the initiation of subduction by crustal extension at a continental margin summary we investigate how subduction may be triggered by continental crust extension at a continental margin the large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement subduction requires the oceanic plate to get submerged in mantle so that negative buoyancy forces may take over and drive further descent this is promoted by two mechanisms loading by continental crust bends the oceanic plate downwards extension in the continental domain induces crustal thinning which acts to raise mantle above the oceanic plate in this model the width of the continental region undergoing extension is an important control parameter the main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading three governing dimensionless parameters are identified one involves the poorly constrained oceanic plate buoyancy we find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant enabling metamorphic reactions and density increase in the oceanic crust another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong the third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness such that a weak plate and a thick crust do not favour subduction thus the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere it is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed

prediction of decadal slope changes in canada by glacial isostatic adjustment modellingthis article is one of a series of papers published in this special issue on the theme geodesy in canada glacial isostatic adjustment gia is the dominant process to cause vertical motion of the land surface a gia model is presented herein that can be used to predict slope changes at given locations in north america where gia is the primary cause for vertical motion uncertainty in pleistocene ice cover and viscosity in the earth s mantle prevent one from picking a single gia model from the literature to predict uplift rates in the region therefore in this study a range of mantle viscosity values as well as two different ice loading histories are used in a forward model of the gia process the combination of viscosities and ice model that gives the best fit to recently available continuous and episodic gps observations is assumed to provide the best prediction of slope changes in north america this model can be used to quantify gia induced vertical deformation in local geomorphologic studies we show that the predicted differential uplift rate in the nelson river with respect to lake winnipe

interannual variability of summer biochemical enhancement in relation to mesoscale eddies at the shelf break in the vicinity of the pribilof islands bering sea abstract this study examined variability of the eddy field and primary production over the bering sea s eastern shelf break area during summer using a satellite multi sensor data set the eddy field and on shelf nutrient flux below 50 m water depth were also investigated using a numerical model the satellite altimeter data analysis and numerical experiments indicated that the eddy field was induced by perturbations of the bering slope current system and or instabilities in the bering sea basin via flow through the aleutian passes the distribution of high primary production roughly coincided with the high variability area of the eddy field numerical experiments showed a 70 increase in net on shelf flux and 54 increase in net on shelf transport by the generation propagation of eddies along the shelf break at pribilof canyon towed ctd fluorometer measurements during the summer of 2003 revealed a stable water column and high fluorescence 10 g l 1 in the area between the 200 and 1000 m isobaths at a time when there was a low level of on shelf flux regardless of the eddy field conversely the eddy induced on shelf flux and stable water column can enhance primary production from spring to summer at the shelf break moreover the eddy induced on shelf nutrient flux probably contributes to the primary productivity at the western shelf of the pribilof islands where the bering sea ecosystem is very dynamic this eddy related shelf break process can be applied to not only the bering sea shelf break but also all shelf break areas in which propagating eddies are found

wave turbulence interactions in the upper ocean part ii statistical characteristics of wave and turbulent components of the random velocity field in the marine surface layer abstract we present the results of an analysis of field data collected by donelan who used a miniature drag sphere to measure velocities beneath wind waves on lake ontario linear statistical techniques are used to separate the velocity into wave and turbulent parts while we mostly aim at demonstrating the effects of surface wind waves on the statistical characteristics of the turbulent field in the upper mixed layer we also interpret several features of the data on the hags of recent theoretical results one of the most intriguing features of the turbulent velocity spectra so obtained is a large peak near the dominant wave frequency we review various possible explanation for this behavior although we prefer a model in which the turbulence is assumed frozen on the timescale of the waves this model requires no new dynamics and gives explicit formulae relating the dissipation rate to the magnitude of the spectral densities for high and low frequencies on this basis we have determined a dissipation leng

a major circulation anomaly in the western bering sea the alaskan stream ceased to flow northward into the western bering sea starting in summer 1990 this unexpected lack of inflow caused by a southward shift in the path of the alaskan stream persisted through at least fall 1991 if the anomalous flow continues substantial changes in the physical environment will occur possibly leading to impacts on the rich fisheries stocks in the region

an inverse modeling study of circulation in the eastern bering sea during 2007 2010 a two way nested 4d variational data assimilation system is implemented in the eastern bering sea ebs to investigate changes in circulation and thermodynamic state for a 3 8 year period assimilated observations include data from 19 moorings deployed on the shelf and in the bering strait 1705 hydrographic stations occupied during eight surveys and remotely sensed sea surface temperature and sea surface height ssh data validation of the presented 4dvar reanalysis against the output of two sequential data assimilative systems the bering ecosystem study ice ocean modeling and assimilation system bestmas and the arctic cap nowcast forecast system acnfs has shown that the product is more consistent with the observed transports in the bering strait and in the ebs interior both in terms of their magnitude and time variability analysis of the data optimized solution quantifies a sequence of wind forced events that resulted in the anomalous heat and freshwater transports through the bering strait including a 28 day long flow reversal that occurred in november of 2009 and carried siberian coastal current water down to the gulf of anadyr lagrangian study of the arctic bound pacific waters indicates the extreme importance of the cross shelf exchange along the path of the bering slope current and quantifies the spectrum of residence times for the waters entering ebs through unimak pass and through aleutian passages residence times in the ebs cold pool are diagnosed to be 2 3 times longer than those in the surrounding waters this article is protected by copyright all rights reserved

post processing removal of correlated errors in grace data 1 gravity fields produced by the gravity recovery and climate experiment grace satellite mission require smoothing to reduce the effects of errors present in short wavelength components as the smoothing radius decreases these errors manifest themselves in maps of surface mass variability as long linear features generally oriented north to south i e stripes the presence of stripes implies correlations in the gravity field coefficients here we examine the spectral signature of these correlated errors and present a method to remove them finally we apply the filter to a model of surface mass variability to show that the filter has relatively little degradation of the underlying geophysical signals we seek to recover

a simplified ice ocean coupled model for the antarctic ice melt season abstract in the antarctic ocean sea ice melts mostly by warming of the ocean mixed layer through heat input mainly solar radiation in open water areas a simplified ice upper ocean coupled model is proposed in which sea ice melts only by the ocean heat supplied from the air the model shows that the relationship between ice concentration i e fraction c and mixed layer temperature t converges asymptotically with time c t relationship which agrees with observed c t plots during summer in the sector 25 45 e this relationship can be used for estimating the bulk heat transfer coefficient between ice and ocean by fitting to observations and a value of 1 2 10 4 m s 1 is obtained the model shows that the ratio of the heat used for melting to the heat input through open water is inclined to be determined as a function of ice concentration for typical conditions in the antarctic ice melt season the ratio ranges mostly between 0 7 and 0 9 when the model is extended to two dimensions in the meri

modeling effects of tidal and wave mixing on circulation and thermohaline structures in the bering sea process studies 1 ocean circulation and the tidal current in the bering sea are simulated simultaneously using a coupled ice ocean model ciom with tidal and parameterized wave mixing to investigate several important physical processes the simulated circulation pattern in the deep basin is relatively stable cyclonic and has little seasonal change the bering slope current is estimated at 5 sv and the kamchatka current at 20 sv the modeled volume transports through the aleutian passes compared reasonably well with observations it is confirmed that subtidal clockwise circulation around st george and st paul islands are driven by tidal rectification the simulation results show that wind wave mixing and tidal stirring are the main factors controlling the formation of the upper and the bottom mixed layers respectively the mechanism of thermocline and the cold pool summer minimum temperature water formation in the middle shelf are investigated in depth the ciom reproduces the cold winter convective water in the middle shelf that forms the bottom cold pool which persists throughout the summer until sea surface cooling and strong wind mixing in fall leading to an unstable vertical water column and eventually to vertically well mixed water in the winter sensitivity experiments show that tidal and wind wave mixing are two important factors in accurately estimating the volume of the cold pool which is the ideal marine habitat for cold water species

