thermohaline variability the effects of horizontal resolution and diffusion abstract an idealized coupled ocean atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the thermohaline circulation a series of experiments ranging from 4 to 0 25 resolution with appropriate horizontal viscosities and diffusivities in each case is performed for both coupled and ocean only models spontaneous internal variability primarily on the decadal timescale is found to exist in the higher resolution cases with the exception of one of the restoring experiments the decadal oscillation whose period varies slightly between cases is described as an advective convective mechanism that is thermally driven and linked to the value of the horizontal diffusivity utilized in the model increasing the diffusivity in the high resolution cases presented in this paper is enough to destroy the variability whereas decreasing the diffusivity in the moderately coarse resolution cases is capable of inducing decadal scale variability as the resol

reconciling the stommel box model with the stommel arons model a possible role for southern hemisphere wind forcing abstract in the stommel box model the strength of the overturning circulation is parameterized in terms of the density and hence the pressure difference between the two boxes straub has pointed out that this parameterization is not consistent with the stommel arons model for the abyssal circulation in particular the zonally averaged density field implied by the stommel arons model is unrelated to the strength or the direction of the meridional overturning circulation here the inconsistency is examined using the abyssal circulation model of kawase and a variant to include the effect of southern hemisphere wind forcing the important parameter is r the ratio of two timescales the timescale for a perturbation to the density field to propagate by either wave or advective processes from a high latitude source to the equator and the timescale for the dissipation of a perturbation to the density field by diapycnal mixing if the model is forced only by a deep water source in the northern basin it is

mechanisms of crustal anatexis a geochemical study of partially melted metapelitic enclaves and host dacite se spain to shed light on the mechanisms of crustal anatexis a detailed geochemical study has been conducted on minerals and glasses of quenched anatectic metapelitic enclaves and their host peraluminous dacites at el hoyazo se spain anatectic enclaves composed of plagioclase biotite sillimanite garnet glass k feldspar cordierite graphite formed during the rapid heating and overstepped melting of a greenschist facies metapelite and finally equilibrated at 850 508c and 5 7 kbar glass appears as melt inclusions within all mineral phases and in the matrix of the enclaves and has a major element composition similar to that of peraluminous leucogranites melt inclusions and matrix glasses have normative quartz orthoclase albite compositions that plot in the vicinity of h2o undersaturated haplogranite eutectics melt inclusions show some compositional variability with high li cs and b low y first row transition elements frte and rare earth elements ree and zircon and monazite saturation temperatures of 665 7508c they are interpreted as melts produced by muscovitebreakdown melting reactions at the onset of the process of rapid melting and mostly under h2o undersaturated conditions compared with melt inclusions matrix glasses show less compositional variability lower large ion lithophile element contents higher y frte and ree and higher zircon and monazite saturation temperatures 695 8158c they are interpreted as former melts recording the onset of biotite dehydration melting matrix glasses in the dacite are compositionally different from glasses in the enclaves hence the genetic connection between metasedimentary enclaves and dacite is not as straightforward as previous petrographic and bulk major element data suggest this opens the possibility for some alternative interpretation this study shows the following 1 melt inclusions provide a window of information into the prograde evolution of anatexis in the enclaves 2 melting occurred for the most part under h2o undersaturated conditions even if because of the rapid heating the protolith preserved most of the structurally bound h2o contained at greenschist facies up to the beginning of anatexis such that the excess h2o maximized the amount of h2o undersaturated melt generated during anatexis 3 although a large proportion of accessory minerals are currently shielded within major mineral phases they have progressively dissolved to a considerable extent into the melt phase along the prograde anatectic path as indicated by the relative clustering of accessory mineral saturation temperatures and closeness of these temperatures to those of potential melting reactions 4 the dacite magma was probably produced by coalescence of melts

the response of an idealized ocean basin to variable buoyancy forcing the response of an idealized ocean basin to variable buoyancy forcing is examined a general circulation model that employs a gent mcwilliams mixing parameterization is forced by a zonally constant restoring surface temperature profile which varies with latitude and time over a period p in each experiment 17 different values of p are studied ranging from 6 months to 32 000 yr the model s meridional overturning circulation moc exhibits a very strong response on all time scales greater than 15 yr up to and including the longest forcing time scales examined the peak to peak values of the moc oscillations reach up to 125 of the steady state maximum moc and exhibit resonance like behavior with a maximum at centennial to millennial forcing periods depending on the vertical diffusivity this resonance like behavior stems from the existence of two adjustment time scales one of which is set by the vertical diffusion and the other of which is set by the basin width furthermore the linearity of the response as well as its lag with the forcing varies with the forcing period the considerable deviation from the quasi equilibrium response at all time scales above 15 yr is surprising and suggests a potentially important role of the ocean circulation for climate even at milankovich time scales

surface climate variations over the north atlantic ocean during winter 1900 1989 abstract the low frequency variability of the surface climate over the north atlantic during winter is described using 90 years of weather observations from the comprehensive ocean atmosphere data set results are based on empirical orthogonal function analysis of four components of the climate system sea surface temperature sst air temperature wind and sea level pressure an important mode of variability of the wintertime surface climate over the north atlantic during this century is characterized by a dipole pattern in ssts and surface air temperatures with anomalies of one sign cast of newfoundland and anomalies of the opposite polarity off the southeast coast of the united states wind fluctuations occur locally over the regions of large surface temperature anomalies with stronger than normal winds overlying cooler than normal ssts this mode exhibits variability on quasi decadal and biennial time scales the decadal fluctuations are irregular in length averaging 9 years before 1945 and 1

laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure an interpretation of the electrical signature of molten rocks within the earth s interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts this paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous 1 3 wt h2o metaluminous obsidians determined in an internally heated pressure vessel in the range of 50 400 mpa and 350 1325 c it is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an arrhenius law applying in the melt and the glass regions this suggests that a similar transport mechanism operates in both melt and glass the determined activation energies are 70 kj mol for the dry obsidian and 65 and 61 kj mol for the 1 and 3 wt h2o melts respectively the activation volume is 20 cm3 mol combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians the temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for na mobility in dry and hydrous rhyolites an increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium as a prospective it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization therefore its electrical signature must reveal its nature and maturity

interdecadal variability in an ocean model driven by a small zonal redistribution of the surface buoyancy flux abstract an idealized three dimensional model of size comparable to the atlantic is used to study interdecadal variability of the thermohaline circulation thc in most of the model experiments salinity is kept uniform and constant the model being driven by surface heat flux only when the model is driven by the surface heat flux diagnosed from a restoring spinup experiment no oscillations occur driving the model by a time independent surface heat flux obtained by applying a small zonal redistribution to the diagnosed flux leads to strong interdecadal oscillations small means that the modification to the diagnosed flux is within the error bars on estimates of surface heat flux based on observations the model sea surface temperature sst anomalies are similar to the observed pattern of sst anomalies in the north atlantic and to the sst anomalies associated with the interdecadal oscillation in the gfdl fully coupled ocean atmosphere model for redistributions that weaken the east west varia

contact metamorphism and anatexis of torridonian arkose by minor intrusions of the rum igneous complex inner hebrides scotland

interdecadal variability of the thermohaline circulation in box ocean models forced by fixed surface fluxes intrinsic modes of decadal variability are analyzed using box geometry ocean models forced by constant surface fluxes an extensive parameter sensitivity analysis of the oscillatory behavior is carried out with respect to the spherical cartesian geometry the b effect the coriolis parameter the parameterization of momentum dissipation and associated boundary conditions the vertical and horizontal diffusivities the convective adjustment algorithm the horizontal and vertical model resolution the forcing amplitude and the basin width the oscillations stand out as a robust geostrophic feature whose amplitude is mainly controlled by the horizontal diffusivity unsuccessful attempts to reproduce the variability in zonally averaged 2d models suggest that the 3d adjustment processes are necessary however the b effect is not necessary for decadal variability to occur and therefore classical rossby waves play no fundamental role in the mechanism various experiments with different geometry and forcing are conducted and do not support the necessity of viscous numerical boundary waves or any boundary in sustaining the oscillations the models show two types of oscillatory behavior 1 temperature anomalies propagating geostrophically westward in the eastward jet northern part of the basin and inducing an opposite anomaly in their wake and 2 stationary temperature anomalies in the northwest quarter that respond to the western boundary current transport changes and reinforce geostrophically this change until the opposite temperature anomaly built on the east finally reverses the meridional overturning anomaly the analysis of the transition from steady to oscillatory states using heat fluxes diagnosed at the equilibrium under restoring boundary conditions and the comparison of the variability under various forcing fields suggest that the oscillations are triggered in the regions of strongest surface cooling finally a simple box model analogy is proposed that captures the crucial phase shift between meridional overturning and north south density gradient anomalies on these decadal timescales

