a subtropical oceanic ring of magnitude storm in the eastern north atlantic physical chemical and biological properties a subtropical oceanic ring of magnitude storm that budded from the azores current was studied in the eastern north atlantic ocean in april 1999 the cyclonic eddy called leticia was centred at 32 4 n 28 7 w and extended across more than 200 km in an east west direction leticia had been previously detected by means of topex poseidon altimetry data and further intensively investigated using expendable bathythermograph and conductivity temperature depth stations the altimeter surface signature of the eddy revealed a 20 cm sea level depression an upward displacement of the 16 c isotherm of 125 m was linked to the eddy centre and geostrophic transport of the amount of water swirling cyclonically was 7 sv the temperature nitrate and oxygen anomalies on the 0 26 5 isopycnal surface inside leticia were 0 4 c 1 mol l 1 and 0 3 ml l 1 respectively with respect to surrounding waters shallowing 50 m of the deep chlorophyll maximum 0 3 mg m 3 was observed at the eddy centre associated with vertical displacements of the isotherms 100 m within the photic layer vertical excursions of the isopycnals and the deep chlorophyll maximum of more than 30 m were observed over temporal scales of a few 3 6 h integrated nitrate concentration over the photic layer was 4 fold higher inside leticia than outside however primary production rates were only slightly higher eddy diffusive fluxes across the nitracline located 30 m below the deep chlorophyll maximum ranged from 0 02 to 0 10 mmol no3 m 2 d 1 and explained 25 of the nitrate required to sustain the estimated new production 0 24 0 48 mmol no3 m 2 d 1

empirical spectral acceleration amplification in the iwate miyagi and niigata regions japan inferred by a spectral ratio method using ground motion prediction equations abstract using the strong ground motion dataset of m w 4 0 6 0 earthquakes in japan from k net and kik net we studied empirically the 5 damped spectral acceleration amplification for periods from 0 05 to 5 s at 75 seismic stations located in the iwate miyagi and niigata regions the method of study is based on estimating the empirical site effect from the spectral ratio of observed and predicted ground motions based on the ground motion prediction equations gmpes our results show a large amplification at most analyzed stations at periods of 0 1 s and shorter at periods of 0 3 s and longer large amplifications generally dominate at basin stations but deamplifications generally dominate at mountain stations empirical amplification factors of spectral acceleration were used to correct the observed strong ground motion data of two crustal earthquakes the 2008 iwate miyagi nairiku earthquake m w 6 9 and the 2004 chuetsu earthquake m w 6 6 the site corrected data fit better with gmpes inferred by less standard error we took advantage of a large body of empirical data to develop a mean empirical site response model for each region these empirical models are superior to models predicted from gmpes using only v s 30 site corrections those relations have shown a low correlation at short periods such as 0 1 s we also found the relations are different for the mountain and basin stations the method is efficient for calculating site effects at observation stations where insufficient velocity structure information and a history of earthquake recordings are available

north atlantic subtropical mode waters and ocean memory in hadcm3 abstract a study of the formation and propagation of volume anomalies in north atlantic mode waters is presented based on 100 yr of monthly mean fields taken from the control run of the third hadley centre coupled ocean atmosphere gcm hadcm3 analysis of the temporal and spatial variability in the thickness between pairs of isothermal surfaces bounding the central temperature of the three main north atlantic subtropical mode waters shows that large scale variability in formation occurs over time scales ranging from 5 to 20 yr the largest formation anomalies are associated with a southward shift in the mixed layer isothermal distribution possibly due to changes in the gyre dynamics and or changes in the overlying wind field and air sea heat fluxes the persistence of these anomalies is shown to result from their subduction beneath the winter mixed layer base where they recirculate around the subtropical gyre in the background geostrophic flow anomalies in the warmest mode 18 c formed on the western

merging and migration of two meddies in this study a scenario is developed of two adjacent mediterranean water eddies meddies as they were observed merging and drifting through the iberian basin observations are based on four rafos floats at 850 1050 dbar two hydrographic surveys centered roughly at 38 n 24 w and trajectories of surface drifters drogued at 100 m in april 1991 the meddy a was identified and labeled by surface drifters during the revisit one month later two meddies were encountered b1 and b2 in the vicinity of the former meddy a the coalescence of b1 subsequently identified as a one month older and b2 is inferred from a simple kinematic model describing the observed movement of the rafos floats for up to three months after the second ctd survey the deduced vorticity front radius 15 km within b1 was of insufficient strength to keep the core waters of b1 isolated and prevent the absorption of b1 by b2 the resulting meddy b1 b2 showed a clear near surface dynamical signal its deep root 1800 m could explain the expulsion from the meddy of the remaining rafos float and surface drifter at the time of the meddy s collision with the josephine seamount for the first time a set of lagrangian and hydrographic observations give direct evidence that neighboring meddies can merge as predicted by theoretical considerations

a tritium box model of the north atlantic thermocline abstract a box model of 1972 tritium observations on isopycnal surface in the main thermocline of the north atlantic subtropical gyre is used to estimate the time scales and volume of exchange of the thermocline with respect to surface waters the flux of water between the surface and the thermocline implied by this model 40 106 m3 s 1 greatly exceeds the downward ekman pumping 8 106 m3 s 1 this suggests that mixing and convective overturning are the dominant mechanisms for exchange between surface waters and the interior geostrophic flow the flux rate is approximately the same size as conventions estimates of the sverdrup transport this suggests that ventilation of the thermocline may occur by recirculation combined with a very efficient exchange across the poleward boundary of the gyre

an eddy resolving numerical model of the ventilated thermocline abstract a three dimensional primitive equation numerical model is used to study the effects of mesoscale eddies within the subtropical thermocline solutions are obtained for an ocean bounded by idealized topography are driven by simple wind and buoyancy forcing at the surface results using an eddy resolving fine grid are compared to those using a noneddy resolving coarser grid relatively little difference is realized in the basic mean flow patterns of the two solutions however more significant differences are seen in the distributions of a passive tracer and the potential vorticity mixing by eddies in the westward flowing sector of the subtropical gyre is quite effective in homogenizing these quantities on isopycnals whereas previous theory has predicted homogenization of potential vorticity on long time scales within recirculating gyres the present model demonstrates homogenization on a much shorter time scale across recirculating ventilating flow boundaries anomalous potential vorticity th

aspects of vertical drain design darcian or non darcian flow the classical consolidation equations developed by barron in 1944 and 1948 are well known among engineers working on soil improvement by preloading in combination with installation of vertical drains these equations have been gradually refined and are generally utilized in conventional design the design may seem simple in theory but is connected with a great number of uncertainties from a practical point of view the problems connected with the choice of consolidation parameters will be commented on and practical advice for the design will be given in this paper a number of case records will be presented and compared with on one hand the conventional method of design and on the other hand the so called method developed by the author in 1960 to take into account the effect of smear the method is based on an exponential correlation between hydraulic gradient and flow velocity we find that the correlation between the conventional theory and practice based on back analysis of field data is les

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

correlation of current and sea level in the eastern equatorial pacific abstract upper ocean current and temperature records from neat 0 110 w are compared with galapagos island sea level measurements 0 3 s 91 28 w for the period march 1980 to july 1981 low frequency periods greater than 30 days near surface zonal currents 20 and 50 m were well correlated with local wind but deeper currents 100 and 150 m and sea level were not response to the local wind was studied in terms of linear dynamics periods where zonal acceleration was in phase with zonal wind stress were found however overall these two series were uncorrelated the direct relation of low frequency wind stress and surface current rather than acceleration suggests a quasi equilibrium response on these time scales the response at deeper levels was investigated by comparing the zonal transport per unit width vertically averaged zonal velocity in the upper 200 m with sea level these series were significantly correlated with 95 confidence at a lag of 10 5 d indicating eastward phase propagation

a view of the canary basin thermocline circulation in winter during january and february 1989 the recirculation of the subtropical gyre in the eastern north atlantic was surveyed with a three ship experiment the analysis of hydrographic measurements and velocity data from a shipboard acoustic doppler current profiler reveals the synoptic scale circulation patterns and water mass distributions in the canary basin the geostrophic transport stream function estimated with a horizontally varying reference level of no motion highlights the major currents in three layers representing the vertical structure of the horizontal circulation the classical circulation scheme is shown by the stream function in the upper 200 m the azores canary and north equatorial currents unlike the deep penetrating azores current the canary current and the north equatorial current are restricted to the upper 200 m both carry north atlantic central water along the water mass boundary with south atlantic central water south atlantic central water flows through the passage between the cape verde archipelago and africa via narrow currents into the area north of 14 5 n at the southern edge of the subtropical gyre we identify an eastward flow of antarctic intermediate water between 700 and 1200 m

