crustal concealing of small scale core field secular variation summary the earth s magnetic field is mainly produced within the earth s liquid and electrically conducting core as a result of a process known as the geodynamo many other sources also contribute to the magnetic signal accessible to observation at the earth s surface partly obscuring the main core magnetic field signal thanks to a series of very successful satellites and to advances in magnetic field modelling techniques considerable progress has however been made in the recent years toward better identifying the signal of each of these sources in particular temporal changes in the field of internal origin happen to be detectable now in spherical harmonic degrees up to perhaps 16 all of these changes are usually attributed to changes in the core field itself the secular variation on the ground that the lithospheric magnetization cannot produce such signals it has however been pointed out on empirical grounds that temporal changes in the field of internal origin produced by the induced part of the lithospheric magnetization could dominate the core field signal beyond degree 22 this short note revisits this issue by taking advantage of our improved knowledge of the small scale field changes and of the likely sources of the lithospheric field we rely on a simple extrapolation of the observed spatial spectrum of the field changes beyond degree 16 and use a forward approach based on a recent geological model of lithospheric magnetization this leads us to confirm that the main cause of the observed changes in the field of internal origin up to some critical degree n c is indeed likely to be the secular variation of the core field but that the signal produced by the time varying lithospheric field is bound to dominate and conceal the time varying core signal beyond that critical degree in very much the same way the permanent component of the lithospheric field dominates and conceals the permanent component of the core field beyond degree 14 all uncertainties taken into account we estimate n c to lie between 22 and 24 we however also note that in practice the main limitation to the observation of the core field small scale secular variation is not so much its concealing by the field of lithospheric origin but its fast changing nature and small magnitude this leads us to conclude that whereas cumulative small scale lithospheric field changes might be detected some day detection of core field secular variation beyond degree 18 is likely to remain a severe challenge for some time

geomagnetic field intensity how high can it get how fast can it change constraints from iron age copper slag article the intensity of the geomagnetic field varies over different time scales yet constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record the purpose of this study is to place firm limits on these fundamental properties by constructing a high resolution archaeointensity record of the levant from the 11th century to the early 9th century bce a period over which the geomagnetic field reached its maximum intensity in eurasia over the past 50 000 years we investigate a 14 c dated sequence of ten layers of slag material which accumulated within an ancient industrial waste mound of an iron age copper smelting site in southern israel depositional stratigraphy constrains relative ages of samples analyzed for paleointensity and 14 c dates from different horizons of the mound constrain the age of the whole sequence the analysis yielded 35 paleointenisty data points with accuracy better than 94 and precision better than 6 covering a period of less than 350 years most probably 200 years we construct a new high resolution quasi continuous archaeointensity curve of the levant that displays two dramatic spikes in geomagnetic intensity each corresponding to virtual axial dipole moment vadm in excess of 200 zam 2

forecasting change of the magnetic field using core surface flows and ensemble kalman filtering 1 accurate forecasting of the change of the earth s internal magnetic field over short intervals of time e g less than five years has many applications for government academic and commercial users forecasting can be achieved by making a number of reasonable assumptions about how the main field interacts with the flow in the liquid outer core in particular the magnetic field can be considered to be entrained in the large scale flow along the core mantle boundary surface over short time periods giving rise to measurable change at the earth s surface the observed change or secular variation at or above the surface of the earth can thus be inverted to produce flow models these can be used to propagate fluid parcels threaded by the field forwards in time to forecast the non linear change of the magnetic field in addition to prediction of field change by flow models it would be advantageous to include observations of the field from satellite measurements or ground based observatories we therefore present a method using ensemble kalman filtering enkf to produce an optimal assimilation between magnetic field change as forecast from core flow models and direct observations of the field we show by assuming a steady flow and assimilating field observations annually it is possible to produce a forecast over five years with less than 30nt root mean square difference from the true field within an assumed error budget the enkf method also allows sensitivity analysis of the field models to noise and uncertainty within the physical representation

geomagnetic intensity spike recorded in high resolution slag deposit in southern jordan article i nfo in paleomagnetism periods of high field intensity have been largely ignored in favor of the more spectacular directional changes associated with low field intensity periods of excursions and reversals hence questions such as how strong the field can get and how fast changes occur are still open in this paper we report on data obtained from an archaeometallurgical excavation in the middle east designed specifically for archaeomagnetic sampling we measured 342 specimens from 72 samples collected from a 6 1 m mound of well stratified copper production debris at the early iron age 12th 9th centuries bce site of khirbat en nahas in southern jordan seventeen samples spanning 200 yr yielded excellent archaeointensity results that demonstrate rapid changes in field intensity in a period of overall high field values the results display a remarkable spike in field strength with sample mean values of over 120 t compared to the current field strength of 44 t a suite of 13 radiocarbon dates intimately associated with our samples tight control of sample location and relative stratigraphy provide tight constraints on the rate and magnitude of changes in archaeomagnetic field intensities

hard and soft bottom thanatofacies from the santa maria di leuca deep water coral province mediterranean thanatofacies and the skeletonized components of the living facies from which they originate have been studied from the santa maria di leuca sml deep water coral mound province faunal analysis was carried out by means of bottom sampling and underwater video observations taking into account all benthic taxonomic groups mostly corals molluscs serpulids bryozoans ostracods foraminifers and barnacles which permitted recognition of six different facies these thanatofacies are easily distinguishable and appear to be largely corresponding and overlapping with related living facies some occur in mound areas others in the intermound ones they are as follows the framework building coral facies fc characterised by colonial corals mostly madrepora oculata the coral rubble facies cr with proximal and distal aspects characterised by large to small sized and densely to loosely packed coral fragments the solitary coral facies sc dominated by different species depending on the availability and dimensions of hard exploitable surfaces the gryphus and isidella facies gi in relatively coarse grained bottoms the mollusc mud facies mm and the foraminifer mud facies fm in comparably homogeneous silty bottoms facies distribution and spatial variability are discussed in relation to hydrology and sea floor topography furthermore the sml facies are compared with living facies from the present day mediterranean and pleistocene sediments of the same area data on bioclastic assemblages can serve for comparison with other recent aphotic non tropical carbonates

the backus gilbert approach to the three dimensional structure in the upper mantle i lateral variation of surface wave phase velocity with its error and resolution summary the backus gilbert method is applied to obtain the phase velocity variations on a sphere from the measured phase velocity narrow peak kernels with radii of about 2000 km are obtained for almost everywhere on the sphere the phase velocity results are thus interpreted as an average within such regions the most trouble comes from the antipodal peak in the resolution kernel this is evaluated as contamination and is incorporated in the error estimation the total error which is a root mean square of contamination from the antipodal peak and statistical error estimated from the data covariance matrix is about 1 per cent of the phase velocity in the average earth model which is the preliminary reference earth model prem however there is about a factor of 2 variation of errors on the sphere maximum variations of phase velocity are about 3 4 per cent of the phase velocity in the average earth model and thus there still remain anomalies which exceed estimated errors the estimated errors correspond to one standard deviation under the assumptions of uncorrelated gaussian distribution for high confidence interval they show that statistically significant anomalies are scarce for the current data set generally love wave phase velocity maps show more resolved features than rayleigh wave maps and we can see in high confidence maps fast velocities in old oceans and old continents and slow velocities in tectonically active regions like the east pacific rise and various back arc regions

can downwelling at the top of the earth s core be detected in the geomagnetic secular variation abstract it has been argued based on recent seismic and mineral physics studies that the top of earth s liquid outer core is stably stratified here i analyze persistent geomagnetic secular variation features on the core mantle boundary to examine whether a kinematic signature of core fluid upwelling downwelling can be detected i focus on regions of intense high latitude geomagnetic flux patches that may be maintained by fluid downwelling in order to identify persistent patterns the radial field and its secular variation are stacked in the flux patch moving reference frame these stacked images are compared with forward solutions to the radial induction equation based on idealized field flow models clear advective secular variation signature below north america indicates that these intense flux patches may exhibit significant mobility stretching signature in the form of persistent positive secular variation correlated with the intense flux patch below the southern indian ocean may be considered as regional scale geomagnetic evidence for whole core convection although pure toroidal flow cannot be outruled