electrical conductivity in a partially molten crust from measurements on metasedimentary enclaves abstract the complex electrical impedance of a garnet biotite sillimanite residual enclave in the neogene dacite of el hoyazo se spain has been determined up to 978 c at 200 300 mpa this well studied material represents a direct sampling of the alboran domain thinned lower crust undergoing partial melting the paragenesis is garnet biotite sillimanite plagioclase graphite cordierite coexisting with widespread 10 wt rhyolitic melt occurring as inclusions and interstitial glass and developed during regional anatexis at 850 50 c and 500 700 mpa the samples were used in cyclic measurements consisting in heating cooling ramps at progressively higher maximum temperature to observe the effect of reactions on conductivity in the first cycle up to 850 c at 2 3 kbar re melting of the interstitial glass is achieved with no additional reactions and logarithmic conductivity s m is up to 1 5 at t 950 c new melt with orthopyroxene ilmenite is produced from biotite partial breakdown and the logarithmic conductivity s m is up to 0 7 application of mixing models as hashin shtrikman bounds or archie s law shows that once interconnected melt controls the electrical conductivity of the enclave the electrical conductivity of the rock can be simulated with the electrical conductivity data obtained on the starting matrix glass of the enclave and with the new melt our experimental results obtained on a graphitic metasedimentary material evidence show that graphite does not contribute to the conductivity enhancement and that the electrical anomaly observed below the betics can be explained as effect of partial melting of a residual crust comparison with previous works on mafic granulites shows that similar conductivity values can be achieved both by melting of a residual felsic crust or by melting of mafic materials and suggests that pelitic rocks can be more common at deep levels than expected

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

heat flow heat production and lithospheric thermal regime in the iberian peninsula abstract the first heat flow and heat production maps of the iberian peninsula and its margins using previously acquired data are presented the surface heat flow map includes 553 determinations carried out on water and mining exploration wells oil exploration wells and on the seafloor the surface heat flow varies noticeably from the iberian mainland 65 10 mw m 2 to the atlantic and the mediterranean margins where the heat flow reaches values of about 40 50 mw m 2 and 80 100 mw m 2 respectively the heat production map consists of 664 determinations carried out on rock samples from the variscan iberian massif and the betics the higher values are obtained for granitic rocks 2 5 3 5 w m 3 whereas metasediments and basic rocks reach values of 1 2 5 w m 3 and nearly zero respectively the lithospheric structure deduced for the iberian peninsula by combining heat flow heat production and elevation data indicates that most of the iberian mainland is characterized by a lithospheric thickness of 110 5 km this value is maintained across the west atlantic margin whereas towards the mediterranean margin the lithospheric thickness decreases down to 60 40 km this study also suggests that the heat production in the southern variscan iberian massif must be noticeably higher 3 7 0 5 w m 3 than in the rest of the areas in order to fit the measured heat flow crustal thickness and elevation data

are some of the deep crustal conductive features found in sw iberia caused by graphite abstract recent results obtained from two dimensional inversion of magnetotelluric mt data acquired in sw iberia reveal high conductive features at the middle lower levels of the crust the top of these anomalous structures correlates very well with the depth 10 13 km of an important seismic interface that has been interpreted as a regional detachment horizon very shallow and relatively narrow conductors in the ossa morena zone appear to correspond to small scale fluid deposited graphite systems in the preorogenic metasedimentary sequences some of the midcrustal conductors can be ascribed to graphite bearing thrust zones the formation of graphite with variable crystallinity being a consequence of variscan shearing processes deep seated conductors are tentatively interpreted as a result of relatively continuous highly crystalline grain boundary graphite films presumably preserved in basement granulite rocks assuming that graphite occurs as interconnected films calculations indicate that a fraction of 0 006 0 02 of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of mt models the role of graphite on the thermal behaviour of the crust is also discussed the results show that low contents of graphite do not significantly change the thermal behaviour of earth materials

a damped decadal oscillation in the north atlantic climate system a simple stochastic atmosphere model is coupled to a realistic model of the north atlantic ocean a north south sst dipole with its zero line centered along the subpolar front influences the atmosphere model which in turn forces the ocean model by surface fluxes related to the north atlantic oscillation the coupled system exhibits a damped decadal oscillation associated with the adjustment through the ocean model to the changing surface forcing the oscillation consists of a fast wind driven positive feedback of the ocean and a delayed negative feedback orchestrated by overturning circulation anomalies the positive feedback turns out to be necessary to distinguish the coupled oscillation from that in a model without any influence from the ocean to the atmosphere using a novel diagnosing technique it is possible to rule out the importance of baroclinic wave processes for determining the period of the oscillation and to show the important role played by anomalous geostrophic advection in sustaining the oscillation

the role of horizontal boundaries in parameter sensitivity and decadal scale variability of coarse resolution ocean general circulation models abstract coarse resolution f plane and plane frictional geostrophic models are used to study the response to restored surface buoyancies and fixed surface buoyancy fluxes with restored surface buoyancies it is found that the overturning and meridional buoyancy transport generally follow the scaling from thermal wind and vertical advective diffusive balance when maximum midbasin rather than maximum overall streamfunction is used as the metric the sensitivity of overturning magnitude to vertical diffusivity agrees quite closely with that of a two dimensional rayleigh frictional model that follows an analogous scaling this measure of meridional overturning as well as the meridional buoyancy transport also roughly follow the predicted f 1 3 scaling and are relatively insensitive to variations in and horizontal viscosity the sensitivity experiments indicate that the coarse resolution model overturning and thermodynamic structure are well characterized by the adjustment of a uniformly rotating viscous

oceanic kelvin waves and tropical atlantic intraseasonal variability 1 kelvin wave characterization 1 oceanic kelvin waves from the equator to the west african coast are investigated in the framework of tropical atlantic intraseasonal variability in order to better highlight the wave propagations a 25 95 day band pass filter was applied to the sea surface height ssh product derived from the topex poseidon altimeter and an ocean general circulation model simulation for the 1993 2000 period in addition to equatorial eastward propagations our analysis reveals recurrent and continuous propagations distinguishable over thousands of kilometers poleward along the coasts as far as about 10 15 latitude a novel result with altimeter data the variance of the filtered ssh signal goes from 1 cm at the equator to 2 cm at the african coast estimates of the phase speed range from 1 5 to 2 1 m s along the equator and the west african coastline such values are very close to those of equatorial kelvin wave propagations likely dominated by the first two baroclinic modes supporting the fact that the coastal propagations are coastally trapped kelvin waves in order to simplify the description of these kelvin waves we present an intraseasonal climatology which reveals regular boreal autumn winter equator to coast propagations an improved description is achieved thanks to the computation of an extended empirical orthogonal function for the boreal autumn winter propagations lag correlation of ssh signals allows for a twofold quantification the phase speed and the importance of remote forcing along the coast the remote forcing effect of intraseasonal kelvin waves is clearly evidenced over coastal upwelling regions as far as 10 15 latitude the physical mechanism associated with the forcing of the kelvin waves and its impacts will be investigated in a future paper

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

a zonally averaged ocean model for the thermohaline circulation part i model development and flow dynamics abstract a two dimensional latitude depth ocean model is developed on the basis of the zonally averaged balance equations of mass momentum heat and salt its purpose is to investigate the dynamics and variability of the buoyancy forced thermohaline circulation for the time scales of interest an annually averaged model is selected and the momentum balance is taken to be diagnostic the east west pressure gradient which arises upon zonally averaging the momentum equations is parameterized in terms of the meridional pressure gradient the thermohaline circulation is driven by mixed surface boundary conditions i e temperatures are relaxed to prescribed values while the salt flux is held constant the dynamics of the flow is investigated in hemispheric and global geometries for both short and long time integrations the latter extending over many thousands of years as has been noted by previous investigators it is possible to perturb a steady state such that a diffusively dominated regime results

integrated two dimensional lithospheric conductivity modelling in the pyrenees using field scale and laboratory measurements abstract recent magnetotelluric mt studies have shown that the lower crust in the pyrenees contains a high conductivity zone consistent with a subducting continental slab whose conductivity is 0 33 s m partial melting has been interpreted to be the most plausible explanation for this high conductivity here we report a two dimensional conductivity model of the lithosphere by integrating field scale and laboratory determinations of the conductivity of continental crustal and mantle rocks the laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature pressure and lithology are known consequently we have also calculated the density lithostatic pressure and several alternative temperature profiles for use in the model from gravity seismic and thermal field data these can be used with a prescribed melt fraction to predict the electrical conductivity at depth which can be compared with the mt conductivity data alternatively the laboratory data can be combined with the mt conductivity data to predict the melt fraction at depth the primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models i waff s model hashin shtrikman hs upper bound ii hs lower bound iii parallel layers iv perpendicular layers v random melt areas and vi a modified archie s law that takes account of the presence of two conducting phases the modelling results indicate that a good match to the mt data can be obtained along the whole profile by the influence of pressure temperature and the fluid phase with the only exception being the subducted slab where a minimum of 4 7 melt fraction is necessary to explain the data

