variations in the age of arctic sea ice and summer sea ice extent 1 three of the past six summers have exhibited record low sea ice extent on the arctic ocean these minima may have been dynamically induced by changes in the surface winds based on results of a simple model that keeps track of the age of ice as it moves about on the arctic ocean we show that the areal coverage of thick multi year ice decreased precipitously during 1989 1990 when the arctic oscillation was in an extreme high index state and has remained low since that time under these conditions younger thinner ice anomalies recirculate back to the alaskan coast more quickly decreasing the time that new ice has to ridge and thicken before returning for another melt season during the 2002 and 2003 summers this anomalously younger thinner ice was advected into alaskan coastal waters where extensive melting was observed even though temperatures were locally colder than normal the age of sea ice explains more than half of the variance in summer sea ice extent

ocean heat transport into the arctic in the twentieth and twenty first century in ec earth the ocean heat transport into the arctic and the heat budget of the barents sea are analyzed in an ensemble of historical and future climate simulations performed with the global coupled climate model ec earth the zonally integrated northward heat flux in the ocean at 70 n is strongly enhanced and compensates for a reduction of its atmospheric counterpart in the twenty first century although an increase in the northward heat transport occurs through all of fram strait canadian archipelago bering strait and barents sea opening it is the latter which dominates the increase in ocean heat transport into the arctic increased temperature of the northward transported atlantic water masses are the main reason for the enhancement of the ocean heat transport the natural variability in the heat transport into the barents sea is caused to the same extent by variations in temperature and volume transport large ocean heat transports lead to reduced ice and higher atmospheric temperature in the barents sea area and are related to the positive phase of the north atlantic oscillation the net ocean heat transport into the barents sea grows until about year 2050 thereafter both heat and volume fluxes out of the barents sea through the section between franz josef land and novaya zemlya are strongly enhanced and compensate for all further increase in the inflow through the barents sea opening most of the heat transported by the ocean into the barents sea is passed to the atmosphere and contributes to warming of the atmosphere and arctic temperature amplification latent and sensible heat fluxes are enhanced net surface long wave and solar radiation are enhanced upward and downward respectively and are almost compensating each other we find that the changes in the surface heat fluxes are mainly caused by the vanishing sea ice in the twenty first century the increasing ocean heat transport leads to enhanced bottom ice melt and to an extension of the area with bottom ice melt further northward however no indication for a substantial impact of the increased heat transport on ice melt in the central arctic is found most of the heat that is not passed to the atmosphere in the barents sea is stored in the arctic intermediate layer of atlantic water which is increasingly pronounced in the twenty first century

thinning and volume loss of the arctic ocean sea ice cover 2003 2008 1 we present our best estimate of the thickness and volume of the arctic ocean ice cover from 10 ice cloud and land elevation satellite icesat campaigns that span a 5 year period between 2003 and 2008 derived ice drafts are consistently within 0 5 m of those from a submarine cruise in mid november of 2005 and 4 years of ice draft profiles from moorings in the chukchi and beaufort seas along with a more than 42 decrease in multiyear my ice coverage since 2005 there was a remarkable thinning of 0 6 m in my ice thickness over 4 years in contrast the average thickness of the seasonal ice in midwinter 2 m which covered more than two thirds of the arctic ocean in 2007 exhibited a negligible trend average winter sea ice volume over the period weighted by a loss of 3000 km3 between 2007 and 2008 was 14 000 km3 the total my ice volume in the winter has experienced a net loss of 6300 km3 40 in the 4 years since 2005 while the first year ice cover gained volume owing to increased overall area coverage the overall decline in volume and thickness are explained almost entirely by changes in the my ice cover combined with a large decline in my ice coverage over this short record there is a reversal in the volumetric and areal contributions of the two ice types to the total volume and area of the arctic ocean ice cover seasonal ice having surpassed that of my ice in winter area coverage and volume became the dominant ice type it seems that the near zero replenishment of the my ice cover after the summers of 2005 and 2007 an imbalance in the cycle of replenishment and ice export has played a significant role in the loss of arctic sea ice volume over the icesat record