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

a numerical model of the ventilated thermocline abstract a steady state numerical solution is found for an idealized rectangular ocean basin driven by wind and surface buoyancy flux a three dimensional primitive equation model is used in agreement with recent analytical modeling the thermocline in the numerical solution consists of three regions quite distinct in their ventilation characteristics forming the greater part of the subtropical thermocline is an unventilated pool zone located in the core of the subtropical gyre and a ventilated zone to the east the unventilated shadow zone lies farther east and toward the equator analysis of potential vorticity on constant density surfaces is used to study the structure of the thermocline a small but intense zone of convection located in the western boundary outflow caused by rapid heat loss to the atmosphere produces source water for the ventilated zone this water of extremely low potential vorticity mode water is distributed widely into the subtropical thermocline the pool forms equ

attenuation of high frequency shear waves in the crust measurements from new york state south africa and southern california we compare the attenuation of high frequency 3 30 hz shear waves for crustal paths in new york state south africa and southern california over source receiver distances of about 10 400 km the data consist of digital recordings of s waves 5 100 km and lg waves 100 400 km produced by earthquakes we use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the s and lg waves over the entire distance range studied 10 400 km the amplitude decay of 3 hz shear wave energy is considerably less for the tectonicaily stable areas of new york and south africa than for the tectonicaily active region of southern california high frequency 30 hz s wave attenuation is significantly less for new york and south africa than for southern california for distances between 15 and 90 km we parameterize the decay with distance r of coda normalized shear wave amplitudes with a frequency independent q and geometrical spreading exponent where geometrical spreading is proportional to r for new york state the s wave amplitude decay 3 30 hz is well described by a frequency independent q of 2100 330 490 and of 1 3 0 1 the decay of lg wave amplitudes from 3 to 15 hz in the new york state region is fit with a frequency independent q of 1600 280 330 of 0 70 0 2 the s wave amplitudes 3 30hz in south africa yield a q of 1500 190 380 and of 1 3 0 1 fixing the geometrical spreading at r 0 5 produces an lg wave q estimate at 3 hz in south africa of 360 50 80 this lg wave q is low considering that south africa is a cratonic tectonicaily stable area the s wave amplitudes from southern california are described with a frequency independent q of 800 150 240 and a large geometrical spreading exponent of of 1 9 0 2 we find an lg wave q at 3 hz of 260 30 for southern california after constraining the geometrical spreading at r 0 5

sea surface signatures of the island mass effect phenomena around madeira island northeast atlantic abstract this is an introductory work that describes the manifestation of the island mass effect phenomena in the atmosphere and at the sea surface for a region of the ocean depleted from oceanographic work the madeira archipelago 33 n 17 w the use of remote sensing tools becomes essential in recognizing some of the sea surface features that characterize the island mass effect phenomena avhrr coastal zone color scanner czcs and seawifs ancillary national center for environmental prediction ncep data together with highly developed processing tools from panorama processing and automatic navigation of real time images allowed the detection of atmospheric von karman vortex streets warm water surface wakes leeward eddies azorean front and localized upwelling results from the remote sensing analysis also helped to promote the reanalysis of historical in situ data for the manifestation of these island effects ipimar cruises carried out between 1979 and 1982 collected temperature salinity density and chlorophyll data which were reinterpolated at the sea surface to study the island mass effect phenomena for the first time results have shown the formation of a warm water wake south of madeira island expanding 400 km offshore the surface signature of the azorean front the so called subtropical front seems to take place within the madeira latitude 33 n north waters were consistently colder than the warm and salty southern waters avhrr data also revealed eddies and fronts being formed in the region leeward eddies were often observed in the flanks of the islands czcs data showed highly productive eddies in the west of madeira island localized cold water with high chlorophyll concentrations was also observed around the island s coast a particularly dynamic area was the underwater ridge that connected madeira and desertas islands future work should continue to sample these regions of interest with the simultaneous use of different satellite sensors vertical characterization of the different phenomena is needed

vertical density gradient in the eastern north atlantic during the last 30 000 years past changes in the density and momentum structure of oceanic circulation are an important aspect of changes in the atlantic meridional overturning circulation and consequently climate however very little is known about past changes in the vertical density structure of the ocean even very extensively studied systems such as the north atlantic here we exploit the physical controls on the settling depth of the dense mediterranean water plume derived from the strait of gibraltar to obtain the first robust observations based probabilistic reconstruction of the vertical density gradient in the eastern north atlantic during the last 30 000 years we find that this gradient was weakened by more than 50 relative to the present during the last glacial maximum and that changes in general are associated with reductions in amoc intensity however we find only a small change during heinrich event 1 relative to the last glacial maximum despite strong evidence that overturning was substantially altered this implies that millennial scale changes may not be reflected in vertical density structure of the ocean which may be limited to responses on an ocean overturning timescale or longer regardless our novel reconstruction of atlantic density structure can be used as the basis for a dynamical measure for validation of model based amoc reconstructions in addition our general approach is transferrable to other marginal sea outflow plumes to provide estimates of oceanic vertical density gradients in other locations

the water masses of the central north atlantic in 1983 84 abstract hydrographic surveys carried out in 1983 84 along both sides of the mid atlantic ridge between 24 and 53 n provide a detailed description of the well known north atlantic water masses with particular emphasis on their meridional distribution and zonal dissymetry in the upper layers the dense horizontal sampling resolves the several narrow gulf stream extensions into the ocean interior giving the image in the central water density range of a mosaic of mode waters separated by fronts at intermediate depths a vertical shear in the distributions of the mediterranean water and labrador sea water stands out both water masses having their lower part displaced southwards relative to their upper parts bottom waters containing nearly 20 percent of pure antarctic bottom water are observed at 50 n in the eastern basin in contrast with the western basin where proportions greater than 10 percent were found only south of 36 n along our section this water mass analysis also gives indications that stron

tenth canadian geotechnical colloquium recent developments in consolidation of natural clays a global analysis of the consolidation of natural clays is realized considering the consolidation process to be a combination of the effects of compressibility and of permeability the compressibil

consolidation by vertical drains a theory is described which can be used in designing a vertical drainage system this theory incorporates important parameters such as vertical discharge capacity remoulding effects during installation and filter resistance the importance of these parameters is widely recognized but very seldom taken into account in the design by engineers six well documented case records related to different drain types are briefly presented experience gained from these case records shows that in many cases even if the overall performance appears successful there are aspects of soil drain behaviour that cannot be explained on the basis of existing theories l article decrit une theorie que l on peut appliquer a la conception d un systeme de drainage vertical cette theorie renferme d importants parametres tels que la capacite de decharge verticale les effets du remaniement pendant l installation et la resistance de filtration l importance de ces parametres est reconnue par bien des ingenieurs mais ils en tien

eddy kinetic energy in the north atlantic from surface drifters one hundred ten satellite tracked freely drifting buoys measured velocities and trajectories of the near surface currents in the north atlantic mean velocity values and the velocity variance about the mean were calculated for different regions a horizontal map of eddy kinetic energy was prepared on a 2 2 grid between latitudes 20 and 55 n maximum eddy energy 3000 cm2 s 2 coincides with the high speed gulf stream jet where it begins large amplitude meanders near 37 n 67 w a tongue of high eddy energy coincides with the stream s path eastward and around the grand banks into the newfoundland basin where values of 1000 cm2 s 2 are found a weaker tongue extends eastward across the mid atlantic ridge near 45 n a second weak extension reaches southeastward from the stream and crosses the mid atlantic ridge between 30 and 35 n north and south of the stream eddy energy diminishes rapidly reaching an e folding at 300 km from the axis values of 200 cm2 s 2 were observed in the mid gyre region and 100 cm2 s 2 in the eastern north atlantic and north equatorial current although the gross distribution of eddy energy is similar to that determined from ship drift measurements there are significant differences eddy energy from drifters amounts to about twice the value measured by ship drift in the gulf stream and one half the ship drift values in the mid gyre it is suggested that these differences are due to the horizontal averaging of mesoscale motion and the errors in navigation both of which are problems with the ship drift technique

the impact of surface flux parameterizations on the modeling of the north atlantic ocean abstract the response of an ocean general circulation model to several distinct parameterizations of the surface heat and freshwater fluxes which differ primarily by their representation of the ocean atmosphere feedbacks is investigated in a realistic configuration for the north atlantic ocean the impact of explicitly introducing oceanic information climatological sea surface temperature into the computation of the heat flux through a haney type restoring boundary condition as opposed to the case in which the flux is based on atmosphere only climatologies and is computed with the full bulk formulation is considered the strong similarity between these two approaches is demonstrated and the sources of possible differences are discussed when restoring boundary conditions are applied to the surface salinity however an unphysical feedback mechanism is being introduced the model s response to this restoring is contrasted to the response to a flux boundary condition that prescribes the freshwater fl

two years in the life of a mediterranean salt lens abstract a lens of mediterranean water meddy was tracked in the eastern north atlantic for two years with sofar floats the meddy was first found between the canary islands and the azores in october 1984 it center moved in an irregular pattern at speeds of a few cm s 1 and translated 1100 km to the south in two years this meddy was surveyed four times by ctd and velocity profilers and once with the microstructure profiler epsonde when observed during the first two surveys the meddy had a core that was stably and smoothly stratified in both salinity and temperature nearly uniform in the horizontal and was saltier than the surrounding ocean by 0 65 psu the meddy was eroded from its edges top and bottom and lost salt and hat with an e folding time of about one year the salinity at the center remained at its original value during the first year and decreased during the second year evidence was seen for mixing by lateral intrusions double diffusion and turbulence the intrusions are thought to