a modeling study of coastal circulation and landfast ice in the nearshore beaufort and chukchi seas using ciom this study investigates sea ice and ocean circulation using a 3 d 3 8 km ciom coupled ice ocean model under daily atmospheric forcing for the period 1990 2008 the ciom was validated using both in situ observations and satellite measurements the ciom successfully reproduces some observed dynamical processes in the region including the bering inflow originated coastal current that splits into three branches alaska coastal water acw central channel branch and herald valley branch in addition the beaufort slope current bsc the beaufort gyre the east siberian current esc mesoscale eddies and seasonal landfast ice are well simulated the ciom also reproduces reasonable interannual variability in sea ice such as landfast ice and anomalous open water less sea ice during the positive dipole anomaly da years vice versa during the negative da years sensitivity experiments were conducted with regard to the impacts of the bering strait inflow heat transport onshore wind stress and sea ice advection on sea ice change in particular on the landfast ice it is found that coastal landfast ice is controlled by the following processes wind forcing bering strait inflow and sea ice dynamics

an ice ocean coupled model an ice model an ocean model and a method of coupling the models are described the ice model is a synthesis with variations and extensions of previous modeling ideas ice thickness concentration velocity and internal energy are prognostic variables the ice thermodynamics are represented by temperatures at the snow surface ice surface the interior and the bottom surface melting and freezing rates are calculated at the ice atmosphere ice ocean and atmosphere ocean interfaces a prescribed portion of summer meltwater can be stored on the surface and refrozen in the fall the ocean model includes a second moment turbulence closure submodel and enables one to solve for oceanic heat flux the interfacial stress and subsurface properties in this paper the model is applied to one dimensional simulations but the equations are cited in a form for implementation by two and three dimensional models in a companion paper kantha and mellor this issue the model is used for two dimensional vertical plane simulations in the bering sea several one dimensional sensitivity studies are performed in the case where the ice model is decoupled from the ocean here the oceanic heat flux and sea surface temperature are prescribed constants the studies reveal the role and sensitivity of surface trapped meltwater ice concentration and ice divergence with the coupled ice ocean model the seasonally varying oceanic heat flux and mixed layer properties are determined by the model some comparisons with observations in the central arctic ocean are possible the role of the molecular sublayer immediately adjacent to the ice is examined frazi ice production is related to the large disparity in the molecular diffusivities for temperature and salinity the mixed layer model contains empirical constants which are known from turbulence data the molecular sublayer parameterization requires one empirical parameter b which is uncertain but from this study is assuredly greater than zero the value implicit in previous models the ice model requires the empirical parameters ci f and ci u to quantitatively account for freezing or melting processes in open leads their values are also uncertain but we present reasoning and sensitivity studies to suggest specific values finally an empirical parameter g is introduced it is the ratio of the value of the ice thickness used to represent average ice volume in the dynamic and thermodynamic equations to the value of the thickness needed in the heat conduction equation estimates of g are made from observed thickness distribution functions sensitivity studies show it to be an important parameter

observation of glacial isostatic adjustment in stable north america with gps 1 motions of three hundred and sixty global positioning system gps sites in canada and the united states yield a detailed image of the vertical and horizontal velocity fields within the nominally stable interior of the north american plate by far the strongest signal is the effect of glacial isostatic adjustment gia due to ice mass unloading during deglaciation vertical velocities show present day uplift 10 mm yr near hudson bay the site of thickest ice at the last glacial maximum the uplift rates generally decrease with distance from hudson bay and change to subsidence 1 2 mm yr south of the great lakes the hinge line separating uplift from subsidence is consistent with data from water level gauges along the great lakes showing uplift along the northern shores and subsidence along the southern ones horizontal motions show outward motion from hudson bay with complex local variations especially in the far field although the vertical motions are generally consistent with the predictions of gia models the horizontal data illustrate the need and opportunity to improve the models via more accurate descriptions of the ice load and laterally variable mantle viscosity

glacial isostatic adjustment observed using very long baseline interferometry and satellite laser ranging geodesy in global space geodetic solutions radial site motions are usually estimated relative to the geocenter the center of figure of the solid earth most geodesists estimate the motion of the geocenter assuming both that sites do not move radially and that sites move laterally as predicted by plate motion model nuvel 1a demets et al 1990 1994 here we estimate the motion of the geocenter assuming that the plate interiors deform radially and laterally as predicted by the postglacial rebound model of peltier 1994 or that of peltier 1996 without assuming a priori knowledge about relative plate motion radial site motions estimated relative to this rebound adjusted geocenter are in the same reference frame as the rebound model predictions whereas site motions estimated without adjusting for rebound are not we further constrain the motion of the rebound adjusted geocenter using satellite laser ranging s sensitivity to the center of mass of the solid earth the oceans and the atmosphere by assuming that the mean velocity between the rebound adjusted geocenter and the center of mass is negligible over the time period of geodetic measurement twenty years of observation with satellite laser ranging and very long baseline interferometry record the isostatic response of the solid earth to the unloading of the late pleistocene ice sheets the misfits of the postglacial rebound model of peltier 1994 and that of peltier 1996 are 34 and 16 less respectively than the misfit of the rigid plate model sites at onsala sweden and algonquin park ontario are observed to be rising at 3 mm yr and 2 mm yr respectively reflecting unloading of the fennoscandian and laurentide ice sheets sites along the east coast of the united states are subsiding at 2 mm yr indicating that the forebulge produced by the laurentide ice sheet is currently collapsing very slowly sites beneath the margins of the ice sheets during the last glacial maximum are currently moving laterally away from the ice sheet centers at 1 5 mm yr in disagreement with the moderately fast outward motion predicted by the model of peltier 1996

impact of climate warming on upper layer of the bering sea the impact of climate warming on the upper layer of the bering sea is investigated by using a high resolution coupled global climate model the model is forced by increasing atmospheric co2 at a rate of 1 per year until co2 reaches double its initial value after 70 years after which it is held constant in response to this forcing the upper layer of the bering sea warms by about 2 c in the southeastern shelf and by a little more than 1 c in the western basin the wintertime ventilation to the permanent thermocline weakens in the western bering sea after co2 doubling the southeastern shelf of the bering sea becomes almost ice free in march and the stratification of the upper layer strengthens in may and june changes of physical condition due to the climate warming would impact the pre condition of spring bio productivity in the southeastern shelf

grace gravity observations constrain weichselian ice thickness in the barents sea the barents sea is subject to ongoing postglacial uplift since the melting of the weichselian ice sheet that covered it the regional ice sheet thickness history is not well known because there is only data at the periphery due to the locations of franz joseph land svalbard and novaya zemlya surrounding this paleo ice sheet we show that the linear trend in the gravity rate derived from a decade of observations from the gravity recovery and climate experiment grace satellite mission can constrain the volume of the ice sheet after correcting for current ice melt hydrology and far field gravitational effects regional ice loading models based on new geologically inferred ice margin chronologies show a significantly better fit to the grace data than that of ice 5g the regional ice models contain less ice in the barents sea than present in ice 5g 5 6 3 m equivalent sea level versus 8 5 m which increases the ongoing difficulty in closing the global sea level budget at the last glacial maximum

