sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

linearized inversion of seismic reflection data this is the first of a series of papers giving the solution of the inverse problem in seismic exploration the acoustic approximation is used together with the assumption that the velocity field has the form the forward problem is then linearized thus neglecting multiple reflected waves and the inverse problem of estimating is set up its rigorous solution can be obtained using an iterative algorithm each step consisting of a classical kirchhoff migration hyperbola summation plus a classical forward modeling step circle summation

sea surface height determination in the arctic ocean from ers altimetry 1 accurate sea surface height measurements have been extracted from ers altimeter data in sea ice covered regions for the first time the data have been used to construct a mean sea surface of the arctic ocean between the latitudes of 60 degreesn and 81 5 degreesn based on 4 years of ers 2 data an rms value for the crossover differences of mean sea surface profiles of 4 2 cm was observed in the ice covered canada basin compared with 3 8 cm in the ice free greenland iceland norwegian seas comparisons are made with an existing global mean sea surface osumss95 highlighting significant differences between the two surfaces in permanently ice covered seas in addition we present the first altimeter derived sea surface height variability map of the arctic ocean comparisons with a high resolution coupled ocean sea ice general circulation model reveal a good qualitative agreement in the spatial distribution of variability quantitatively we found that the observed variability was on average a factor of 3 4 greater than model predictions

source inversion of intensity patterns of earthquakes a destructive shock in 1936 in northeast italy 1 we demonstrate that it is possible to retrieve geometric and kinematic information about the source of a destructive past earthquake by inverting its regional macroseismic intensity patterns in fact in the case study the inversion results agree with the seismological instrumental measurements of the 1930s and with neotectonic evidence this leads to the hope that more knowledge about preinstrumental events can be obtained a key toward improving the calculation of seismic hazard mostly in the old world after validating our technique on the 1987 whittier narrows california earthquake pettenati and sirovich 2003 gentile et al 2004 we achieve the present results by investigating an earthquake that happened in 1936 in northeast italy the automatic inversions were performed by using a simplified formula for body waves that radiate from a linear source the inversion shows two minima on the hypersurface of the minimum residuals calculated observed intensity at all sites in the multiparameter model space these two minimum variance source models resemble the two auxiliary planes of the same theoretical fault plane solution similar to that given by the standard use of the first p wave arrivals the present result encourages us to treat more cases and to explore new inversion techniques for quantitatively treating the intensity patterns of earthquakes because at least in some cases they carry geometric and kinematic information about their source

tomographic determination of velocity and depth in laterally varying media estimation of reflector depth and seismic velocity from seismic reflection data can be formulated as a general inverse problem the method used to solve this problem is similar to tomographic techniques in medical diagnosis and we refer to it as seismic reflection tomography seismic tomography is formulated as an iterative gauss newton algorithm that produces a velocity depth model which minimizes the difference between traveltimes generated by tracing rays through the model and traveltimes measured from the data the input to the process consists of traveltimes measured from selected events on unstacked seismic data and a first guess velocity depth model usually this first guess model has velocities which are laterally constant and is usually based on nearby well information and or an analysis of the stacked section the final model generated by the tomographic method yields traveltimes from ray tracing which differ from the measured values in recorded data by approximately 5 ms root mean square the i

sea ice atmosphere and upper ocean variability in the weddell sea antarctica a frequency domain singular value decomposition is performed on 20 years 1979 1998 of monthly sea ice concentration sea ice drift and sea level pressure data in the weddell sea antarctica interannual oscillations with periods of around 3 4 years are found to dominate the variability in this region anomalous atmospheric patterns periodically reach the weddell sea from the west and perturb the sea ice circulation and distribution in the weddell gyre through changes in the intensity and direction of the climatological winds sea ice accumulates in the southeastern weddell sea every 3 4 years owing to two atmospherically driven processes 1 weak ice export to the north due to a weak northward branch of the gyre driven by weak southerly winds and 2 large ice import from the east due to a strong east wind drift driven by strong easterly winds along the coast the opposite situation gives rise to a depletion of sea ice in the same region half a cycle later sea ice anomalies are then advected north northeastward before turning eastward in the gyre circulation the eastward propagation of ice anomalies along the ice margin accounts for the passage of the antarctic circumpolar wave through the atlantic sector of the southern ocean a low frequency signal is also detected in the weddell sea variations albeit rather speculatively in this 20 year long record sea ice variability on this timescale appears to be associated with a change in the shape and characteristics of the weddell gyre circulation around 1990 this mode of variability implicates feedbacks between the gyre and weddell deep water temperature variations whose impact is observed near the maud rise topographic feature

evidence of nonlinear dynamics in the eastward shift of the nao 1 hilmer and jung 2000 have identified an eastward shift in the sea level pressure slp pattern associated with interannual variability of the north atlantic oscillation nao that took place around 1980 we investigate the nature of the eastward shift using a primitive equation dry atmospheric model driven by diabatic forcing diagnosed from observations the model results reveal the nonlinear dependence of the spatial pattern of the nao on the nao index the pattern being shifted to the east west for high low nao index general agreement is found between the model and observations we suggest that the eastward shift noted by hilmer and jung 2000 is a consequence of the trend towards higher nao index during the last several decades of the 20th century

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

crustal velocity structure in italy from analysis of regional seismic waveforms abstract in this article we use regional seismic waveforms recorded by the recently installed istituto nazionale di geofisica e vulcanologia ingv national network and the mediterranean very broadband seismographic network mednet stations to develop 1d crustal velocity models for the italian peninsula about 55 000 p wave and 35 000 s wave arrival times from 4727 events are used to derive average seismic parameters in the crust and uppermost mantle we define four regions according to geological constraints and recent travel time tomography results based on the average seismic parameters we combine broadband seismic waveforms and travel times of regional phases to model crustal structures for the four regions by applying the genetic algorithm our results indicate smooth velocity gradients with a depth beneath the apennines and a deep moho beneath the central alps green s functions from the regionalized 1d velocity models are used to determine source depths and focal mechanisms for 37 events with a magnitude larger than 3 5 by a grid search technique our results show that normal and strike slip faulting source mechanisms dominate the apenninic belt and that most thrust faulting events occur in the adriatic sea and the outer margin of the northern apennines

a new regional hypocenter location method a new regional hypocenter location method is presented in this article to address location problems associated with sparse station distributions and lack of velocity information for an arbitrary slowness structure an unknown parameter slowness deviation is introduced to construct a more precise slowness model than a homogeneous slowness model by incorporating a genetic algorithm the new method yields reasonable solutions for epicenters and origin times both inside and outside the seismic network the synthetic data tests indicate that outside the network the new method gives excellent results for epicenter locations compared with geiger s method even when geiger s method is used with the true velocity structure the relocation of the loma prieta mainshock and 26 aftershocks occurring within the first 24 h after the mainshock by using the new hypocenter location method places the aftershocks an average of 2 29 km to the southwest of the locations published by the northern california earthquake data center the new locations are in better agreement with the geodetic measurements

atmospheric forcing on the barents sea winter ice extent abstract the atmospheric forcing on the barents sea ice extent during winter december february djf has been investigated for the period 1967 2002 the time series for the sea ice extent is updated and includes the winter of 2005 which marks a new record low in the wintertime barents sea ice extent and a linear trend of 3 5 decade 1 in the ice extent was found covariability between the barents sea ice extent and the atmospheric mean seasonal flow and the synoptic cyclones has been discussed separately for the mean flow linear correlations and regression analysis reveal that anomalous northerly southerly winds prevail in the nordic seas during winters with extensive sparse barents sea ice extent some of the variability in the mean flow is captured by the north atlantic oscillation nao however the wintertime link between the barents sea ice extent and the nao is moderate by studying the cyclone activity in the high latitude northern hemisphere using a dataset of individual cyclones two

