a high resolution simulation of the ocean during the pomme experiment simulation results and comparison with observations 1 a modeling study of physical processes occurring in an area of the northeast atlantic 21 33 15 33 w 38 00 45 00 n that was extensively sampled during the programme ocean multidisciplinaire meso echelle pomme october 2000 september 2001 is carried out the model is a mesoscale version of the ocean general circulation model opa developed at the laboratoire d oceanographie dynamique et de climatologie in paris it is used in a three dimensional limited area domain with a high resolution grid approximately 5 km horizontal spacing 69 vertical levels and realistic boundary conditions initial state air sea fluxes open boundary fluxes and bottom topography the objectives of the study are to properly simulate the upper ocean dynamics particularly mesoscale activity and mixed layer evolution during a key period restratification of the pomme experiment pomme 1 and pomme 2 from february to may 2001 and to compare model results with oceanographic observations collected during the experiment in order to establish confidence in the model some results provided by the high resolution simulation in particular features related to mixed layer depth and vertical velocities are also presented there is no pronounced north south mixed layer depth gradient but strong filament shaped structures associated with stirring at the periphery of eddies are present mixed layer restratification is simulated it is associated with submesoscale mixed layer depth structures and intense vertical velocity filaments in the upper ocean correlated with the relative vorticity gradient field

subduction of carbon nitrogen and oxygen in the northeast atlantic 1 northeast atlantic mode waters neamw are formed by subduction in a region known to be a strong sink of atmospheric co2 this study investigates the biological and physical carbon pumps involved in this sink for that purpose we estimate the annual transfer of carbon nitrogen and oxygen from the surface into the ocean interior through neamw subduction our estimates are based on a model simulation constrained with data collected during the programme ocean multidisciplinaire meso echelle pomme we found that subduction accounts for a large proportion of carbon export below the mixed layer in the pomme region 38 n 45 n 16 w 22 w fifty percent of labile organic carbon is exported by the biological pump and 50 by subduction 98 of the total carbon flux below the mixed layer is due to carbon subduction essentially in the form of dissolved inorganic carbon and of more refractory dissolved organic matter this is because they are the dominant pools of carbon and are passing through the winter mixed layer depth gradient that characterizes the region our results emphasize that the net export of carbon represents a small fraction of the fluxes of carbon across the mixed layer through the processes of subduction and obduction moreover our results indicate that neamw subduction occurs before the spring bloom this implies that neamw is mostly subducted with end winter characteristics with in particular relatively high nutrient content this reflects a relative inefficiency in nutrient utilization and contrasts with subtropical mode waters which are subducted during the bloom and are thus relatively poor in nutrients when they are subducted

does stommel s mixed layer demon work abstract stommel argued that the seasonal cycle leads to a bias in the coupling between the surface mixed layer and the main thermocline of the ocean he suggested that a demon operated that effectively only allowed fluid at the end of winter to pass from the mixed layer into the main thermocline in this study stommel s hypothesis is examined using diagnostics from a time dependent coupled mixed layer primitive equation model of the north atlantic cme the influence of the seasonal cycle on the properties of the main thermocline is investigated using two methods in the first the rate and timing of subduction into the main thermocline is diagnosed using kinematic methods from the 1 resolution cme fields in the second tracer diagnostics of the cme and idealized experiments using a date tracer identifying the timing of subduction are performed over the subtropical gyre both approaches generally support stommel s hypothesis that fluid is only transferred from the mixed layer into the main therm

inferring the subduction rate and period over the north atlantic abstract the annual rate at which mixed layer fluid is transferred into the permanent thermocline that is the annual subduction rate sann and the effective subduction period x1d4af eff is inferred from climatological data in the north atlantic from its kinematic definition sann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer geostrophic velocity fields computed from the levitus climatology assuming a level of no motion at 2 5 km are used the vertical velocity at the base of the mixed layer is deduced from observed surface ekman pumping velocities and linear vorticity balance a plausible pattern of sann is obtained with subduction rates over the subtropical gyre approaching 100 m yr twice the maximum rate of ekman pumping the subduction period x1d4af eff is found by viewing subduction as a transformation process converting mixed layer fluid into stratified thermocline fluid the effective

