stability limits of hydrous minerals in sediment and mid ocean ridge basalt compositions implications for water transport in subduction zones phase relationships in hydrated natural sediment and mid ocean ridge basalt morb compositions were investigated experimentally from 6 to 15 gpa and 700 c to 1400 c in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle phengite and hydrous aluminum silicates topaz oh and phase egg are stable in the sediment composition at depths greater than 150 km and constitute h2o reservoirs similarly lawsonite is an h2o reservoir in morb composition at depths greater than 150 km according to mass balance calculations based on experimental run products the weight proportions of phengite and h2o at 6 gpa in the sediment are about 50 and 2 respectively at pressures from 8 to 12 gpa topaz oh al2sio4 oh 2 appears as the hydrous mineral and the sediment retains about 0 5 0 7 wt h2o at higher pressures phase egg alsio3 oh appears as the hydrous phase with a large stability field the h2o content in the sediment is about 0 5 wt at pressures greater than 12 gpa in natural morb compositions lawsonite is the hydrous phase at pressures lower than 10 gpa about 1 wt h2o in the morb composition is retained by lawsonite at 6 gpa and 800 c above 6 gpa a lawsonite out reaction is controlled by compositional changes of garnet and clinopyroxene if the oceanic crust descends with an overlying sediment layer water can be carried down into the transition zone by the sediment the amount of transported water is controlled by the chemical compositions of the subducting sediments water in the morb layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle causing the generation of a peridotite region including hydrous phases chondrodite clinohumite phase a and phase e if this hydrous peridotite is dragged down by the descending oceanic crust water can be transported into the transition zone by the subduction process the amount of transported water is controlled by the thermal state of the slab and the chemical composition of morb

high temperature stability limit of phase egg alsio3 oh the stability relations of phase egg alsio3 oh have been investigated at pressures from 7 to 20 gpa and temperatures from 900 to 1700 c in a multi anvil apparatus at the lower pressures phase egg breaks down according to the univariant reaction phase egg stishovite topaz oh which extends from 1100 c at 11 gpa to 1400 c at 13 gpa where it terminates at an invariant point involving corundum at pressures above the invariant point the stability of phase egg is limited by the breakdown reaction phase egg stishovite corundum fluid which extends from the invariant point to 1700 c at 20 gpa stishovite crystallized in the al2o3 sio2 h2o system contains al2o3 and the amount of al2o3 increases with increasing temperature it is inferred that the al2o3 content is controlled by the charge balanced substitution of si4 by al3 and h aluminum bearing stishovite coexisting with an h2o rich fluid may contain a certain amount of water therefore phase egg and stishovite in a subducting slab could transport some h2o into the deep earth

cloud layer overlap characteristics derived from long term cloud radar data assumptions regarding the vertical overlap characteristics of horizontal cloudy layers have been shown to be important to both the radiation transfer and the cloud microphysics that are predicted in general circulation models certain reasonable assumptions regarding cloud layer overlap have been applied in models up to now where vertically continuous cloudy layers were assumed to overlap maximally leading to a minimum in cloud cover and layers separated by noncloudy layers were assumed to overlap randomly however these assumptions have not been systematically evaluated with a comprehensive dataset that can resolve simultaneously occurring cloud layers presented here is an analysis of cloud layer overlap characteristics derived from 103 months of cloud radar data collected by continuously operating millimeter wavelength instruments deployed at the atmospheric radiation measurement arm sites in the tropics middle latitudes and the arctic using an approach recently proposed by hogan and illingworth it is shown that an assumption of random overlap for layers separated by noncloudy layers is supported by observations however that the overlap characteristics of vertically continuous layers cannot be considered maximal is also shown indeed vertically continuous cloudy layers do not appear to lend themselves to a simple overlap assumption therefore to avoid significant biases in diagnosed cloud cover the overlap properties of these layers in models will need to be parameterized it is shown that the cloud layer overlap characteristics in the middle latitudes do appear to be a strong function of season suggesting that an overlap parameterization in terms of cloud system type may be possible

