the pressure fields that define this recirculation coupled with the potential vorticity fields associated with the recirculating flow provide observational evidence for basin scale eddy driven flow in the global ocean the recirculations are intimately tied to the waters carried southward by the deep western boundary current and are therefore likely to affect the distribution of climatic anomalies in the north atlantic

zonal jets in the equatorial atlantic ocean abstract we use position data from argo floats smoothed out over 400 km 200 km zonal ellipses and interpolated onto a 0 5 grid to investigate the zonal jet structure of the flow field at the sea surface and on three subsurface layers central waters cw 200 m antarctic intermediate waters aaiw 1000 m upper north atlantic deep waters unadw 1500 m in the equatorial atlantic ocean 15 s to 15 n the annual mean fields exhibit narrow zonal jets typically 4 5 wide at the sea surface and only 2 at the subsurface levels with directions alternating in latitude and maximum speeds about 0 5 m s 1 at the surface 0 1 m s 1 at cw and unadw and 0 03 m s 1 at aaiw the available data also allows us to explore the seasonal variability of these jets at the surface and aaiw levels the surface currents are dominated by an annual cycle between 4 n and 10 n and to a lesser degree by a semi annual contribution close to the equator this variability is an outcome of evolving zonal recirculations with the north equatorial countercurrent necc arising from the diversion of the northern branch of the south equatorial current nsec the diversion begins in the eastern atlantic and propagates west between april and august following the inter tropical convergence zone itcz the aaiw current field is largely affected by westward propagating anomalies most visible near 3 s 0 3 n and 7 n which give rise to current reversals annual averaging produces the illusion of more 5 instead of 3 and slower peak values about 0 03 m s 1 instead of 0 1 m s 1 jets than found on any month

using a cfc effective age to estimate propagation and storage of climate anomalies in the deep western north atlantic ocean 1 1990s chlorofluorocarbon cfc observations are used to estimate an effective age of 20 years for north atlantic deep water nadw at the equator the effective age is found by subtracting a relic age from pcfc 11 age the effective age is several to 10 years younger at the equator than other methods from the effective age equatorward effective spreading rates of 1 2 cm s are found which are similar for the nadw components effective spreading rates take into account recirculation gyres along equator flow and mixing which slow spreading of climate anomalies from the north atlantic to the southern hemisphere the difference between direct velocities and tracer effective spreading rates suggests that the buffering effect due to storage in the western north atlantic and equatorial region is about 20 years even so warming and freshening due to greenhouse gases will be first felt in the deep waters of the western north atlantic

improving the utility of ocean circulation models through adjustment of the momentum balance we propose a new method to improve the utility of three dimensional ocean circulation models the method uses climatological temperature and salinity data to adjust the momentum balance of the model while leaving the tracer equations fully prognostic and unconstrained the adjustment is accomplished by replacing density in the hydrostatic equation by a linear combination of model computed and climatological density the procedure is equivalent to adding a forcing term to the horizontal momentum equation through a modification of the model s horizontal pressure gradient term the forcing term modifies the model computed velocity field which in turn affects the model computed temperature and salinity fields through the advection term there is no adjustment of the tracer equations carried by the model assuming the linear combination coefficient to be invariant in time and space we suggest a statistical approach to estimating its optimal value we apply this semiprognostic method to the northwest atlantic a primitive equation circulation model is initialized with january climatological temperature and salinity and is forced by monthly mean comprehensive ocean atmosphere data set surface wind stress and heat flux by restoration of the surface salinity to monthly mean climatology and by flows through the open boundaries both the model computed tracer and velocity fields produced using the semiprognostic method show significant improvement over those produced by a purely prognostic calculation drift of the tracer and velocity fields away from climatology is greatly reduced further convective mixing is explicitly represented thus improving the utility of results over those obtained from pure diagnostic calculations the velocity fields obtained with the new approach are somewhat more realistic than those obtained from pure diagnostic calculations the method reproduces many well known circulation features in the region including the labrador current the gulf stream and the north atlantic current more significantly the method reproduces reasonably well the seasonal evolution of temperature and salinity in the region despite the fact that the model s tracer fields are not constrained directly by the new method this result suggests that the semiprognostic approach will be useful for examining the evolution of tracers that are not easily determined by observations

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

artificial radionuclide tracer supply to the denmark strait overflow between 1972 and 1981 measurements of the concentrations of the artificial radionuclides 3h 137cs and 90sr in the northern irminger sea in 1972 and 1981 are reported in both years tracer measurements from this area included data from samples of the dense overflow water from the north through denmark strait all three tracers were strongly correlated inversely with salinity in the dense outflows the tracer maxima being related directly to the salinity minimum when the tracer characteristics in the outflows in 1972 and 1981 were compared concentrations of all in 1981 were observed to be about double the 1972 values the individual tracer concentrations on a decay corrected density normalized basis were higher in increasing order 90sr 93 3h 115 137cs 150 the relatively greater increases for 3h and 137cs were attributed to contributions of new sources of these tracers in northern surface waters the 3h source is argued to derive from atmospheric hydrological recycling whereas the 137cs source is identified as the input to the greenland and iceland seas of advected european nuclear fuel reprocessing wastes both the tracer and hydrographic data are used to identify northern locations of intermediate water formation capable of supplying the observed dense overflow water characteristics it is argued from the time taken for the overflow water to reflect the new surface 137cs source that transport from the source to the overflow can be quite rapid about 2 years