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

dynamical analysis of a satellite observed anticyclonic eddy in the northern bering sea the characteristics and evolution of a satellite observed anticyclonic eddy in the northern bering sea during march and april 1999 are investigated using a three dimensional princeton ocean model pom the anticyclonic like current pattern and asymmetric feature of the eddy were clearly seen in the synthetic aperture radar sar sea surface temperature and ocean color images in april 1999 the results from model simulation reveal the three dimensional structure of the anticyclonic eddy its movement and dissipation energy analysis indicates that the barotropic instability bti is the main energy source for the growth of the anticyclonic eddy the momentum analysis further reveals that the larger magnitude of the barotropic pressure gradient in the meridional direction causes the asymmetry of the anticyclonic eddy in the zonal and meridional directions while the different magnitudes of the meridional baroclinic pressure gradient are responsible for the different intensity of currents between the northern and southern parts of the anticyclonic eddy

the bering slope current system revisited mean circulation and water properties within the aleutian basin of the bering sea are investigated using hydrographic and subsurface park pressure displacement data from a regional array of 14 profiling ctd floats after 10 days drifting at 1000 dbar each float measures temperature and salinity profiles as it rises to the surface and then transmits these data via satellites which also make several fixes of the float surface positions before it sinks again every fourth cycle the floats sink from 1000 dbar to a 2000 dbar target just prior to ascent to measure deeper profiles the 1000 dbar displacements estimated from the float surface position fixes reveal a coherent few centimeters per second northwestward flow along the northeastern boundary the deep signature of the bering slope current middepth water property distributions are consistent with cyclonic advection of warm water from the south around the basin eastward in the aleutian north slope current and then northwestward in the bering slope current geostrophic transport estimates relative to 1000 dbar also show cyclonic motion although with significant noise likely owing to the influence of mesoscale eddies the mean alongslope geostrophic transport of the bering slope current is determined between 0 and 1900 dbar relative to 1000 dbar and then combined with mean along slope velocities at 1000 dbar the result is an absolute geostrophic transport estimate with 95 confidence intervals for the along slope current offshore of the 1000 m isobath and between 0 and 1900 dbar of 5 8 61 7 3 106 m3 s 21

inference of mantle viscosity from grace and relative sea level data summary gravity recovery and climate experiment grace satellite observations of secular changes in gravity near hudson bay and geological measurements of relative sea level rsl changes over the last 10 000 yr in the same region are used in a monte carlo inversion to infer mantle viscosity structure the grace secular change in gravity shows a significant positive anomaly over a broad region 3000 km near hudson bay with a maximum of 2 5 gal yr 1 slightly west of hudson bay the pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ice 5g deglaciation history strongly suggesting a postglacial rebound origin for the gravity change we find that the grace and rsl data are insensitive to mantle viscosity below 1800 km depth a conclusion similar to that from previous studies that used only rsl data for a mantle with homogeneous viscosity the grace and rsl data require a viscosity between 1 4 1021 and 2 3 1021 pa s an inversion for two mantle viscosity layers separated at a depth of 670 km shows an ensemble of viscosity structures compatible with the data while the lowest misfit occurs for upper and lower mantle viscosities of 5 3 1020 and 2 3 1021 pa s respectively a weaker upper mantle may be compensated by a stronger lower mantle such that there exist other models that also provide a reasonable fit to the data we find that the grace and rsl data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle

numerical study on tidal mixing along the shelf break in the green belt in the southeastern bering sea 1 tidal mixing and its associated iron and nutrients flux from a subsurface layer along the shelf break has been considered as one of the key processes to maintain the high summertime biological productivity in the green belt in the southeastern bering sea in the present study tidal mixing near the shelf break is examined with a three dimensional high resolution numerical model to quantify the enhanced mixing and to reveal the underlying physical mechanisms based on the different characteristics of diurnal and semidiurnal internal waves strong turbulent mixing with energy dissipation rates of over 1 10 8 w kg is reproduced along the green belt in the numerical model forced by barotropic diurnal and or semidiurnal tides at open boundaries the energy dissipation off the shelf break near the sea surface is enhanced due to the semidiurnal internal waves whereas the dissipation near the bottom of the shelf break is enhanced by the diurnal topographically trapped waves an additional experiment with stratification representative of winter conditions shows a significant influence of stratification on the energy dissipation off the shelf break due to the change in features of both diurnal and semidiurnal internal waves low mode semidiurnal baroclinic energy flux from the aleutian passes is shown to increase the energy dissipation along the shelf slope between pribilof and zhemchug canyons these results suggest that the strong vertical mixing along the shelf break induced by the diurnal and semidiurnal tides both play important roles in maintaining the iron and nutrients supply along the green belt

winter currents and hydrographic conditions on the northern central bering sea shelf current observations obtained from the northern bering shelf region in winter 1984 1985 indicate that midwinter circulation patterns differed from those during the rest of the year annual mean circulation in this region is dominated by vigorous northward currents from 10 cm s in shpanberg strait to more than 50 cm s in bering strait these northward currents are accompanied by a weak northwestward coastal flow south of st lawrence island in february april 1985 however flow was weakly 0 5 cm s southward through shpanberg strait and southeastward south of st lawrence island while remaining northward through anadyr and bering straits comparison with current data obtained from previous years suggests that midwinter reversal of flow in shpanberg strait is a recurrent winter feature it generally occurs at about the same time that the winter ice cover undergoes its major advance and it is associated with strong northerly winds northward flow through anadyr strait persists conversely through the winter and is the northward extension of a western intensified northward flow over the bering shelf its source is bering sea water farther south rather than water from the northern central shelf the resultant trapping of shelf water on the northern central shelf coupled with intensive regional ice formation during the same period as the flow reversal led in 1985 to significant about 1 5 brine enrichment this salinity increase agrees qualitatively with historical data which suggest that the northern central bering shelf is the site both of sluggish winter circulation and of elevated salinities due to brine enrichment from ice formation

seasonal variations of sea ice and ocean circulation in the bering sea a model data fusion study 1 a 9 km coupled ice ocean model ciom was implemented in the entire bering sea to investigate seasonal cycles of sea ice and ocean circulation under atmospheric forcing sea ice cover with a maximum of 0 6 106 km2 in february to late march was reasonably reproduced by the bering ciom and validated by special sensor microwave imager ssm i measurements the model also captures some important spatial variability and downscaling processes such as polynyas and ridging which the ssm i measurements cannot reproduce because of their coarse 25 km resolution there are two distinct surface ocean circulation patterns in winter and summer on the bering shelves because of the dominant winds which are northeasterly in winter and southwesterly in summer summer low temperature high salinity water mass 3 c on the bering shelf is formed locally during winter because of strong vertical convection caused by salt injection when ice forms wind and wind wave mixing on the shelf the northward volume transport across the 62 5 n line with an annual mean of 0 8 0 33 sv 1 sv 106 m3 s 1 that is consistent with the measurements in the bering strait has barotropic structure which transports heat flux with an annual mean of 7 74 tw 1 tw 1012 w northward the anadyr current advects warmer saltier water northward during summer nevertheless it reverses its direction to southward during winter because of predominant northeasterly and northerly wind forcing therefore the anadyr current advects cold salty water southward the volume transport on the broad midshelf is northward year round advecting heat 3 3 2 4 tw and freshwater 8 10 104 practical salinity unit psu m3 s 1 northward one important finding is that the anadyr current and the midshelf current are out of phase in volume and heat transports the alaskan coastal current also transports heat and freshwater northward on an annual basis the bering ciom also captures the winter dense water formation along the siberian coast which is promoted by the downwelling favorable northeasterly wind and the summer upwelling due to the basin scale upwelling favorable southwesterly wind which brings up the cold salty and nutrient rich water from the subsurface to the surface within a narrow strip along the west coast this upwelling found in the model was also confirmed by satellite measurements in this study