intensive shipboard observations of the flow through the strait of gibraltar during the european project canary island azores gibraltar experiment canigo intensive shipboard observations were carried out in april 1996 and october 1997 in order to observe the spatial and temporal variability of the flow and of the water mass structure in the strait of gibraltar at the sill and the eastern and western entrances to the strait repeated cross strait sections and station time series of the flow and of t s profiles were measured using vessel mounted and lowered acoustic doppler current profilers adcp and conductivity temperature depth probes ctd expendable bathythermographs xbt yielding new views of the rapid changes over tidal cycles and of approximate tidal means it is argued that transport observations might be easier to carry out away from the sill in the eastern part of the strait even though maximum resolved shears were comparable in both places 0 03 0 04 s 1 in the vertical and 0 001 0 016 s 1 in the horizontal in the east coordinated changes in the stratification and the flow field are documented via four time series over m 2 tidal cycles showing a sharpening diffusing of the vertical gradients in the water masses and the flow maximum shear and maximum water mass gradients do not always coincide there and both are much shallower 50 80 m than the delimiter between inflow and outflow 120 150 m the mean salinity of the outflow core decreases from 38 43 in the east to 38 17 west of the sills as a result of the mixing processes the internal bore was followed and directly observed with rapid ctd yoyo stations and with xbt vessel mounted adcp measurements it generates extreme changes in currents and shears on timescales of minutes with directly measured vertical velocities reaching 50 cm s 1 patches of density inversions were observed as the bore passed by consistent with active turbulent mixing along the interface the time series of flow and ctd measurements allow the direct calculation of froude numbers at various locations and over tidal cycles these and along strait sections suggest that the exchange through the strait is maximal in april 1996 and submaximal in october 1997 supporting the expectations of garrett et al 1990

high pressure behavior of kyanite decomposition of kyanite into stishovite and corundum the pressure stability of kyanite was experimentally reversed with the use of a multi anvil apparatus kyanite was found to decompose into its oxides stishovite and corundum between 14 or 0 5 gpa at 1000 degrees c and 17 5 or 1 0 gpa at 2000 degrees c reliable thermodynamic calculations can be performed to temperatures of approximately 1500 degrees c up to this temperature the location of the equilibrium kyanite corundum stishovite determined in this study constrains the equilibrium coesite stishovite a set of thermodynamic data was calculated by linear programming from the kyanite breakdown reaction and the coesite stishovite equilibrium feasible values for the fitted thermo dynamic properties are 28 5 to 26 3 mpa k for the temperature derivative of the bulk modulus dk dt p of kyanite 815254 to 813635 j mol for g 0 sub 1298 of stishovite and 24 6 to 26 3 j mol k for s 0 sub 1298 of stishovite the experimental results indicate 1 that in peraluminous eclogites of basaltic or sedimentary origin stishovite may coexist stably with corundum at a depth greater than 420 450 km and 2 that in an inhomogeneous al enriched mantle corundum could be a minor constituent in the lower mantle

a near uniform basin wide sea level fluctuation of the mediterranean sea abstract a new basin wide oscillation of the mediterranean sea is identified and analyzed using sea level observations from the ocean topography experiment topex poseidon satellite altimeter and a numerical ocean circulation model more than 50 of the large scale nontidal and non pressure driven variance of sea level can be attributed to this oscillation which is nearly uniform in phase and amplitude across the entire basin the oscillation has periods ranging from 10 days to several years and has a magnitude as large as 10 cm the model suggests that the fluctuations are driven by winds at the strait of gibraltar and its neighboring region including the alboran sea and a part of the atlantic ocean immediately to the west of the strait winds in this region force a net mass flux through the strait of gibraltar to which the mediterranean sea adjusts almost uniformly across its entire basin with depth independent pressure perturbations the wind driven response can be explained in part by wind setup