buoyancy and mixed layer effects on the sea surface height response in an isopycnal model of the north pacific abstract an isopycnal model of the north pacific is used to demonstrate that the seasonal cycle of heating and cooling and the resulting mixed layer depth entrainment and detrainment cycle play a role in the propagation of wind driven rossby waves the model is forced by realistic winds and seasonal heat flux to examine the interaction of nearly annual wind driven rossby waves with the seasonal mixed layer cycle comparison among four model runs one adiabatic without diapycnal mixing or explicit mixed layer dynamics one diabatic with diapycnal mixing and explicit mixed layer dynamics one with the seasonal cycle of heating only and one with only variable winds suggests that mixed layer entrainment changes the structure of the response substantially particularly at midlatitudes specifically the mixed layer seasonal cycle works against ekman pumping in the forcing of first mode rossby waves between 17 and 28 n south of there the mixed layer seasonal cycle has little influence on the rossby waves

multivariate projection of ocean surface data onto subsurface sections a multivariate projection of ocean surface data onto subsurface fields is presented the method is applied to the projection of sea surface height and sea surface temperature anomalies onto the three dimensional temperature field the results are compared to the conventional univariate regression between sea surface height and the underlying temperature structure the multivariate method clearly outperforms the univariate approach and succeeds in capturing the equatorial anomaly structures even during the early nineties a period in which the univariate projection encounters great difficulties a multivariate scheme projects onto linear combinations of different vertical modes depending on the contribution of each input variable thus vertical patterns different not only in amplitude but also in shape can be affected as opposed to the univariate case during the enso cycle two different vertical temperature modes become dominant to address both modes two linearly independent variables are required therefore the combination of ssh and sst contains additional information about the vertical temperature structure that cannot be extracted from either sea surface height measurements or sea surface temperature only this is of relevance in the context of enso predictions and it may help to improve the initialization of enso forecast models

the ventilation of the world s oceans maps of the potential vorticity field abstract maps of potential vorticity q within four different density layers are presented for the north and south atlantic north and south pacific and indian oceans potential vorticity is evaluated as f 0 z a prescription that is valid for large slow scales and that allows its mapping from hydrographic data alone here f is the coriolis frequency p0 the potential density and z the vertical coordinate it is shown that the character of a layer within the thermocline of a subtropical gyre will vary greatly depending on the extent to which the layer is isolated from surface boundary conditions and forcing fluid particles recirculating within ventilated layers have their potential vorticity and other properties reset with virtually every circuit round the gyre when they pass through the outcrop zone by contrast particles within unventilated or dynamically isolated layers have time to share their properties with neighboring particles and approach a common homogenized state the north

assimilating altimetric data into a south china sea model sea surface heights from the topex poseidon altimeter are assimilated into a three dimensional primitive equation model to derive the circulation in the south china sea with data assimilation the model resolves not only the basin wide circulation but also a dipole off vietnam and a low high feature near the luzon strait mesoscale features are missing in the simulation without data assimilation because of poor resolution in the wind field and inadequate knowledge of the transport through the luzon strait compared to the case without data assimilation data assimilation reduces the root mean square error between the simulated and observed sea surface heights by a factor of 2 3 circulation derived from data assimilation under climatological conditions is contrasted with that during el nino in the normal winter of 1993 1994 flow at 50 m depth is strong and cyclonic flow at 900 m depth is cyclonic as well the deep cyclone persists into the following summer during the 1994 1995 el nino winter features in the flow field at 50 m depth either weaken or disappear and circulation at 900 m depth is anticyclonic in the summer of 1995 the dipole and the eastward jet off vietnam at 50 m depth are missing and the anticyclonic circulation at 900 m depth persists temperature at 65 m shows significant warming from fall 1994 to summer 1995 a weakened flow field and warming in the upper ocean are consistent with findings from earlier el nino events

mixed layer thermocline interaction in a three dimensional isopycnic coordinate model abstract the annual buildup and obliteration of the seasonal thermocline and the associated ventilation of the permanent thermocline in a wind and thermally driven ocean basin are simulated numerically the model developed for this purpose is a combination of a single layer model of the oceanic mixed layer based on a simple closure of the turbulence kinetic energy equation and a three dimensional isopycnic coordinate model of the stratified oceanic interior the joint model set in a rectangular ocean basin is forced by annually varying wind stress and radiative plus turbulent heal fluxes approximating zonnaly averaged conditions over the north atlantic special emphasis is placed on the description of the mixed layer detrainment process which requires distributing mixed layer water of continuously variable density among constant density interior layers the truncation errors associated with this process are found to be numerically tolerable the quasi lagrangian character of the model s vertical coo

assimilation of altimeter eddy fields in a limited area quasi geostrophic model abstract techniques for the synoptic analysis vertical inference dynamical adjustment and forecast of altimetric and deeper in situ data are presented as a first step towards the design of continuous assimilation schemes in limited area oceanic domains a year long time series of streamfunction maps denoted mark 2 drawn in the polymode area of the western north atlantic is used as a benchmark for various tests and simulations an original projection extension scheme using empirical modes of density and or pressure anomalies is used to obtain a first guess of the three dimensional structure of the currents starting from surface topography only in the mark 2 domain this technique works well since the first empirical mode is surface intensified and largely dominant an alternative approach is to incorporate deeper data e g float trajectories below the main thermocline the first guess currents are specified as initial and boundary condition of the harvard 6 level quasi geostrophic open ocean mode

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

observations on regional variability in ground motion amplitudes for small to moderate earthquakes in north america abstract the regional variability in earthquake ground motion amplitudes for a given magnitude and distance in western north american environments was examined using shakemap data from small to moderate events the abundance of data for small to moderate events in california allows average ground motion levels as a function of magnitude and distance to be resolved with a high level of confidence ground motion amplitudes in northern california are lower on average than those for southern california for events of the same magnitude at distances in the range from 120 to 250 km over all frequencies the observed regional variations could be indicative of regional differences in attenuation effects or site effects an unexpected result of the study is the finding that ground motions for events of m m m m

a semi empirical method for simulating strong ground motions based on variable slip rupture models for large earthquakes abstract variable slip rupture models for large earthquakes obtained by the source inversion of long period 4 sec seismic waves are taken into account in a semi empirical method for simulating broadband 0 25 hz source spectrum of the p q th subfault is inferred by the 2 mode with two circular corner frequencies the first is dpq v pq d pq due to the temporal integration of the slip velocity time function where v pq is the maximum slip velocity and d pq is the final slip the other is spq 2 pq pq due to the spacial integration of the slip velocity time function on the subfault where pq is the s wave velocity of the medium and pq is the equivalent radius of the subfault here v pq d pq pq and pq are specified by the long period source inversion results first we describe this new method by applying it to the variable slip rupture model for the 1985 michoacan mexico earthquake of m s 8 1 obtained by mendoza and hartzell 1989 the simulated accelerations and velocities at cal caleta de campos and vil la villita both located above the ruptured zone are in good agreement with the observed ones next the method is applied to the variable slip rupture model for the 1923 kanto japan earthquake of m s 8 2 obtained by wald and somerville 1995 this earthquake is one of the most important earthquakes for the mitigation of earthquake disaster in the tokyo metropolitan area unfortunately strong motion records for this earthquake were off scaled in the region of strong shaking and significant damage the pseudo velocity response spectrum of the simulated acceleration of tok tokyo jma averages 60 cm sec in the period range of 0 5 to 10 sec and is consistent with that of the kanto earthquake record observed at hng hongo tokyo whose off scaled parts were restored as well as possible by yokota et al 1989 the instrumental jma seismic intensities jma 1996 of the simulated accelerations at tok and yok yokohama jma are consistent with the observed jma seismic intensity 6 jma 1983 the instrumental jma seismic intensity of the simulated accelerations at kns soil site in odawara is also consistent with the jma seismic intensity 7 estimated from the ratio of collapsed houses mononobe 1925 the simulated broadband 0 1 to 10 sec motions will be useful in the mitigation of earthquake disaster in the tokyo metropolitan area

a circumpolar gravest empirical mode for the southern ocean hydrography historical hydrographic data from the southern ocean are projected into a baroclinic stream function space generating a three dimensional gravest empirical mode gem parameterized by pressure geopotential height and longitude the low dimensional gem projection is intrinsically steady vertically coherent and equivalent barotropic the gem fields capture more than 97 of the total density and temperature variance in the antarctic circumpolar current region

