tracing of water masses using a multi isotope approach in the southern indian ocean anthropogenic radionuclides 3 h 14 c and 129i stemmed from nuclear weapons tests were found in 1999 to be very abundant in the surface of the southern indian ocean comparable to those in the subtropical northwest pacific ocean the observed radionuclide variations with latitude longitude in the southern indian ocean are not due to deposition patterns of global fallout but due to transport of water masses from the western pacific through the indonesian seas and different water fronts present in the crozet basin of the indian ocean high radionuclide concentrations observed in the latitudinal belt of 20 40 s are associated with the indian ocean subtropical gyre which acts as a reservoir of radionuclides maintaining their high concentrations on a time scale of several decades 14 c data documents that the southern indian ocean is an important for sink of anthropogenic carbon the isotopic tracers reveal the evidence of the most intense surface gradients and presence of several water masses in the southern indian ocean which makes the region one of the most dynamic places of the world ocean

a linear decomposition of the southern ocean thermohaline structure the thermohaline structure of the southern ocean is deeply influenced by the presence of the antarctic circumpolar current acc where water masses of the world ocean are advected transformed and redistributed to the other basins it remains a challenge to describe and visualize the complex 3d pattern of this circulation and its associated tracer distribution here a simple framework is presented to analyze the southern ocean thermohaline structure a functional principal component analysis pca is applied to temperature and salinity s profiles to determine the main spatial patterns of their variations using the southern ocean state estimate sose this study determines the vertical modes describing the southern ocean thermohaline structure between 5 and 2000 m the first two modes explain 92 of the combined s variance thus providing a surprisingly good approximation of the thermohaline properties in the southern ocean the first mode 72 of total variance accurately describes the north south property gradients the second mode 20 mostly describes salinity at 500 m in the region of antarctic intermediate water formation these two modes present circumpolar patterns that can be closely related with standard frontal definitions by projecting any given hydrographic profile onto the sose based modes it is possible to determine its position relative to the fronts the projection is successfully applied on the hydrographic profiles of the woce sr3 section the southern ocean thermohaline decomposition provides an objective way to define water mass boundaries and their spatial variability and has useful application for comparing model output with observations

a quasi synoptic view of the frontal circulation in the crozet basin during the antares 4 cruise abstract with the major objective of estimating the secondary vertical circulation a fine grid survey of hydrographic and current fields across the crozet basin frontal zone was made as a physical component of the multidisciplinary antares 4 cruise this frontal zone forms the boundary of the antarctic circumpolar current and the agulhas return current with the quasi permanent existence of strong eddy activity prior to any quantitative diagnosis of the vertical circulation from the fine grid data it is necessary to place the mesoscale circulation and its time variability of the survey area within the basin wide context we give a quasi synoptic view of the ambient frontal circulation during the cruise period using satellite derived chlorophyll concentrations and sea surface temperature particular emphasis is placed on evaluation of the degree of synopticity of our fine grid measurements by comparing mesoscale features on satellite images the limited survey area 1 5 latitude 2 longitude lies precisely within the southernmost and easternmost convergent zone of the two current systems cutting across mesoscale meanders having typical wavelengths of 400 km and periods of 4 months both these satellite images and topex poseidon sea surface height data indicate little temporal variability of the survey area during the 14 days of measurement although fortuitous this quasi steady state of circulation enables us to use the ensemble of fine grid data as a set of synoptic data for diagnosing the vertical circulation using quasi geostrophic dynamics preliminary analysis of the horizontal and vertical hydrographic structure of the fine grid hints at the existence of wave like cross frontal intrusions having a mean along front wavelength of about 30 km

seasonal variability of qscat derived wind stress over the southern ocean 1 seasonal variability of surface wind stress derived from seawinds onboard the nasa s quikscat satellite is mapped over the latitude range 30 60 s in the southern ocean for the period from august 1999 to july 2003 seasonal variability is pronounced on basin wide scale which dominates the atlantic indian ocean sector during austral winter harmonic analysis reveals that seasonal harmonic captures larger than 60 of the variance on regional scales the cyclonic wind stress curl induces divergence around antarctica and facilitates acc meandering equatorward in the atlantic indian ocean sector of the southern ocean

direct observations of the antarctic circumpolar current transport on the northern flank of the kerguelen plateau 1 the standing meander in the antarctic circumpolar current acc found on the northern flank of the kerguelen plateau was investigated during the southern ocean finestructure cruise in november december 2008 an 18 year time series of surface geostrophic currents from satellite altimetry shows that the meander as observed during this survey is typical of the region hydrographic stations were occupied between 65 75 e and 43 48 s on the shelf 200 m depth and slope into the deep ocean to the north of kerguelen 4700 m providing the most detailed survey of this region to date geostrophic shears are referenced to lowered acoustic doppler current profiler velocities to give the first estimate of the total volume transport in this region and the transport budget is closed around the survey box the subtropical front subantarctic front and a northern branch of the polar front together have an associated transport of 174 22 sv eastward while 174 sv is large compared with typical drake passage transports it is reconciled with other estimates of the total transport with the additional 15 sv of the indonesian throughflow baroclinic transport referenced to the deepest common level between station pairs is 119 sv consistent with other estimates of the baroclinic transport in this area at this longitude the fronts of the acc are exceptionally close together we discuss the exchange of properties across the fronts

the δ15n of nitrate in the southern ocean nitrogen cycling and circulation in the ocean interior we report analyses of the nitrogen isotopic composition of nitrate in the eastern indian and pacific sectors of the southern ocean in this paper we focus on the subsurface data as well as data from the deep waters of other ocean basins nitrate 15n is relatively invariant in much of the abyssal ocean i e below 2 5 km with a value of 4 8 0 2 observed in lower circumpolar deep water north atlantic deep water and central pacific deep water the isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean the southern ocean surface being an important exception in the subantarctic zone north of the polar frontal zone the nitrate 15n of upper circumpolar deep water is 0 7 greater than that of lower circumpolar deep water this isotopic enrichment appears to result from denitrification in the low latitude water masses with which upper circumpolar deep water communicates the isotopic enrichment of upper circumpolar deep water is diminished in the antarctic probably because of the remineralization of sinking organic n which has a low 15n in the antarctic relative to the other water masses of the southern ocean the subantarctic thermocline has a very low nitrate 15n for its nitrate concentration because of exchange with the low latitude thermocline where this isotopic signature appears to originate this signature of the low latitude thermocline has two probable causes 1 mixing with low nitrate surface water and 2 the oxidation of newly fixed n

fe and h2o2 distributions in the upper water column in the indian sector of the southern ocean total dissolvable iron tdfe particulate iron pfe and hydrogen peroxide h2o2 measurements were performed along a n s transect in the upper 250 m in the southern ocean 62 00e 66 42s 49 00s antares ii cruise february 1994 tdfe was organically extracted apdc dddc chloroform and analysed by graphite furnace atomic absorption spectrometry gfaas pfe was analysed by gfaas following a strong mixed acid leach and h2o2 was analysed on board by fluorometry the respective detection limits are equal to 0 13 nmol kg 0 02 nmol kg and 3 0 nmol kg tdfe concentrations vary from 0 4 to 6 2 nmol kg and profiles are not completely depleted in the surface pfe concentrations vary from 0 02 to 0 2 nmol kg iron carbon fe c uptake ratios for phytoplankton were calculated either from seawater or particle measurements they are variable along the transect but are consistent when they could be compared all the observed ratios are within the range of values proposed for the fe c uptake ratios by phytoplankton using our uptake ratio calculated in the permanent open ocean zone 4 10 6 mol mol we estimate that the primary production which can be supported by the iron input flux into the surface waters is two times higher than the measured primary production in the same area in the surface waters h2o2 concentrations vary from 5 0 to 19 7 nmol kg such low concentrations are due to strong vertical mixing low dissolved organic matter concentrations and the latitude of the site

topographical and geological amplifications determined from strong motion and aftershock records of the 3 march 1985 chile earthquake site response experiments were performed 5 months after the ms 7 8 central chile earthquake of 3 march 1985 to identify amplification due to topography and geology topographical amplification at canal beagle a subdivision of vina del mar was hypothesized immediately after the main event when extensive damage was observed on the ridges of canal beagle using frequency dependent spectral ratios of aftershock data obtained from a temporarily established dense array it is shown that there is substantial amplification of motions at the ridges of canal beagle the data set constitutes the first such set depicting topographical amplification at a heavily populated region and correlates well with the damage distribution observed during the main event dense arrays established in vina del mar also yielded extensive data which are quantified to show that in the range of frequencies of engineering interest there was substantial amplification at different sites of different geological formations to substantiate this spectral ratios developed from the strong motion records of the main event are used to show the extensive degree of amplification at an alluvial site as compared to a rock site similarly spectral ratios developed from aftershocks recorded at comparable stations qualitatively confirm that the frequency ranges for which the amplification of motions occur are quite similar to those from strong motion records in case of weak motions the denser arrays established temporarily as described herein can be used to identify the frequency ranges for which amplification occurs to quantify the degree of frequency dependent amplification and used in microzonation of closely spaced localities

substantial advective iron loss diminishes phytoplankton production in the antarctic zone 1 after 1 decade of research it is a well established fact that iron limits photosynthetic co2 fixation and phytoplankton growth in the southern ocean intense blooms are scarce however the input of iron to the southern ocean is considerable an important factor for diminished phytoplankton production refers to the meridional circulation of the southern ocean intense spatially relatively homogeneous upwelling of upper circumpolar deep water ucdw causes a large iron flux into the surface layer however the main entrainment of upwelled ucdw into the surface layer occurs in autumn and winter which strongly restricts the usefullness of iron supply for phytoplankton due to unfavorable light conditions moreover the meridional transport within the ekman layer is intense enough to export at least 25 of the iron input away from the antarctic zone before it can be used by phytoplankton this also depresses the potential phytoplankton primary production by at least 25 most iron that crosses the polar front unused probably leaves the surface ocean north of the polar front because the surface water participates in antarctic intermediate water mode water formation

kerguelen plateau quaternary late pliocene palaeoenvironments from diatom silicoflagellate and sedimentological data abstract gravity cores from the south kerguelen plateau contain sections representing two quaternary intervals 1

