mesoscale features create hotspots of carbon uptake in the antarctic circumpolar current the influence of eddy structures on the seasonal depletion of dissolved inorganic carbon dic and carbon dioxide co2 disequilibrium was investigated during a trans atlantic crossing of the antarctic circumpolar current acc in austral summer 2012 the georgia basin downstream of the island of south georgia 54 55 s 36 38 w is a highly dynamic region due to the mesoscale activity associated with the flow of the subantarctic front saf and polar front pf satellite sea surface height and chlorophyll a anomalies revealed a cyclonic cold core that dominated the northern georgia basin that was formed from a large meander of the pf warmer waters influenced by the saf formed a smaller anticyclonic structure to the east of the basin both the cold core and warm core eddy structures were hotspots of carbon uptake relative to the rest of the acc section during austral summer this was most amplified in the cold core where greatest co2 undersaturation 78 atm and substantial surface ocean dic deficit 5 1 mol m 2 occurred in the presence of high wind speeds the cold core eddy acted as a strong sink for atmospheric co2 of 25 5 mmol m 2 day 1 waters of the warm core displayed characteristics of the polar frontal zone pfz with warmer upper ocean waters and enhanced co2 undersaturation 59 atm and depletion of dic 4 9mol m 2 a proposed mechanism for the enhanced carbon uptake across both eddy structures is based on the ekman eddy pumping theory i the cold core is seeded with productive high chlorophyll a waters from the antarctic zone and sustained biological productivity through upwelled nutrient supply that counteracts dic inputs from deep waters ii horizontal entrainment of low dic surface waters biological uptake from the pfz downwell within the warm core and cause relative dic depletion in the upper water column the observations suggest that the formation and northward propagation of cold core eddies in the region of the pf could project low dic waters towards the site of antarctic intermediate water formation and enhance co2 drawdown into the deep ocean

body wave traveltimes and a spherically symmetric p and s wave velocity model summary the derivation of a spherically symmetric reference earth model is a subject which has recently regained interest after a decade of studies on lateral heterogeneity we now have the tools to approach the problem an iaspei working group on traveltime tables was formed in 1988 with the goal of deriving a reliable and up to date radial model of seismic velocities following the guidelines developed by that working group we have investigated the body wave traveltimes for several branches frequently reported in the isc bulletins and propose here a model for p and s wave velocities consistent with these data approximately 16000 best recorded shallow focus earthquakes are selected from 24 years of isc bulletins 1964 1987 and relocated using among others corrections for lateral heterogeneity from this dataset we derive summary traveltime curves specified for each degree of epicentral distance for p pcp p pkpab pkpbc pkikp and s to constrain velocity at the top of the outer core we include data for sks and skks sks from hales roberts 1970 1971 our discrete time distance curves are obtained by evaluating the shape of the residual density distribution independently for each 1 interval to limit contamination due to the interference of other branches precursors or late readings in the upper mantle the resulting model sp6 differs from iasp91 kennett engdahl 1991 only by slightly higher p and s velocities between 410 and 660 km in the lower mantle new velocity profiles have lower velocity gradients this is also true in comparison with prem dziewonski anderson 1981 at the top of the outer core vp is smaller than in prem and the velocity jump at the inner core boundary is reduced to the value of 0 62 km s 1 the new inner core radius is 1215 km our model predicts traveltimes that differ from the iasp91 tables considerably although generally less than 1 s except for sks and skks for which the differences are greater

double beam imaging mapping lower mantle heterogeneities using combinations of source and receiver arrays we present a new technique for imaging spatially distributed heterogeneities using information from combinations of source and receiver arrays the method is based on the single scattering assumption and is closely related to the double beam method of kruger et al 1993 1995 1996 in that it exploits amplitude delay time slowness and azimuth information in two arrays simultaneously a crucial step in the method is the application of static time corrections for a chosen reference phase here pcp one type of image is obtained as a spatial likelihood distribution for prescribed source and receiver array slowness and azimuth values and delay times with respect to a reference phase this allows the determination of the spatial origin of coherent phases in the p coda in a complementary approach we perform a double beam stack migration according to the theoretical slowness and azimuth values for candidate scatterers distributed over a three dimensional 3 d grid the scattering strength is expressed as the resulting beam power beam amplitude or semblance in a time window determined from the theoretical delay time with respect to the reference phase while likelihood mapping provides an image reflecting the probability of a region to explain observed kinematic phase properties by single scattering double beam migration provides information about the scattering strength the method is applied to nuclear explosion sources in eastern kazakhstan recorded at the yellowknife array in northern canada we identify several scattering volumes within the lower mantle below the arctic producing individual anomalous lower mantle phases in the p coda these anomalies range in depth from the core mantle boundary cmb up to 500 km into the lowermost mantle and are mainly located under the eurasian side of the arctic the joint interpretation of these results with the results of previous studies suggests a connection of these anomalies to the cenozoic and mesozoic subduction of the pacific plate and the kula plate the determination of the nature of such 3 d anomalies necessitates further array studies with better azimuthal coverage

observations of ekman currents in the southern ocean abstract largely zonal winds in the southern ocean drive an equatorward ekman transport that constitutes the shallowest limb of the meridional overturning circulation of the antarctic circumpolar current acc despite its importance there have been no direct observations of the open ocean ekman balance in the southern ocean until now using high resolution repeat observations of upper ocean velocity in drake passage a mean ekman spiral is resolved and ekman transport is computed the mean ekman currents decay in amplitude and rotate anticyclonically with depth penetrating to 100 m depth above the base of the annual mean mixed layer at 120 m the rotation depth scale exceeds the e folding scale of the speed by about a factor of 3 resulting in a current spiral that is compressed relative to predictions from ekman theory transport estimated from the observed currents is mostly equatorward and in good agreement with the ekman transport computed from four different gridded wind products the mean tempe

absence of short period ulvz precursors to pcp and scp from two regions of the cmb we present results from a study of the core mantleboundary cmb underneathmexico centralamer ica and the northeast pacic using core reflected phases pcpand scp which provide a direct means of sampling the deep mantle precursor arrivals to these phases pro duced by reflection from the top of an ultra low velocity layer ulvz at the base of themantle would contain infor mation about the thickness of such a layer and the velocity reductions within it stacks of these phases from 17 earth quakes recorded by the northern and southern california seismographic networks do not exhibit coherent or inco herent precursor arrivals from a ulvz in precursor time windows ratios of stack amplitudes to those of the cmb reflected phases fall below predicted values from published ulvz models we conclude that ulvzs if present in these regions havevp andvs reductions less than 10 are less than ve km thick or have transitions greater thanve km wide at their upper boundaries

p wave propagation heterogeneity and earthquake location in the mediterranean region summary we analyse p wave traveltimes for the mediterranean area using both teleseismic and regional arrivals for shallow earthquakes reported in the bulletins of the international seismological centre we model delays between pairs of 0 5 0 0 5 0 cells obtaining a detailed representation of the p traveltime heterogeneities examination of these anomalies shows the clear presence of geographically coherent patternsconsistent with known geological featuresdue to signi cant structure in the upper mantle we present a scheme based on an empirical heterogeneity correction ehc to p wave traveltimes to improve earthquake location this method provides similar bene ts to those of a location procedure based on ray tracing in a 3 d model but it is simpler and computationally more e cient the de nition of the traveltime heterogeneity model being based on a statistical procedure bypasses most of the critical points and possible instabilities involved in model inversion ehc relocation applied to mediterranean earthquakes allows one to predict about 70 per cent of the estimated signal due to heterogeneity and produces epicentral and origin time shifts of respectively 4 22 km and 0 35 s rms from a synthetic experiment in which we use the proposed algorithm to retrieve known source locations we estimate that the rms improvement achieved by the ehc relocation over a simpler standard 1 d location is more than 20 per cent for both epicentral mislocation and origin time shifts