environmental changes and growth history of a cold water carbonate mound propeller mound porcupine seabight on and off mound sediment cores from propeller mound hovland mound province porcupine seabight were analysed to understand better the evolution of a carbonate mound the evaluation of benthic foraminiferal assemblages from the off mound position helps to determine the changes of the environmental controls on propeller mound in glacial and interglacial times two different assemblages describe the holocene and marine isotope stage mis 2 and late mis 3 31 kyr bp the different assemblages are related to changes in oceanographic conditions surface productivity and the waxing and waning of the british irish ice sheet biis during the last glacial stages the interglacial assemblage is related to a higher supply of organic material and stronger current intensities in water depth of recent coral growth during the last glaciation the benthic faunas showed high abundances of cassidulinid species implying cold bottom waters and a reduced availability of organic matter high sedimentation rates and the domination of elphidium excavatum point to shelf erosion related to sea level lowering 50 m and the progradation of the biis onto the shelf a different assemblage described for the on mound core is dominated by discanomalina coronata gavelinopsis translucens planulina ariminensis cibicides lobatulus and to a lower degree by hyrrokkin sarcophaga these species are only found or show significantly higher relative abundances in on mound samples and their maximum contribution in the lower part of the record indicates a higher coral growth density on propeller mound in an earlier period they are less abundant during the holocene however this dataset portrays the boundary conditions of the habitable range for the cold water coral lophelia pertusa which dominates the deep water reefal ecosystem on the upper flanks of propeller mound the growth of this ecosystem occurs during interglacial and interstadial periods whereas a retreat of corals is documented in the absence of glacial sediments on mound glacial conditions with cold intermediate waters a weak current regime and high sedimentation rates provide an unfavourable environmental setting for lophelia corals to grow a late pleistocene decrease is observed in the mound growth for propeller mound which might face its complete burial in the future as it already happened to the buried mounds of the magellan mound province further north

persistent westward flow in the azores current as seen from altimetry and hydrography temporal and spatial variability of the eastward flowing jet like azores current is investigated by applying a new method for combining altimetry with hydrography in order to produce absolute surface geostrophic currents challenor et al 1996 the altimeter surface measurements contain information about both the geoid and the geostrophic ocean circulation the geoid signal dominates and is generally not sufficiently well known at the small length scales required to identify the mesoscale ocean signal in this study hydrography is used to distinguish the mean ocean signal from the geoid along an ers 1 satellite track south of the azores during both 3 day repeat phases and thereby determine cross stream absolute dynamic topography of the azores current at high temporal resolution hydrographic data collected near 28 w in march 1992 is combined with ers 1 altimeter data from january march 1992 and january to early april 1994 time series of 3 day repeat profiles of absolute surface geostrophic velocity perpendicular to the altimeter cruise track are examined the azores current absolute velocity profile along this track is rather coherent the current variability is less than the mean surprisingly a persistent westward flow of 25 cm s 1 is observed near 35 n 28 w this flow appears to be a retroflection from the main eastward flowing current an isopycnal analysis reveals the westward flow to be cooler by 0 5 c and fresher by 0 15 practical salinity units than the main body of the azores current the flow appears to persist throughout each of the above 3 month periods in late winter early spring but the question of whether this is a seasonal feature or more permanent remains open it is suggested that the westward flow is topographically controlled

tracking three meddies with sofar floats abstract three meddies were tracked for up to two years in the canary basin using neutrally buoyant sofar floats these meddies have cores of warm salty mediterranean water and are approximately 100 km in diameter 800 m thick and are centered at a depth of 1100 m meddy 1 was tracked for two years 1984 86 with five floats as it drifted 1090 km southward with a mean velocity of 1 8 cm s 1 four shipboard surveys made during these two years revealed the nearly total decay of meddy 1 by gradual mixing processes meddy 2 drifted 530 km southwestward over 8 5 months with a mean velocity of 2 3 cm s 1 until it collided with hyeres seamount near 31 n 29 w the floats trapped in this meddy then stopped looping abruptly implying a major disruption of this meddy meddy 3 drifted 500 km southwestward for a year and a half with a mean translation velocity of 1 1 cm s 1 a comparison of the velocity of meddies to the velocity of nearby floats at 1100 m depth outside of the meddies shows clearly that all three m

on the origin of the azores current the azores current south of the azores archipelago is part of the subtropical north atlantic gyre using an international hydrographic data set we analyze mean and seasonal geostrophic transport fields in the upper 800 m of the ocean in order to determine the origin of the azores current in the western basin and seasonal changes in the related flow geostrophic currents are obtained by using the method applied by stramma 1984 in the eastern basin the azores current is found to originate in the area of the southwest newfoundland rise figure 10 in winter an almost uniform current connects this region of origin with the azores current while a branching into two current bands is observed in summer with the southern band forming a marked cyclonic loop within the upper 800 m all of the transport in the northern band and about 70 of the transport in the southern band recirculates in the eastern basin additionally expendable bathythermograph data from the azores current region indicate an increase of eddy potential energy from winter to summer

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

sea surface dynamics and palaeoenvironmental changes in the north atlantic ocean iodp site u1313 during marine isotope stage 19 inferred from coccolithophore assemblages abstract coccolithophore assemblages recovered from integrated ocean drilling program iodp site u1313 are investigated to reconstruct the palaeoceanographic evolution of a sector of the north atlantic during marine isotope stage mis 19 790 760 ka at orbital suborbital and millennial time scales the end of the glacial mis 20 is marked by the arrival of cold waters deriving from ice melting at higher latitudes as recorded by coccolithus pelagicus pelagicus the mis 19c is characterised by an extension of warmer north atlantic transitional waters natw reflecting the intensification of subtropical gyre influence on site u1313 identified by an increase in warm species umbilicosphaera sibogae the influence on the site of cooler natw is inferred from higher percentages of gephyrocapsa margereli which starts increasing from 779 ka finally the transition from mis 19a to mis 18 is characterised by a southward shift of the palaeoceanographic system due to a southward extension of the subpolar front as indicated by an increase of cold subspecies c pelagicus pelagicus the evolution of the palaeoproductivity proxy records number of coccoliths g of sediment and nannofossil accumulation rate nar and the abundances of small gephyrocapsa g margereli u sibogae and c pelagicus pelagicus show that fluctuations in sea surface productivity and alternations between cooling and warming phases occurred on the precessional timescale spectral analyses of u sibogae number of coccoliths g of sediment and nar indicate a significant concentration of variance close to half precession 10 kyr this reflects a nonlinear response of the mid latitude north atlantic surface ocean to low latitude insolation forcing furthermore millennial to multi centennial instability in sea surface dynamics is highlighted by spectral and wavelet analyses of u sibogae g margereli and c pelagicus pelagicus percentages number of coccoliths g of sediment and nar which reveal the occurrence of rapid events related to stadial and interstadial type conditions

quasi synoptic transport budgets and water mass transformation in the azores gibraltar strait region during summer 2009 abstract we describe the circulation patterns in the azores gibraltar strait region north eastern atlantic during the 2009 caibox cruise on the basis of hydrographic and direct current velocity measurements this study offers new data for a region where importation of central waters subpolar and subtropical modes of eastern north atlantic central water and exports of mediterranean water are strongly related to large scale dynamics in the north atlantic ocean azores current mediterranean water system the description is backed up quantitatively by the results of a box inverse model which was used to obtain absolute water mass transport values consistent with thermal wind equations and with conservation of volume salt and heat the contributions of water masses were determined in an extended optimum multiparameter analysis from a quasi synoptic point of view providing detail in addition to volume salt and heat transport the surface subsurface large scale current system in the region consists of the azores current 13 1 2 5 sverdrup sv 1 sv 10 6 m 3 s 1 the azores counter current 5 2 2 1 sv the portugal current 4 5 1 4 sv and the canary current 7 1 1 1 sv broadly speaking central waters are imported into the caibox region at a rate of 1 6 0 9 sv and mediterranean water is exported at a rate of 1 5 0 4 sv the downwelling of central waters west of gibraltar strait was quantified at 1 1 sv not all this volume participates in mw formation but 0 8 sv of entrained central waters of which 0 5 sv are from central waters of subpolar origin and 0 3 sv from subtropical central waters of the 4 9 sv of subtropical central waters advected by the azores current about 0 7 sv would reach the gulf of cadiz region either to take part in central water entrainment or to flow across the gibraltar strait as part of the atlantic inflow to the mediterranean sea

monsoonal forcing of european ice sheet dynamics during the late quaternary the dynamics of northern hemisphere ice sheets during late quaternary glacials have yet been dominantly examined from a laurentide ice sheet perspective which helped shaping the idea of moisture starved glacials and small scale ice volume variability however the waxing and waning of the european ice sheet eis casts doubt on this perception understanding eis dynamics under glacial boundary conditions is crucial because its meltwater pulses influence global climate by weakening deepwater formation in the north atlantic ocean here we show that the advection of subtropical water toward the continental margin of western europe lead to enhanced moisture availability on the continent and fueled the growth of eis lobes during glacials this warm water pooling was caused by monsoonally paced enhanced mediterranean outflow water mow entrainment that dragged subtropical surface waters toward the european margin this mechanism presents a yet unrecognized marine terrestrial pathway that allows low latitude forcing to shape high latitude glaciations