tracing the thermohaline circulation in the western north atlantic using chlorofluorocarbons abstract measurement of chlorofluorocarbons cfcs f 11 and f 12 were obtained from vertical sections extending across the deep western boundary current dwbc along the western margin of the north atlantic ocean between 32 45 n in 1983 and 1986 these sections revealed an upper and a deep cfc maximun with highest concentrations along the continental slope the upper maximum had a potential temperature ranging from 4 2 4 6 c a core density of 1 5 34 56 and occurred at a depth of about 800m adjacent to the continental slope water with classical labrador sea water s characteristics occurs just above the cfc minimum between the upper and lower maxima and indicates that a significant volume of classical labrador sea water has not entered the region south of the grand banks for the last 3 decades the deep cfc maximum is from dense water overflowing the greenland iceland scotland ridge it occurred at about 3500m depth and had a potential temperature range of 2 02 2 18 c and a core density of 4 45 87 a source function of f 11 and f 12 for the deep maximum was estimated using temperature salinity and tracer data from previous studies the core of the dwbc defined by the deep cfc maximum was estimated to consist of roughly 2 3 gibbs fracture water and 1 3 denmark strait overflow water however 80 or more of the f 11 and f 12 signal was derived from denmark strait overflow water ages for the upper and lower cfc maxima in the dwbc were estimated from the cfc data these ages ranged from 11 12 years at the northernmost section to 18 years at the southernmost section for both features this corresponds to apparent mean current speeds of about 0 8cm s 1 for the upper maximum and 1 4cm s 1 for the deep maximum an age of 8 9 years with a corresponding apparent mean current speed of 2 2 2 5cm s 1 was also estimated for the deep cfc maximum from the observed increase in the f 11 concentration at 41 n between 1983 and 1986 the apparent mean current speeds are significantly less than current speeds measured with long term current meter moorings two reasons for this are 1 mixing between the dwbc core and adjacent water causes ages estimated from cfc data to be overestimated and hence the associated current speed to be underestimated pickart hogg and smethie 1989 and 2 water propagating along the western boundary spends some time in recirculation gyres the northern recirculation gyre south of the eastern north american continental margin is clearly apparent in the lateral distributions of f 11 and f 12 the apparent mean current speed in the northern recirculation gyre was estimated to be 1 3cm s 1 from the cfc data seaward of the dwbc north of the new england seamount chain high cfc low silica parcels of water were observed interspersed with low cfc high silica parcels of water along the bottom the high cfc low silica water is overflow water with a slightly greater component of denmark strait overflow water than contained in the dwbc deep cfc maximum the low cfc high silica water is of southern origin and has a component of antarctic bottom water

seasonal variability from a model of the tropical atlantic ocean abstract a numerical model incorporating a single baroclinic mode and realistic coastline geometry is used to analyze the linear dynamic response to estimates of the seasonal wind field over the tropical atlantic ocean the forced periodic response consists of a spatially dependent combination of a locally forced response kelvin waves rossby waves and multiple wave reflections the seasonal displacements of the model pycnocline are compared with observed dynamic height annual and semiannual fluctuations dominate the seasonal signal throughout the basin in general the distribution of amplitude and phase are similar for annual changes in dynamic height and pycnocline depth major features of the seasonal response are reproduced e g east west changes in pycnocline depth about a nodal point at the equator the seasonal pycnocline movement along the northern and southern coast of the guinea gulf and a significant changes of phase in the ocean variability north and south of the itcz the relative impo

the north atlantic tritium and radiocarbon transients 1972 1983 deep ventilation and water mass transformation processes in the north atlantic on decadal time scales are illustrated by the evolving distribution patterns of anthropogenic tritium and radiocarbon data from two quasi synoptics surveys of the interior of the north atlantic the geochemical ocean sections study geosecs and complementary oceanographic observation projects in 1972 and the transient tracers in the oceans tto in 1981 and 1983 are presented in comparative cross sections and maps representing conditions roughly one and two decades respectively after the first major tracer injections the discussion emphasizes comparisons of the decay corrected tritium concentration fields which show several distinct regimes of transient evolution including examples of surprising constancy in some regional patterns while largely supportive of previous qualitative ideas about transport patterns these results also suggest that the patterns of deep water injection in high latitudes must have undergone a major change around or soon after the time of geosecs 1972 involving a major increase in supply of upper north atlantic deep water the near surface waters show strong signatures of tropical low tritium influence in the southwestern and western part of the subtropical low tritium influence in the southwestern and western part of the subtropical gyre while the northeasternmore part is influenced by continued tritium input from arctic surface waters strong evidence for distinct sub basin scale interior circulation domains is found in the fact that the major distribution regime transitions seen in the geosecs data are observed again in tto and thus maintained through the second decade of the transient less

seasonal response of the sea surface to the wind in the equatorial atlantic from february 1983 through september 1984 18 inverted echo sounder deployments yielded time series from 10 locations as part of the seasonal response of the equatorial atlantic sequal program using historical and in situ data the sounders travel time data are interpreted as dynamic height 0 relative to 500 dbar and then related to representations of the synoptic wind field the seasonal variations of the zonal pressure gradient along the equator and the height of the dynamic ridge along 3 n relative to the equator and 9 n are described the pressure gradient between 10 w and 34 w has a minimum each april and a maximum each july its value ranges between 1 4 not significantly different from zero and 8 3 10 5 dyn g 1 during the observational period the zonal pressure gradient is correlated with zonal wind stress from an in situ wind sensor at midfetch and a 10 day lag time is indicated the north equatorial countercurrent trough attains a maximum in the late boreal summer and fall and a minimum in the spring it ranges from 39 to 8 dyn cm deep at 38 5 w and from 18 to 1 dyn cm at 28 w a two layer model calculation using monthly mean winds derived by servain et al 1986 for the appropriate time period generally reproduces the observed seasonal variation along the two meridians and the large zonal variation between them tracking the individual terms month by month illustrates the rate of the sverdrup adjustment to the local wind stress with surface elevation rising as fast as 10 cm per month

determining the strength of the deep western boundary current using the chlorofluoromethane ratio abstract the dilution of a passive tracer during deep water formation and the subsequent advection mixing in a deep boundary current is modeled with application to chlorofluoromethanes cfms in the north atlantic two different types of boundary currents are considered a uniform flow and a simple shear flow in each case the core of the flow mixes with surrounding water which continually accumulates cfms in an extreme case the coupled systems predicts that the cfm ratio in the current is unaltered from the ratio of its source water save for a time lag more realistic cases however suggests that the ratio is not a conserved quantity but is substantially altered in both the overflow basin and boundary current matching the model results to cfm data collected near the grand banks gives a predicted average core speed of 5 10 cm s 1 for the deep western boundary current and provides a constraint on the transport and diffusivity of the flow as well