does eddy subduction matter in the northeast atlantic ocean 1 mesoscale eddies are an important contributor to subduction in the gulf stream region and the antarctic circumpolar current but is eddy subduction also important in the relatively quiescent interior of the world s subtropical gyres observations from the subduction experiment of the northeast atlantic do not have the spatial resolution necessary to calculate eddy subduction and answer this question regional numerical models can diagnose subduction but their representativeness is unknown furthermore water mass budgets in an open ocean domain show that the simulated properties of subducted water directly depend upon uncertain open boundary conditions and surface fluxes to remedy these problems a state estimate of the ocean circulation is formed by constraining an eddy permitting general circulation model to observations by adjusting the model parameters within their uncertainty the resulting estimate is self consistent with the equations of motion and has the necessary resolution for diagnosing subduction in the northeast atlantic during 1991 1993 the time variable circulation contributes less than 1 sv of net subduction while the total subduction is 4 sv eddy volume fluxes of 40 m yr in the north equatorial current and the azores current however are significant and rival the subduction by ekman pumping locally furthermore a state estimate at 1 6 resolution has 2 3 sv more subduction in the density bands centered around 24 0 kg m3 and 26 0 kg m3 than a 2 state estimate this result implies that the inability to accurately simulate mesoscale phenomena and surface fluxes in climate models would lead to an accumulation of errors in water mass properties over 10 20 years even in the interior of the subtropical gyre

introduction to the pomme special section thermocline ventilation and biogeochemical tracer distribution in the northeast atlantic ocean and impact of mesoscale dynamics conducted in the northeast atlantic ocean 15 20 21 20 w 38 n 45 n the programme ocean multidisciplinaire meso echelle pomme is a research project aimed at a better understanding of the biological production and the carbon budget of the region in relation to the formation mechanisms of the 11 12 c mode water of the northeast atlantic with the help of two research vessels several tens of floats and drifters and nine moorings the field experiment was carried out between autumn 2000 and autumn 2001 with a more intensive phase in the winter and early spring of 2001 the field experiment resolved small several kilometers to regional several hundred kilometers scales and daily to seasonal variability a first analysis of the rich data set focused on the large scale and the mesoscale variability it shows that the distribution of water mass characteristics and biological activity is strongly influenced by the mesoscales in this supposedly quiet transition zone between the subtropical and subpolar gyres the seasonal variability however presents an imprint of the large scale structures with a clear north south gradient in properties and budgets this region is found on an annual average to be a sink of atmospheric co2 smaller scales associated with fronts and filaments were clearly observed in many fields temperature but also chlorophyll oxygen biogenic particles etc with modeling studies suggesting that they play a significant role in subduction ventilation and transport of biogeochemical tracers in the pomme region

deep convection in the labrador sea as observed by lagrangian floats abstract during the winters of 1997 and 1998 a total of 24 lagrangian floats were deployed in the labrador sea these floats were designed to match the buoyancy and compressibility of seawater they measured temperature and three dimensional position pressure for vertical position and rafos acoustic tracking for latitude and longitude as they followed water motions three dimensionally this data provides direct observation of mixed layer depth and excellent estimates of vertical velocity floats were repeatedly carried across the convecting layer by vertical velocities averaging several centimeters per second with vertical excursions of up to one kilometer in the horizontal several scales of eddy motion were resolved as was a possible float predilection toward remaining in water preconditioned for convection heat flux estimates from this data reveal entrainment and surface heat fluxes similar in magnitude the mixed layer acts as a vertical conveyor belt of temperature transporting heat from depth