earthquake location genetic algorithms for teleseisms kennen b l n and sambndge m s 1992 earthquake location genetic algorithms for teleselsms phys earth planet inter 75 103 110 the location of earthquakes requires the estimation of the spatial and temporal components of the hypocentre this can he achieved by a direct mmlmtsatlon of a measure of the misfit between observed and calculated travel times and also slownesses and azimuths if array data are available an efficient means of carrying out this optlmlsatlon procedure is to make use of genetic algorithms this technique is based on the use of many estimates of the hypocentre location at once and the properties of the cluster of estimated locations in four dimensions are exploited m the course of the optlmtsatlon process each estimate of the hypocentral location is represented on a local discrete grid by a bit string and successive iterations generate new bit strings and hence location estimates by operations based on biological analogues these operations are the replication of the best fining bit strings the cross over of reformation between pairs of bit strings and the mutation of indwldual bits in a string the non local character of the information on the misfit function carried in the cloud of hypocentral estimates is usually sufficient to prevent the location being trapped in local minima of the misfit surface convergence to the global misfit minimum can be achieved with a very limited sampling of the original spatml and temporal grid no derivatives of the seismic phase information are reqmred and so the technique is easily generahsed to three dimensional velocity models and can be used with any statable measure of the quahty of an earthquake location by the choice of the misfit criterion between observed and calculated quantities

hypocentral relocation using clustering along planes constraints implications for fault geometry summary hypocentre location is an ill posed inverse problem even assuming that the velocity model is known because different sets of hypocentre locations may satisfy the fitting criterion we present a regularized hypocentre inversion in which the constraints of spatial proximity of the hypocentres to target planes are used this constraint introduces the geological bias that earthquakes might occur along fault planes here the target planes may be either 1 planes specified by the interpreter or 2 planes fitting groups of events we assume also that initial estimates of hypocentres and origin times are available then the initial hypocentre estimates origin times and target planes are used as input to an inversion problem to relocate the hypocentres so that the maximum possible clustering of events along the given planes is attained matching the observed traveltimes we use l1 norm for data fitting l2 norm for the plane proximity criterion and a polytope algorithm to minimize the functional results from synthetic and real data indicate that the plane proximity constraint allows for hypocentre relocation presenting a high degree of clustering along planes the real data example is an intraplate earthquake sequence in ne brazil our methodology defined the geometry and strike of fault segments close to known geology and focal mechanism data in addition the new method indicates that the fault is characterized by a splay geometry in its southern end and that more than three fault segments are necessary to explain the hypocentre distribution

storm driven mixing and potential impact on the arctic ocean 1 observations of the ocean atmosphere and ice made by ice ocean environmental buoys indicate that mixing events reaching the depth of the halocline have occurred in various regions in the arctic ocean our analysis suggests that these mixing events were mechanically forced by intense storms moving across the buoy sites in this study we analyzed these mixing events in the context of storm developments that occurred in the beaufort sea and in the general area just north of fram strait two areas with quite different hydrographic structures the beaufort sea is strongly influenced by inflow of pacific water through bering strait while the area north of fram strait is directly affected by the inflow of warm and salty north atlantic water our analyses of the basin wide evolution of the surface pressure and geostrophic wind fields indicate that the characteristics of the storms could be very different the buoy observed mixing occurred only in the spring and winter seasons when the stratification was relatively weak this indicates the importance of stratification although the mixing itself was mechanically driven we also analyze the distribution of storms both the long term climatology and the patterns for each year in the past 2 decades the frequency of storms is also shown to be correlated but not strongly to arctic oscillation indices this study indicates that the formation of new ice that leads to brine rejection is unlikely the mechanism that results in the type of mixing that could overturn the halocline on the other hand synoptic scale storms can force mixing deep enough to the halocline and thermocline layer despite a very stable stratification associated with the arctic halocline the warm subsurface thermocline water is not always insulated from the mixed layer

contrasts in sea ice deformation and production in the arctic seasonal and perennial ice zones 1 four years 1997 2000 of radarsat geophysical processor system rgps data are used to contrast the sea ice deformation and production regionally and in the seasonal siz and perennial piz ice zones ice production is of seasonal ice in openings during the winter three day estimates of these quantities are provided within lagrangian elements initially 10 km on a side a distinct seasonal cycle is seen in both zones with these estimates highest in the late fall and with seasonal minimums in the midwinter regional divergence over the winter could be up to 30 spatially the highest deformation is seen in the siz north of coastal alaska both ice deformation and production are higher in the siz deformation related ice production in the siz 0 5 m is 1 5 2 3 times that of the piz 0 3 m this is connected to ice strength and thickness atmospheric forcing and boundary layer structure contribute to only the seasonal and interannual variability seasonal ice growth in ice fractures accounts for 25 40 of the total ice production of the arctic ocean uncertainties in these estimates are discussed by itself this deformation ice production relationship could be considered a negative feedback when thickness is perturbed however the overall effect on ice production in the face of increasing seasonal and thinner weaker ice coverage could be modified by local destabilization of the water column promoting overturning of warmer water due to increased brine rejection and the upwelling of the pynocline associated with increased occurrence of large shear motion in sea ice divergence is shown to be negligibly correlated to cyclonic motion in summer and winter in both ice zones

a model to interpret driftwood transport in the arctic abstract driftwood is frequently used to estimate past changes of sea ice extent and circulation in the arctic nevertheless driftwood observations are difficult to interpret because of the potentially complex relation with climate change in order to determine the origin of the observed changes we built a driftwood transport model dtm simulating the driftwood trajectories from the boreal forest to arctic coasts the model is driven by three main variables which are the sea ice velocity concentration and the sea surface current velocity that can be derived from observations or climate model outputs e g from a general climate model gcm overall the dtm model agrees with the observations although this comparison needs to be taken with caution because of the sparse data and the uncertainties of driftwood provenance through simulations performed with the dtm model we confirm the strong influence of the variability of the atmospheric circulation on the spatial driftwood distribution model simulations of the mid holocene period driven by six gcms show that small local changes in sea ice circulation a westward shift in the transpolar drift and a reduced beaufort gyre during the mid holocene compared to the present period suffice to explain the driftwood landing change during the mid holocene with a non negligible contribution from reduced sea ice concentration consequently a change in driftwood deposit should not be directly interpreted as large modifications in atmospheric circulation and the complexity of the response of driftwood trajectories to past climate changes clearly highlights the interest of using a model to interpret driftwood records

modeled winter sea ice variability and the north atlantic oscillation a multi century perspective the relationship between winter sea ice vari ability and the north atlantic oscillation nao is examined for the time period 1860 2300 this study uses model output to extend recently reported observational results to multi century time scales nine ensemble members are used in two global climate models with forcing evolving from pre industrial conditions through the so called a1b scenario in which carbon dioxide stabilizes at 720 ppm by 2100 throughout the nao generates an east west dipole pattern of sea ice concentration sic anomalies with oppositely signed centers of action over the labrador and barents seas during the positive polarity of the nao sic increases over the labrador sea due to wind driven equatorward advection of ice and sic decreases over the barents sea due to wind driven poleward transport of heat within the mixed layer of the ocean although this nao driven sic variability pattern can always be detected it accounts for a markedly varying fraction of the total sea ice variability depending on the strength of the forced sea ice extent trend for the first half of the 20th century or 1990 control conditions the nao driven sic pattern accounts for almost a third of the total sic var iance in the context of the long term winter sea ice retreat from 1860 to 2300 the nao driven sic pattern is robustly observable but accounts for only 2 of the total sic vari ance the nao driven sic dipole retreats poleward with the retreating marginal ice zone and its barents sea center of action weakens results presented here underscore the idea that the nao s influence on arctic climate is robustly observable but time dependent in its form and statistical importance