multi and quasi decadal variations of sea surface temperature in the north atlantic abstract the multi and quasi decadal variabilities of the sea surface temperature sst anomaly over the north atlantic are investigated by means of two dimensional propagating ceof forty six years of the coads sst dataset from 1947 to 1992 are used after removing the monthly climatology and four year low pass filtering the sst anomaly is submitted to ceof the total variance of the three largest modes accounts for 87 0 of the total variance mode 1 is a very slow oscillation with an approximate 42 yr period and has basinwide spatial evolution with alternate warm and cold anomalies appearing off newfoundland and migrating northward until they disappear south of greenland the center of action of the anomalies both occur in the vicinity of the labrador sea newfoundland and south of greenland where deep water formation takes place mode 2 3 is a quasi decadal fluctuation with an approximate 14 yr period mode 2 3 contains quasi decadal timescales with alternating warm and cold anomalies propagati

seismic properties of rocks from an exposure of extended continental crust new laboratory measurements from the ivrea zone abstract compressional wave velocities for new samples from the ivrea verbano zone ivz and strona ceneri zone scz have been measured to 600 mpa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains these data coupled with previous measurements show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of scz and ivz schists and gneisses ivz paragneisses form the basis of a linear velocity density relationship for metagraywackes of various metamorphic grade this relationship falls close to that expected for mafic rocks and overlaps velocity density data for ivz mafic gneisses one dimensional near vertical reflection models based on these data and recent structural reconstructions of the terrain show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite granulite facies paragneiss mafic gneiss and ultramafic rocks transparent upper crustal levels in the model result from the overall similarity of velocities for schists gneisses and granites from the structurally and lithologically complex scz a mean lower crustal velocity of 7 1 km s is calculated for the part of the ivz composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex the velocity density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from p wave velocity data in the absence of other constraints

electrical conductivity of orthopyroxene and plagioclase in the lower crust the electrical conductivities of lower crustal orthopyroxene and plagioclase as well as their dependence on water content were measured at 6 12 kbar and 300 1 000 c on both natural and pre annealed samples prepared from fresh mafic xenolith granulites the complex impedance was determined in an end loaded piston cylinder apparatus by a solarton 1260 impedance gain phase analyzer in the frequency range of 0 1 106 hz the spectra usually show an arc over the whole frequency range at low temperature and an arc plus a tail in the high and low frequency range respectively at high temperature the arc is due to conduction in the sample interior while the tails are probably due to electrode effects different conduction mechanisms have been identified under dry and hydrous conditions for the dry orthopyroxene the activation enthalpy is 105 kj mol and the conduction is likely due to small polarons e g electrons hopping between fe2 and fe3 for the dry plagioclase the activation enthalpy is 161 kj mol and the conduction may be related to the mobility of na for the hydrous samples the activation enthalpy is 81 kj mol for orthopyroxene and 77 kj mol for plagioclase and the electrical conductivity is markedly enhanced probably due to proton conduction for each mineral the conductivity increases with increasing water content with an exponent of 1 and the activation enthalpies are nearly independent of water content combining these data with our previous work on the conductivity of lower crustal clinopyroxene the bulk conductivity of lower crustal granulites is modeled which is usually 10 4 s m in the range of 600 1 000 c we suggest that the high electrical conductivity in most regions of the lower crust especially where it consists mostly of granulites can be explained by the main constitutive minerals particularly if they contain some water contributions from other highly conducting materials such as hydrous fluids melts or graphite films are not strictly necessary to explain the observed conductivities

interdecadal variability in a zonally averaged ocean model an adjustment oscillator abstract a meridional plane hemispherical ocean model is developed to study interdecadal variability of the thermohaline circulation thc the model differs from previous formulations of zonally averaged ocean models by using a prognostic equation to calculate the meridional velocity this allows the incorporation of an adjustment timescale comparable to the advective timescale of the meridional overturning an interdecadal oscillation is documented for an idealized ocean of homogeneous salinity forced by a time independent surface heat flux the governing equations are linearized about a basic state in order to isolate the effect of parameter changes on the oscillation an eigenvalue analysis reveals that the frequency of the oscillation is independent of the advective timescale instead the interdecadal timescale of the oscillation is set by the slow adjustment of the overturning intensity to a meridional pressure gradient anomaly the oscillatory instability on the other hand is dependent on both

oceanic responses to gradual transitions of equator to pole temperature gradients summary responses of an ocean model to gradual transitions of equator to pole gradients show that several quasiequilibrium states within the thermally driven regime are generated under identical forcing conditions these different states are associated with different patterns the location and the penetration depth of oceanic convection small changes in the thermal forcing conditions can push the system into an oscillatory regime once in this regime oscillations appear to proceed as internal processes one important implication of these results is that the presentday ocean climate may not recover fully if the atmospheric coz level after doubling is lowered to the present day level

lithospheric structure beneath the alboran basin results from 3d gravity modeling and tectonic relevance a three dimensional gravity modeling combined with integrated heat flow and elevation modeling is conducted to map out the crustal and lithospheric mantle thickness in the alboran basin in the westernmost mediterranean a sediment corrected bouguer anomaly has been derived using a depth to the basement map and densities determined from well logs and seismic data the gravity effect of the base of the lithosphere has been removed from the sediment corrected bouguer anomaly to obtain a crustal bouguer anomaly which has been inverted for crustal thickness the resulting lithospheric structure is further constrained by elevation data under the assumption of local isostasy the low residual elevation anomalies obtained 100 m in average suggest that the area is in local isostasy particularly the medium and long wavelength topography features variations in crustal thickness range from 36 km underneath the betic and rif chains to 12 km beneath the easternmost part of the alboran sea basin in the transition to the south balearic basin in western alboran the moho lies at a rather constant depth of 18 km deepening sharply toward the gibraltar strait down to 30 32 km the base of the lithosphere shallows from 140 km depth in the gibraltar strait to 45 km depth in the easternmost alboran sea lithospheric thinning penetrates to the southeastern side of the iberian margin crosscutting topographic highs of the central and eastern betic chain our results favor mantle delamination produced by detachment and subsequent peeling away of the lithospheric mantle rather than convective removal of the lithospheric mantle either by orogenic collapse or detachment and sinking of a lithospheric slab

on steady salinity distribution and circulation in partially mixed and well mixed estuaries abstract perturbation analysis based on small ra0 23fm0 9 where ra is the rayleigh number and fm is the froude number is used to study steady state circulation and salinity distribution in estuaries the classical hansen and rattray s similarity solution is obtained for the special case of linear variation of longitudinal dispersion coefficient kh in a channel of constant width b and depth d it is argued that kh b and d must vary in real estuaries in such a way that the general solution is regular throughout the length of the estuary and shows a salinity structure which resembles that observed in a real estuary it is shown that hansen and rattray s theory for predicting the importance of upstream salt transport due to the vertical gravitational circulation in estuaries is valid to a good degree of approximation for arbitrary longitudinal variations in width depth fresh water discharge wind stress and various dispersion and mixing coefficients this finding is checked against available observa

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

evidence from borehole samples for the role of accessory minerals in lower crustal conductivity the high electrical conductivity of the lower continental crust as inferred from geophysical measurements1 has defied a simple explanation both pore saturating brines2 and interconnected carbon films3 5 have been proposed as conducting pathways but their relative importance remains uncertain and may vary from place to place6 so far attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures7 10 although these rocks almost certainly originated in the lower crust chemical alteration during their transport to and residence at the surface is likely to have affected their conducting properties here we report conductivity data for pristine crustal rocks recovered from depths of up to 4 6 km in the ktb boreholes in southern germany our results show that the electrical conductivity of accessory phases such as fe ti oxides and sulphides can enhance or even surpass the high conductivity produced by saline fluids

electrical conductivity measurement of gneiss under mid to lower crustal p t conditions abstract we conducted electrical conductivity measurements perpendicular and parallel to mineral foliation in dry gneiss at up to 1000 k and a constant pressure of 1 gpa the analyzed gneisses were obtained from the higo metamorphic belt kyushu japan as the metamorphic conditions of these rocks have been well determined by previous studies we were able to select samples that were representative of the middle to lower crust prior to the conductivity measurements the samples were maintained at the maximum temperature for a long period until the electrical conductivity had stabilized our experiment results reveal linear and reproducible conductivity data between temperatures of 600 and 1000 k conductivity measured perpendicular and parallel to foliation differ by an order of magnitude over the same temperature window a plausible explanation for this discrepancy in conductivity is the contrasting configuration of minerals in the two sample orientations as observed by backscattered electron image bei and electron probe microanalysis epma we evaluated the conductivity and computed activation energy for each of the samples and compared the results with those of previous studies our results are consistent with the conductivity values reported for other types of rocks we also compared the experiment results with data derived from electromagnetic em soundings electrical conductivity measurements undertaken perpendicular to foliation can account for the subsurface conductivity structure beneath central kyushu japan

a high electrical conductive zone at lower crustal depth beneath the betic chain spain abstract magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the central betic chain interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two dimensional resistivity model along a nw se profile from the guadalquivir foreland basin and across the boundary between the external and the internal betics elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected the conductors in the upper crust coincide with the areal distribution of the cenozoic and mesozoic sedimentary rocks from the guadalquivir basin and the external betics and the conductors in the middle crust with the major miocene extensional detachments affecting the basement rocks of the internal betic zone a high conductivity zone at lower crustal levels beneath the outer internal betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective moho this conductor was interpreted as partial melting of a southeast dipping subducted iberian lower crust the geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the alboran domain or to the iberian plate in the former case it would correspond to a major plate boundary and in the latter to an intraplate feature