sources and transport of the deep western boundary current east of the kerguelen plateau east of the kerguelen plateau a deep western boundary current in the australian antarctic basin brings cold dense waters north from the margins of antarctica geostrophic velocities referenced to acoustic doppler cur rent proler data both shipboard and lowered suggest the flow is unidirectional throughout the water column with es timated northwestward transport below potential temper ature 1 co f 28 7x1 0 6 m 3 s 1 and 49 9x1 0 6 m 3 s 1 respectively hydrographic and acoustic doppler current proler data show that the deep boundary current is sup plied by northward flow emerging from the confluence of westward flow along the antarctic continental slope and eastward flow of weddell basin waters through the princess elizabeth trough to the south of the kerguelen plateau 1 0 to 0 5 0 2 1 1 3 1 4 61 7 8 42 5 0 5 to 0 0 2 0 0 3 3 2 8 41 9 14 52 3 0 0 to 0 5 8 1 3 6 1 4 15 03 2 26 04 4 1 0 to 0 5 10 3 2 2 7 7 28 06 7 48 99 4

environmental reconstructions of the upper 500 m of the southern indian ocean over the last 40 ka using radiolarian protista proxies abstract in 2007 we demonstrated that radiolarians are proxies for a wide range of oceanic physico chemical properties from the surface to depths of up to 500 m below sea level in this study our results are refined and correspondence analysis ca scores derived from census counts of radiolarian subfossils from southern indian ocean core tops are correlated with the physico chemical properties of the region obtained from the 2005 world ocean database calibration and regression techniques are employed to reconstruct palaeoenvironmental conditions spanning the last 40 ka for four indian ocean cores md88 769 46 04 s 90 06 e md88 770 46 01 s 96 27 e md94 102 43 30 s 79 50 e and md94 103 45 35 s 86 31 e all from close to the southeast indian ridge for the first time reconstructions of temperature salinity dissolved oxygen and the silicate nitrate and phosphate concentrations for a range of water depths are proved possible changes of the oceanic environment and the movement of water masses over the last 40 ka as suggested by these reconstructions are discussed during marine isotope stages 2 and 3 mis 2 and mis 3 the water column at some of the core sites has similar characteristics to the waters south of the polar front today at the mis 1 mis 2 transition the development of the subantarctic mode water is apparent temperature reconstructions include evidence of the antarctic cold reversal and the holocene optimum

fe binding dissolved organic ligands near the kerguelen archipelago in the southern ocean indian sector abstract during the ke rguelen o cean and p lateau compared s tudy keops january february 2005 cruise the area southeast of the kerguelen archipelago in the indian sector of the southern ocean was investigated to identify the mechanisms of natural iron fertilization of the kerguelen plateau in this study the organic speciation of fe is described samples were determined immediately on board using competing ligand adsorptive cathodic stripping voltammetry cl adcsv the dissolved organic ligands were always in excess of the dissolved fe concentration increasing the residence time in the water column and the potential availability for phytoplankton the concentration of the dissolved organic ligands ranged from 0 44 to 1 61 n eq of m fe complexation site for fe with an average concentration of 0 91 n eq of m fe s d 0 28 n 113 and a mean logarithm of conditional stability constant log k of 21 7 s d 0 28 n 113 a second weaker dissolved organic ligand group was detected in 32 of the samples with fe binding characteristics at the edge of the detection window of the applied method the occurrence of the highest concentrations of dissolved organic ligands in the wind mixed surface layer and near the sediment at the bottom of the water column indicated that both phytoplankton and the sediment act as sources both sources are in concert with the general conclusions from the keops research on the sources of fe where fe was regenerated organic fe binding ligands were formed in the upper layers and both fe and ligands were supplied by the sediment

contrasting high and intermediate pressures of metamorphism in the archaean sargur schists of southern india the archaean karnataka craton of southern india contains eastern and western crustal blocks separated by a major thurst in which sargur schists occur as lenses within tonalitic peninsular gneisses the schist complex comprises pelites quartzitic psammites carbonates and calc silicates iron formations and basic rocks and thus provides many mineral assemblages ideal for the calculation of pt conditions with their gneisses the sargur rocks are unconformably overlain by the dharwar greenstone belts and are generally thought to be older than 3 000 my

energetics of internal tides around the kerguelen plateau from modeling and altimetry a barotropic tidal model with a parameterization term to account for the internal wave drag energy dissipation is used to examine areas of possible m2 internal tide generation in the kerguelen plateau region barotropic energy flux and a distribution of wave drag dissipation are computed the results suggest important conversion of barotropic energy into baroclinic tide generation over the northern kerguelen plateau shelf break consistent with a theoretical criterion based on ocean stratification tidal forcing frequency and bathymetric gradients the sea surface height signatures of time coherent internal tides are studied using topex poseidon and jason 1 altimeter data whose ascending tracks cross nearly perpendicular to the eastern and western kerguelen plateau shelf break oscillations of a few centimeters associated with phase locked internal tides propagate away fromthe plateau over distances of several hundred kilometers with a 110 km wavelength when reaching the frontal area of the antarctic circumpolar current the internal tide cannot be identified because of the aliasing of mesoscale variability into the same alias band as m2 finally using altimeter data we estimate the m2 barotropic tidal power converted through the internal tide generation process we find consistent values with the barotropic model parameterization estimation which is also in good agreement with global internal tide model estimates combined with modeling this study has shown that altimetry can be used to estimate internal tide dissipation

dissolved trace metals ni zn co cd pb al and mn around the crozet islands southern ocean a phytoplankton bloom shown to be naturally iron fe induced occurs north of the crozet islands southern ocean every year providing an ideal opportunity to study dissolved trace metal distributions within an island system located in a high nutrient low chlorophyll hnlc region we present water column profiles of dissolved nickel ni zinc zn cobalt co cadmium cd lead pb aluminium al and manganese mn obtained as part of the nerc crozex program during austral summer 2004 2005 two stations m3 and m1 were sampled downstream north of crozet in the bloom area and near the islands along with a control station m2 in the hnlc zone upstream south of the islands the general range found was for ni 4 64 6 31 nm zn 1 59 7 75 nm co 24 49 pm cd 135 673 pm pb 6 22 pm al 0 13 2 15 nm and mn 0 07 0 64 nm vertical profiles indicate little island influence to the south with values in the range of other trace metal deprived regions of the southern ocean significant removal of ni and cd was observed in the bloom and zn was moderately correlated with reactive silicate si indicating diatom control over the internal cycling of this metal higher concentrations of zn and cd were observed near the islands pb al and mn distributions also suggest small but significant atmospheric dust supply particularly in the northern region

observations of the antarctic circumpolar current over the udintsev fracture zone the narrowest choke point in the southern ocean an up to date map of the antarctic circumpolar current acc fronts is constructed from the latest version of mean dynamic topography mdt from satellite altimetry and reveals the narrowest acc width in the udintsev fracture zone ufz with the strongest concentration of the three major acc fronts within a limited distance as short as 170 km about 40 narrower than that at drake passage at 144 w at the entrance of the ufz which lies between the pacific antarctic ridge par and its eastwardly offset segment offset par segment there is a triple confluence of the subantarctic front polar front and southern acc front downstream of this longitude the subantarctic front progressively meanders northward over the relatively shallow offset par segment before channeling through the eltanin fracture zone thus diverging from the polar front which proceeds through the ufz in situ observations from two recent cruises at 144 w confirm the satellite altimetry derived frontal circulation in the ufz region and yield a baroclinic transport relative to the bottom of 113 10 6 m 3 s comparable to that through drake passage the hydrographic sections show no antarctic bottom water colder than 0 2 c characteristics of major water masses are described and the implications for their potential downstream modifications at drake passage are discussed in terms of the meridional overturning circulation across the acc mesoscale eddy activity with periods shorter than 90 days is predominantly concentrated in the immediate downstream area of the offset par segment suggesting a substantial poleward eddy heat flux there

location and dynamics of the antarctic polar front from satellite sea surface temperature data the location of the antarctic polar front pf was mapped over a 7 year period 1987 1993 within images of satellite derived sea surface temperature the mean path of the pf is strongly steered by the topographic features of the southern ocean the topography places vorticity constraints on the dynamics of the pf that strongly affect spatial and temporal variability over the deep ocean basins the surface expression of the pf is weakened and the pf meanders over a wide latitudinal range near large topographic features width and temperature change across the front increase and large scale meandering is inhibited elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the pf encounters large topographic features the strong correlations between topography and pf dynamics can be understood in the context of the planetary potential vorticity ppv or f h field mean ppv at the pf varies by more than a factor of 2 along its circumpolar path however at the mesoscale the pf remains within a relatively narrow range of ppv values around the local mean away from large topographic features the pf returns to a preferred ppv value of 25 10 9 m 1 s 1 despite large latitudinal shifts the mean paths of the surface and subsurface expressions of the pf are closely coupled over much of the southern ocean

variability of the antarctic circumpolar current transport through the fawn trough kerguelen plateau abstract the kerguelen plateau is a major topographic obstacle to the eastward flowing antarctic circumpolar current acc while approximately two third of the acc transport is diverted to the north most of the remaining flow engulfs in the fawn trough the only deep passage across the plateau as part of the track transport across the kerguelen plateau project three mooring lines of current meters were deployed in the fawn trough for one year in february 2009 underneath ground track 94 of the jason 2 satellite altimeter full depth ctd ladcp casts carried out during the deployment cruise were previously analyzed to provide a comprehensive description of the regional circulation featuring in particular a volume transport of 43 sv across the fawn trough park et al 2009 here we present a time series of the transport in the fawn trough estimated from current meter observations featuring a mean eastward transport of 34 sv possibly biased low by at most 5 sv and a root mean squared variability of 6 sv consistent with ladcp estimates 43 sv in february 2009 and 38 sv in january 2010 in addition we analyze to what extent the transport can be directly monitored from along track satellite altimeter data which would enable study of the variability of the fawn trough current from a now 20 year long archive the ability to reconstruct the flow from a limited set of moored instruments as well as from altimeter derived surface geostrophic velocity is further assessed from synthetic data extracted from a high resolution peri antarctic simulation while a canonical method to derive transport from altimetry previously applied to the malvinas current gives here unsatisfactory comparisons with in situ estimates an ad hoc approach using only the two northernmost mooring lines yields an estimate well correlated 0 8 with in situ transport at subseasonal time scales during the one year period of observations at interannual time scales however both methods provide significantly correlated 0 7 transport estimates suggesting that long term transport fluctuations across the kerguelen plateau can be confidently estimated from altimetry these consistently indicate a measurable impact of the outstanding 1997 1998 el nino southern oscillation enso event yielding an increase of the annual mean transport of 3 sv possibly with a one year lag the transport estimate based on the ad hoc approach is significantly correlated 0 6 with the southern annular mode sam index at interannual time scales suggesting that an intensification of the circumpolar winds drives an increase in the transport across the kerguelen plateau