the influence of magnitude range on empirical ground motion prediction a key issue in the assessment of seismic hazard in regions of low to moderate seismicity is the extent to which accelerograms obtained from small magnitude earthquakes can be used as the basis for predicting ground motions due to the larger magnitude events considered in seismic hazard analysis in essence the question is whether empirical ground motion prediction equations can be applied outside their strict range of applicability as defined by the magnitude and distance ranges covered by the datasets from which they are derived this question is explored by deriving new spectral prediction equations using an extended strong motion da taset from europe and the middle east covering the magnitude range mw 3 0 7 6 and comparing the predictions with previous equations derived using data from only mw 5 0 and above events the comparisons show that despite their complex func tional form including quadratic magnitude dependence and magnitude dependent attenuation the equations derived from larger magnitude events should not be extra polated to predict ground motions from earthquakes of small magnitude moreover the results suggest not only that ground motion prediction equations cannot be used outside the ranges of their underlying datasets but also that their applicability at the limits of these ranges may be questionable although only tested for smaller magni tudes the results could be interpreted to suggest that predictive equations also cannot be reliably extrapolated to higher magnitudes than those represented in the dataset from which they are derived a finding that has important implications for seismic hazard analysis the conclusion of the study is that empirical derivation of ground motion pre diction equations should be based on datasets extending at least one unit below the lower limit of magnitude considered in seismic hazard calculations the inclusion of small magnitude recordings results in a significant increase in the aleatory varia bility of the equations although it is yet to be established whether this is due to greater uncertainty in the associated metadata or whether ground motion variability is gen uinely dependent on earthquake magnitude

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

north atlantic modeling of low frequency variability in mode water formation abstract the generation of interannual and near decadal variability in the formation of mode waters in the western north atlantic is investigated in the realistic framework of an isopycnic coordinate ocean model forced with atmospheric data from 1946 to 1988 at bermuda the model reproduces quite well the observed potential vorticity and isopycnal depth anomalies associated with the subtropical mode water stmw heat storage and preconditioning of the convective activity are found to be the important factors for the generation of stmw variability with persistence of cold warm conditions associated with anomalous heat loss gain over the western subtropics being more significant for the generation of the simulated variability than are strong anomalous events in isolated years in the labrador sea the model captures the phase and order of magnitude of the observed near decadal variability in the convective activity if not its maximum amplitude the simulated potential vorticity anomalies are as o

effects of nao variability on the north atlantic ocean circulation 1 the ocean response to an idealised north atlantic oscillation like wind stress is evaluated as a function of the atmospheric forcing frequency in order to elucidate the relationship between internal and forced variability the nao is modulated with two specific timescales 10 and 50 years which characterise the spectrum of the system s internal variability different timescales of atmospheric variability select distinct sea surface temperature sst and large scale circulation patterns under a 50 year nao forcing period a lagged sst response is excited in the gulf stream extension region which is consistent with the spin up of the gyre circulation the thermohaline circulation varies in phase with the nao and shows a strong sensitivity to the forcing frequency a dipole mode of the overturning is excited by a 10 year modulation of the nao while an enhanced overturning response emerges under a 50 year nao with low enough lateral mixing the ocean exhibits an irregular response to a regular nao like forcing

back calculation of consolidation parameters from field measurements at a reclamation site a pilot test was carried out at a land reclamation project site to study the effect of vertical drains on the rate of consolidation of a marine clay deposit in singapore theoretical and numerical analyses were conducted to interpret the field measurements of pore pressure and settlement and consolidation parameters were back calculated the back calculated preconsolidation pressure is similar to that obtained from the laboratory consolidation test on undisturbed samples but the field deduced compression index is generally larger than that measured in the laboratory the coefficient of consolidation back calculated from the settlement measurement is larger than that from the pore pressure measurement the coefficient of consolidation for horizontal flow obtained from the rowe cell test which is comparable to that back calculated from the field settlement measurement is recommended for the preliminary design of vertical drains without the need of considering the smear effect key words clays consolida

an isopycnic model study of the north atlantic part i model experiment abstract this paper describes a 30 yr spinup experiment of the north atlantic ocean with the miami isopycnic coordinate ocean model which when compared with previous experiments possesses improved horizontal resolution surface forcing functions and bathymetry and is extended to higher latitudes overall there is a conversion of lighter to heavier water masses and waters of densities 1027 95 and 1028 05 kg m 3 are produced in the greenland lceland norwegian basin and of density 1027 75 kg m 3 in the labrador and irminger basins these water masses flow primarily southward the main purpose of this present study however is to investigate the ventilation of the subtropical gyre the role of ekman pumping and lateral induction in driving the subduction process is examined and the relative importance of the latter is confirmed the paper also illustrates how the mixed layer waters are drawn southward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging

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

lagrangian transport pathways in the northeast atlantic and their environmental impact this study focused on mapping the general transport pathways of the northeast atlantic ocean by using the regional oceanic modeling system to calculate ocean current velocity components u v w and ariane an off line fortran code dedicated to the computation of three dimensional streamlines from velocity fields to calculate the transport of particles around the numerical model domain the study was undertaken using a 10 year climatic simulation statistical comparisons with satellite and in situ data showed that the ocean circulation model captured known regional oceanographic features four depth ranges showed different lagrangian transport pathways 0 10 m 20 200 m 300 500 m and 600 2000 m confirming that these routes are consistent with the known ocean circulation patterns our results were supported by multiple sources i connectivity between the african coast and the canary islands for sardine sardina pilchardus anchovy engraulis encrasicolus ii panmixia of lobsters scyllarides latus and palinurus elephas and european conger eel conger conger iii connectivity between azores and canary archipelagos for sponge phorbas fictitius and iv observed drifting of crude oil from the prestige oil tanker spill these results should help guide future observational campaigns as well as the interpretation of open ocean transport patterns and the distribution of marine organisms and chemical tracers in the northeast atlantic region

isopycnal empirical orthogonal functions eofs in the north and tropical atlantic and their use in estimation problems we investigated the vertical variability of the thermal dynamical structure of the north and tropical atlantic ocean from a set of historical hydrographic data reynaud et al 1998 the analysis was performed in 10 geographic bins in terms of vertical empirical orthogonal functions eofs in isopycnal coordinates as first proposed by gavart and de mey 1997 hereinafter referred to as gdm97 in the isopycnal coordinate system the state vector is made up of isopycnal displacements and the compensated potential temperature along isopycnals gdm97 showed that in the azores madeira region the isopycnal eofs consistently help to better separate physical processes in the water column and to better project the dynamic height anomaly downward in comparison with the classical eof analysis in depth coordinates in the present study we extended their work geographically and the connection with state estimation and data assimilation was more formalized from our results smooth spatial variations of isopycnal eofs were found in the north atlantic and confirmed the large scale relevance of isopycnal eofs in the ocean north of 20 n the dominant isopycnal eof was associated with a quasihomogeneous vertical displacement of the isopycnal surfaces 1 with a quasiconservation of compensated potential temperature on those surfaces 0 that mode was strongly modified by thermocline intensified baroclinic effects along the path of the gulf stream up to 40 w some physical interpretations of the vertical eofs were also suggested in several areas as far as the isopycnal formulation was concerned e g some thermal fronts the mediterranean outflow water lenses or meddies this work confirmed the validity of the ideas of gdm97 over the entire north atlantic ocean the potential use of the isopycnal eofs in state estimation problems was discussed they are possible candidates to reduce the order of state estimation problems provided that they have good observability properties these observability properties were studied in the case of satellite altimetry that is we examined how the dominant isopycnal eofs were connected to sea level anomalies in most bins it was found that the order of an estimation problem could be truncated to one primarily north of 20 n or two modes the dominant eof was found to account for most of the surface dynamic height variability with a rather normally distributed error of only a few centimeters dynamically more robust results were also presented performing a univariate isopycnal analysis in leaving coherent thermal effects out as a result the dominant character of mode 1 was further enhanced everywhere these univariate isopycnal eofs can be used for purely dynamical state estimation more generally we believe that our results can be used to set up reduced order data assimilation schemes in the north atlantic or in other regions with good historical data

subsurface geostrophic velocities inference from altimeter data application to the sicily channel mediterranean sea 1 coupled pattern reconstruction cpr and multivariate empirical orthogonal function reconstruction meof r techniques have been applied to infer the vertical structure of the sea from surface integrated measurements in the sicily channel mediterranean sea the data used to train the methods are the ctd casts present in the medar medatlas climatology the independent test and validation data set consisted of topex poseidon sea level measurements and ctd profiles collected during the synoptic mesoscale plankton experiment symplex 1996 and 1998 cruises meof r was found to give the more accurate and promising results being able to discriminate and reconstruct the main processes that drive the variability inside the channel in particular the analysis of the covariance of steric heights temperature and salinity profiles confirmed that mesoscale processes are predominant in fact the first meof steric height pattern closely resembles the first baroclinic normal mode estimated from the mean stratification profile a critical discussion on the signals contributing to the sea level variability has been done before applying the meof r to altimetric data the steric contribution of the upper 300 dbar has then been estimated from topex poseidon measurements the effects of the adjustment of altimeter data with a synthetic mean dynamic topography have also been quantified