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

baroclinic instability an oceanic wavemaker for interdecadal variability numerical simulations of coarse resolution idealized ocean basins under constant surface heat flux are analyzed to show that the interdecadal oscillations that emerge naturally in such configurations are driven by baroclinic instability of the mean state and damped by horizontal diffusion when the surface heat fluxes are diagnosed from a spinup in which surface temperatures are strongly restored to apparent atmospheric temperatures the most unstable regions diagnosed by large downgradient eddy heat fluxes are located in the basin northwest corner where the surface heat losses are largest the long wave limit of the baroclinic instability of idealized mean flows in a three layer model with vertical shears as observed in the gcms demonstrates that growth rates of order one cycle per year can be produced locally large enough to amplify thermal anomalies in the face of lateral diffusion the proposed instability mechanism that favors surface intensified perturbations also explains the lack of oscillations if the restoring to a surface climatology is too strong to assess whether this instability process of oceanic origin is robust enough to cause interdecadal variability of coupled ocean atmosphere models a four box ocean atmosphere model is constructed given the large heat capacity of the ocean as compared to the atmosphere the dynamical system that governs the model evolution is reduced to only two degrees of freedom the oceanic overturning thermohaline circulation and the interior north south temperature gradient the authors show that when the baroclinic instability growth rate exceeds the overall dissipation caused by turbulent eddy diffusion in the atmosphere and ocean and infrared back radiation the dynamical system undergoes a hopf bifurcation and interdecadal oscillations emerge through a limit cycle

large scale melt depletion in granulite terranes an example from the archean ashuanipi subprovince of quebec this study uses field petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane and to determine what the grain and outcrop scale melt flow paths were during the melt segregation process the ashuanipi subprovince located in the north eastern superior province of quebec is a large 90 000 km2 metasedimentary terrane in which 85 of the metasediments are of metagreywacke composition that was metamorphosed at mid crustal conditions 820 900 c and 6 7 kbar in a late archean dextral transpressive orogen decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction biotite quartz plagioclase melt orthopyroxene oxides using melt orthopyroxene ratios for this reaction derived from experimental studies the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol partial melting the metagreywackes are enriched in mgo cao and feot and depleted in sio2 k2o rb cs and u have lower rb sr higher rb cs and th u ratios and positive eu anomalies compared to their likely protolith these compositions are modelled by the extraction of between 20 and 40 wt granitic melt from typical archean low grade metagreywackes a simple mass balance indicates that about 640 000 km3 of granitic melt was extracted from the depleted granulites the distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary porous flow from the site of melting across the layers and into bedding planes between adjacent layers in other rocks pervasive grain boundary flow of melt occurred along the layers for a few to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits visible as the net like array of leucosomes in some outcrops the leucosomes contain very little residual material 5 biotite orthopyroxene indicating that the melt fraction was well separated from the residuum left in situ as melt depleted granulite only 1 3 vol percentage melt remained in the melt depleted granulites hence the extraction of melt generated by biotite dehydration melting in these granulites was virtually complete under conditions of natural melting and strain rates in a contractional orogen

a two level model of a thermally forced ocean basin abstract some simple solutions mostly analytic are presented for the large scale baroclinic response to thermal forcing on a mid latitude beta plane surface heat flux is parameterized as ta tt tau with atmospheric temperature ta prescribed as a function of latitude varying ocean surface temperature tt and equilibration time for long times decades benthic forcing is included using a similar representation the model allows horizontal density variations at each level when there are no meridional boundaries there is only a local response to the forcing a geostrophic baroclinic zonal flow is driven by the north south temperature gradient but it has no associated advection or divergence effects this picture is greatly changed when east and or west coasts are added kelvin waves pass information rapidly about 200 km day 1 along coasts and rossby waves travel slowly offshore most effectively from the cut with speed c 1 km day 1 for spin up problems e g the response to a change in for

electrical conductivity and partial melting of mafic rocks under pressure we demonstrate the importance of electric conductivity measurements of partially molten mafic rocks by examining of oman gabbro karelia olivinite ronda and spitzbergen peridotites the electrical conductivities of these rocks were estimated using the impedance spectroscopy at temperatures between 800 c and 1450 c and at pressures between 0 3 and 2 gpa in experiments performed in a piston cylinder apparatus at temperatures below and above melting samples were equilibrated during durations on the order of 200 h our results show that a jump in electrical conductivity can be correlated with the temperature range slightly above the solidus due to the delayed formation of an interconnected melt phase thin sections of quenched samples were used to estimate volume fractions and chemical compositions of the partial melts the increase of the electrical conductivity compares well with the connectivity of melt in partially molten samples above the solidus the electrical conductivity increases by 1 to 2 orders of magnitude in comparison with the conductivity of non melted rock below solidus when a complete melt connectivity is established the charge transport follows the network of the formed melt films at grain boundaries durations of up to 200 h are required in order to reach a steady state electrical resistance in a partially molten rock sample the experimental results were compared with the conductivity data obtained from magnetotelluric mt and electromagnetic em measurements in the northern part of the mid atlantic ridge where a series of axial magma chambers amc are presumably located there is good agreement between the measured electric conductivity of gabbroic samples with a melt fraction of 10 to 13 vol and the conductivity estimated at amc beneath the central part of reykjanes ridge as well as between the conductivity of partially molten peridotites and the source zone beneath the mid atlantic ridge at 60 km

oceanic wave dynamics and interdecadal variability in a climate system model western boundary disturbances detected in the first multicentury integration of the national center for atmospheric research climate system model are analyzed and the role they play in the model interdecadal variability is investigated the boundary signals propagate southwestward along the coast of north america at depth they reach the equator and propagate along the equator but with decreasing amplitude away from the western boundary their southwestward propagation is associated with the evolution of the western boundary current system the gulf stream system in the upper part of the water column and the deep western boundary current at depth from weaker than average to stronger than average conditions thus the boundary disturbances seem to play an important role in the model thermohaline circulation variability at interdecadal timescales they show similarities with the boundary waves found in ocean only models exhibiting interdecadal variability and their role in the model s response to changes in the rate of sinking appears to be in agreement with the theory of the spin up of the deep ocean circulation the main result of this study is that the similarities with ocean only results and theories support the idea that interdecadal timescales of variability originate from the ocean dynamics

migmatites formed by water fluxed partial melting of a leucogranodiorite protolith microstructures in the residual rocks and source of the fluid abstract the opatica subprovince in the canadian shield is a late archaean 2761 2702 ma plutonic arc formed above a north dipping subduction zone anatexis 2690 2677 ma of leucogranodiorite and leucotonalite orthogneisses in the opatica generated migmatites in an area of north vergent back thrusts visible at the surface and in l ithoprobe seismic profile 48 schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them the modal mineralogy microstructure and whole rock major trace and oxygen isotope compositions of the protolith and migmatites were investigated to 1 determine the melting reaction 2 find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome and 3 to determine the source of the fluid involved in anatexis partial melting of the protolith did not change the mineral assemblage but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks quartz plagioclase and microcline show evidence for dissolution and biotite does not thus water fluxed melting of quartz plagioclase microcline occurred a mass balance indicates 25 30 partial melting the melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase extraction of 80 90 of the melt left a thin film of melt on the grain boundaries and crystallization of these in the plagioclase domains created diagnostic microstructures microcline fills the last remaining pore space and forms high aspect ratio crystals between plagioclases or triangular crystals at grain junctions quartz shows a range of morphologies from high aspect ratio films through the string of beads to isolated rounded grains as the microstructure progressively equilibrated after crystallisation most accessory phases including zircon remained in the residuum however almost all the schollen migmatites have high contents of th u nb ta and ree relative to the protolith due to contamination by accessory phases derived from mafic rocks disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed average whole rock 18 o for the protolith and migmatites are similar ca 8 2 and the small difference between melt rich 8 6 and residuum rich rocks 8 0 is consistent with fractionation thus the fluid that caused melting was probably of metamorphic origin with 18 o similar to the protolith the seismic profile shows several reflectors extending to a present depth of 20 km ca 40 km in the late archaean under the migmatites these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface a new type of neosome reported in this study may have formed along fractures that the fluids migrated along however these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites

proximity of the present day thermohaline circulation to an instability threshold the relation between the mean state of the thermohaline circulation thc and its stability is examined using a realistic geometry primitive equation coupled ocean atmosphere ice global general circulation model the main finding is that a thermohaline circulation that is 25 weaker and less dominated by thermal forcing than that of today s ocean is unstable within this coupled gcm unstable initial ocean climates lead in the coupled model to an increase of the thc to strong oscillations or to a thc collapse the existence of an unstable range of weak states of the thc provides a natural explanation for largeamplitude thc variability seen in the paleo record prior to the past 10 000 years a weakening of the thc due to an external forcing e g ice melting and freshening of the north atlantic may push it into the unstable regime once in this regime the thc strongly oscillates due to the inherent instability of a weak thc hence the strong thc variability in this scenario does not result from switches between two or more quasi stable steady states

the damping effect of bottom topography on internal decadal scale oscillations of the thermohaline circulation by comparing the response of flat and bowl shaped basins to fixed heat fluxes of various magnitudes it is determined that coastal topography has a considerable damping influence upon internal decadal oscillations of the thermohaline circulation it is proposed that this is because the adjustment of baroclinic currents to the no normal flow boundary condition at weakly stratified coasts is aided in the topography case by the generation of substantial barotropic flow

metal and nutrient behavior in the raritan estuary new jersey u s a the effect of multiple freshwater and industrial waste inputs abstract a geochemical analysis of the raritan estuary during high and low river flow is presented several statistical and graphical approaches in addition to a hydrodynamic model of the raritan estuary are used to demonstrate the effects of lateral inputs on trace element distribution in a complicated fluvial marine system results from factor and cluster analysis show that nutrient salinity distributions on both sampling dates are controlled primarily by freshwater saltwater mixing industrial and municipal waste sources within the estuary are important in controlling dissolved organic carbon at low flow and dissolved and bottom sediment trace metals biological and physico chemical reactions have a significant but secondary effect on nutrient and trace metal distributions with salinity apparent flux calculations and property property plots show that for dissolved phosphate nitrate and inorganic carbon the raritan estuary can be divided into two mixing zones with the raritan river controlling nutrient concentrations in the lower salinity stretches and the south river controlling their distributions at intermediate and higher salinities high enrichment factors of most metals in estuary bottom sediment reveal that this is an important and semi permanent sink for trace metals in the raritan system previous work on suspended sediment in the estuary and river substantiates that this load is also an important sink for trace metals however many of these metals are in leachable modes which are more susceptible to release and incorporation into the food chain

adjustment of the ocean under buoyancy forces part ii the role of planetary waves abstract a numerical ocean general circulation model is used to investigate the early stages in the adjustment to equilibrium of an ocean initially at rest with imposed uniform meridional potential temperature gradients which yield density gradients representative of those observed in the north atlantic the main feature of the adjustment during the early stages the first year is the formation and decay of a subtropical warm core and a subpolar cold core density gyre the gyres are formed by the irreversible winding up of the initially zonal isotherms by first baroclinic mode coastally trapped dissipative kelvin waves this phase and the north south asymmetry arising from the variation in viscous diffusive kelvin wave properties with latitude were discussed in part i of this series within a month effects become significant especially in the evolution of the southern gyre which develops a distinct east west asymmetry through western intensification and long planetary wave propagation of b

new geoelectrical characterisation of a continental collision zone in the west central pyrenees constraints from long period and broadband magnetotellurics abstract continental collision dominates the development of a large number of mountain ranges on earth although the geological evolution of these regions is highly studied lithospheric scale physical processes are less well characterised and their interpretations frequently differ a new magnetotelluric profile crossing the west central pyrenees with broadband and long period magnetotelluric data constrains the geoelectrical features of this continental collision zone confining the geological and physical processes that take place at a lithospheric scale three geoelectrical structures associated with partial melting and its melt migration are imaged for the first time in the west central pyrenees suggesting an iberian subducted lower crust reaching the depth of 70 km this result supported by an analysis of the thermal evolution of the iberian subducted lower crust reinforces the hypothesis of partial melting in this region from the long period magnetotelluric data the asthenosphere has been imaged as a geoelectrical structure with electrical resistivity values between 20 m and 70 m the lithosphere asthenosphere boundary close to the collision zone has been constrained between 90 km and 110 km depth below the iberian plate and between 120 km and 160 km depth below the transition zone between plates and the european plate in agreement with the previous geophysical data additionally other geoelectrical anomalies imaged have been associated with an unexpected major variscan boundary located below the ebro basin silurian sediments and free fluids in the gavarnie thrust and with a thick sequence of jurassic and early cretaceous sediments situated below the north pyrenean thrust sheet

a multidisciplinary geophysical study in the betic chain southern iberia peninsula abstract the combined analysis of magnetotelluric measurements tomographic velocity models and deep seismic reflection images confirms that the betics orogen consists of the juxtaposition of two crustal domains characterized by distinctive physical properties at depth these data sets show evidence for a non coincidence of the petrological and the seismic moho beneath the betics chain the data sets reveal the geophysical properties of the alboran domain internal betics and the iberian massif external betics according to this the iberian crust features a relatively high seismic velocity is seismically transparent in the seismic reflection images and is electrically resistive the alboran domain crust is characterized by a low average velocity displays high reflectivity in the seismic reflection images and is electrically conductive the outcrops of the metamorphic complexes alpujarride and nevado filabride showing relatively high velocities coupled with low v p v s values 1 67 derived from the wadati slopes suggest the existence of rocks rich in silica beneath the alboran domain crust an interpreted detachment at 12 km depth imaged by deep seismic reflection suggests that these rocks could be related to the iberian upper crust partial melts and fluids are proposed to explain the high conductivity observed at deep crustal levels these would account also for the reflectivity and the low v p v s ratios mapped beneath the alboran domain

hydrogeochemistry of the new jersey coastal plain 2 transport and deposition of iron aluminum dissolved organic matter and selected trace elements in stream ground and estuary water the geochemical controls on the transport and deposition of iron organic carbon and certain trace elements have been examined in bogs streams groundwater and estuaries of the new jersey coastal plain surface waters are unusually dilute total dissolved solids 25 ppm and acidic ph 4 5 and contain relatively high concentrations of organic c and fe liquid chromatographic analyses show that from 10 to 70 of total dissolved fe 0 2 m fe oxyhydroxide or oxyhydroxides admixed with insoluble organic material the proportion of organically complexed fe present varies seasonally with the doc content of the water these stream waters are near saturation with respect to amorphous fe oh 3 groundwaters are uniformly undersaturated with respect to amorphous fe oh3 both stream and groundwaters are near saturation with respect to halloysite slightly supersaturated with respect to gibbsite and highly supersaturated with respect to kaolinite bog iron deposits are common in streams and swamps where fe precipitation is catalyzed by iron oxidizing bacteria fe and al precipitate rapidly within both estuaries with ionic strength rather than increased ph exerting the prime control on estuarine deposition estuarine floccules contain very little or no organic matter suggesting that inorganic fe and al oxyhydroxides are the primary precipitates the high contents of dissolved 0 2 m fe and al suggest that both are either organically complexed or in the form of small 0 2 m oxyhydroxide colloids in the estuaries

anomalies of meridional overturning mechanisms in the north atlantic abstract optimal observations are used to investigate the overturning streamfunction in the north atlantic at 30 n and 900 m depth those observations are designed to impact the meridional overturning circulation moc in numerical models maximally when assimilated and therefore establish the most efficient observation network for studying changes in the moc they are also ideally suited for studying the related physical mechanisms in a general circulation model optimal observations are evaluated here in the framework of a global 1 model over a 10 yr period hydrographic observations useful to monitor the moc are primarily located along the western boundary north of 30 n and along the eastern boundary south of 30 n additional locations are in the labrador irminger and iberian seas on time scales of less than a year variations in moc are mainly wind driven and are made up through changes in ekman transport and coastal up and downwelling only a small fraction is buoyancy driven and constitutes a sl

on the influence of dsow in a numerical model of the north atlantic general circulation abstract two numerical experiments regarding the north atlantic circulation are compared the two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland sea one case climatological wind stress sst and surface salinity to force the model it reproduces the atlantic circulation found in similar studies a change in surface boundary values in the iceland sea in order to improve the denmark strait overflow water properties in the second case results in major changes in circulation and hydrography the meridional overturning more than doubles and horizontal gyres are intensified recirculation cells occur north and south of the gulf stream the gulf stream separates from the north american coast in a more realistic manner and its further course is in better agreement with observations than in the reference run the subsurface temperature distribution differs by several kelvin in the subpolar and