geodetic slip rates in the southern san andreas fault system effects of elastic heterogeneity and fault geometry 1 we use high resolution interferometric synthetic aperture radar and gps measurements of crustal motion across the southern san andreas fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths geodetically measured strain rates are asymmetric with respect to the mapped traces of both the southern san andreas and san jacinto faults two possibilities have been proposed to explain this observation large contrasts in crustal rigidity across the faults or an alternate fault geometry such as a dipping san andreas fault or a blind segment of the san jacinto fault we evaluate these possibilities using a two dimensional elastic model accounting for heterogeneous structure computed from the southern california earthquake center crustal velocity model cvm h 6 3 the results demonstrate that moderate variations in elastic properties of the crust do not produce a significant strain rate asymmetry and have only a minor effect on the inferred slip rates however we find that small changes in the location of faults at depth can strongly impact the results our preferred model includes a san andreas fault dipping northeast at 60 and two active branches of the san jacinto fault zone in this case we infer nearly equal slip rates of 18 1 and 19 2mm yr for the san andreas and san jacinto fault zones respectively these values are in good agreement with geologic measurements representing average slip rates over the last 10 10 years implying steady long term motion on these faults

eastern north atlantic mode waters during pomme september 2000 2001 four hydrographic surveys in the northeast atlantic 38 45 n 15 21 w carried out during the programme ocean multidisciplinaire meso echelle pomme experiment september 2000 2001 are used to investigate the water masses in the upper layers corresponding to eastern north atlantic mode waters a large meridional gradient in isopycnal temperature salinity is witnessed which has no counterpart in climatology and is associated with an anomalous eastward current near 41 n waters tend to be warmer more salty somewhat more stratified and less oxygenated on isopycnal surfaces in the southern part of the domain in addition there is an overlying zonal gradient of properties in particular north of the jet consistent with earlier descriptions however these large scale distributions also change significantly through the pomme year in particular between the february and early april surveys which we attribute to 1 the arrival of a 2001 vintage of mode waters in particular in the northeastern part of the domain and to 2 stirring by the mesoscale motions which contribute to down gradient transport equivalent diffusion coefficient of 2000 m 2 s for s 27 10 on a smaller scale there are noticeable changes in properties between surveys some of which can be shown by quasi lagrangian diagnostics to result from advection by the horizontal circulation in particular in the southwest corner of the domain between the surveys p1 february 2001 and p2 april 2001 horizontal isopycnal advection is responsible for a large cooling freshening and oxygen increase or in the northeast corner where new oxygenated and freshly ventilated water in particular for surface s q 27 05 kg m 3 penetrates during that period in other regions or for a longer period water masses are often difficult to track as a result of dispersion or stirring by the eddies and too small of a pomme domain

response of the atmospheric boundary layer to a mesoscale oceanic eddy in the northeast atlantic fields of air sea turbulent fluxes and bulk variables were derived from satellite sensor data from february to april 2001 over a region of the northeast atlantic where a field experiment programme oce an multidisciplinaire meso echelle pomme was conducted the satellite products are in good agreement with in situ data in terms of heat fluxes sea surface temperature and wind speed the central part of the experimental domain presented a cyclonic eddy in the ocean which corresponded to a cold sea surface temperature sst anomaly winds were weaker within the eddy than outside of it with lower latent and sensible heat loss in order to analyze the relationship between the sst and wind anomalies three numerical experiments were conducted with a regional atmospheric model three 3 month runs of the model were performed using a realistic sst field a smoothed sst field in which the cold sst was not present reference run and an sst field where the cold anomaly was increased by two degrees successively the fields simulated with the realistic sst were consistent with satellite sensor derived observations in particular the weak wind area over the cold sst anomaly was successfully rendered whereas it was not present in the forcing fields taken individually the three runs did not reveal the presence of secondary circulations however anomalous secondary circulations were clearly identified with respect to the reference run the origin of the latter circulations was investigated with the giordani and planton generalization of the sawyer eliassen equations according to our results differential heating induced by the cold sst anomaly mostly altered the vertical wind through the effect of friction and only marginally through pressure gradient forces in the upper part of the boundary layer the wind speed increased decreased over downstream the cold sst we found that stability was the main factor that induced the simulated patterns of the friction term in the diagnostic equations therefore our results show that mesoscale wind patterns were significantly affected by sst gradients through the effect of stability in a region of low oceanic eddy activity