comparison of in situ bottom pressure data with grace gravimetry in the crozet kerguelen region two time series of deep ocean bottom pressure records bprs in between the crozet islands and kerguelen are compared with grace gravity recovery and climate experiment equivalent water heights an analysis of the correlation is performed for four time series 1 monthly averages of the equivalent water height at the crozet islands 2 the same near the kerguelen islands 3 the mean of the two preceding series and 4 the difference between the two locations expressed in terms of geostrophic transport we find that smoothed grace solutions are strongly correlated with the bpr data with correlation coefficients in the order of 0 7 0 8 consequently grace measures real oceanic mass variations in this region

the little red hill seismic experimental study topographic effects on ground motion at a bedrock dominated mountain edifice the kinematic response of a bedrock dominated mountain edifice to seismic shaking near lake coleridge in the southern alps of new zealand was examined a small elongated and bedrock dominated mountain ridge little red hill was equipped with a seismic array seven earss instruments mark l 4 3d seismometers were installed along the ridge crest the flank and at the base of the 210 m high 500 m wide and 800 m long mountain edifice from february to july 2006 seismic records of local and regional earthquakes were used to provide information on amplification and deamplification effects as well as on the frequency response at different parts on the edifice the ground motion records were analyzed using three different methods comparisons of peak ground accelerations power spectral density analysis and standard spectral ratio analysis time and frequency domain analyses show that site amplification is concentrated along the elongated crest of the edifice where amplifications of up to 1100 were measured relative to the motion at the flat base the field experiment also highlighted the extreme variations in amplification with regard to closely located sites but at contrasting topographic locations theoretical calculations and frequency analyses of field data indicate a maximum response along the ridge crest of little red hill for frequencies of about 5 hz which correlate to wavelengths approximately equal to the half width or height of the edifice the consequence of amplification effects on the stability and degradation of rock masses can be seen areas showing high amplification effects overlap with the spatial distribution of seismically generated block fields at little red hill it is concluded that topography geometry and distance to the seismic source play key roles in causing amplification effects of seismic ground motion and supporting dilation and degradation of rock mass across bedrock dominated mountain edifices

ground motion observations at hotel montana during the m 7 0 2010 haiti earthquake topography or soil amplification unusually severe structural damage was reported during the 2010 m 7 0 haiti earthquake in the vicinity of hotel montana located on top of a ridge in the district of petionville prompted by the observations u s geological survey seismic stations were deployed and aftershock recordings indicated ground motion amplification on the top of the hill compared to adjacent stations on reference site conditions the presence of topographic relief has been shown to significantly aggravate the consequences of strong ground motion during past events and topographic effects were brought forward to explain the observations in this paper we test the hypothesis of topographic amplification as the dominant factor that contributed to the damage concentration in the vicinity of hotel montana we initially conduct numerical simulations of the ridge seismic response assuming elastic homogeneous site conditions and show that numerical predictions of topographic amplification disagree with the field data both in amplitude and in frequency conversely while 1d ground response analyses for the site conditions at the hilltop predict amplification in the same frequency range as the field data they significantly underestimate the recorded amplitude we then conduct numerical simulations of the foothill ridge response to seismic motion while accounting for soil layering and qualitatively demonstrate that the recorded amplification is most likely attributed to coupled site topographic amplification effects namely to seismic waves trapped in the soft soil layers of the near surface amplified as a consequence of reverberations and further modified due to diffraction and scattering upon incidence on the irregular ground surface parametric investigations of the topography soil amplification coupling effects are then conducted and our results show that when accounting for a hypothetical soil bedrock interface at 100 m depth predictions are in excellent agreement with the observed motion

effects of realistic surface topography on seismic ground motion in the yangminshan region of taiwan based upon the spectral element method and lidar dtm we combine light detection and ranging lidar digital terrain model dtm data and an improved mesh implementation to investigate the effects of high resolution surface topography on seismic ground motion based upon the spectral element method in general topography increases the amplitude of shaking at mountain tops and ridges whereas valleys usually have reduced ground motion as has been observed in both records from past earthquakes and numerical simulations however the effects of realistic topography on ground motion have not often been clearly characterized in numerical simulations especially the seismic response of the true ground surface here we use lidar dtm data which provide two meter reso lution at the free surface and a spectral element method to simulate three dimensional 3d seismic wave propagation in the yangminshan region in taiwan incorporating the effects of realistic topography a smoothed topographic map is employed beneath the model surface in order to decrease mesh distortions due to steep ground surfaces numerical simulations show that seismic shaking in mountainous areas is strongly affected by topography and source frequency content the amplification of ground motion mainly occurs at the tops of hills and ridges whilst the valleys and flat topped hills experience lower levels of ground shaking interaction between small scale to pographic features and high frequency surface waves can produce unusually strong shaking we demonstrate that topographic variations can change peak ground accel eration pga values by 50 in mountainous areas and the relative change in pga between a valley and a ridge can be as high as a factor of 2 compared to a flat surface response this suggests that high resolution realistic topographic features should be taken into account in seismic hazard analysis especially for densely populated moun tainous areas

topographic site effects and hvsr a comparison between observations and theory we present a comparison between observed and predicted amplification on a topographic feature our data come from an ad hoc experiment carried out during the summer of 1989 in epire northern greece ten digital autonomous three component stations recorded a total of 68 small earthquakes as these data were inadequate to compute empirical transfer functions using the standard spectral ratio ssr technique we explored the possible use of horizontal to vertical spectral ratios hvsr first we compare hvsr computed from theoretical transfer functions for incident p sv and rayleigh waves with the horizontal transfer function for each incidence type second we compute hvsr for our weak motion data set and show that the results are stable with a scatter not larger than that usually accepted in ssr studies we observe significant differences between n s and e w motion when we compare hvsr with theoretical transfer functions we obtain a better agreement for sv motion than for sh motion amplification levels are similar between hvsr and predicted amplification even for the sh case this suggests that discrepancies that have been signaled for the topographic amplification between observations and predictions could be due to site effects being present in the reference station used to computed ssr our results support the use of hvsr to determine topographic site effects thus hvsr may have a large potential to evaluate site effects in a wider variety of geological conditions than previously thought without the shortcomings of ssr finally we show that hvsr computed from noise recordings gives a reasonable estimate of hvsr computed from weak motion data although this conclusion should be better substantiated our preliminary results are encouraging

detailed characterization of a cold antarctic eddy 1 eddy formation in the southern ocean plays a critical role in meridional exchanges of heat and salt it has also been shown to have a significant impact on primary productivity and on the creation of preferred feeding regions for birds and other predators this eddy formation is not geographically uniform but is concentrated in certain specific regions particularly where the antarctic circumpolar current crosses shallow bathymetry one such region is at the south west indian ridge where eddies are formed that subsequently drift past the prince edward islands we have studied such an eddy in detail it was at least 1000 m deep had a radius of 60 90 km a heat content of 5 40 1019 j and rotated at a rate of 1 revolution every 5 days altimetric observations show that it was formed at 51 s 32 e and that its intensity as expressed as its sea level anomaly declined by 40 of the original value within 144 days this relatively rapid dissipation suggests that these eddies do not have a lifetime of more than 14 months during that period they will have carried their anomalous heat and salt content about 1 5 degrees of latitude

microphyte pigments and resting spores at the water sediment interface in the subantarctic deep sea indian sector of the southern ocean during the antares i cruise april may 1993 in the indian sector of the southern ocean deep sea sediments were collected using a multiple gravity corer in the polar frontal zone pfz and frontal zone between crozet and kerguelen islands and in the permanently open ocean zone pooz south west of kerguelen and heard islands microphyte biomass indicators chl a b c and related phaeopigments measured by spectrofluorometry tests on natural fluorescence and detection of encysted diatoms observed by scanning electron microscopy sem were carried out on both sediment and overlying fluff whenever present evaluation of the phytoplankton biomass in the overlying surface water masses also was performed at each sampling station the phytoplankton biomass was on the whole very low and decreased from north to south 0 25 0 10 g 1 1 chl a with a maximum on the frontal zone whereas the sediment water interface of the southern sampling stations pooz was pigment enriched a consistent bioclastic fluffy layer was only present in the pooz stations up to 13 g 1 1 chl a 157 g 1 1 phaeo a biodeposition of this type was very thin or absent north of the pf polar front illustrating stronger advection in the northern part of the study area hydrodynamic studies of the crozet basin and the presence of benthic pennate coastal diatoms at the sediment interface of these northern stations further suggest increased advective activity in the north surficial sediments first 5 mm also were enriched with pigments in the pooz up to 0 15 g g 1 dw chl a 3 5 g g 1 dw phaeo a whereas in the pfz the chl a concentrations were insignificant pigment gradients in the sediments particularly well marked in the pooz indicate the absence of a secondary intense reworking such as bioturbation or resuspension nevertheless also in the pooz an unusual and well preserved megafaunal faecal cast was sampled and analysed sediments at southern stations 50 55 s 56 75 e 3600 4700 m depth were the most enriched with encysted microphytic cells mainly diatoms belonging to the genus chaetoceros this was related primarily to the presence of a fluffy layer composed mainly of well preserved detritic frustules of various diatom genera including robust nitzschia and thalassiosira as well as chains of delicate chaetoceros corethron and rhizosolenia well preserved coccoliths and silicoflagellates also were noticeable in the fluffs revival tests cultures on f2 medium initiated with samples from the water sediment interface were positive for the southern stations leading to low diversity diatom assemblages dominated by the genera chaetoceros and nitzschia

time mean flow as the prevailing contribution to the poleward heat flux across the southern flank of the antarctic circumpolar current a case study in the fawn trough kerguelen plateau the major mechanisms of the oceanic poleward heat flux in the southern ocean are still in debate the long standing belief stipulates that the poleward heat flux across the antarctic circumpolar current acc is mainly due to mesoscale transient eddies and the cross stream heat flux by time mean flow is insignificant this belief has recently been challenged by several numerical modeling studies which stress the importance of mean flow for the meridional heat flux in the southern ocean here this study analyzes moored current meter data obtained recently in the fawn trough kerguelen plateau to estimate the cross stream heat flux caused by the time mean flow and transient eddies it is shown that the poleward eddy heat flux in this southern part of the acc is negligible while that from the mean flow is overwhelming by two orders of magnitude this is due to the unusual anticlockwise turning of currents with decreasing depth which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough the circumpolar implications of these local observations are discussed in terms of the depth integrated linear vorticity budget which suggests that the six topographic features along the southern flank of the acc equivalent to the fawn trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region as eddy activity on the southern flank of the acc is too weak to transport sufficient heat poleward the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task