high pressure phase relations and thermodynamic properties of caal4si2o11 cas phase abstract phase relations in caal4si2o11 were examined at 12 23 gpa and 1000 1800 c by multianvil experiments a three phase mixture of grossular kyanite and corundum is stable below about 13 gpa at 1000 1800 c at higher pressure and at temperature below about 1200 c a mixture of grossular stishovite and corundum is stable indicating the decomposition of kyanite above about 1200 c caal4si2o11 cas phase is stable at pressure higher than about 13 gpa the triple point is placed at 14 7 gpa and 1280 c the equilibrium boundary of formation of cas phase from the mixture of grossular kyanite and corundum has a small negative slope and that from the mixture of grossular stishovite and corundum has a strongly negative slope while the decomposition boundary of kyanite has a small positive slope enthalpies of the transitions were measured by high temperature drop solution calorimetry the enthalpy of formation of caal4si2o11 cas phase from the mixture of grossular kyanite and corundum was 139 5 15 6 kj mol and that from the mixture of grossular stishovite and corundum was 94 2 15 4 kj mol the transition boundaries calculated using the measured enthalpy data were consistent with those determined by the high pressure experiments the boundaries in this study are placed about 3 gpa higher in pressure and about 200 c lower in temperature than those by zhai and ito zhai s ito e 2008 phase relations of caal4si2o11 at high pressure and high temperature with implications for subducted continental crust into the deep mantle phys earth planet inter 167 161 167 combining the thermodynamic data measured in this study with those in the literature dissociation boundary of cas phase into a mixture of ca perovskite corundum and stishovite and that of grossular into ca perovskite plus corundum were calculated to further constrain the stability field of cas phase the result suggests that the stability of cas phase would be limited at the bottom of transition zone and top of the lower mantle when sediments are subducted into the deep mantle it is also suggested that cas phase may be stable at the depth of the upper part of the lower mantle when partial melting of basalt occurs at the depth

mineral inclusions in diamonds associations and chemical distinctions around the 670 km discontinuity three new mineral associations have been discovered within diamonds from the juina district of brazil these include a previously unrecorded na al mg fe sio3 phase associated with ferropericlase and the tetragonal almandine pyrope phase tapp also reported are an association of corundum with aluminous pyroxene and an olivine composition phase associated with ferropericlase the minerals in each association often occurred within the same diamonds in addition to being recovered from individual diamonds in different combinations high pressure experimental data indicate that these associations formed at different depths within a region ca 60 km on either side of the upper mantle lower mantle boundary mineral compositions show that for the regions sampled the deep transition zone and lower mantle are chemically distinct and inhomogeneous importantly in the shallow lower mantle al is not solely accommodated within perovskite structured mg fe sio3 as some recent experimental studies have suggested

integral measurements of mass transport and heat content in the strait of gibraltar from acoustic transmissions three view the mathml sourceacoustic transceivers were deployed on either side of the eastern entrance of the strait of gibraltar during april may 1996 to determine the feasibility of using acoustic methods to make routine rapidly repeated horizontally integrated measurements of flow and temperature in straits reciprocal transmissions between the transceivers were used to test the feasibility of using traditional ray differential travel times to monitor the component of flow along the acoustic paths transmissions directly across the strait were used to test the feasibility of using horizontal arrival angle fluctuations and acoustic intensity scintillations to monitor the flow perpendicular to the acoustic path the geometry was selected to provide ray paths that only sample the lower layer mediterranean water so that the feasibility of monitoring the mediterranean outflow using the various methods could be evaluated the acoustic scintillation method did not yield useful current estimates but the experimental parameters were not optimized for this approach since the low frequency variability in log amplitude was found to be highly correlated at receivers view the mathml source apart it is possible that acoustic scintillation measurements using different receiver spacings and more rapid sampling might yield better results the horizontal deflection method gave encouraging results at the time of neap tides but less so during spring tides for this approach both theoretical estimates and measured phase differences between the horizontally separated receivers suggest that internal wave induced horizontal arrival angle fluctuations may fundamentally limit the precision with which arrival angles can be measured further work is needed to determine if a smaller horizontal spacing and higher signal to noise ratios would yield better results reciprocal travel time measurements diagonally across the strait performed the best of the three methods giving absolute flow estimates consistent with those derived from current meter data the fractional uncertainty variance for the lower layer tidal transport from a single tomographic path was estimated to be 0 017 i e 98 of the a priori tidal transport variance was resolved the spatial scales of the sub tidal flow are thought to be significantly shorter than those of the tidal flow however which means that a more elaborate monitoring network is required to achieve the same performance for sub tidal variability finally sum travel times from the reciprocal transmissions were found to provide good measurements of the temperature and heatcontent in the lower layer

