direct velocity measurements in the malvinas current we present a set of current meter measurements collected in the southwest atlantic from december 1993 to june 1995 four moorings were set out to monitor the malvinas falkland current at 40 41 s and one mooring was located within the brazil malvinas front area at 38 s the mean flow of the malvinas current has an equivalent barotropic structure whereas the vertical structure of the variable part of the flow is dominated by a barotropic like empirical mode for regions shallower than 2500 m surface intensified which appears as a linear combination of barotropic and baroclinic modes suggesting mode coupling due to the steep topography this mode appears to gradually become nearly exactly barotropic toward shallower depths higher empirical modes bottom intensified and associated with higher frequencies suggest waves trapped by the conjugated effects of topography and stratification the time variability is discussed in terms of along and cross isobath flows the along shelf flow features substantial energy in the 50 to 70 day band there is no evidence of an annual cycle nor of a distinct semiannual cycle but there is a suggestion of significant energy at periods a little shorter 135 days the cross shelf variability is important with a quite clear annual cycle associated with excursions of the subantarctic front its phase coincides with that of the brazil current transport suggesting that annual migrations of the front are predominately determined by the strength of the latter whereas the intensity of the malvinas current would only have marginal impact a conclusion that may not hold at other timescales

geometry and p and s velocity structure of the african anomaly 1 we constrain the geometry and p and s velocity structure of a low velocity anomaly in the lower mantle beneath southern africa we term it the african anomaly on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the east pacific rise to the japan sea our collected data set consists of direct s direct p sdiff scs pcp sks and skks phases recorded by three temporary broadband passcal seismic arrays deployed in africa between 1994 and 2002 the tanzania seismic array 1994 1995 the kaapvaal seismic array 1997 1999 and the ethiopia kenya seismic array 2000 2002 for earthquakes occurring in the east pacific rise drake passage south sandwich islands iran hindu kush xinjiang and the japan sea the seismic data provide excellent sampling of the african anomaly in the lower mantle along the specific great arc in order to accurately account for the contributions from the african anomaly we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the global seismographic network and correct for the contributions from the seismic heterogeneities outside the african anomaly the seismic observations suggest that the african anomaly locally extends 1300 km above the core mantle boundary beneath southern africa around 25 n 27 e and exhibits a bell like geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth the base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions the seismic data can best be explained by a shear velocity structure with average velocity decreases of 5 in the base and 2 to 3 in the mid lower mantle above the base and a compressional velocity structure with a uniform s to p velocity perturbation ratio of 3 1 for the entire african anomaly these geometric and velocity features suggest that the mid lower mantle portion of the african anomaly is an integral component of the very low velocity province and the african anomaly is compositionally distinct and geologically stable

strong heterogeneity caused by deep mantle layering 1 numerical simulations are used to characterize the volumetric heterogeneity associated with proposed chemical layering in the deep mantle a globally continuous undulating layer generates enormous amounts of volumetric heterogeneity which is caused by lateral variations in the layer boundary depth and by strong hot upwellings arising from the top of the layer in the middle lower mantle region in contrast the heterogeneity in the deepest mantle is very weak partial cancellation of the temperature and compositional contributions to seismic velocity could mask the layer itself but the strong signatures of the thermochemical boundary layer and induced upwellings remain even when fields are filtered to approximate the resolution of global seismic tomographic models this distinctive heterogeneity is not observed in global seismic tomographic models dense material that is restricted to discontinuous piles generates heterogeneity that is more consistent although not entirely with seismic observations with strong heterogeneity at the core mantle boundary radial correlation functions display a characteristic narrowing in the vicinity of any chemical layer but are very sensitive to the applied seismic tomographic filtering

probabilistic lowermost mantle p wave tomography from hierarchical hamiltonian monte carlo and model parametrization cross validation bayesian methods powered by markov chain monte carlo estimates of posterior densities have become a cornerstone of geophysical inverse theory these methods have special relevance to the deep earth where data are sparse and uncertainties are large we present a strategy for efficiently solving hierarchical bayesian geophysical inverse problems for fixed parametrizations using hamiltonian monte carlo sampling and highlight an effective methodology for determining optimal parametrizations from a set of candidates by using efficient approximations to leave one out cross validation for model complexity to illustrate these methods we use a case study of differential traveltime tomography of the lowermost mantle using short period p wave data carefully selected to minimize the contributions of the upper mantle and inner core the resulting tomographic image of the lowermost mantle has a relatively weak degree 2 instead there is substantial heterogeneity at all low spherical harmonic degrees less than 15 this result further reinforces the dichotomy in the lowermost mantle between relatively simple degree 2 dominated long period s wave tomographic models and more complex short period p wave tomographic models

empirical evidence for latitude dependence and asymmetry of geomagnetic spatial variation in mainland china spatiotemporal geomagnetic variation is a significant research topic of geomagnetism and space physics generated by convection and flows within the fluid outer core latitude dependence and asymmetry as the inherent spatiotemporal properties of geomagnetic field have been extensively studied we apply and modify an extension of existing method hidden markov model hmm which is an efficient tool for modeling the statistical properties of time series based on ground magnetic measurement data set in mainland china first we find the parameters of hmm can be used as the geomagnetic statistical signature to represent the spatiotemporal geomagnetic variations for each site the results also support the existence of the geomagnetic latitude dependence more apparently furthermore we provide solid empirical evidence for geomagnetic asymmetry relying on such ground magnetic measurement data set