influences of sea ice on the eastern bering sea ncar cesm simulations and comparison with observations abstract we examine the influences of sea ice on the eastern bering sea ebs regional oceanography on seasonal and inter annual time scales using the national center for atmospheric research community earth system model ncar cesm simulations comparing the modeling results with satellite and in situ observations when possible while the modeled mean seasonal cycle of ice cover in the ebs middle shelf is generally within the uncertainty range of satellite observations in the northern domain north of 59 n the simulation reaches its annual maximum in april instead of in march as observed by satellite remote sensing modeled ice reduction in late spring in the region is also slower than observations despite this bias the simulation captures the observed seasonal transit of freshwater from the north to the south via ice advection en route the sea ice melts cooling and freshening the local water column on inter annual time scales modeling results suggest that extensive ice cover persisting into spring in the central ebs leads to cold anomalies in the bottom water especially on the middle and inner shelves of the southern domain the corresponding salinity anomalies are positive in the northern coastal domain and weak but negative in the southern middle shelf the associated 10 m ocean current anomalies are southward on the shelf and directed offshore in the slope region comparing years 1961 2005 versus years 2005 2050 the probability distribution function of ice cover on the ebs middle shelf shifts northward by 2 latitude

antarctic sea ice climatology variability and late twentieth century change in ccsm4 abstracta preindustrial control run and an ensemble of twentieth century integrations of the community climate system model version 4 ccsm4 are evaluated for antarctic sea ice climatology modes of variability trends and covariance with related physical variables such as surface temperature and sea level pressure compared to observations the mean ice cover is too extensive in all months this is in part related to excessively strong westerly winds over 50 60 s which drive a large equatorward meridional ice transport and enhanced ice growth near the continent and also connected with a cold bias in the southern ocean in spite of these biases in the climatology the model s sea ice variability compares well to observations the leading mode of austral winter sea ice concentration exhibits a dipole structure with anomalies of opposite sign in the atlantic and pacific sectors both the el nino southern oscillation and the southern annular mode sam project onto this mode in twentieth century inte

sea ice response to an extreme negative phase of the arctic oscillation during winter 2009 2010 3 the long term annual mean arctic slp field features an anticyclone centered over the northern beaufort sea known as the beaufort sea high bsh and a trough of low pressure extending from the icelandic low northeastward into the kara sea associated surface winds drive the clockwise beaufort gyre ice motion and the transpolar drift stream tds representing ice motion from the siberian coast across the arctic and then into the north atlantic through fram strait when the winter ao is in its positive mode slp over the icelandic low region and extending into the arctic is anomalously low and the bsh is weak promoting a cyclonic counter clockwise sea ice circulation anomaly this is expressed as decreased ice transport from the beaufort sea westward across the dateline into the chukchi sea increased ice transport out of the arctic ocean through fram strait and increased transport of ice away from the siberian coast leaving open water areas that foster new iceformation rigoretal 2002 bypromotingmorethinice in spring the positive ao sets the stage for negative summer ice extent anomalies conversely during a negative ao phase slp is above normal over the arctic most prominent in the vicinity of the icelandic low ice motion tends to have an anticyclonic clockwise anomaly ice flow through fram strait is reduced and the beaufort gyre is stronger leading to enhanced ice transport from the western to the eastern arctic where ice thickens by ridging and rafting against the siberian coast the stronger beaufort gyre also sequesters and thickens ice in the canada basin collectively these processes favor survival of sea ice through summer

nares strait hydrography and salinity field from a 3 year moored array 1 nares strait to the west of greenland facilitates the exchange of heat and freshwater between the arctic and atlantic oceans this study focuses on salinity temperature and density measurements from nares strait from a mooring array deployed from 2003 to 2006 innovative moorings requiring novel analysis methods measured seawater properties near 80 5 n at spacing sufficient to resolve the internal rossby deformation radius the 3 year mean geostrophic velocity has a surface intensified southward flow of 0 20 m s 1 against the western side of the strait and a secondary core flowing southward at 0 14 m s 1 in the middle of the strait data show warm salty water on the greenland side and cold fresher water on the ellesmere island side especially in the top layers there was a clear difference in hydrographic structure between times when sea ice was drifting and when it was land fast ice was drifting in late summer fall and early winter with a strong surface intensified geostrophic flow in the middle of the strait ice was land fast in late winter spring and early summer when there was a subsurface core of strong geostrophic flow adjacent to the western side of the strait salinity variations of about 2 psu in time and space reflect a variable freshwater outflow from the arctic ocean one particularly strong pulse occurred at the end of july 2005 for several days steeply sloping isohalines indicated strong geostrophic flow down the middle of the strait coinciding with an amplified ice export from the arctic due to strong southward winds