tracer dating and ocean ventilation the interpretation of transient tracer observations depends on difcult to obtain information on the evolution in time of the tracer boundary conditions and interior distributions recent studies have attempted to circumvent this problem by making use of a derived quantity age based on the simultaneous distribution of two complementary tracers such as tritium and its daughter helium 3 the age is defined with reference to the surface such that the boundary condition takes on a constant value of zero we use a two dimensional model to explore the circumstances under which such a combination of conservation equations for two complementary tracers can lead to a cancellation of the time derivative terms an interesting aspect of this approach is that mixing can serve as a source or sink of tracer based age we define an idealized ventilation age tracer that is conservative with respect to mixing and we explore how its behavior compares with that of the tracer based ages over a range of advective and diffusive parameters transient tracers such as tritium and tritiugenic helium 3 chlorofiuorocarbons especially cfc 11 and cfc 12 and radiocarbon produced by nuclear bomb tests have entered the oceans primarily during the past 30 years the inherent time scale of these tracers matches very well typical time scales for thermocline ventilation as well as other important oceanic processes such as deep water formation they have been used with some success to study the time scales of thermocline ventilation e g sarrniento 1983 jenkins 1987 and oxygen utilization rates our e g jenkins 1977 1980 1982 1987 as well as other processes such as equatorial upwelling rates broecker et al 1978 quay et al 1983 wunsch 1984 and transport of upper north atlantic deep water weiss et al 1985 the problem of coping with the nonsteady state aspect of these tracers has been dealt with in many ways few of them truly satisfactory for example rooth and ostlund 1972 chose to estimate the diapycnic diffusivity in a region where there was some evidence that the tracer distributions were approximately in steady state at the time of their measurements wunsch 1984 used a similar steady state assumption in his study of equatorial upwelling using bomb radiocarbon such an argu ment cannot be applied in most regions of the ocean however indeed one can question whether it applies even in the above situations therefore one generally would expect that it would be necessary to know something about the evolution of the tracer concentration in time as well as the time history of the boundary conditions for that tracer over the same span of time for example sarrniento s 1983 tritium box model of the thermocline solved the conserva tion equation for the period of time between 1952 before nuclear bomb tests had injected a significant amount of tritium into the oceans and 1972 when a single realization of the tracer field was obtained by a series of field studies the surface concentration time history of dreisigacker and roether 1978 was used as a boundary condition such an l now at deutsche forschungs und versuchasanstalt ftir luft

eddy subduction in a model of the subtropical gyre abstract subduction is the process by which fluid transfers from the mixed layer to the interior of the ocean south of the gulf stream extension 18 mode water is formed in a region of high subduction rates in this region there is high mesoscale eddy activity in the present study the role of eddies in modifying the large scale subduction into mode water is investigated an eddy resolving isopycnic ocean model with mixed layer physics included and coupled to an atmospheric anomaly model is used for this purpose the geometry and forcing of the model are idealized annual mean subduction rates into mode water up to 200 m yr 1 are found south of the gulf stream extension the eddy contribution to the annual subduction is estimated by comparing the annual mean subduction rates obtained from monthly mean quantities with the total subduction rates i e eddy plus mean contributions the latter are determined by integrating the detrainment rates over the period when fluid is irreversibly detrained eddy

a 1 year mesoscale simulation of the northeast atlantic mixed layer heat and mass budgets during the pomme experiment 1 a simplified oceanic model is used to perform a realistic yearlong simulation to study fine scale mixed layer processes in an area of the northeast atlantic 21 15 w 38 45 n during the programme ocean multidisciplinaire meso echelle pomme experiment september 2000 october 2001 this simplified three dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis derived geostrophic currents ug called guide into the momentum equation by the substitution of the horizontal pressure gradient the realism of the simulated mesoscale structures was validated by a comparison with in situ mesoscale and submesoscale data and results from the geostrophic adjustment to the guide with this adjustment being applied at all scales this model was able to simulate energetic small horizontal scales taking place around stirring eddies and frontal regions moreover filamentary mixed layer depth structures were simulated and correlated to intense vertical velocities confined in the mixed layer the circulation structure during pomme consisted of three quasi permanent mesoscale eddies and a front the mixed layer heat and mass budgets were thus strongly driven by the horizontal advection and the vertical transport associated with this mesoscale circulation although no effective detrainment or entrainment occurred at the domain scale three regions were found to be the site of intense detrainment around anticyclonic and cyclonic mesoscale eddies and along an axis of strong currents in which the lateral induction and the vertical velocity at the mixed layer base are the major components driving the detrainment respectively