lateral variations in cmb heat flux and deep mantle seismic velocity caused by a thermal chemical phase boundary layer in 3d spherical convection article i nfo numerical simulations of thermo chemical multi phase compressible mantle convection in a three dimensional spherical shell are used to investigate the relationship between lateral variations in seismic shear wave velocity vs above the core mantle boundary cmb and lateral variations in heat flux across the cmb qcmb when compositional variations and the post perovskite phase transition are included for simple thermal convection the vs qcmb relationship is reasonably but not perfectly linear the post perovskite transition introduces a non linearity that amplifies fast vs anomalies in cold regions but there is still a unique mapping between vs and qcmb lateral variations in composition such as piles of dense material introduce another non linearity that affects hot upwelling regions and introduces a non uniqueness in vs qcmb if the dense material e g morb is seismically fast compared to the surrounding material in this case dense piles are ringed by sharp low vs anomalies if the cmb is covered by a global dense layer than variations in vs and qcmb are reduced but so is the mean value of qcmb in all cases the peak to peak lateral variation in qcmb is similar to or larger than twice the mean value which might create problems for generating a dynamo according to existing numerical dynamo simulations analytical scalings are developed to explain the observed trends

simulated geomagnetic reversals and preferred virtual geomagnetic pole paths summary the question of whether virtual geomagnetic poles vgps recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists one possible mechanism for such vgp clustering is the heterogeneity of heat flux at the core mantle boundary cmb we use 3 d convection driven numerical dynamo models with imposed non uniform cmb heat flow that show stochastic reversals of the dipole field we calculate transitional vgps for a large number of token sites at the earth s surface in a model with a simple heat flux variation given by a y22 harmonic the vgp density maps for individual reversals differ substantially from each other but the vgps have a tendency to fall around a longitude of high heat flow the mean vgp density for many reversals and excursions shows a statistically significant correlation with the heat flow in a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean vgp density at american and east asian longitudes roughly consistent with the vgp paths seen in several palaeomagnetic studies we find that low latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated they contribute to the equatorial dipole component and bias its orientation in longitude during reversals the equatorial dipole part is not necessarily dominant at the earth s surface but is strong enough to explain the longitudinal preference of vgps as seen from different sites

brazil malvinas frontal system as seen from 9 years of advanced very high resolution radiometer data 1 surface thermal fronts in the southwestern atlantic swa ocean are examined using 9 years 1987 1995 of advanced very high resolution radiometer data fronts are detected considering a gradient based edge detector sea surface temperature gradients are calculated from 4 km resolution 5 day composite images covering the western argentine basin south of 30 s variability in the position and intensity of the fronts from seasonal to interannual timescales is characterized in six regions including different parts of the brazil current front bcf and the subantarctic front saf shelf break brazil sb b brazil malvinas collision bm c brazil current overshoot bc o shelf break malvinas sb m malvinas return flow mrf and falkland escarpment fe fronts in the sb b sb m mrf and fe regions are controlled by the bathymetry in the bm c region the bcf and saf appear to merge as a single front this front does not present large seasonal north south excursions as previously described though it pivots seasonally around a fixed point located approximately at 39 5 s 53 5 w changing its orientation from n s in winter to nw se in summer consequently on average the front intersects the 1000 m isobath at 38 30 s in summer and north of 37 s in winter in the bc o region the bcf has a u shape centered at 53 w the intensity of the fronts in each region except in the fe region presents large seasonal variability in the sb b bm c and bc o regions the frontal intensity is highest 0 35 c km during austral winter the annual components explain 83 67 and 71 of the total variance respectively in the sb m and mrf regions the saf is most intense 0 25 c km in summer and fall the annual component of the intensity fluctuations explains 29 and 38 of the total variance respectively in the fe region the annual component of frontal intensity explains only 17 of the variability in the six regions important interannual variability is found the zapiola rise centered at 45 s 43 w appears as a gradient free region closed planetary potential vorticity contours in this area suggest it is dynamically isolated

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

on the implementation of open boundary conditions for a general circulation model the three dimensional case this article discusses the results of an experimental survey on the performance of a select group of open boundary conditions obcs for three dimensional primitive equation models the focus is on passive conditions where the model response at the open boundaries is dictated by the interior dynamics the performance of the obcs is tested in a series of numerical experiments conducted in a rotating channel which includes variable bottom topography and density stratification the experiments are sele cted to enhance nonlinear phenomena that comprises advection and the propagation of dispersive waves the first two experiments study coastal upwelling with constant and time varying wind forcing the third experiment analyses the barotropic and baroclinic response of a coastal ocean due to the passage of a traveling storm the open boundary condition with the best overall performance is the flow relaxation scheme on sea surface elevation and transport a radiation condition for internal velocities and a combined scheme of flow advection and relaxation for the temperature field a modified gravity wave radiation scheme provides reasonable responses when combined with radiation conditions for internal velocities and an advection equation for temperature if the forcing is not changing direction rapidly at the open boundary schemes using the method of characteristics or traditional wave advection schemes fail when combined with radiation conditions for the baroclinic mode