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

double beam analysis of anomalies in the core mantle boundary region seismic p waves from nuclear explosions at the kazakhstan test site in eastern kazakhstan kts observed at the yellowknife array yka canada are used to analyze the structure at the base of the mantle under severnaya zemlya arctic sea 18 events at the degelen subsite and 9 events at the shagan river subsite of kts are studied the simple waveforms and well controlled source parameters of nuclear explosions allow the use of the events as source arrays in addition to the usual receiver array configuration a new method double beamforming integrating both concepts is presented this method increases the resolution considerably for the degelen yka path one anomalous arrival 1 9 s before pcp slowness between p and pcp slowness is observed smaller anomalous arrivals are found 6 5 s degelen yka and 8 1 s before pcp shagan river yka these anomalous phases cannot be explained with standard earth models they are produced by reflections from an inhomogeneity in the lowermost mantle at 82 3n 107 5e under the area north of severnaya zemlya the strong variations between the wavefields for the two source receiver combinations indicate a heterogeneous structure in the lower mantle the anomaly found here is located close to an anomaly in the lowermost mantle beneath the nansen basin

polysynthetic twin width distributions in anorthoclase the widths of just under 7000 albite and pericline lamellae were measured in a suite of 20 polysynthetically twinned anorthoclase megacrysts collected at two sites in new mexico the majority of the twins were formed during the inversion of the anorthoclase crystals from their high temperature monoclinic modification to their present triclinic form textural evidence suggests that a few of the lamellae postdate the transformation and are mechanical in origin twin density is exceedingly inhomogeneous on all scales and the related black and white lamella sets are unequally developed within even a single crystal this type of inhomogeneity is probably a common feature of all inversion twinning since it arises as a natural consequence of the complex and rapidly changing conditions within a crystal undergoing transformation

alternatives to core mantle boundary topography abstract nuclear explosions at the french test sites in the south pacific have been recorded at the regional seismic networks in the western united states a total of approximately 600 good quality short period seismograms were selected from an initial set of 900 the dense receiver spacing particularly in california allows the division of the networks into numerous subarrays and the data from each subarray can be analysed using conventional array techniques such as beam forming and frequency wave number analysis the seismic rays provide dense sampling of a small area of the core mantle boundary cmb beneath the eastern pacific from 5 to 13 n and 133 to 126 w pcp has a shallow incidence angle on the cmb at an epicentral distance of 65 and d is sampled over a large swathe from 2 s to 24 n and from 134 to 123 w beam traces have been formed for the p and pcp phases for each of the subarrays using slowness and azimuth values derived from array methods good quality beams have been selected for further analysis by applying a matched filter to the beamed traces an accurate set of pcp p differential travel times has been determined and compared with iasp91 the residuals show a smooth trend over the area sampled for the given epicentral distances p and pcp have similar paths through the crust and upper mantle so we make the assumption that near source and near receiver structures have negligible effect on the differential times we also assume that the midmantle is relatively homogenous many previous studies have interpreted pcp p residuals in terms of cmb topography by mapping the residuals directly to topography a range in elevation of 40 km over a lateral extent of about 700 km results alternatively we attempted to interpret the residuals in terms of velocity heterogeneity in d a range of d models have been tested including lateral velocity variation in a d layer with constant thickness the pcp p residuals may be explained by allowing the p wave velocity to vary laterally by a factor of 2 over the region sampled with most of the region being marginally fast this suggests that d heterogeneity has large spectral power at relatively short wavelengths in this scenario previous tomographic inversions for cmb topography will have large errors since in these inversions corrections for d structure are based on low order spherical harmonic models

seismic p waves from novaya zemlya explosions seeing double summary the p wave seismograms generated by an underground explosion on 11 october 1980 at the northern novaya zemlya test site are quite different from the seismograms normally recorded from explosions there however the pcp seismograms are similar to the pcps normally observed a simple deconvolution of the seismograms yields pulse shapes that are interpreted as being generated by two simultaneous explosions some 7 km apart the spatial separation causes the appearance of the p wave seismograms to change with azimuth but such variation is not observed in pcp because the steeply descending wavefronts from each explosion are superpositioned the observed p wave seismograms from the double explosions are successfully modelled using the observed pcp as the source function

equivalent monopole source of the geomagnetic south atlantic anomaly the south atlantic magnetic anomaly saa is an important feature of the present geomagnetic field in this paper we model the space time evolution of this anomaly over the past 400 years in terms of the resultant between a decrease of a global axial dipole and an increase of a virtual local monopole source certain characteristics of this evolution are investigated and some considerations are made in the light of a possible special state of the global geomagnetic field dynamical regime among the possible speculations one is made regarding the topography of the core mantle boundary cmb and its possible aspect beneath the saa region in terms of simple sinusoidal undulations met by the monopole source during its centennial motion

probing the core mantle boundary beneath europe and western eurasia a detailed study using pcp summary we use pcp the core reflected p phase recordings of deep earthquakes and nuclear explosions from the grafenberg germany and norsar norway arrays to investigate the core mantle boundary region beneath europe and western eurasia we find evidence for a previously unknown ultra low velocity zone 600 km south east of moscow located at the edge of a middle size low shear velocity region imaged in seismic tomography that is located beneath the volga river region the observed amplitude variations of pcp can be modelled by velocity reductions of p and s waves of 5 and 15 respectively with a density increase of 15 travel time delays of pre and postcursors are indicating a thickness of about 13 km for this ultra low velocity region ulvz however our modelling also reveals highly ambiguous amplitude variations of pcp and a reflection off the top of the anomaly for various ulvzs and topography models accordingly large velocity contrasts of up to 10 in vp and 20 in vs cannot be excluded in general the whole volga river region shows a complex pattern of pcp amplitudes caused most likely by cmb undulations further pcp probes beneath paris kiev and northern italy indicate likely normal cmb conditions whereas the samples below finland and the hungary slovakia border yield strongly amplified pcp signals suggesting strong cmb topography effects we evaluate the amplitude behaviour of pcp as a function of distance and several ulvz models using the 1d reflectivity and the 2d gauss beam method the influence of the velocity and density perturbations is analysed as well as the anomaly thickness the dominant period of the source wavelet and interface topographies strong variation of the pcp amplitude are obtained as a function of distance and of the impedance contrast we also consider two types of topographies undulations atop the cmb in the presence of flat ulvzs and vice versa where a broad range of cmb topography dimensions lead to large pcp amplitude variations only large ulvz undulations generate significant amplitude scattering consequently this indicates that topography effects of anomalies may mask the true medium parameters as well as the ulvz thickness moreover there might be a possibility of misinterpreting the precursor as pcp in particular for thin ulvzs

spectral comparisons between explosion p signals from the tuamotu islands nevada and eastern kazakh a comparison is presented between explosion generated teleseismic p signals from single events of comparable magnitude in the tuamotu islands nevada and e kazakh recorded by the wake island hydrophone array spectral analysis of these signals indicates that attenuation presumably under the source is greatest for the tuamotu travel path and least for the e kazakh travel path the high frequencies 3 hz present in e kazakh signals which enhance the detectability of these signals on the ocean bottom near wake where noise levels are low in this range are generally absent in signals from nevada and the tuamotus