two circulation regimes of the wind driven arctic ocean the major goal of this paper is to demonstrate the existence in the arctic ocean of two regimes of wind forced circulation we simulated the vertically averaged currents sea level heights and ice drift in the arctic ocean from 1946 to 1993 using a two dimensional wind forced barotropic model that includes frictional coupling between the ocean and ice the model has a spatial resolution of 55 5 km and is driven by winds river runoff and an imposed but realistic sea level slope between the pacific and the atlantic oceans there is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics the results indicate that wind driven motion in the central arctic alternates between anticyclonic and cyclonic circulation with each regime persisting for 5 7 years based upon our analysis of the modeled sea level and ice motion anticyclonic wind driven motion in the central arctic appeared during 1946 1952 1958 1963 1972 1979 and 1984 1988 and cyclonic motion appeared during 1953 1957 1964 1971 1980 1983 and 1989 1993 shifts from one regime to another are forced by changes in the location and intensity of the icelandic low and the siberian high the two regimes may help explain the significant basin scale changes in the arctic s temperature and salinity structure observed recently the great salinity anomaly and the variability of ice conditions in the arctic ocean

sources of sediment found in sea ice from the western arctic ocean new insights into processes of entrainment and drift patterns 1 the geochemical fingerprint of entrained fe oxide mineral grains in arctic ocean sea ice is used to determine precise sources of this sediment this approach substantiates the importance of the laptev sea as a source of sea ice and even the presence of russian ice in the beaufort sea off alaska fe oxide grains from the laptev sea were found in floes in the beaufort sea chukchi borderland and central arctic ocean demonstrating the importance of sea ice in distributing itself throughout the arctic ocean and the potential of transporting sediment from russian rivers to north american shelves banks island shelf is the most important source of sediment sampled from ice floes in the beaufort sea northern chukchi sea and chukchi borderland area although most of the entrained sediment fits the criteria for suspension freezing in shallow water the presence of winter polynyas with offshore winds and not the size of shallow areas appears to be the critical factor for sea ice entrainment seven of the 18 ice floes sampled contained fe oxide grains from more than one source area the two most common sources that are found in the same ice floes are banks island and the laptev sea multiple sources in ice floes suggest that either mingling of fragmented ice floes occurs or that a source area containing grains from both these sources has yet to be located the addition of fine sand sized windblown sediment is not thought to be significant

sea ice motion from satellite passive microwave imagery assessed with ers sar and buoy motions observing the motion of sea ice from space is analogous to observing wind stress over the wet oceans both provide surface forcing for modeling ocean dynamics ice motion also directly provides the advective component of the equations governing the mass balance of the sea ice cover thus its routine observation from space would be of great value to understanding ice and ocean behavior to demonstrate the feasibility of creating a global multidecadal ice motion record from satellite passive microwave imagery and to quantitatively assess the errors in the estimated ice motions we have tracked ice every 3 days in the arctic ocean and daily in the fram strait and baffin bay during the 8 winter months from october 1992 to may 1993 and daily in the weddell sea during the 8 winter months from march to october 1992 the method which has been well used previously involves finding the spatial offset that maximizes the cross correlation of the brightness temperature fields over 100 km patches in two images separated in time by from 1 to 3 days the resulting ice motions are compared with contemporaneous buoyand sar derived ice motions the uncertainties in the displacement vectors between 5 and 12 km are better than the spatial resolution of the data both 85 ghz data with 12 km spatial resolution and 37 ghz data with 25 km resolution are tracked these trials with the 37 ghz data are new and show quite surprisingly that the error is only about 1 km larger with these data than with the 12 km 85 ghz data errors are typically larger than average in areas of lower ice concentration in the most dynamic regions particularly near the ice edge in the barents and greenland seas and in zones of high shear these passive microwave ice motions show a large increase in spatial detail over motion fields optimally interpolated from buoy and wind observations especially where buoy data are virtually absent such as near coasts and in some passages between the arctic ocean and its peripheral seas the feasibility of obtaining ice motion from the 37 ghz data in addition to the 85 ghz data should allow an important record of ice motion to be established for the duration of the scanning multichannel microwave radiometer smmr special sensor microwave imager ssm i and future microwave sensors that is from 1978 into the next millenium

dipole anomaly in the winter arctic atmosphere and its association with sea ice motion abstract this paper identified an atmospheric circulation anomaly dipole structure anomaly in the arctic atmosphere and its relationship with winter sea ice motion based on the international arctic buoy program iabp dataset 1979 98 and datasets from the national centers for environmental prediction ncep and the national center for atmospheric research ncar for the period 1960 2002 the dipole anomaly corresponds to the second leading mode of eof of monthly mean sea level pressure slp north of 70 n during the winter season october march and accounts for 13 of the variance one of its two anomalous centers is stably occupied between the kara sea and laptev sea the other is situated from the canadian archipelago through greenland extending southeastward to the nordic seas the dipole anomaly differs from one described in other papers that can be attributed to an eastward shift of the center of action of the north atlantic oscillation the finding shows that the dipole anomaly also differs from

variability in arctic sea ice drift 1 analysis of arctic sea ice drift from 1979 1997 using a lagrangian perspective shows the complexities of ice drift response to variations in atmospheric conditions changes in ice dynamics influence the redistribution of ice and any transported material from different source areas sources of ice exported to fram strait shifted in about 1986 87 from dominance of the kara sea and severnaya zemlya to the new siberian islands east siberian sea and chukchi sea average travel time of multiyear ice within the perennial pack of the central arctic basin reached a maximum in 1987 88 and decreased by at least 1 year between 1984 1989 and 1990 1997 consistent with the observations of other investigators this decrease in ice travel time occurred following a major export or surge of old ice to fram strait from the beaufort gyre in 1988 through 1990 which decreased the fraction of thick ridged ice within the central basin

recent changes in arctic ocean sea ice motion associated with the north atlantic oscillation examination of a new ice motion dataset of the arctic ocean over a recent eighteen year period 1978 1996 reveals patterns of variability that can be linked directly to the north atlantic oscillation the intensity of the icelandic low one of its centers of action modulates the sea level pressure distribution over a broad region of the arctic ocean and the greenland iceland norwegian and barents seas over the winters of 1988 through 1995 the oscillation has remained in its positive phase contributing to coherent large scale changes in the intensity and character of ice transport in the arctic ocean the significant changes include the weakening of the beaufort gyre the increase in ice export through the fram strait the increase in ice import from the barents kara seas the enhanced eastward transport of sea ice from the laptev sea the weakening of the transpolar drift stream and the reduction in ice extent in the nordic seas all of these changes affect the regional and total sea ice mass balance of the arctic ocean

new observations of gulf of gdansk seismic events abstract seismicity of northern poland and of the adjacent area of the baltic sea has been considered weak if any historical reports mention just a couple of seismic events felt in the region while observatory seismology has recorded just a single swarm of seismic events in the gulf of gdansk the lack of information is caused not only by the weak seismicity of the area but also by the lack of seismological observatories in the region this situation is changing with the installment of a new seismological station suwalki in northeastern poland and two temporary very broadband stations of the geofon network one in northwestern poland and the other on oeland island sweden just months after the installations we are able to detect and locate numerous seismic events in the gulf of gdansk this does not necessarily mean that gulf of gdansk is a seismically active area as there are indications that the events may be caused by human activity one of the objectives of this paper was to obtain reliable location solutions with the best possible location error control since only a few stations are in operation in the nearby area and little is known about the velocity structure the bayesian inversion method has been used to locate event epicenters this method allows to minimize location errors caused by insufficient knowledge of local crust velocity structure and non gaussian statistics of measurement modeling errors the method being fully nonlinear does not introduce instabilities typical for inverse approaches which uses linearization

has arctic sea ice rapidly thinned abstract reports based on submarine sonar data have suggested arctic sea ice has thinned nearly by half in recent decades such rapid thinning is a concern for detection of global change and for arctic regional impacts including atmospheric time series ocean currents and river runoff into an ocean ice snow model show that the inferred rapid thinning was unlikely the problem stems from undersampling varying winds that readily redistribute arctic ice create a recurring pattern whereby ice shifts between the central arctic and peripheral regions especially in the canadian sector timing and tracks of the submarine surveys missed this dominant mode of variability although model derived overall thinning from the 1960s to the 1990s was less than hitherto supposed there is also indication of accelerated thinning during the early mid 1990s