thermochemical flows couple the earth s inner core growth to mantle heterogeneity the uppermost 100 km of the earth s inner core is subject to an east west divide seismic waves travel more rapidly and are attenuated more severely in the eastern than in the western hemisphere and the west is more anisotropic that is seismic waves travel at different speeds in different directions than the eastern hemisphere the origin of this hemispherical dichotomy has remained enigmatic aubert et al now show that a model incorporating thermochemical convection and dynamo action can account for these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core have revealed that the eastern hemisphere is seismically faster more isotropic and more attenuating than the western hemisphere it is now shown that a single model of thermo chemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core reveal that the eastern hemisphere 40 e 180 e is seismically faster1 2 more isotropic2 3 and more attenuating4 than the western hemisphere the origin of this hemispherical dichotomy is a challenging problem for our understanding of the earth as a system of dynamically coupled layers previously laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core5 which in turn can induce textural heterogeneity on the inner core solidification front6 the resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo specifically magnetic flux concentrations7 8 in the time average palaeomagnetic field9 10 over the past 5 myr and preferred eddy locations11 in flows imaged below the core mantle boundary by the analysis of historical geomagnetic secular variation12 here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle the main feature of this thermochemical wind is a cyclonic circulation below asia which concentrates magnetic field on the core mantle boundary at the observed location and locally agrees with core flow images this wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary suggesting that lateral seismic anomalies at the top of the inner core result from mantle induced variations in its freezing rate

the patagonian shelf tides the patagonian shelf tides constitute one of the strongest regimes among the biggest in the world where a significant amount of global tidal energy is dissipated by the effect of bottom friction in this paper the tidal response of the patagonian shelf is described by means of information generated by a numerical model and computations of energy fluxes and dissipation rates are presented since measurements of tidal elevations and currents on the patagonian shelf are scarce and mostly limited to coastal locations the information obtained from the model offers valuable insight in a region where a detailed description was not available particularly in the context of global tidal modelling the tidal regime on the patagonian shelf is dominated by the principal lunar semidiurnal tide m2 with contributions from other semidiurnal diurnal and fourth diurnal constituents this regional study shows that dissipation is highly localized and that the integrated value of the dissipation rate over the model domain constitutes a significant fraction of the estimated global dissipation

shelf break upwelling driven by brazil current cyclonic meanders analysis of hydrographic data and results of numerical modeling show the occurrence of upwelling associated with the cyclonic meanders of the brazil current in the region known as the southeast brazil bight or santos bight the temperature and salinity distributions collected during summer and winter confirm the idea that meander induced shelf break upwelling plays an important role in the pumping of south atlantic central water from the slope region onto the continental shelf

coastal oceanographic regimes of the northern argentine continental shelf 34 43 s the oceanographic regimes of the northern argentine continental shelf nacs 34 e43 s are derived from advected waters of subantarctic origin local sources of continental run off and a locally generated salinity maximum based on 3690 ctd profiles monthly mean wind fields at coastal stations and river discharge data we define the oceanographic regimes over the shelf by analyzing salinity characteristics and spatial distribution 1 a maximum in salinity 33 7e34 2 originating from the gulf of san matias 2 a relative salinity minimum 30 0e33 3 of the el rincon estuarine system 3 a salinity minimum 0e33 0 originating in the rio de la plata and 4 waters of the continental shelf 33 5 and 33 7 temperature over the shelf is controlled by seaeair heat exchange coupled with bathymetry an analysis of the simpson parameter of stability f provided an objective definition of a vertically homogenous coastal zone separated from seasonally stratified shelf waters south of 37 s bottom temperature gradients and synoptic sections in the winter and spring indicate the presence of a shallow sea front at the 40e50 m isobaths south of 37 s persistent throughout the year we define two seasonal periods autumnewinter and springesummer based on seasonality in monthly mean winds fields continental run off fresh water balance and the spatial distribution of salinity signals maximum seasonal variation in the extent and location of the oceanographic regimes occurs within the coastal zone in the autumnewinter period we observe a northward extension of the rio de la plata and gulf of san matias waters as well as a reduction of the el rincon and continental shelf waters near the coast the springesummer period is characterized by rio de la plata waters flowing to the south and east a reduction of gulf of san matias waters and an invasion of el rincon and continental shelf waters into the coastal areas in a general sense waters across the nacs undergo a seasonal oscillation in distribution and extension that implies a springesummer reversal of the characteristic shelf wide northenortheastward direction of flow within the coastal zone 2005 elsevier ltd all rights reserved

paleomagnetic volcanic data and geometric regularity of reversals and excursions mostly on the basis of paleomagnetic sedimentary data it has been suggested that maps of virtual geomagnetic poles vgps corresponding to directions of the magnetic field at each site tend to fall along american and asian longitudes during reversals and excursions such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled however studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields in this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 myr volcanic eruptions occur sporadically thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions but we prefer not to use averaging methods of previous investigators who discarded or combined directions when they appeared to be similar we find that volcanic data give intermediate vgps which tend to fall along american and asian longitudes roughly consistent with the sedimentary data this result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites provided the appropriate polarity is assigned to intermediate vgps we find that icelandic vgps tend to fall along asian longitudes other patterns in the data for example latitudinal clustering of vgps or distinguishing longitudinal preferences of excursions from reversals are not resolved however it appears that in general transitional fields are nondipolar