mean circulation and eddy kinetic energy in the eastern north atlantic sixty two satellite tracked buoys have been released in the northern part of the western and central north atlantic during 1981 and 1982 by the institut fur meereskunde universitat kiel federal republic of germany additionally six buoys deployed in spring 1980 by the bedford institute have been taken over mean values and variances about the mean have been calculated for 3 by 3 areas the mean velocities confirm the large scale gyre circulation as deduced from hydrographie data and numerical models the center of the gyre is located at about 33 n eddy kinetic energy decreases east and southward and is concentrated along the north atlantic current it appears that the north atlantic current polar front is the main source of eddy kinetic energy for the northern north atlantic whereas the mid atlantic ridge is no source of eddy energy eddy kinetic energy along the north atlantic current decreases from about 1000 cm2 s 2 near newfoundland to 300 cm2 s 2 west of scotland a homogeneous pool of low eddy kinetic energy less than 100 cm2 s 2 appears in the central north atlantic east of the mid atlantic ridge the level increases towards the european and african shelf where the mean southward flow also becomes stronger the results concern the upper ocean

anomalous propagation of long period ground motions recorded in tokyo during the 23 october 2004 mw 6 6 niigata ken chuetsu japan earthquake unusually large 5 cm and prolonged shaking associated with long period ground motions at periods of about 7 sec were observed in central tokyo during the mw 6 6 niigata ken chuetsu earthquake of 23 october 2004 the long period ground motions caused significant resonance in high rise buildings of about 70 floors in height thus it is an urgent matter to understand the development and amplification properties of long period ground motions in tokyo associated with large earthquakes in this study we use numerous waveform records from 585 stations in a nationwide accelerometer network k net kik net and 495 intensity meters in the area around tokyo the data reveal that the long period ground motion is characterized in most part by a surface rayleigh wave generated at the northern edge of kanto basin and the surface wave is developed as propagating through a thick cover of sediments 3000 4000 m that overlies rigid bedrock to complement the observational data we conducted a large scale computer simu lation of seismic wave propagation by employing the earth simulator supercomputer with a detailed source slip model and a high resolution 3d sedimentary structural model of central japan the results of the computer simulation demonstrate that the anomalously prolonged ground shaking of the long period signal recorded in the center of tokyo occurred because of the stagnation of seismic energy resulting from the multipathing and focusing of rayleigh waves toward the bottom of the kanto basin from surrounding mountain regions with interaction to the 3d basin structure

physical oceanography of the azores front abstract there is a considerable body of evidence for the existence of a permanent front sw of the azores surveys in the summer of 1981 show the structure to penetrate to a depth of 2 000 m and suggest that its meandering is related to the alignment of the atlantis gt meteor seamount chain the total volume transport above 2 000 m is between 10 and 12 106 m3 s 1 towards the east the main water mass contrasts across the front are confined to the upper 400 m of the water column only a small change in mediterranean water strength was seen across the front satellite tracked drogued buoys seeded into the front showed surface speeds of up to 40 cm sec 1 and revealed the formation of a closed cold core eddy some 100 115 km in diameter which propagated westward at 2 2 km day 1 over a 200 day period sea surface temperature contrasts across the frontal boundary are small and together with frequent cloud cover prevent the detection of the front and associated eddies by satellite infra red imagery the front apparently marks the eastward extension of the southern branch of the gulf stream recirculation and the northern limit of the 18 c water mass eddies formed at the front could propagate westward into the interior of the sargasso sea but have not yet been observed to cross the mid atlantic ridge

continuous assimilation in an open domain of the northeast atlantic 1 methodology and application to athena 88 a scheme to perform suboptimal intermittent assimilation in an open ocean quasi geostrophic model is presented and applied to the assimilation of altimeter data in a domain of the northeast atlantic west of ireland both simulated and real altimeter data are used three dimensional 3 d synoptic observation fields and error variance fields are derived from synoptic dynamic topography anomaly estimates using an empirical orthogonal mode eof vertical extension technique the 3 d estimates are suboptimally combined with the time dependent part of the forecast fields suboptimality here means that the combination does not directly use spatial correlations of errors however the 3 d synoptic observation estimates used in the combination do contain error spatial statistical information the resulting fields added to a mean model climatology are used as initial conditions for the model which is integrated until a new dynamic topography anomaly estimate is available locally the combination is optimal in the sense that it is based on the observational error variances which reflect the measurement noise and the space time distribution of data and that it minimizes the error variance of the results a similar combination scheme using past as well as future observations is used to update the boundaries during model integration simulated surface topography anomaly maps typical of the northeast atlantic are assimilated every 20 days for a 300 day period with different noise characteristics typical of altimetric sampling errors in a three level version of the model the assimilation fields especially the vorticity at deeper levels converge toward the reference fields the convergence is obtained after o 100 days regardless of the observational noise levels tested up to 2 db using a relevant observational error model seems to matter in particular the error level should not be underestimated in the case of an 80 day gap in the data inflow the model predictability limits the reliability of the forecast beyond 20 days the scheme makes the model converge back as soon as new observations are entered with limited convergence loss due to the gap the case of an unknown model climatology is also studied finally the scheme is applied to geosat altimeter data added to the robinson bauer and schroeder annual climatology in the athena 88 cruise area which is contained in the model box the assimilation results compare favorably with hydrographical data from the cruise and help the synoptic interpretation of the observed phenomena

the sensitivity of a coupled climate model to its ocean component abstract the control climates of two coupled climate models are intercompared the first is the third climate configuration of the met office unified model hadcm3 while the second the coupled hadley isopycnic model experiment chime is identical to the first except for the replacement of its ocean component by the hybrid coordinate ocean model hycom both models possess realistic and similar ocean heat transports and overturning circulation however substantial differences in the vertical structure of the two ocean components are observed some of which are directly attributed to their different vertical coordinate systems in particular the sea surface temperature sst in chime is biased warm almost everywhere particularly in the north atlantic subpolar gyre in contrast to hadcm3 which is biased cold except in the southern ocean whereas the hadcm3 ocean warms from just below the surface down to 1000 m depth a similar warming in chime is more pronounced but shallower and confined to the up

high attenuation rate for shallow small earthquakes in japan we compared the attenuation characteristics of peak ground accelerations pgas and velocities pgvs of strong motion from shallow small earthquakes that occurred in japan with those predicted by the equations of si and midorikawa j struct constr eng 523 63 70 1999 the observed pgas and pgvs at stations far from the seismic source decayed more rapidly than the predicted ones the same tendencies have been reported for deep moderate and large earthquakes but not for shallow moderate and large earthquakes this indicates that the peak values of ground motion from shallow small earthquakes attenuate more steeply than those from shallow moderate or large earthquakes to investigate the reason for this difference we numerically simulated strong ground motion for point sources of mw 4 and 6 earthquakes using a 2d finite difference method the analyses of the synthetic waveforms suggested that the above differences are caused by surface waves which are predominant at stations far from the seismic source for shallow moderate earthquakes but not for shallow small earthquakes thus although loss due to reflection at the boundaries of the discontinuous earth structure occurs in all shallow earthquakes the apparent attenuation rate for a moderate or large earthquake is essentially the same as that of body waves propagating in a homogeneous medium due to the dominance of surface waves

inferring circulation and lateral eddy fluxes in the arctic ocean s deep canada basin using an inverse method abstractthe deep waters in the canada basin display a complex temperature and salinity structure the evolution of which is poorly understood the fundamental physical processes driving changes in

space and time scales of mesoscale motions in the eastern north atlantic abstract the tourbillon experiment carried out in 1979 80 over the porcupine abyssal plain in the eastern north atlantic is the fist experiment providing data that allow an efficient description of the spatial structure of the mesoscale low frequency flows in an eastern midlatitude basin the eddies appear to scale in the horizontal as the internal rossby radius of deformation and although of an energy level comparable to those observed during mode they are intrinsically more nonlinear because of reduced horizontal scale in much the same way the time scales are less than what is observed in western basin experiments mode polymode iii a b suggesting some kind of turbulent dispersion relation with frequency proportional to wavenumber in the eddy energy containing range we observe in tourbillon an equipartition of eddy kinetic and potential energy the frequency kinetic energy spectra have slopes of order 2 in a log log form hence are more white than in western basins 2 5 3 the distribution of

salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

north atlantic potential vorticity and its relation to the general circulation abstract maps and sections of the large scale north atlantic potential vorticity q are presented here q is fd dz where f is the coriolis frequency the potential density and z the vertical coordinate they bear on the general circulation and on geostrophic waves instability and turbulence in many ways both eulerian and lagrangian mean circulations proceed along isostrophes q constant in a zero dissipation region in a resting fluid q varies simply as the sine of the latitude but we show here that the wind driven circulation reshapes the q field creating bowls and plateaus which allow the flow to cross latitude circles the implied nature of the western boundary current is very different than in classical frictional theory the maps show a region of uniform potential vorticity in the wind gyre 26 5 27 0 which fills the ocean between 15 37 n and 20 80 w such regions were prominent features of a circulation theory of rhines and young 1982a b at deeper levels and close to surface out