a numerical study of the circulation of the bering sea basin and exchange with the north pacific ocean abstract a limited area primitive equation three layer hydrodynamic model with realistic coastlines and bathymetry and resolution is used to investigate the circulation of the bering sea basin and the adjacent north pacific ocean the westward flowing alaskan stream to the south of the aleutian island chain is specified as a boundary condition at inflow and outflow ports with a constant throughput of 15 sv sv 1 106 m3 s 1 atmospheric forcing is based an the hellerman and rosenstein monthly climatological wind field the model is spun up over 50 years and the statistics of the final decade are described the general features of the model circulation as discussed below are consistent with available hydrographic and buoy drift observations the model alaskan stream separates from the aleutian island chain near 175 beyond this point there is strong interannual variability associated with meandering and occasional eddy shedding along the northern arm of the western subarctic gyre there is a gen

trapped motion around the pribilof islands in the bering sea observations in the region of the pribilof islands and canyon pic reveal a clockwise circulation around the group of islands and around each of the two largest islands st paul and st george six current meters deployed around st paul island revealed a steady clockwise flow around the island which was strongest south of the island and weakest to the east we use a high resolution tidal model in the pic region to show that this flow pattern results from tidal rectification over the shallow topography tides tidal residual currents of 10 15 cm s were predicted by the model compared to mean currents of 5 20 cm s observed at the mooring sites both diurnal and semidiurnal tidal oscillations contribute to the clockwise circulation around the islands in the diurnal band the enhanced currents occur also at the shelf slope where a tidal wave generates a shelf wave due to resonance with topography in the pic region the main shelf wave occurs in the pribilof canyon where both observations and measurements show enhancement of the diurnal tidal currents

scattered wave imaging of the lithosphere asthenosphere boundary several recent studies have imaged discontinuities interpreted as the lithosphere asthenosphere boundary using receiver function methods these are exciting results since this boundary is fundamental and yet relatively elusive its location is not well imaged nor are its properties well understood the receiver function method provides high depth resolution and the multitude of studies using this method have increased lateral resolution a better global understanding of the location and nature of the boundary is developing which ultimately will have implications for mantle dynamics and planetary evolution however these new results do not all form a clear and consistent picture and in some cases introduce new complexities and questions some apparent contradictions may represent unmodeled earth structure overall interpretation and synthesis requires careful consideration of the approaches and sensitivities of individual studies

modeling 3 d crustal velocities in the united states and canada a numerical model for three dimensional 3 d crustal velocities has been derived for most of the united states and canada primarily from repeated geodetic data this model provides a foundation for a prototype of the trans4d software trans4d is being developed to enable geospatial professionals and others to transform 3 d positional coordinates across time the derived model reveals several macroscopic features of the 3 d velocity field including the pervasive presence of the glacial isostatic adjustment associated with the past melting of the ice fields that formed more than 19 000 years ago during the last glacial maximum in this study the present day 3 d velocity field associated with this melting as estimated via the recently published ice 6g c vm5a model was subtracted from this study s total 3 d velocity field to identify features of the residual velocity field in particular this study introduces the na ice 6g reference frame in which residual horizontal velocities have magnitudes that are less than 2 mm yr everywhere east of longitude 104 w and south of latitude 60 n except in southern texas residual horizontal velocities of greater magnitude are found west and or north of these two boundaries and they are due mostly to interactions among tectonic plates with localized pockets due to other geophysical phenomena large residual vertical velocities some with values exceeding 30 mm yr are found in southeastern alaska the uplift occurring here is due to present day melting of glaciers and ice fields formed during the little ice age glacial advance that occurred between 1550 a d and 1850 a d

on exchange of water between the gulf of alaska and the bering sea through unimak pass we present the first long term current and bottom pressure observations from unimak pass alaska and adjacent locations on the gulf of alaska and bering sea shelves vector mean current recorded between march and august 1980 was 12 cm s toward 284 t or onto the bering sea shelf however magnitude decreased from 20 cm s to 6 cm s between the first and second halves of the record at shorter periods 3 10 days current fluctuations in the pass were strongly coherent k2 0 7 with the pressure difference measured along the axis of the pass and this was coherent with geostrophic wind estimates the results indicated that wind induced sea level perturbation was the dominant forcing mechanism for fluctuations relations among current bottom pressure pressure difference and geostrophic wind time series also showed that dynamic variaton on the gulf of alaska shelf was primarily responsible for current fluctuations in the pass hydrographic data indicated that a baroclinic current existed along the gulf of alaska coast and this flow became the long term mean flow through the pass we suggest that this feature was the extension of the kenai current the historic supposition that water is transported from the gulf of alaska into the bering sea was verified however the waters are of coastal origin rather than from the alaskan stream

summer transport estimates of the kamchatka current derived as a variational inverse of hydrophysical and surface drifter data 1 the quasistationary summer bering sea circulation is reconstructed as a variational inverse of the hydrographic and atmospheric climatologies transport estimates through the bering strait and surface drifter data our results indicate the splitting of the kamchatka current in the vicinity of the shirshov ridge this branching is in agreement with independent argo drifter observations it was also found that transport of the kamchatka current gradually increases downstream from 14 sv in the olyutorsky gulf to 24 sv in the kamchatka strait which is twice higher than previous estimates

modeling the seasonal variation of sea ice in the labrador sea with a coupled multicategory ice model and the princeton ocean model we use a multicategory sea ice model coupled to the princeton ocean model which is driven by monthly climatological atmospheric forcing to study the seasonal variation of ice cover in the labrador sea initial ocean conditions are derived from a gridded objectively analyzed temperature salinity data set that provides improved resolution of gradients in the vicinity of the shelf break the model produces a realistic seasonal variation of sea ice there is ice growth over the inner shelf and ice melt over the outer shelf and slope over the inner shelf advection and diffusion decrease the ice mass over the outer shelf advection and diffusion increase the ice mass which maintains the location of the ice edge near the offshore ice edge the melt rate exceed s1mp er month and the heat to melt ice together with the heat lost to the atmosphere exceeds 500 wm 22 the heat lost at the ocean surface is compensated for by advection of heat from an offshore convective region the dominant heat source for the spring retreat of ice in the south is shortwave radiation over the open water fraction marginal ice zones are regions of dynamic interaction be tween atmosphere ice and ocean in the labrador marginal ice zone the ice edge is located near the labrador current and its associated shelf break front wintertime convection in the western labrador sea is thought to form labrador intermedi ate water clarke and gascard 1983 since the ice edge bor ders the convective region it is important to clarify the pro cesses that limit the ice extent and to examine the heat and salt fluxes in the vicinity of the ice edge in this study we investigate the seasonal evolution of sea ice over the labrador shelf with a coupled ice ocean model our objective is to assess the capability of the model in simulating the evolution of the ice cover and to identify model deficien cies we examine the processes that limit the ice extent labrador sea ice has been the subject of a number of studies with numerical models most recently ikeda et al 1996 cou pled a two category ice model hibler 1979 with the geo physical fluid dynamics laboratory gfdl ocean model bryan 1969 cox 1984 there was overall agreement with the seasonal cycle as well as with interannual variations the re sults showed that shoreward advection of relatively warm off shore water is an important source of heat to melt ice how ever the model ice decayed too rapidly partly because the two category ice model underestimates ice concentration there are a number of differences between the present model and the model of ikeda et al 1996 we have imple mented a multicategory ice model and used the sigma coordi nate princeton ocean model blumberg and mellor 1987 mel lor 1996 with its embedded turbulence closure submodel we use a climatology from the national centers for environmen tal prediction ncep national center for atmospheric re