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

geomagnetic forecasts driven by thermal wind dynamics in the earth s core there exists a fundamental as well as practical interest in being able to accurately forecast the future evolution of earth s magnetic field at decadal to secular ranges this work enables such forecasts by combining geomagnetic data with an earth like numerical model of a convection driven fluid dynamo the underlying data assimilation framework builds on recent progress in inverse geodynamo modelling a method which estimates an internal dynamic structure for earth s core from a snapshot of the magnetic field and its instantaneous rate of change at the surface and takes advantage of linear relationships and long range correlations between observed and hidden state variables here the method is further evolved into a single epoch ensemble kalman filter in order to initialise at a given epoch an ensemble of states compatible with the observations and representative of the uncertainties in the estimation of hidden quantities the ensemble dynamics obtained by subsequent numerical integration of the prognostic model equations are found to be governed by a thermal wind balance or equilibrium between buoyancy forces the coriolis force and the pressure gradient the resulting core fluid flow pattern is a quasi steady eccentric gyre organised in a column parallel to earth s rotation axis in equilibrium with a longitudinal hemispheric convective density anomaly pattern the flow provides induction for the magnetic field which also undergoes a realistic amount of diffusion predictions of the present magnetic field from data taken within the past century show that the ensemble has an average retaining good consistency with the true geomagnetic evolution and an acceptable spread well representative of prediction errors up to at least a secular range the predictability of the geodynamo thus appears to significantly exceed previous theoretical expectations based on the chaotic divergence of ensemble members the assimilation generally outperforms the linear mathematical extrapolations from a 30 yr prediction range onwards with a 40 per cent improvement in earth surface error at a secular range the geomagnetic axial dipole decay observed over the past two centuries is predicted to continue at a similar pace in the next century with a further loss of 1 1 0 3 t by year 2115 the focal or minimum intensity point of the south atlantic geomagnetic anomaly is predicted to enter the south pacific region in the next century with the anomaly itself further deepening and widening by year 2065 the minimum intensity is predicted to decrease by 1 46 0 4 t at the earth surface and the focal point to move 12 8 1 4 deg westwards with a slight northward component this corresponds to a drift rate of 0 26 deg yr 1 similar to the typical geomagnetic westward drift observed over the past four centuries the same drift rate is also predicted until 2115 with a further but more uncertain intensity decrease

circulation of the chukchi sea shelfbreak and slope from moored timeseries abstract data from a year long mooring array across the shelfbreak upper slope of the chukchi sea are used to describe and quantify the circulation and water masses of the region the timeseries revealed the year round existence of the eastward flowing shelfbreak jet and seaward of this the westward flowing chukchi slope current in the mean the slope current is estimated to transport 0 57 0 04 sv of pacific water while the bottom intensified shelfbreak jet transports 0 009 0 003 sv towards barrow canyon the slope current is surface intensified in summer and fall and in winter and spring it becomes middepth intensified moves shoreward and weakens two extreme states of the circulation were identified 1 an enhanced slope current and reversed westward flowing shelfbreak jet and 2 a strong eastward flowing shelfbreak jet and weak slope current the former state occurs when the wind stress curl on the chukchi shelf is positive and the latter state occurs when the curl is negative a simple theoretical model is used to determine the changes in sea surface height due to such wind stress curl forcing which is consistent with the observed changes in flow seaward of the shelf both in amplitude and phase via geostrophic set up shelfbreak upwelling occurred throughout the year but there was no correlation between the regional wind conditions and the upwelling furthermore there was no apparent relationship between upwelling and the extreme slope current shelfbreak jet events a comparison of water mass signals between the chukchi slope array and a mooring at the head of barrow canyon supports the notion that the slope current is fed by the outflow of pacific water from the canyon

reconstruction and analysis of the chukchi sea circulation in 1990 1991 1 the chukchi sea cs circulation reconstructed for september 1990 to october 1991 from sea ice and ocean data is presented and analyzed the core of the observational data used in this study comprises the records from 12 moorings deployed in 1990 and 1991 in u s and russian waters and two hydrographic surveys conducted in the region in the fall of 1990 and 1991 the observations are processed by a two step data assimilation procedure involving the pan arctic ice ocean modeling and assimilation system employing a nudging algorithm for sea ice data assimilation and the semi implicit ocean model utilizing a conventional four dimensional variational 4d var assimilation technique the reconstructed cs circulation is studied to identify pathways and assess residence times of pacific water in the region quantify the balances of volume freshwater and heat content and determine the leading dynamical factors configuring the cs circulation it is found that in 1990 1991 high ao index and a cyclonic circulation regime pacific water transiting the cs toward the canada basin followed two major pathways namely via herald canyon herald branch of circulation 0 23 sv and between herald shoal and cape lisburne central branch of circulation and alaskan coastal current 0 32 sv the annual mean flow through long strait was negligible 0 01 sv typical residence time of pacific water in the region varied between 150 days for waters entering the cs in september and 270 days for waters entering in february march momentum balance analysis reveals that geostrophic balance between barotropic pressure gradient and coriolis force dominated for most of the year baroclinic effects were important for circulation only in the regions with large horizontal salinity gradients associated with the fresh alaskan and siberian coastal currents and the cape lisburne and great siberian polynyas in the polynyas the baroclinic effects were due to strong salinification and convection processes associated with sea ice formation

cosmonaut polynya in the southern ocean structure and variability along the far eastern margin of the weddell gyre is a persistent feature in the middle of the ice pack which we previously reported and called the cosmonaut polynya a study of polynya occurrences from 1973 to 1993 reveals that since 1986 the polynya has become more active with an average size of about 7 2 104 km2 and an average location at 52 e and 65 s satellite observations indicate that the polynya has recurred several times during winter in recent years with intervals ranging from a few days to a few weeks the centroid of the polynya varies only slightly with each formation during the year and from one year to another suggesting a controlling influence of the ocean and bottom topography that may be initially induced by wind the daily time series indicates two primary modes of formation one that is initiated in the early winter during a storm at a site usually preceded by an embayment of the ice edge and another that occurs during midwinter often preceded by a coastal polynya event adjacent to cape ann the cosmonaut polynya region is characterized in this study by compression of the westward flowing coastal current and the eastward flowing southern edge of the antarctic circumpolar current following the principle of conservation of potential vorticity vertical stretching of the water column would ensue enhancing upwelling such a process accelerates the injection of relatively warm salty deep water into the surface layer inhibiting sea ice growth and causing the polynya formation this theory appears to explain the general behavior of the polynya in terms of frequency duration size and location

observational and modeling evidence of seasonal trends in sediment derived material inputs to the chukchi sea author posting american geophysical union 2020 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 125 5 2020 e2019jc016007 doi 10 1029 2019jc016007

pacific ventilation of the arctic ocean s lower halocline by upwelling and diapycnal mixing over the continental margin 1 pacific winter waters a major source of nutrients and buoyancy to the arctic ocean are thought to ventilate the arctic s lower halocline either by injection isopycnal or penetrative of cold saline shelf waters or by cooling and freshening atlantic waters upwelled onto the shelf although ventilation at salinity s 34 psu has previously been attributed to hypersaline polynya waters temperature salinity nutrient and tracer data suggest instead that much of the western arctic s lower halocline is in fact influenced by a diapycnal mixing of pacific winter waters with s 33 1 psu and denser eastern arctic halocline atlantic waters the mixing taking place possibly over the northern chukchi shelf slope estimates from observational data confirm that sufficient quantities of atlantic water may be upwelled to mix with the inflowing pacific waters with volumes implying the halocline over the chukchi borderland region may be renewed on timescales of order a year

early summer water mass transformation in the eastern chukchi sea abstract pacific water entering the chukchi sea through bering strait can be modified by physical processes such as surface heating and cooling and internal mixing here we document the existence of a previously unobserved water mass in the eastern chukchi sea during the early summer using hydrographic and velocity data from the june july 2010 icescape survey the water mass in question is as salty as pacific winter water pww but is much warmer with temperatures typical of summer waters we label the water mass dense chukchi summer water dcsw which is dense enough to ventilate the upper halocline of the western arctic ocean normally ventilated by the much colder pww satellite sea surface temperature data long term mooring data from bering strait and the beaufort shelfbreak and atmospheric reanalysis fields are used to characterize the spatial and temporal variability of the dcsw and explore potential forcing scenarios that can lead to its formation in 2010 the dcsw did not enter the chukchi sea through bering strait hence it had to be formed locally on the shelf two distinct forcing mechanisms are considered 1 mixing of very warm alaskan coastal water with the cold pww when the two water masses come in contact in the energetic alaskan coastal current and 2 warming of pww when it experiences solar heating within the coastal polynya in the northeastern chukchi sea during the spring melt season finally we explore the conditions under which the springtime polynya has formed over the past three decades as it relates to the formation of dcsw