the greenland sea in water 1993 and 1994 preconditioning for deep convection abstract we present here a detailed analysis of the time dependent large scale and mesoscale features observed in the greenland sea during winter and spring 1993 and 1994 based on a comprehensive data set this study gives particular emphasis to the data from hydrology and rafos vcm floats in both years the basin underwent an overturning to 1000 and 800 m depth respectively with extensive horizontal variability these inhomogeneities are shown to be closely linked with mesoscale eddies identified at a late stage during the preconditioning phase to deep convection it is suggested that these eddies are remnants of previous convective events one of them found at 500 m depth in 1994 was thoroughly studied the dynamical coupling between the eddy and the modified atlantic water maw advected over it from the northwest led to the deepest convected water column found within the basin during this particular year sea ice absent in the gyre in winter 1994 but partly present in the colder 1993 winter does not appear to be a strong prerequisite for deep convection heat loss at the surface combined with salt input by the maw and remnant subsurface mesoscale eddies of past convective events are the most likely agents that determine the depth reached by convection and the characteristics of the new greenland arctic intermediate water formed in the greenland gyre in 1993 and 1994

oceanic ventilation in the eastern north atlantic abstract ventilation of the eastern north atlantic by subpolar mode water is illustrated by recent hydrographic section and analyzed using a large scale stationary thermocline model including a surface mixed layer the distributions of potential vorticity along the hydrographic lines and the geostrophic transports across them confirm the circulation pattern of the mode water that was previously deduced from older data that is arrival from the west at intergyre latitudes and southward subduction of the lightest part at about 42 n the model approach emphasizes the large scale advective thermodynarnic equilibrium which controls the shape of the thick 500 m winter mixed layer observed in the eastern basin following recent theories the process of ventilation is interpreted in terms of exchanges between the mixed layer and the stratified thermocline mode water is formed where thermocline water is entrained into a thick mixed layer and is subducted where mixed layer water is injected into the thermocli

central water vortices of the eastern north atlantic abstract a set of 102 high resolution hydrographic lines mostly xbt lines in the eastern north atlantic is used to study baroclinic vortices dynamically intensified within the central water layer first for the sake of comparison with recent results from altimetry spatial statistics of the main thermocline dynamic height are computed the dynamic height wavenumber spectrum slope close to k 3 at wavelengths shorter than 300 km and the decrease with latitude of eddy scales deduced from its autocorrelation function are found to be consistent with previous local altimetric results then using a criterion of local temperature anomaly relative to the large scale trend along each line 162 central water mesoscale vortices are detected their distribution is heterogeneous vortices of different size sign and intensity being found in most places they have a mean apparent diameter chord along the line of 110 50 km it is estimated that a range of 100 145 central water vortices coexist at the same t

a simplified 3d oceanic model assimilating geostrophic currents application to the pomme experiment abstract a simplified oceanic model is developed to easily perform cheap and realistic mesoscale simulations on an annual scale this simplified three dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis derived geostrophic currents ug into the momentum equation in substitution of the horizontal pressure gradient simplification is provided by a time sequence of ug called guide which is used as a low resolution and low frequency interpolator this model is thus necessarily coupled to systems providing geostrophic currents that is ocean circulation models analyzed reanalyzed fields or climatologies in this model the mass and currents fields are constrained to adjust to the geostrophic guide at all scales the vertical velocity is deduced from the vorticity equation which ensures the coherence between the vertical motion and the geostrophic structures evolution horizontal and vertical advection are the coupling processes that can b

