a numerical study of the mediterranean sea circulation abstract a primitive equation ocean model that makes use of a curvilinear orthogonal grid and a sigma coordinate system was used to simulate the mediterranean sea the model was forced with monthly climatological values of wind stress heat and salinity flux with the help of the curvilinear horizontal grid the larger scales of the entire mediterranean sea are modeled and the topography around the narrow and shallow straits of gibraltar is also reasonably well represented the resulting model inflow and outflow seems to mimic the real mediterranean often in considerable detail levantine intermediate water is formed in the levantine basin and exits through the strait of sicily and the strait of gibraltar deep water formation processes are clearly represented by the model the model results indicate that in the western mediterranean the wind stress is very important in establishing the summer northward shift of the atlantic inflow lateral boundary runoff surface salinity and heat fluxes are necessar

circulation in the algerian basin during june 1986 hydrographic measurements made in the algerian basin during june 1986 as part of the western mediterranean circulation experiment together with satellite infrared images are used to describe the flow of major water types in the region modified atlantic water flows eastward along the algerian coast as the algerian current to 4 e where it breaks into a series of eddies and spreads far offshore it then flows partly northward into the ligurian provencal basin and partly eastward through the strait of sardinia the subsurface levantine intermediate water turns sharply northward after flowing westward through the strait of sardinia and forms a narrow vein along the sardinian coast winter intermediate water flows southward around the balearic islands its eastern branch recirculates back to the north whereas part of the western branch contributes to the algerian current by flowing eastward beneath the modified atlantic water two large long lived eddies are confirmed in the area the easternmost of these and other features with unresolved horizontal structure between sardinia and menorca extend from the surface through the pycnocline

satellite investigations of a cyclonic gyre in the central tyrrhenian sea western mediterranean sea thermal satellite images related to surface temperature of the tyrrhenian sea western mediterranean sea show an elliptic cold patch suggesting the presence of a wind induced cyclonic gyre with well defined meanders of comparatively constant wavelength in order to gain insight into the dynamics of the gyre these data are compared with results of two classical linear models that of crepon and richez on wind induced upwellings and that of gill on currents due to a buoyancy discontinuity in rotating channels the large scale gyre meanders are analyzed using theoretical information on baroclinic instability models it results that the gyre is due to winds blowing eastward from the mouths of bonifacio and to the buoyancy due to a thermal front along 41 e with this general procedure from satellite images an estimate of the deep hydrologic quantities is obtained this technique is of interest also for other similar systems

monitoring the width of hydraulic fractures with acoustic waves during scaled hydraulic fracturing experiments in our laboratory the fracture growth process is monitored in a time lapse experiment with ultrasonic waves we observe dispersion of compressional waves that have propagated across the hydraulic fracture this dispersion appears to be related to the width of the hydraulic fracture this means that we can apply the dispersion measurements to monitor the width of the hydraulic fracture in an indirect manner for a direct determination of the width the resolution of the signal is required to distinguish the reflections that are related with two distinct fluid solid interfaces delimiting the hydraulic fracture from its solid embedding to make this distinction the solid fluid interfaces must be separated at least one eighth of a wavelength and represent sufficient impedance contrast the applicability of the indirect dispersion measurement method however extends to a fracture width that is in the order of 1 of the incident wavelength the time lapse ultraso

adjustment of the ocean under buoyancy forces part i the role of kelvin waves abstract the early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model the initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the north atlantic the main adjustment in the time examined here the first few months is by coastal kelvin waves the first baroclinic mode dominates so the behavior proves to be very similar to that in a shallow water model also the dynamics is locally close to that for an f plane but account must be taken of the way f varies with latitude the essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin especially at the level where the first mode has the maximum effect this study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations namely in a model with too coarse a horizontal resolution and an artif

temporal variability of currents in the eastern ligurian sea surface and intermediate waters enter the ligurian sea from the tyrrhenian and algero provencal basins via northward flows along the east and west coasts of corsica the temporal variability of these flows is examined by means of a 1 year time series of current data october 1986 to september 1987 and hydrographic data from december march and may cruises the flow through the corsica channel increases suddenly during december and then gradually decreases after march reaching its lowest values in late summer the seasonal variability in the channel seems associated mainly with the steric sea level differences between the tyrrhenian sea and ligurian sea the west corsican current appears more stable throughout the one year of data the two flows converge just north of corsica into the ligurian current because of the water masses differences between the two currents in the area of convergence considerable smaller scaled structure is observed during winter the less dense tyrrhenian water provides available potential energy to the ligurian shelf area and thereby helps sustain the large scale cyclonic circulation in the ligurian basin during the other seasons the spatial structures over the ligurian shelf are less organized and the spatial scale of the flow is a few tens of kilometers

seasonal variability of gyre scale circulation in the northern tyrrhenian sea the principal hydrographic structures and dynamic conditions of the northern tyrrhenian sea are studied through the comparison of in situ field data and a modeling simulation the field observations are long term eulerian and lagrangian current measurements associated with two hydrographic campaigns conducted in late summer and in winter the data indicate that a well marked cyclonic gyre affecting the water column from the surface to the bottom is present in the northern tyrrhenian sea in both periods the gyre undergoes significant seasonal changes in both shape and size following the indications of past studies a numerical quasi geostrophic model driven by the wind jet blowing from the strait of bonifacio was able to reproduce the principal gyre characteristics however the wind jet alone does not explain all the gyre features especially its seasonal variation a study of the model results and field data indicates that these variations are linked to the seasonal variability of the northward surface current flowing along the eastern boundary of the tyrrhenian sea