spatial and temporal structure of the barotropic response of the scotian shelf and gulf of maine to surface wind stress a model based study the barotropic current and pressure field responses of the scotian shelf and gulf of maine region to spatially uniform wind stress are examined using a three dimensional finite element numerical model the model is linear and harmonic with spatially varying eddy viscosity and bottom friction coefficients obtained from a nonlinear m2 tidal solution the solutions for low frequency 20 day period wind stress and upstream boundary forcing are generally consistent with previous depth averaged model results showing a substantially stronger response for along shelf compared to cross shelf stress and strong topographic shaping of the associated arrested topographic waves with their expanded domain and dimensionality the present solutions reveal important additional features of the responses such as the northeast channel being the primary conduit for cross shelf subsurface flow compensating the cross shelf ekman transport over the scotian shelf widespread areas of near bottom upwelling and downwelling along the coast and shelf break and leakage of the response across the shelf break model solutions for stress forcing with periods in the storm band 2 5 days show similar structures and elevation current gains to those in the low frequency solutions with the primary exception that the large northeast channel transport for along shelf stress excites a topographic wave over the continental slope south of georges bank the strength of bottom stress dependent on the magnitudes of both the bottom friction coefficient and near bottom viscosity in the model has a significant quantitative influence on the inner shelf response coastal elevation amplitudes and cross shelf decay scales in particular the implications of the model solutions for the interpretation of various observational features of the region s response to wind stress are discussed including coastal elevations offshore bottom pressures currents transports upwelling zones frequency dependences and seasonal and frictional influences

a laboratory model for convection in earth s core driven by a thermally heterogeneous mantle thermal convection experiments in a rapidly rotating hemispherical shell suggest a model in which the convection in earth s liquid outer core is controlled by a thermally heterogeneous mantle experiments show that heterogeneous boundary heating induces an eastward flow in the core which at a sufficiently large magnitude develops into a large scale spiral with a sharp front the front separates the warm and cold regions in the core and includes a narrow jet flowing from the core mantle boundary to the inner core boundary the existence of this front in the core may explain the pacific quiet zone in the secular variation of the geomagnetic field and the longitudinally heterogeneous structure of the solid inner core

bottom currents derived from a shipborne adcp on woce cruise a11 in the south atlantic abstract this paper is divided into two parts the first describing an enhancement of the shipborne adcp system and the second describing information about bottom currents that have been obtained therefrom on woce cruise a11 in the south atlantic punta arenas to cape town rrs discovery has been fitted with a gps3df receiver and an array of antennas about its bridge which determine the instantaneous attitude of the ship the heading record from this instrument has been compared with the ship s gyrocompass throughout the 40 day cruise differences have been calculated and examined according to heading speed and movements underway differences are found insensitive to heading except in a southerly sector but on station differences exhibit a sinusoidal dependence on heading with range 3 steaming from a station led to transient behavior with recognizable effects for 1 hour the results are generally consistent with earlier reports and are attributed to gyro error an improved heading measurement has a

a numerical study of the southwestern atlantic shelf circulation stratified ocean response to local and offshore forcing 1 this article discusses the results of a suite of numerical simulations of the oceanic circulation in the southwestern atlantic shelf region that are aimed to characterize its mean circulation and seasonal variability and to determine the dynamical mechanisms controlling them our experiments indicate that south of 40s the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf which is controlled by the discharges from the magellan straits tidal mixing wind forcing and the offshore influence of the malvinas current farther north the region from 40 st o 33s presents the highest seasonal variability with intrusions of cold sub antarctic waters

local and deep ocean forcing contributions to anomalous water properties on the west florida shelf 1 material property distributions on continental shelves result from the mixing and modifications of estuarine and deep ocean source waters how this occurs depends on the momentum and buoyancy that are input either locally on the shelf or from the deep ocean at the shelf break we address this question of local versus deep ocean forcing for the west florida shelf wfs using in situ data and a numerical circulation model the spring and summer seasons of 1998 and 1999 show distinctively different water properties on the shelf and at the shelf break we account for these differences by a combination of local forcing independent of the adjacent gulf of mexico loop current and interactions of the loop current with the shelf the primary role of the deep ocean is to set the height of material isopleths along the shelf slope whether or not these material isopleths broach the shelf break is then a consequence of local shelf wide wind and buoyancy forcing the subsequent along and across shelf distributions are accomplished through a combination of local and deep ocean effects with the bottom ekman layer being the major conduit for the across shelf transport of ecologically important materials

three dimensional structures and dynamics in the deep mantle effects of post perovskite phase change and deep mantle layering 1 three dimensional numerical simulations of thermo chemical mantle convection with multiple phase transitions olivine spinel perovskite post perovskite are used to investigate the morphology of compositional and post perovskite ppv boundary structures above the core mantle boundary cmb compositionally dense subducted morb partially segregates above the cmb forming ridge like dense piles morphologically similar to structures in previous spherical or cartesian studies in which a dense layer was inserted a priori an anti correlation is observed between regions where a thick post perovskite layer occurs and regions containing piles of dense material as in our previous two dimensional study lateral variations in the thickness or occurrence of a ppv layer cause very strong seismic shear wave velocity anomalies which may dominate thermal or composition related seismic heterogeneity in the depth range where the variable ppv boundary occurs and also generate sharp sided structures as observed seismologically both compositional and ppv effects may be necessary to simultaneously explain large scale heterogeneity and the global distribution of the discontinuity atop d

correlation of earth s magnetic field with lower mantle thermal and seismic structure abstract variations in the earth s lower mantle appear to influence the geodynamo operating in the liquid core we present a solution to the full dynamo equations with lateral variations in heat flux on the outer boundary defined by the shear wave velocity of the lowermost mantle the magnetic field is almost stationary and locked to the boundary with 4 symmetrical concentrations of flux sited beneath cold mantle this allows for the first time a direct comparison between a dynamo solution and the main features of the present geomagnetic field of the four main equatorially symmetric flux lobes two the siberian pair are centered within 5 of the corresponding earth s pair the other two the canadian pair are not quite so close but are more mobile as the corresponding earth s pair have been in the last 300 years our study strongly suggests that geomagnetic field morphology is dominated not only by geometry related to the inner core but also by structure in the bottom few hundred kilometres of the mantle notably the seismically fast ring beneath the pacific rim and large fast anomalies beneath siberia and canada tighter locking of one of the pairs of flux lobes suggests the seismic anomaly beneath the siberian side of the ring is in some way stronger than the one on the canadian side these locked solutions only occur for a limited parameter range with the large ekman numbers available to numerical experiments which explains why none have been found earlier this solution provides an important starting point for further searches for dynamos with realistic geomagnetic fields