circulation and water mass transformation in a model of the chukchi sea 1 the circulation and water mass transformation in a regional ocean ice model of the chukchi sea are discussed the model has horizontal resolution of o 4 km is forced by fluxes derived from daily ncep reanalysis fields and has seasonally varying transport temperature and salinity imposed at bering strait many of the observed characteristics of the mean circulation and seasonal cycle in the chukchi sea are reproduced the discussion focuses on the branching of the inflow transport into pathways following herald canyon central channel and the alaskan coast the pattern of ice melt and the water mass transformation and formation of winter water and hypersaline water the ice melt pattern and timing is strongly influenced by advection through bering strait high frequency forcing results in a larger region of ice melt particularly over the shoals and in the northern chukchi sea compared to monthly mean forcing in the model the seasonal cycle of salinity in the southern and central chukchi sea is dominated by advection through bering strait while local atmospheric forcing and brine rejection are more important north of herald and hanna shoals and in barrow canyon however since the residence time in the chukchi sea is generally less than 1 year interannual variability in the bering strait salinity will be reflected in the salinity across the chukchi sea and at barrow canyon

geomagnetic jerks as chaotic fluctuations of the earth s magnetic field 1 the geomagnetic field is chaotic and can be characterized by a mean exponential time scale after which it is no longer predictable it is also ergodic so time analyses can substitute the more difficult phase space analyses taking advantage of these two properties of the earth s magnetic field a scheme of processing global geomagnetic models in time is presented to estimate fluctuations of the time scale here considering that the capability to predict the geomagnetic field is reduced over periods of geomagnetic jerks we propose a method to detect these events over a long time span this approach considers that epochs characterized by relative minima of fluctuations in time scale i e those periods when a geomagnetic field is less predictable are possible jerk occurrence dates we analyze the last 400 years of the geomagnetic field covered by the gufm1 model to detect minima of fluctuations i e epochs characterized by low values of the time scale most of the well known jerks are confirmed through this method and a few others have been suggested finally we also identify some short periods when the field is less chaotic more predictable than usual naming these periods as steady state geomagnetic regime to underline their opposite behavior with respect to jerks

circulation on the north central chukchi sea shelf abstract mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central chukchi shelf previous studies indicated that pacific waters crossed the chukchi shelf through herald valley in the west and barrow canyon in the east we find a third branch through the central channel onto the outer shelf the central channel transport varies seasonally in phase with bering strait transport and is 0 2 sv on average although some of this might include water entrained from the outflow through herald valley a portion of the central channel outflow moves eastward and converges with the alaskan coastal current at the head of barrow canyon the remainder appears to continue northeastward over the central outer shelf toward the shelfbreak joined by outflow from herald valley the mean flow opposes the prevailing winds and is primarily forced by the sea level slope between the pacific and arctic oceans current variations are mainly wind forced but baroclinic forcing associated with upstream dense water formation in coastal polynyas might occasionally be important winter water mass modification depends crucially on the fall and winter winds which control seasonal ice development an extensive fall ice cover delays cooling limits new ice formation and results in little salinization in such years bering shelf waters cross the chukchi shelf with little modification in contrast extensive open water in fall leads to early and rapid cooling and if accompanied by vigorous ice production within coastal polynyas results in the production of high salinity 33 shelf waters such interannual variability likely affects slope processes and the transport of pacific waters into the arctic ocean interior

observed increases in bering strait oceanic fluxes from the pacific to the arctic from 2001 to 2011 and their impacts on the arctic ocean water column 1 mooring data indicate the bering strait throughflow increases 50 from 2001 0 7 sv to 2011 1 1 sv driving heat and freshwater flux increases increase in the pacific arctic pressure head explains two thirds of the change the rest being attributable to weaker local winds the 2011 heat flux 5 1020j approaches the previous record high 2007 due to transport increases and warmer lower layer ll temperatures despite surface temperature sst cooling in the last decade warmer ll waters arrive earlier 1 6 1 1 days yr though winds and sst are typical for recent decades maximum summer salinities likely set in the bering sea remain remarkably constant 33 1 psu over the decade elucidating the stable salinity of the western arctic cold halocline despite this freshwater flux variability strongly driven by transport exceeds variability in other arctic freshwater sources remote data winds sst prove insufficient for quantifying variability indicating interannual change can still only be assessed by in situ year round measurements

grimm the gfz reference internal magnetic model based on vector satellite and observatory data summary in this paper the new gfz reference internal magnetic model grimm is presented the model has been derived from nearly 6 yr of champ satellite data and 5 yr of observatory hourly means at high latitudes full vector satellite data are used at all local times which allows a separation between on one hand the fields generated by ionosphere and field aligned currents and on the other hand the fields generated in the earth s core and lithosphere this selection technique leads to a data set without gaps during the polar summers resulting in a core field model that has an unprecedented time resolution the modelled static core field secular variation and lithospheric field are all in good agreement with previously published magnetic field models order five b splines are used to model the variation in time of the core field the energy in the secular acceleration has therefore a smooth behaviour in time and increases continuously from 2003 5 mapping the field acceleration from 2001 5 to 2005 5 reveals its rapid and complex evolution over this time period at the earth s surface due to the applied regularization technique the acceleration energy in spherical harmonics 6 11 is significantly larger than for other models and we show that such a spectrum is acceptable

vertical structure and temporal variability of currents over the chukchi sea continental slope abstract observations from a single mooring site on the northern chukchi sea continental slope near the 1000 m isobath are presented this site was occupied consecutively for three years spanning september 2014 august 2017 vertically the flow divides into three depth ranges the upper 200 m 200 850 m and near bottom flow in the upper 200 m the mean flow was northwestward and strongest in the summer months during winter months currents decreased in magnitude and in some years even reversed in direction satellite tracked drifter trajectories drogue depth 30 m show this along slope flow persists at least from 156 to 165 w with an average velocity of 17 cm s 1 this northwestward flowing current is the chukchi slope current from 250 m to 850 m the flow reversed this weak flow is the arctic wide cyclonic boundary current advecting atlantic water the mean flow at 900 m is weak and on an annual time scale not significantly different from 0 cm s 1 it consists of arctic deep water in the upper two layers currents vary on the scale of days to seasons with short term reversals common currents below 40 m were not significantly correlated with local winds nor wind stress curl we hypothesize that the northwestward flowing chukchi slope current is a consequence of dynamics associated with the beaufort gyre

mapping geomagnetic secular variation at the core mantle boundary summary data from recent satellite missions have vastly increased the resolution of models of the geomagnetic field and itsfirst and second time derivatives secular variation sv and secular acceleration sa the spectra of both sv and sa are blue at the core mantle boundary both well fit by functions proportional to l l 1 where l is the spherical harmonic degree the ratio of the two spectra defines a timescale for geomagnetic variations of approximately 10 yrs for all resolvable harmonic degrees the blue spectra should prevent meaningful maps of the sv being generated nevertheless the coherence of the maps up to harmonic degree 13 suggests that it is possible to obtain useful insight from their examination low sv is confirmed under the pacific but also revealed under the north atlantic and antarctica these features are more readily explained in terms of dynamo control through thermal core mantle coupling than by electromagnetic screening comparison with maps from measurements prior to the recent satellites using the comprehensive model suggests that models back to at least 1970 are sufficiently good to enable direct comparison of the sv

distribution of atlantic and pacific waters in the upper arctic ocean implications for circulation the atlantic and pacific oceans provide source waters for the arctic ocean that can be distinguished by their differing nitrate and phosphate concentration relationships using these relationships we estimate the amount of atlantic and pacific waters in the surface layer top 30 m of the arctic ocean atlantic source water is dominant in most of the eurasian basin and is present in significant amounts in the makarov basin north of the east siberian sea pacific source water is dominant in most of the canadian basin and is present in significant amounts in the amundsen basin north of greenland we deduce circulation patterns from the distributions of atlantic and pacific source waters in the surface layer of the arctic ocean and conclude that the flow within the surface layer differs from ice drift along the north american and european boundaries of the polar basin