mid to late holocene strengthening of the east greenland current linked to warm subsurface atlantic water the relatively fresh and cold east greenland current egc connects the arctic with the subpolar north atlantic ocean its strength and influence on the freshwater balance in the north atlantic affects both the subpolar gyre dynamics and deep convection in the labrador sea enhanced freshwater and sea ice expansion in the subpolar north atlantic is suggested to modify the northward heat transport within the north atlantic current high resolution palaeoceanographic reconstructions based on planktic and benthic foraminifera assemblage data from the central east greenland shelf foster bugt reveal distinct centennial to millennial scale oceanographic variability that relates to climatic changes during the mid to late holocene the last c 6 3 ka bp our data highlight intervals of cooling and freshening of the polar surface egc waters that accompany warming in the subsurface atlantic waters which are a combination of chilled atlantic intermediate water aiw from the arctic ocean and of the return atlantic current rac from the west spitsbergen current wsc mid holocene thermal optimum conditions prevailed until c 4 5 ka bp a thin absent surface polar water layer low drift sea ice occurrence and strong contribution of recirculating warm atlantic waters at the subsurface suggest a relatively weak egc during this period subsequently between 1 4 and 4 5 ka bp the water column became well stratified as the surface polar water layer thickened and cooled indicating a strong egc this egc strengthening parallelled enhanced subsurface chilled aiw contribution from the arctic ocean after c 4 5 ka bp which culminated from 1 4 to 2 3 ka bp this coincides with warming identified in earlier work of the north atlantic current the irminger current and the west greenland current we link the enhanced contribution of chilled atlantic water during this period to the time of the roman warm period the observed warming offshore east greenland centred at c 1 8 ka bp likely occurred in response to changes in the interactions of i a weakened subpolar gyre ii increased northward heat advection in the north atlantic current and iii a predominant positive north atlantic and arctic oscillation mode prevailing during the time of the roman warm period

an estimate of the mean field of arctic sea ice motion the observational basis for our understanding of sea ice motion comes from the trajectories of particular ice floes these ice floes may have been marked by a manned research station or more recently by an automatic data buoy since 1893 almost 100 station years of drift data have been recorded using the principles of optimal linear estimation these data are used to obtain a quantitative estimate of the field of mean motion

arctic sea ice circulation and drift speed decadal trends and ocean currents 1 we examine the basinwide trends in sea ice circulation and drift speed and highlight the changes between 1982 and 2009 in connection to regional winds multiyear sea ice coverage ice export and the thinning of the ice cover the polarity of the arctic oscillation ao is used as a backdrop for summarizing the variance and shifts in decadal drift patterns the 28 year circulation fields show a net strengthening of the beaufort gyre and the transpolar drift especially during the last decade the imprint of the arctic dipole anomaly on the mean summer circulation is evident 2001 2009 and enhances summer ice area export at the fram strait between 2001 and 2009 the large spatially averaged trends in drift speeds winter 23 6 decade summer 17 7 decade are not explained by the much smaller trends in wind speeds winter 1 46 decade summer 3 42 decade notably positive trends in drift speed are found in regions with reduced multiyear sea ice coverage over 90 of the arctic ocean has positive trends in drift speed and negative trends in multiyear sea ice coverage the increased responsiveness of ice drift to geostrophic wind is consistent with a thinner and weaker seasonal ice cover and suggests large scale changes in the air ice ocean momentum balance the retrieved mean ocean current field from decadal scale average ice motion captures a steady drift from siberia to the fram strait an inflow north of the bering strait and a westward drift along coastal alaska this mean current is comparable to geostrophic currents from satellite derived dynamic topography

distribution and biomass transport of ice amphipods in drifting sea ice around svalbard abstract diversity and distribution of ice amphipods were determined in drifting sea ice around svalbard during may august 2003 2005 sea ice concentrations were determined visually and by satellite recordings backward trajectories for ice drift based on satellite observations indicated that multi year ice in the area had originated 3 5 years earlier in the kara sea and western laptev sea whereas some of the younger ice 1 2 years may have originated in franz josef land area quantitative collections of ice amphipods were obtained by scuba divers from flat areas and ridges below sea ice the abundance and biomass were generally higher on ridges particularly for the large gammarus wilkitzkii whereas apherusa glacialis was more abundant on flat areas abundance and biomass varied among seasons may august and years with low values in may and higher values in july the high values were similar to abundance and biomass values from july 1996 redundancy analysis showed that 33 and 52 of the variability in respective species abundance and biomass could be explained by environmental variables with ice draft and snow cover being the most important the overall mean biomass for flats 0 30 0 05 g m 2 and ridges 1 64 0 46 g m 2 were used to calculate ice associated biomass transport based on revised mean annual ice flux through fram strait 662 000 km2 yr 1 and into the barents sea total 228 000 km2 yr 1 about 478 103 t ww 57 103 t c of ice amphipods are transported annually through fram strait and 194 103 t ww 23 103 t c into the northern barents sea implying that 71 of the amphipod biomass in drifting sea ice passes through fram strait and 29 enters the barents sea climate induced reduction in ice thickness and extent will likely decrease this southward biomass transport and thus the current carbon input of about 80 103 t c yr 1 into these marginal seas ice amphipod populations probably cannot be sustained if the summers become ice free in the arctic ocean and particularly reduction of multi year ice in the arctic ocean will affect long lived species such as gammarus wilkitzkii

holocene fluctuations in arctic sea ice cover dinocyst based reconstructions for the eastern chukchi sea cores from site hly0501 05 on the alaskan margin in the eastern chukchi sea were analyzed for their geochemical organic carbon 13corg corg n and caco3 and palynological dinocyst pollen and spores content to document oceanographic changes during the holocene the chronology of the cores was established from 210pb dating of near surface sediments and 14c dating of bivalve shells the sediments span the last 9000 years possibly more but with a gap between the base of the trigger core and top of the piston core sedimentation rates are very high 156 cm ka allowing analyses with a decadal to centennial resolution the data suggest a shift from a dominantly terrigenous to marine input from the early to late holocene dinocyst assemblages are characterized by relatively high concentrations 600 7200 cysts cm3 and high species diversity allowing the use of the modern analogue technique for the reconstruction of sea ice cover summer temperature and salinity results indicate a decrease in sea i

validation of the automatic nonlinear source inversion of the u s geological survey intensities of the whittier narrows 1987 earthquake in this work we validated the automatic inversion of the u s geological survey usgs intensity data set of the whittier narrows m l 5 9 1987 earthquake this validation of our new technique was obtained by comparing the source inversion results with the principal source parameters coming from instrumental measurements independent of our study to perform the inversion first we used a genetic algorithm ga with a population of 20 000 individuals i e sources second because the problem is bimodal we also used a niching genetic algorithm nga with four demes of 2 000 individuals this gave us almost the same results the site intensities were calculated by our kinematic kf function twelve source parameters were involved in the inversions the most sensitive of which are the epicentral coordinates and the fault plane solution two minimum variance models were determined by both the ga and the nga inversions 1 one east west trending dip slip source which is in agreement with that already known from instrumental measurements and 2 one almost coinciding with its auxiliary plane in the fault plane solution these findings almost coincide with those produced by the grid search inversion method but the ga nga inversion is much faster and does not need strong constraints this confirms that it is possible to get an approximate idea of the source of the studied earthquake also by automatically inverting the regional pattern of the usgs intensities this result encourages us to validate our inversion technique with more well documented earthquakes and to treat intensities of preinstrumental earthquakes which are the principal target of our work manuscript received 30 july 2003