wet fault or dry fault a laboratory approach to remotely monitor the hydro mechanical state of a discontinuity using controlled source seismics stress variation and fluid migration occur in deformation zones which are expected to affect seismic waves reflected off or propagating across such structures we developed a basic experimental approach to monitor the mechanical coupling with respect to seismic coupling across a single discontinuity between a granite sample in contact with a steel platen piezoceramics located on the platen were used to both generate and record the p and s wave fields reflected off the discontinuity at normal incidence this way normal b n and tangential b t compliances were calculated using schoenberg s linear slip theory schoenberg j acoust soc am 68 1516 1521 1980 when the roughness the effective pressure p eff up to 200 mpa and the nature of the filling gas or water vary we observe that increasing the effective pressure decreases b n and b t which is interpreted as the effect of the closure of the voids at the interface permitting more seismic energy to be transmitted across the interface values of b n are significantly higher than those of b t at low p eff 60 80 mpa in dry conditions and significantly drop under water saturated conditions the water filling the voids therefore helps to transmit the seismic energy of compressional waves across the interface these results show that the assumption b n b t commonly found in some theoretical approaches does not always stand the ratio b n b t actually reflects the type of saturating fluids and the effective pressure in agreement with other experimental studies however we illustrate that only the relative variations of this ratio seem to be relevant not its absolute value as suggested in previous studies consequently the use of b n against b t plots may allow effective pressure variation and the nature of the pore fluid to be inferred in this respect this experimental approach at sample scale helps to pave the way for remotely monitoring in the field the hydro mechanical state of deformation zones such as seismogenic faults fractured reservoirs or lava conduits

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

a strait outflow circulation process study the case of the alboran sea abstract a three dimensional primitive equation model is used to investigate the physical processes governing the exchanges of water between the mediterranean sea and the atlantic ocean at the level of the strait of gibraltar the circulation is driven by connecting two reservoirs filled with waters of different densities the motion starts from rest and is initiated by removing the dam separating them at the initial time the results are in agreement with observations in the strait the light atlantic water flows into the mediterranean basin in the surface layer while the denser mediterranean water moves toward the ocean as a deep current after a spinup time interval of three days the flow transport in the strait reaches a quasi stationary value of 0 9 sv sv 106 m3 s 1 as it enters the alboran sea the flow of atlantic water experiences a transition regime where the surface current is transformed into a shallow buoyant jet after several days of integration this flow intrudes into the alboran bas

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

location and variation of the summertime upper troposphere temperature maximum over south asia the upper troposphere temperature maximum uttm over south asia is a pronounced feature in the northern hemisphere summer its formation mechanism is still unclear this study shows that the latitude location of the upper tropospheric warm center t coincides with the subtropical anticyclone and its longitude location is determined by the zonal distribution of vertical gradient of heating cooling q z q z which is different from the gill s model since both convective heating and radiation cooling decrease with height in the upper troposphere the heating cooling generates vertical northerly southerly shear leading to a warm cold center being developed between heating in the east west and cooling in the west east the location of the uttm coincides with the south asian high sah and is between a radiation cooling in the west and the asian monsoon convection heating source in the east the uttm is sensitive to this convective heating increased heating in the source region in a general circulation model causes intensification of both the sah and uttm and imposing periodic convective heating there results in oscillations in the sah uttm and vertical motion to the west with the same period diagnoses of reanalysis indicate that such an inherent subtropical t q z relation is significant at interannual timescale during the end of the twentieth century rainfall increase over south china is accompanied by an increasing northerly flow aloft and intensification in the sah and uttm to the west results demonstrate that the feedback of atmospheric circulation to rainfall anomalies is an important contributor to the regional climate anomaly pattern

the echo of a fault or fracture the seismic response of single faults joints or fractures of large planar extent is analyzed these are modeled as nonwelded interfaces in spite of the large range of scale all are assumed to behave according to linear slip theory such a model has been considered theoretically and experimentally before the aim of this paper is to give a physical interpretation to such a linear slip interface to provide simple analytical formulae for the scattering response even when the fracture is embedded in an anisotropic background medium and to relate the properties of this scattering response in the isotropic case to the physical features of the fracture the analysis shows that the reflectivity and transmissivity of the fracture depend on slowness along the fracture and on frequency the frequency dependence arises from the fact that even though the fracture is assumed to be an interface of zero thickness it still has at least two characteristic widths that provide the length scales necessary for scatt