changes across 66 w the caribbean sea and the western boundaries of the north atlantic subtropical gyre abstract a world ocean circulation experiment hydrographic program along with a global repeat hydrography program occupied sections along 66 w in the north west atlantic ocean in 2003 and 2012 hydrographic variables tracers and ladcp measurements were collected in situ this section together with the north american and south american coasts create a closed volume excluding any flow through the panama canal combining mass silica and ladcp information with constraints in an inverse box model we obtain a strong eastward flow of the gulf stream 100 1 4 6 sv in 2003 and 123 8 4 4 sv in 2012 and westward flows of the caribbean current 24 4 1 0 sv in 2003 and 24 2 1 1 sv in 2012 and deep western boundary current dwbc the dwbc carries the ventilated waters of the labrador sea and the nordic seas comparing the results of 2003 and 2012 an insignificant reduction is observed in the dwbc transport 17 3 2 9 sv and 15 0 2 5 sv for 2003 and 2012 respectively the net heat fluxes do not show major changes through the section during both cruises 0 23 0 08 pw in 2003 and 0 21 0 12 pw in 2012 in contrast freshwater fluxes present positive values in the dry season 0 24 sv in spring 2012 and negative values in the wet season 0 32 sv in fall 2003 the difference between both cruises may be due to seasonal variability in the area

circulation characteristics in three eddy permitting models of the north atlantic a systematic intercomparison of three realistic eddy permitting models of the north atlantic circulation has been performed the models use different concepts for the discretization of the vertical coordinate namely geopotential levels isopycnal layers terrain following sigma coordinates respectively although these models were integrated under nearly identical conditions the resulting large scale model circulations show substantial differences the results demonstrate that the large scale thermohaline circulation is very sensitive to the model representation of certain localised processes in particular to the amount and water mass properties of the overflow across the greenland scotland region to the amount of mixing within a few hundred kilometers south of the sills and to several other processes at small or sub grid scales the different behaviour of the three models can to a large extent be explained as a consequence of the different model representation of these processes

where and how long ago was water in the western north atlantic ventilated maximum entropy inversions of bottle data from woce line a20 1 a maximum entropy me method is used to deconvolve tracer data for the joint distribution of locations and times since last ventilation the deconvolutions utilize world ocean circulation experiment line a20 repeat hydrography for cfc 11 potential temperature salinity oxygen and phosphate as well as global ocean data analysis project glodap radiocarbon data combined with surface boundary conditions derived from the atmospheric history of cfc 11 and the world ocean atlas 2005 and glodap databases because of the limited number of available tracers the deconvolutions are highly underdetermined leading to large entropic uncertainties which are quantified using the information entropy of relative to a prior distribution additional uncertainties resulting from data sparsity are estimated using a monte carlo approach and found to be of secondary importance the me deconvolutions objectively identify key water mass formation regions and quantify the local fraction of water of age or older last ventilated in each region ideal mean age and radiocarbon age are also estimated but found to have large entropic uncertainties that can be attributed to uncertainties in the partitioning of a given water parcel according to where it was last ventilated labrador irminger seawater l water is determined to be mostly less than 40 a old in the vicinity of the deep western boundary current dwbc at the northern end of a20 but several decades older where the dwbc recrosses the section further south pointing to the importance of mixing via a multitude of eddy diffusive paths overflow water lies primarily below l water with young waters 40 a at middepth in the northern part of a20 and waters as old as 600 a below 3500 m

the western boundary undercurrent off the bahamas abstract two tritium sections through the deep western boundary current east of the bahamas taken in late 1980 and early 1981 are presented tritium from the bomb tests in the late 1950s and early 1960s is used to identify recently formed deep waters in the sections high concentrations are found in the north atlantic deep water low tritium values occur in the labrador sea water found above the core of this deep water this is consistent with the suggestion by talley and mccartney that this water mass has not been ventilated at the temperatures observed in these sections since the mid 1950s tritium in the sections is correlated with maxima in potential vorticity this is inconsistent with deep convection as a direct source for the water mass the potential vorticity maxima may be associated with plume dynamics near the overflow regions or with the dynamics of the deep western boundary current the sections are south of the section discussed by jenkins and rhines where high tritium concentrations were fo

the role of oscillating southern hemisphere westerly winds global ocean circulation abstractin the framework of a sea ice ocean general circulation model coupled to an energy balance atmospheric model an intensity oscillation of southern hemisphere sh westerly winds affects the

response of the surface tropical atlantic ocean to wind forcing abstract we use 10 years of satellite data sea level pressure surface winds and absolute dynamic topography adt together with argo inferred monthly mean values of near surface velocity and water transport to examine how the tropical system of near surface zonal currents responds to wind forcing the data is analyzed using complex hilbert empirical orthogonal functions confirming that most of the variance has annual periodicity with maximum amplitudes in the region spanned by the seasonal displacement of the inter tropical convergence zone itcz the adt mirrors the shape of the upper isopycnals hence becoming a good indicator of the amount of water stored in the upper ocean within about 3 from the equator where the coriolis force is small there is year long meridional ekman transport divergence that would lead to the eastward transport of the equatorial undercurrent and its northern and southern branches beyond 3 of latitude and at least as far as 20 the convergence of the ekman transport generally causes a poleward positive adt gradient which sustains the westward south equatorial current sec the sole exception occurs in summer between 8 n and 12 n when an ekman transport divergence develops and depletes de amount of surface water resulting in an adt ridge valley system which reverses the adt gradient and drives the eastward north equatorial countercurrent necc at latitudes 4 9 n in late fall divergence ceases and the necc drains the adt ridge so the adt gradient again becomes positive and the sec reappears the seasonal evolution of a tilted itcz controls the surface water fluxes the wind induced transports set the surface divergence convergence which then drive the adt and through the adt gradients create the geostrophic jets that close the water balance