strontium isotopic constraints on the origin of proterozoic anorthosites abstract the proterozoic giles complex central australia contains an almost complete range of anorthosite types from minor or major layers in gabbronorite intrusions to large anorthosite troctolite bodies to small orthopyroxene anorthosite massifs each type has a distinctive sr isotopic signature anorthosite dominated masses have a regular relationship between ferromagnesian mineralogy initial 87 sr 86 sr and anorthite contents in plagioclase anorthosite troctolite bodies have significant olivine relatively low initial 87 sr 86 sr 0 7038 0 7043 and an 50 69 orthopyroxene dominant anorthosites have relatively high 87 sr 86 sr 0 7045 0 7063 and an 45 60 the pattern is found worldwide detailed study of one intrusion demonstrates that contamination by wall rock granulite produces the higher 87 sr 86 sr values anti correlation between 87 sr 86 sr and an and determines olivine orthopyroxene proportions olivine bearing anorthosites form from a primary aluminous tholeiite magma with plagioclase dominating the liquidus progressive contamination of this parent magma produces a gradation to orthopyroxene anorthosites

physical pathways for carbon transfers between the surface mixed layer and the ocean interior 1 although they are key components of the surface ocean carbon budget physical processes inducing carbon fluxes across the mixed layer base i e subduction and obduction have received much less attention than biological processes using a global model analysis of the preindustrial ocean physical carbon fluxes are quantified and compared to the other carbon fluxes in and out of the surface mixed layer i e air sea co2gas exchange and sedimentation of biogenic material model based carbon obduction and subduction are evaluated against independent data based estimates to the extent that was possible we find that climatological physical fluxes of dissolved inorganic carbon dic are two orders of magnitude larger than the other carbon fluxes and vary over the globe at smaller spatial scale at temperate latitudes the subduction of dic and to a much lesser extent 10 the sinking of particles maintain co2undersaturation whereas dic is obducted back to the surface in the tropical band 75 and southern ocean 25 at the global scale these two large counter balancing fluxes of dic amount to 275 5 pgc yr 1 for the supply by obduction and 264 5 pgc yr 1 for the removal by subduction which is 3 to 5 times larger than previous estimates moreover we find that subduction of organic carbon dissolved and particulate represents 20 of the total export of organic carbon at the global scale we evaluate that of the 11 pgc yr 1 of organic material lost from the surface every year 2 1 pgc yr 1 is lost through subduction of organic carbon our results emphasize the strong sensitivity of the oceanic carbon cycle to changes in mixed layer depth ocean currents and wind

a high resolution simulation of the ocean during the pomme experiment mesoscale variability and near surface processes 1 this article is the second part of a modeling study of upper ocean mesoscale physical processes in an area of the northeast atlantic that was extensively sampled during the programme ocean multidisciplinaire meso echelle pomme experiment october 2000 september 2001 the model is a high resolution regional version of the ocean general circulation model opa integrated from february to may 2001 the mesoscale upper ocean dynamics has been carefully validated in a first paper paci et al 2005 in the present article the simulation is used to analyze the contribution of mesoscale eddies to the mixed layer processes during the period that controls the annual subduction rate and the characteristics of the subducted water mesoscale eddies account for a significant part of the mixed layer heat salt and water budgets through mesoscale and submesoscale structures acting on the horizontal advective terms they represent respectively 52 and 66 of the horizontal advection of temperature and salinity and 70 of the lateral induction they also induce an earlier and more intense restratification the domain averaged detrainment appears to be reduced by more than 15 by mesoscale eddies and reaches about 0 8 sv however these eddies increase the density of the detrained water some dense water likely to be involved in the formation of subpolar mode water is detrained by them in a way that maximizes its chance to be effectively subducted in the area

effects of the mixed layer time variability on kinematic subduction rate diagnostics abstract an eddy resolving primitive equation general circulation model is used to estimate water mass subduction rates in the north atlantic ocean subtropical gyre the diagnostics are based on the instantaneous kinematic approach which allows the calculation of the annual rate of water mass subduction at a given density range following isopycnal outcrop positions over the annual cycle it is shown that water mass subduction is effected rapidly 1 2 months as the mixed layer depth decreases in spring consistent with stommel s hypothesis and occurs mostly over the area of deep late winter mixed layers 150 m across the central north atlantic in the density range 26 27 2 annual subduction rates o 100 200 m yr 1 are found south and east of the gulf stream extension in the density range of subtropical mode waters from roughly 26 2 to 26 6 in the northeastern part of the subtropical gyre annual subduction rates are somewhat larger o 250 m yr 1 from a density of about 26 9 east of the nort