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

modelling late cenozoic isostatic elevation changes in the barents sea and their implications for oceanic and climatic regimes preliminary results late cenozoic isostatic changes in the elevation of the barents sea are simulated using a numerical model isopach maps of the deposits off present day bear island and storfjorden troughs made earlier are used to calculate the thickness of sediment cover removed from the respective drainages basins at various time intervals during the last 2 3 ma results indicate that barents sea was subaerially exposed at 2 3 ma and major parts of it became submarine after 1 ma barents sea today receives around 40 of the warm and saline north atlantic waters flowing into the scotland greenland ridge and about half of the atlantic water entering the arctic ocean it thus has an important role to play in the present day ocean circulation pattern in the polar north atlantic region and water mass transformations that take place in the greenland iceland norwegian sea and the arctic ocean the effects of an uplifted barents sea on the oceanic regime and the arctic sea ice cover under the present day forcings fields are studied using the miami isopycnic coordinate ocean model preliminary results indicate that a subaerial barents sea causes an increased input of warm atlantic waters into the arctic ocean through the fram strait which results in warming of the atlantic water masses in the arctic ocean followed by a reduction in the sea ice cover the obtained findings can be used to explain the apparent discrepancy in the late cenozoic record of the sub arctic and arctic regions whereby fennoscandia iceland and greenland are envisaged to have been covered by major ice sheets during late pliocene whereas high arctic areas such as svalbard and ne greenland were apparently free of any major ice r 2002 elsevier science ltd all rights reserved

seasonal dynamics in the azores gibraltar strait region a climatologically based study abstract annual and seasonal mean circulations in the azores gibraltar strait region north eastern atlantic are described based on climatological data an inverse box model is applied to obtain absolute water mass transports consistent with the conservation of volume salt and heat and the equations of the thermal wind the large scale gyre circulation azores current azores counter current canary current and portugal current is well represented in climatological data the azores current annual mean transport was estimated to be 6 5 0 8 sv 1 sv 106 m3 s eastward exhibiting a seasonal signal with minimum transport in the spring 5 3 0 8 sv and maximum transport in autumn 7 3 0 8 sv the azores current transport is twice that of the azores counter current in spring and autumn and is four times higher in summer and winter the southward portugal and canary currents show similar seasonal cycles with maximum transports in spring 3 5 0 6 and 6 6 0 4 sv respectively the overturning circulation within the area has an annual mean magnitude of 2 2 0 1 sv and two seasonal extremes the highest in summer 2 6 0 1 sv and the lowest in winter 1 7 0 1 sv of the annual mean about two thirds 1 4 sv of the overturning circulation results from water mass transformation west of the strait of gibraltar the downwelling and recirculation of upper central water 0 6 sv in the intermediate layer the entrainment of central water 0 6 sv into the mediterranean outflow and the contribution of antarctic intermediate water 0 2 sv to the mediterranean outflow the remaining 0 8 sv relates to the overturning in the mediterranean sea through the two layer exchange at the gibraltar strait accordingly the density level dividing the upper inflowing and lower outflowing limbs of the overturning circulation was found to be 1 31 65 kg m 3 1 potential density referred to 1000 db which is above the isopycnal that typically separates central and mediterranean water 1 31 8 kg m 3 in terms of water masses we describe quantitatively the water mass composition of the main currents focusing on the spread of mediterranean water we found that when the northward mediterranean water branch weakens in spring and autumn the westward mediterranean water vein strengthens and vice versa the maximum net transports of mediterranean water across the northern and western sections of the box were estimated at 1 9 0 6 sv summer and 0 8 0 2 sv spring respectively within the error bar 0 2 sv we found no significant net volume transport of mediterranean water across the southern section

interaction of mediterranean water eddies with sedlo and seine seamounts subtropical northeast atlantic abstract during years 2003 and 2004 both sedlo and seine seamounts lying in the area of influence of the mediterranean water outflow in the northeast atlantic were investigated in the framework of the eu funded project oasis analysis of ctd data shows that the levels of major variability in temperature and salinity were confined to the upper ocean layer which is subject to seasonal air sea heat exchange and to the mediterranean water mw level in this paper we concentrate on the latter case and show that the variability observed resulted from mediterranean water eddies meddies colliding with the seamounts combined data from ctd surveys current meter moorings ship mounted adcp and satellite altimetry suggest that during the period of observations there was only one meddy interacting with each of the seamounts on sedlo seamount intensive interaction resulted in the loss by the meddy of about 25 of the salt around the seamount whereas on seine the meddy or portion of it escaped the seamount after performing several rotations around the summit without any significant interaction

isopycnal eofs in the azores current region a statistical tool fordynamical analysis and data assimilation abstract an empirical orthogonal function analysis of the vertical space time variability of the thermohaline structure of the ocean in summer and fall 1993 in the azores current region has been performed the analysis used data from ctd casts of the semaphore intensive experiment referred to as a seasonally varying climatology the relative merits of depth coordinate and isopycnal coordinate representations have been examined in regard to the quality of identification of physical processes and to the effectiveness of the extrapolation from a sea level anomaly in an attempt to improve the treatment of the vertical dimension in oceanic assimilation problems the isopycnal eofs consistently proved more efficient in capturing the vertical structure of both dominant processes in the area of investigation the variability linked to the mediterranean water and the coherent physical system made up of the azores front and the azores current the isopycnal analysis also proved more robust when meddies mediterra

an isopycnic model study of the north atlantic part ii interdecadal variability of the subtropical gyre abstract in a companion paper a spinup integration of the north atlantic ocean with the miami isopycnic coordinate model was presented the wintertime mixed layer in the central north atlantic was subject to relatively little change in salinity or depth but cooled markedly most probably because of heat loss associated with a partial surface relaxation to climatological sea surface temperatures in a region in which the gulf stream was too far to the north this mixed layer cooling caused the isopycnic layers in the model ventilated subtropical gyre to rise and surprisingly to warm while the experiment was not an attempt to simulate changes in the real atlantic ocean it nevertheless appears from observations that in recent decades the mixed layer in nature has undergone a change similar to that exhibited by the model mixed layer since it is expected that changes in the ventilated subtropical gyre will be governed largely by changes in the mixed layer in the central north atlantic from where the ve

the eddy field associated with the azores front east of the mid atlantic ridge as observed by the geosat altimeter two years of geosat data november 1986 to december 1988 for the azores madeira region of the north atlantic are analyzed the objective is to study the eddy field associated with the terminal retroflection of the azores current the rms variability of sea level anomaly is 4 to 8 cm with maximum values between 33 degrees and 35 degrees n being associated with the azores current and located on its northern flank higher energy is observed during winter and during the year november 1987 to november 1988 analysis of velocities at crossovers reveals anisotropy with larger meridional velocity variances this is the signature of meanders and or eddies elongated in the north south direction the eddy mean flow interaction is studied through the analysis of the reynolds tensor meridional and zonal accelerations of the mean by the eddy field are consistent with retroflection of the azores current to the south and to the west spectral analyses show that the dominant signals have periods and wavelengths larger than 100 days and 300 km respectively propagation velocities are westward but slow 1 5 km d synoptic mapping of the eddy field shows the wavelike structure of the variability on the northern flank of the current it is likely that the wave generation is not directly due to an instability of the azores current but is attributable to a cause more to the east once formed the wave interacts with the current in winter and forms meanders seasonal variations of the surface circulation are finally analyzed from the 2 year long time series of synoptic maps there is a clear seasonality in the surface circulation which generally appears to be consistent with historical in situ measurements in winter there is a strong meandering and a southward branching while in summer the mean front is narrower and penetrates further to the east

3h and 3he in the beta triangle observations of gyre ventilation and oxygen utilization rates abstract isopycnal maps of 3h 3he age with about 100 km resolution have been obtained for a 1000 km scale area in the eastern subtropical north atlantic the midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m 2 s 1 analysis of the distributions within the context of advection diffusion equations reveals that the larger scale 3h 3he age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of ekman downwelling thus isopycnal velocities can be estimated from 1 but record only the velocity component normal to the outcrop the velocities thus obtained agree well with geostrophic estimates although the comparison is flawed by the fundamental mismatch in timescales between the two techniques backward extrapolation of the maps to zero age outcrop positions indicate that the contribution of unventilated older recirculated water to the gyre flow above 600 m depth is small in this area no more than 20