variability of fram strait ice flux and north atlantic oscillation we estimate the winter sea ice export through the fram strait using ice motion from satellite passive microwave data sea ice motion october may is obtained by tracking the displacement of common features in sequential 85 and 37 ghz brightness temperature fields the average winter area flux over the 18 year record 1978 1996 is 670 000 km2 7 of the area of the arctic ocean the winter area flux ranges from a minimum of 450 000 km2 in 1984 to a maximum of 906 000 km2 in 1995 the daily monthly and interannual variabilities of the ice area flux are high there is an upward trend in the ice area flux over the 18 year record the average winter volume flux over the winters of october 1990 through may 1995 is 1745 km3 ranging from a low of 1375 km3 in the 1990 flux to a high of 2791 km3 in 1994 the sea level pressure gradient across the fram strait explains more than 80 of the variance in the ice flux over the 18 year record we use the coefficients from the regression of the time series of area flux versus pressure gradient across the fram strait and ice thickness data to estimate the summer area and volume flux the average 12 month area flux and volume flux are 919 000 km2 and 2366 km3 we find a significant correlation r 0 86 between the area flux and positive phases of the north atlantic oscillation nao index over the months of december march correlation between our 5 years of volume flux estimates and the nao index gives r 0 56 during the high nao years a more intense icelandic low increases the gradient in the sea level pressure by almost 1 mbar across the fram strait thus increasing the atmospheric forcing on ice transport correlation is reduced during the negative nao years because of decreased dominance of this large scale atmospheric pattern on the sea level pressure gradient across the fram strait

application of genetic algorithms to constrain near source velocity structure for the 1989 sichuan earthquakes abstract when a complex earthquake source process is studied using broadband body waveforms a common problem is finding an adequate layered velocity model for the near source structure tests with several existing crustal models are not sufficient or objective but an enumerative search scheme to search through a whole possible model space is not practical we study the feasibility of applying genetic algorithms ga to rapidly and effectively explore the model space to find an optimal model for the near source structure our data set consists of multi station broadband body waveforms of a simple event recorded at teleseismic distances the near source velocity structure is approximated by a layered model and the model parameters include the layer thicknesses and velocities as well as the source depth forming a multi parameter search space test results with synthetic data indicate that optimal models can be found using ga depending on the noise level when the noise level is low optimal models are well recovered with small uncertainty results with the data of the 1989 earthquakes of sichuan province china indicate that a three layer velocity model obtained with ga is in agreement with regional models and better explains the observed waveforms

constrained dix inversion we propose a stable inversion method to create geologically constrained instantaneous velocities from a set of sparse irregularly picked stacking or rms velocity functions in vertical time the method is primarily designed for building initial velocity models for curved ray time migration and initial macromodels for depth migration and tomography it is mainly applicable in regions containing compacted sediments in which the velocity gradually increases with depth and can be laterally varying inversion is done in four stages establishing a global initial background velocity trend applying an explicit unconstrained inversion performing a constrained least squares inversion and finally fine gridding the method can be applied to create a new velocity field create mode or to update an existing one update mode in the create mode initially the velocity trend is assumed an exponential asymptotically bounded function defined locally by three parameters at each lateral node and calculated from a reference datum surface velocity picks related to nonsediment rocks such as salt flanks or basalt boundaries require different trend functions and therefore are treated differently in the update mode the velocity trend is a background velocity field normally used for time or depth imaging the unconstrained inversion results in a piecewise constant residual instantaneous velocity with respect to the velocity trend and is mainly used for regularizing the input data the constrained inversion is performed individually for each rms velocity function in vertical time and the lateral and vertical continuities are controlled by the global velocity trend function a special damping technique suppresses vertical oscillations of the results finally smoothing and gridding interpolation are done for the resulting instantaneous velocity to generate a regular fine grid in space and time this method leads to a stable and geologically plausible velocity model even in cases of noisy input rms velocity or residual rms velocity data

locating regional seismic events with global optimization based on interval arithmetic more than 1000 seismic events in northern europe at distances of up to 400 km from the detecting network are located using an optimization method in which the global minimum of the traveltime function residuals is searched for using an interval arithmetic ia method epicentres are determined using p waves detected by the finnish national seismic network up to 15 stations were used in the analysis the ia results coincide with locations provided by the university of helsinki bulletins with a median location bias of 7 6 km a second data set of 59 explosions in the siilinjarvi mine in central finland was examined in detail because the locations of the explosions were known exactly in this case the median difference of ia locations was 3 8 km from the average location of mine explosions while all 59 events were located within 9 km of the true epicentres the corresponding median error of the university of helsinki locations was smaller 3 2 km but some helsinki locations were well over 10 km from the mine the convergence towards the global optimum using interval arithmetic was fast when compared with the conventional least squares approaches for epicentre determinations

climate response to basin specific changes in latitudinal temperature gradients and implications for sea ice variability we use experiments with the giss general circulation model to investigate how changes in latitudinal temperature gradients affect atmospheric circulation in different ocean basins with particular attention paid to the implications for high latitude sea ice the results are relevant to both estimated past climate changes current climate gradient changes e g el nino southern oscillation events and proposed future climate responses to greenhouse gases sea surface temperature gradients are increased decreased in all ocean basins and in the pacific and atlantic separately without changing sea ice or global average temperature additional experiments prescribe sea ice growth reduction with global cooling warming as expected increased gradients strengthen the subtropical jet stream and deepen the subpolar lows in each hemisphere but results in the northern and southern hemispheres differ in fundamental ways in the northern hemisphere increased storm intensities occur in the ocean basin with the increased gradient in the southern hemisphere the deeper storms occur in the ocean basin with the decreased gradient alterations of the gradient in one ocean basin change longitudinal temperature gradients an increased gradient in one basin from tropical heating results in subsidence in the tropics in the other basin mimicking the effect of a decreased gradient in that basin the subtropical jet is therefore strengthened over the basin with the increased gradient and decreased over the other ocean basin hence in many respects regional effects such as the strength of subpolar lows in an individual basin are amplified when the gradient changes are of opposite sign in the two ocean basins the southern hemisphere response occurs because gradient increases in one ocean basin by strengthening the subtropical jet shift storm tracks equatorward and away from the potential energy source associated with cold air advection from antarctica at the same time with a weaker subtropical jet in the other basin storms move poleward and strengthen this latter effect may explain observed sea ice variations that are out of phase in the atlantic and pacific ocean basins in the southern hemisphere referred to as the antarctic dipole as well as upper ocean variability in the weddell gyre gradient changes produce little effect on sea level pressure in the arctic unless sea ice is changed with arctic sea ice reductions the sea ice response acts as a positive feedback inducing cyclonic circulation changes that would enhance its removal as may be occurring due to the current high phase of the north atlantic oscillation

recent arctic sea ice variability connections to the arctic oscillation and the enso 1 trends in the satellite derived arctic sea ice concentrations 1978 2002 show pronounced decreases in the barents kara seas between the chukchi and beaufort seas the central sea of okhotsk and a portion of the hudson baffin bay by 2 8 per decade exceeding the 95 confidence level qualitatively speaking positive phases of the arctic oscillation ao and el nino southern oscillation enso produce similar ice changes in the western arctic but opposite ice changes in the eastern arctic the manner in which the ice changes are related to the ao and enso are demonstrated over the last 24 years the magnitude of the ice changes associated with the positive ao trend and the negative enso trend is much smaller than the regional ice trends thus more local or less understood large scale processes should be investigated for explanations