interdecadal variability and oceanic thermohaline adjustment changes in the strength of the thermohaline overturning circulation are associated by geostrophy with changes in the east west pressure difference across an ocean basin the tropical polar density contrast and the east west pressure difference are connected by an adjustment process in flat bottomed ocean models the adjustment is associated with viscous baroclinic kelvin wave propagation weak high latitude stratification leads to the adjustment having an interdecadal timescale we reexamine model interdecadal oscillations in the context of the adjustment process for both constant flux and mixed surface boundary conditions under constant surface flux interdecadal oscillations are associated with the passage of a viscous kelvin wave around the model domain we carry out experiments suppressing wave propagation along each of the model boundaries suppressing wave propagation along either the tropical or eastern boundary does not eliminate the oscillation but increases both its period and amplitude suppressing wave propagation along either the polar or the western boundary eliminates the oscillation our results suggest the oscillations can be self sustained by perturbations to the western boundary current arising from the southward boundary wave propagation mixed boundary condition oscillations are characterized by the eastward cross basin movement of salinity dominated density anomalies and the westward return of these anomalies along the northern boundary we suggest the latter is associated with viscous kelvin wave propagation under both types of boundary conditions the strength of the thermohaline overturning and the tropical polar density contrast vary out of phase we show how the phase relationship is related to the boundary wave propagation box models and zonally averaged models assume that the east west and north south pressure gradients vary in phase and are proportional to one another we suggest this assumption is valid only on timescales long compared to the adjustment timescale the importance of boundary regions indicates an urgent need to examine the robustness of interdecadal variability in models as the resolution is increased and as the representation of the coastal shelf slope wave guide is improved

pathways of manganese in an open estuarine system the distribution of mn was examined in the bottom sediments and water column suspended paniculate matter of the laurentian trough gulf of st lawrence a characteristic profile of mn with depth in the sediment consisted of a mn enriched surface oxidized zone less than 20 mm thick and a mn depleted subsurface reducing zone a subsurface mn maximum occurred within the oxidized zone below this maximum the concentration dropped sharply to nearly constant residual levels in the reducing zone the accumulating estuarine sediments are deficient in mn compared to the river input of suspended matter and are definitely not the ultimate sink for manganese manganese escapes from the sediment by diffusion and resuspension forming mn enriched fine grained particles which are flushed out in the estuarine circulation 5 0 109gyr 1 of mn or 50 more than the river input of dissolved mn are exported to the open ocean in spite of the efficient mobilization and export of mn the quantity exported is a small fraction 0 2 of the total flux to the deep sea sediments this is related to the low levels of paniculate matter transported by the st lawrence river the export phenomenon however is probably true of many coastal regions of muddy sediments and thus has interesting implications for the oceanic budget of mn

signal propagation related to the north atlantic overturning 1 changes of the meridional overturning circulation moc due to surface heat flux variability related to the north atlantic oscillation nao are analyzed in various ocean models i e eddying and non eddying cases a prime signature of the forcing is variability of the winter time convection in the labrador sea the associated changes in the strength of the moc near the subpolar front 45 n are closely related to the nao index leading moc anomalies by about 2 3 years in both the eddying and non eddying simulation further south the speed of the meridional signal propagation depends on model resolution with lower resolution non eddying case 4 3 resolution the moc signal propagates equatorward with a mean speed of about 0 6 cm s similar as spreading rates of passive tracer anomalies eddy permitting experiments 1 3 show a significantly faster propagation with speeds corresponding to boundary waves thus leading to an almost in phase variation of the moc transport over the subtropical to subpolar north atlantic

great salinity anomalies in the north atlantic abstract we revisited the great salinity anomaly of the 1970s gsa 70s dickson et al 1988 and documented the newly identified great salinity anomaly of the 1980s hence termed gsa 80s both propagated around the north atlantic in a similar fashion the advective mechanism initially proposed to explain the observed sequence of low salinity low temperature events during the gsa 70s apparently holds also for the gsa 80s the latter was successively observed in the west greenland current 1982 the labrador current 1983 the flemish pass 1984 near the charlie gibbs fracture zone 1984 1985 in the rockall channel 1985 south of iceland 1985 1988 in the north sea 1986 1987 norwegian sea 1987 1988 barents sea 1988 1989 and iceland sea 1989 1990 the advection speed of the gsa 80s seems to be greater than the one of the gsa 70s the 1980s anomaly reached the barents sea 6 to 7 years after peaking in the west greenland current while the 1970s anomaly traveled the same route in 8 to 10 years these anomalies however seem to be of different origin the gsa 70s was apparently boosted remotely by a freshwater sea ice pulse from the arctic via fram strait consequently the gsa 70s was accompanied by a large sea ice extent anomaly in the greenland and iceland seas which propagated into the labrador sea in contrast the gsa 80s was likely formed locally in the labrador sea baffin bay mainly because of the extremely severe winters of the early 1980s but supplemented with a possible contribution of the arctic freshwater outflow via the canadian archipelago facilitated by strong northerly winds which would have enhanced stability and ice formation this anomaly was also associated with a positive sea ice extent anomaly in the labrador sea baffin bay which however had no upstream precursor in the greenland sea thus the gsas are not necessarily caused solely by an increased export of freshwater and sea ice from the arctic via fram strait these results are corroborated by the early 1990s data when a new fresh cold anomaly was formed in the labrador sea and accompanied by a large positive sea ice extent anomaly the harsh winters of the early 1990s were however confined to the labrador sea baffin bay area while the atmospheric and oceanic conditions in the greenland iceland and irminger seas were normal the labrador sea baffin bay area appears therefore to play a key role in formation of gsas as well as in propagation of the gsas formed upstream a likely contribution of the enhanced canadian archipelago freshwater outflow to the gsa formation also seems to be significant two major modes of the gsa origin are thus identified remote generated by an enhanced arctic ocean freshwater export via either fram strait or the canadian archipelago and local resulting from severe winters in the labrador sea baffin bay both modes should be taken into account to model decadal variability of the coupled ocean ice atmosphere system in the north atlantic arctic basin

fe 3 reduction during biotite melting in graphitic metapelites another origin of co 2 in granulites the fe 3 fetot of all fe bearing minerals has been analysed by mossbauer spectroscopy in a suite of biotite rich to biotite free graphitic metapelite xenoliths proxies of an amphibolite granulite transition through progressive biotite melting biotite contains 9 to 16 fe 3 fetot whereas garnet cordierite and ilmenite are virtually fe 3 free 0 1 fe 3 fetot in all samples regardless of biotite presence under relatively reducing conditions graphite bearing assemblages biotite is the only carrier of fe 3 during high temperature metamor phism therefore its disappearance by melting represents an important event of iron reduction during granulite formation because haplogranitic melts usually incorpo rate small amounts of ferric iron iron reduction is accompanied by the oxidation of carbon and the production of co2 according to the redox reaction 2fe2o bt 3 c gr 4feo crd grt ilm opx co fluid melt crd 2 depending on the nature of the peritectic fe mg mineral produced garnet cordierite orthopyroxene the co2 can either be present as a free fluid component or be completely stored within melt and cordierite the oxi dation of graphite by iron reduction can account for the in situ generation of co2 implying a consequential ra ther than causal role of co2 in some granulites and migmatites this genetic model is relevant to graphitic rocks more generally and may explain why co2 is present in some granulites although it is not required for their formation

flow properties in rotating stratified hydraulics abstract this paper discusses three distinct features of rotating stratified hydraulics using a reduced gravity configuration first a new upstream condition is derived corresponding to a wide almost motionless basin and this is applied to flow across a rectangular sill and compared with the case of a zero potential vorticity upstream condition for this geometry it is shown that unidirectional flow permits more water to pass through the sill than bidirectional flow second the general problem is considered of flow from any upstream configuration that passes through sills that vary slowly in depth cross sill and so are effectively many deformation radii wide only two flow configurations permit any realistic amount of flux across the sill either the fluid occupies a narrow region within the sill with a small flux or the fluid occupies a wide region with sluggish geostrophic flow except for at boundary layers at each side in the latter case hydraulic control is not likely to occur the zero

a thermal oscillation under a restorative forcing summary the authors report an interdecadal oscillation in a wind and thermally driven ocean general circulation model ogcm the oscillation is tantalizing in that it occursunder a relatively strong thermal damping 26 3 w m 2k examinations involving a two dimensional ogcm a simple thermal flip flop model and a three dimensional ogcm with and without the nonlinear effect of temperature in the state equation of sea water demonstrate that the oscillation is not driven by mechanisms such as the so called convective oscillator or the advective overshooting oscillator instead the oscillation is associated with the propagation of modelled long rossby waves with westward phase velocity it is found that the north south basin extent of the ogcm is an important factor a larger northsouth basin extent is conducive to the generation of thermal wind flows crucial for initiating the propagation of disturbances the geophysical relevance of these results is discussed