investigating the base of the mantle using differential travel times abstract several techniques using differential seismic travel times to map lateral structure in the lowermost mantle are discussed results are shown for recent studies involving the established techniques of core reflected phases scs s and pcp p and diffracted phase profiles sdiff and new techniques involving the differential times of both core transmitted and core diffracted phases pkp pdiff and sdiff sks skks are described the recent databases of digital seismograms have allowed for a study of d velocities in the eastern hemisphere using scs s and sscs ss differential times from the many western pacific earthquakes the result is an image at a resolution of a few hundred kilometers of a slow velocity anomaly of 2500 km width beneath micronesia 2 relative to the preliminary reference earth model prem that is surrounded on three sides by fast d rock that is 3 faster than prem a study using the differential arrivals of core diffracted s waves sdiff from digital records is providing information about long wavelength variations in d shear velocities though the rigorous earthquake station geometry requirements limit the study to particular regions of the globe another study is using over 40 000 pcp p differential travel times as reported to the international seismological centre to map global p velocities at the base of the mantle and it shows that global coverage of the core mantle boundary cmb is very poor though there are some regions northern asia northern pacific central america with enough data sampling to allow a quantification of average d p velocities with a total robust range of 4 lateral variation they cover only a small portion of the total cmb as a means of increasing our understanding of the long wavelength variations of seismic velocities a description is given of two techniques that will take advantage of totally different sets of earthquake station geometries from the core reflected phase studies in the distance range from 120 to beyond 165 the differential times of pkp and pdiff can be used to map long wavelength d p velocities these two phases are very different in shape and frequency content so the differential times are found by a waveform cross correlation with reflectivity synthetic counterparts in the distance range from 105 to beyond 135 the differential times of sdiff sks and skks can be examined simultaneously to test models of velocity structure above and below the cmb also through comparison with synthetic counterparts

coherence of antarctic sea levels southern hemisphere annular mode and flow through drake passage 1 it is known from small sets of tide gauges that sub surface pressure sea level corrected for the inverse barometer effect around antarctica varies coherently around about half of the continent and that this coherent signal is related to atmospheric forcing in the form of the antarctic oscillation or southern hemisphere annular mode we here confirm that this coherence extends to a more extensive network of tide gauges and to parts of the continental shelf far from the shore as measured by bottom pressure gauges we use time series from an eddy permitting ocean model with realistic forcing to relate the coherent mode to fluctuations in transport through drake passage and confirm using a 1 resolution barotropic model that the fluctuations are predominantly due to barotropic dynamics although baroclinic dynamics are expected to play an increasing role at interannual timescales

theory and observations of ekman flux in the chlorophyll distribution downstream of south georgia 1 a large phytoplankton bloom occurs downstream of south georgia an island on the northern edge of the scotia sea atlantic sector of the southern ocean this is due to natural iron fertilisation being advected downstream in the antarctic circumpolar current mapping of seawifs chl a in dynamic height longitude space reveals the trajectory of surface water under the dual influences of geostrophic flow and ekman flux a theoretical estimate of the trajectory shows good agreement with observations the trajectory is dependent on the wind stress mixed layer depth ratio but independent of the latitudinal separation of dynamic height contours the chl a distribution suggests that iron reaches 20 w below the mixed layer away from the effects of ekman flux but beyond the iron is confined to the surface layer chl a reduces monotonically east of south georgia with the signal detectable to 15 e over 3500 km downstream indicating the basin scale biogeochemical influence of the iron supply

fronts and habitat zones in the scotia sea abstract the fronts in the antarctic circumpolar current acc comprise the southern acc front saccf the polar front pf and sub antarctic front saf which together with the southern boundary sb separate zones with distinct water mass properties in the southern ocean they are important for describing habitat ranges of oceanic species over the range of trophic levels and are also key habitat zones in their own right for a variety of species frontal positions were studied along a transect running s to n across the scotia sea during three hydrographic cruises in spring 2006 summer 2008 and autumn 2009 subsurface data from these cruises and argo data are here combined with aviso satellite absolute dynamic topography data to locate these fronts facilitating study of their ecological significance within the scotia sea northern and southern dynamic height limits are found for each front as opposed to the more usual one dimensional line thus the sb was found between 132 to 116 dyn cm the saccf between 115 to 99 dyn cm and the pf between 71 to 45 dyn cm argo data allow the sensitivity of the dynamic heights found to be tested against seasonal interannual and longitudinal effects and each is found to be small although the saf lay beyond the northern extent of the transect we sampled along argo profiles allowed us to determine its limits as lying between 5 and 29 dyn cm the dynamic height limits found are thus suitable for assessing the location in relation to fronts of any sample or tracked animal within this area the chlorophyll distribution across the scotia sea is assessed relative to frontal positions and known iron sources the physical conditions sea surface temperature sea ice and mixed layer depth along the sampling transect are also presented allowing the oceanographic conditions of the stations occupied across the scotia sea to be assessed

scp a probe of ultralow velocity zones at the base of the mantle the core reflected phase scp is a sensitive probe of the properties of an ultralow velocity zone ulvz at the base of the mantle in synthetic experiments a ulvz with large depressions in p and s velocities results in scp having two diagnostic precursors and one later arrival whose timing depends on ulvz vp vs and thickness amplitudes of the additional arrivals can be large if ulvz shear velocity has extreme reductions such as 30 and greater some data examples are presented for southwest pacific events recorded at short period and broadband arrays in hawaii which show evidence for a ulvz resolving all three additional arrivals will help in addressing the strength and ratio of vs and vp reductions in ulvz and thus the issue of partial melt

inference of core mantle boundary topography from isc pcp and pkp traveltimes we invert isc pcp and pkp absolute and differential traveltimes in an attempt to infer the long wavelength topography of the core mantle boundary cmb the data selection and processing methods are described and evaluated these travel time data are very noisy and the geographic distribution of the data is highly non uniform inhibiting reliable inference of cmb topography spatial averaging enhances the coherent component of the residual variance related to heterogeneity however the random component of the variance is much larger than the coherent component we show that for pcp data the coherent signal due to mantle heterogeneity overshadows that arising from the cmb and that the effects of mantle heterogeneity are mapped into our inferred cmb solutions the pcp data are not correlated across the spatial averaging bins and seem to have a strong bias due to small scale structure and or noise the non uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the cmb spatial patterns of cmb models inferred from different phases do not agree amplitudes of seismically inferred cmb undulations vary greatly the sensitivity of inferred cmb models to the processing spatial averaging procedure and inversion techniques are investigated topographic amplitudes increase strongly with increasing input residual variance the power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher based on this work we conclude that reliable inference of long wavelength cmb topography is not likely with the current isc data set or with a spherical harmonic expansion truncated to degree and order 6

scattering and attenuation abstract an extensive existence of strong small scale heterogeneity in the earth is suggested by the great variability of direct waves and remarkable stability of scattered waves both in the case of teleseismic p coda and in the case of local earthquake s coda quantitative studies of small scoale heterogeneities have been made by the following three methods a lower bound of heterogeneity in the lithosphere has been estimated by the deterministic or stochastic modeling of travel time and amplitude fluctuation of teleseismic p waves across a large aperture seismic array an upper bound of heterogeneity has been estimated by modeling the attenuation of s waves as a scattering loss due to heterogeneity these two approaches suggest the existence of heterogeneity with rms velocity fluctuations from 3 to 10 per cent the third method is based on the estimation of heterogeneity from the amplitude of coda waves we found a significant discrepancy between the result obtained by the third method and those from the first two and attribute the discrepancy to the assumption of a single scattering model adopted in the coda generation theory the need for including multiple scattering as well as for considering vector elastic waves rather than scalar waves is emphasized