implications of the deep circulation and ventilation of the indian ocean on the renewal mechanism of north atlantic deep water in the indian ocean sector of the great ocean conveyor scheme the north atlantic deep water nadw replacement is realized by the transformation of the sinking circumpolar deep water cdw from the southern ocean to a prevailingly upward flow in the northern indian ocean this water mass transformation scenario has been studied by using hydrographie data including potential temperature salinity dissolved oxygen and nutrients in a water mass mixing model and by calculation of dianeutral velocity the model comprises a mixing system of three major deep water masses nadw cdw and north indian deep water this third water mass is introduced as a virtual water mass to represent the aged cdw the assumption is feasible as the artificial water mass can be eliminated on the deepest n 28 12 neutral surface characterized by isopycnal mixing the experiment is fulfilled when a conservative variable initial phosphate po40 is introduced and all conservative parameters are used in the mixing scheme the n 28 12 surface provides a major isopycnal transition path of cdw to the northern indian ocean and thus cdw is transformed to an upward flow dominant dianeutral up welling is detected on all four neutral surfaces north of 30 s showing increasingly strong dianeutral velocity toward shallower surfaces with the water mass mixing pattern and spreading path dianeutral circulation and dynamical information of the acceleration potential 10 m2 s 2 mapped on neutral surfaces a schematic of deep water circulation and ventilation of the indian ocean emerges from this study the contribution of nadw is largely limited to the southwest indian ocean the northward cdw is found mainly in two paths one in the western indian ocean apparent between 2000 and 3000 m through the western crozet and madagascar basins and farther north into the somali basin and arabian basin and another in the central indian basin apparent below 3000 m where ridge blocking becomes effective in the west and nadw salinity reaches a maximum while the major path switches from west to the central basin in the deeper layer 3000 3500 m the flow in the western indian ocean reverses southward north of the southeast indian ridge the deep path bifurcates from which point the eastern branch feeds the deep western boundary flow in the west australian basin except for part of the transformed cdw which is contributed to the upward flow in the north the return flow of cdw is found on the eastern side of these basins

cross frontal exchange of antarctic intermediate water and antarctic bottom water in the crozet basin abstract the hydrographic survey made across the antarctic circumpolar current acc in the southwestern crozet basin during the april may 1993 antares 1 cruise revealed unprecedented strong cross frontal injections of newly formed antarctic intermediate water aaiw into the subtropical zone north of the acc the acc in this area is mostly associated with the subantarctic front saf and subtropical front stf which are tightly merged into one structure bordered to the north with the eastward extension of the agulhas return current the oxygen and salinity extrema of the newly injected aaiw at a depth of 1200 m well north of the stf reach as high as 6 17 ml l 1 and 34 15 these values coincide almost exactly with those in the aaiw source found at a depth of 500 m just south of the saf but surpass considerably oxygen by 1 35 ml l 1 and salinity by 0 23 psu those values corresponding to the old aaiw at the nearby subtropical stations such spectacular cross frontal injections of new aaiw were not continuous in space and time but were found to be rather local intermittent and impulsive this was closely correlated with the highly perturbed meandering frontal zone due to the frequent passages of strong atmospheric depressions during the cruise there is also clear evidence of the northward penetration of antarctic bottom water aabw through the crozet kerguelen gap and its northward flow along the deep western boundary of the crozet basin supporting previous observations moreover our observations indicate more precisely the bottom water pathway which is concentrated against the continental rise west of 56 e following roughly the 4000 m isobath it is concluded that the crozet basin frontal zone acts as a barrier only for the upper layer water masses and that it is dynamically transparent for the cross frontal exchange of intermediate and deeper layer water masses of potential density anomalies greater than 27 1 including aaiw and aabw

an experimental approach for estimating seismic amplification effects at the top of a ridge and the implication for ground motion predictions the case of narni central italy abstract from march to september 2009 a velocimetric network was installed in narni central italy a village on the top of a limestone ridge the aim was to investigate local site effects due to the 220 m high ridge which is characterized by slopes ranging from 22 to 35 to investigate amplification without and with a reference site three stations were installed at the base of the hill and seven at the crest the network recorded 702 earthquakes many of them from the 2009 l aquila sequence to determine the dependence of amplification on the morphological features the spectra were computed for horizontal components rotated into a range of azimuths both the ratio of the horizontal to vertical component spectra and the ratio of the spectra at the ridge crest with respect to a reference station at the base of the ridge showed amplification by a factor of circa 4 5 for frequencies between 4 hz and 5 hz the highest amplifications were seen for the directions of the ground motion perpendicular to the main elongation of the ridge finally considering events with an epicentral distance less than 30 km empirical ground motion models were calibrated for maximum horizontal peak ground acceleration pga velocity and acceleration response spectra 5 damping up to 1 s to estimate the site corrective coefficients for topographic amplification the data show corrective coefficients between 0 35 and 0 48 log 10 scale amplification 2 2 3 0 for the spectral ordinates between 0 2 s and 0 3 s

isotopic evidence for nitrification in the antarctic winter mixed layer we report wintertime nitrogen and oxygen isotope ratios 15n and 18o of seawater nitrate in the southern ocean south of africa depth profile and underway surface samples collected in july 2012 extend from the subtropics to just beyond the antarctic winter sea ice edge we focus here on the antarctic region south of 50 3 s where application of the rayleigh model to depth profile 15n data yields estimates for the isotope effect the degree of isotope discrimination of nitrate assimilation 1 6 3 3 that are significantly lower than commonly observed in the summertime antarctic 5 8 the 18o data from the same depth profiles and lateral 15n variations within the mixed layer however imply o and n isotope effects that are more similar to those suggested by summertime data these findings point to active nitrification i e regeneration of organic matter to nitrate within the antarctic winter mixed layer nitrite removal from samples reveals a low 15n for nitrite in the winter mixed layer 40 to 20 consistent with nitrification but does not remove the observation of an anomalously low 15n for nitrate the winter data and the nitrification they reveal explain the previous observation of an anomalously low 15n for nitrate in the temperature minimum layer remnant winter mixed layer of summertime depth profiles at the same time the wintertime data require a low 15n for the combined organic n and ammonium in the autumn mixed layer that is available for wintertime nitrification pointing to intense n recycling as a pervasive condition of the antarctic in late summer

frequency scaled curvature as a proxy for topographic site effect amplification and ground motion variability we introduce a new methodology to predict the topographic site effect amplification ground motions obtained from a large database of 3d earthquake simulations show that the curvature of the earth s surface defined as the second spatial derivative of the elevation map is correlated with the topographic site amplification the highest correlation between the frequency dependent topographic amplification and the topographic curvature is reached when the curvature is smoothed over a characteristic length equal to the s wavelength divided by two i e frequency scaled curvature fsc this implies the amplification is caused by topographic features for which horizontal dimensions are similar to half of the s wavelength the largest ground motion variabilities are found at sites located on slopes and on the largest summits whereas intermediate variabilities occur over narrow ridges and a stable behavior in the bottom valleys the fsc proxy allows the identification of topographic features with similar characteristic dimensions and probabilistic estimates of amplification values accounting on the variability of ground motions due to source site interactions amplification estimates using the fsc proxy are robust and easily computed from digital elevation maps provided that reasonable values of s wave velocities are available in the area of interest

slope currents around the kerguelen islands from demersal longline fishing records 1 the kerguelen plateau constitutes a natural obstacle for the eastward progress of the antarctic circumpolar current especially around the kerguelen islands however there is little quantitative knowledge of the current field around the islands due to lack of long term current measurements we performed a systematic analysis of a total of 28917 points of fishing gear drifts from setting and recovery positions of demersal longlines deployed between 2002 and 2007 for patagonian toothfish dissostichus eleginoides fisheries in kerguelen waters this enabled the construction of a realistic field of depth averaged time mean slope currents along the 1000 m isobath all around the kerguelen islands the strongest depth mean velocities of 25 cm s 1 are associated with the polar front which rounds the islands from the south and flows northward along the inner continental slope immediately east of the islands strongly supporting previous hydrographic evidence these results demonstrate the potential for hitherto unexploited historic longline drift data from demersal fishing grounds to provide valuable quantitative information on the regional circulation

observations of watermass modification by mesoscale eddies in the subtropical frontal region of the indian ocean sector of southern ocean abstract two eddies one cyclonic and the other anticyclonic in the subtropical frontal region stf of the indian ocean sector of southern ocean so and their role on the watermass modification are described here using the hydrographic data collected during the austral summer february of 2012 and 2013 and also by using argo profiles sampled during the austral summer from 2004 to 2012 the cyclonic eddy transports cold fresh and deeper antarctic intermediate water aaiw to much shallower depths but the anticyclonic eddy pushes the warm more saline subtropical surface water stsw to deeper depths significant changes in the properties of watermasses in terms of temperature and salinity viz stsw colder by 1 c and less saline by 1 psu and subtropical mode water stmw temperature decreased by 0 5 c and salinity by 0 7 psu were noticeable in the centre of cyclonic eddy on the other hand no such changes were found in the stmw properties in the anticyclonic eddy region the observed changes in the properties of watermasses were confirmed with the argo profiles sampled from the cyclonic and anticyclonic eddy regions of the stf a striking feature observed both in the cyclonic eddy and the anticyclonic eddy was the freshening of aaiw and this is consistently evident both in the cruise based observations as well as in the argo profiles it is hypothesized that the freshening in the aaiw at a given depth in the cyclonic eddy is due to shoaling of the pycnocline causing further intrusion of fresh saline water from deeper depths whereas the corresponding freshening in the anticyclonic eddy is due to the conservation of potential vorticity

frontal structure water masses and circulation in the crozet basin due to topographic steering by the crozet and kerguelen plateaus the antarctic circumpolar current acc core in the crozet basin area is shifted to its northernmost position in the southern ocean along the southern limb of the south indian subtropical gyre here the jet like current is embedded within a narrow band 26 5 kg m3 in the western half of the basin and heavier mode 26 7 kg m3 in its eastern half these different varieties of crozet basin mode water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone the area north of the kerguelen plateau appears to be an important source of water mass modification antarctic intermediate water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher oxygen richer colder subantarctic water energetic eddy activities observed along the frontal zone likely enhance such cross frontal exchange of water masses equatorward spreading of the modified thermocline water masses mode water antarctic intermediate water occurs as part of the anticyclonic circulation in the south indian subtropical gyre which is centered within the basin strong injection of north indian deep water into the acc south of amsterdam island is clearly evidenced for the first time this deep oxygen minimum water centered at about 3000 m can be traced to the east of madagascar from where it spreads southward through the eastern part of the crozet basin due to the influence of this water mass north atlantic deep water characteristics of the circumpolar deep water are significantly altered in kerguelen amsterdam passage our results indicate a cyclonic deep level circulation pattern in the crozet basin with a northward flow 9 sv of circumpolar deep water antarctic bottom water as a deep western boundary current and a partially compensating southward flow 3 sv of north indian deep water through the eastern part of the basin it is concluded that the crozet basin constitutes an important passage for the exchange of water masses between the southern ocean and the indian ocean