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

evidence for rapid thinning of sea ice in the western arctic ocean at the end of the 1980s examination of springtime ice drafts obtained from submarine profiles in a narrow band of the western arctic ocean from offshore alaska to 89 n indicates that the mean ice draft decreased 1 5 m between the mid 1980s and early 1990s no similar trend was evident in ice drafts near the north pole the 1980s drafts were composed largely of ice exceeding 3 5 m while the early 1990s drafts contained more ice in thinner categories the differences in drafts between the two periods appear to be related largely to ice dynamics effects associated with the presence and strength of the beaufort gyre which weakened considerably in the early 1990s

retreat of the cold halocline layer in the arctic ocean we present a comparison of arctic ocean hydrographic data sets from the 1990s with a focus on changes in the upper few hundred meters of the eurasian basin the most recent observations discussed here were collected during the spring 1995 scientific ice expedition scicex 95 the second in a series of scientific cruises to the arctic ocean aboard u s navy nuclear submarines although the 1990s have seen an abundance of synoptic cruises to the arctic this was the only one to take place in winter spring conditions other data considered here were collected during the first scicex cruise in summer 1993 scicex 93 and during an icebreaker cruise to the eurasian basin in summer 1991 oden 91 a new russian american winter climatology is also used as a reference these comparisons reveal that the eurasian basin cold halocline layer has retreated during the 1990s to cover significantly less area than in previous years specifically we find a retreat from the amundsen basin back into the makarov basin the latter is the only region with a true cold halocline layer during scicex 95 changes are also seen in other halocline types and in the atlantic water layer heat content and depth since the cold halocline layer insulates the surface layer and thus the overlying sea ice from the heat contained in the atlantic water layer this should have profound effects on the surface energy and mass balance of sea ice in this region using a simple mixing model we calculate maximum ice ocean heat fluxes of 1 3 w m 2 in the eurasian basin where during scicex 95 the surface layer lay in direct contact with the underlying atlantic water layer the overall cause of water mass changes in the 1990s might have been a shift in the atmospheric wind forcing and resulting sea ice motion during the late 1980s which we speculate influenced the location where fresh shelf waters flow into the deeper basins of the arctic ocean finally we discuss two different mechanisms that have been proposed for cold halocline water formation and we propose a compromise that best fits these data

an attempt to integrate reflection seismics and balanced profiles different techniques in geophysics and geology are used to derive the structure of the subsurface they are based on different data sets i e seismic and geological data and a combination of these techniques should produce better earth models the case study presented in this paper is based on data of the german continental reflection program dekorp collected in the munsterland basin and the rhenish massif located at the northern border of the rhenohercynian fold and thrust belt of the mid european variscides in this study we present an attempt to integrate balanced profiles i e structural geology and reflection seismics the integration is performed by synthetically modelling seismic waves according to the acquisition of the field data where the velocity model is based on the balanced profile the synthetic data are compared with the field observations differences between observed data and field data are either caused by velocity errors in the model or by errors in the balanced profile criteria are developed to interpret these differences in order to improve the joint model of geologists and geophysicists the case study presented in this paper shows that the combination of balanced profiles and reflection seismics may lead to shortcuts in the determination of seismic velocities of the subsurface these shortcuts can reduce processing times and processing costs of reflection seismic data

holocene cooling culminates in sea ice oscillations in fram strait a reconstruction of holocene sea ice conditions in the fram strait provides insight into the palaeoenvironmental and palaeoceanographic development of this climate sensitive area during the past 8500 years bp organic geochemical analyses of sediment cores from eastern and western fram strait enable the identification of variations in the ice coverage that can be linked to changes in the oceanic and atmospheric circulation system by means of the sea ice proxy ip25 phytoplankton derived biomarkers and ice rafted detritus ird increasing sea ice occurrences are traced along the western continental margin of spitsbergen throughout the holocene which supports previous palaeoenvironmental reconstructions that document a general cooling a further significant ice advance during the neoglacial is accompanied by distinct sea ice fluctuations which point to short term perturbations in either the atlantic water advection or arctic water outflow at this site at the continental shelf of east greenland the general holocene cooling however seems to be less pronounced and sea ice conditions remained rather stable here a major neoglacial increase in sea ice coverage did not occur before 1000 years bp phytoplankton ip25 indices pip25 index are used for more explicit sea ice estimates and display a mid holocene shift from a minor sea ice coverage to stable ice margin conditions in eastern fram strait while the inner east greenland shelf experienced less severe to marginal sea ice occurrences throughout the entire holocene highlights biomarker and ird data give insight into holocene sea ice conditions in fram strait we find increasing sea ice coverage off west spitsbergen throughout the holocene oceanic atmospheric variability caused neoglacial sea ice fluctuations ice conditions along east greenland shelf remain rather stable until 1000 years bp

reexamination of fram strait sea ice export and its role in recently accelerated arctic sea ice retreat using the high resolution global ocean sea ice model mitgcm ecco2 variability and changes of arctic sea ice export through fram strait during 1979 2012 are examined the simulated annual mean ice export is about 3216 km3 year which is 12 7 of the 34 year averaged arctic sea ice volume with the maximum and minimum occurring in 1994 95 and 1984 1985 respectively winter october march ice export is much more than summer april september and exhibits a greater interannual variation this study suggests that a significant regime shift of fram strait ice export from high to low value occurs around the mid 1990s further analysis shows that the regime shift of the atmospheric circulation from conventional arctic oscillation north atlantic oscillation ao nao to the dipole structure arctic rapid change pattern arp plays an important role on the regime shift of ice export this shift of atmospheric circulation pattern dominates the variability of ice motion and changes the main source region of ice outflow combined with the decreased ice thickness and the less ice outflow related to the weakened northerly northeasterly winds near the strait sea ice export decreased eventually generating a regime shift around the mid 1990s the distinct impact of changes in fram strait ice export on the arctic sea ice inside the basin before and after mid 1990s indicates that the recent continuing loss of arctic sea ice was mainly induced by the accelerated ice melting in the arctic ocean rather than the ice outflow through fram strait

synchronous variability in the southern hemisphere atmosphere sea ice and ocean resulting from the annular mode zonally symmetric fluctuations of the midlatitude westerly winds characterize the primary mode of atmospheric variability in the southern hemisphere during all seasons this is true not only in observations but also in an unforced 15 000 yr integration of a coarse resolution r15 coupled ocean atmosphere model here it is documented how this mode of atmospheric variability known as the southern annular mode sam generates ocean circulation and sea ice variations in the model integration on interannual to centennial timescales that are tightly in phase with the sam the positive phase of the sam is associated with an intensification of the surface westerlies over the circumpolar ocean around 608s and a weakening of the surface westerlies farther north this induces ekman drift to the north at all longitudes of the circumpolar ocean and ekman drift to the south at around 308s through mass continuity the ekman drift generates anomalous upwelling along the margins of the antarctic continent and downwelling around 458s the anomalous flow diverging from the antarctic continent also increases the vertical tilt of the isopycnals in the southern ocean so that a more intense circumpolar current is also closely associated with positive sam in addition the anomalous divergent flow advects sea ice farther north resulting in an increase in sea ice coverage finally positive sam drives increases in poleward heat transport at about 308s while decreases occur in the circumpolar region ocean and sea ice anomalies of the opposite sign occur when the sam is negative the ocean and sea ice fluctuations associated with the sam constitute a significant fraction of simulated ocean variability poleward of 308s year round the robustness of the mechanisms relating the sam to oceanic variability suggests that the sam is likely an important source of large scale variability in the real southern hemisphere ocean

semi automated feature tracking of east antarctic sea ice from envisat asar imagery while feature tracking of sea ice using cross correlation methods on pairs of satellite synthetic aperture radar sar images has been extensively carried out in the arctic this is not the case in the antarctic this is due to the dynamic nature of antarctic pack ice its microwave signature the tendency for sar swath paths to be poorly aligned with the often narrow sea ice zone around the continent and inadequate satellite sampling a semi automated system known as ipads imcorr imagecorrelation processing analysis and display system has been developed to map fast ice and pack ice in antarctica using multiple pairs of sar images the software processing pipeline uses overlapping image pairs which are geocoded and roughly registered using only data contained in the image headers next fast ice maps are rapidly generated using zero motion features located within ocean regions this also provides precise image registration finally the same image pairs are re examined for pack ice motion in a slower off line batch process the pack and fast ice are identified using a cluster based search method which compares both location and motion information each image pair generates a netcdf file which adds to a growing database of antarctic sea ice motion and ice roughness five image pair examples are presented to illustrate the methods used as well as their strengths and limitations substantial pack ice motion can often be detected in the marginal ice zone on sar images only a few days apart

arctic sea ice seasonal prediction by a linear markov model abstracta linear markov model has been developed to predict sea ice concentration sic in the pan arctic region at intraseasonal to seasonal time scales which represents an original effort to use a reduced dimension statistical model in forecasting arctic sea ice year round the model was built to capture covariabilities in the atmosphere ocean sea ice system defined by sic sea surface temperature and surface air temperature multivariate empirical orthogonal functions of these variables served as building blocks of the model a series of model experiments were carried out to determine the model s dimension the predictive skill of the model was evaluated by anomaly correlation and root mean square errors in a cross validated fashion on average the model is superior to the predictions by anomaly persistence damped anomaly persistence and climatology the model shows good skill in predicting sic anomalies within the arctic basin during summer and fall long term trends partially contribute to the