reply to comment on teleseismic p wave attenuation and nuclear explosion source functions inferred from yellowknife array data by kin yip chun tianfei zhu and gordon f west

the core mantle boundary under the gulf of alaska no ulvz for shear waves the earth s core mantle boundary cmb marks the boundary between the hot molten iron core and the silicate mantle and is a thermal chemical and flow boundary previous observations of very slow compressional wavespeeds suggest that thin ultra low velocity zones ulvzs possibly composed of a mixture of molten iron and silicates exist at the base of the mantle a molten or partially molten layer would cause a large shear wavespeed decrease however this velocity drop has not been observed here we use core reflected scp phases to investigate the shear properties of ulvzs these phases reveal at least two distinct regions one region under central america which is distinctly average prem and iasp91 like and a second region under the entire gulf of alaska that produces large scp reflections with amplitudes of up to 30 times larger than calculated with average earth models the large amplitudes suggest a combination of focusing by cmb topography high shear wavespeeds at the bottom of the mantle and low attenuation high q along the scp path high shear wavespeeds and low attenuation are opposite from what would be expected from a shear wave ulvz a ulvz in compressional wavespeeds ultra low vp has previously been observed in this region however in addition to the large amplitudes the short period scp waveforms show no complexity that can be related to a ulvz thus either there is not a ulvz under the gulf of alaska or if it does exist it is restricted to compressional wavespeed changes precluding interpretation as partial melt but rather suggesting a chemical origin fl 2000 published by elsevier science b v all rights reserved

modelling seismic reflections from d using the kirchhoff method abstract the observation of seismic reflections from the top of d is restricted to the epicentral distance range near the critical angle where the amplitude is large and a phase distortion occurs this phase advance in the reflected wavelet is examined and implications for commonly used seismic processing techniques such as phase picking stacking and deconvolution are made kirchhoff synthetics are generated to study several models of d reflectors such as steeply inclined interfaces and laterally varying velocity contrasts observations of strong amplitude variations in seismic reflections the bounce points of which lie well within a fresnel zone are successfully explained in a search for reflections from d investigators have exclusively used models of the d layer parallel to the core mantle boundary we speculate about a steep slope as it may be produced by deposited slab material and model the influence of such features on travel time and waveform

high frequency response of wind driven currents measured by drifting buoys and altimetry over the world ocean 1 we investigate the wind driven current response over the world ocean using svp drifting buoys ers 1 and ers 2 and topex altimetry data and ecmwf wind stress fields wind driven ageostrophic currents are estimated from drifting buoys by removing altimetry deduced geostrophic currents from observed drifting velocities removing the geostrophic signal enhances the coherence with wind stress at periods longer than 10 days for superinertial periods lower than 20 days a two parameter i e angle and amplitude model is fitted to the isolated ageostrophic signal to study the upper ocean response to surface wind stress in that frequency band the model which can explain up to 30 of the variance in some areas indicates that the currents spiral to the right left of the wind with depth in the northern southern hemisphere in agreement with the ekman theory the absolute value of the angle parameter is found to exhibit significant spatial and seasonal variability the vertical eddy viscosity and the ekman layer depth as deduced from ekman theory are compared to previous results their seasonal variability is also studied model results are then used to compute the mean and standard deviation of ageostrophic currents in 5 boxes over the world ocean the equatorial divergence and the subpolar and subtropical convergence zones are well reproduced

whole earth tomography from delay times of p pcp and pkp phases lateral heterogeneities in the outer core or radial anisotropy in the mantle we perform tomographic inversions of compressional wave travel time measurements in order to image the laterally varying radial anisotropy of p wave velocity in the entire mantle in order to achieve an adequate sensitivity to both horizontal and vertical p velocity throughout the mantle we include in our data set travel time measurements corresponding to ray paths of very different geometries specifically we use data associated with the phases p pcp core reflected and pkpbc pkp df core refracted all extracted from the bulletins of the international seismological centre 1964 1993 pcp pkpbc and pkpdf data are also sensitive to the topography of the core mantle boundary cmb and pkp only to the velocity structure of the earth s core these circumstances require that we also allow for cmb topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core e g ritzwoller et al 1986 stevenson 1987 wahr and de vries 1989 vasco and johnson 1998 the solution models that we propose include alternatively a laterally heterogeneous outer core a radially anisotropic mantle or both so that trade offs between those features can also be evaluated we conclude that only minor and very localized radially anisotropic anomalies are present in the middle and lower mantle the most prominent being located in the deepest 400 km under the western pacific we find a discrepancy particularly evident by a comparison between different models of the cmb topography between the travel time measurements that are sensitive to the earth s core and those that are not we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and or lateral heterogeneity of the outer core

on the temporal variability of the transport through drake passage a recently obtained series of bottom pressure measurements from various positions in the drake passage dp has been used to estimate the transport variability of the antarctic circumpolar current acc under the assumption that the variability is predominantly barotropic and thus proportional to the north minus south pressure difference the standard deviations in derived transport range from 5 3 sv in 1993 to 8 9 sv in 1990 10 day filter 1 sv 106 m3 s 1 all values are less than the 10 sv 10 day filter obtained during the international southern ocean studies isos program at dp between the mid 1970s and early 1980s whitworth and peterson 1985 although some of this discrepancy is due to differences in procedure in dealing with gaps in the data our data suggest that the acc transport can be persistently less variable over several years than was previously thought in particular the isos data set contains two large amplitude changes in pressure difference over timescales less than 2 weeks while no similar events were observed in the more recent data although the more recent pressure gauges were deployed at different positions to those used in isos their reliability as indicators of acc transport changes has been established by examination of simultaneous pressure measurements within dp

substantial primary production in the land remote region of the central and northern scotia sea the scotia sea area has high productivity relative to the southern ocean as a whole but this displays strong latitudinal and longitudinal gradients elucidating the extent of these from a single cruise is problematic given the high variability of bloom timing and location in this region therefore this study used data from transects across the central scotia sea in spring summer and autumn of 2006 2008 and 2009 combined with satellite data to obtain a larger scale appreciation of the latitudinal contrasts in phytoplankton standing stock and primary production across the region concentrations of nitrate phosphate and particularly silicic acid increased towards the south of the transect with the latter showing a step change at the southern antarctic circumpolar current front saccf changes in seasonal nutrient concentrations indicated increasing phytoplankton uptake north of 57 s that peaked at 53 s in the georgia basin based on seasonal depletions of nitrate relative to phosphate the highest relative nitrate uptake occurred northwest of south georgia on the periphery of the georgia basin indicating efficient nitrate use here due to iron replete conditions an integrative approach to examine these gradients was with the use of 10 year satellite climatology data these showed that the lowest mean chlorophyll a chl a values were in the central northern scotia sea but these were still substantial values 67 of values within the georgia basin bloom cruise data on chl a and on microplankton biomass from cell counts support this finding of substantial biomass in the central scotia sea since these averaged half of values in the iron fertilised bloom of the georgia basin downstream of south georgia given that our transect was nearly 1000 km long and in parts was land remote with low iron concentrations the relatively high production in the central and northern scotia sea is surprising iron levels may be maintained here by efficient recycling and irregular injections possibly for example from dust deposition or shelf derived inputs from the south the moderate chl a concentration across the mid scotia sea and southwest of south georgia reflect periodic non ice associated blooms that occur in some years and not others these may provide a connection between the large populations of krill inhabiting the northern and southern fringes of the scotia sea