the seasonal characteristics of the circulation in the north mediterranean basin and their relationship with the atmospheric climatic conditions the greater heat losses sustained by the ligurian provencal basin in winter appear to be the major factor leading to the periodic intrusions of the warmer tyrrhenian water from the east this is the main conclusion of a 3 year oceanographic investigation carried out in the eastern ligurian provencal basin in order to monitor the long term characteristics of its exchanges with the tyrrhenian sea the analysis of the whole data set collected in this period shows that two currents converge north of corsica after having flowed along the northwestern side of corsica the west corsican current and through the corsican channel the tyrrhenian current while the former does maintain nearly steady properties year round the latter crosses the strait only during the coldest season thus showing a clear seasonal trend that begins again every year the comparison with the meteorological parameters of the ligurian provencal basin indicates that the winter intrusions of the tyrrhenian current are tightly associated with the heat and water losses through the rise induced in the upper basin density in addition to reintegrating the heat lost to the atmosphere the tyrrhenian flow might also restore the volume of the basin upper layer involved in the deep water formation processes this enables us to infer that a unique circulation pattern mostly driven by atmospheric climatic conditions involves the northern part of the western mediterranean sea

elastic wave propagation in fractured media using the discontinuous galerkin method abstractwe have formulated and implemented a discontinuous galerkin method dgm for elastic wave propagation that allows for discontinuities in the displacement field to simulate fractures or faults the approach is based on the interior penalty formulation of dgm and the fractures are simulated using the linear slip model which is incorporated into the weak formulation by including an additional term that is similar to the penalty term but uses the fracture compliance instead of an arbitrary penalty parameter we have calibrated our results against an analytic solution of fracture induced anisotropy for a set of elongated horizontal fractures and we have evaluated numerical examples that simulate the reflection and transmission of waves at a fracture and at fracture interface waves this method can further be used with models containing intersecting fractures and multiple fracture sets in 2d or 3d domains

dominant control of the south asian monsoon by orographic insulation versus plateau heating heat emitted from the tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the south asian summer monsoon but new work suggests that in fact it is the neighbouring mountains that are the major influence william boos and zhiming kuang use an atmospheric model to show that flattening the tibetan plateau has little effect on the monsoon so long as the himalayas and surrounding mountain ranges remain the plateau does boost rainfall locally along its southern edge but it is the build up of hot moist air over india insulated from colder drier air by the himalayas that drives large scale monsoon circulation the elevation of the tibetan plateau is thought to cause its surface to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales here however an atmospheric model is used to show that flattening of the tibetan plateau has little effect on the monsoon provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved the tibetan plateau like any landmass emits energy into the atmosphere in the form of dry heat and water vapour but its mean surface elevation is more than 5 km above sea level this elevation is widely held to cause the plateau to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales1 2 3 4 5 observations of the present climate however do not clearly establish the tibetan plateau as the dominant thermal forcing in the region peak upper tropospheric temperatures during boreal summer are located over continental india south of the plateau here we show that although tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model the large scale south asian summer monsoon circulation is otherwise unaffected by removal of the plateau provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm moist air over continental india from the cold and dry extratropics these results call for both a reinterpretation of how south asian climate may have responded to orographic uplift and a re evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades

water masses and circulation in the western alboran sea and in the straits of gibraltar abstract some new elements of alboran sea circulation and water masses in close connection with flows through the straits of gibraltar are presented a general description is given of water masses found in 1981 during mediprod iv cruise from the gulf of cadiz to the gulf of lions followed by a brief review of the conditions found in the straits of gibraltar based on 1981 and 1961 observations the hydrological structure vertical and horizontal and surface and bottom currents typical of the western alboran sea are analysed we present some detailed observations about vertical currents in the deep waters near the straits and finally we analyse the characteristics of the mediterranean overflow through the straits our conclusions in part confirm those of bryden and stommel 1982 and in part shed light on the general pattern of the mediterranean currents entering the alboran sea from the east however considering the suggestion of stommel bryden and mangelsdorf 1973 that the deep currents in the interior of the western mediterranean basin are basically controlled by a bernoulli flow induced by the intense currents in the straits and the weak vertical stratification in the mediterranean we suggest that an interaction of a large scale circulation pattern and the topography of the alboran basin which is much shallower than the western mediterranean basin may play a major role in controlling hydraulically the deep mediterranean waters overflow a detailed water mass analysis of 1981 data and comparison with previous data revealed a time and space variability in the mediterranean waters the scales of this variability are unknown but the s variations make it difficult to make assumptions as to the depth of origin of the deep waters found in the alboran sea which flow into the atlantic ocean over the gibraltar sill

the atlantic inflow in the western alboran sea abstract an extensive dataset collected in the alboran sea during the 1982 donde va experiment is used to characterize the kinematics and dynamics of the inflow of atlantic water into the mediterranean sea the veering of the inflow toward the ene after leaving the strait of gibraltar and the existence of an anticyclonic gyre that fills much of the western alboran sea in the upper 200 m are confirmed in the mean inflow variability with periods of 2 to 10 days is described particularly striking is one interval of about 9 days during which the gyre was not present and the inflow adopted a southerly course after leaving the strait the observations are interpreted in terms of vorticity conservation and in the light of earlier theoretical and numerical results