cloud overlap statistics the u s air force three dimensional nephanalysis 3dneph has been employed to analyze the vertical distribution of clouds and cloud overlap statistics during january 1979 over the north atlantic ocean 40 to 60 n the 3dneph integrates both satellite and conventional observations and gives what is probably the best available information on the vertical distribution of clouds this region was chosen because of the predominance of layered clouds and the high density of conventional observations the grid size of the 3dneph is 45 km and the data set was additionally averaged over three different grid sizes 90 221 and 442 km three cloud overlap assumptions that have been commonly used were tested namely the maximum minimum and random overlap assumptions for adjacent layers of the 3dneph that contained cloud the maximum overlap assumption performed the best accurately determining the total cloud cover to within the roundoff error for the layered cloud amounts in 82 of the cases for two or three cloud layers that were separated by clear interstices the random overlap assumption performed the best overall for all resolutions although there were systematic biases in the predicted cloud fraction which depend on the cloud fraction of the layers and on grid size for grid sizes 9o km random overlap resulted in a systematic underestimation of total cloud cover 5 random overlap performed worst when there were two layers with intermediate cloud fractions 30 70 the results of this study indicate that while random overlap performs reasonably well on average the systematic bias which depends on grid resolution and random discrepancies could result in significant errors when this approximation is used in general circulation modeling and cloud climatologies

corundum inclusions in diamonds discriminatory criteria and a corundum compositional dataset abstract mineral inclusions of corundum are reported from diamonds from alluvial deposits of tributaries of the rio aripuana juina brazil we present the first recorded occurrence of sapphire as an inclusion in diamond and expand on the database of ruby and white corundum inclusions ruby inclusions are found to occur both as isolated and touching grains with aluminous pyroxene and associated with ferropericlase mineral chemistry and phase relations place the origin of such ruby bearing diamonds within the lower mantle at 770 km mineral associations indaving other corundum inclusions were not observed hence their depth of origin is less certain compositions of corundum samples were characterised by electron and ion microprobe given the scarcity of literature data corundum samples from a variety of other geological settings were also analysed samples comprised corundums associated with granitic emplacement metasomatism amphibolite facies and granulite facies rocks gem and industrial synthetic origins and carmine coloured corundums recovered from kimberlite drill cores in addition to variable amounts of cr fe ti mg and si measurable quantities of other transition elements and high field strength elements were also detected corundums from similar geological settings show very similar compositions and are easily distinguishable from other settings irrespective of locality rubies from norwegian tanzanian and kenyan amphibolite facies rocks are compositionally indistinguishable additionally corundums from metasomatised zones associated with contact metamorphism from arizona and japan were very similar particularly characterised by unusually high abundance of mobile zr and nb tens of ppm all juina inclusions are particularly distinguishable from other corundums by high concentrations of ni 18 171 ppm weight typically at least an order of magnitude enriched over the same corundum varietal types from elsewhere furthermore the sapphire inclusion exhibited much larger ratios of ga and ge to hfse elements compared to otherwise similar samples and ruby inclusions are distinguished by high mg fe ratios 0 27 1 56 by weight compositional differences between inclusions in diamonds and corundums from other settings in addition to corundum s physical and chemical durability suggest that with the employment of rapid identification tools such as energy dispersive spectrometry eds and laser icpms corundum has promise as an indicator of diamond prospectivity