analysis of topographic amplification of inclined shear waves in a steep coastal bluff abstract the effect of inclined shear waves on the seismic response of a steep bluff is analyzed using generalized consistent transmitting boundaries the results of the frequency domain analysis of a stepped half space subjected to incident shear waves inclined from 0 to 30 show that the motion at the crest of the slope is amplified for waves traveling into the slope and attenuated for waves traveling away from the slope as compared to the motion in the free field behind the crest of the slope this amplification can be as much as twice that observed for vertically propagating waves a time domain analysis of bluffs at seacliff state beach california is used to estimate the effect of topography using realistic conditions taking into account wave inclination and site effects the analysis of the site shows that although topographic amplification does in fact nearly double the amplitude of the motion in some cases this amplification is offset by reduced site amplification and by wave splitting at material interfaces thus the actual peak acceleration occurring at the crest of the slope changes little with incident angle as compared to the amplification of the free field motion and actually decreases in many cases though a more general study is recommended these results suggest that wave orientation and inclination substantially increase topographic amplification however it may be adequate to only account for vertically propagating waves for site response and slope stability analyses where only the magnitude of acceleration is considered

thermal evolution of the indian shield and its subjacent mantle abstract the estimate of the paleothermal state at 3 ga of the indian shield is deduced using pressure and temperature estimates from the mineral assemblages combined with models of the heat transport mechanism and depth distribution of radiogenic heat in the crust we find that at 3 ga ago the mean thermal gradient within the crust reduced heat flow and mean mantle temperature were 28 c km 1 08 h f u and 2350 c respectively

determination of the surface geostrophic velocity field from satellite altimetry 1 presently available marine geoid models are not accurate enough to extract the mean surface circulation directly from satellite altimetry a novel method for estimating the mean velocity field of major ocean current systems is derived from the free surface boundary condition with a given quasi geostrophic balance for the horizontal surface flow a scaling analysis of this boundary condition indicates that although the vertical velocity w is mostly balanced by the local change of the free surface t useful information on the mean current is contained in a small ageostrophic departure t w our method consists in the development of a simple algebraic equation with two unknowns and an adjustable parameter associated with t assuming that the latter is proportional to t w most interestingly t and all other coefficients of the equation can be determined from altimetry the altimeter data used is combined topex poseidon ers gridded data and the solution is obtained by least squares minimizing the contribution from the time variable part of the parameter and prescribing the zonal direction of the mean current the method which is found to be particularly useful for quasi zonal high energy current systems has been validated against direct observations in the gulf stream and southern ocean comparisons with direct observations and monte carlo experiments suggest an overall solution error of about 10 cm s 1 once calibrated against regional velocity statistics this method will be able to determine from altimetry the mean or instantaneous surface velocity field down to the frontal scale with a realism that has been inaccessible because of the geoid constraint

amplification of seismic response of a large deep seated landslide in tokushima japan abstract to better understand seismic site responses of deep seated landslides we carried out long term seismic monitoring of a target landslide reactivated by a rainstorm in 2004 accompanying typhoon namtheun in naka town tokushima prefecture japan five seismometers with high sensitivity were installed at different locations on landslide areas with different elevations by examining numerous records of earthquake events we distinguished the effects of material contrast and topography on these localized amplifications for different areas and summarized their features with help of detailed geological and geophysical surveys to analyze differences in amplification the hvsr method calculating the horizontal to vertical spectral ratio of ground motion was applied the amplification on a landslide block with an hvsr peak amplitude smaller than 5 and resulting from topography is predominantly perpendicular to the direction of elongation of the mountain ridge however the amplification revealed by hvsr peak amplitude of 3 to 5 resulting from material contrasts in multiple strata showed differing amplification directions by examining the relationship between the hvsr peak amplitude and shear velocity profiles we found that the amplifications at different frequencies are greatly affected by the extent of material contrasts this may explain why the toe part of the slope with ancient landslide deposits generally shows the greatest ground motion characterized by hvsr peak amplitude normally 10 this type of strong amplification due to material contrasts may result in severe damage to residential areas in mountainous areas during an earthquake and should be considered for the construction of buildings on landslide areas

high moho temperature in the indian shield abstract moho temperatures in the indian shield region are estimated using step exponential and power law models of the depth variation of radiogenic heat with temperature dependent thermal conductivity the moho temperatures and heat flows are around 550 600 c and 0 8 hfu 33 mwm 2 in the southern part of the indian shield and 850 900 c and 0 95 hfu 40 mwm 2 in its northern part it is argued that such high temperatures and their lateral variations may contain signatures of deccan trap volcanism and himalayan orogeny the implications of the temperature estimates on some related crustal parameters viz p n wave velocity curie isotherm depth and melting point temperature at moho depth are also indicated

subtropical mode waters in the south atlantic ocean this systematic study of the subtropical mode waters in the south atlantic is based on hydrological data the basin wide south atlantic ventilation experiment oceanus and ajax sections and the world ocean circulation experiment woce sections in the brazil malvinas confluence region the woce expendable bathythermograph data set and the atmospheric forcing produced by the national centers for environmental prediction reanalysis the main ventilation window of the subtropical gyre lies in the southwest and corresponds to the southern part of the recirculation region and to the overshoot of the brazil current bc some ventilation occurs along the southern branch of the gyre through deep mixed layers in harsh winters and in the east through the rings shed by the agulhas current however the ventilation from the eastern border concerns a small volume of water with highly variable characteristics three main types of subtropical mode water are observed the lightest around the 26 2 density surface and the second and third around the 26 5 and 26 7 density surfaces respectively the 26 2 subtropical mode water stmw1 corresponds to potential temperatures between 16 and 18 c and salinities between 35 9 and 36 2 the stmw1 thickness does not exceed 150 m the volume of the 26 5 mode water stmw2 is larger it is produced through the formation of deep mixed layers reaching 300 m or more in the active ventilation window over the bc overshoot its potential temperature ranges between 14 and 16 c and its salinity ranges between 35 5 and 35 9 its thickness can be above 300 m the last component observed around the 26 7 density surface stmw3 has a potential temperature between 12 and 14 c and a salinity of about 35 2 to 35 5 it is formed on the eastern flank of the bc overshoot and during harsh winters in the center of the southern branch of the gyre within the main ventilation window the stmws exhibit quite a significant variability in characteristics and volume there is a large interannual variability in the atmospheric forcing over the bc recirculation and bc extension and the observed variations of the subtropical mode waters are qualitatively correlated to the atmospheric forcing variations

initial investigation of site and topographic effects at robinwood ridge california abstract following the 1989 loma prieta earthquake a dense array of seven digitally recorded three component seismograph stations was deployed on robinwood ridge 7 3 km northwest of the epicenter the purpose of this array was to investigate the cause of high levels of structural damage and ground cracking observed on the ridge crest aftershocks recorded by the array allow a comparison of ground motion up the slope of the ridge from the base to the crest the data present an extremely complicated pattern of ground motion that demonstrates the importance of the three dimensionality of the problem slowness analysis of p wave trains show initial arrivals propagating away from the source with small angles of incidence and large apparent velocities consistent with direct arrivals after 0 5 sec propagation azimuths become more random and apparent velocities drop indicating nearly horizontal wave propagation and multiply reflected and diffracted phases within the ridge slowness analysis and particle motion diagrams of horizontal components of motion show dramatic variations in ground motion with changes in azimuth of the source and a complicated interaction between body waves and rayleigh and love waves results suggest that the larger amplitude more coherent arrivals at the array stations favor a propagation direction parallel to the ridge axis an amplification factor of from 1 5 to 4 5 is seen for frequencies from 1 0 to 3 0 hz with wavelengths comparable to the base of the ridge part of which may be caused by local site effects and part by topographic amplification in addition amplifications of up to a factor of 5 are seen at higher frequencies and are attributed to local site effects these effects are most notable from 4 to 8 hz on the vertical components and from 6 to 9 hz on the horizontal components the entire robinwood ridge area may also have been situated in a region of heightened mainshock ground motion due to source directivity and radiation pattern effects

an experimental topographic amplification study at los alamos national laboratory using ambient vibrations abstract an experimental study aimed at investigating potential topographic amplification of seismic waves was conducted on a 50 m tall and 185 m wide soft rock ridge located at los alamos national laboratory near los alamos new mexico ten portable broadband seismograph stations were placed in arrays across the ridge and left to record ambient vibration data for 9 hours clear evidence of topographic amplification was observed by comparing spectral ratios calculated from ambient noise recordings at the toe slope and crest of the instrumented ridge the inferred resonance frequency of the ridge obtained from the experimental recordings was found to agree well with several simple estimates of the theoretical resonance frequency based on its geometry and stiffness results support the feasibility of quantifying the frequency range of topographic amplification solely using ambient vibrations rather than strong or weak ground motions additionally comparisons have been made between a number of widely used experimental methods for quantifying topographic effects such as the standard spectral ratio median reference method and horizontal to vertical spectral ratio differences in the amplification and frequency range of topographic effects indicated by these methods highlight the importance of choosing a reference condition that is appropriate for the site specific conditions and goals associated with an experimental topographic amplification study

fronts transport and weddell gyre at 30 e between africa and antarctica a detailed description of the frontal structure of major currents and estimates of transport between africa and antarctica at 30 e were made on the basis of a finely resolved hydrographic section made during the 1996 civa 2 cruise particular emphasis was put on a refinement of the eastern boundary of the weddell gyre by analyzing also supplementary hydrographic data from the 1993 civa 1 cruise and the best available historical hydrographic data in the weddell and enderby basins contrary to the general belief our results show that the weddell gyre extends far beyond 30 e reaching at least as far east as the enderby land promontory 53 e from this point westward currents along the antarctic continental margin constitute the gyre s southern boundary thus closing the cyclonic circulation of the gyre at its easternmost end as the most convincing evidence for this refined circulation scheme significant westward transport south of 65 s has been identified from the civa 1 acoustic doppler current profiler data we have shown also using an inertial jet model that the blocked feature of the weddell cold regime 57 s to the west of 25 e is due to the topographic control of flow by local bathymetry during the civa 2 cruise 70 of the antarctic circumpolar current acc was concentrated near the polar front 51 s with surface speeds reaching up to 70 cm s 1 the baroclinic current field in the subpolar area south of the weddell front 58 s for civa 2 and 57 s for civa 1 is featureless 3 cm s 1 but yielding a nonnegligible eastward transport of 15 sv 1 sv 106 m3 s 1 this bottom referenced baroclinic transport is entirely compensated by a substantial westward barotropic transport south of 65 s the acc transport across the 30 e section during the civa 2 cruise is estimated to be 160 sv