on the resolution of density within the earth roughly 30 years have passed since the last publication of a linear resolution calculation of density inside the earth since that time the data set of free oscillation degnerate frequencies has been completely re estimated taking into account the biassing effects of splitting and coupling due to 3d structure this paper presents a new resolution analysis based on the new data and focuses on two particular issues 1 the density jump at the inner core boundary which is important in discussions of the maintenance of the geodynamo and 2 a possible density excess in the lowermost mantle which might be indicative of a hot abyssal layer we find that the density jump at the inner core boundary is 0 82 0 18 mg m 3 which is significantly larger than previously thought we also find little support for an excess density in the lowermost mantle though an increase

large geomagnetic field anomalies revealed in bronze to iron age archeomagnetic data from tel megiddo and tel hazor israel abstract geomagnetic field measurements from the past few centuries show heightened secular variation activity in the southern hemisphere associated with the south atlantic anomaly saa it is uncertain whether geomagnetic anomalies at a similar scale have existed in the past owing to limited coverage and uncertainties in the paleomagnetic database here we provide new evidence from archaeological sources in the levant suggesting a large positive northern hemisphere anomaly similar in magnitude to the saa during the 9th 8th centuries bce called levantine iron age anomaly we also report an additional geomagnetic spike in the 8th century the new dataset comprises 73 high precision paleointensity estimates from ca 3000 bce to 732 bce and five directional measurements between the 14th and the 9th centuries bce well dated pottery and cooking ovens were collected from twenty archaeological strata in two large contemporaneous stratigraphical mounds tells in israel tel megiddo and tel hazor the new data are combined with previously published data and interpreted automatically using the pmagpy thellier gui program the tel megiddo and tel hazor data sets demonstrate excellent internal consistency and remarkable agreement with published data from mesopotamia syria the data illustrate the evolution of an extreme geomagnetic high that culminated in at least two spikes between the 11th and the 8th centuries bce iron age in the levant the paleomagnetic directional data of the 9th century bce show positive inclination anomalies and deviations of up to 22 from the averaged geocentric axial dipole gad direction from comparison of the levantine archaeomagnetic data with igrf model for 2015 we infer the levantine iron age anomaly between the 10th and the 8th centuries bce is a local positive anomaly the eastward extent of the anomaly is currently unknown

circulation of winter water on the chukchi shelf in early summer abstract using a variety of data sources we investigate the properties and pathways of pacific origin winter water as it spreads across the eastern chukchi shelf in early summer the focus is on the time period june july 2011 during which an extensive shipboard hydrographic velocity survey was undertaken as part of the impacts of climate on ecosystem and chemistry of the arctic pacific environment icescape program a revised circulation scheme is constructed revealing that the transport pathways on the chukchi shelf are more complex than previously thought notably the well known branch progressing northward from the central channel bifurcates as it reaches hanna shoal flowing around both sides of the shoal and dividing into smaller filaments that continue towards barrow canyon mass is conserved in the circulation scheme with approximately 1 sverdrup flowing poleward across the chukchi shelf within these pathways then exiting barrow canyon the salinity of the winter water varied on the shelf in 2011 with saltier water found in the upstream portion of what is defined as the central pathway using sea ice concentration data and atmospheric reanalysis fields we argue that salinization of the winter water in the central pathway occurred via brine rejection as the parcels progressed north and passed through the cape lisburne polynya this demonstrates that winter water pervading the interior shelf can be transformed by convective overturning north of bering strait presumably stirring up nutrients from the sediments and thereby influencing primary productivity in the region of hanna shoal

derivation and use of core surface flows for forecasting secular variation improving forecasts of the temporal and spatial changes of the earth s main magnetic field over periods of less than 5 years has important scientific and economic benefits various methods for forecasting the rate of change or secular variation have been tried over the past few decades ranging from the extrapolation of trends in ground observatory measurements to computational geodynamo modeling with data assimilation from historical magnetic field models we examine the utility of an intermediate approach using temporally varying core surface flow models derived from relatively short periods of magnetic field data to produce by advection secular variation estimates valid for the earth s surface we describe a new method to compute a core flow changing linearly with time from magnetic secular variation and acceleration data we invert a combination of data from the champ satellite mission and ground observatories over the period 2001 0 to 2010 0 for a series of such models we assess their ability to forecast magnetic field changes by comparing them to chaos 4 a state of the art model using data from 1997 to 2014 5 we show that the magnetic field predictions tend to have a lower root mean square difference from chaos 4 than the international geomagnetic reference field or world magnetic map series of secular variation models

the 2003 geomagnetic jerk and its relation to the core surface flows abstract in this paper we examine the core surface flow obtained by an inversion of a continuous model of the geomagnetic field and its temporal variation using the diffusion less induction equation the continuous chaos model is derived from satellite data up to spherical harmonic degree 14 and covers the period 1999 to 2006 the chaos secular variation when downward continued to the core surface shows stripe like features which can be attributed to spherical harmonic degree 12 and higher these contributions are removed by applying a tapering method and the resulting tapered model is then inverted for the core surface flow satellite based field models have a high spatial resolution however their temporal resolution is limited in order to enhance the temporal resolution of the flow we additionally constrain the flow to fit the secular variation from ground based observatory data a range of solutions subject to different constraints are computed two flow hypotheses being considered purely toroidal flow and tangentially geostrophic flow we show that both flow types provide similar results however the purely toroidal flow provides a better fit to the secular variation in the equatorial region than the tangentially geostrophic flow we then analyze the residuals between observed secular variation and its predictions from the flow we note larger residuals for the tangentially geostrophic flow where strong radial secular variation and a weak radial field are observed although diffusive effects cannot be ruled out as a potential source of the mismatch we attribute the larger residuals to be caused by a flawed estimation of the poloidal flow we also seek to relate temporal changes in the fluid flow to the geomagnetic jerk which occurred at the beginning of 2003 it is found that this geomagnetic jerk coincides with variations in the zonal flow components of both flow types suggesting a possible link to torsional oscillations however we argue that other flow components are important to obtain a detailed fit of the observed secular variation finally we compare observed changes in the length of day and the predictions from the flow solutions

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

the chukchi slope current abstract using a collection of 46 shipboard hydrographic velocity transects occupied across the shelfbreak and slope of the chukchi sea between 2002 and 2014 we have quantified the existence of a current transporting pacific origin water westward over the upper continental slope it has been named the chukchi slope current which is believed to emanate from barrow canyon the current is surface intensified order 50 km wide and advects both summer and winter waters it is not trapped to a particular isobath but instead is reminiscent of a free jet there is no significant variation in pacific water transport with distance from barrow canyon a potential vorticity analysis suggests that the flow is baroclinically unstable consistent with the notion that it meanders the current is present during all synoptic wind conditions but increases in strength from summer to fall presumably due to the seasonal enhancement of the easterly winds in the region its transport increased over the 12 year period of data coverage also likely in response to wind forcing in the mean the slope current transports 0 50 0 07 sv of pacific water this estimate allows us to construct a balanced mass budget of the chukchi shelf inflows and outflows our study also confirms the existence of an eastward flowing chukchi shelfbreak jet transporting 0 10 0 03 sv of pacific water towards barrow canyon

ekman veering internal waves and turbulence observed under arctic sea ice the ice ocean system is investigated on inertial to monthly time scales using winter 2009 10 observations from the first ice tethered profiler itp equipped with a velocity sensor itp v fluctuations in surface winds ice velocity and ocean velocity at 7 m depth were correlated observed ocean velocity was primarily directed to the right of the ice velocity and spiraled clockwise while decaying with depth through the mixed layer inertial and tidal motions of the ice and in the underlying ocean were observed throughout the record just below the ice ocean interface direct estimates of the turbulent vertical heat salt and momentum fluxes and the turbulent dissipation rate were obtained periods of elevated internal wave activity were associated with changes to the turbulent heat and salt fluxes as well as stratification primarily within the mixed layer turbulent heat and salt fluxes were correlated particularly when the mixed layer was closest to the freezing temperature momentum flux is adequately related to velocity shear using a constant ice ocean drag coefficient mixing length based on the planetary and geometric scales or rossby similarity theory ekman viscosity described velocity shear over the mixed layer the ice ocean drag coefficient was elevated for certain directions of the ice ocean shear implying an ice topography that was characterized by linear ridges mixinglengthwasbestestimatedusingthewavenumber ofthebeginningoftheinertialsubrangeoravariable drag coefficient analyses of this and future itp v datasets will advance understanding of ice ocean interactions and their parameterizations in numerical models