mantle driven geodynamo features effects of compositional and narrow d anomalies abstract lower mantle heterogeneity could cause deviations from axial symmetry in geodynamo properties global tomography models are commonly used to infer the pattern of core mantle boundary heat flux via a linear relation that corresponds to a purely thermal interpretation of lower mantle seismic anomalies ignoring both non thermal origins and non resolved small scales here we study the possible impact on the geodynamo of narrow thermal anomalies in the base of the mantle originating from either compositional heterogeneity or sharp margins of large scale features a heat flux boundary condition composed of a large scale pattern and narrow ridges separating the large scale positive and negative features is imposed on numerical dynamos we find that hot ridges located to the west of a positive large scale core mantle boundary heat flux anomaly produce a time average narrow elongated upwelling a flow barrier at the top of the core and intensified low latitudes magnetic flux patches when the ridge is located to the east of a positive core mantle boundary heat flux anomaly the associated upwelling is weaker and the homogeneous dynamo westward drift leaks precluding persistent intense low latitudes magnetic flux patches these signatures of the core mantle boundary heat flux ridge are evident in the north south component of the thermal wind balance based on the pattern of lower mantle seismic tomography masters et al 2000 we hypothesize that hot narrow thermal ridges below central asia and the indian ocean and below the american pacific coast produce time average fluid upwelling and a barrier for azimuthal flow at the top of the core east of these ridges below east asia and oceania and below the americas time average intense geomagnetic flux patches are expected

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

on the origins of the variability of the malvinas current in a global eddy permitting numerical simulation 1 this article investigates the origins of the variability of the malvinas current mc transport using the results of an eddy permitting ocean general circulation model first the dynamical links between the variabilities of the mc and the antarctic circumpolar current acc transports at the drake passage are established time series analyses indicate that the connection between the transport variations of the acc and the mc is masked by high frequency oscillations although a substantial portion of this highfrequency variability is produced and dissipated locally there are also anomalies that propagate between those regions the most conspicuous of these signals has an average speed of 6 7 cm s 1 and a period of 150 days second the contribution of the wind forcing to the variability of the acc and the mc is determined from principal estimator patterns analysis the analysis indicates that the variance of the transport of both currents is more directly related to the variability of the wind stress than to the wind stress curl the transport variability is particularly sensitive to zonal wind stress anomalies in the latitudinal range of the drake passage that extends farther north over the south indian ocean a rather counterintuitive result of our analysis is that the wind stress forcing over the south atlantic ocean is unimportant to the variability of the mc transport

on the absence of an ultralow velocity zone in the north pacific 1 using an unusually large earthquake near the big island of hawaii we study the core mantle boundary cmb beneath the north northeastern pacific between hawaii and north america a dense sampling of the cmb is achieved using the core reflected phase pcp recorded at a large number of high quality stations in north america including networks in california oregon washington and alaska as well as at earthscope s usarray stations we apply an adaptive stacking technique for optimal record alignment on specific phases namely p and pcp and subsequently stack seismograms to obtain summation traces possessing relatively high signal to noise ratios of pcp and p anomalous ultralow velocity zone ulvz layering at the cmb has been noted to exist in various parts of the pacific beneath the lowermost mantle large low shear velocity province imaged by tomography ulvz structure produces anomalous pcp waveform variations in the form of precursors to pcp these pcp data however lack precursory energy indicating either that 1 ulvz layering is lacking or 2 that a ulvz is present and thinner than our detection threshold i e less than a few kilometers thick we use synthetic waveform modeling to establish the sensitivity and utility of investigating the time window ahead of pcp for precursors generated from fine scale cmb layering these results combined with evidence for ulvz structure in other parts of the pacific suggest that ulvzs are intermittent and possibly only detectable in regions where mantle currents collect ulvz material whether or not partially molten presumably beneath or near upwellings or plumes

sea surface velocities from sea surface temperature image sequences 2 application to the brazil malvinas confluence area an inverse variational model is applied to four exceptionally cloud free sea surface temperature image sequences of the brazil malvinas confluence area for the purpose of estimating sea surface velocity fields the velocity fields are constructed by minimizing a mixed layer integrated form of the heat balance equation while the horizontal divergence and vorticity of the flow field are controlled the source terms involved in the heat balance are determined by fitting a polynomial to diurnal temperature variations as a function of temperature and longitude a sensitivity study of the solution to perturbations of the weights imposed to the constraints on divergence and vorticity as well as of the representation of the source terms yields error fields associated with the velocity a comparison of the estimated velocity fields with available concomitant in situ velocity measurements shows that the fields are realistic errors are of the order of 15 20 in magnitude and 20 25 in direction these errors fall within the error bars of 25 30 in magnitude and of 30 35 in direction except for the summer image series surface velocities of 0 5 0 6 m s 1 are obtained across the malvinas current they are 1 m s 1 in the frontal region in the brazil current overshoot and in warm and cold core rings and vary between 0 3 and 0 8 m s 1 in the brazil current with some hypotheses on the vertical shear of the horizontal flow the transport of the malvinas current is estimated to be 25 5 sv between 41 and 40 s