glacial interglacial dust and export production records from the southern indian ocean abstract we present 230th normalized dust and export production fluxes for two contrasted marine sediment cores spanning the antarctic polar front close to the kerguelen plateau in the southern indian ocean covering the last glacial cycle we report glacial lithogenic fluxes comparable to the south atlantic and higher than in the south pacific sectors of the southern ocean structural and temporal discrepancies with dust reconstructions from antarctic ice cores and the atlantic and pacific sectors of the subantarctic zone saz point towards southern africa and or the kerguelen plateau as an additional source of lithogenic material to the southern indian ocean during the last ice age in the saz export production proxies respond to iron fe fertilization with total organic carbon toc fluxes as high as those previously reported from the atlantic sector of the southern ocean however the correlation between export production and dust proxies is weaker than in the other sectors and shows a muted response of export production during peak glacials we hypothesize that this muted response may be related to macronutrient co limitation imposed on phytoplankton growth possibly induced by a northward displacement of wind driven upwelling and or the polar frontal system during peak glacials the antarctic zone az record depicts the typical pattern of enhanced export production during interglacials and comparatively low productivity during glacials suggesting a decrease in the supply of macronutrients to the az surface during ice ages compared to warm periods as previously proposed however a muted response of opal fluxes during marine isotope stage mis 5e argues for a southward migration of the frontal system during warmer mis 5e possibly causing silicic acid co limitation at this site these results illustrate the important role of fe fertilization and changes in wind driven upwelling intensities in modulating the strength and efficiency of the biological carbon pump in the southern ocean but also highlight the potential role of frontal movements in explaining export production variability at sites located close to the antarctic circumpolar current acc fronts overall our new datasets provide new insights on the regional differences in dust and export production patterns that need to be considered in order to evaluate the efficiency of carbon sequestration in the different sectors of the southern ocean during ice ages

ground motion amplifications in small size hills case study of gokgang ri south korea during the 2017 ml 5 4 pohang earthquake sequence

ocean circulation and frontal structure near the southern kerguelen plateau the physical context for the kerguelen axis ecosystem study oceanographic observations from ships floats and remote sensing are used to describe the ocean circulation and frontal structure from the southern kerguelen plateau to the antarctic margin the flow of the antarctic circumpolar current acc is largely zonal upstream of the kerguelen plateau the southern branch of the polar front and the northern branch of the southern acc front saccf are deflected to the north by the topography and cross the plateau through the fawn trough the southern branch of the saccf and the southern boundary of the acc pass to the south of banzare bank through the princess elizabeth trough these fronts turn sharply to the north on the eastern flank of the plateau then retroflect to the south before continuing eastward satellite altimetry and float trajectories suggest the acc regime to the east of the plateau is eddy rich and characterised by extensive meandering of the acc fronts in contrast to the low eddy energy and zonal flow observed west of the plateau the antarctic slope current flows west along the upper continental slope with hydrographic data indicating some of the flow turns offshore and feeds the southern boundary flow in the western boundary current flow over the plateau itself is relatively quiescent and floats in this region have a long residence time 1 year the regional circulation is closely associated with the distribution of biological productivity in surface waters as revealed in satellite measurements of ocean colour including high productivity in subpolar waters along the eastern edge of the plateau and over the antarctic continental shelf moderate productivity over the plateau and low productivity in acc waters in the fawn trough and north of the saccf

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

geochemistry of suspended particles in the subantarctic and polar frontal zones south of australia constraints on export and advection processes filtered particles from a southern ocean meridional transect from 42 s to 54 s along 142 e off tasmania were studied between surface and 600 m water depth saz 1998 expedition february april 1998 particulate ba sr ca p u th ti al mn light rare earth elements and zr contents were analyzed by inductively coupled plasma mass spectrometry and inductively coupled plasma atomic emission spectroscopy mesopelagic baxs total ba minus a lithogenic fraction estimated from al contents were higher within the polar front zone pfz in comparison with the northern areas indicating that c export to the intermediate and deep ocean is more important in the pfz average surface uxs versus latitude covaries with mesopelagic baxs contrasting with organic matter proxies such as pp and chlorophyll a this similar behavior suggests that u might be preferentially adsorbed on organic material which subsequently becomes involved with barite precipitation srp and cap contents decrease sharply with depth following total particle content for the northern area 42 47 s surface ce anomaly reflects essentially a lithogenic signature 0 6 ce ce 1 ascribed to the supply from the tasmanian continental shelf suspended matter from surface waters at stations south of 47 s shows an enhanced seawater signature 0 3 ce ce 0 6 in all stations ce anomalies increase with depth toward values close to 1 this is likely due to the increase of lithogenic authigenic ratio of particles with depth rather than redox processes finally an advective event is identified at 45 s below 200 m by a strong increase of lithogenic element contents which affects baxs

carbon isotope evidence for changes in antarctic intermediate water circulation and ocean ventilation in the southwest pacific during the last deglaciation 1 deep sea sediment core fr1 97 gc 12 is located 990 mbsl in the northern tasman sea southwest pacific where antarctic intermediate water aaiw presently impinges the continental slope of the southern great barrier reef analysis of carbon 13c and oxygen 18o isotope ratios on a suite of planktonic and benthic foraminifera reveals rapid changes in surface and intermediate water circulation over the last 30 kyr during the last glacial maximum there was a large 13c offset 1 1 between the surface dwelling planktonic foraminifera and benthic species living within the aaiw in contrast during the last deglaciation termination 1 the 13cplanktonic benthic offset reduced to 0 4 prior to an intermediate offset 0 7 during the holocene we suggest that variations in the dominance and direction of aaiw circulation in the tasman sea and increased oceanic ventilation can account for the rapid change in the water column 13cplanktonic benthic offset during the glacial interglacial transition our results support the hypothesis that intermediate water plays an important role in propagating climatic changes from the polar regions to the tropics in this case climatic variations in the southern hemisphere may have led to the rapid ventilation of deep water and aaiw during termination 1 which contributed to the postglacial rise in atmospheric co2

phytoplankton pigment distribution from seawifs data in the subtropical convergence zone south of africa a wavelet analysis the agulhas system agulhas current retroflection agulhas return current arc south of africa constitutes a complex energetic system subject to high frequency mesoscale and low frequency variability using chlorophyll pigment concentration data collected by the seawifs ocean color sensor and sea surface height satellite data we demonstrate the utility of wavelet analysis in jointly analyzing ocean color and altimetric signals two distinct dynamical scenarios of the agulhas system october 1997 and january 1998 yield different dominant wavelengths 620 and 500 km for the arc rossby wave they are simultaneously retrieved from the chlorophyll and sea level anomaly signals longitudinal extension of the power resulting from the wavelet analysis also depends on the dynamical regime

circulation pathways and transports of the southern ocean in the vicinity of the southwest indian ridge circulation pathways and transports of the antarctic circumpolar current acc and agulhas return current arc around the complex bathymetry of the southwest indian ridge and del cano rise have been established from two hydrographic surveys and two year long moorings after crossing the southwest indian ridge in a 110 sv 1 sv 106 m3s 1 flow concentrated at 48 s the acc fragments before being reconcentrated into several branches by the bathymetry in particular 30 40 sv turns north around the eastern flank of the del cano rise turns west as part of anticyclonic flow round the del cano rise finally returning eastward as part of the crozet front eddy variability from both satellite data and moorings is low over the bathymetry of the del cano rise so the strong currents found between the del cano rise and the crozet plateau are believed to be permanent features this largely barotropic time independent transport is the major pathway for the acc to enter the crozet basin the anticyclonic flow also transfers water from the mozambique basin across the southwest indian ridge to the south side of the del cano rise given the evidence for fragmentation of the acc where it crosses the southwest indian ridge and the concentration of several jets into the crozet front from the acc subtropical front and agulhas return current it is misleading to relate standard frontal definitions such as the polar front and subantarctic front to the major transport pathways

interannual changes of austral summer coccolithophore assemblages and southward expanse in the southern indian ocean abstract coccolithophore ecology and biogeography in the southern indian ocean was studied during the austral summer of 2009 soe09 late austral summer 2010 soe10 mid austral summer and 2012 isea12 early austral summer during the soe09 and soe10 surface water samples 55 and 54 water samples respectively were collected from two different transects between mauritius and antarctica 48 e 57 3 e whereas during the isea12 22 sea surface water samples were collected between cape town and prydz bay highest coccolithophore diversity was during soe10 36 taxa followed by soe09 and isea12 12 and 7 taxa respectively whereas highest coccolithophore abundance was reported during the isea12 up to 2203 103 coccospheres l followed by soe10 up to 742 103 coccospheres l and soe09 up to 217 103 coccospheres l during each expedition elevated coccolithophore diversity was documented at the subtropical zone whereas the highest abundance was recorded at the subantarctic zone which was dominated by emiliania huxleyi differences in the occurrence and southward expanse of coccolithophores were documented during each expedition i e maximum southward expanse during the soe10 and lowest during the isea12 we show that biogeographic boundaries of coccolithophores are highly variable in the southern indian ocean which is controlled by environmental factors in the early and mid austral summer and grazing pressure in the late austral summer in addition the physical forcing also may play a key role in transport of coccoliths and coccospheres in the high latitudes we indicate that southward expanse of coccolithophores is significant and may occur for short periods during austral summer

ground motion in the presence of complex topography earthquake and ambient noise sources to study the influence of topography on ground motion eight seismic recorders were deployed for a period of one year over poverty ridge on the east side of the san francisco bay area california this location is desirable because of its proximity to local earthquake sources and the significant topographic relief of the array 439 m topographic amplification is evaluated as a function offrequency using a variety of methods including reference site based spectral ratios and single station horizontal to vertical spectral ratios using both shear waves from earthquakes and ambient noise field observations are compared with the predicted ground motion from an accurate digital model of the topography and a 3d local velocity model amplification factors from the theoretical calculations are consistent with observa tions the fundamental resonance of the ridge is prominently observed in the spectra of data and synthetics however higher frequency peaks are also seen primarily for sources in line with the major axis of the ridge perhaps indicating higher resonant modes excitations of lateral ribs off of the main ridge are also seen at frequencies consistent with their dimensions the favored directions of resonance are shown to be transverse to the major axes of the topographic features