ocean dynamics associated with rapid j2 fluctuations importance of circumpolar modes and identification of a coherent arctic mode 1 fluctuations of the earth s dynamic oblateness j2 are a measure of global redistributions of mass between equatorial and polar regions sensitive to the component which is symmetric about the equator cox and chao recently published a time series of monthly values of j2 based on satellite orbit determination which showed rapid fluctuations from month to month as well as a large change in j2 in 1997 1998 we here investigate the ocean dynamics associated with these rapid fluctuations using a barotropic ocean model forced by atmospheric wind stress and pressure we find that the model has significant skill in reproducing the observations though not the change in 1997 1998 much of the model variability is related to coherent circumpolar fluctuations in arctic and antarctic bottom pressure which despite their apparent symmetry are forced by quite different mechanisms the antarctic mode is forced by circumpolar winds whereas the arctic mode is forced by northward winds over the entrances to the polar sea comparison with in situ observations shows that the model has skill in reproducing the polar modes arctic tide gauge records confirm the presence of a coherent mode at short periods and at long periods over at least the eurasian arctic shelf if barotropic and representative of the whole arctic this would be large enough to make a major contribution to measured j2 fluctuations but does not account for the 1997 1998 event

detection of p wave reflector in d beneath the south western pacific using double array stacking the double array event and receiver array stacking is applied to the p wave data from 6 fiji is events recorded by a local network of short period seismographs in japan to investigate vp structure in the lowermost mantle a signal which may be interpreted as the reflection from a discontinuity at 170 km above the cmb core mantle boundary is observed between p and pcp in the stacked record as compared with the receiver array stacking with a small number of stations applied to a single event the double array stacking is effective to detect the weak signal of pdp

kirchhoff synthetics for seismic reflections and diving waves in two dimensions and application to the d layer summary the kirchhoff helmholtz kh integration has been used to model the reflected and the diving waves from an interface with a positive velocity gradient the modelling is carried out for a spherical boundary and for a sinusoidal topography with a long scale wavelength an artefact which is a major problem in modelling the seismic response using the kh integration has been reduced by introducing a hilbert transform sign manipulation cleaner synthetic seismograms with correct amplitudes have been produced by this method a discretization in larger surface elements has been made possible by introducing a smoothing factor that suppresses the noise that normally follows the constructed signal if a large element size is taken

pcp and scp beneath the western pacific detected by a deep ocean bottom hydrophone array a deep ocean bottom hydrophone array near wake island in the western pacific has detected pcp and scp from several deep focus earthquakes that occurred beneath the sea of okhotsk and beneath the fiji basin two representative events have been analyzed the pcp of the okhotsk event is very strong and sharp with a predominant frequency of 2 hz whereas the pcp of the fiji event is weak with a predominant frequency closer to 1 hz the strong contrast between pcp from these two deep focus earthquakes demonstrates differences in reflectivity at the core mantle boundary as well as heterogeneities in seismic velocity and attenuation in the lower mantle travel times and spectral analyses show that seismic velocity attenuation factor q and reflectivity are all high beneath the northwest pacific basin and normal beneath the equatorial pacific

coherent sea level response to the antarctic oscillation 1 large scale sea level variations around antarctica were studied and their relationship with atmospheric variations was investigated tide gauge data at five coastal stations were used coherent sea level variations were clearly detected for all stations on intraseasonal time scales the coherent variations had significant negative correlations with an index of the atmospheric annular mode variation antarctic oscillation coherence was significant for periods from 10 to 100 days with negligible time lag the negative correlation is consistent with the mechanism that a high low westerly anomaly leads to a stronger weaker northward ekman drift and causes divergence convergence around antarctica

p and pcp travel time tomography for the core mantle boundary pcp arrival times reported to the international seismological centre isc are inverted for vertical travel time anomaly at the core mantle boundary cmb we first invert about 2 106 isc first p arrival data for the three dimensional 3 d structure of the mantle and hypocenter locations iteratively the pcp data are corrected for the relocated hypocenters and the 3 d structure this correction has reduced the observed pcp residuals the corrected pcp data are then inverted for the vertical travel time anomalies at the cmb which are due either to velocity anomaly or topographic anomaly the spherical average of the inverted structure requires either a velocity reduction in the boundary layer just above the cmb by 3 if the layer is 20 km thick or a lowering of the cmb by 3 km the heterogeneity pattern of the cmb is drastically different from that of the overlying d layer negative vertical travel time anomalies appear in the northern high latitude region and positive anomalies appear under southeast asia and middle to south america the lateral dimensions of these anomalies are of the order of 40 the anomalies can be interpreted as either a velocity perturbation in the boundary layer 7 if the layer is 20 km thick or an undulation of the cmb 7 km various tests including the error and resolution estimations indicate that these three anomalies are the significant resolvable features of the cmb these anomalies are also corroborated by the result of the joint inversion of p and pcp for both the 3 d mantle and the 2 d cmb assuming that the layer is laterally heterogeneous in iron content and deforms at the base in isostatic equilibrium with the outer core the pcp data are also inverted for the isostatic figure of the boundary layer the velocity perturbation in this case is 5 6 if the layer is 20 km thick and the cmb undulation is only 1 2 km regardless whether the cmb heterogeneity is due to velocity and or topographic perturbations it is dominant in the zonal component of degree 2 while the degree 2 pattern in the lower mantle including the d is dominant in sectorial component

yield estimation from spectral amplitudes of direct p and p coda recorded by the wake island deep ocean hydrophone array abstract spectral amplitudes of direct p and p coda between 0 5 and 6 hz have been measured for 14 eastern kazakh test explosions using 8 hydrophones of the wake island array and a new model for these data has been developed to estimate relative yields of those explosions each log spectral amplitude is considered to be the sum of four parameters and an error term three parameters are frequency dependent and represent 1 the average eastern kazakh spectrum at the wake island array 2 the spectral shape characteristics unique to each event and 3 the station or hydrophone corrections the fourth parameter represents relative yield a total of 346 parameters including the 14 relative yields were needed to model all 901 spectral amplitudes that were measured and first order linear regression techniques were used to invert the data for these parameters standard deviations of the computed relative yields are very small averaging only 0 015 m b units deviations between the relative yields and their corresponding relative neis body wave magnitudes however are larger averaging 0 14 m b units this difference is interpreted to be an indicator of the level of inhomogeneity in the worldwide pattern of energy radiated from eastern kazakh tests there is evidence that this pattern can be correlated with the precise epicentral location of each explosion within the test site and is due to focusing and defocusing near the source it is proposed that the yield of a large explosion might be disguised by siting the explosion at a location that selectively defocuses energy towards continents where most seismic stations are located the precision with which a relative yield is determined from the data of any single hydrophone averages 0 035 m b units thus the model appears to provide a very stable measure of yield errors associated with frequencies greater than 2 hz are only slightly greater than errors associated with frequencies less than 2 hz verifying the utility of using the higher frequencies observed in direct p and p coda to estimate yield

spectra of nuclear explosions earthquakes and noise from wake island bottom hydrophones spectral characteristics of p phases from 4 shallow focus earthquakes and 8 underground explosions and of 52 samples of ocean bottom background noise are examined by using tape recordings of ocean bottom hydrophones near wake island from july 1979 through march 1981 significant differences are found between spectra of large shallow focus earthquakes and explosions 5 7 6 hz observed in the teleseismic p phases indicate high q values for the deep mantle ambient noise levels on the ocean bottom near wake are comparable to levels at the quietest continental sites for frequencies between 3 and 15 hz also high levels of coherence at least as high as 0 85 have been observedmore for p phases recorded on sensors with 40 km separation less