density of morb eclogite in the upper mantle in order to study the role of eclogite subducted oceanic crust in geodynamics we carried out high pressure experiments on natural mid oceanic ridge basalt morb using a multi anvil apparatus we determined phase relations of morb as a function of pressure and temperature in the temperature range of 1200 1700 c and pressures from 5 to 14 gpa the present results differ from a previous high pressure study of morb earth planet sci lett 77 1986 245 conducted at 1200 c as follows 1 coesite coe or stishovite st appears throughout the studied pressure range and 2 the clinopyroxene garnet cpx gar ratio is smaller particularly in the low pressure range 12 gpa our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 gpa due to the coesite stishovite phase transition the density is considerably higher than estimates based on the previous experiments at pressures less than 12 gpa thus the role of eclogite in driving subduction should be more important than previously thought on the other hand the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases a result of the increase in clinopyroxene garnet sio2 ratio in eclogite with increasing temperature as eclogite is still denser than peridotite even under subsolidus conditions recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes

validation and sensitivities of frontal clouds simulated by the ecmwf model abstract clouds simulated by the european centre for medium range weather forecasts ecmwf model are composited to derive the typical organization of clouds surrounding a midlatitude baroclinic system comparison of this composite of about 200 cyclones with that based on satellite data reveals that the ecmwf model quite accurately simulates the general positioning of clouds relative to a low pressure center however the optical depths of the model s high low clouds are too small large relative to the satellite observations and the model lacks the midlevel topped clouds observed to the west of the surface cold front sensitivity studies with the ecmwf model reveal that the error in high cloud optical depths is more sensitive to the assumptions applied to the ice microphysics than to the inclusion of cloud advection or a change of horizontal resolution from 0 5625 to 1 69 lat this reflects the fact that in the ecmwf model gravitational settling is the most rapid process controlling the abundance of ic

a speculation on the structure of the d layer the growth of anti crust at the core mantle boundary through the subduction history of the earth abstract the growth curve of the continental crust shows that large amounts of continental crust formed in the early part of the earth history are missing in order to test a hypothesis that the former crust was subducted to the deep mantle we performed phase assemblage analysis in the systems of mid oceanic ridge basalt morb anorthosite and tonalite trondhjemite granite ttg down to the core mantle boundary cmb conditions results show that all these materials can be subducted to the cmb leading to the development of a compositional layering in the d layer we speculate that there could be five layers of feo enriched melt from partial melting of morb morb crust anorthosite ttg and slab or mantle peridotite in ascending order although the polymorphic transformation of perovskite to post perovskite in mg fe sio3 may explain the seismic discontinuity at the top of the d layer d discontinuity the effects of solid solution on the sharpness of the transformation suggest that the compositional layering is more plausible for the origin of the d discontinuity the d layer can be an anti crust made up mostly of ttg anorthosite derived from the former continental crust tectonic style of the anti crust at the cmb is similar to that at the surface at both places chemically distinct layers are density stratified and are also characterized by the processes of accretion magmatism and metasomatism

high pbehavior of anorthite composition and some phase relations of the cao al2o3 sio2 system to the lower mantle of the earth and their geophysical implications 1 multianvil experiments with long experimental durations have been made with the anorthite composition caal2si2o8at pressure temperature p t conditions of 14 25 gpa and 1400 2400 c at subsolidus conditions these experiments demonstrated three phase assemblages grossular gr kyanite ky stishovite st at 14 gpa gr calcium alumino silicate phase cas st at 18 gpa and cas casio3 perovskite capv st at above 20 gpa which are related by the reactions gr ky cas st and gr st cas capv following the method of schreinemakers we combined our data with the literature data to deduce ap tphase diagram for a portion of the cao al2o3 sio2system at subsolidus conditions which subsequently helped to solve some long lasting discrepancies in the high pbehavior of the compositions of anorthite and grossular the crystal chemistry of the cas and capv solid solutions was examined and new substitution mechanisms were firmly established along the solidus the melting reaction at 14 gpa is peritectic while that at 22 gpa is eutectic for both pressures st is the first phase to melt out and the melt is generally andesitic for the an composition its density starts to be significantly higher than the density of pyrolite at 2 5 gpa a much lower pressure than that for the or ab or qtz composition 7 5 10 gpa so that the an enriched continental crust material should readily plunge into the upper mantle