mean sea surface height of the antarctic circumpolar current from geosat data method and application the mean sea surface height across the antarctic circumpolar current has been reconstructed from height variability measured by the geosat altimeter without assuming prior knowledge of the geoid for this study an automated technique has been developed to estimate mean sea surface height for each satellite ground track using a meandering gaussian jet model and errors have been estimated using monte carlo simulation the results are objectively mapped to produce a picture of the mean subantarctic and polar fronts which together comprise the major components of the antarctic circumpolar current the meandering jet model explains between 40 and 70 of the height variance along the jet axes the results show that the fronts are substantially steered by topography and that the jets have an average gaussian width of about 44 km in the meridional direction and meander about 75 km to either side of their mean locations the average height difference across the subantarctic front saf is 0 7 m and across the polar front pf 0 6 m the mean widths of the fronts are correlated with the size of the baroclinic rossby radius

directional topographic site response at tarzana observed in aftershocks of the 1994 northridge california earthquake implications for mainshock motions abstract the northridge earthquake caused 1 78 g acceleration in the east west direction at a site in tarzana california located about 6 km south of the mainshock epicenter the accelerograph was located atop a hill about 15 m high 500 m long and 130 m wide striking about n78 e during the aftershock sequence a temporary array of 21 three component geophones was deployed in six radial lines centered on the accelerograph with an average sensor spacing of 35 m station c00 was located about 2 m from the accelerograph we inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion we identified a 3 2 hz resonance that is a transverse oscillation of the hill a directional topographic effect the top base amplification ratio at 3 2 hz is about 4 5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill this resonance is seen most strongly within 50 m of c00 other resonant frequencies were also observed a strong lateral variation in attenuation probably associated with a fault caused substantially lower motion at frequencies above 6 hz at the east end of the hill there may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill causing particle motions not spectral ratios at the top of the hill to be rotated about 20 away from the direction transverse to the hill the resonant frequency but not the amplitude of our observed topographic resonance agrees well with theory even for such a low hill comparisons of our observations with theoretical results indicate that the 3d shape of the hill and its internal structure are important factors affecting its response the strong transverse resonance of the hill does not account for the large east west mainshock motions assuming linear soil response mainshock east west motions at the tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill probable variations in surficial shear wave velocity do not account for the observed differences among mainshock acceleration observed at tarzana and at two different sites within 2 km of tarzana

three dimensional isoneutral potential vorticity structure in the indian ocean the three dimensional isoneutral potential vorticity structure of the indian ocean is examined using world ocean circulation experiment and national oceanic and atmospheric administration conductivity temperature depth data and historical bottle data the distribution of the potential vorticity is set by the indian ocean s source waters and their circulation inside the basin the lower thermocline has a high potential vorticity signal extending westward from northwest of australia and a low signal from the subantarctic mode water in the south the antarctic intermediate water inflow creates patches of high potential vorticity at intermediate depths in the southern indian ocean below which the field becomes dominated by planetary vorticity indicating a weaker meridional circulation and weaker potential vorticity sources wind driven gyre depths have lower potential vorticity gradients primarily due to same source waters homogenization and western shadow zones are not observed the effect dominates the effect of the somali current and the red sea water on the potential vorticity distribution isopleths tilt strongly away from latitude lines in the deep and abyssal waters as the circumpolar deep water fills the basins in deep western boundary currents indicating a strong meridional circulation north of the antarctic circumpolar current the lower gradient intermediate layer surrounded vertically by layers with higher meridional potential vorticity gradients in the subtropical indian ocean suggests that rossby waves will travel 1 3 times faster than standard theory predicts to the south several pools of homogenized potential vorticity appear in the upper 2000 m of the southern ocean where gyres previously have been identified south of australia the abyssal potential vorticity structure is set by a combination of the antarctic circumpolar current and the bathymetry

large scale circulation over and around the northern kerguelen plateau abstract the kerguelen plateau is known to constitute a major barrier to the eastward flowing antarctic circumpolar current however there is limited knowledge on the regional circulation due to sparse observations especially in the northern kerguelen plateau between the fawn trough and the kerguelen islands as part of the physical component of the multidisciplinary keops cruise systematic ctd measurements along three sw ne oriented sections were made on the latter plateau cutting through the annual phytoplankton bloom across the eastern flank of the plateau into deep water major hydrographic features and geostrophic velocities perpendicular to the sections were first documented from these in situ observations in order to interpret the results within the general circulation context and to trace the upstream pathways of the observed water masses we established a synthetic picture of the large scale circulation over and around the entire northern kerguelen plateau using the combination of new data from keops with historical hydrographic data together with all available information on mid depth and near surface velocities derived from autonomous floats surface drifting buoys and satellite altimetry this synthesis shows that the time mean geostrophic flow over the shallow platform is sluggish of the order of 3 5 cm s 1 with a general anticyclonic circulation roughly following the local bathymetry this weak flow is bordered to the east by relatively strong up to 18 cm s 1 northwestward flow along the eastern flank of the northern kerguelen plateau due to a northward bifurcation of cold antarctic waters of eastern enderby basin origin carried by the powerful fawn trough current we have shown the utmost importance of the general circulation for the spatial distribution of primary productivity in that the sluggish circulation over the complex shallow topography of the northern kerguelen plateau may precondition the recurrence of annual blooms in contrast to neighboring strong advective regimes without blooming in deep water

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

location of the antarctic polar front from amsr e satellite sea surface temperature measurements abstract the location of the southern ocean polar front pf is mapped from the first 3 yr of remotely sensed advanced microwave scanning radiometer for the earth observing system amsr e sea surface temperature sst measurements in agreement with previous studies the mean path of the antarctic pf and its standard deviation are strongly influenced by bottom topography however the mean pf path diverges slightly from previous studies in several regions where there is high mesoscale variability although the sst and sst gradient at the pf show spatially coherent seasonal variations with the highest temperature and the lowest temperature gradient during summer the seasonal variations in the location of the pf are not spatially coherent the temporal mean sst at the pf corresponds well to the mean pf path the temperature is high in the atlantic and indian ocean sections and is low in the pacific ocean section where the pf has a more southerly position the relationship of the wind field with the antar

how much deep water is formed in the southern ocean three tracers are used to place constraints on the production rate of ventilated deep water in the southern ocean the distribution of the water mass tracer po4 phosphate star in the deep sea suggests that the amount of ventilated deep water produced in the southern ocean is equal to or greater than the outflow of north atlantic deep water from the atlantic radiocarbon distributions yield an export flux of water from the north atlantic which has averaged about 15 sv over the last several hundred years cfc inventories are used as a direct indicator of the current production rate of ventilated deep water in the southern ocean although coverage is as yet sparse it appears that the cfc inventory is not inconsistent with the deep water production rate required by the distributions of po4 and radiocarbon it has been widely accepted that the major part of the deep water production in the southern ocean takes place in the weddell sea however our estimate of the southern ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the weddell sea which lie in the range 1 5 sv possible reasons for this difference are discussed

major nutrients and dissolved oxygen as indicators of the frontal zones in the atlantic sector of the southern ocean 1 the chemical tracers of the main frontal zones of the atlantic sector of the southern ocean are considered before the beginning of the spring bloom frontal zones are distinguished by lateral gradients of dissolved oxygen phosphate nitrate and silicate during the spring bloom the smoothing of nutrient concentrations on both sides of the fronts weakens lateral gradients of chemical properties the position of surface gradients of nutrients within the subtropical frontal zone stfz does not coincide with the location of temperature and salinity gradients as a result fronts in this region have a stepped character the best chemical indicator of the northern stfz front is the dissolved oxygen gradient which coincides with the temperature and salinity gradients the southern boundary of the stfz is distinguished by the gradient of nitrate the chemical criterion for identifying the subantarctic front is the gradient of oxygen which ranges from 0 5 to 4 0 mol kg 1 per km the polar front is identified by the gradient of silicate 0 56 to 2 78 m per km at the surface the weddell scotia confluence wsc is distinguished not by the temperature and salinity but by chemical parameters the best year round criterion is the lateral gradient of silicate to phosphate atomic ratio which ranges from 25 to 35 other markers of the wsc are the gradients of silicate at the surface oxygen at the upper boundary of the circumpolar deep water and the depths of its location

polar front around the kerguelen islands an up to date determination and associated circulation of surface subsurface waters the circulation of iron rich shelf waters around the kerguelen islands plays a crucial role for a climatically important annually recurrent phytoplankton spring bloom over the sluggish shelf region and its downstream plume area along the antarctic circumpolar flow however there is a long standing confusion about the polar front pf in the kerguelen region due to diverse suggestions in the literature for its geographical location with an extreme difference over 10 of latitude based on abundant historical hydrographic data the in situ hydrographic and current measurements during the 2011 keops2 cruise satellite chlorophyll images and altimetry derived surface velocity fields we determine and validate an up to date location of the pf around the kerguelen islands artificial lagrangian particle trajectories computed from altimetric velocity time series are analyzed for the possible pathways and sources of different surface subsurface waters advected into the chlorophyll bloom area east off the islands studied during the keops2 cruise the pf location determined as the northernmost boundary of the winter water colder than 2 c which is also associated with a band of strong currents appears to be primarily controlled by topography the pf rounds the kerguelen islands from the south to deflect northward along the eastern escarpment up to the northeastern corner of the kerguelen plateau before making its southward retroflection it is shown that the major surface subsurface waters found within the deep basin east of the kerguelen islands originate from the shelf around the heard island rather than from the shallow shelf north of the kerguelen islands

the stratification and water masses at drake passage the waters at drake passage are known to be separated horizontally into four distinct zones of relatively small geostrophic shear by three fronts in which the shear and eastward flow are large the two northern fronts subantarctic front and polar front that bound the polar frontal zone comprise most of the transport of the antarctic circumpolar current vertical sections of closely spaced 50 km oceanographic stations made across drake passage in the summers of 1975 and 1976 produced quality measurements of temperature salinity oxygen silicate nitrate and phosphate based on these data a traditional description of characteristic distributions is presented good sampling resolution in the vertical allows maxima in hydrostatic stability to be traced laterally approximately along isopycnals between individual stations eleven strata of maximum stability are identified most vertically separate distinct water masses the most pronounced stability stratum is associated with the seasonal pycnocline north of the polar front it separates the subantarctic surface waters from subantarctic mode water to the south of the polar front the seasonal pycnocline is found slightly above the temperature minimum characteristic of the antarctic surface waters in austral summer this stability stratum extends northward near the base of the subantarctic mode water another stability stratum extending across the entire passage forms the upper boundary for the circumpolar deep water south of the polar front this stratum is at the base of the antarctic surface water to the north it is the lower boundary of the antarctic intermediate water although subantarctic mode water and antarctic intermediate water are separated by a stability stratum north of the polar front there is no such separation between antarctic intermediate water and the temperature minimum of the antarctic surface water south of the front this suggests that subantarctic mode water and antarctic intermediate water are distinct and that the latter derives from the surface waters south of the polar front at greater depths a horizontally discontinuous stratum is found between the upper and lower circumpolar deep water two horizontally continuous stability strata are found near the base of the circumpolar deep water marking the transition to a high silicate variety of denser deep water from the southeast pacific basin the densest water mass found is weddell sea deep water which flows through a gap in the south scotia ridge and along the southern scotia sea to fill the trough in the southern drake passage it is overlain by a laterally continuous stratum of relative maximum stability