toward a warmer arctic ocean spreading of the early 21st century atlantic water warm anomaly along the eurasian basin margins we document through the analysis of 2002 2005 observational data the recent atlantic water aw warming along the siberian continental margin due to several aw warm impulses that penetrated into the arctic ocean through fram strait in 1999 2000 the aw temperature record from our long term monitoring site in the northern laptev sea shows several events of rapid aw temperature increase totaling 0 8 c in february august 2004 we hypothesize the along margin spreading of this warmer anomaly has disrupted the downstream thermal equilibrium of the late 1990s to earlier 2000s the anomaly mean velocity of 2 4 2 5 0 2 cm s was obtained on the basis of travel time required between the northern laptev sea and two anomaly fronts delineated over the eurasian flank of the lomonosov ridge by comparing the 2005 snapshot along margin data with the aw pre 1990 mean the magnitude of delineated anomalies exceeds the level of pre 1990 mean along margin cooling and rises above the level of noise attributed to shifting of the aw jet across the basin margins the anomaly mean velocity estimation is confirmed by comparing mooring derived aw temperature time series from 2002 to 2005 with the downstream along margin aw temperature distribution from 2005 our mooring current meter data corroborate these estimations

differentiating between first year and multiyear sea ice in the arctic using microwave retrieved ice emissivities polarized emissivities of the sea ice over the arctic were retrieved at advanced microwave scanning radiometer eos amsr e 10 65 18 7 23 8 and 36 5 ghz channel frequencies results indicate that retrieved emissivities are consistent with other emissivity estimates however errors in the retrieved emissivity for multi year sea ice at 23 8 and 36 5 ghz can be large up to 8 and 20 respectively because of ignoring the freeboard ice scattering and the use of the same emission layer as in 6 925 ghz it is shown that the emissivity slope for first year ice between 10 65 and 18 7 ghz is opposite to that for multiyear sea ice enabling a distinction between first year ice and multiyear ice using these differences in spectral features with ice types an evd vertically polarized emissivity difference between 10 65 and 18 7 ghz was devised to differentiate between first year sea ice and multiyear sea ice a comparison with the ice status information obtained from cold regions research and engineering laboratory crrel buoy measurements demonstrates that the method can separate first year ice from multiyear ice implying that this technique enables us to obtain instantaneous and pixel level ice type information from space based passive microwave measurements

remapping the thickness distribution in sea ice models in sea ice models with multiple thickness categories the ice thickness distribution evolves in time the evolution of the thickness distribution as ice grows and melts is analogous to one dimensional fluid transport and can be treated by similar numerical methods one such method remapping is applied here thickness categories are represented as lagrangian grid cells whose boundaries are projected forward in time the thickness distribution is approximated as a linear or quadratic polynomial in each displaced category and ice area and volume are transferred between categories so as to restore the original boundaries in simple test problems and in a single column model with forcing typical of the central arctic remapping performs significantly better than methods previously used in sea ice models it is less diffusive than a scheme that fixes the ice thickness in each category and behaves better numerically than a scheme that represents the thickness distribution as a set of delta functions also remapping converges faster i e with fewer thickness categories than the alternative schemes with five to seven categories the errors due to finite resolution of the thickness distribution are much smaller than the errors due to other sources linear remapping performs as well as the more complex quadratic version and is recommended for climate modeling its computational cost is minimal compared to other sea ice model components

characteristics of arctic sea ice thickness variability in gcms abstractskillful arctic sea ice forecasts may be possible for lead times of months or even years owing to the persistence of thickness anomalies in this study sea ice thickness variability is characterized in fully coupled gcms and sea ice ocean only models ioms that are forced with an estimate of observations derived from atmospheric reanalysis and satellite measurements overall variance in sea ice thickness is greatest along arctic ocean coastlines sea ice thickness anomalies have a typical time scale of about 6 20 months a time scale that lengthens about a season when accounting for ice transport and a typical length scale of about 500 1000 km the range of these scales across gcms implies that an estimate of the number of thickness monitoring locations needed to characterize the full arctic basin sea ice thickness variability field is model dependent and would vary between 3 and 14 models with a thinner mean ice state tend to have ice thickness anomalies that are generally shorter lived and s