western arctic shelfbreak eddies formation and transport abstract the mean structure and time dependent behavior of the shelfbreak jet along the southern beaufort sea and its ability to transport properties into the basin interior via eddies are explored using high resolution mooring data and an idealized numerical model the analysis focuses on springtime when weakly stratified winter transformed pacific water is being advected out of the chukchi sea when winds are weak the observed jet is bottom trapped with a low potential vorticity core and has maximum mean velocities of o 25 cm s 1 and an eastward transport of 0 42 sv 1 sv 106 m3 s 1 despite the absence of winds the current is highly time dependent with relative vorticity and twisting vorticity often important components of the ertel potential vorticity an idealized primitive equation model forced by dense weakly stratified waters flowing off a shelf produces a mean middepth boundary current similar in structure to that observed at the mooring site the model boundary current is also highly v

comment on can core surface flow models be used to improve the forecast of the earth s main magnetic field by stefan maus luis silva and gauthier hulot

a stochastic model for palaeomagnetic field variations summary regeneration of the earth s magnetic field by convection in the liquid core produces a broad spectrum of time variation relative palaeointensity measurements in marine sediments provide a detailed record over the past 2 myr but an explicit reconstruction of the underlying dynamics is not feasible a more practical alternative is to construct a stochastic model from estimates of the virtual axial dipole moment the deterministic part of the model drift term describes time averaged behaviour whereas the random part diffusion term characterizes complex interactions over convective timescales we recover estimates of the drift and diffusion terms from the sint2000 model of valet et al and the padm2m model of ziegler et al the results are used in numerical solutions of the fokker planck equation to predict statistical properties of the palaeomagnetic field including the average rates of magnetic reversals and excursions a physical interpretation of the stochastic model suggests that the timescale for adjustments in the axial dipole moment is set by the dipole decay time d w e obtain d 29kyr from the stochastic models which falls within the expected range for the earth s core we also predict the amplitude of convective fluctuations in the core and establish a physical connection to the rates of magnetic reversals and excursions chrons lasting longer than 10 myr are unlikely under present day conditions however long chrons become more likely if the diffusion term is reduced by a factor of 2 such a change is accomplished by reducing the velocity fluctuations in the core by a factor of 2 which could be attributed to a shift in the spatial pattern of heat flux from the core or a reduction in the total core heat flow

a numerical study of dense water formation and transport on a shallow sloping continental shelf the circulation and transport of dense water generated by an idealized coastal polynya is studied using a three dimensional primitive equation model starting with a homogeneous quiescent ocean a constant negative buoyancy flux is imposed at the surface over a half elliptical region adjacent to the coastal boundary on a gently sloping continental shelf the flow response can be divided into the following three phases geostrophic adjustment instability and offshore eddy transport during geostrophic adjustment the fluid within the forcing region becomes denser and the flow at the edge of the forcing region accelerates in response to the strong density gradient there eventually the flow at the leading edge of the forcing region relative to kelvin wave propagation becomes unstable and a train of counterrotating eddies develops these eddies then form a complex three dimensional flow field and rapidly transport dense water offshore across isobaths the density within the forcing region reaches a maximum which remains fairly constant after the eddies begin to transport the dense fluid offshore the results are qualitatively insensitive to weakening of the negative buoyancy forcing and to changing the bottom slope eddy scales and velocities are consistent with observations in the arctic the results suggest that instability processes and eddy fluxes are important in transporting dense water off continental shelves and into marginal seas

jerks abound an analysis of geomagnetic observatory data from 1957 to 2008 we present a two step method for the removal of external field signals and the identification of geomagnetic jerks in magnetic observatory monthly mean data providing quantitative uncertainty estimates on jerk occurrence times and amplitudes with minimal a priori information we apply the method to the complete time series of x y and z components at up to 103 observatory locations in the period of 1957 2008 we find features fitting the definition of jerks in individual components to be frequent and not globally contemporaneous identified regional jerks have no consistent occurrence pattern and the most widespread in any given year is identified at 30 of observatories worldwide whilst we identify jerks throughout the period of study relative peaks in the global number of jerk occurrences are found in 1968 71 1973 74 1977 79 1983 85 1989 93 1995 98 and 2002 03 with the suggestion of further poorly sampled events in the early 1960s and late 2000s the mean uncertainties on individual jerk occurrence times and amplitudes are found to be 0 3 yrs and 2 1 nt yr2 respectively for all field components jerk amplitudes suggest possible periodic trends across europe and north america which may be related to the 6 yr periods detected independently in the secular variation and length of day

evolution and dynamics of the flow through herald canyon in the western chukchi sea abstract the flow of summer and winter pacific water masses through herald canyon is investigated using data from a high resolution hydrographic velocity survey conducted in summer 2004 the survey was part of the russian american long term census of the arctic rusalca program and consisted of four cross canyon transects occupied over a 2 day period at the time of the survey dense winter water was entering the western side of the canyon from the chukchi sea flowing alongside a poleward jet of summer water on the canyon s eastern flank as the dense water progressed northward it switched sides of the canyon and underwent a sudden increase in layer thickness this coincided with vertical mixing near the interface of the winter and summer water producing a new water mass mode exiting the canyon all of these features are consistent with the notion of hydraulic activity occurring in the canyon a three layer hydraulic theory is applied to the flow which suggests that it is supercritical and that hydraulic control is likely a lock exchange formulation accurately predicts the northward transport of the winter water the origin of the winter water and the manner in which it drains into the canyon is investigated using satellite ice concentration data atmospheric re analysis fields historical in situ data and a simple circulation model finally the fate of the pacific water exiting the canyon and its connection to the chukchi shelfbreak current is discussed

geomagnetic intensity variations for the past 8 kyr new archaeointensity results from eastern china abstract in this study we have carried out paleointensity experiments on 918 specimens spanning the last 7 kyr including pottery fragments baked clay and slag collected from shandong liaoning zhejiang and hebei provinces in china approximately half of the specimens yielded results that passed strict data selection criteria and give high fidelity paleointensities the virtual axial dipole moments vadms of our sites range from 2 10 22 to 13 10 22 am 2 at 2250 bce our results suggest a paleointensity low of 2 10 22 am 2 which increases to a high of 13 10 22 am 2 by 1300 bce this rapid less than 1000 yrs six fold change in the paleointensity may have important implications for the dynamics of core flow at this time our data from the last 3 kyr are generally in good agreement with the arch3k 1 model but deviate significantly at certain time periods from the cals3k 4 and cals10k 1b model which is likely due to differences in the data used to constrain these models at ages older than 3 ka where only the cals10k 1b model is available for comparison our data deviate significantly from the model combining our new results with the published data from china and japan we provide greatly improved constraints for the regional model of eastern asia when comparing the variations of geomagnetic field in three global representative areas of eastern asia the middle east and southern europe a common general trend of sinusoidal variations since 8 ka is shown likely dominated by the dipole component however significant disparities are revealed as well which we attribute to non dipolar components caused by movement of magnetic flux patches at the core mantle boundary

making sound inferences from geomagnetic sounding abstract we examine the nonlinear inverse problem of electromagnetic induction to recover electrical conductivity as this is an ill posed problem based on inaccurate data there is a critical need to find the reliable features of the models of electrical conductivity we present a method for obtaining bounds on earth s average conductivity that all conductivity profiles must obey our method is based completely on optimization theory for an all at once approach to inverting frequency domain electromagnetic data the forward modeling equations are constraints in an optimization problem solving for the electric fields and the conductivity simultaneously there is no regularization required to solve the problem the computational framework easily allows additional inequality constraints to be imposed allowing us to further narrow the bounds we draw conclusions from a global geomagnetic depth sounding data set and compare with laboratory results inferring temperature and water content through published boltzmann arrhenius conductivity models if the upper mantle is assumed to be volatile free we find it has an average temperature of 1409 1539 c for the top 1000 km of the lower mantle we find an average temperature of 1849 2008 c these are in agreement with generally accepted mantle temperatures our conclusions about water content of the transition zone disagree with previous research with our bounds on conductivity we calculate a transition zone consisting entirely of wadsleyite has 0 27 wt water and as we add in a fraction of ringwoodite the upper bound on water content decreases proportionally this water content is less than the 0 4 wt water required for melt or pooling at the 410 km seismic discontinuity