on the origins of deep and bottom waters of the indian ocean the characteristics of the deep and bottom waters of the indian ocean when illustrated on potential density anomaly surfaces indicate that the waters enter from both the atlantic and pacific oceans the paths of spreading are constrained by the complex topography and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments especially in the northern indian ocean the weddell sea contributes to the densest waters found in the western basins and the ross sea and adelie coast to the densest waters found in the eastern basins both dense water varieties are altered by and incorporated in the less dense water above initially water carried by the circumpolar current then water from the north atlantic and finally by deep water whose characteristics are derived in the northern indian ocean contact with the sediments increases the silica content of the bottom water in the southern ocean in the northern indian ocean the sediments alter the silica of the water at the bottom and together with enhanced salinity from diffusion of saline overflows from the marginal seas above imprint unique markers to the deep water that flows back to the south at middepths the series of ridges between madagascar and australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern indian ocean characteristics southward within the circumpolar current low oxygen deep water from the pacific is carried across the atlantic and into the indian ocean south of africa part flows around the cyclonic weddell sea gyre and part extends across the southern ocean water from another pacific source can be seen near 2000 m extending westward from the tasman sea south of aus tralia and across the indian ocean and perhaps to the aghulas current region southeast of africa in this study we show some maps of deep and bottom water characteristics of the indian ocean which may be used in a qualitative sense to infer pathways of spreading of water below a depth of about 2000 m the indian ocean has numerous ridges and basins figure 1 that severely restrict possible pathways of deep and bottom wa ter flow in the following we will begin by looking at char acteristics at the bottom of the ocean to reveal the path ways of spreading of the densest bottom waters from basin to basin then a series of six potential density anomaly surfaces at progressively shallower depths from about 4000 m up to about 2000 m will be shown to see what character istics influence the deep waters and how pathways of flow are constrained by the bathymetry of the indian ocean the choice of the specific density levels table 1 was based primarily on various property extrema shown by the 18os atlantis ii cruise 93 colored sections that accompany warren s 1981 detailed discussion of the major features of the central indian ocean between madagascar and aus tralia the sections approximately midway between antarctica and the northern indian ocean boundaries are well positioned to show features originating from both the north and the south as well as some brought into the indi an ocean by the antarctic circumpolar current the full ocean area isopycnal maps provide essential evidence on the origins and fates of features seen in the sections and

physical and biological patchiness of an upper ocean transect from south africa to the ice edge near the greenwich meridian an undulating ctd seasoar was towed from the subtropical front at 42 s 12 e to the ice edge at 57 5 s 2 w the instrument measured temperature salinity pressure and light in the top 400 m of the water column fluorescence and particle counts also were recorded fluorescence was calibrated to chlorophylla and particle counts were converted to biovolume to provide estimates of phyto and zooplankton respectively the size of particles counted covered the range 0 25 8 0 mm three major fronts subtropical subantarctic and polar were intersected and identified by their physical characteristics the polar front showed surface and subsurface signatures separated by nearly 300 km considerable small scale structure was observed between the fronts in particular a large 150 km diameter warm core eddy was found in the northern part of the subantarctic zone both fronts and eddy were associated with significant biological signals phyto and zooplankton correlated well in the frontal regions and south of the polar front the polar frontal zone was associated with elevated concentrations of biomass whereas concentrations were low at the subantarctic front and south of 55 s in contrast the warm core eddy in the subantarctic zone contained the highest zooplankton biovolume of the section while chlorophyll concentration was only moderate we hypothesise that such a distribution results from grazing the overall distribution of phyto and zooplankton changed across the subantarctic front the vertical extent of biomass deepened from north to south across the front zooplankton tended to be concentrated in relatively narrow deep extending bands to the north but were more widely spread to the south the small scale hydrographic structure correlated significantly with the distribution of plankton suggesting strong physical controls on both small zooplankton and phytoplankton

the subantarctic and polar fronts in relation to deep water masses through the southwestern atlantic the two principal fronts within the antarctic circumpolar current acc the subantarctic front saf and the polar front pf are investigated with respect to their spatial patterns and relations to deep water masses from the northern drake passage to the western and southern argentine basin observations from four recent cruises in adjoining and overlapping regions are supplemented with satellite infrared imagery and trajectories from surface drifters east of drake passage the width of the polar frontal zone pfz increases rapidly when the saf turns sharply northward as a part of the falkland malvinas current while the pf remains oriented mainly toward the northeast found within this region of the pfz are mesoscale eddies the upper layers of which consist of subantarctic mode water from the pacific the contrasts in upper layer water properties that identify the saf can be traced the entire length of the falkland current to the brazil falkland confluence zone and then southward to the southern argentine basin there the saf turns eastward and later but perhaps not always merges with the pf north of ewing bank to form a single intense current core strongly influenced by bottom topography at times this eastward current loops southward through a gap in the falkland ridge into the georgia basin thereby supplying relatively warm and salty subantarctic water to the antarctic zone east of the gap the two fronts become separated and are once again distinct features in the vicinity of the islas orcadas rise at depth within drake passage and the western scotia sea the saf and pf are not lateral boundaries between distinct water masses but are instead identified by enhanced vertical displacements of property isopleths with the northward turn of the saf east of drake passage a thick layer of circumpolar deep water cdw is advected over the falkland plateau into the argentine basin there it is joined by waters entering the argentine basin via a deep spreading route through the georgia basin denser cdw deep water from the weddell sea and episodically deep water from the southeastern pacific ocean together these waters form a northward flow off the patagonian shelf that extends from the sea surface to the bottom most of which turns back toward the south with the falkland current return also flowing south along the seaward side of the falkland current return well away from the western boundary is the main core of north atlantic deep water nadw detatched masses of nadw are observed within the cdw at the western boundary as well as within the acc south of the saf in the south central argentine basin

sub mesoscale structure and the development of an eddy in the subantarctic front north of the crozet islands two stations only 20 km apart were observed to have quite different biological and biogeochemical characteristics the first site had low concentrations of chlorophyll and sufficient nutrients to support phytoplankton growth the second site had high concentrations of chlorophyll depleted nutrient concentrations and significant export of phytodetrital material had taken place the two sites were located in the polar frontal zone of the southern ocean to the northwest of the crozet islands the main physical difference between the two sites was in the depth of the mixed layer at the first site the mixed layer was deep and well mixed whereas the second site had a shallow stratified layer otherwise the horizontal gradients of physical properties were weak survey data from the surrounding area showed that the productive site was located on the edge of a filament of water drawn into a developing meander of the subantarctic front remotely sensed data provided a history of the growth of the meander in the subantarctic front and its development into an eddy in the polar frontal zone the dynamics associated with the filament in the meander were clearly important in driving the primary productivity as an intrusion of saline water into adjacent fresher water generated a shallow mixed layer and conditions suitable for phytoplankton growth the dynamics promoting conditions favourable to phytoplankton growth continued to operate as chlorophyll was enhanced in the eddy after the main bloom had died away later measurements suggested that the surface layer had changed from diatom dominated to a coccolithophore or calcite rich community

large scale circulation around the crozet plateau controls an annual phytoplankton bloom in the crozet basin the circulation in the vicinity of the crozet plateau in the southwest indian ocean sector of the southern ocean is examined using hydrographic sections argo floats surface drifters and satellite altimetry all four techniques confirm that a major branch of the antarctic circumpolar current the subantarctic front saf flows anticyclonically round the del cano rise west of the crozet plateau i e eastward to the south of the del cano rise then northward and sometimes northwestward into the crozet basin before turning back eastward in a combined front with the agulhas return current and the subtropical front this s bend in the saf is a permanent feature controlled by the bathymetry as has been inferred previously by pollard and read pollard r t read j f 2001 circulation pathways and transports of the southern ocean in the vicinity of the southwest indian ridge journal of geophysical research 106 c2 2881 2898 similar but much weaker anticyclonic flow is found round the crozet plateau itself with no more than 5 10 106 m3 s 1 turning north to the east of the crozet islands circulation north of the crozet plateau between the plateau and the s bend of the saf is extremely weak fed only by anticyclonic meanders breaking off the saf into the area from the west or north and occasional input from the northward partially wind driven i e ekman flow south and east of the islands in consequence of the weak circulation dissolved iron from the land or sediments of the crozet plateau and islands can build up during the winter in the polar frontal zone between crozet and the saf which gives rise to an annual bloom in this area biological evidence from satellite images and from phytoplankton and zooplankton distributions supports the circulation pattern we develop this pattern confirms that patchiness of productivity in the bloom area results from close juxtaposition of water that has entered the area from the west from the saf and from the south and east after flowing past the islands flow from the south and east has had a chance to entrain iron from the islands and sediments whereas flow from the west has not

the seismic response of a steep slope high resolution observations with a dense three component seismic array we deployed an array of 35 three component seismographs to measure site effects associated with a 50 m high slope that is the highwall of an abandoned surface coal mine near sullivan indiana the strip mine pit had filled with water to within 10 m of the top of the highwall our array was deployed over this slope and recorded 10 underwater explosions detonated in the lake 800 m away the raw data show a site resonance revealed by ringy waveforms observed at stations near the slope crest the ringing is seen on the vertical component immediately following the p arrival and on the horizontal components immediately following what we interpret as a secondary p arrival formed by a direct wave propagating through the water three dimensional particle motion analysis demonstrates that the p wave signal is linearly polarized on the flat ground behind the slope but becomes strongly elliptical for stations on the slope the direct wave through the water is nearly horizontally polarized and shows a pronounced amplification of about a factor of 2 for stations at the top of the slope we quantify the variations in the site resonance by using spectral ratios we observe a factor of 2 amplification at about 9 hz that we interpret as a fundamental mode of the slope face online material animations of 3d and 4d visualizations manuscript received 8 june 2001