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

mesoscale dynamics and vertical motion in the alboran sea abstract the inflow of atlantic water through the strait of gilbraltar usually exhibits a stationary wavelike pattern in the alboran sea we use an objective analysis technique for quantitative scale separation to investigate the interaction of scales in the region the large scale is clearly dominated by an anticyclonic gyre smaller scale analysis shows the existence of several mesoscale cyclonic eddies along the northern boundary of the western alboran sea anticyclonic gyre the relationship between the large scale mesoscale variability and the induced ageostrophic vertical motion is established using the q vector formulation we find that on the macroscale upward motion occurs upstream of the anticyclonic gyre upstream of a wave crest while downward motion takes place downstream upstream of a trough we also show that the vertical motion associated with the mesoscale eddies is an order of magnitude higher than the large scale vertical motion these patterns of large scale and mesoscale vertical m

summertime quadruplet heating pattern in the subtropics and the associated atmospheric circulation 1 a quadruple heating pattern is found over each subtropical continent and its adjacent oceans in summer based on data diagnosis the ocean region to the west is characterized by strong longwave radiative cooling lo the western and eastern portions of the continent are dominated by sensible heating se and condensation heating co respectively and the ocean region to the east is characterized by double dominant heating d with lo prevailing co these compose a losecod heating quadruplet its general feature is heating over the continent and cooling over the oceans a distinct circulation pattern accompanies this heating pattern in the upper troposphere anticyclonic circulation over the continent is accompanied by cyclonic circulations over the oceans on its western and eastern sides near the surface cyclonic circulation over the continent is accompanied by anticyclonic circulations over the oceans on both sides this circulation feature is interpreted as the atmospheric thermal adaptation to the quadruplet heating it is further demonstrated that the global summer subtropical heating and circulation may be viewed as mosaics of such a quadruplet heating and circulation patterns respectively

the role of the interface in exchange through the strait of gibraltar five cross strait hydrographic sections repeated several times during the gibraltar experiment in 1985 1986 are used to examine the structure of the interface layer between the inflowing atlantic waters and outflowing mediterranean waters in the strait of gibraltar the interface is 60 100 m thick with a strong vertical salinity gradient identified by fitting individual salinity profiles to a piecewise linear three layer model the interface is deeper thicker fresher and colder on the west end of the strait than in the narrows where there is a minimum in thickness and a maximum in salinity gradient farther east the interface thickens again and continues to get saltier warmer and shallower property variations in all three layers are also cast in terms of the three principal water types involved in the exchange the traditional knudsen model of exchange is extended to three layers assuming that the interface is a transport carrying third layer with uniform vertical shear as much as half of the inflowing or outflowing transport occurs in the interface layer transport converges in both the upper and lower layers implying over the length of the strait vertical exchange between layers that is comparable to about half the horizontal exchange the richness of structure and complexity of interaction between the interface and the upper and lower layers argues against the use of two layer models to characterize exchange through the strait of gibraltar

some features of the algerian current an analysis of infrared images leads to a new conception of the dynamics of the atlantic water in the algerian basin in situ current measurements are not yet available but coherent hypotheses are presented with the aim to initiate further experimental and theoretical works the algerian current flows steadily along the coast near 0 and becomes unstable near 1 2 e eddies of both signs are generated that are advected by the mean current but only the anticyclonic eddies increase in size diameter about 100 km as they extract more energy from the mean current they are advected more slowly and they may detach from the coast and drift for several weeks in the algerian basin they grow as deep as 1000m at least and they are able to pull lenses of intermediate water seaward from the sardinian continental slope coastal upwelling cells are generated near 1 2 e between the cyclonic and the anticyclonic eddies they are also advected by the mean current and are definitively not wind induced these turbulent structures create intense mixing between the atlantic and mediterranean waters which account for the large eastward gradient of salinity encountered in the surface layer therefore the algerian basin is characterized by a large mesoscale variability mainly due to the instability of the algerian current it appears to be a reservoir in which the water of atlantic origin is amassed and it forms a buffer zone which disconnects the flow of atlantic water coming in at gibraltar from the flow of hygly modified atlantic water exiting the basin through the strait of sardinia and into the ligurian sea

seismic reflection coefficients of faults at low frequencies a model study we use linear slip theory to evaluate seismic reflections at non welded interfaces such as faults or fractures sandwiched between general anisotropic media and show that at low frequencies the real parts of the reflection coefficients can be approximated by the responses of equivalent welded interfaces whereas the imaginary parts can be related directly to the interface compliances the imaginary parts of low frequency seismic reflection coefficients at fault zones can be used to estimate the interface compliances which can be related to fault properties upon using a fault model at normal incidence the expressions uncouple and the complex valued p wave reflection coefficient can be related linearly to the normal compliance as the normal compliance is highly sensitive to the infill of the interface it can be used for gas fluid identification in the fault plane alternatively the tangential compliance of a fault can be estimated from the complex valued s wave reflection coefficient the tangential compliance can provide information on the crack density in a fault zone coupling compliances can be identified and quantified by the observation of ps conversion at normal incidence with a comparable linear relationship