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

greenland mass balance from grace 1 we use 22 monthly grace gravity recovery and climate experiment gravity fields to estimate the linear trend in greenland ice mass during 2002 2004 we recover a decrease in total ice mass of 82 28 km3 of ice per year consistent with estimates from other techniques our uncertainty estimate is dominated by the effects of grace measurement errors and errors in our post glacial rebound pg correction the main advantages of grace are that it is sensitive to the entire ice sheet and that it provides mass estimates with only minimal use of supporting physical assumptions or ancillary data

measurements of time variable gravity show mass loss in antarctica using measurements of time variable gravity from the gravity recovery and climate experiment satellites we determined mass variations of the antarctic ice sheet during 2002 2005 we found that the mass of the ice sheet decreased significantly at a rate of 152 80 cubic kilometers of ice per year which is equivalent to 0 4 0 2 millimeters of global sea level rise per year most of this mass loss came from the west antarctic ice sheet

a glacial isostatic adjustment model for the central and northern laurentide ice sheet based on relative sea level and gps measurements the thickness and equivalent global sea level contribution of an improved model of the central and northern laurentide ice sheet is constrained by 24 relative sea level histories and 18 present day gps measured vertical land motion rates the final model termed laur16 is derived from the ice 5g model by holding the timing history constant and iteratively adjusting the thickness history in four regions of northern canada in the final model the last glacial maximum lgm thickness of the laurentide ice sheet west of hudson bay was 3 4 3 6 km conversely east of hudson bay peak ice thicknesses reached 4 km the ice model thicknesses inferred for these two regions represent respectively a 30 per cent decrease and an average 20 25 per cent increase to the load thickness relative to the ice 5g reconstruction which is generally consistent with other recent studies that have focussed on laurentide ice sheet history the final model also features peak ice thicknesses of 1 2 1 3 km in the baffin island region a modest reduction relative to ice 5g and unchanged thicknesses for a region in the central canadian arctic archipelago west of baffin island vertical land motion predictions of the final model fit observed crustal uplift rates well after an adjustment is made for the elastic crustal response to present day ice mass changes of regional ice cover the new laur16 model provides more than a factor of two improvement of the fit to the rsl data 2 measure of misfit and a factor of nine improvement to the fit of the gps data mean squared error measure of fit compared to the ice 5g starting model laur16 also fits the regional rsl data better by a factor of two and gives a slightly better fit to gps uplift rates than the recent ice 6g model the volume history of the laur16 reconstruction corresponds to an up to 8 m reduction in global sea level equivalent compared to ice 5g at lgm

circulation in the eastern bering sea inferences from a 2 km resolution model abstract a 2 km resolution model of the eastern bering sea is developed to capture dynamical processes on the scale of the rossby radius of deformation on tidal to seasonal time scales the model spans the region from 178 e to the alaskan coast and from roughly 50 to 66 n including the aleutian islands in the south and the bering strait in the north the high resolution throughout ensures that the mesoscale dynamics of significant subregions of the domain such as the aleutian island passes bering sea slope and the shelf canyons are captured simultaneously without the concern for loss of interconnectivity between regions simulations are performed for the ice free season june october of 2009 with tidal and atmospheric forcing the model compares favorably with observations from avhrr and envisat satellites argo drifters and bering sea shelf moorings the mesoscale dynamics of the mixing and exchange flow through the eastern aleutian island passes which exhibit strong diurnal and two week variability are well represented the two week oscillation in volume flux through the largest of these passes amukta pass is found to be out of phase with the transport through the neighboring passes e g seguam and samalga passes mesoscale structure is also found to be ubiquitous along the mixing front of the cold pool structures at the scale of o 20 km persist and play a role in determining the pattern of erosion of the water mass as the shelf warms and mixes on the bering sea shelf tidal motions are dominant and variability on the horizontal scale of the first mode internal tide develops o 30 km from the shelf break to the onshore edge of the bering shelf cold pool

wave induced mixing in the upper ocean distribution and application to a global ocean circulation model 1 from the reynolds stress expression the wave induced vertical viscosity or diffusivity bv is defined which can be used as a parameter to estimate the strength of wave induced mixing in addition a parameter d5 is introduced to represent a wave induced mixing penetration depth the global distribution of bv averaged over the upper 20 m is calculated and its latitudinal transects in boreal summer and winter is discussed the results show that in summer the wave induced mixing is strong in the southern oceans south of 30 s and in winter it is strong in the north pacific and the north atlantic north of 30 n as well as in the southern oceans south of 40 s adding bv to the vertical diffusivity in a global ocean circulation model yields a temperature structure in the upper 100 m that is closer to the observed climatology than a model without the wave induced mixing

closure of the budget of global sea level rise over the grace era the importance and magnitudes of the required corrections for global glacial isostatic adjustment abstract the budget of global sea level rise includes contributions from several distinct factors including thermosteric effects the wasting of small ice sheets and glaciers and the loss of mass by the great polar ice sheets and by the continents due to desiccation since the former contribution may be estimated on the basis of both hydrographic survey data and more recently using argo float data the second may be estimated on the basis of mass balance measurements on existing ice fields and the latter on the basis of modern grace based time dependent gravity field measurements the inputs to the globally averaged rate of sea level rise may be directly constrained since grace also provides a measurement of the rate at which mass is being added to the oceans we are now in a position to ask whether this rate of mass addition to the oceans matches the rate at which mass is being removed from the continents as demonstrated herein the mass component of the budget of global sea level is closed within the observational errors when the mass derived contribution is added to the thermosteric contribution it is furthermore shown that the inference of the net rate of global sea level rise by the altimetric satellites topex poseidon and jason 1 is also reconcilable over the grace era it is noted those individual terms in the budget especially the contribution from small ice sheets and glaciers remains insufficiently accurate it is demonstrated that the lingering influence of the late quaternary ice age upon sea level is profound and that closure of the budget requires an accurate model of its impact

analysis of gps measurements in eastern canada using principal component analysis continuous global positioning system cgps position time series from eastern north america constrain the pattern and magnitude of regional crustal deformation initial analysis delineates consistent uplift patterns as expected from glacial isostatic adjustment gia predictions but the associated horizontal deformation is not definitive in part due to the short time periods for a significant number of the available stations we employ an eigenpattern decomposition in order to define a unique finite set of deformation patterns for this continuous gps data similar in nature to the empirical orthogonal functions historically employed in the analysis of atmospheric and oceanographic phenomena the method derives the eigenvalues and eigenstates from the diagonalization of the correlation matrix using a karhunen loeve expansion kle the kle technique is used to identify the important modes in both time and space for the cgps data modes that potentially include signals such as horizontal and vertical gia tectonic strain and seasonal effects here we filter both the vertical and horizontal velocity patterns on different spatiotemporal scales in order to study the potential geophysical sources after the removal of correlated and random noise the method is successful in disaggregating the linear vertical signal from more variable and less spatially correlated signals the vertical and horizontal results are compared to the predictions of the ice 3g gia loading model with a number of plausible mantle viscosity profiles the horizontal velocity analysis allows for qualitative differentiation between several potential gia models and suggests that with longer time series this technique can be employed to remove correlated noise and improve estimates of crustal strain patterns and their sources

seesaw structure of subsurface temperature anomalies between the barents sea and the labrador sea 1 using a coupled ocean sea ice model of the pan arctic and north atlantic ocean we investigate the response of the arctic and subarctic thermohaline structure to seasonal extreme atmospheric forcing associated with the winter arctic oscillation ao during the positive phase of ao significant surface cooling occurs in the labrador sea but there is no substantial surface warming in the barents sea i e no seesaw pattern in sea surface temperature sst anomaly between the barents sea and labrador sea a possible explanation is that arctic sea ice export into the barents sea melts locally and lowers the sst however a seesaw structure in subsurface below the mixed layer 40 100 m water temperature anomaly between the two regions is found exceeding the 95 significance level corresponding to the positive phase of the winter ao a significant warming of the subsurface water in the barents sea and a concurrent cooling in the northwestern labrador sea are seen in the model results which is analogous to the seesaw structure in both surface air temperature sat and sea ice extent anomalies the mechanism leading to the anomalous subsurface temperature seesaw is consistent with the northward advection of the warm atlantic water into the barents sea and the southward advection of the cold arctic and sub arctic water into the labrador sea from the david strait and baffin bay hydrographic data are analyzed and the resulting temperature distribution supports this new finding