a geophysical model of the variscan orogenic root bohemian massif implications for modern collisional orogens abstract a new model of the structure and composition of the variscan crust in the bohemian massif is proposed based on 3d gravity modelling geological data seismic refraction cel09 and reflection 9hr sections the bohemian massif crust is characterized by a succession of positive and negative anomalies of about 60 80 km wavelength for nearly constant moho depths the south western part of the bohemian massif displays a large negative bouguer anomaly corresponding to high grade rocks granulites and migmatites of the palaeozoic crustal root represented by the moldanubian domain the adjacent neo proterozoic bruno vistulian microcontinent displays an important gravity high reflecting mafic and intermediate medium grade metamorphic and magmatic rocks the deep crustal boundary between the root domain and the bruno vistulian microcontinent is represented by a strong gradient located 50 to 70 km westwards from the surface boundary between these units indicating that the high density basement rocks are covered by a thin sheet of low density granulites and migmatites north west from the moldanubian domain occurs an important gravity high corresponding to the neo proterozoic basement of the tepla barrandian unit limited in the north by southeast dipping reflectors of the tepla suture which is characterized by high density eclogites and ultramafics the footwall of the suture corresponds to low density felsic crust of the saxothuringian basement the reflection and refraction seismics and gravity modelling suggest a complex lithological structure of the moldanubian domain marked by a low density 5 10 km thick lower crustal layer located above the moho a 5 10 km thick heavy mafic layer a 10 km thick mid crustal layer of intermediate density and a locally developed 2 5 km thick low density layer at the surface the low density lower crust correlates well with low p wave velocities in the range 6 0 6 4 km s 1 in the cel09 section this complex geophysical structure and surface geology are interpreted as a result of carboniferous partial overturn of low density lower crust and high and intermediate density crust in the area of central root and by viscous extrusion of low density orogenic lower crust over the high density bruno vistulian continent comparison of these data with geophysical profiling of the andean and the tibetan plateaus suggests that modern orogenic systems reveal comparable deep crustal geophysical pattern based on these similarities we propose that the variscan root represents a deep crustal section of above mentioned plateaus which may have develop by the same orogenic process

global teleconnections of meridional overturning circulation anomalies there is a wide range of evidence from both models and palaeoclimatic data that indicates the possibility of abrupt changes in the oceanic meridional overturning circulation moc however much of our dynamical understanding of the moc comes from steady state models that rely upon the assumption of thermodynamic equilibrium and are therefore only valid on millennial time scales here a dynamical model for the global teleconnections of moc anomalies on annual to multidecadal time scales is developed it is based on a linear theory for the propagation of zonally integrated meridional transport anomalies in a reduced gravity ocean and allows for multiple ocean basins connected by a circumpolar channel to the south the theory demonstrates that the equator acts as a low pass filter to moc anomalies as a consequence moc anomalies on decadal and shorter time scales are confined to the hemispheric basin in which they are generated and have little impact on the remainder of the global ocean the linear theory is compared with the results of a global nonlinear numerical integration which it reproduces to a good approximation

microstructures and composition of melt inclusions in a crustal anatectic environment represented by metapelitic enclaves within el hoyazo dacites se spain abstract this article reports microstructures and compositions of melt inclusions in anatectic metapelites found as enclaves within el hoyazo dacites in the neogene volcanic province of southeastern spain the enclaves represent fragments of continental crust partially melted at 800 850 c and 5 7 kbar and brought to surface rapidly within the host volcanics while they still were in a molten state rapid cooling produced the solidification of silicate melt to glass in the rock matrix and in inclusions within minerals melt inclusions mi are present within nearly all mineral phases in the enclaves and their associated microstructures indicate a primary or pseudo secondary origin the entrapment mechanisms of mi within plagioclase were associated with 1 the presence of micron sized solids mostly graphite on the surface of growing crystals 2 partial resorption of crystals generation of embayments and later growth in the presence of melt and 3 entrapment of melt during crystal growth within crystallographic controlled planar structures e g on crystal faces melt inclusions in garnet are commonly associated with regularly oriented planar discontinuities filled with glass interpreted either as spaces left between adjacent mineral growth spirals or as crystallographic controlled cracks generated in between stages of mineral growth melt was trapped or percolated along these microstructures and later on necking down phenomena individualized inclusions with negative crystal shape melt inclusions in biotite are abundant show negative crystal shape and parallel cleavage planes melt inclusions in cordierite alkali feldspar and ilmenite are isolated sparse and sometimes rather large the composition of glass from all mi is leucogranitic and peraluminous with small differences among glasses in each mineral host there is some variation in glass composition within each textural location as well compositional heterogeneity can be partially explained by a combination of processes such as generation of boundary layers during mineral growth crystallization of daughter phases and hydrogen loss via diffusion through the host mean glass compositions in the several textural locations however can be tentatively interpreted as reflecting the evolution of melt chemistry during the prograde low to medium pressure anatexis of quartz poor metapelites

the influence of partial melting on the electrical behavior of crustal rocks laboratory examinations model calculations and geological interpretations abstract the complex electrical impedance of a granulite has been determined over the frequency range 10 1 to 105 hz at temperatures between 600 and 1200 c at normal pressure and different oxygen fugacities in the stability field of magnetite the impedance spectroscopic is measurements are compared with the results of melting experiments performed under the same experimental conditions and with the same rock sample as the is measurements a strong increase in the electrical conductivity of about one and a half orders of magnitude is observed during partial melting this increase is explained by the formation of an interconnected network of melt the wetting of the grain surfaces starts slightly above the solidus however complete interconnection of the network is detected at higher temperatures 1070 c and hence higher melt fractions 10 vol this is explained by deviations from the equilibrium fabric of the partially molten sample with a modified brick layer model mblm the electrical conductivity is calculated as a function of the observed melt portion this model is valid if the melt forms a complete interconnected network at the grain boundaries the electrical conductivity at different temperatures was modeled using the results of the melting experiments and the mblm melt 10 s m with ea melt 1 ev solid 0 01 s m with ea solid 1 4 ev the results of the laboratory examinations were applied to different crustal regions where partially molten rocks are assumed to exist the same mblm is used to estimate the melt fraction that is necessary to explain the high conducting zones hczs observed in the crust beneath the andean orogen the pyrenees and the tibetan plateau at least 14 4 and 4 vol of interconnected melt are necessary to produce the observed conductivity respectively the actual amount of melt should be higher due to perturbation problems and melt trapped in pockets

signature of the atlantic meridional overturning circulation in sea level along the east coast of north america in this letter we examine the relationship between the north atlantic meridional overturning circulation moc and sea level sl along the east coast of north america in the eddy permitting ocean model occam we find a distinctive topography following pattern of sl variability in the western north atlantic that is closely linked with the changing strength of the moc with a 2 cm drop in sl along the us east coast corresponding to a 1sv increase in the moc we find a similar pattern of sl variability in the altimetry record and show that this meridionally coherent sl mode dominates interannual sl variability at tide gauges along the north american east coast between 40 50n hence we conclude that north american coastal sea level may indeed be a useful indicator of moc variability on interannual timescales allowing an observationally based estimate of the likely range of interannual moc fluctuations to be determined citation bingham r j and c w hughes 2009 signature of the atlantic meridional overturning circulation in sea level along the east coast of north america geophys res lett 36 l02603 doi 10 1029 2008gl036215

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

c o h s fluid composition and oxygen fugacity in graphitic metapelites c 0 h fluid produced by the equilibration of hzo and excess graphite must maintain the atomic h o ratio of water 2 1 this constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric isothermal conditions the o h20 and co fugacities j fhto and fc of such fluids have been estimated from equations of state and fit as a function of pressure and temperature these fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration e g pelitic lithologies because there are no compositional degrees of freedom for graphite saturated fluids produced entirely by dehydration the variance of the dehydration process is not increased in comparison with that in non graphitic systems thus compositional buffering of c 0 h fluids by dehydration equilibria a common petrological model requires that redox reactions decarbonation reactions or external h o 2 fluid sources perturb the evolution of the metamorphic system such perturbations are not likely to be significant in metapelitic environments but their tendency will be to increase the fo of the fluid phase at high metamorphic grades pyrite desulphidation reactions may cause a substantial reduction of fhz0 and slight increases info and fcs relative to sulphur free fluid at low metamorphic grade sulphur solubility in h o s 2 fluids is so low that pyrite decomposition must occur by sulphurconserving reactions that cause iron depletion in silicates a common feature of sulphidic pelites with increasing temperature and sulphur solubility pyrite desulphidation may be driven by dehydration reactions or infiltration of h o rich fluids the absence of magnetite and the assemblages carbonate aluminosilicate or pyrite pyrrhotite ilmenite from most graphitic metapelites is consistent with an h o 2 model for gcoh s fluid for graphitic rocks in which such a model is inapplicable a phase diagram variable that defines the h o ratio of gcoh s fluid is more useful than the conventional