exchange through the strait of gibraltar to measure the exchange between the atlantic and mediterranean through the strait of gibraltar an array of current meter moorings was deployed for a year in the strait during 1985 1986 a novel aspect of these measurements is the inclusion of conductivity as well as temperature and pressure sensors on each current meter so that the salinity of the flows could be monitored continuously these salinity measurements determine the water mass characteristics of the flows crossing the sill they allow definition of the 37 psu isohaline as the interface between inflowing fresher atlantic water and outflowing saltier mediterranean water and they enable time series to be developed for the depth of this interface for the upper layer inflow and for the lower layer outflow from these measurements the time averaged outflow of mediterranean water is estimated to be 0 68sv 1sv 1 106m3s 1 and the outflow salinity transport defined to be the outflow times the salinity excess above a basic atlantic water salinity of 36 1 psu is estimated to be 1 50 103m3s 1 1sv 1psu 1 103m3s 1 equivalent to a net evaporation over the mediterranean basin of 52cm y 1 extrapolated measurements of the inflow from current meters generally deployed below 100m depth yield an estimate for the time averaged inflow of 0 93sv which is believed to be unrealistically high in view of the better measured outflow and net evaporation thus a more realistic estimate of the inflow is 0 72sv equal to the sum of the outflow and net evaporation as required by the mass budget for the mediterranean sea such estimates of the exchange are smaller by almost a factor of 2 than previous values for the exchange by lacombe and richez 1982 the exchange across the gibraltar sill is found to be due in nearly equal parts to the mean currents and to the tidal fluctuations the mean currents are smaller than had been expected reaching a peak value of only about 60 cm s 1 in the deep outflow over the sill the tidal exchange is due to a strong correlation over the tidal period between the depth of the interface and the strength of the inflowing currents for the m2 tide at the sill the amplitude of the interface depth is 51 m and the amplitude of the tidal currents is 1 2m s 1 furthermore the inflow and interface depth have similar phases as a consequence the upper layer is deep on the inflowing tide so that a large slug of atlantic water crosses the sill into the mediterranean on the outflowing tide the interface is shallow so that a large slug of mediterranean water crosses the sill into the atlantic similar processes occur for the s2 o1 and k1 tides though the amplitude are smaller in this manner tidal oscillations lead to a time averaged exchange of water masses across the gibraltar sill the inflow and outflow defined to be the instantaneous transports above and below the 37psu isohaline interface exhibit m2 tidal amplitudes of 2 3 sv and 1 3sv respectively thus the tides are large enough to reverse the mean upper layer inflow and lower layer outflow daily averaged inflow and outflow transports exhibit low frequency fluctuations with standard deviations of 0 37sv and 0 22sv respectively such low frequency fluctuations have been shown previously to be associated with barotropic flows through the strait of gibraltar compensating for sea level variations over the mediterranean due to atmospheric pressure fluctuations candela winant and bryden 1989 finally from these measurements there appear to be little fortnightly or annual period fluctuations in the exchange through the strait of gibraltar

a sensitivity study of the general circulation of the western mediterranean sea part ii the response to atmospheric forcing abstract this paper investigates the influence of sea surface thermohaline fluxes and wind stress on the circulation of the western mediterranean sea using a high resolution 3d primitive equation model an 18 year experiment was forced with the daily output of a fine grid mesh numerical weather prediction model the major characteristics of the circulation are well reproduced the basin surface circulation is cyclonic over all of the basin the two anticyclonic alboran gyres are present the instabilities of the algerian current generate large anticyclonic eddies that invade the whole algerian basin the liguro provencal catalan current is well marked deep water convection down to the bottom only occurs during the first 3 years then winter intermediate water is produced the north south gradient of the atmospheric thermohaline fluxes induces a northward surface transport of water from the algerian basin into the liguro provencal basin this pattern can be associated with the balearic front sensitivity

a sensitivity study of the general circulation of the western mediterranean sea part i the response to density forcing through the straits abstract in this paper the influence of the density gradients at the straits of gibraltar and sicily on the large scale circulation of the western mediterranean sea is investigated through a 3d numerical model the topography constraint is analyzed by comparing an experiment with a flat bottom with an experiment with a realistic bathymetry the results show the ability of the density gradients at the straits to force a cyclonic surface circulation in the whole basin particularly in the northern basin where the transport in the liguro provencal current is about 40 of the observed one these experiments point out the key role played by the strait of sicily and its topography there the eastward surface flow separates into two branches one enters the eastern mediterranean sea while the other flows along the italian coasts to feed the cyclonic circulation of the northern basin in the alboran sea which is a priori the region where the dynamics is the most strongly induced by the transport through the st

the large scale circulation and heat sources over the tibetan plateau and surrounding areas during the early summer of 1979 part i precipitation and kinematic analyses abstract the time evolution of the large scale precipitation low level 850 mb wind moisture and vertical motion fields over the tibetan plateau and surrounding areas during a 40 day period from late may to early july 1979 is studied based on the objectively analyzed fgge level ii b data set during this period the general circulation over east asia undergoes a distinct change characterizing the onset of the summer monsoon circulation the tibetan plateau exerts profound orographic and thermal influences upon the low level wind field the inflow towards the eastern part of the plateau with a marked diurnal change in its intensity is the most prominent feature of the low level wind field the areas of organized precipitation are well related to synoptic systems seen in the 850 mb flow the quasi stationary burma india trough the disturbances forming on the trough the mei yu baiu front and the transverse trough extending eastward from the plateau there is a sharp contrast between the western an