detecting thermal boundary control in surface flows from numerical dynamos the geomagnetic field and secular variation exhibit asymmetrical spatial features which are possibly originating from an heterogeneous thermal control of the earth s lower mantle on the core the identification of this control in magnetic data is subject to several difficulties some of which can be alleviated by the use of core surface flow models using numerical dynamos driven by heterogeneous boundary heat flux we confirm that within the parameter space accessible to simulations time average surface flows obey a simple thermal wind equilibrium between the coriolis and buoyancy forces the lorentz inertial and viscous forces playing only a secondary role even for elsasser numbers significantly larger than 1 furthermore we average the models over the duration of three vortex turnovers and correlate them with a longer time average which fully reveals the signature of boundary heterogeneity this allows us to quantify the possibility of observing mantle control in core surface flows averaged over a short time period a scaling analysis is performed in order to apply the results to the earth s core we find that three vortex turnovers could represent between 100 and 360 years of earth time and that the heat flux heterogeneity at the core mantle boundary could be large enough to yield an observable signature of thermal mantle control in a time average core surface flow within reach of the available geomagnetic data

chlorophyll a variability off patagonia based on seawifs data 1 seasonal to interannual variability of satellite derived chlorophyll a over the patagonia shelf and shelf break in the western south atlantic are studied based on 7 years of ocean color data 1998 2004 from the sea viewing wide field of view sensor seawifs strong chlorophyll a seasonal variability 4 mg m 3 is observed compared to the open ocean 3 5 mg m 3 extends from the midshelf to the shelf break between 37 s and 44 s and southward to 51 s along a narrow shelf break band in summer the shelf break maximum persists from 37 s to 51 s and two inner shelf blooms develop off valdes peninsula and along a near coastal band between 46 s and 52 s 3 mg m 3 chlorophyll a concentrations in the northern midshelf sharply decay in late spring reaching lowest concentrations in summer february and march and a secondary maximum in early winter june though all regions present substantial interannual variations the bloom locations are stable the shelf break maximum is located inshore of the front between the low salinity shelf waters and the cold salty and nutrient rich malvinas current waters the inner shelf maxima are offshore of fronts separating well mixed coastal waters from the stratified midshelf north of 41 s the midshelf bloom is also associated to a bottom trapped thermal front thus all the high chlorophyll a regions are associated to well defined fronts

time independent and time dependent behaviour of high latitude flux bundles at the core mantle boundary 1 using a numerical dynamo model we examine whether quasi stationary high latitude flux bundles which are observed in the geomagnetic field over the last 300 years and possibly over much longer time periods result from thermal core mantle interactions we do so by imposing lateral heat flux variations at the core mantle boundary based upon the dominant pattern of seismic heterogeneity in the lowermost mantle we find that the time averaged field then has a similar arrangement of flux bundles to that observed during the last few hundred years however the field is time dependent and this arrangement of flux bundles is not a feature of the field at all times

vertical velocities and dynamical balances in gulf stream meanders a triangular array of current meter moorings was deployed within the cyclonic flank of the gulf stream in the south atlantic bight from september 1981 to april 1982 using velocity and temperature data and the nondiffusive heat equation a time series of vertical velocity was derived a mean vertical velocity of 0 013 cm s 1 and standard deviation of 0 078 cm s 1 were obtained to better understand the subsurface structure of gulf stream meanders the time series of horizontal and vertical velocities and temperature were examined at times when events were passing through the array the flow along the trailing edge of a gulf stream meander and within the leading portion of a cold core frontal eddy was found to have an upward component positive w while that within the trailing portion of the frontal eddy and along the leading edge of a meander had a downward component negative w using the horizontal and vertical velocity time series cross stream and along stream momentum balances were calculated the downstream flow was found to be in geostrophic balance in contrast the coriolis and nonlinear terms were found to contribute equally to the determination of the cross stream flow

on the upwelling of downwelling currents abstract the term downwelling currents refers to currents with a downslope mass flux in the bottom boundary layer examples are the malvinas and southland currents in the southern hemisphere and the oyashio in the northern hemisphere although many of these currents generate the same type of highly productive ecosystems that is associated with upwelling regimes the mechanism that may drive such upwelling remains unclear in this article it is postulated that the interaction between a downwelling current and the continental slope generates shelfbreak upwelling the proposed mechanism is relatively simple as a downwelling current flows along the continental slope bottom friction and lateral diffusion spread it onto the neighboring shelf thus generating along shelf pressure gradients and a cross shelf circulation pattern that leads to shelfbreak upwelling at difference with previous studies of shelfbreak dynamics e g gawarkiewicz and chapman chapman and lentz and pickart the shelfbreak upwelli

effects of thermally heterogeneous structure in the lowermost mantle on the geomagnetic field strength abstract we have conducted a study of numerical dynamos in a rapidly rotating spherical shell with prescribed non uniform heat flux patterns at the outer boundary to examine effects of thermal structure at the core mantle boundary cmb on the geodynamo especially on the magnetic field strength large heat flux heterogeneity with equatorial symmetry enhances strength of the dipolar magnetic field for a sufficiently small ekman number whereas that with equatorial anti symmetry does not change the strength but affects the tilt of the magnetic dipole axis the thermal wind induced by heat flux heterogeneity at the cmb has strong influence on the flow structure and on the magnetic field intensity these results suggest that thermally heterogeneous structure of the lowermost mantle might give rise to an anomalously strong geomagnetic field such as that during the cretaceous normal superchron