shock induced formation of kyanite al2sio5 from sillimanite within a dense metamorphic rock from the ries crater germany a dense 3 34 g cm 3 garnet sillimanite rich metamorphic rock from the suevite breccia of the ries impact crater was studied by scanning electron microscopy and raman microprobe spectroscopy in the strongly shocked rock clast kyanite was formed from sillimanite under momentary high pressures of natural shock waves kyanite aggregates were found as thin 0 3 2 0 m seams along grain boundaries between and fractures within sillimanite grains within these seams kyanite c axes are oriented perpendicular to original grain boundaries and fractures in addition larger up to 10 m isolated kyanite grains were rarely found within host sillimanite filamentary kyanite aggregates and isolated crystals typically show shrinkage cracks due to volume decrease 10 locally broad interstices between sillimanite crystals are filled with aluminosilicate glass containing a high volume fraction of sub micrometer sized euhedral crystals the silica rich glass suggests incongruent melting of sillimanite at local post shock temperatures significantly higher than 1 300 c the edges of adjacent sillimanite grains are thermally and chemically altered the local generation of temperature spikes is attributed to strong shock wave interactions due to very high shock impedance contrasts

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

hydraulic criticality of the exchange flow through the strait of gibraltar abstract the hydraulic state of the exchange circulation through the strait of gibraltar is defined using a recently developed critical condition that accounts for cross channel variations in layer thickness and velocity applied to the output of a high resolution three dimensional numerical model simulating the tidal exchange the numerical model uses a coastal following curvilinear orthogonal grid which includes in addition to the strait of gibraltar the gulf of cadiz and the alboran sea the model is forced at the open boundaries through the specification of the surface tidal elevation that is characterized by the two principal semidiurnal and two diurnal harmonics m2 s2 o1 and k1 the simulation covers an entire tropical month the hydraulic analysis is carried out approximating the continuous vertical stratification first as a two layer system and then as a three layer system in the latter the transition zone generated by entrainment and mixing between the atlantic and mediterranean flows

subduction of continental crust and terrigenous and pelagic sediments an experimental study abstract an important role for the recycling of terrigenous sediments in the mantle was proposed by armstrong 1 2 aspects of this hypothesis have been tested via an experimental study of phase equilibria in a composition similar to that of average upper continental crust over the pressure interval 6 24 gpa this composition is sufficiently close to those of the principal argillaceous and siliceous classes of pelagic sediments to use the results to interpret the behaviour of these latter sediments in addition to terrigenous sediments during subduction the subsolidus phase relationships displayed by lithologies derived from these materials are complex with major roles being played by k hollandite garnet na clinopyroxene and stishovite continentally derived lithologies achieve densities similar to or greater than surrounding mantle at depths greater than 200 km the geodynamical implications of these buoyancy relationships are discussed a reconnaissance study has also been made to assess the melting and element partitioning behaviours in the subducted continental crust lithologies during partial melting at relatively low pressures 5 10 gpa orthoclase wadeite and k hollandite are eliminated near the solidus whereas the stability field of na clinopyroxene extends to temperatures well above the solidus liquids formed by small to moderate degrees of partial melting of subducted continentally derived lithologies in this pressure interval have high k na ratios and high sio2 contents at higher pressures 16 24 gpa the stability fields of k hollandite and stishovite extend towards the liquidus whereas sodium bearing phases are eliminated much closer to the solidus resultant partial melts consequently possess relatively low k na ratios and sio2 contents lead possesses a high crystal liquid partition coefficient in k hollandite whereas uranium is excluded the broad stability field of k hollandite during partial melting of these lithologies at pressures above 15 gpa therefore has the capacity to cause fractionation of lead from uranium the petrological and geochemical implications of these results are discussed in the context of the partial melting of terrigenous materials during subduction and the interactions of the partial melts with the surrounding mantle