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

strengthening kuroshio observed at its origin during november 2010 to october 2012 direct measurements of kuroshio at its origin 18 degrees n east of the luzon island are conducted from november 2010 to october 2012 it is found that the depth averaged kuroshio between 200 and 700 m has increased over 15 cm s 1 during the 2 year observational period and it is accompanied by the pronounced southward shift of the north equatorial current nec bifurcation further analysis indicates that the kuroshio s strengthening is confined to the upstream segment east of the luzon island while the kuroshio decreased as it passed the luzon strait due to a dipole like sea surface height ssh trend between 15 degrees n and 23 degrees n it is demonstrated that the 2 year strengthening of the kuroshio as well as the dipole like ssh trend can be adequately reproduced by a 1 5 layer nonlinear reduced gravity model suggesting an important role of upper ocean response to low frequency wind forcing in the western pacific salinity at 500 m depth is also found to increase during the concurrent 2 years this subthermocline salinity increase is a combined outcome of vertical basin scale isopycnal surface movement and horizontal advections i e strengthened kuroshio due to changes in the large scale wind driven ocean circulation

the forced annual reversal of the atlantic north equatorial countercurrent abstract we analyze the variability of the thermal structure associated with the atlantic north equatorial countercurrent necc and its relation to the seasonally varying winds the analysis performed allows us to establish the period of time and the region where the necc reverses direction west of 25 w the depth of the thermocline in the southern side of the necc 4 to 7 n annually oscillates 180 out of phase to the oscillations on the northern side 7 to 10 n east of 25 w the thermocline rises and falls nearly in phase across the countercurrent if the flow can be uniquely presumed from the thermal structure the necc disappears during the boreal spring in the western basin the reversal of phase from north to south over one part of the ocean but no the other is shown to be mirrored in the annual variation of the curl of the stress induced at the surface by the winds in order to quantify this result the different terms of the vorticity equation are calculated from the data we conclude that t

zonal circulation in the nw atlantic and caribbean from a meridional world ocean circulation experiment hydrographic section at 66 w a world ocean circulation experiment hydrographic program section along 66 w in the north atlantic was made in 1997 in addition to the usual variables hydrographic and tracer measured in woce onetime sections we made lowered acoustic doppler current profiler ladcp measurements at nearly all of the stations the section closed off a portion of the western north atlantic to the west of the line making a closed volume for constraining the circulation in addition the deep portions of the caribbean do not communicate with the rest of the basin by combining mass silica and ladcp information as constraints an inverse calculation obtained reveals a strong eastward transport in the gulf stream bounded by westward flowing water on either side within these energetic flows we see evidence for recently ventilated classical labrador sea water which has not reached the deep western boundary current north of puerto rico in any significant amounts within the caribbean our major new finding is a deep cyclonic circulation below sill depth in excess of 100 times the inflow of deep atlantic water through the major deep sill the anegada jungfern passage the signature of the deep atlantic source water is most prominent in cfcs and both a bottom and mid depth maximum are present in the caribbean to the south of puerto rico off the coast of venezuela however only the deeper cfc maximum is found for the entire section the net overturning circulation heat flux and freshwater fluxes are all consistent with expectations based on water mass formation and air sea exchanges to the west of our section but the annual mean air sea fluxes of heat and freshwater from comprehensive ocean atmosphere data sets appear somewhat too small in comparison with our single section result

a method for improved representation of dense water spreading over topography in geopotential coordinate models abstract a new technique for incorporating processes of near bottom tracer transport over sloping topography in geopotential coordinate ocean circulation models is presented it is based on a hybrid approach coupling a simple terrain following bottom boundary layer model to the standard ocean general circulation model tests in simplified configurations show enhanced downslope spreading of passive and active density tracers the method is helpful for both coarse resolution global climate studies and biogeochemical coastal modeling studies

seasonal cycle of velocity in the atlantic north equatorial countercurrent as measured by surface drifters current meters and ship drifts this report describes the general circulation and seasonal variation of currents in the equatorial atlantic concentrating on the north equatorial countercurrent necc using data collected as part of the seasonal response of the equatorial atlantic and programme francais ocean et climat dans l atlantique equatorial experiments plus historical ship drifts during 1983 1985 the lagrangian circulation was studied by launching and tracking 30 freely drifting drogued buoys in the north equatorial countercurrent and 23 in the south equatorial current sec in addition continuously recording current meters were moored for 20 months at depths from 20 to 300 m near the center of the necc at 6 n 28 w these measurements are the longest series ever obtained in this current and provide information about its seasonal and interannual variations and zonal meridional and vertical structure the seasonal cycle of the necc is very regular from year to year each year the necc starts up in may june it flows eastward across the atlantic with surface speeds of up to 143 cm s in the west extending down to 350 m at 28 w and flowing into both the guinea current and north equatorial current it disappears or reverses from about january june west of 18 w some near surface water of the necc is inferred to downwell and flow equatorward toward the equatorial undercurrent the sec flows into the north brazil current which during spring continues up the coast into the caribbean during fall however the whole north brazil current retroflects or turns back on itself between 45 and 50 w forming the western necc the retroflection establishes a meander pattern in the necc that slowly propagates westward during fall at a speed of 4 cm s the meanders have a displacement of about 300 km in latitude a wavelength of 900 km and meridional velocity fluctuations of up to 100 cm s the swift currents and time dependent meanders in the western necc cause a high eddy kinetic energy 2400 cm2 s2 equivalent to that of the energetic part of the gulf stream

impact of an adiabatic correction technique on the simulation of cfc 12 in a model of the north atlantic ocean 1 a model of the north atlantic ocean is used to simulate the spreading of cfc 12 from the labrador sea deep convection site the standard version of the model fails to capture the local maximum in cfc 12 concentration that is observed along the continental slope of the western boundary hydrographic data are used to apply a simple correction to the model s horizontal momentum equations the corrected model is much more successful at capturing the nearslope maximum in cfc 12 concentration than the uncorrected model and also exhibits a 50 increase of the deep southward export of cfc 12 at 24 n the difference between the two model runs is shown to be a consequence of the different paths taken by the deep western boundary current in the two model versions

on the total input of antarctic waters to the deep ocean a preliminary estimate from chlorofluorocarbon measurements 1 deep ocean inventories of dissolved chlorofluorocarbon 11 cfc 11 along representative sections off antarctica provide the first estimate of the overall strength of all dense water sources in the southern ocean their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the thermohaline circulation thc the bottom layer is supplied via sinking of antarctic bottom water aabw produced at a few continental shelves the middle layer receives the offshore injection of ventilated modified circumpolar deep water mcdw produced along much of the lengthy antarctic slope front the top layer is ventilated by northward export of antarctic surface water into the upper circumpolar deep water of the antarctic circumpolar current average southern ocean inputs to the upper two layers of the deep ocean for the 1970 1990 period are derived on the basis of the cfc 11 distributions along meridional sections the mean cfc 11 saturations of all water mass ingredients and the inferred mixing leading to production of dense source waters about 5 4 1 7 sv 1 sv 10 6 m 3 s 1 of near freezing shelf water ventilate the bottom layer and 4 7 1 7 sv and 3 6 1 3 sv of cold antarctic surface water ventilate the middle and top layers therefore the total contribution of ventilated southern ocean waters to the lower limb of the global thc is about 14 sv this is close to the about 17 sv estimated for north atlantic near surface sources from cfc 11 inventories their entrainment of 7 4 2 4 sv of cfc poor subsurface lower circumpolar deep water during the formation and sinking of aabw and mcdw raises the total southern ocean input to the deep ocean to about 21 sv