tomographic filtering of geodynamic models implications for model interpretation and large scale mantle structure 1 the resolution operator r is a critical accompaniment to tomographic models of the mantle r facilitates the comparison between conceptual three dimensional velocity models and tomographic models because it can filter these theoretical models to the spatial resolution of the tomographic model we compute r for the tomographic model s20rts ritsema et al 1999 2004 and two companion models that are based on the same data but derived with different norm damping values the three models explain within measurement uncertainty s sks and s skks travel times equally well to demonstrate how artifacts distort tomographic images and complicate model interpretation we apply r to 1 a thermochemical and 2 an isochemical model of convection in the mantle that feature different patterns of shear velocity heterogeneity in the deep mantle if we assume that shear velocity heterogeneity is caused by temperature variations only r suppresses short wavelength structures removes strong velocity gradients and introduces artificial stretching and tilting of velocity anomalies temperature anomalies in the thermochemical model resemble the spatial extent of low seismic velocity anomalies and the shear velocity spectrum in the d region better than the isochemical model however the thermochemical model overpredicts the amplitude of shear velocity variation and places the african and pacific anomalies imperfectly we suspect that inaccurate velocity scaling laws and uncertain initial conditions control these mismatches extensive hypothesis testing is required to identify successful models

rapidly changing flows in the earth s core variations in the earth s magnetic field over a span of a few months can be resolved despite the potential filtering effects of the electrically conducting mantle and are indicative of rapid flow in the earth s outer core

equatorially dominated magnetic field change at the surface of earth s core slow temporal variations in earth s magnetic field originate in the liquid outer core we analyzed the evolution of nonaxisymmetric magnetic flux at the core surface over the past 400 years we found that the most robust feature is westward motion at 17 kilometers per year in a belt concentrated around the equator beneath the atlantic hemisphere surprisingly this motion is dominated by a single wavenumber and persists throughout the observation period this phenomenon could be produced by an equatorial jet of core fluid by hydromagnetic wave propagation or by a combination of both discrimination between these mechanisms would provide useful constraints on the dynamics of earth s core

stationary oscillating or drifting mantle driven geomagnetic flux patches 1 the impact of the heterogeneous lower mantle on the geomagnetic field is under debate especially the question of whether high latitude intense geomagnetic flux patches currently observed at the core surface are stationary oscillating or drifting on longer time scales while the correlation between the location of these patches with that of similar patches found in the time averaged paleomagnetic field may suggest stationary behavior their variability over archaeomagnetic time scales together with their weaker signature in the average paleomagnetic field relative to the present geomagnetic field precludes such a scenario here we use numerical dynamos with an imposed heat flux boundary condition based on lower mantle tomography to study the behavior of such intense magnetic flux patches we design an algorithm to detect centers of intense flux patches and track their time evolution we find that the time dependent nature of those patches comprises oscillatory motion about statistically preferred locations imposed by mantle control with episodic drift from one preferred location to the other corresponding to an azimuthal migration of fluid downwelling structures that concentrate surface magnetic flux this statistical behavior provides a possible explanation for both the observed variability of high latitude patches on the archaeomagnetic time scale and the similar locations of the current patches and the weaker patches seen in the paleomagnetic field our simulations also show that the patches exhibit more time dependence and less coherency in the southern hemisphere leading to a weaker time averaged patch signature in that hemisphere

magnetic boundary layers in numerical dynamos with heterogeneous outer boundary heat flux abstract it has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core mantle boundary bloxham 1986 in this process intense horizontal field lines are concentrated below the outer boundary introducing small radial length scales and consequently strong radial diffusion here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell in our models mean magnetic boundary layer thickness ranges 200 400 km and decreases with increasing magnetic reynolds number extrapolation or interpolation to earth s core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of 220 and 260 330 km respectively we find magnetic boundary layers associated with the azimuthal field at the equatorial region whereas magnetic boundary layers associated with the meridional field are found at mid latitudes negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below africa and the pacific while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the americas and east asia furthermore we find a tendency of the azimuthal field to low latitudes of the northern hemisphere our results suggest that the local diffusion time is on the order of several kyr and the local magnetic reynolds number is on the order of 10 both much smaller than classical estimates

topographically induced upwelling off eastern australia abstract a high resolution numerical study of an idealized western boundary current flow over variable topography is presented with application to the east australian current eac the results indicate that alongshelf topographic variations off australia s east coast cause the eac to accelerate over the narrowing continental shelf near cape byron this acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundary layer bbl which would usually cause the eac driven bbl to shut down consequently a region of persistent high bottom stress was established off cape byron which was responsible for driving an upwelling bbl it is shown that the enhanced vertical mixing associated with a low richardson number flow beneath the eac reduced the local stratification consequently the burger number is decreased resulting in a long shutdown timescale of the bbl which enables a nearshore thermal to be established and maintained such fronts are commonly observed in the region as a p

the influence of the plata river discharge on the western south atlantic shelf 1 the influence of the plata the second largest river in south america extends along a coastal strip of 1300 km historical hydrographic and wind data and numerical simulations are combined to determine the seasonal and interannual variability of the plata plume and its relationship to the magnitude of the river discharge and the intensity and direction of the wind stress our results indicate that the seasonal variability of the river plume is controlled by the alongshore component of the wind stress during el nino the effects of the wind and precipitation anomalies tend to compensate each other preventing anomalous northeastward plume extensions associated to large outflow events numerical experiments confirm this finding and indicate that during el nino the discharge from the plata river spreads offshore