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

mediterranean outflow mixing and dynamics the mediterranean sea produces a salty dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern gulf of cadiz the current accelerates to 1 3 meters per second which raises the internal froude number above 1 and is intensely turbulent through its full thickness the outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline north atlantic central water within 100 kilometers downstream the current is turned by the coriolis force until it flows nearly parallel to topography in a damped geostrophic balance the mixed mediterranean outflow continues westward slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass

effect of improving representation of horizontal and vertical cloud structure on the earth s global radiation budget part i review and parametrization a poor representation of cloud structure in a general circulation model gcm is widely recognised as a potential source of error in the radiation budget here we develop a new way of representing both horizontal and vertical cloud structure in a radiation scheme this combines the tripleclouds parametrization which introduces inhomogeneity by using two cloudy regions in each layer as opposed to one each with different water content values with exponential random overlap in which clouds in adjacent layers are not overlapped maximally but according to a vertical decorrelation scale this paper part i of two aims to parametrize the two effects such that they can be used in a gcm to achieve this we first review a number of studies for a globally applicable value of fractional standard deviation of water content for use in tripleclouds we obtain a value of 0 75 0 18 from a variety of different types of observations with no apparent dependence on cloud type or gridbox size then through a second short review we create a parametrization of decorrelation scale for use in exponential random overlap which varies the scale linearly with latitude from 2 9 km at the equator to 0 4 km at the poles when applied to radar data both components are found to have radiative impacts capable of offsetting biases caused by cloud misrepresentation part ii of this paper implements tripleclouds and exponential random overlap into a radiation code and examines both their individual and combined impacts on the global radiation budget using re analysis data copyright c

phase transition of zircon at high p t conditions in situ observations of the zircon reidite transition in zrsio4 were carried out using a multianvil high pressure apparatus and synchrotron radiation the phase boundary between zircon and reidite was determined to be p gpa 8 5 0 0017 t 1200 k for temperatures between 1100 1900 k when subducted slabs including igneous rocks and sediments descend into the upper mantle the zircon in the subducted slab transforms into reidite at pressures of about 9 gpa corresponding to a depth of 270 km reidite found in an upper eocene impact ejecta layer in marine sediments is thought to have been transformed from zircon by a shock event the peak pressure generated by the shock event in this occurrence is estimated to be higher than 8 gpa

in situ observation of the decomposition of kyanite at high pressures and high temperatures in situ observations of the decomposition of kyanite al 2 sio 5 were carried out in a multi anvil high pressure apparatus using synchrotron radiation where the phase change from kyanite to stishovite corundum was observed at high pressures and high temperatures the phase boundary of this decomposition at t 1200 1900 k and p 5 15 gpa was determined to be p gpa 10 2 0 0016 t k previous studies using the quench method showed a discrepancy in the transition pressure of the decomposition our results using in situ observations resolve this dispute

maximal two layer exchange over a sill and through the combination of a sill and contraction with barotropic flow the analysis of two layer exchange flow through contractions with a barotropic component treated by armi farmer 1986 is extended to include exchange flows over sills and through a combination of a sill and contraction it is shown that exchange over a sill is fundamentally different from exchange through a contraction control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer this asymmetry in the control results in asymmetrical flows the interface depth above the crest is not one half the total depth as assumed in other studies by analogy with flow through contractions but is somewhat deeper the maximal exchange rate is less than for flow through a contraction of equal depth when both a sill and a contraction are present the contraction influences control at the sill crest only if it lies between the sill and the source of denser water the response to barotropic flow is also asymmetrical the transition to single layer flow occurs at much lower speeds for a barotropic component in one direction than the other results of the analysis are applied to exchange flow through the strait of gibraltar which includes both a sill and a contraction it is shown that maximal exchange conditions apply throughout part of the tidal cycle and observations illustrate several of the analytical predictions for barotropic flows including the formation of fronts single layer flow submaximal exchange and reverse flow