an electron microscopic study of anorthoclase spherulites abstract spherulites consisting of fibrous alkali feldspar and silica minerals are produced by devitrification of rhyolite glass under hydrothermal conditions the alkali feldspars ab 72 5 or 23 0 an 4 5 ab 81 7 or 14 0 an 4 3 in spherulites from two localities in japan consist of triclinic anorthoclase showing fine cross hatched twinning and monoclinic sanidine showing fine cross hatching not attributable to twinning the cross hatching which corresponds to albite and pericline twinning is produced in the process of transition from a monoclinic to a triclinic phase the spherulite may develop at a temperature lower than about 200 c because the co existing silica mineral is not quartz but metastable tridymite according to the phase diagram of the alkali feldspars by mackenzie 1952 the alkali feldspars should have been triclinic during growth however the textures show that the alkali feldspar grew as a disordered monoclinic phase because of the high growth rate the al si disordered structure was produced during growth and afterwards transformed into a triclinic structure with cross hatched twinning

azimuth energy q and temperature variations on p wave amplitudes in the united states mean relative p wave 1 s amplitude characteristics are determined for standardized seismic stations in the united states data are compiled from three azimuths to the north explosions in russia to the northwest earthquakes along the asian pacific arc and to the south southeast earthquakes in south america characteristic receiver amplitudes are found to be azimuthally dependent high amplitude areas occur in the central united states and lowest amplitudes occur along the rocky mountain front range and the rio grande rift zone amplification effects of low velocity sediments are significant in the central united states azimuthal amplitude variations in the eastern united states are consistent with an anomalous attenuating or defocusing body in the lower mantle beneath colombia south america nonazimuthal amplitude studies are tied to the azimuthal data set to increase station amplitude coverage the regional and local stability of amplitudes is analyzed amplitudes are stable with an average variation of a factor of about 1 3 around the regional mean for seismic stations situated on hard bedrock the squared amplitude variations are related to energy variations in the p wave fronts acknowledging the likely importance of elastic focusing defocusing and scattering effects at individual stations variations of anelastic attenuation are derived that are consistent with the observed energy amplitude variations assuming that the variations of anelastic attenuation occur in the upper 400 km of the earth high amplitude stations may be unattenuated low amplitude stations are consistent with q 110 and a mean amplitude implies q 220 240 the range of variation of q is mapped into temperature variations assuming a thermally activated q mechanism and an olivine activation energy for the upper mantle the apparent q variations are consistent with temperature variations of 135 287 c between high and low amplitude seismic station sites geophysical and physiographic correlations with the p wave amplitude variations are discussed accounting for the motion of the north american plate low p wave amplitudes along the rocky mountain front may be related to the uplift of the colorado plateau low amplitudes extending from northern texas to new england lie along the approximate boundary of the continental craton

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

upper ocean temperature and the baroclinic transport stream function relationship in drake passage repeat hydrographic sections across the antarctic circumpolar current acc in drake passage are used to derive an empirical relationship between upper ocean temperature and the baroclinic transport stream function cross validation shows this relationship can be used to infer baroclinic transport above and relative to 2500 m from expendable bathythermograph xbt temperature measurements with an error of a few per cent transport errors of less than 2 sv are obtained if temperature at depths between 600 and 1600 m is used to define the relationship temperature at depths above 300 m provides an unreliable index of transport because of variability in temperature salinity t s properties produced by air sea interaction the scatter in the relationship between temperature and stream function from repeat observations along a single line is similar in magnitude to the scatter observed when data from the broader drake passage area are considered in both cases variability about the mean temperature stream function relationship reflects advection of water with anomalous t s properties the tight relationship between temperature and stream function in drake passage and south of australia suggests baroclinic transports can be inferred from xbt temperatures with high accuracy in the southern ocean providing a cost effective means of monitoring acc variability however care must be taken at the end points particularly in the drake passage where the strong flow of the subantarctic front sometimes lies over the continental slope

whole earth structure estimated from seismic arrival times the arrival times of 10 compressional and shear phases p s pp pcp ss scs pk pab pk pbc pk pdf sk sac and two sets of differential times ss s410s and ss s660s are used to infer the earth s three dimensional structure with the exception of ss sds the raw data are composed of arrival times from the international seismological centre isc catalogue the isc events have been relocated using p and s phases and the ak135 radial earth model a total of 818 655 summary arrival times constrain lateral velocity variations in the crust mantle and core we also determine relocation parameters for all events stations corrections for each phase boundary topography 410 km 660 km core mantle boundary inner core boundary and inner core anisotropy variations our model of p and s velocity variations in the crust and mantle contains the general features observed in other body wave inversions a low velocity ring around the pacific ocean basin in the depth range 35 400 km high velocity anomalies associated with subducting slabs most prominent in the depth range 660 1070 km and high velocity anomalies surrounding the pacific basin just above the core mantle boundary in the outer core there is evidence of a latitudinal variation in velocity approaching 0 6 in magnitude with lower p velocities at the poles and higher velocities near the equator this pattern is observed throughout the outer core but is most prominent in the depth range 3671 4071 km and near the inner core boundary synthetic tests indicate that outer core features are moderately resolvable and inversions of random arrival times indicates that the anomalies are a factor of 2 above the estimated parameter noise level lateral heterogeneity in the inner core trades off with anisotropy resulting in poor resolution of isotropic velocity variations anisotropy in the inner core is predominantly subparallel to the earth s axis and of the order of 3 but this value can decrease if more inner core isotropic lateral heterogeneity is allowed a comparison of structure above and below the core mantle boundary cmb indicates that velocity heterogeneity is uncorrelated across the cmb but there is a moderate correlation between cmb topograpy and outer core heterogeneity an inversion in which the pk pdf residuals were absent indicates that the latitudinal pattern of outer core velocity variations is not due to the influence of inner core velocity structure

zoned ternary feldspars in the klokken intrusion exsolution microtextures and mechanisms the microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the klokken intrusion described in the preceding paper were determined by tem and their origin and evolution deduced the feldspars normally have a plagioclase core and an alkali feldspar rim cores become smaller and rims larger and the an content of both decrease with distance from the contact of the intrusion the following microtextural sequence was observed the inner plagioclase cores are homogeneous oligoclase andesine with albite growth twins only but are crypto antiperthitic towards the outer core at first small platelets of low sanidine a few nanometres thick and up to 10 nm long occur sporadically only on albite twin composition planes with further increase in bulk or they are homogeneously distributed in the plagioclase thicker through going plates in platelet free areas are found which induce albite twins in the surrounding plagioclase the microtextures in the rims are regular cryptomesoperthitic with 601 lenses or lamellae depending on the bulk or content of low sanidine in albite twinned low oligoclase andesine albite and pericline twins in plagioclase in an m twin relationship together with lenticular low sanidine were found in only one small area the overall diffraction symmetry of the mesoperthites is monoclinic showing that exsolution started in a monoclinic feldspar whereas that of the antiperthites is triclinic the intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres the cryptomesoperthites are very regular where or rich and probably arose by spinodal decomposition the platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere near the intermediate zone they coarsened to give larger plates which induced albite twins in the plagioclase because of the zoning microtextures that were initiated in areas of given composition can propagate laterally into zones of different composition a diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly cooled rocks