seasonal probability dispersion maps in the mediterranean sea obtained from the mediterranean forecasting system eulerian velocity fields 1 this paper presents a method to build seasonal probability dispersion maps from numerical lagrangian trajectories as well as a user friendly web site containing the results the main aim is to provide useful tools for the description of dispersion properties at the mediterranean sea surface trajectories were obtained integrating with an off line lagrangian algorithm the eulerian velocity fields from january 2000 to december 2004 provided by the ocean general circulation model modular ocean model used in the mediterranean forecasting system project the statistics of dispersion properties over the entire surface of the mediterranean sea is calculated from the time evolution of clusters of numerical particles released in nonoverlapping grid boxes some examples of seasonal probability dispersion maps taken from the web site are shown and discussed the results are checked against drifters of the mediterranean surface drifter database 1986 1999 by comparing the statistics of numerical and observed trajectories the validation shows both deficiencies and skills of the methodology proposed for estimating lagrangian dispersion variability it also gives a starting point for discussion about future developments and improvements

spatial structure of anticyclonic eddies in the algerian basin mediterranean sea analyzed using the okubo weiss parameter abstract three algerian eddies open sea big anticyclonic eddies in the algerian basin western mediterranean sea are studied in detail using altimetric and in situ data to characterize their spatial structure we make use of the okubo weiss parameter which allows to separate the flow into vorticity dominated and deformation dominated regions the application of this parameter to geostrophic velocity fields obtained from sea level anomaly maps show that the observed eddies have a spatial structure in close resemblance to that found in coherent vortices of two dimensional turbulence these eddies appear to have a rather coherent vorticity dominated core region surrounded by an outer region dominated by deformation with a very complicated and variable in time structure this analysis is also applied to ctd data collected across three eddies sampled between 1997 and 1998 from these data eddy cores are equivalently defined as the interior region enclosed by the maximum of azimuthal velocity and are consistent with the horizontal structures seen in sla maps the distinction between the core and the outer region circulation cell is particularly important to understand the role of transport and stirring of properties associated to eddies as illustrated by the observed salinity structure some selected sst images and drifter trajectories the core is characterized by very small isopycnal salinity gradients while the surrounding cell is characterized by a strong slope of isohalines with respect to isopycnals persistent stagnation points are observed around the eddy and correspond to regions of high values of deformation with respect to vorticity these regions allow analyzing characteristic patterns seen in the corresponding sst images in addition drifters trajectories exhibit very different behavior when crossing such regions

a 130 000 year record of levantine intermediate water flow variability in the corsica trough western mediterranean sea abstract sortable silt particle size data and stable isotope analyses from the corsica trough western mediterranean sea provide a continuous palaeoceanographic record of the inflow ventilation and vertical fluctuations of the levantine intermediate water liw in the northern tyrrhenian sea for the last 130 000 years the results presented herein reveal that climate changes drive the mediterranean intermediate circulation on milankovitch to millennial time scales intensified intermediate inflow and ventilation in the corsica trough occurred throughout the last glacial interval with a cold faster warm slower pattern existing between the dansgaard oeschger climatic oscillations including heinrich events and the liw variability by contrast a weak intermediate ventilation characterised the holocene and the last interglacial period especially during insolation maxima and the sapropel deposition in the eastern mediterranean this variability probably reflects the changes of the eastern mediterranean net evaporation as well as the propagation to the western mediterranean of the profound hydrographic adjustments of the levantine sea and adjacent areas to climate forcing the implications for the formation and ventilation of the western mediterranean deep water wmdw in the northwestern mediterranean basin as well as for mediterranean atlantic exchange through the strait of gibraltar are discussed

interaction of a coastal current with a gulf application to the shelf circulation of the gulf of lions in the mediterranean sea abstract the interaction of a coastal current with a shallow shelf is studied using analytical model and numerical simulations first the wave propagations along a steplike bottom topography in a two layer fluid are investigated the authors focus on the case in which the interface between the two layers is deeper than the shelf depth and show analytically that two waves propagate along the shelf break one is represented by a barotropic baroclinic double kelvin wave the other by a baroclinic wave trapped at the shelf break the velocity of the latter is proportional to the difference between the depth of the shelf and the depth of the upper layer off the shelf a primitive equation model was also used to study more realistic configurations 1 with an idealized shallow gulf and 2 with a realistic topography like that of the gulf of lions it is shown that the steady component of the coastal current follows the shelf break a transient cross shelf transport can be induced by a gulf scale gyre associated

eddy transport of western mediterranean intermediate water to the alboran sea during the second cruise of the eu funded omega project the towed undulating vehicle seasoar was deployed to survey the upper 350 m of the water column in the eastern alboran sea and extreme western algerian basin with an effective along track resolution of 4 km the data sets enabled a detailed description of the different upper ocean water types and the fronts that separate them the almeria oran front forms at the eastern boundary of the alboran sea gyre system in the upper 150 200 m of the water column and separates waters of predominantly atlantic origin from those formed in the western mediterranean sea below these surface waters but above the levantine intermediate water western mediterranean intermediate waters believed to be formed to the north of the balearic sea are normally observed in this region however to our knowledge this is the first time a discrete eddy of western mediterranean intermediate water a weddy has been described in the extreme western algerian basin repeated surveys of the region allowed us to observe the evolution of the eddy over a period of 40 d a climatological analysis of historical data in the medar medatlas database provides evidence for the repeatability of this observation and the significance of the estimated transport