transport estimates in the strait of gibraltar with a tidal inverse model to estimate the volume transport through the strait of gibraltar and to study the spatial structure of the time variable flow a varying number of current meter moorings were maintained at the eastern entrance of the strait between october 1994 and april 1998 and was complemented with intensive shipboard measurements during the european union project canary island azores gibraltar experiment canigo a tidal inverse model is used to merge these data sets in order to investigate the flow at the eastern entrance of the strait the two dimensional structure of the tidal flow was described by simple analytical functions harmonics with the seven most important tidal frequencies were used as temporal functions with this model the tidal currents can be predicted for any time and location at the eastern entrance of the strait and more than 92 of the variance of the lower layer flow is explained it was used to remove the tidal currents from the individual measurements and to calculate the mean flow through the strait from the residuals combined with a similar inverse model for determining the depth of the interface between mediterranean and atlantic water the volume transport was estimated to be 0 81 0 07 sv for the upper layer and 0 76 0 07 sv for the lower layer the correlation of the tidal currents and the fluctuations of the interface accounts for 7 of the transport at the eastern entrance

deriving cloud overlap statistics from radar the predictions of general circulation models gcms are sensitive to the assumed cloud overlap within a vertical column of model grid boxes but until now no reliable observations of the degree of cloud overlap have been available in this note we derive the overlap characteristics of clouds from 71 days of high vertical resolution 94 ghz cloud radar data in the uk it is found that contrary to the assumption made in most models vertically continuous clouds tend not to be maximally overlapped rather the overlap of clouds at two levels tends to fall rapidly as their vertical separation is increased and for levels more than 4 km apart overlap is essentially random a simple inverse exponential expression for the degree of overlap as a function of level separation is proposed that could once results become available from a variety of other locations and seasons be implemented in current gcms with relatively little difficulty

the flow of atlantic water through the strait of gibraltar abstract we describe and analyze observations of the water exchange through the strait of gibraltar the primary observations were taken in april 1986 and included data from moored recording instruments at four locations in the strait together with an intensive survey by ship these measurements included extensive ctd profiling acoustic doppler current profiling numerous profiles from expendable instruments and imaging using a high frequency echo sounder the analysis focuses on the internal hydraulics of the strait and in particular the presence of hydraulic controls and their influence on the exchange our observations during april 1986 show that the maximal exchange condition in which a subcritical flow is bounded by supercritical flow at both ends of the strait did occur although with various subtleties not explicitly incorporated in our previous theoretical developments the atlantic water moving east along the surface encounters a control at tarifa narrows in the eastern part of the strait the mediterranean water passes through controls both at camarinal sill and at spartel sill further west at our westernmost mooring at spartel the outflowing mediterranean water is continuously supercritical with little tidal variability the control at camarinal sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide tangier basin bounded by camarinal and spartel sills acts as an internal reservoir for the outflowing mediterranean water the interface rising and falling through each tidal cycle during spring tides the tangier basin interface rises high enough to flood the control at camarinl sill and produces a reverse flow of the lower layer when control is lost at camarinal sill a travelling bore is released this bore may modify the location of the control acting on the surface layer in tarifa narrows the atlantic water enters the alboran sea as a jet which may have a well defined northern boundary where it separates from the european coast this boundary may be identified by the presence of a slick although the internal jump west of camarinal sill is almost certainly an area of intense mixing frictional effects do not appear to be dominant in the subcritical portion of the flow

implementation of subgrid cloud vertical structure inside a gcm and its effect on the radiation budget abstract the giss goddard institute for space studies gcm general circulation model predicts stratiform and convective cloud cover and optical thickness at nine atmospheric levels in horizontal grid boxes of 4 lat 5 long until now the radiative fluxes were calculated once per grid box assuming clear sky or a complete cloud cover here a refinement of the radiative flux calculation is explored by introducing a horizontal subgrid cloud overlap scheme in which cloud blocks are formed by adjacent cloud layers using maximum overlap different cloud blocks are separated by an atmospheric level of clear sky and are assumed to overlap randomly inside the grid box this subgrid cloud structure allows determination of the occurrence probabilities of columns with different vertical structures inside each horizontal grid box then radiative fluxes are calculated for each of these columns the radiative fluxes of each horizontal grid box are obtained as the occurrence probability weighted sum of the colum