reconstructing the oceanic 13c suess effect the anthropogenic 13c change for the time period 1968 to 1991 was determined based on calculations of the preformed 13c 12c of dissolved inorganic carbon dic distributions on isopycnal surfaces in the main thermocline of the pacific north atlantic and south indian oceans the time rate of change of preformed 13c the 13c suess effect along isopycnals was calculated using cfc derived water ages and yields a time history of the surface water 13c change at the isopycnal outcrop location the surface ocean suess effect recorded on isopycnals decreased with increasing outcrop latitude from approximately 0 2 decade 1 within the subtropics to around 0 1 decade 1 in the subpolar oceans in the pacific ocean these surface 13c change rate reconstructions agree both in magnitude and meridional trend with direct observations of surface ocean 13c changes reported from time series measurements and from comparisons of surface water 13c of dic measurements in 1970 and 1993 a global ocean average surface 13c rate of change of 0 15 0 04 decade 1 is determined which is slightly smaller than a previous time series data and model based estimate 0 171 decade 1 bacastow etal 1996 depth integrations of the 13c reconstructions in the pacific indian and atlantic oceans when combined with these previous individual depth profile comparisons and geochemical ocean sections study bomb 14c inventories quay et al 1992 imply a global depth integrated 13c change rate of 9 7 2 4 m yr 1 over the time period 1970 1990 these results imply a net oceanic co2 uptake rate of 1 9 0 9 gt c yr 1 over the time period 1970 1990 when applied to an atmospheric 13co2 and 12co2 budget

advection of north atlantic deep water from the labrador sea to the southern hemisphere recently formed labrador seawater lsw and overflow water from denmark strait dsow are main components of the atlantic meridional overturning circulation both exhibit a distinct chlorofluorocarbon cfc maximum here we use 25 years of cfc observations in the atlantic to study the main features of the circulation of lsw and dsow from the cfc data the age and fraction of young deep water are inferred due to the superior spatial data resolution compared to former attempts regional differences in the spreading velocity and pathways of young deep water become evident dependent on the regional circulation the observed distributions of young lsw and dsow showed that the dwbc is the fastest pathway to reach the southern hemisphere the downstream decrease of the fractions of young lsw in the dwbc is slower compared to model studies from 47 n to 42 n dwbc transports of young lsw and dsow decrease by 44 and 49 respectively at 26 n the dwbc transport of young water is still 39 of the lsw formation rate and 44 of the dsow overflow transport interior pathways also exist especially in the subpolar north atlantic and in the transition zone between the subpolar and subtropical gyre compared to dsow the distributions indicate a higher tendency for lsw to follow additional interior pathways north of 45 n the major part of lsw is younger than 20 years the general weakening of new lsw formation since the 1990s worked toward a homogenization between the lsw in the western and the eastern subpolar north atlantic

transequatorial flow of antarctic bottom water in the western atlantic ocean abyssal geostrophy at the equator abstract in its general northward flow along the western trough of the atlantic antarctic bottom water aabw must pass over several sills separating the various abyssal basins at the equator the western trough is deformed by major east west offsets of the mid atlantic ridge and the continental margin of brazil forming a newly zonal abyssal channel about 250 km wide centered at the equator and extending approximately 1000 km along its axis in which the aabw is confined thus the general northward flow of aabw is topographically constrained to be westward as it crosses the equator a hydrographic section across this channel at 37 w shows the aabw isopycnals to be bowl shaped within and beneath the level of the channel walls the equatorial geostrophic relation permits us to compute a zonal velocity from the well defined parabolic distribution of dynamic height relative to a reference level at the transition between aarw and the overlying deep water here 4 3 106 m3 s 1 is estimated for the wes

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

a satellite derived climatology of the itcz abstract this paper presents fundamental climatological characteristics of the intertropical convergence zone itcz in a simple concise manner using the highly reflective cloud hrc dataset this satellite derived dataset uses both visible and infrared observations to measure the frequency of occurrence of large scale convective systems over the global tropics at a 1 spatial resolution these dataset characteristics make the hrc particularly well suited for climatological analysis of the itcz because the dataset is based on estimates of organized deep convective cloud systems rather than observations of clouds as a whole and it provides the spatial resolution needed to identify thew large wale convective structures furthermore the dataset covers a time period extending nearly two decades which provides for a fairly robust climatology and the opportunity to examine seasonal and interannual variability of both the convection and the latitude of the itcz

the formation of labrador sea water part iii the evolution of oxygen and nutrient concentration abstract oxygen nutrient and tritium concentrations observed in the western labrador sea in march 1976 during deep convective renewal of labrador sea water are analyzed to show how a newly formed water mass obtains its characteristics common to other winter observations of deep mixed layers the oxygen concentrations are some 6 undersaturated even in the upper 20 m a gas transfer model coupled to a simple mixed layer model illustrates the difficulty of transferring sufficient oxygen across the air sea boundary to fully oxygenate the mixed layer when the mixed layer depth exceeds a few hundred meters the nutrient concentrations of the mixed layers are fairly well mixed as is consistent with the fairly narrow range of nutrient concentrations of the source waters only the tritium concentrations exhibit any structure within the mixed layer and it is argued that this is due to the much larger range of tritium concentrations in the source waters that make up the mixed layer

the formation of labrador sea water part i large scale processes abstract data obtained in the western labrador sea during march 1976 by hudson are analysed to show that new labrador sea water was being formed at this time on the basis of hydrographic and moored current meter data it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western labrador sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to labrador sea water to take place using a model it is demonstrated that water columns found along the western boundary of the labrador sea can be modified by cooling evaporation and mixing to form deep mixed layers with the properties of labrador sea water approximately 105 km3 of new labrador sea water was formed in 1976 an estimate that is consistent with earlier estimates of mean annual production rates this water 2 9 c 34 84 is some 0 6 c cooler and 0 06 fresher than that defined by lazier 1973 from his data collected in 1966 the variation of labrador sea water