modeling the baroclinic circulation in the area of the sicily channel the role of stratification and energy diagnostics 1 aspects of the circulation of the sicily channel region are studied with a high resolution primitive equation sigma coordinate model this work is an attempt to study the process governing the purely baroclinic dynamics and its sensitivity to seasonal stratification thus no atmospheric forcing is used two numerical experiments are analyzed using as initial conditions diagnostic velocity fields derived from the winter and summer climatological hydrologies the quasi steady state is characterized by circulation patterns in the depth range of the modified atlantic water maw and the levantine intermediate water liw strongly controlled by mesoscale eddies and topography seasonal difference is found in the surface current which exhibits the major variability on the african side downstream of adventure bank and the ionian slope where the absolute maximum occurs the intermediate circulation instead does not show relevant seasonal differences liw enters through the two passages south of malta plateau and preferentially flows along the sicilian shelf edge exiting over the northern sill its pathway is strongly controlled by subsurface cyclonic and anticyclonic cells the results are in good agreement with observations an energy diagnostics is applied to model results and sheds important light on dynamical characteristics of the mesoscale eddy field and indicates seasonal differences in the energetic exchange with the mean flow the analysis of the mean mke and eddy eke kinetic energy the transfer terms between energy compartments and the eddy momentum flux divergence indicates how eddies are strongly energetic in summer and exchange energy more efficiently with the mean flow in the quasi steady state maw volume transport and mke and eke fluctuations are strongly correlated this result suggests that mesoscale motions can control up to 40 of volume transport variability

observation of a deep convection regime with acoustic tomography in the winter of 1991 1992 a convection experiment was conducted in the western mediterranean gulf of lions combining a variety of observational techniques an essential component was an acoustic tomography array consisting of six moorings designed to observe the time evolution of the large scale processes believed to be relevant in a convection regime here two dimensional slice inversions in three directions from the central mooring are used to estimate the volume of convected water and the mean convection depth and to observe the preconditioning and restratification processes before and after the convection the near surface layer is well sampled by the acoustics which show cooling and subsequent entrainment of the warmer levantine intermediate water liw from below in agreement with mixedlayer calculations during the 2 months prior to the main convection event the total heat loss of the large scale field is in approximate agreement with the surface heat fluxes showing that little net warm water advection takes place from outside the convection region the implied confinement of water by the local circulation should be an important factor in setting the location and extent of the deep convection patch the volume of water modified by convection in this winter is estimated to correspond to an area of order 60 km radius and 1500 m depth it is argued that the homogenized area implies an annual mean deep water replenishment of 0 3 sv the restratification by the return of less dense water in the surface and liw layers occurs first by rapid capping in the near surface region thereafter the southern and eastern parts of the region restratify in the deeper layers on a 40 day timescale while a dense core of 50 60 km diameter remains in the northwest until the end of the experiment

the role of winter intermediate waters in the spring summer circulation of the balearic sea 2 a sensitivity numerical study a primitive equation model is used to investigate the role of winter intermediate waters in the surface circulation of the balearic sea in spring summer the interior solution is forced by restoring to thermohaline fields in four reservoirs schematizing the adjacent basins several simulations are carried out with different initial and boundary conditions which evidence the sensitivity of the circulation to the thermohaline structure in the basin and to the inflows through the channels the accumulation of winter intermediate waters to the north of the ibiza channel in spring is found to have a drastic effect on the circulation and on the water exchange through the channels the continental current is deflected away from the slope toward the balearic islands hence reducing the southward transport through the ibiza channel while enhancing the transport through the mallorca channel a simultaneous northward energetic inflow of modified atlantic waters through the balearic channels happens to amplify this effect and can lead to a major blocking of the continental current which veers back to the north feeding the balearic current during several months model results compare favorably with observations and help interpret some data sets

structure and variability of the alboran sea frontal system physical characteristics of the alboran sea in the western mediterranean were examined in october 1977 during a survey by ship and aircraft the main features observed were a distinct oceanic front coinciding with the jet of incoming north atlantic water and an associated anticyclonic gyre in the western half of the basin the front was confined to the upper 200 m and was a continuous feature extending from the strait of gibraltar 500 km eastward to the prime meridian of particular interest was the observed variability of the anticyclonic gyre in 10 days between two surveys the center of the gyre shifted 50 km westward this shift may correspond to changes in strength of the north atlantic inflow as inferred by local wind and the average atmospheric pressure over the western mediterranean

observation and modeling of the winter coastal oceanic circulation in the gulf of lion under wind conditions influenced by the continental orography fetch experiment 1 hydrological and currentmeter observations were collected on the continental shelf and slope of the gulf of lion during the fetch experiment 13 march to 15 april 1998 results from the first part of the cruise characterized by strong northern winds are presented the hydrological structures evidence well mixed water masses on the eastern and western ends of the shelf in the central part the situation is more complex with the influence of the rhone river s freshwater plume in the first 40 m of the water column and closer from the bottom with the confrontation of downwelled coastal cold water and upwelled warmer and saltier slope water current measurements show the path of the cyclonic circulation along the slope which is part of the general circulation of the western mediterranean and suggest the presence of large and temporary eddies on the shelf this oceanic circulation is simulated with a free surface three dimensional model using realistic forcing the model outputs are in agreement with the main hydrological and circulation patterns the results further indicate that coastal eddies are generated by the mesoscale structure of the wind field