observed mean and mesoscale upper ocean circulation in the midlatitude northeast atlantic 1 lagrangian instruments and altimetry data are used to investigate the upper 0 to 500 m mean and mesoscale circulations in a region of the midlatitude northeast atlantic 36 47 n 13 23 w a formation and subduction area of subpolar mode water lagrangian data were collected between september 2000 and september 2001 with isobaric subsurface floats totaling 34 0 float years and surface drifting buoys totaling 27 9 buoy years the near surface 0 100 m mean circulation is dominated by two weak 5 cm s eastward currents which veer southward in the domain in the 100 to 500 m deep layer these eastward mean currents retroflect westward and form two anticyclonic circulation cells opened to the west and separated by a cyclonic cell eddy kinetic energy is decreasing from the northwest to the southeast of the area from 100 200 cm s 2 to 20 30 cm s 2 long lived anticyclones and cyclones display mean coherent displacements both to the southwest westward eddy propagation speeds close to that of long rossby waves increase southward and are consistent with a superposition of beta effect and advection by mean currents southward eddy displacements are coherent with the observed mean currents in the north anticyclones are seen to consist of central and subpolar mode water eddies most of them likely originating from the eastern boundary whereas in the south most anticyclonic features result from the influence of mediterranean water eddies meddies on the upper layers cyclonic activity is concentrated in cyclonic zones which are seen to exchange floats and water properties

eddy induced mixed layer shallowing and mixed layer thermocline exchange the effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high resolution model of an unstable front a spring scenario is considered with a mixed layer maintained against surface warming by wind mixing tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front giving a mixed layer as shallow as 10 m in some places compared with the equilibrium monin obukhov depth of 30 m away from the front variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field vertical velocities associated with the growing instabilities push tongues of a ventilation tracer initially only present within the surface layer down to depths of up to 100 m after 24 days a quarter of this ventilation tracer has been deposited in the thermocline conversely the vertical motions pull nutrients from the thermocline up into the mixed layer giving enhanced surface values along the front in the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of o 3 10 10 4m2s 1

a three dimensional ocean mesoscale simulation using data from the semaphore experiment mixed layer heat budget a primitive equation model is used to simulate the mesoscale circulation associated with a portion of the azores front investigated during the intensive observation period iop of the structure des echanges mer atmosphere proprietes des heterogeneites oceaniques recherche experimentale semaphore experiment in fall 1993 the model is a mesoscale version of the ocean general circulation model ogcm developed at the laboratoire d oceanographie dynamique et de climatologie lodyc in paris and includes open lateral boundaries a 1 5 level order turbulence closure scheme and fine mesh resolution 0 11 for latitude and 0 09 for longitude the atmospheric forcing is provided by satellite data for the solar and infrared fluxes and by analyzed or reanalyzed for the wind atmospheric data from the european centre for medium range weather forecasts ecmwf forecast model the extended data set collected during the iop of semaphore enables a detailed initialization of the model a coupling with the rest of the basin through time dependent open boundaries and a model data comparison for validation the analysis of model outputs indicates that most features are in good agreement with independent available observations the surface front evolution is subject to an intense deformation different from that of the deep front system which evolves only weakly an estimate of the upper layer heat budget is performed during the 22 days of the integration of the model each term of this budget is analyzed according to various atmospheric events that occurred during the experiment such as the passage of a strong storm this facilitates extended estimates of mixed layer or relevant surface processes beyond those which are obtainable directly from observations surface fluxes represent 54 of the heat loss in the mixed layer and 70 in the top 100 m layer while vertical transport at the mixed layer bottom accounts for 31 and three dimensional processes account for 14