interaction of the south american monsoon system and the southern westerly wind belt during the last 14 kyr abstract surface currents and sediment distribution of the se south american upper continental margin are under the influence of the south american monsoon system sams and the southern westerly wind belt swwb both climatic systems determine the meridional position of the subtropical shelf front stsf and probably also of the brazil malvinas confluence bmc we reconstruct the changing impact of the sams and the swwb on sediment composition at the upper rio grande cone off southern brazil during the last 14 cal kyr bp combining sedimentological geochemical micropaleontological and rock magnetic proxies of marine sediment core geob 6211 2 sharp reciprocal changes in ferri and paramagnetic mineral content and prominent grain size shifts give strong clues to systematic source changes and transport modes of these mostly terrigenous sediments our interpretations support the assumption that the sams over se south america was weaker than today during most of the late glacial and entire early holocene while the swwb was contracted to more southern latitudes resembling modern austral summer like conditions in consequence the stsf and the bmc were driven to more southern positions than today s favoring the deposition of fe rich but weakly magnetic la plata river silts at the rio grande cone during the mid holocene the northern boundary of the swwb migrated northward while the stsf reached its northernmost position of the last 14 cal kyr bp and the bmc most likely arrived at its modern position this shift enabled the transport of antarctic diatoms and more strongly magnetic argentinean shelf sands to the rio grande cone while sediment contributions from the la plata river became less important during the late holocene the modern el nino southern oscillation set in and the sams and the austral tradewinds intensified causing a southward shift of the stsf to its modern position this reinforced a significant deposition of la plata river silts at the rio grande cone these higher magnetic silts with intermediate fe contents mirror the modern more humid terrestrial climatic conditions over se south america

holocene shifts of the subtropical shelf front off southeastern south america controlled by high and low latitude atmospheric forcings over the uruguayan shelf and uppermost slope the coalescence of northward flowing subantarctic shelf water and southward flowing subtropical shelf water forms a distinct thermohaline front termed the subtropical shelf front stsf running in a sw direction diagonally across the shelf from the coastal waters at 32 s toward the shelf break at ca 36 s the stsf represents the shelfward extension of the brazil malvinas confluence zone this study reconstructs latitudinal stsf shifts during the holocene based on benthic foraminifera 18o and 13c total organic carbon carbonate contents ti ca and grain size distribution from a high accumulation sedimentary record located at an uppermost continental slope terrace our data provide direct evidence for 1 a southern stsf position to the south of the core site at the beginning of the early holocene 9 4 cal ka bp linked to a more southerly position of the southern westerly winds in combination with restricted shelf circulation intensity due to lower sea level 2 a gradual stsf northward migration bypassing the core site toward the north primarily forced by the northward migration of the southern westerly winds from 9 4 cal ka bp onward 3 a relatively stable position of the front in the interval between 7 2 and 4 0 cal ka bp 4 millennial scale latitudinal oscillations close to 36 s of the stsf after 4 0 cal ka bp probably linked to the intensification in el nino southern oscillation and 5 a southward migration of the stsf during the last 200 years possibly linked to anthropogenic influences on the atmosphere

wind induced ice floe rotation and icequakes north of the alaskan beaufort sea coast linear sea ice canopy fractures leads and ice floe rotation appear to be interrelated this study focused on a highly persistent storm wind event as the wind stress increased over the ice pack a separate rotation threshold for four floe attached buoys was observed the northwestern most buoy recorded the start of clockwise ice floe rotation just as hydrophones recorded increased oceanic ambient noise several days later when the surface wind speeds and computed stress at all four buoys dropped to 60 and e 1 of their respective peak storm values a counterclockwise rotation began at each buoy with a concomitant impulsive ambient noise peak the rotationally induced noise bursts are called icequakes in an analogy with earthquakes

implications of post perovskite transport properties for core mantle dynamics abstract recent evidence on perovskite to post perovskite phase change in the lowermost mantle suggests that post perovskite piles or lens should be present in the relatively cold downwelling areas while the roots of the hot upwelling plumes consist of perovskite post perovskite is often believed to be deformed predominantly by the dislocation creep and there are some hints that the activation parameters of the dislocation creep in post perovskite induce lower viscosity than that of perovskite at the same pressure and temperature conditions that can even result in a viscosity paradox in the lowermost mantle viscosity in cold downwellings transformed to post perovskite might be lower than the viscosity of warm perovskite plumes such a viscosity structure was indeed recently suggested by the geodynamical inversions of the geoid rheologically weak areas at the base of the subducted slabs may have important consequences for the slab deformation in the d we investigate the dynamics of the cold slabs transformed to post perovskite in the lowermost mantle by means of the simulations of thermal convection in a 2d cartesian model of the lower mantle with composite rheology including diffusion creep and dislocation creep different creep parameters are used for the perovskite lower mantle and for the post perovskite lens or layer respectively while perovskite is considered to be deformed via linear diffusion creep post perovskite is modeled either as a constant viscosity material with viscosity ranging from 1020 pa s to 1023 pa s or it is deformed purely by the dislocation creep the presence of the rheologically distinct post perovskite not only strongly influences the slab deformation above the core mantle boundary cmb but also results in different dynamic regimes of the cmb region characterised by e g the different length scales of the upwelling plumes depending on the creep parameters of post perovskite further the presence of very low viscosity lens or a layer above the cmb strongly enhances the cmb heat flux in the cold slab areas according to the amount of post perovskite present in the d and the rheology of post perovskite the heat flux across the cmb can vary considerably with time which may have important implications for the core mantle coupling and the character of the earth s geomagnetic field