simultaneous inversion of steep angle observations of pcp and scp in europe what can we learn about the core mantle boundary summary at epicentral distances less than 30 the steep angle reflections pcp and scp from the core mantle boundary cmb are normally difficult to identify and rarely reported the reason is that the reflection coefficient at the cmb for these phases is relatively small and pcp and scp onsets are often disturbed by interference with other more prominent phases however the absolute traveltimes of pcp and scp in the steep angle range are the most sensitive seismic phases for depth undulations of the cmb and in addition of all cmb reflections these phases are influenced least by lateral heterogeneities along their ray paths during this study a set of more than 200 such reflections from the earth s core observed at european stations with reflection points below europe was collected and analysed starting from observations at the geres array in germany pcp and scp could also be identified at other european seismic stations the stations used for this study were the fennoscandian arrays norsar nores and fines the grafenberg array in germany and the stations of the german regional seismic network grsn with the combined investigation of pcp and scp a map of reflection points of the cmb below europe could be constructed to reduce the influence of location and other errors traveltime differences were measured between the onsets of p or pn pcp and scp these traveltime differences show positive and negative deviations from theoretical values of up to several seconds cmb undulations of up to 6 km as discussed in the literature change the absolute traveltimes of pcp or scp only by about 0 9 or 1 2 s for an epicentral distance of 20 the observed residuals can be explained by several factors still unresolved location errors lateral heterogeneities along the whole ray paths of all investigated phases or undulations of the depth of the cmb all observed traveltime effects must be known at least one order of magnitude better than the expected effects to resolve statistically reliable cmb undulations inverting the residuals for possible cmb undulations shows that if there is any cmb signal in the data it is very small most of the observed residuals can easily be explained by smaller changes in event depth lateral heterogeneities in the source region and along the whole ray paths and or station corrections but the observations do not require any cmb topography if such undulations of up to 6 km exist all other traveltime effects will mask them coming to this disillusioning result by using the most sensitive data to resolve a cmb topography can only lead to this conclusion all models of cmb undulations derived from traveltime data must be taken as very unsure and any far reaching interpretations of such models should be avoided

observations of q from the northwest pacific subduction zone recorded at teleseismic distances twenty seven earthquakes in the northwest pacific with mb 5 4 are used to study upper mantle attenuation the data were recorded at seismometer arrays in australia canada india and scotland the short period instruments used in the arrays are sufficiently broadband and the signal to noise enhancement resulting from beamforming the data is good enough that seismic noise is above system noise out to between 3 and 5 hz and in some cases to 8 hz the instrument response has been removed and a power law 2 farfield source correction has been assumed and applied the remaining fall off in the spectra above 1 hz which is above the corner frequency fc is assumed to be due to attenuation including losses due both to anelasticity and to scattering an 3 spectral fall off preferred by some would result in lower estimates of attenuation conservative estimates of the average attenuation along the whole path have been made from the spectral fall off despite the imperfect knowledge of the source spectra the high sensitivity of spectral amplitudes to attenuation at high frequencies yields quite reliable and stable attenuation estimates the scatter in the data is small resulting in estimates of mean t t av with an average standard error of only 21 where t av is defined as the ratio of the travel time t to the effective mean quality factor q av only 14 of the 64 measurements show a t av above 0 5 sec within the given error bounds all estimates of t av and q av refer to p wave attenuation only the results for t av from the northwest pacific subduction zone cannot be explained simply by the traditional high q subducting slab in a low q upper mantle a low q region in the uppermost 100 km approximately of the subduction zone has been identified source corrected t av yield upper mantle q av estimates of about 1200 for predominantly oceanic paths australia and india and 1500 for mixed continental and oceanic paths canada and scotland

changes in the ocean transport through drake passage during the 1980s and 1990s forced by changes in the southern annular mode 1 we present the first direct evidence that interannual changes in ocean transport through drake passage are forced by variability in the southern annular mode sam this evidence is derived from two decades 1980s and 1990s of subsurface pressure measurements from the tide gauge at faraday station western antarctic peninsula combined with the output of an ocean general circulation model in recent decades the sam has moved toward a higher index state stronger circumpolar winds this trend is not simply monotonic but is the product of a long term change in the seasonality of the sam whilst we cannot address directly the effect of the long term trend on circumpolar transport bottom pressure data from drake passage during the 1990s demonstrate that ocean transport showed the same changes in seasonality as did the sam this offers a mechanism for atmospheric climate change to influence directly the largescale ocean circulation indexterms 4207 oceanography general arctic and antarctic oceanography 4215 oceanography general climate and interannual variability 3309 4532 oceanography physical general circulation 1635 global change oceans 4203 4556 oceanography physical sea level variations citation meredith m p p l woodworth c w hughes and v stepanov 2004 changes in the ocean transport through drake passage during the 1980s and 1990s forced by changes in the southern annular mode geophys res lett 31 l21305 doi 10 1029 2004gl021169

p and s wave reflections from anomalies in the lowermost mantle summary seismic p and s waves recorded at the grf array in germany are used to study the inhomogeneous structure of the boundary layer d at the base of the mantle the use of the seismic array allows the detection of small scale anomalies in the lowermost mantle the lateral resolution attainable is about 100 to 400 km i e more than 10 times better than with tomographic methods the analysis of 13 years of grf broad band array data yields 255 events with high signal to noise ratio which are used to map the lowermost mantle 74 of these events show anomalous p waves pdp which arrive 3 6 s after the direct p wave and have a slowness 0 7 0 8 s smaller than the slowness of direct p other events close to the events with pdp do not have such an anomalous phase using slowness traveltime amplitude and waveform information it is demonstrated that pdp is caused by an anomalous lower mantle velocity structure below the turning point of the p wave if the pp phase i e a p wave first reflected at the free surface near the source is used together with the p phase distinct and well sepaarted p velocity anomalies can be determined under the nansen basin the kara sea and northern siberia the areas of the bounce points of pdp in the lower mantle have a lateral extension of about 100 by 200 km but this is not the size of the anomaly since the resolution of p waves at 1 hz in this depth is 130 km by 260 km fresnel zone the accuracy to which the depth of the reflector 2612 km under the nansen basin and 2605 km under northern siberia respectively can be determined for 1 d models is 10 to 20 km the velocity contrast at the lower mantle discontinuity is about 3 per cent 1 to 1 5 per cent areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas 2 d models of the anomalies reveal the range of adequate models and possible trade offs if the lateral extension of the anomaly is about 7 the reflector has to be 40 km deeper than in the 1 d model if the dip of the reflector in the lowermost mantle is only about 1 5 it is difficult to resolve if the reflector is tilted towards the source or towards the receiver for the anomaly under the nansen basin deviations from the great circle path are observed for pdp indicating 3 d effects below the three anomalies the core mantle boundary cmb can be located by pcp with a depth in agreement with standard earth models under northern siberia has a region where p and s velocity anomalies coincide but also a region where only an s velocity anomaly but no p velocity anomaly is observed this results in changes of the poisson ratio from 5 9 per cent to 4 8 per cent 1 5 per cent across the discontinuity in the lowermost mantle for the regions studied the analysis of the s waves s sds and scs reveals two s velocity anomalies in the lowermost mantle under northern siberia the first anomaly coincides with the p velocity anomaly under northern siberia and can be explained by a 1 d model with a reflector depth of 2610 km 15 km and a velocity contrast of 2 3 per cent the second s velocity anomaly is in an area where no p velocity anomaly can be detected the corresponding 1 d s velocity model has a reflector depth of 2575 km 15 km and an s velocity contrast of 2 6 per cent the smallest structures that can be resolved with the s waves in this depth are about twice as large as for the p waves i e 230 km by 460 km fresnel zone the joint analysis of p and s waves therefore shows a region with a p velocity anomaly together with weak indications for an s velocity anomaly nansen basin and a second region with a p but no s anomaly kara sea the lowermost mantle