large and mesoscale variability of the water masses and the deep chlorophyll maximum in the azores front 1 coupling between the physical and biological fields in the upper ocean around the azores front af was studied in summer 1998 during an oceanographic cruise to the southern azores islands 30 n 38 n and 20 w 33 w on the basis of conductivity temperature depth ctd acoustic doppler current profiler adcp and fluorescence data several large scale mesoscale and submesoscale physical features are described the adcp data and geostrophic field revealed an eastward transport associated with the azores current ac and weak countercurrents to the south and north of the ac the water mass field shows a strong latitudinal variation in the af with an eastward modification from the surface to 300 dbar the saline distribution strongly defined the af as a very sharp vertical front the horizontal thermohaline gradient increased to the east and reached a maximum at approximately 150 dbar the 16 2 c isotherm and 36 2 isohaline at 150 dbar precisely located the af along the whole area in agreement with the dynamic field to the west of 26 w the azores current was 140 km wide while the af was only 15 25 km wide to the east of 26 w the ac underwent a strong meandering the water mass characteristics in the southern part of the af show an eastward evolution between 18 c seawater and madeira mode water data from a ctd fluorometer mounted on an undulator system allowed the small scale structure of af to be mapped describing several eddies and meanders the deep chlorophyll maximum dcm showed highest values 0 9 to 1 2 mg chl m 3 toward the west 32 w on the southern side of the af chlorophyll concentrations at the dcm 0 4 0 6 mg chl m 3 on the northern side of the study area were moderate and fairly constant whereas low values 0 3 mg chl m 3 were measured at the very deep dcm 120 140 m in the southeast associated with high saline waters examination of a cold eddy showed similar chlorophyll concentrations to those found on the northern side of the af the thermohaline structure of the eddy was similar to the af with northern water enclosed within saline and warm southern water

vivaldi 1991 a study of the formation circulation and ventilation of eastern north atlantic central water abstract a synoptic hydrographic data set comprising 32 full depth ctd casts and 2500 ctd seasoar profiles to 500 m is used to describe the s properties and circulation of central water east of the mid atlantic ridge and between 39 n and 54 n eastward transport of 20 10 6 m 3 s 1 in the north atlantic current turns entirely northwards to the west of 54 n 20 w this transport consists in the upper layers of western north atlantic water freshened at temperatures below 10 c by mixing with subarctic intermediate water northern and southern branches of the north atlantic current are well defined and both turn northwards west of 20 w a further 10 10 6 m 3 s 1 of eastern north atlantic water forms and recirculates anticyclonically to the west of spain south of the north atlantic current and north of 40 n eastern north atlantic water is most weakly stratified east of 20 w and there is clear correlation between weakly stratified pycnostads and positive salinity anomalies relative to western north atlantic water thus eastern north atlantic water is a winter mode water in which strong winter cooling has increased the density and hence also the salinity anomaly at a given temperature near the southern entrance to the rockall trough there is evidence that salinities are also increased by mediterranean water influence circulation south of the north atlantic current is complex there is no evidence for direct ventilation southwards across 40 n where water properties s potential vorticity and cfc 113 and historical data all indicate westward ventilation east of 24 w with weak southward ventilation occurring further west in the vicinity of the azores the circulation pattern suggested is remarkably similar to that proposed by helland hansen and nansen in 1926 the eastern north atlantic geophysiske publicajoner 4 1 76 with anticyclonic circulation of colder eastern north atlantic water north of 40 n meeting warmer water from south of 40 n circulating cyclonically north of the azores current the distribution of pycnostads and s properties between 20 w and 35 w north of the azores indicates alternate bands of western and eastern north atlantic water moving eastward and westward respectively including evidence for westward motion immediately south of the southern branch of the north atlantic current possibly by westward propagation of anticyclonic eddies containing deep pycnostads

in situ and laboratory stress strain relationships the question of whether or not the end of primary consolidation curve from laboratory oedometer tests is representative of in situ behaviour has been the subject of much discussion the behaviour of the berthierville test embankment is described and in situ effective stress strain curves are defined for two well defined clay sublayers these compressibility curves are then compared with laboratory end of primary consolidation curves similar comparisons are also made for sublayers under three other well instrumented embankments all the results show that the laboratory end of primary consolidation curve strongly underestimates in situ settlements and thus that theory a is not valid on s est souvent demande si la courbe de la fin de consolidation primaire obtenue a partir d essais oedometriques en laboratoire represente effec tivement le comportement en place l article decrit le comportement du remblai experimental de berthierville et definit les courbes contrainte deformation effectives en place pour de

the influence of air sea interaction on the ventilated thermocline abstract air sea interaction influences the ventilated thermocline by forcing the mixed layer to deepen and cool poleward when there is flow from the mixed layer into the interior the mixed layer depth and density fields help to set the potential vorticity of the subducted fluid the importance of this process is assessed by incorporating a depth varying mixed layer in a ventilation model which is forced by ekman pumping and implied surface heating the formulation of the ventilation problem is simplified by only allowing density surfaces to outcrop along latitude circles and by assuming that there is no zonal inflow along the eastern boundary the surface heating enables a cross isopycnal flow within the mixed layer the volume of ventilated fluid within the subtropical gyre is increased by including the depth varying mixed layer and this fluid partly originates from the western boundary as well as from the ekman layer the depth varying mixed layer increases the depth at which isopycnals are subduc

on the reasons for the formation and variability of the azores current abstract recent studies have shown that the formation of the well defined zonally oriented azores current may be the result of water mass transformation associated with the mediterranean outflow in the gulf of cadiz as the denser mediterranean water descends down the continental slope it entrains overlying north atlantic central water it is believed that the azores current then forms as part of the horizontal recirculating gyre generated through the plume mechanism in this study the authors further explore this hypothesis by performing a series of numerical experiments these experiments are based on a high resolution general circulation model that includes the mediterranean sea and that realistically simulates the water mass exchange through the strait of gibraltar and the transport and variability of the azores current the authors show that the divergence of the relative vorticity flux and the planetary vorticity flux associated with planetary waves are the main factors determining the variab

the madeira mode water quasi homogeneous layers in vertical profiles of temperature and salinity in the eastern north atlantic near madeira indicate the existence of a subtropical mode water in the eastern basin temperature sections show a maximum horizontal extent of at least 500 km the frequency distribution analysis of homogeneous layers in a historical xbt dataset shows a mode water formation region near and to the north of madeira this mode water is found at increasing depths and displaced to the west and southwest during the course of the year after its formation by wintertime convection it disappears almost completely due to mixing before the next winter volume estimates suggest that this madeira mode water in the eastern atlantic accounts for 15 20 of the total central water formation in the corresponding density range as obtained from tracer studies in the north atlantic gyre

reliability of settlement prediction case history a geotextile reinforced embankment was built over soft ground staged construction and vertical prefabricated drains were used to obtain strength gain via consolidation of the foundation material settlement and pore pressure were measured as a part of construction control settlement differential settlement and rate of consolidation were predicted by simple models uncertainties in the predictions were evaluated to assess the reliability of the predictions the estimated errors were compared with the measured values and major contributors to prediction errors were identified

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

on the role of the azores current in the ventilation of the north atlantic ocean three high resolution ocean circulation models of the north atlantic differing chiefly in their description of the vertical coordinate are used to examine the ventilation of the north atlantic subtropical gyre all the models produce mode waters of realistic densities in the sargasso sea and the european basin but have azores currents of differing strengths which are categorised as strong of realistic transport intermediate and weak these differences have a critical impact upon the ventilation of the gyre most importantly the strong azores current forms an effective barrier to the southward ventilation of eastern north atlantic water from the northern european basin initially driving it southwestwards into the central gyre before turning it back eastwards again in a general cyclonic circulation north of the current the intermediate and weak azores currents instead allow the southward ventilation of this water mass near the european and african coasts the situation in nature appears to be intermediate between these two cases with the azores current acting as a partial block the study also raises the possibility of the azores current forming an advective connection between the sargasso sea mode waters in the western basin and modes of similar densities found in the eastern basin on the southern side of the current although there are high levels of variability in the extent of these lighter modes in the eastern basin in nature this postulate is supported by a number of observational studies in addition the present study also provides some support for the local production of madeira mode water in the eastern basin associated with retroflecting current branches on the southern side of the azores current overall the azores current is therefore likely to have a critical impact upon the ventilation of the subtropical gyre over a large area rather than just locally affecting the potential vorticity and density structure of the upper ocean between subtropical latitudes and the northern european basin the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models

the upper oceanic response to overflows a mechanism for the azores current abstract the oceanic response to overflows is explored using a two layer isopycnal model overflows enter the open ocean as dense gravity currents that flow along and down the continental slope while descending the slope overflows typically double their volume transport by entraining upper oceanic water the upper oceanic layer must balance this loss of mass and the resulting convergent flow produces significant vortex stretching overflows thus represent an intense and localized mass and vorticity forcing for the upper ocean in this study simulations show that the upper ocean responds to the overflow induced forcing by establishing topographic plumes that are aligned more or less along isobaths and that have a transport that is typically a few times larger than that of the overflows for the topographic plume driven by the mediterranean overflow the occurrence of eddies near cape st vincent portugal allows the topographic plume to flow across isobaths the modeled topographic plume circu

meandering of the subtropical front south east of the azores a subtropical front was observed in the area south and southeast of the azores during cruises of fs meteor and fs poseidon in early 1982 the front has a basically west east extension with considerable meandering observed meso scale eddies are found on both sides the overall flow pattern corresponds to earlier results on geopotential differences in the upper northeast atlantic but the baroclinic transport of the order of 107 m3 s 1 is found to be concentrated in a 60 km wide jet we suggest here that the current band is part of the gyre circulation resulting from a branching of the north atlantic current