mantle viscosity and ice age ice sheet topography ice age paleotopography and mantle viscosity can both be inferred from observations of earth s response to the most recent deglaciation event of the current ice age this procedure requires iterative application of a theoretical model of the global process of glacial isostatic adjustment results demonstrate that the iterative inversion procedure converges to a paleotopography that is extremely close to that from the ice 4g model the accompanying mantle viscosity profile is furthermore shown to reconcile the requirements of aspherical geoid anomalies related to the mantle convection process thus resolving a fundamental issue concerning mantle rheology the combined model also explains postglacial sea level histories for the east coast of the united states

heat flow in the trans hudson orogen of the canadian shield implications for proterozoic continental growth fourteen new heat flow and radiogenic heat production measurements have been obtained in the paleo proterozoic trans hudson orogen of the canadian shield this orogen which consists of several distinctive belts corresponds to a pulse of crustal growth through island arc magmatism between 1 9 and 1 8 ga the data now available include 17 previously published measurements heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen the average and standard deviation of heat flow values for the entire orogen 425 11 mw m 2 are identical to those of the older superior province and of the younger grenville province for the orogen as a whole heat flow is weakly correlated to the heat production of surface rocks high heat flow values are found in the thompson belt consisting of metasedimentary rocks deposited on the ancient continental margin of the superior craton there the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of rdioelements in large volumes of rocks the heat flow is low in the belts that expose juvenile proterozoic crust consisting mostly of arc related volcanic rocks in the flin flon snow lake belt the average heat flow is the same as the average of the orogen the low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed a t the surface are thin and rest on a basement richer in radioelements in the lynn lake belt the heat flow is significantly lower than the average for the orogen although the surface heat production is not low the heat flow data require a thin 10 kin surface layer overlying the mid and lower crust depleted in ra dioelements around the town of lynn lake heat flow is consistently low over a distance of m40 kin the coincidence between this cold spot and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures the evolution of the trans hudson orogen exemplifies the interplay between the processes generating rocks of evolved composition which require crustal thickening and those forming normal continental crust with average thickness which require crustal flow and soft crustal rheology

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

recent observations of variability in the western bering sea current system data from two cruises in the western bering sea are presented and examined during spring 1990 conditions were typical and the kamchatka current had a transport of 11 sv on the other hand during fall 1990 this flow had weakened to 6 sv although a strong flow near the komandorski islands occurred various eddylike features were common during both cruises and were particularly developed in fall although some aspects of the flow patterns may be seasonal in nature the major weakening of transport probably represents an interannual change

long wavelength gravity field constraint on the lower mantle viscosity in north america the long wavelength negative gravity anomaly over hudson bay coincides with the area depressed by the laurentide ice sheet during the last glacial maximum suggesting that it is at least partly caused by glacial isostatic adjustment gia additional contributions to the static gravity field stem from surface dynamic topography core mantle boundary cmb topography and density anomalies in the subsurface previous estimates of the contribution of gia to the gravity anomaly range from 25 to more than 80 however these estimates did not include uncertainties in all components that contribute to the gravity field in this study we develop a forward model for the gravity anomaly based on density models and dynamic models investigating uncertainty in all components we derive lithospheric densities from equilibrium constraints but extend the concept of lithospheric isostasy to a force balance that includes the dynamic models the largest uncertainty in the predicted gravity anomaly is due to the lower mantle viscosity uncertainties in the ice history the crustal model the lithosphere asthenosphere boundary and the conversion from seismic velocities to density have a smaller effect a preference for lower mantle viscosities 1022 pa s is found in which case at least 60 of the observed long wavelength gravity anomaly can be attributed to gia this lower bound on the lower mantle viscosity has implications for inferences based on models for mantle convection and gia

tidal currents in the st lawrence island region 1 vertical and spatial variability of tidal currents in the northern part of the bering sea near st lawrence island were examined using observations made at 17 new and historical current meter mooring locations the semidiurnal currents dominate over the diurnal currents semidiurnal currents exhibit a seasonal modulation with currents being somewhat stronger during the summer and fall when the density structure displays two layer stratification diurnal currents exhibit a weak semiannual modulation examination of the vertical structure of tidal currents shows that maximum semidiurnal currents are observed in the upper or middle layers down to a depth of about 20 m and below this depth the energy of the semidiurnal currents decreases rapidly however the energy of diurnal band currents remains almost constant with depth decreasing only in close proximity to the bottom rotary spectral analysis applied to the records shows further differences the m2 clockwise cw component of rotation dominates in the upper and middle water column but the counterclockwise component ccw dominates near the bottom interpretation of these observed results points to the different bottom boundary layers bbl for the cw and ccw motions in addition to the role of density stratification in organizing the bbl we compare results of a vertically averaged two dimensional model to the observationally derived tidal ellipse parameters and show that the barotropic formulation provides a first order description of the tidal regime within the domain

ocean circulation and exchanges through the northern bering sea 1979 2001 model results abstract we have developed and run a model with sufficiently high resolution 9 km and 45 levels and a large enough spatial domain to allow for realistic representation of flow through the narrow and shallow straits in the northern bering sea this is potentially important for quantification of long term mean and time dependent ocean circulation and water mass and property exchanges between the pacific and arctic oceans over a 23 year interval 1979 2001 mean transport through bering strait is estimated to be 0 65 sv comparison of our model results with published observations indicates that ocean circulation is not only variable at seasonal to interdecadal scales but it is also responsive to short term atmospheric forcing one of such events occurred during the winter of 2000 2001 with reversed oceanic flow in some areas and much reduced sea ice cover analyses of eddy kinetic energy fields identify some high biological productivity regions of the chirikov basin coincident with persistent high energy up to 2700 cm 2 s 2 in the surface layer and up to 2600 cm 2 s 2 at mid depth throughout the annual cycle model output in the bering strait region is validated against several time series of moored observations of water mass properties comparison with shipboard observations of near bottom salinity from late winter through autumn indicates that the model reasonably represents the major water mass properties in the region the modeled vertical water column structure in the northern bering sea allows increased understanding of the mechanisms of water transformation and transport northward through bering strait into the chukchi and beaufort seas we conclude that the long term model results for the northern bering sea provide important insights into the ocean circulation and fluxes and they are a useful frame of reference for limited observations that are short term and or cover only a small geographic region

omori like decay of postseismic velocities following continental earthquakes various mechanisms have been proposed to explain the transient enhanced surface deformation rates following earthquakes unfortunately these different mechanisms can produce very similar surface deformation patterns leading to difficulty in distinguishing between them here we return to the observations themselves and compile near field postseismic velocity measurements following moderate to large continental earthquakes we find that these velocities have a remarkably consistent pattern with velocity inversely proportional to time since the earthquake this suggests that postseismic velocities show an omori like decay and that postseismic displacements increase logarithmically over time these observations are inconsistent with simple linear maxwell or burgers body viscoelastic relaxation mechanisms but are consistent with rate and state frictional afterslip models and power law shear zone models the results imply that near field postseismic surface deformation measurements are primarily the result of fault zone processes and therefore that the inference of lower crustal viscosities from near field postseismic deformation requires care