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

a probabilistic seismic model for the european arctic 1 the development of three dimensional seismic models for the crust and upper mantle has traditionally focused on finding one model that provides the best fit to the data while observing some regularization constraints in contrast to this the inversion employed here fits the data in a probabilistic sense and thus provides a quantitative measure of model uncertainty our probabilistic model is based on two sources of information 1 prior information which is independent from the data and 2 different geophysical data sets including thickness constraints velocity profiles gravity data surface wave group velocities and regional body wave traveltimes we use a markov chain monte carlo mcmc algorithm to sample models from the prior distribution the set of plausible models and test them against the data to generate the posterior distribution the ensemble of models that fit the data with assigned uncertainties while being computationally more expensive such a probabilistic inversion provides a more complete picture of solution space and allows us to combine various data sets the complex geology of the european arctic encompassing oceanic crust continental shelf regions rift basins and old cratonic crust as well as the nonuniform coverage of the region by data with varying degrees of uncertainty makes it a challenging setting for any imaging technique and therefore an ideal environment for demonstrating the practical advantages of a probabilistic approach maps of depth to basement and depth to moho derived from the posterior distribution are in good agreement with previously published maps and interpretations of the regional tectonic setting the predicted uncertainties which are as important as the absolute values correlate well with the variations in data coverage and quality in the region a practical advantage of our probabilistic model is that it can provide estimates for the uncertainties of observables due to model uncertainties we will demonstrate how this can be used for the formulation of earthquake location algorithms that take model uncertainties into account when estimating location uncertainties

physical and biological characteristics of the pelagic system across fram strait to kongsfjorden abstract the fram strait is very important with regard to heat and mass exchange in the arctic ocean and the large quantities of heat carried north by the west spitsbergen current wsc influence the climate in the arctic region as a whole a large volume of water and ice is transported through fram strait with net water transport of 1 7 3 2 sv southward in the east greenland current and a volume ice flux in the range of 0 06 0 11 sv the mean annual ice flux is about 866 000 km2 yr 1 the kongsfjorden krossfjorden fjord system on the coast of spitsbergen or at the eastern extreme of fram strait is mainly affected by the northbound transport of water in the wsc mixing processes on the shelf result in transformed atlantic water in the fjords and the advection of atlantic water also carries boreal fauna into the fjords the phytoplankton production is about 80 g c m 2 yr 1 in fram strait and has been estimated both below and above this for kongsfjorden the zooplankton fauna is diverse but dominated in terms of biomass by calanoid copepods particularly calanus glacialis and c finmarchicus other important copepods include c hyperboreus metridia longa and the smaller more numerous pseudocalanus p minutus and p acuspes microcalanus m pusillus and m pygmaeus and oithona similis the most important species of other taxa appear to be the amphipods themisto libellula and t abyssorum the euphausiids thysanoessa inermis and t longicaudata and the chaetognaths sagitta elegans and eukrohnia hamata a comparison between the open ocean of fram strait and the restricted fjord system of kongsfjorden krossfjorden can be made within limitations the same species tend to dominate but the fram strait zooplankton fauna differs by the presence of meso and bathypelagic copepods the seasonal and inter annual variation in zooplankton is described for kongsfjorden based on the record during july 1996 2002 the ice macrofauna is much less diverse consisting of a handful of amphipod species and the polar cod the ice associated biomass transport of ice amphipods was calculated based on the ice area transport at about 3 55 106 ton wet weight per year or about 4 2 105 t c yr 1 this represents a large energy input to the greenland sea but also a drain on the core population residing in the multi year pack ice myi in the arctic ocean a continuous habitat loss of myi due to climate warming will likely reduce dramatically the sympagic food source the pelagic and sympagic food web structures were revealed by stable isotopes the carbon sources of particulate organic matter pom being ice pom and pelagic pom revealed different isotopic signals in the organisms of the food web and also provided information about the sympagic pelagic and pelagic benthic couplings the marine food web and energy pathways were further determined by fatty acid trophic markers which to a large extent supported the stable isotope picture of the marine food web although some discrepancies were noted particularly with regard to predator prey relationships of ctenophores and pteropods

locating oceanic earthquakes the influence of regional models and location criteria summary earthquakes in oceanic areas are normally located using traveltime tables which are representative of continental paths since most seismic stations lie on continents it should therefore be possible to improve such locations by employing a set of traveltimes more appropriate to paths from oceanic events to continental stations a comparison has therefore been made between locations for a number of oceanic events using the recent iasp91 global traveltimes and the times for the pac91 model derived from observations of events in the pacific although there were often significant differences in the location estimates for the two models these were often no larger than the shifts induced by changing the misfit criterion used for determining the location for events in purely oceanic regions such as tonga and the marianas with little nearby continent the results from the pac91 model either provided a significantly better fit to the data or produced depth estimates in close accord with independent constraints e g centroid moment tensor locations in these cases the use of a specific set of oceanic traveltimes can be recommended however for marginal zones and island arcs the situation is less clear and it is probably best to employ the global traveltime set with the use of additional phases to improve depth estimates

errors in hypocenter location picking model and magnitude dependence abstract the location procedures of seismic events are influenced by two major classes of errors the error in picking individual seismic phases and modeling error due to the departure of the real earth from the reference model used in the location both classes of error influence the estimate of location and it is difficult to separate them the role of picking errors can be assessed by a nonlinear analysis using a monte carlo procedure arrivals times are perturbed with random numbers drawn from a normal distribution and the event is relocated using these perturbed arrival times by repeating the procedure many times a cluster of locations is obtained which can be used to investigate the effects of picking errors on the hypocenter this analysis is insensitive to velocity model errors as these are fixed for a given combination of stations and phases some care must be exercised when analysing multidimensional distributions in two dimensional slices because of a projection effect the modeling error due to the influence of lateral heterogeneity in the earth is examined by comparing the locations of the same event using different combinations of phases and network geometries which reinforces the need to use arrivals other than p for accurate depth resolution the sensitivity of p arrivals to changes in depth are swamped by model errors and inclusion of depth sensitive phases such as pp is highly recommended the effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only p arrivals are used consequently the monte carlo analysis which is primarily aimed at picking errors only is most appropriate when multiple phases have been used to more accurately constrain the hypocenter especially for the depth component altering the type of phase data used in the location plays a similar role in changing the network geometry in that both are mechanisms that influence the nature of the constraint on the hypocenter by relocating events with network geometries corresponding to the different magnitudes it is found that the location of the event can be affected significantly by the magnitude and when using robust statistics to describe earthquake residuals the mislocation can occur in a systematic manner the effect is marked in regions with significant lateral variations in seismic velocities for example low magnitude events in the flores sea are found to be dragged toward australia as a result of the fast paths to australian stations relative to the iasp 91 reference velocity model