geomagnetic field analysis v determining steady core surface flows directly from geomagnetic observations summary geomagnetic secular variation is a result of inductive effects of fluid motion at the top of the earth s liquid core which can be mapped using the frozen flux approximation and inverting mathematical models of the secular variation nonuniqueness may be removed by assuming the flow to be stationary the use of an intermediate mathematical model of secular variation makes comparison of the derived models with observation unsatisfactory and here we devise a new method inverting the geomagnetic measurements directly to obtain steady core motion the method is applied to observatory annual mean data for the 30yr period 1960 1990 to produce six separate models of steady flow three that are stationary over 10yr intervals 1960 1970 1970 1980 and 1980 1990 two 20 yr models 1960 1980 and 1970 1990 and one 30yr model 1960 1990 the method is not restricted to continuous time series or linear x y z data the 10 yr models fit the weighted data with misfits of 1 0 1 3 the 20 yr models fit less well and the 30yr model has a misfit of 1 7 when constrained to have the same norm as the 10yr models all models appear to fit the central decade with a gross misfit close to unity suggesting that changes in core motion may be associated with the two geomagnetic jerks that occurred around 1969 and 1978 furthermore close inspection of the fits at individual observatories shows that steady core motions with this norm cannot produce the secular accelerations required at a large number of high quality observatories steady flows can predict magnetic fields that follow the trend at most observatories but cannot follow the detailed time variations of the past three

pathways of pacific water across the chukchi sea a numerical model study 1 pathways of pacific water flowing from the north pacific ocean through bering strait and across the chukchi sea are investigated using a two dimensional barotropic model in the no wind case the flow is driven only by a prescribed steady northward flow of 0 8 sv through bering strait the resulting steady state circulation consists of a broad northeasterly flow basically following the topography with a few areas of intensified currents about half of the inflow travels northwest through hope valley while the other half turns somewhat toward the northeast along the alaskan coast the flow through hope valley is intensified as it passes through herald canyon but much of this flow escapes the canyon to move eastward joining the flow in the broad valley between herald and hanna shoals another area of slightly intensified currents there is a confluence of nearly all of the flow along the alaskan coast west of pt barrow to create a very strong and narrow coastal jet that follows the shelf topography eastward onto the beaufort shelf thus in this no wind case nearly all of the pacific water entering the chukchi sea eventually ends up flowing eastward along the narrow beaufort shelf with no discernable flow across the shelf edge toward the interior canada basin travel times for water parcels to move from bering strait to pt barrow vary tremendously according to the path taken e g less than 6 months along the alaskan coast but about 30 months along the westernmost path through herald canyon this flow field is relatively insensitive to idealized wind forcing when the winds are from the south west or north in which cases the shelf transports tend to be intensified however strong northeasterly to easterly winds are able to completely reverse the flows along the beaufort shelf and the alaskan coast and force most of the throughflow in a more northerly direction across the chukchi sea shelf edge potentially supplying the surface waters of the interior canada basin with pacific water the entire shelf circulation reacts promptly to changing wind conditions with a response time of 2 3 days the intense coastal jet between icy cape and pt barrow implies that dense water formed here from winter coastal polynyas may be quickly swept away along the coast in contrast there is a relatively quiet nearshore region to the west between cape lisburne and icy cape where dense water may accumulate much longer and continue to become denser before it is carried across the shelf

localized analysis of polar geomagnetic jerks abstract a new method is introduced to find the relatively sudden temporal changes or jerks of the geomagnetic field over the polar regions geomagnetic jerk events during the 20th century have mostly been identified from direct measurements at mid latitude geomagnetic observatories recently however global magnetic spherical harmonic models like cm4 and chaos have been found effective in identifying the antarctic events of 1969 1978 1985 1991 and 2003 that were followed by jerks in the arctic region with time delays of one to three years the present study extends these events into earlier decades from 1900 to 1985 using the global harmonic model gufm1 with localized spherical coefficients or slepian functions for the polar regions power spectral minima for the localized coefficients correlate with and thus help identify the abrupt changes in geomagnetic secular acceleration models the time delay between the antarctic and arctic geomagnetic jerks was also confirmed

circulation of summer pacific halocline water in the arctic ocean 1 we present an analysis of arctic ocean hydrographic and sea ice observations from the 1990s with a focus on the circulation of water that originates in the north pacific ocean previous studies have shown the presence of two varieties of relatively warm summer halocline water in the vicinity of the chukchi sea i e the relatively fresh alaskan coastal water acw and the relatively saltier summer bering sea water sbsw here we extend these studies by tracing the circulation of these waters downstream into the arctic ocean we find that acw is generally most evident in the southern beaufort gyre while sbsw is strongest in the northern portion of the beaufort gyre and along the transpolar drift stream we find that this separation is most extreme during the early mid 1990s when the arctic oscillation was at historically high index values this leads us to speculate that the outflow to the north atlantic ocean through the canadian archipelago and fram strait may be similarly separated as arctic oscillation index values fell during the later 1990s acw and sbsw began to overlap in their regions of influence these changes are evident in the area north of ellesmere island where the influence of sbsw is highly correlated with a 3 year lag with the arctic oscillation index we also note the presence of winter bering sea water wbsw which underlies the summer varieties all together this brings the number of distinct pacific water types in our arctic ocean inventory to three acw sbsw and wbsw

source and formation of the upper halocline of the arctic ocean 1 the upper halocline of the arctic ocean has a distinct chemical signature with high nutrient concentrations as well as low oxygen and ph values this signature is formed in the chukchi and east siberian seas by a combination of mineralization of organic matter and release of decay products to the sea ice brine enriched bottom water salinity and total alkalinity data show that the fraction of sea ice brine in the nutrient enriched upper halocline water in the central arctic ocean is up to 4 in the east siberian sea the bottom waters with exceptional high nutrient concentration and low ph have typically between 5 and 10 of sea ice brine as computed from salinity and oxygen 18 values on the continental slope over bottom depths of 150 200 m the brine contribution was 6 at the nutrient maximum depth 50 100 m at the same location as well as over the deeper basin the silicate maximum was found over a wider salinity range than traditionally found in the canada basin in agreement with earlier observations east of the chukchi plateau a detailed evaluation of the chemical and the temperature salinity properties suggests at least two different areas for the formation of the nutrient rich halocline within the east siberian sea this has not been observed before 2004 and it could be a sign of a changing marine climate in the east siberian sea caused by more open water in the summer season followed by more sea ice formation and brine production in the fall winter

very rapid geomagnetic field change recorded by the partial remagnetization of a lava flow 1 a new paleomagnetic result from a lava flow with a distinctive two part remanence reinforces the controversial hypothesis that geomagnetic change during a polarity reversal can be much faster than normal the 3 9 m thick lava flow 20 is exposed in the sheep creek range north central nevada and was erupted during a reverse to normal r n geomagnetic polarity switch at 15 6 ma flow 20 began to acquire a primary thermoremanence while the field was pointing east and down but was soon buried reheated and partially remagnetized in a north down direction by the 8 2 m thick flow that succeeded it a simple conductive cooling calculation shows that the observed remagnetization could not have occurred unless flow 20 was still warm about 150 c near its base when buried and that the 53 change from east down to north down field occurred at an average rate of approximately 1 week several orders of magnitude faster than typical of secular variation