anthropogenic chlorofluoromethanes in the greenland and norwegian seas the concentrations of two industrially produced chlorofluoromethanes ccl3f f 11 and ccl2f2 f 12 have been measured in the water column and in the marine atmosphere of the greenland and norwegian seas measurable concentrations of these two chlorofluoromethanes have penetrated to the deep basins of both of these regions and the general characteristics of their vertical distributions are similar to those of the bomb produced radioisotopes injected into the atmosphere on a similar time scale the data have been fitted to a time dependent box model based on deep convective mixing in the greenland sea and lateral exchange between the deep basins the model calculations for the two chlorofluoromethanes in the greenland sea give similar results with a time scale for deep convection of about 40 years the time scale for lateral mixing between the deep greenland sea and the deep norwegian sea is estimated to be 20 to 30 years although the agreement between the calculations for the two chlorofluoromethanes is limited by analytical uncertainties at the low concentrations found in the deep norwegian sea and by uncertainties in the model assumptions

the formation of labrador sea water part ii mesoscale and smaller scale processes abstract in a previous paper clarke and gascard argued that the formation of labrador sea water was taking place in a cyclonic gyre set up each winter in the western labrador sea within the gyre and at its boundaries a number of different scales of organization are believed to be important in the formation processes the longest of these scales is the mesoscale 50 km which appears to be related to topographic rossby waves generated in the labrador current and propagating offshore the next smaller scale is an eddy scale 20 km believed to arise because the mesoscale is baroclinically unstable as shown by applying a two layer model of tang this instability is believed to promote mixing by generating frontal structures and vertical motions along them thus bringing subsurface t s maxima nearer the surface then within the mesoscale and eddy scale structures intense vertical convective cells take place at scales which are probably of the order of 1 km in three dimensions these events are short liv

the arrival of recently formed labrador sea water in the deep western boundary current at 26 5 n the deep western boundary current dwbc of the north atlantic is a principal conduit between the formation region for labrador sea water lsw and the oceanic interior to the south time series 1985 1997 of hydrographic properties obtained in the dwbc at 26 5 n show that prior to 1994 temperature salinity and transient tracer properties within the lsw density range showed little indication of recently formed parcels properties characteristic of a newer version of lsw cooler fresher and higher tracer concentrations were observed beginning in 1994 and continuing through 1997 longer time series of temperature and salinity developed from a regional data base show both the 1994 and a 1980 1981 event in the abaco region both events are consistent with anomalies in the labrador sea that occurred some 10 years earlier the 10 year transit time from the labrador sea to 26 5 n is less than the 18 year transit time inferred from earlier studies

paleocene sequence stratigraphy of southwestern alabama abstract in southwestern alabama the paleocene consists of about 1 300 ft of marginal marine and marine terrigenous and carbonate sediments based on regional stratigraphic sedimentologic and paleontologic data eight unconformity bounded depositional sequences resulting from relative changes in coastal onlap during the paleocene are recognized in these strata these sequences are in ascending order the tp1 1a pine barren member of the clayton formation the tp1 1b turritella rock beds of the pine barren member and mcbryde limestone member of the clayton formation and calcareous clays and limestones of the lower member of the porters creek formation the tp1 1c marls and clays of the lower member of the porters creek formation the tp1 2 cross bedded sands of the lower member and matthews landing marl member of the porters creek formation and oak hill member of the naheola formation the tp1 3 coal bluff marl member of the naheola formation the tp2 1 gravel creek sand ostrea thirsae beds and grampian hills members of the nanafalia formation and lower beds of the tuscahoma sand the tp2 2 greggs landing marl member and middle beds of the tuscahoma sand and the tp2 3 bells landing marl member and upper beds of the tuscahoma sand planktonic foraminiferal zone boundaries recognized for these strata correspond to relative rises in sea level and usually occur at sequence boundaries or transgressive surfaces zonal boundaries occurring in this fashion include subbotina pseudobulloides interval zone morozovella uncimata interval zone morozovella angulata interval zone planorotalites pusilla pusilla interval zone planorotalites pseudomenardii range zone and morozovella velascoensis interval zone the zonal boundary for the subbotina trinidadensis interval zone also corresponds to a relative rise in sea level and occurs in transgressive or condensed section deposits

a unique lower paleocene shelf sea chalk in the eastern u s gulf coastal plain clayton formation western alabama implications for depositional environment sea level dynamics and paleogeography abstract the deposition of carbonate oozes in shelf sea settings so prevalent during the cretaceous generally ceased by the onset of the paleogene in response to sea level fall however the lower paleocene danian clayton formation exposed at moscow landing in western alabama u s a contains a thin chalk bed the clayton chalk that records a later final phase of shelf sea carbonate ooze deposition in the u s eastern gulf coastal plain sedimentologic character ichnology and foraminiferal assemblages indicate deposition in an outer shelf setting comparable to that for cretaceous chalks exposed in the gulf region i e campanian demopolis chalk at depths of 90 m or more during a brief transgressive pulse peaking circa 63 8 63 2 ma this transgression occurred 1 myr after the maximum expansion of the danian cannonball sea in the north american western interior presumably reflecting different controls on sea level dynamics eustasy vs laramide tectonism and isostasy although regional paleogeography for the early paleocene is poorly constrained deposition of the clayton chalk suggests that the eastern gulf seaway at times extended much further north than implied by the presently mapped updip limit of preserved paleocene marine strata the apparent geographic restriction of this unique paleocene shelf sea chalk indicates that relative sediment starvation and associated carbonate ooze deposition may have been influenced by local structural controls operating along the eastern margin of the mississippi embayment

tracking labrador sea water property signals along the deep western boundary current observations of the deep western boundary current dwbc at line w on the western north atlantic continental slope southeast of cape cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the labrador sea this is most evident in the cold dense and deep class of labrador sea water dlsw that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987 1994 the arrival of this record cold fresh and low potential vorticity anomaly at line w lags its formation in the labrador sea by 3 7 years complementary observations along the path of the dwbc provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the flemish cap finally the consistency of the data with realistic advective and mixing time scales is assessed using the waugh and hall 2005 model framework the data are found to be best represented by a mean transit time of 5 years from the labrador sea to line w with a leading order role for both advection by the dwbc and mixing between the boundary flow and interior waters