climatic analysis of sea ice variability in the canadian arctic from operational charts 1980 2004 we present a complete climatic analysis of sea ice conditions in the canadian arctic based on digitized operational charts from the canadian ice service for the period 1980 2004 the seasonal climatology spatial variance and linear trends in total ice concentrations ct were calculated the maximum reduction rate in ct was found in the beaufort sea 24 decade 1 and in the davis strait labrador sea region 18 decade 1 during spring and summer empirical orthogonal function eof analysis performed on monthly ct deviations yielded four significant eof modes explaining 32 of the total variance the spatial pattern temporal behaviour and seasonality of these four eof modes are discussed and correlated with fields of sea level pressure surface winds surface air temperature and sea surface temperature monthly anomalies these results point to the dominant influence of the north atlantic oscillation on ct decadal anomalies during the cold season while climate variability in the pacific influences ct variations in the beaufort sea region during spring summer

a seesaw structure in slp anomalies between the beaufort sea and the barents sea 1 this paper reveals a new mode of winter october march monthly mean atmospheric intraseasonal variability characterized by a seesaw structure in sea level pressure slp anomalies between the barents sea and the beaufort sea with an equivalent barotropic structure the beaufort high varies out of phase with the siberian high compared to the arctic oscillation ao and the dipole anomaly the effect of this mode on fram strait sea ice flux is the most important so far although it accounts for only 9 1 of the variance the beaufort high itself is not crucial for sea ice flux through fram strait

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

anomalies of sea ice transports in the arctic in the arctic sea ice motion and ice export are prominent processes and good indicators of arctic climate system variability sea ice drift is simulated using a dynamic hermodynamic sea ice model validated with retrievals from ssm i satellite observations both datasets agree well in reproducing the main arctic drift patterns in order to study inner arctic transports and ice volume anomalies the arctic ocean is split by ten boundaries separating the central arctic ocean from the nordic and marginal seas it is found that the already dominant sea ice export through fram strait has increased at the expense of export through the barents sea in the most recent years investigated furthermore ice export from the eurasian marginal seas increased slightly followed by greater ice production during the winter in contrast to this the sea ice volume moved within the beaufort gyre distinctly decreased in total the ice volume in the central arctic decreased during the 40 year period covered by this study the changes in the ice volume correspond to two wind driven circulation regimes of the arctic sea ice motion which recur approximately every 11 years for the volume anomalies we derived a correlation of 0 59 to the north atlantic oscillation nao index lagging the naoby 2 years

arctic salinity anomalies and their link to the north atlantic during a positive phase of the arctic oscillation abstract many of the changes observed during the last two decades in the arctic ocean and adjacent seas have been linked to the concomitant abrupt decrease of the sea level pressure in the central arctic at the end of the 1980s the decrease was associated with a shift of the arctic oscillation ao to a positive phase which persisted throughout the mid 1990s the arctic salinity distribution is expected to respond to these dramatic changes via modifications in the ocean circulation and in the fresh water storage and transport by sea ice the present study investigates these different contributions in the context of idealized ice ocean experiments forced by atmospheric surface wind stress or temperature anomalies representative of a positive ao index wind stress anomalies representative of a positive ao index generate a decrease of the fresh water content of the upper arctic ocean which is mainly concentrated in the eastern arctic with almost no compensation from the western arctic sea ice contributes to about two third of this salinification another third being provided by an increased supply of salt by the atlantic inflow and increased fresh water export through the canadian archipelago and fram strait the signature of a saltier atlantic current in the norwegian sea is not found further north in both the barents sea and the fram strait branches of the atlantic inflow where instead a widespread freshening is observed the latter is the result of import of fresh anomalies from the subpolar north atlantic through the iceland scotland passage and enhanced advection of low salinity waters via the east icelandic current the volume of ice exported through fram strait increases by 20 primarily due to thicker ice advected into the strait from the northern greenland sector the increase of ice drift velocities having comparatively less influence the export anomaly is comparable to those observed during events of great salinity anomalies and induces substantial freshening in the greenland sea which in turn contributes to increasing the fresh water export to the north atlantic via denmark strait with a fresh water export anomaly of 7 msv the latter is the main fresh water supplier to the subpolar north atlantic the canadian archipelago contributing to 4 4 msv the removal of fresh water by sea ice under a positive winter ao index mainly occurs through enhanced thin ice growth in the eastern arctic winter sat anomalies have little impact on the thermodynamic sea ice response which is rather dictated by wind driven ice deformation changes the global sea ice mass balance of the western arctic indicates almost no net sea ice melt due to competing seasonal thermodynamic processes the surface freshening and likely enhanced sea ice melt observed in the western arctic during the 1990s should therefore be attributed to extra winter atmospheric effects such as the noticeable recent spring summer warming in the canada alaska sector or to other modes of atmospheric circulations than the ao especially in relation to the north pacific variability

a holocene record of changing arctic ocean ice drift analogous to the effects of the arctic oscillation 1 the arctic oscillation ao controls the configuration of the transpolar drift tpd if thicker ice from the beaufort gyre were exported the volume of fresh water sea ice in the nordic seas would significantly increase decreasing the formation of north atlantic deep water this would cool large parts of the northern hemisphere and affect global climate therefore in order to understand how the global climate system functions it is imperative to know how the tpd changed over the last millennium or more the provenance of grains in a sediment core located near the confluence of the tpd and the beaufort gyre provides a direct proxy for changes in seaice drift owing to these circulation systems the core has more than 200 cm of holocene sediment with intervals dominated by grains from the russian shelves alternating with intervals of abundant grains from north american sources grains matched to russian shelves indicate that the tpd was shifted toward north america similar to what occurs during a more positive phase of the ao this condition alternated with intervals where few grains matched to russian sources presumably because the tpd was restricted to the russian half of the arctic far from the core site during the last 1300 years increased influx of russian grains occurred approximately every 50 150 years this fluctuation might represent the long term oscillation of the ao which modulates the same tpd shift today

decadal variability in the arctic ocean shown in hydrochemical data 1 the recent distribution of russian hydrochemical data collected from the arctic basin provides useful information on ocean interior variabilities among various chemical components silicate is the most valuable for showing the boundary between the silicate rich pacific water and the opposite atlantic water here it is assumed that the silicate distribution receives minor influence from seasonal biological productivity and siberian rivers outflow it shows a clear maximum around 100 m depth in the canada basin the vertical gradient below 100 m provides information on the vertical motion of the upper boundary of the atlantic water at a decadal time scale i e the boundary shifts upward downward as realized by the silicate reduction increase at a fixed depth responding to a more intense weaker polar vortex or the positive negative phase of the arctic oscillation this valuable hydrochemical data set has provided a tool for verifying arctic ocean models with decadal responses to atmospheric variabilities

outflow of arctic ocean sea ice into the greenland and barents seas 1979 2007 abstract twenty nine years of arctic sea ice outflow into the greenland and barents seas are summarized outflow is computed at three passages fram strait between svalbard and franz josef land s fjl and between franz josef land and severnaya zemlya fjl sz ice drift at the flux gates has been reprocessed using a consistent and updated time series of passive microwave brightness temperature and ice concentration ic fields over the record the mean annual area outflow at the fram strait is 706 113 103 km2 it was highest in 1994 95 1002 103 km2 when the north atlantic oscillation nao index was near its 29 yr peak the strength of the transpolar drift stream tds was high during the late 1980s through the mid 1990s there is no statistically significant trend in the fram strait area flux even though there is a positive trend in the gradient of cross strait sea level pressure the outflow has not increased because of a negative trend in ic seasonally the area outflow during recent sum

holocene ice rafting and sediment transport from the glaciated margin of east greenland 67 70 n to the n iceland shelves detecting and modelling changing sediment sources abstract we examine variations in the ice rafted sources for sediments in the iceland east greenland offshore marine archives by utilizing a sediment unmixing model and link the results to a coupled iceberg ocean model surface samples from around iceland and along the e ne greenland shelf are used to define potential sediment sources and these are examined within the context of the down core variations in mineralogy in the