ventilation of the north atlantic and north pacific subduction versus obduction abstract ventilation in the north atlantic and north pacific is examined by analyzing the levitus climatological data and the hellerman and rosenstein wind stress data ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes subduction and obduction subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above veatilation in the north atlantic and north pacific can be clarified into four physically different regions the subductive region the obductive region the ambiductive region where both subduction and obduction take place and the insulated region where neither subduction nor obduction occurs although the total subduction rates in these two oceans are comparable the total obduction rates are considera

oceanic restratification forced by surface frontogenesis potential vorticity pv conservation implies a strong constraint on the time evolution of the mean density at a given depth the authors show that on an f plane and in the absence of sources and sinks of pv it only depends on two terms namely the time evolution of the product between density anomaly and relative vorticity and the vertical pv flux this primitive equation result which applies at any depth suggests that the ageostrophic dynamics induced by baroclinic eddies strongly affect the mean oceanic stratification profile this result is illustrated for two simple initial value simulations of a baroclinic balanced jet for initial situations propitious to surface frontogenesis the simulations show a restratification over the whole water column characterized by the amplification in time of the brunt vaisala frequency in the upper oceanic layers in the absence of surface frontogenesis such as when the jet is initialized at the middepth of the water column the restratification is much weaker and slower because both simulations have similar kinetic energy and growth rate of baroclinic instability the results clearly reveal that the restratification is driven by surface frontogenesis in the first case and by vertical pv flux in the interior in the second case the authors also point out that the dynamics of the interior pv is tightly related to the surface dynamics because of total mass conservation

the effects of mesoscale eddies on the stratification and transport of an ocean with a circumpolar channel abstract the effects of eddies in a primitive equation ocean model configured in a single hemisphere domain with circumpolar channels at their poleward ends are investigated in particular two regimes for the mass balance in the channel are investigated with small overlying winds the channel stratification is largely set by diffusion operating in the gyre portion of the domain the depth scale varies with a fractional power of the diffusivity but has little dependence on the wind stress as the winds are increased the depth becomes increasingly controlled by a tendency toward small residual circulation in this limit a scaling theory is derived for the stratification in the channel that predicts the overall depth of the thermocline as a power of the wind stress and that allows the eddy length scale to differ from the channel length scale the predicted depth depends on the details of the closure chosen for the eddy buoyancy flux but in general it varies as some fractional power of the wind stress a

shallow pycnoclines and mode water subduction in the eastern north atlantic abstract a lagrangian one dimensional model is used to study the subduction of subpolar mode water in the eastern north atlantic and to analyze recently observed hydrological features related to this process considering a southward moving column of mode water subduction occurs when the upper part of the column starts gaining buoyancy in an annual budget the seasonal pycnocline on top of the column can no longer be completely eroded in winter and it is shown how its remnant forms a shallow secondary pycnocline at about 200 m depth which isolates the lower part of the fluid column from the atmosphere this mechanism for the subduction of subpolar mode water induces a strong meridional gradient in the winter mixed layer depth the various components of the buoyancy input are thoroughly studied horizontal advection in the shallow ekman layer and vertical advection along the column are shown to modify significantly both the thermal and haline contents of the column with magnitudes comparable to the ai

observations of open ocean deep convection in the labrador sea from subsurface floats abstract the occurrence and extent of deep convection in the labrador sea in winters 1996 97 and 1997 98 is investigated from measurements of over 200 neutrally buoyant subsurface profiling autonomous lagrangian circulation explorer palace and sounding oceanographic lagrangian observer solo floats in addition to providing drift velocity data and vertical profiles of temperature and salinity 55 floats are equipped with vertical current meters vcms time series of vertical velocity derived from measured pressure and vertical flow past the float and temperature are obtained from the vcm floats mixed layer depths estimated from profile measurements indicate that convection reached depths greater than 1300 m in 1997 but no deeper than 1000 m in 1998 deep mixed layers were concentrated in the western basin although a number of deep mixed layers were observed southwest of cape farewell and also north of 60 n the highest variance in vertical velocity and the lowest mean temperatures were found in t