adcp referenced geostrophic circulation in the bering sea basin abstract a month long circumnavigation of the bering sea basin in august 1991 was designed to study the basin scale circulation for the first time in this region vessel mounted acoustic doppler current profiler adcp measurements provided an absolute reference for geostrophic currents transport in the alaskan stream 01 1500 db was 24 sv sv 106 m3 s 1 westward considerably higher than the 10 15 sv usually inferred from geostrophic calculations referred to 1500 db but in good agreement with previous work relative to moored current meters as previously reported the stream separated from the aleutian ridge forming an anticyclonic eddy that restricted inflow to the bering sea flow through the aleutians varied substantially across the passes the kamchatka current was traceable for 800 km until it left the bering sea carrying 14 sv 0 3000 db eddies were detected from the clustering of centers of curvature of sequential adcp vectors incompletely spanned eddies can bias transports counterrotat

a space geodetic target for mantle viscosity discrimination horizontal motions induced by glacial isostatic adjustment a model of mantle viscosity recently obtained on the basis of the inversion of glacial isostatic adjustment data is herein shown to predict a highly characteristic signature in tangential motion due to this cause

constraints on the subsurface structure of europa abstract the identification of anti jovian wedge shaped bands as tension cracks separating thin lithospheric blocks that have rotated apart is used with accepted failure criteria and ductile flow laws to quantitatively derive conductive thermal gradients in the brittle crust and minimum amounts of differentiation of water on europa the interpretation of wedge shaped bands as tension cracks that have allowed the rotation and translation of large blocks of lithosphere is based on offsets of preexisting lineations and the ability to fit blocks of europa s surface back together without gap or overlap the size of the blocks and their undeformed state suggest the lithosphere was greater than a few kilometers thick but less than a few tens of kilometers thick to have allowed the rotation to occur and that it was decoupled from the silicate interior at the time of rotation analysis of the griffith criterion for tensile and shear failure of intact materials and byerlee s law for the shear failure of prefractured materials shows that the slip on preexisting fractures with dips greater than 25 will occur at lower stress differences than those required to break intact ice or rock the formation of tension cracks on europa requires a intact lithosphere with no preexisting fractures or subsurface mechanical discontinuities confining pressure and the tensile strength of ice limit the depth to which tension cracks can form and thus the thickness of the brittle lithosphere under europan conditions to 6 km with a corresponding maximum stress difference of 10 mpa below the brittle lithosphere brittle ductile tr transition the ice deforms by ductile creep which is thermally activated the ductile yield stress curve must intersect the tensile failure curve no deeper than 6 km or tension cracks would not have formed shear failure occurs below this depth this tensile brittle ductile intersection depth constrains thermal gradients on europa to 3 7 km for tensile lithospheres of 6 3 km thick respectively for the assumed ductile flow laws and material constants the estimate of 3 km is a minimum because faster strain rates than assumed and strengthening of ice by inclusion of silicates each raises gradients by about a third and possible though unlikely fluid pressure can raise gradients by about a degree per kilometer for tensile lithospheres less than 6 km thick given a fluid pressure ratio of 0 5 our results limit the possible effects of surface warming processes such as insulating regoliths or a solid state greenhouse to less than 10 30 on europa at the time of fracturing a minimum of about one quarter of europa s water is required to have differentiated to have allowed complete decoupling of the ice lithosphere from the underlying silicate mantle

direct evidence for northward flow on the northwestern bering sea shelf during the summer of 1994 a satellite tracked drifter transited from the southeastern bering sea slope through bering strait by a route westward along the slope and then northward through anadyr canyon and strait the trajectory emphasizes the importance of a western location of northward flow on the bering sea shelf the transit time was 2 months from cape navarin to bering strait with northward drift velocities of 5 40 cm s

the angular velocities of the plates and the velocity of earth s centre from space geodesy summary using space geodetic observations from four techniques gps vlbi slr and doris we simultaneously estimate the angular velocities of 11 major plates and the velocity of earth s centre we call this set of relative plate angular velocities geodvel for geodesy velocity plate angular velocities depend on the estimate of the velocity of earth s centre and on the assignment of sites to plates most geodetic estimates of the angular velocities of the plates are determined assuming that earth s centre is fixed in an international terrestrial reference frame itrf and are therefore subject to errors in the estimate of the velocity of earth s centre in itrf2005 and itrf2000 earth s centre is the centre of mass of earth oceans and atmosphere cm the velocity of cm is estimated by slr observation of lageos s orbit herein we define earth s centre to be the centre of mass of solid earth ce we determine the velocity of ce by assuming that the portions of plate interiors not near the late pleistocene ice sheets move laterally as if they were part of a rigid spherical cap the geodvel estimate of the velocity of ce is likely nearer the true velocity of cm than are the itrf2005 and itrf2000 estimates because 1 no phenomena can sustain a significant velocity between cm and ce 2 the plates are indeed nearly rigid aside from vertical motion and 3 the velocity of cm differs between itrf2005 and itrf2000 by an unacceptably large speed of 1 8 mm yr 1 the velocity of earth s centre in geodvel lies between that of itrf2000 and that of itrf2005 with the distance from itrf2005 being about twice that from itrf2000 because the geodvel estimates of uncertainties in plate angular velocities account for uncertainty in the velocity of earth s centre they are more realistic than prior estimates of uncertainties geodvel differs significantly from all prior global sets of relative plate angular velocities determined from space geodesy for example the 95 per cent confidence limits for the angular velocities of geodvel exclude those of revel sella et al for 34 of the 36 plate pairs that can be formed between any two of the nine plates with the best constrained motion the median angular velocity vector difference between geodvel and revel is 0 028 myr 1 which is up to 3 1 mm yr 1 on earth s surface geodvel differs the least from the geodetic angular velocities that altamimi et al determine from itrf2005 geodvel s 95 per cent confidence limits exclude 11 of 36 angular velocities of altamimi et al and the median difference is 0 015 myr 1 geodvel differs significantly from nearly all relative plate angular velocities averaged over the past few million years including those of nuvel 1a the difference of geodvel from updated 3 2 myr angular velocities is statistically significant for all but two of 36 angular velocities with a median difference of 0 063 myr 1 across spreading centres eight have slowed down while only two have sped up we conclude that plate angular velocities over the past few decades differ significantly from the corresponding angular velocity averaged over the past 3 2 myr

eddy induced cross slope exchange maintaining summer high productivity of the bering sea shelf break 1 eddy related cross slope exchange along the bering sea shelf break was investigated using a hydrographic observations data set and a numerical model results of observations in summer of 2001 showed a shelf break front that formed at a shelf break near an anticyclonic eddy high nitrate nitrite concentrations in the subsurface layer and high chlorophyll a chl a concentrations 6 mg m 3 in the surface layer a hydrographic observation in summer of 2002 exhibited relatively high chl a concentrations at the surface around the anticyclonic eddy tracer experiments revealed two types of cross slope exchange under isopycnals nutrient rich water in the basin is transported to the shelf and there is about a 64 53 increase in integrated nitrate nitrite on shelf flux 50 m depth bottom when mesoscale eddies are formed and propagated along the shelf break at the surface high chl a waters in the shelf are advected to the deep basin area by eddy transport and propagation these indicate that 1 mesoscale eddies supply nutrients and sustain primary productivity at the shelf break and 2 eddies expand the high chl a area to the basin then to the highly productive area so that the green belt is maintained