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

excepftonally strong near bottom flows on the continental rise of nova scotia recent current velocity measurements across the lower continental rise of nova scotia show a deep equatorwardflow with speeds maximum 73 centimeters per second among the highest recorded for the deep sea silicate measurements indicate that this flow usually consists of southern source antarctic bottom water these measurements confirm the existence of a second and deeper western boundary flow that was earlier inferredfrom geological observations

eustatic cyclicity in the paleocene and eocene data from the brazos river valley texas abstract documentation of the geologic history of global sea level change requires the analysis of numerous geographically diverse sections the paleocene through eocene section of the brazos river valley eastern texas is an area suitable for such analysis which has yet to be studied analysis of the brazos river valley section for sea level changes during the paleocene and eocene was accomplished by the application of sequence stratigraphie analysis using seismic well log and outcrop data the information obtained from this study was then used in evaluating the published sea level record a total of fifteen sequence boundaries and ten maximum flooding surfaces indicating conditions of falling and rising sea level respectively were identified in the paleocene and eocene strata of the brazos river valley results of this study show an excellent correlation with the published record of sea level change in the paleocene poor correlation in the early eocene good correlation in the middle eocene and fair correlation in the late eocene the excellent correlation between the results of this study and the published record during the paleocene provide strong support for eustalic nature of these events poor to fair correlation during the early and late eocene is attributed to the resolution of the data base and neither supports nor refutes the published record results of this study in the middle eocene provide strong support lor the published record with two important exceptions published records show a cycle of sea level change in the earliest part of the middle eocene which was not observed in this study and a rise in sea level near the end of the middle eocene bartonian may have occurred earlier than that indicated by the published record

formation process of intermediate water in baroclinic current under cooling the formation process of intermediate water in baroclinic current under cooling is investigated using a nonhydrostatic numerical model after baroclinic instability develops into finite amplitude in a short time strong downdrafts with a horizontal scale of 1 km are generated near the density front and subduct surface water to depths 400 m along isopycnals as a result patches of ventilated water of 10 20 km horizontal scale with anticyclonic circulation are formed at intermediate depths combined effects of baroclinic instability and convection are key dynamics for these phenomena convection acts as an initiator for baroclinic instability at the onset and accelerates its subsequent growth by reducing stratification developed baroclinic wave forms an intense density front and downdraft along isopycnals is generated through the frontogenetic process density change due to convection intensifies this frontal downdraft by strengthening the geostrophic forcing tendency to destroy the geostrophic balance and by reducing potential vorticity static stability further symmetric instability induced by the density change due to convection and intensified by the frontogenetic process drives slantwise convection which in turn enhances the frontal downdraft consequent downward velocity becomes 20 times as large as that of the frontal downdraft without convection cooling and twice larger than that of pure convection since the intensified frontal downdraft moves its position with time and induces divergent flow at depths a patch of ventilated water with a horizontal scale much larger than that of the frontal downdraft is formed the role of convection cooling in the formation process of intermediate water in this context is to enhance the frontal downdraft rather than to deepen the mixed layer this scenario is quite different from the one realized when baroclinic instability and convection do not coexist

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

thermocline displacement across the atlantic north equatorial countercurrent during 1983 two time series of thermocline displacement across the north equatorial countercurrent are derived from pairs of inverted echo sounders which straddle the countercurrent along the 28 w and 38 30 w meridians the observations reported are from february to september 1983 from mid may to the end of the record the thermocline deepens to the south and shallows to the north of the countercurrent suggesting a continual strengthening of the countercurrent the rate of increase of the thermocline slope at the western meridian is about twice that at 28 w as expected from historical data an empirical orthogonal functions eof analysis of the four sounders finds the dominant mode accounting for 64 of the variance this is compared to an eof analysis of model results by philander and pacanowski where the forcing is provided by the historical average wind field with the model the dominant mode is almost the entire response 92 of the variance the time dependence of the observed mode tracks that derived from the model during the overlapping seasons

eddies of newly formed upper labrador sea water a new type of submesoscale eddy has been observed south of the labrador sea during late winter embedded in equatorward flow along the western boundary the eddy radius is 20 km with a weak dynamic signature swirl speed of 0 5 cm s the center of the eddy is characterized by weak stratification elevated concentrations of oxygen and anthropogenic tracers and low tracer ages all indicative of newly ventilated water strong lateral intrusions distort the shape of the feature the water mass contained in the eddy is not classical labrador sea water from the central labrador sea but is significantly fresher and hence lighter it is of the correct density to be the source of the high chlorofluorocarbon layer of the shallow deep western boundary current observed further south and hence is termed upper labrador sea water using a combination of hydrographic data sets along the western boundary to implement a simple lateral diffusion model it is shown that such eddies decay of the order of several months and are difficult to observe equatorward of the grand banks of newfoundland this is in contrast to deeper lenses of classical labrador sea water which persist further equatorward tracer derived ages of the upper labrador sea water eddy range from 3 to 5 years much older than the lifetime deduced from their lateral diffusion a simple convection model of tracer age shows that this age discrepancy is caused by gas exchange being unable to maintain equilibrium between the deep convecting mixed layer and the atmosphere during formation

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

labrador sea water tracked by profiling floats from the boundary current into the open north atlantic fifteen profiling floats were injected into the deep boundary current off labrador they were ballasted to drift in the core depth of labrador sea water lsw at 1500 m depth and were deployed in two groups during march and july august 1997 initially for about three months the floats were drifting within the boundary current and the flow vectors were used to determine the mean horizontal structure of the deep labrador current which was found to be about 100 km wide with an average core speed of 18 cm s 1 north of flemish cap the boundary current encounters complicated topography around orphan knoll and there the lsw outflow splits up into different routes one obvious lsw path is eastward through the charlie gibbs fracture zone and another route is a narrow recirculation toward the central labrador sea a surprising result was that none of the floats were able to follow the boundary current southward to the grand banks area and exit into the subtropics trajectories and temperature profiles of the eastward drifting floats indicate the importance of the north atlantic current for dispersing the floats even at the level of lsw