on symmetry and anisotropy of earth core flows 1 quasi geostrophic qg flows are a recently developed and very promising paradigm for modeling decadal secular variation sv here we examine the effects of allowing anisotropy and departures of the flow from quasigeostrophy we perform dedicated numerical experiments of the flow dynamics and magnetic induction inside the earth s liquid core at time scales characteristic of secular variation of the geomagnetic field obtained results motivate new flow inversion regularization featuring an equatorially anti symmetric component superimposed to quasi geostrophic columns and stronger latitudinal than longitudinal flow gradients applying these constraints allows to explain the observed sv for the whole period 1840 2010 and most significantly provides a clearly improvement in prediction for decadal length of day variations for the period 1980 2000 furthermore the trace of the inner core appears clearly without any assumption for the 1997 2010 period covered by satellite geomagnetic data our results support qg being the appropriate description of the force balance within the core on decadal time scales and large spatial scales

mechanisms of pacific summer water variability in the arctic s central canada basin pacific water flows northward through bering strait and penetrates the arctic ocean halocline throughout the canadian basin sector of the arctic in summer pacific summer water psw is modified by surface buoyancy fluxes and mixing as it crosses the shallow chukchi sea before entering the deep ocean measurements from ice tethered profilers moorings and hydrographic surveys between 2003 and 2013 reveal spatial and temporal variability in the psw component of the halocline in the central canada basin with increasing trends in integrated heat and freshwater content a consequence of psw layer thickening as well as layer freshening and warming it is shown here how properties in the chukchi sea in summer control the temperature salinity properties of psw in the interior by subduction at isopycnals that outcrop in the chukchi sea results of an ocean model forced by idealized winds provide support to the mechanism of surface ocean ekman transport convergence maintaining psw ventilation of the halocline

the siberian coastal current a wind and buoyancy forced arctic coastal current we describe circulation and mixing in the siberian coastal current scc using fall shipboard measurements collected between 1992 and 1995 in the western chukchi sea the scc forced by winds siberian river outflows and ice melt flows eastward from the east siberian sea it is bounded offshore by a broad 60 km front separating cold dilute siberian coastal water from warmer saltier bering sea water the alongshore flow is incoherent because the current contains energetic eddies and squirts probably generated by frontal baroclinic instabilities these enhance horizontal mixing and weaken the cross shore density gradient along the scc path eventually the scc converges with the northward flow from bering strait whereupon it deflects offshore and mixes with that inflow deflection occurs where the alongshore pressure gradient vanishes that location varies on synoptic and seasonal timescales because this gradient depends on the winds buoyancy fluxes and the sea level difference between the pacific and arctic oceans deflection usually occurs on the chukchi shelf but the scc occasionally flows southward through bering strait such events are short lived 1 10 days and occur mainly in fall and winter under northerly winds scc transport is likely small 0 1 sv but its dilute waters could substantially freshen the bering strait inflow and affect the disposition of pacific waters in the arctic ocean arctic river outflows should preferentially form surface advected fronts rather than bottom advected fronts because vertical mixing energy is low on arctic shelves surface advected fronts are more susceptible to upwelling winds and for the scc the pressure gradient between the pacific and arctic oceans than bottom advected fronts the scc never developed in fall 1995 because of anomalously steady upwelling winds the western chukchi shelf could have formed upper halocline source water in the winter of 1995 1996

geomagnetic jerks in the polar regions 1 the occurrence of geomagnetic jerks over the arctic and antarctic regions is here investigated maps of geomagnetic secular acceleration over the polar regions are produced from the cm4 and chaos models and the occurrence of geomagnetic jerks is associated with jumps in secular acceleration the obtained results confirm that in antarctica geomagnetic jerks systematically follow geomagnetic jerks in the arctic region with a time delay from one to three years evidence is found of an abrupt change in secular acceleration in both polar regions around 1985 suggesting that the 1985 local jerk could actually be a worldwide event combining our results with the results previously obtained on the occurrence of a geomagnetic jerk at low mid latitudes around 2003 we support the hypothesis of a global extension of an event occurred at the beginning of the 21st century

on the geographical distribution of induced time varying crustal magnetic fields 1 a long standing question in geomagnetism is whether the time variation of the induced crustal field is a detectable quantity and if so at which spatial wavelengths we tackle this problem with the help of a forward modeling approach using a vertically integrated susceptibility vis grid of the earth s crust for spherical harmonic degrees 15 90 we estimate the root mean square of the crustal magnetic field secular variation to amount 0 06 0 12 nt yr at the terrestrial surface between epochs 1960 2002 5 the geographical distribution of the signal shows absolute values reaching 0 65 1 30 nt yr over south america unfortunately most of the world magnetic observatories currently lie on quasi stationary features where the crustal field signal variations are expected to be very low however this long sought signal could be detected over well chosen regions provided that satellite observatory and repeat station measurements are available over several decades

on the nature and origin of water masses in herald canyon chukchi sea synoptic surveys in summer 2004 2008 and 2009 abstract hydrographic and velocity data from three high resolution shipboard surveys of herald canyon in the northwest chukchi sea in 2004 2008 and 2009 are used to investigate the water masses in the canyon and their possible source regions both summer and winter pacific waters were observed in varying amounts in the different years although in general the summer waters resided on the eastern side of the canyon while the winter waters were located on the western flank the predominant summer water was bering summer water although some alaskan coastal water resided in the canyon in the two later years likely due to wind forcing both newly ventilated and remnant winter waters were found in the canyon but the amount lessened in each successive survey using mooring data from bering strait it is shown that a large amount of bering summer water in the western channel of the strait follows a relatively direct route into herald canyon during the summer months with an estimated advective speed of 10 20 cm s however while the winter water observed in 2004 was consistent with a bering strait source with a slower advective speed of 5 8 cm s the dense water in the canyon during 2008 and 2009 was more in line with a northern source this is consistent with sections to the west of the canyon and with previously reported measurements implying winter water formation on the east siberian shelf large scale wind patterns and polynya activity on the shelf are also investigated it was found that the former appears to impact more strongly the presence of dense water in herald canyon

distributional patterns along the mediterranean continental margin upper bathyal using gryphus vitreus brachiopoda densities abstract the gryphus vitreus biocoenosis occurs in two types of profiles of the bottom topography on the continental slope the density of g vitreus can be divided into five zones directly related to the velocity of the bottom current the direction of the current over the gryphus vitreus biocoenosis suggests three models according to the distributional limits of the density zones the upper limit is always the continental shelf edge 100 120 m and the zonal limits are related to the slope gradient below 150 m the bathymetric and spatial distribution of the density zones are analysed in relation to the current velocity and direction

temporal variations of strength and location of the south atlantic anomaly as measured by rxte abstract the evolution of the particle background at an altitude of 540 km during the time interval between 1996 and 2007 is studied using the particle monitor of the high energy x ray timing experiment on board nasa s rossi x ray timing explorer a special emphasis of this study is the location and strength of the south atlantic anomaly saa the size and strength of the saa are anti correlated with the 10 7 cm radio flux of the sun which leads the saa strength by 1 year reflecting variations in solar heating of the upper atmosphere the location of the saa is also found to drift westwards with an average drift rate of about 0 3 yr following the drift of the geomagnetic field configuration superimposed to this drift rate are irregularities where the saa suddenly moves eastwards and where furthermore the speed of the drift changes the most prominent of these irregularities is found in the second quarter of 2003 and another event took place in 1999 we suggest that these events are previously unrecognized manifestations of the geomagnetic jerks of the earth s magnetic field

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

geomagnetic jerks and rapid hydromagnetic waves focusing at earth s core surface geomagnetic jerks are abrupt changes in the second time derivative the secular acceleration of earth s magnetic field that punctuate ground observatory records as their dynamical origin has not yet been established they represent a major obstacle to the prediction of geomagnetic field behaviour for years to decades ahead recent jerks have been linked to short lived temporally alternating and equatorially localized pulses of secular acceleration observed in satellite data associated with rapidly alternating flows at earth s core surface here we show that these signatures can be reproduced in numerical simulations of the geodynamo that realistically account for the interaction between slow core convection and rapid hydromagnetic waves in these simulations jerks are caused by the arrival of localized alfven wave packets radiated from sudden buoyancy releases inside the core as they reach the core surface the waves focus their energy towards the equatorial plane and along lines of strong magnetic flux creating sharp interannual changes in core flow and producing geomagnetic jerks through the induced variations in magnetic field acceleration the ability to numerically reproduce jerks offers a new way to probe the physical properties of earth s deep interior geomagnetic jerks in the earth s magnetic field are caused by the arrival of hydromagnetic waves and could be generated by sudden releases of buoyancy in the earth s core suggest geodynamic numerical model simulations