slowness corrections one way to improve idc products the first step to identify and locate a seismic event is the association of observed onsets with common seismic sources this is especially important in the context of monitoring the comprehensive nuclear test ban treaty ctbt at the international data center idc being developed in vienna austria well defined slowness measurements are very useful for associating seismic phases to presumed seismic events shortly after installation of the first seismic arrays systematic discrepancies between measured and theoretically predicted slowness values were observed and therefore slowness measurements of seismic stations should be calibrated the observed slownesses measured with small aperture arrays some of which will be included in the international monitoring system ims now being implemented for verifying compliance with the ctbt show large scatter and deviations from theoretically expected values however in this study a method is presented by which mean slowness corrections can be derived which show relatively stable patterns specific to each array the correction of measured slowness values of these arrays clearly improved the single array location capabilities applying slowness corrections with seismic phases observed by arces fines geres and nores and associated to seismic events in the bulletins of the prototype international data center pidc in arlington va also clearly demonstrates the advantages of these corrections for arrays with large slowness deviations that are due to the influence of a dipping layer the corrections were modeled with a sine function depending on the measured azimuth in addition the measured values can be weighted with the corresponding uncertainties known from the process of deriving the mean corrections

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

circumpolar structure and distribution of the antarctic circumpolar current fronts 1 mean circumpolar paths 1 high resolution hydrographic sections and maps of the gradient of sea surface height ssh reveal that the antarctic circumpolar current acc consists of multiple jets or frontal filaments here we use a 15 year time series of ssh observations to determine the circumpolar structure and distribution of the acc fronts the jets are consistently aligned with particular streamlines along the entire circumpolar path confirming and extending the results of an earlier study restricted to the region south of australia the intensity of the fronts as measured by the cross front gradient of ssh varies along the fronts and the individual branches merge and diverge often in response to interactions with bathymetry maps of absolute velocity at 1000 m depth derived from argo trajectories confirm the existence of multiple current cores throughout the southern ocean high resolution hydrographic sections and profiles of temperature and salinity from argo floats are used to show that the front locations derived from fitting ssh contours to maps of ssh gradient are consistent with locations inferred from the traditional criteria based on water mass properties suitably modified to account for multiple frontal branches three regions are examined in detail the crozet plateau the kerguelen plateau and the scotia sea these examples show how recognition of the multiple jets of the acc can help resolve discrepancies between previous studies of acc fronts

dissolved iron in the vicinity of the crozet islands southern ocean the annual phytoplankton bloom occurring north of the crozet plateau provides a rare opportunity to examine the hypothesis that natural iron fertilization can alleviate high nutrient low chlorophyll hnlc conditions normally associated with the southern ocean therefore during crozet natural iron bloom and export experiment crozex a large multidisciplinary study performed between november 2004 and january 2005 measurements of total dissolved iron dfe0 2 m were made on seawater from around the islands and atmospheric iron deposition estimated from rain and aerosol samples dfe concentrations were determined by flow injection analysis with n n dimethyl p phenylenediamine dihydrochloride dpd catalytic spectrophotometric detection dfe concentrations varied between 0 086 and 2 48 nm with low values in surface waters enrichment of dissolved iron 1 nm at close proximity to the islands suggests that the plateau and the associated sediments are a source of iron waters farther north also appear to be affected by this input of coastal and shelf origin although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of 0 07 nm km 1 at the time of sampling using lateral and vertical diffusion coefficients derived from ra isotope profiles and also estimates of atmospheric inputs it was then possible to estimate a dfe concentration of 0 55 nm to the north of the islands prior to the bloom event which is sufficient to initiate the bloom the lateral island source being the largest component a similar situation is observed for other sub antarctic islands such as kerguelen south georgia that supply dissolved iron to their surrounding waters thus enhancing chlorophyll concentrations

detailed investigation of a spot on the core mantle boundary using digital pcp data abstract a number of deep earthquakes from the fiji tonga region were used to study the properties of the core mantle boundary cmb the investigation was restricted to an area with a diameter of only a few hundred kilometers at the cmb the use of a cluster of neighboring events yielding redundant sampling and rather good resolution the components of the investigation are i a classical stacking procedure to search for additional reflectors near the cmb ii a deconvolution of seismograms and a stack of the inferred reflector series for the same purpose iii a matched filter method to determine pcp travel times and pcp p amplitude ratios precisely and iv a comparison with synthetic seismograms to check the consistency of derived travel time residuals and pcp amplitude variations in the presence of cmb topography while we do not see any additional reflector for epicentral distances around 35 we detect by stacking seismograms recorded at 70 an additional phase which might be caused by a reflection from the top of d we show that pcp travel time residuals must not be interpreted as being caused exclusively by cmb topography an additional investigation of synthetics provides some constraints on dominant wavelengths and corresponding amplitudes of cmb undulations for wavelengths of 50 400 km the elevation cannot exceed 2 3 km if pcp is observed in this particular area

models of the core mantle boundary and the travel times of internally reflected core phases topographic maps of the core mantle boundary cmb obtained by inverting different subsets of international seismological centre arrival time data involving either waves refracted through the boundary pkp or waves reflected from the topside pcp do not agree we report on the implications for models of cmb structure when data from bottomside reflections notably pkkp are included in the inversion travel time residuals with respect to a reference earth model prem of all core phases are positively biased but the mean residual is different for different phases and must be accounted for prior to inversion the new data illustrate that models of cmb topography can explain only a small fraction of the data variance the new cmb model involves both topography and a laterally varying boundary layer the cmb itself is smooth compared to previous results

amplitude of the core mantle boundary topography estimated by stochastic analysis of core phases abstract the core mantle boundary cmb topography is an important parameter for constraining the models of mantle dynamics and core mantle coupling however the various large wavelength seismological models of the cmb topography which have been obtained up to now are poorly correlated moreover their maximum amplitudes vary considerably from one model to another with values ranging from 4 to 12 km these large discrepancies may be due to the difficulty to separate in the travel time anomalies the contribution of the cmb topography from that of the highly heterogeneous d region at the base of the mantle in order to better constrain the amplitude of the cmb topography we perform a stochastic analysis of the core phases which sample the cmb as transmitted waves and or as reflected waves in particular we analyse underside reflected pkkp phases which help to discriminate between cmb topography and d structure and have in addition a great sensitivity to cmb topography the other phases used are the upperside reflected waves pcp and the transmitted waves pkp the analysis is performed on the travel time residual file obtained by engdahl et al engdahl e r van der hilst r buland r p 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seismol soc am 88 722 743 after earthquake relocation and phase re identification after careful travel time data selection the stochastic analysis allows us to separate coherent signal from random signal in the data at different length scales ranging from 300 to 1500 km then the cmb topography variance is obtained at the different length scales from a joint analysis of the coherent signal in the different core phases in taking into account their different sensitivities to cmb topography the estimated cmb topography variance has a significant signal in the wavelength range that we have investigated showing that 95 of cmb topography amplitude is in the range 4 0 km for wavelengths larger than 300 km this value decreases to 1 5 km for wavelengths larger than 1200 km indicating that the long wavelength cmb topography is significantly lower than previously proposed however it has not been possible to invert the data for deriving a map of the cmb topography a checkerboard pattern analysis reveals that despite the introduction of the pkkp phase the relative contributions of d heterogenieties and cmb topography cannot be separated with the presently available data