variations in electrical conductivity of rocks above metamorphic conditions abstract this study of the electrical conductivity of metamorphic rocks as a function of temperature highlights the importance of conductivity variations associated with dehydration we observed conductivity changes for anhydrous rocks and hydrous amphibolites up to 1100 k at 0 5 gpa our experimental results revealed that dehydration takes place and that there is a non linear variation above metamorphic temperatures to evaluate the observed changes in conductivity after dehydration numerical calculations were conducted the calculation results showed that the bulk conductivity of rock plus pure water cannot account for the conductivity values of reacted rocks if the crust dehydrates however the data for 0 1 m nacl solutions are consistent with those for amphibolites between 530 and 1100 k in this temperature range the estimated volume fractions of dewatered saline solutions vary between 0 04 and 0 36 vol these results show that although dehydration is insignificant and only small amounts of high salinity fluids are formed in the crust the total conductivity of amphibolites is high after dehydration

groundwaters of florence italy trace element distribution and vulnerability of the aquifers geochemical and hydrogeological research has been carried out on 109 wells in the alluvial plain of florence in order to evaluate conductivity and main chemistry of ground waters the pattern of some possible pollutant chemical species fe mn cr cu pb zn no2 no3 and the vulnerability of the aquifers the plain is made up of plio quaternary alluvial and lacustrine sediments for a maximum thickness of 600 m silts and clays sometimes with lenses of sandy gravels are dominant while considerable deposits of sands pebbles and gravels occur along the course of the arno river and its tributary streams and represent the most important aquifer of the plain the groundwaters analyzed belong to this aquifer or to the smaller ones hosted in the gravel lenses most waters show conductivity values around 1000 1200 s and almost all of them have an alkaline earth bicarbonate chemical character these features are consistent with the mainly calcareous lithology of the aquifers in the western areas a higher salt content of the groundwaters is evident probably related to the presence of industrial activities which use water desalinators heavy metal and no2 no3 analyses point out that no important pollution phenomena affect the groundwaters all the mean values of the chemical considered species are below the maximum admissible concentration mac fixed by the european community for drinkable waters nevertheless some anomalies of no2 no3 fe mn and zn are present in the plain apart from mn which seems to be released by certain calcareous gravels the other anomalies have a local influence since they disappear even in the nearest wells the most plausible causes can be recognized in losses of the sewage system no2 3 4 mg t use of nitrate compounds in agriculture no3 60 70 mg l oxidation of well pipes fe 20 mg l zn 6 mg l as regards cr cu and pb all the observations are below the mac therefore the median values of 3 3 9 and 1 1 g l respectively could be considered reference concentrations for groundwaters circulating in calcareous lithotypes under undisturbed natural conditions finally a map of vulnerability related to the most superficial and important aquifer has been elaborated on the basis of thickness and permeability of the covers the map shows that the areas near the arno river are highly vulnerable for the minimum thickness or lacking of sediments covering the aquifer on the other hand in the case of pollution several factors not considered in the map could significantly increase the self purification capacity of the aquifer such as the dilution of groundwaters caused by the feeding of the rivers the bacteria oxidation of nitrogenous species and the sorption capacity of clay minerals and organic matter with respect to trace metals

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

identification of the mechanism of interdecadal variability in the north atlantic ocean the aim of this paper is to identify the physical mechanism of interdecadal variability in simulations of the north atlantic ocean circulation with the modular ocean model of the geophysical fluid dynamics laboratory to that end a hierarchy of increasingly complex model configurations is used the variability in the simplest case that of viscous purely thermally driven flows in a flat bottom ocean basin with a box shaped geometry is shown to be caused by an internal interdecadal mode the westward propagation of temperature anomalies and the phase difference between the anomalous zonal and meridional overturning that characterize the interdecadal mode are used as fingerprints of the physical mechanism of the variability in this way the variability can be followed toward a less viscous regime in which the effects of continental geometry and bottom topography are also included it is shown that although quantitative aspects of the variability like period and spatial pattern are changing the physical mechanism of the interdecadal variability in the more complex simulations can be attributed to the same processes as in the simplest model configuration

a two level wind and buoyancy driven thermocline model abstract a simple two level model is designed to simulate the thermocline equations applicable for large scale steady oceanic flow the model serves two functions first it replaces problems with the interpretation of slablike dynamics e g luyten et al 1983 by using continuously horizontally varying buoyancy but at the cost of reducing the vertical resolution drastically the equations used are geostrophy plus a small linear drag to close a stommel like western boundary layer mass conservation and buoyancy conservation with a small but necessary horizontal diffusion inclusion of vertical diffusion has little effect the ocean is driven by an ekman layer whose functions are to provide a given surface input of mass through ekman pumping and buoyancy through a specified buoyancy in the ekman layer i e to maintain the same boundary conditions as in classical thermocline studies sidewall conditions are not well understood and are almost certainly over specified in this formulation s

evidence for present day leucogranite pluton growth in tibet several geophysical surveys carried out in southern tibet have revealed the occurrences of bright spots of high electrical conductivity at 15 20 km depth corresponding to zones of seismic attenuation and correlated to high heat flow at the surface such zones have been variably interpreted as intruding magmas partially melted middle crust or concentrations of free fluids here we show that experimentally derived electrical conductivities of hydrous granite melts under pressure temperature h2o conditions appropriate for crust derived magmas are in perfect agreement with those inferred from magnetotelluric data strongly suggesting that the tibetan bright spots image present day pluton assembly in the middle crust in a manner analogous to what happened during the miocene to form the leucogranite plutons now exposed southward in the high himalaya range

on the robustness of the interdecadal modes of the thermohaline circulation ocean models in box geometry forced by constant surface fluxes of density have been found to spontaneously generate interdecadal oscillations of the thermohaline circulation this paper analyzes the sensitivity of these oscillations to various physical effects including the presence of mesoscale turbulence various thermal surface boundary conditions and the presence of wind forcing or bottom topography the role of unstable long baroclinic waves is also reexamined in an attempt to understand the oscillation period in idealized geometry it is found that the low frequency variability of the thermohaline circulation under quasi constant surface fluxes is a robust feature of the large scale circulation it is not strongly affected by energetic mesoscale turbulence the oscillation period is relatively invariant with respect to varying resolution and momentum and tracer horizontal mixing coefficients although it loses some regularity as shorter and longer periods of variability emerge when the mesoscale activity increases in strength with smaller mixing coefficients the oscillations are also retained as the ocean model is coupled to an interactive atmospheric energy balance model the thermohaline modes are robust to a range of exchange coefficients that widens with the amplitude of the mean circulation the presence of an additional wind forced component generally weakens the oscillation and depending on the relative strength of thermodynamic and dynamic forcings the oscillation may be completely killed a simple interpretation is given highlighting the role of upward ekman pumping in damping density anomalies finally the interaction of these baroclinic modes with bottom topography depends strongly on the relative directions of the mean topographic features and the mean currents and baroclinic waves but usually results in a damping influence

response of circulation and heat transport in the north atlantic to changes in thermohaline forcing in northern latitudes a model study to avoid an explicit simulation of the overflows across the greenland scotland ridge many models of the large scale ocean circulation seek to include the net effect of the inflowing dense water masses by restoring temperature and salinity near the ridge to observed conditions in this paper the authors examine the effect of different datasets for the northern restoring condition in two versions eddy resolving and non eddy resolving of the model of the north and equatorial atlantic that has been developed in recent years as a community modeling effort for woce it is shown that the use of smoothed climatological fields of temperature and salinity south of the denmark strait leads to strong deficiencies in the simulation of the deep flow field in the basin a switch to actual hydrographic data from the denmark strait ignites a rapid dynamic response throughout the north atlantic affecting the transport and vertical structure of the deep western boundary current and by virtue of the jebar efffect the transport of the horizontal gyres meridional overturning and northward heat transport too weak in the cases with climatological boundary conditions increase to more realistic levels in the subtropical north atlantic the initial response to switches in the high latitude thermohaline forcing is mediated by fast waves along the westurn boundary leading to changes in the deep western boundary current in low latitudes after about two years in the non eddy resolving cast the initial timescale depends on the horizontal grid spacing of the model in the high resolution case the first signal reaches the equator in a few months the adjustment to a new dynamic quasi equilibrium involves kelvin waves along the equator and rossby wave in the interior and is attained in less than two decades throughout the north atlantic it is suggested that these fast dynamic adjustment processes could play an important role in possible fluctuations of the thermohaline circulation or transitions between different equilibrium states of the coupled ocean atmosphere system and may have determined the timescale of the observed climatic transitions before and during the last deglaciation