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

stable isotope profiles ca o c through modern brachiopod shells of t septentrionalis and g vitreus implications for calcium isotope paleo ocean chemistry brachiopod shells are widely used as an archive to reconstruct elemental and isotopic composition of seawater studies focused on oxygen and carbon isotopes over the last decades are increasingly extending to the emerging calcium isotope system to date only little attention has been paid to test the reliability of fossil brachiopods on their modern counterparts in this context the present study investigates two modern brachiopods terebratulina septentrionalis eastern canada 5 30 m depth 7 1 c seasonal temperature variation two layer shell and gryphus vitreus northern mediterranean 200 m depth constant all year round temperature three layer shell both species were sampled along the ontogenetic growth direction and calcium oxygen and carbon isotopes as well as elemental concentration were measured calcium isotopes were analyzed on tims the elemental composition was analyzed by la icp ms and icp aes the results indicate an intra specimen 44 40ca variation ranging from 0 16 to 0 33 pointing to a fairly homogenous distribution of calcium isotopes in brachiopod shells however in the light of the suggested 0 7 increase in calcium isotopes over the phanerozoic such intra specimen variations constrain ocean reconstruction 44 40ca values of t septentrionalis do not seem to be affected by growth rate calcium isotopic values of g vitreus are heavy in the central part of the shell and trend towards lighter values in peripheral areas approaching the maximum isotopic composition of t septentrionalis the maximum inter species 44 40ca difference of 0 62 between t septentrionalis and g vitreus indicates that care should be taken when using different taxa species with different strontium content or brachiopods with specialized shell structure such as g vitreus for ocean water reconstruction in terms of ca isotopic composition t septentrionalis may record ca isotopic fractionation related to seasonal seawater temperature variations in its shell but this is difficult to resolve at the current analytical precision average 18o derived temperatures of the two investigated species are close to on site measured temperatures

a new approach to directly determine the secular variation from magnetic satellite observations 1 observatory monthly means provide an excellent opportunity to study the temporal changes of the geomagnetic field at a given location unfortunately determination of the global pattern of changes using observatory data is hampered by their uneven distribution satellite data provide excellent global coverage but the spacecraft movement makes direct comparisons of satellite and observatory data difficult to investigate short period secular variation in observatory and satellite data we developed an approach to extract satellite monthly means for virtual observatories at 400 km altitude using champ magnetic measurements comparison of these virtual observatory monthly means with the corresponding ground values shows a remarkably well correlated signal at time scales of months to years which is beyond the temporal resolution limit of recent global models here we describe this newly developed approach its validation and discuss how it can be used to understand short period changes of the recent geomagnetic field

new constraints on the variation of the geomagnetic field during the late neolithic period archaeointensity results from sichuan southwestern china we have carried out an archaeomagnetic study on a late neolithic locality liujiazhai in sichuan southwestern china we pull together various dating techniques including radiocarbon analysis optically stimulated luminescence dating stratigraphic information as well as archaeological and archaeomagnetic estimations to constrain the age of the studied samples rock magnetic results indicate thermally stable fine grained magnetite or titanomagnetite as the dominant magnetic carriers more than half of the specimens 141 246 in the paleointensity experiment pass the selection criteria and are considered to record robust intensity values the virtual axial dipole moments range from approximately 2 8 to 7 8 1022 am2 with an average of 5 9 1022 am2 indicating that the geomagnetic intensity around 3000 before the common era b c e is overall lower than the present field intensity 9 8 1022 am2 of this area the new results from liujiazhai are generally consistent with the published data of similar age but deviate from the only available model of cals10k 1b at certain time periods making them important for future improvements of the model those data are significant for constraining the variation of geomagnetic field intensity between 3100 and 2600 b c e and improving the regional model of eastern asia

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

investigation of a secular variation impulse using satellite data the 2003 geomagnetic jerk abstract observatory monthly means provide an excellent opportunity to study the temporal changes of the magnetic field at a given location unfortunately the uneven distribution of the present observatory network makes it difficult to determine the global field change pattern recently we have developed an approach to extract satellite monthly means at a regular network of virtual observatories at 400 km altitude based on champ magnetic measurements using monthly means for 2001 2005 from those virtual observatories we investigate the space time structure of the short period variation of the earth s magnetic field by means of a spherical harmonic expansion followed by a separation into external magnetospheric and internal part this allows for the first time to study the secular variation globally and directly from satellite magnetic data analyzing the time series of the magnetic field at the virtual observatories as well as those of the spherical harmonic expansion coefficients we detect a secular variation impulse an abrupt jump in the second time derivative of the magnetic field in the champ satellite data during the first months of the year 2003 the jerk occurred simultaneously in the northern and southern hemispheres in a rather limited area near 90 e with maximum jerk strength at about 30 latitude a region also characterized by a strong secular acceleration second time derivative of the magnetic field we show that the 2003 geomagnetic jerk is not worldwide in occurrence and that there is an evidence for this event in the length of day variation

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

circulation dense water formation and outflow on the northeast chukchi shelf we investigated circulation and water mass modification processes in the chukchi sea using 1 temperature salinity and velocity data collected between september 1991 and september 1992 from moorings in bering strait and the northeast shelf and 2 meteorological data and ice concentrations derived from special sensor microwave imager ssm i imagery in october 1991 and from february to august 1992 the mean monthly circulation was steady and northward from november to january strong northeast winds diverted the low salinity bering inflow onto the western shelf and weakened and reversed flow over the northeast shelf the winds also opened extensive polynyas wherein cold hypersaline salinity 34 waters formed the fall winter circulation enhanced dense water formation by diverting low salinity waters away from and prolonging the residence time of water within the polynyas alongshore convergence of the coastal flow swept some of the cold hypersaline water offshore across isobaths but the coastal current carried most of it into the arctic ocean through barrow canyon the dense outflow mixed little during its descent through the upper canyon where bottom friction and rotation but not entrainment were important in the plume momentum balance if the outflow is not diluted by mixing in the lower canyon where theory suggests this will occur then the densest shelf waters can ventilate layers deeper than the halocline salt rejection estimates using ssm i data and surface heat budget calculations agree within a factor of 2 with independent estimates from the mooring hence remote sensing techniques can monitor winter salt rejection rates in the chukchi sea

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

summertime circulation in the eastern chukchi sea abstract the transport of pacific origin water across the eastern chukchi sea during summer is studied using shipboard hydrographic and acoustic doppler current profiler data from multiple surveys the study identifies two major transport pathways in this region the well known alaskan coastal current acc flows poleward along the alaska coast while a more recently discovered and slower current flows northward through the central channel between herald and hanna shoals the two currents separate past point hope and appear to merge again in barrow canyon via previously unresolved pathways flowing west to east across the chukchi sea the collective flow that transports pacific water from bering strait to barrow canyon is termed the bering to barrow current system bbcs the coastal branch of the bbcs which encompasses the acc is a weakly baroclinic flow with a maximum speed in excess of 80 cm s inside barrow canyon the central channel branch of the bbcs is considerably weaker with a maximum flow speed under 20 cm s a portion of the pacific water that flows towards herald canyon to the west may also contribute to the bbcs the amount but the amount is within the margin of error of our data the difference in the advective time scales of the various branches allows water masses of different seasonal characteristics to be transported simultaneously by the bbcs summer water masses occupy most of the southern and central chukchi sea while winter water masses occupy hanna shoal and shelfbreak regions in the northern chukchi sea during summer the total bbcs volume transport in summer is measured at four different locations in the eastern chukchi sea and found to be generally consistent the overall mean transport is 0 86 sv compared to the summer bering strait transport of 1 08 sv this study suggests that the bbcs is the main transport pathway delivering pacific water masses into the western arctic basin

signal from noise in geomagnetic field modelling denoising data for secular variation studies summary we analyse the residuals between a continuous geomagnetic field model of the internal field c3fm and observed secular variation a large part of the residual variations correlate closely with the dst index suggesting an origin from unmodelled external field variations removal of this signal enhances the resolution of fine scale detail in secular variation this is useful in considering the phenomenology of geomagnetic jerks the residual variations between different observatories show even better correlation suggesting the possibility of the construction of a proxy for the dst index notable cross correlation is also seen between the residuals and dst index with a lag of about 55 months