lagrangian observations of the deep western boundary current in the north atlantic ocean abstract in this study the authors analyze the trajectories of 18 rafos floats launched in the deep western boundary current dwbc between the grand banks and cape hatteras to investigate the kinematics and dynamics in the region where the dwbc crosses under the gulf stream near 36 n the crossover region floats deployed in the chlorofluorocarbon cfc maximum associated with upper labrador sea water depth 800 m illustrate the entrainment process of this water mass into the gulf stream the behavior of the floats and fluid parcels in the crossover region is strongly dependent on the meandering of the gulf stream when the stream is close to its mean position fluid parcels entrained from the upper dwbc travel along the northern edge of the stream when a meander trough is present downstream of the entrainment location dwbc fluid parcels cross the gulf stream and sometimes are expelled on the south side this represents a previously unrecognized mechanism for transporting upper labrador sea w

chlorofluorocarbon constraints on north atlantic ventilation the north atlantic ocean vigorously ventilates the ocean interior thermocline and deep water masses are exposed to atmospheric contact there and are sequestered in two principal classes subtropical mode water stmw 26 5 le sigma thetas le 26 8 and subpolar mode water spmw 26 9 le sigma thetas le 27 8 these ventilation rates and pathways are uncertain and a powerful way to estimate them is to monitor the penetration of chlorofluorocarbon cfc tracers here a cfc dataset of over 44 000 observations taken between 1982 and 1998 is combined with a non eddy resolving resolution general circulation model of the north atlantic ocean the cfc data are assimilated with the model by optimizing the uncertain air ndash sea cfc flux the assimilated cfc fields are then systematically compared with the observations to identify the best fit and hence the most realistic ventilation three gcm experiments are performed this way to find the dependence on model thickness diffusivity each gcm solution is close to being statistically consistent with the cfc observations and likely sources of error lower diffusivity gives the best match to data although some systematic bias in sequestering tropospheric cfc remains lower diffusivity around 150 m2 s ndash 1 permits a stronger circulation with a more realistic north atlantic current for this experiment the subduction rate is around 16 sv sv equiv 106 m3 s ndash 1 in the subtropics and eastern subpolar atlantic 26 35 le sigma thetas le 27 13 averaged over 1975 ndash 95 around 26 sv is formed in the labrador and irminger seas 27 58 le sigma thetas le 27 8 only about 40 percnt of the cfc carried into the subpolar interior by this flux remained there in 1998 however the rest was returned to the subpolar mixed layer after an average period of 6 ndash 8 yr in contrast 70 percnt of the cfc subducted into the subtropical interior remained there in 1998

a mechanism for the latitudinal dependence of peak spectrum sea surface height variability previous studies have shown that the power spectrum of satellite observed sea surface height ssh variability peaks at a certain frequency or a wave number band at a given latitude lin et al 2008 attributed this latitudinal dependence to the critical frequency of the first baroclinic mode rossby waves in the tropical and subtropical oceans their study was based on the linear rossby wave theory and focused on ssh variability in the tropical and subtropical oceans since the altimetry data do not adequately resolve lengths of baroclinic rossby waves at and near the critical frequency in high latitudes in this study we expand their analysis to high latitude oceanic basins and to include nonlinear eddy effects by using a linear wave model and a high resolution model output from the ogcm for the earth simulator ofes it is found that the linear wave mechanism by and large remains valid in the tropical and subtropical oceans in higher latitudes as well as in some regions in the western tropical and subtropical oceans other mechanisms like nonlinear eddy play more important role in determining the ssh variability

diagnosing the role of eddies in driving the circulation of the northwest atlantic ocean 1 in this letter we present a variation of the recently proposed semi prognostic method for use with ocean models the new version has the advantage that model drift is effectively prevented while at the same time the meso scale eddy field is free to evolve we use the method to probe the importance of the eddy driven circulation in the northwest atlantic ocean for the particular model we use here it is shown that the eddies strongly reinforce the eastward gulf stream jet and the northern recirculation in the slope region with over 50 of the total transport of this recirculation being directly eddy driven the eddies also play a role in setting the temperature and salinity properties of the northwest corner southeast of newfoundland

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

low frequency meandering of the atlantic north equatorial countercurrent four 19 month time series of indirect measurements of dynamic height were obtained in the tropical atlantic along 28 w at 0 3 6 and 9 n with three inverted echo sounders and one current meter mooring the series were analyzed to study the time latitude variability of the north equatorial countercurrent necc the eastward flow associated with the necc was present at 28 w from 3 to 9 n during most of the observed period except in march april 1983 and april may 1984 periods that coincided with the onset of the wind at the equator the amplitude of the necc s annual cycle was maximum at 6 n and was larger in 1983 than in 1984 the analysis of the time latitude variability of differences in dynamic height shows a long period meridional shift of the necc the core of the current attains its northernmost location during august september in both years and its southernmost location during march april in 1983 and march in 1984 the location of the core is directly related to the position of the intertropical convergence zone from the time series of dynamic height obtained from the indirect measurements geostrophic velocities and transports were estimated and compared with direct observations of currents and values obtained from hydrographic casts

iodine 129 cfc 11 transit times for denmark strait overflow water in the labrador and irminger seas 1 iodine 129 discharged from nuclear fuel reprocessing plants in france and the united kingdom is transported into the nordic seas on timescales of 3 5 years tracer 129i is subsequently injected into intermediate waters that overflow the sills between greenland iceland and scotland and ventilate the north atlantic deep waters nadw during the early 1990s discharges of 129i increased by 600 resulting in a large well resolved tracer front whose passage through the nordic seas is presently being observed by a time series of 129i measurements on the woce ar7w section in the labrador sea the highest 129i levels were measured below depths of 3000 m in denmark strait overflow water dsow these levels increased by about 300 between 1997 and 2001 to values 40 107 atoms l indicating that the leading edge of the tracer front from the early 1990s was being observed entering the nadw using a simple mixing advection model 129i and cfc 11 results were used to calculate transit times of 0 4 2 6 years for the flow of dsow from formation regions in the nordic seas to the labrador sea measurements on samples collected in the irminger sea in 2001 gave transit times of 0 3 years for core dsow immediately south of denmark strait and about 2 years for locations off the southern tip of greenland an increase in dsow transit times observed in the labrador sea between 1999 and 2001 probably reflects a more general weakening in the subpolar gyre of the north atlantic during the 1990s