a possible influence of the north atlantic oscillation on the circulation of the western mediterranean sea a current time series spanning from 1985 to 1996 in the corsica channel shows that seasonal and interannual variabilities are stable components of the circulation of the northern part of the western mediterranean sea for the first time these characteristics are related to the state of the north atlantic oscillation during winter we have found that this large scale control is mainly maintained through the air sea interaction processes which take place in the northern basin these results provide new elements for a consistent explanation of the major features of the basin circulation such as the seasonal increase of the boundary current and deep water formation

variations in the structure of the anticyclonic gyres found in the alboran sea historical satellite aircraft and in situ data have shown that two anticyclonic gyres the western and eastern alboran gyres are major ocean features of the alboran sea an examination of several years of satellite imagery indicates that large variations in the surface expression of these two gyres occur and that on occasion one or the other gyre disappears the disappearance of both gyres at the same time was not seen the initial disappearance of either gyre occurs on a time scale of a week to 2 weeks whereas the return may take from 3 weeks to 2 months various forcing mechanisms i e winds mass flux inflow through the straits of gibraltar and sicily and or density have been used in numerical ocean circulation models to study the dynamics of the western mediterranean sea various model results show relationships similar to those shown by the satellite imagery however no single forcing mechanism has been positively identified as the source of the disappearances and the events may be a result of a combination of forcing mechanisms

a study of an intense density front in the eastern alboran sea the almeria oran front abstract studies of satellite imagery and space shuttle photographs of the western mediterranean have indicated that the main path of inflowing atlantic water is around two large anticyclonic gyres in the alboran sea and along the algerian coast these studies have also shown that a strong ocean front is present between almeria spain and oran algeria which is part of the easternmost segment of the eastern alboran gyre based on these satellite studies the first in situ investigation of the front called here the almeria oran front was conducted in march 1986 as part of the winter campaign of the western mediterranean circulation experiment wmce analyses of the resulting data show that the almeria oran front is a large scale density front formed by the convergence of two distinct water masses and controlled by the geographic position and strength of the eastern alboran gyre physical and biochemical data indicate that the front is limited to the upper 300 m with a strong southward baroclinic jet

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency

modeling the dispersal of levantine intermediate water and its role in mediterranean deep water formation this paper demonstrates the importance of levantine intermediate water liw in the deep water formation process in the mediterranean using the modular ocean general circulation model at 0 25 resolution 19 vertical levels over the entire mediterranean with an open gibraltar strait liw formation is strongly prescribed in the rhodes gyre region by haney 1971 relaxation while in other regions surface salinity relaxation is much reduced by applying the mixed thermohaline surface boundary conditions isopycnal diagnostics are used to trace water mass movements and volume fluxes are monitored at straits low viscosity and diffusion are used to permit baroclinic eddies to play a role in water mass dispersal the overall water budget is measured by an average flux at gibraltar of 0 8 sv of which 0 7 sv is exchanged with the eastern basin at sicily liw density around 28 95 spreads rapidly after formation throughout the entire levantine due to baroclinic eddies toward the west liw accumulates in the northern and central ionian with some entering the adriatic through otranto and some mixing southward in eddies and exiting to the western mediterranean through sicily liw is converted to deep water in the south adriatic at an average rate of 0 4 sv water exchange through the otranto strait appears to be buoyancy driven with a strong bias to the end of winter march april while at sicily the exchange has a strong symmetric seasonal cycle with maximum transport of 1 1 sv in december indicating the effects of wind driving liw pathways in the west are complex and variable in the tyrrhenian intermediate water becomes uniform on isopycnal surfaces due to eddy stirring west of sardinia two liw boundary currents are formed in the balearic basin one flows northward up the west coast of sardinia and corsica and one westward along the northern african coast the northward current is consistent with observations while the westward current is intermittent for the first 10 years often breaking up into eddies which enter the basin interior some observations of high salinity waters near the african coast may support this interpretation liw retains a subsurface salinity maximum of 38 4 38 5 practical salinity units psu when reaching the northwestern mediterranean contrasting with surface waters fresher than 38 0 psu west mediterranean deep water is formed below 1500 m depth with climatological characteristics when it is mixed and cooled during winter convection in lions gyre

horizontal ocean circulation forced by deep water formation part i an analytical study abstract the horizontal ocean circulation generated by vertical convection is investigated analytically the stratification is parameterized by a two layer ocean and attention is focused on the spinup phase when the phenomena can be considered as linear it is found that the response of the ocean to a two dimensional idealized thermohaline forcing is baroclinic a cyclonic gyre is generated in the upper layer an anticyclonic gyre in the lower layer variation of the coriolis parameter with latitude causes the center of the gyre to drift westward at the speed of long baroclinic rossby waves