a model data study of the 1999 st lawrence island polynya in the bering sea 1 a coupled ice ocean model ciom and in situ measurements were used to investigate sea ice and the st lawrence island polynya slip in the bering sea in 1999 the modeled 1999 seasonal cycle of ice cover compared well with satellite measurements the simulated maximum sea ice coverage was 0 8 106 km2 and the simulated maximum sea ice volume was 344 km3 the polynya south of st lawrence island was captured by the ciom and investigated in depth against the measurements it was found that an offshore wind was necessary but not sufficient on its own for the development of the slip it was found that a strong offshore wind offshore surface water velocity and the angle 60 between wind and water current are the three major factors for the development of the slip multiple variable linear regression models were developed to confirm these three mechanisms yearly potential sea ice production in the slip area was estimated to be about 95 7 km3 which accounts for 2 8 of the total potential production of 3393 km3 in the whole bering sea sea ice contributes to approximately 63 of winter salinity changes in the bering shelf 200 m while the slip can contribute more than twice the local salinity changes the relationships among wind sea ice and surface ocean current were examined the classic ekman drift theory that surface water velocity drifts 45 to the right of the wind direction is modified to be 50 4 on the ice covered bering shelf due to the year round existence of a background northward ocean transport

glacial isostatic adjustment in the static gravity field of fennoscandia in the central part of fennoscandia the crust is currently rising because of the delayed response of the viscous mantle to melting of the late pleistocene ice sheet this process called glacial isostatic adjustment gia causes a negative anomaly in the present day static gravity field as isostatic equilibrium has not been reached yet several studies have tried to use this anomaly as a constraint on models of gia but the uncertainty in crustal and upper mantle structures has not been fully taken into account therefore our aim is to revisit this using improved crustal models and compensation techniques we find that in contrast with other studies the effect of crustal anomalies on the gravity field cannot be effectively removed because of uncertainties in the crustal and upper mantle density models our second aim is to estimate the effects on geophysical models which assume isostatic equilibrium after correcting the observed gravity field with numerical models for gia we show that correcting for gia in geophysical modelling can give changes of several kilometer in the thickness of structural layers of modeled lithosphere which is a small but significant correction correcting the gravity field for gia prior to assuming isostatic equilibrium and inferring density anomalies might be relevant in other areas with ongoing postglacial rebound such as north america and the polar regions

subsidence along the atlantic coast of north america insights from gps and late holocene relative sea level data the atlantic coast of north america is increasingly affected by flooding associated with tropical and extratropical storms exacerbated by the combined effects of accelerated sea level rise and land subsidence the region includes the collapsing forebulge of the laurentide ice sheet high quality records of late holocene relative sea level rsl rise are now available allowing separation of long term glacial isostatic adjustment induced displacement from modern vertical displacement measured by gps we compare geological records of late holocene rsl to present day vertical rates from gps for many coastal areas there is no significant difference between these independent data exceptions occur in areas of recent excessive groundwater extraction between virginia 38 n and south carolina 32 5 n the present day subsidence rates in these areas are approximately double the long term geologic rates which has important implications for flood mitigation tide gauge records therefore should be used with caution for studying sea level rise in this region

characteristics and variability of the inner front of the southeastern bering sea the inner front of the southeastern bering sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms differences in forcing mechanisms sea ice advance retreat and storm strength and timing and early spring water properties result in strong interannual variability in biological chemical and physical features near the front we have developed a simple model based on surface heat flux and water column mixing to explain the existence of cold belts cont shelf res 19 14 1999 1833 associated with such fronts hydrography fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs and this can prolong primary production at the inner front the effectiveness of this process depends on two factors the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column

a numerical simulation of sea ice cover in hudson bay abstract hibler s dynamic thermodynamic sea ice model with viscous plastic rheology is used to simulate the seasonal cycle of sea ice motion thickness compactness and growth rate in hudson bay under monthly climatological atmospheric forcing and a prescribed ocean surface current field the sea ice motion over most of the domain is driven mainly by the wind stress wintertime sea ice velocities are only of the order of 1 5 10 4 m s 1 due to the nearly solid ice cover and the closed boundary constraint of hudson bay however the velocities rise to 0 10 0 20 m s 1 during the melting and freezing seasons when there is partial ice cover the simulated thickness distribution in mid april the time of heaviest ice cover ranges from 1 3 m in james bay to 1 7 m in the northern part of hudson bay which compares favorably with observations the area averaged growth rate computed from the model is 1 5 0 5 cm day 1 from december to march is negative in may indicative of melting and reaches its minimum v

holocene sea level database for the atlantic coast of the united states we have constructed a database of holocene relative sea level rsl observations for the atlantic coast of the united states the database contains 492 index points which locate the position of rsl in time and space and 344 limiting dates which constrain the minimum or maximum limit of former sea level the majority of the index points in the database are from 6 ka bp to present with only 7 older than 6 ka bp spatially index points are distributed between maine and south carolina but there is an absence of data from georgia and the atlantic coast of florida the database is sub divided into 16 regions based on the distance from the former laurentide ice sheet and are classified depending upon their susceptibility to compaction the index points and limiting data demonstrate that rsl did not exceed present 0 m during the holocene except potentially in regions 1 and 2 eastern maine and southern maine rates of rsl change were highest during the early holocene and have decreased over time due to the diminishing response of the earth s mantle to glacial isostatic adjustment and reduction of ice equivalent meltwater input along the atlantic coast of the united states the linear rate of rsl rise prior to 4 ka bp ranged from 0 5 to 4 5 mm a 1 compared to 0 6 1 8 mm a 1 from 4 ka bp to ad 1900 the database suggests minimal 0 3 mm yr changes in these rates of rsl rise during the late holocene deglaciation of the laurentide ice sheet caused the spatial variability captured by the database the maximum rate of late holocene and ongoing rsl rise occurred in mid atlantic regions new jersey and inner delaware because of collapse of the peripheral forebulge

rheological stratification of the lithosphere a direct inference based upon the geodetically observed pattern of the glacial isostatic adjustment of the north american continent 1 a sequence of new analyses of the process of glacial isostatic adjustment gia is described the focus is upon the resolution of a recognized flaw in the currently prevalent model of this process that denoted ice 5g vm2 the flaw concerns a previously noted inability of the model to simultaneously reconcile the vlbi and gps measured rates of vertical and horizontal motion in the region of the north american continent that lay outboard and to the south of the laurentide ice sheet lis at last glacial maximum this characteristic misfit of the model to the data has been suggested to be reconcilable only by recourse to models that explicitly incorporate the influence of lateral viscosity heterogeneity it is demonstrated herein that on the contrary this flaw is entirely and unambiguously attributable to the rheological stratification of the lithosphere an influence not previously accounted for in global models of the gia process but which must exist on a priori grounds

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

holocene relative sea level changes and glacial isostatic adjustment of the u s atlantic coast the first quality controlled holocene sea level database for the u s atlantic coast has been constructed from 686 sea level indicators the database documents a decreasing rate of relative sea level rsl rise through time with no evidence of sea level being above present in the middle to late holocene the highest rates of rsl rise are found in the mid atlantic region we employ the database to constrain an ensemble of glacial isostatic adjustment models using two ice ice 5g ice 6g global ice sheet reconstructions and two mantle viscosity models vm5a vm5b vm radial variation of viscosity in the sublithospheric mantle variations to assess whether the spherically symmetric viscoelastic models are able to survive intercomparison with a more refined database of postglacial rsl history we identify significant misfits between observations and predictions using ice 5g with the vm5a viscosity profile ice 6g provides some improvement for the northern atlantic region but misfits remain elsewhere decreasing the upper mantle and transition zone viscosity by a factor of 2 to 0 25 10 21 pa s vm5b removes significant discrepancies between observations and predictions along the mid atlantic coastline although misfits remain in the southern atlantic region these may be an indication of the importance of laterally heterogeneous viscosity in the upper mantle