beaufort gyre freshwater reservoir state and variability from observations 1 we investigate basin scale mechanisms regulating anomalies in freshwater content fwc in the beaufort gyre bg of the arctic ocean using historical observations and data collected in 2003 2007 specifically the mean annual cycle and interannual and decadal fwc variability are explored the major cause of the large fwc in the bg is the process of ekman pumping ep due to the arctic high anticyclonic circulation centered in the bg the mean seasonal cycle of liquid fwc is a result of interplay between the mechanical ep and thermal ice transformations factors and has two peaks one peak occurs around june july when the sea ice thickness reaches its minimum maximum ice melt the second maximum is observed in november january when wind curl is strongest maximum ep and the salt input from the growing ice has not yet reached its maximum interannual changes in fwc during 2003 2007 are characterized by a strong positive trend in the region varying by location with a maximum of approximately 170 cm a 1 in the center of ep influenced region decadal fwc variability in the period 1950 2000 is dominated by a significant change in the 1990s forced by an atmospheric circulation regime change the center of maximum fwc shifted to the southeast and appeared to contract in area relative to the pre 1990s climatology in spite of the areal reduction the spatially integrated fwc increased by over 1000 km 3 relative to climatology

the role of ocean atmosphere coupling in the zonal mean atmospheric response to arctic sea ice loss abstractthe role of ocean atmosphere coupling in the zonal mean climate response to projected late twenty first century arctic sea ice loss is investigated using community climate system model version 4 ccsm4 at 1 spatial resolution parallel experiments with different ocean model configurations full depth slab and no interactive ocean allow the roles of dynamical and thermodynamic ocean feedbacks to be isolated in the absence of ocean coupling the atmospheric response to arctic sea ice loss is confined to north of 30 n consisting of a weakening and equatorward shift of the westerlies accompanied by lower tropospheric warming and enhanced precipitation at high latitudes with ocean feedbacks the response expands to cover the whole globe and exhibits a high degree of equatorial symmetry the entire troposphere warms the global hydrological cycle strengthens and the intertropical convergence zones shift equatorward ocean dynamics are fundamental to producing this equatorially symmetric pattern

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

influence of the sea ice thickness distribution on polar climate in ccsm3 abstract the sea ice simulation of the community climate system model version 3 ccsm3 t42 gx1 and t85 gx1 control simulations is presented and the influence of the parameterized sea ice thickness distribution itd on polar climate conditions is examined this includes an analysis of the change in mean climate conditions and simulated sea ice feedbacks when an itd is included it is found that including a representation of the subgrid scale itd results in larger ice growth rates and thicker sea ice these larger growth rates represent a higher heat loss from the ocean ice column to the atmosphere resulting in warmer surface conditions ocean circulation most notably in the southern hemisphere is also modified by the itd because of the influence of enhanced high latitude ice formation on the ocean buoyancy flux and resulting deep water formation changes in atmospheric circulation also result again most notably in the southern hemisphere there are indications that the itd also modifies simulated sea

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

last millennium climate and its variability in ccsm4 abstractan overview of a simulation referred to as the last millennium lm simulation of the community climate system model version 4 ccsm4 is presented the ccsm4 lm simulation reproduces many large scale climate patterns suggested by historical and proxy data records with northern hemisphere nh and southern hemisphere sh surface temperatures cooling to the early 1800s common era by 0 5 c nh and 0 3 c sh followed by warming to the present high latitudes of both hemispheres show polar amplification of the cooling from the medieval climate anomaly mca to the little ice age lia associated with sea ice increases the lm simulation does not reproduce la nina like cooling in the eastern pacific ocean during the mca relative to the lia as has been suggested by proxy reconstructions still dry medieval conditions over the southwestern and central united states are simulated in agreement with proxy indicators for these regions strong global cooling is associated with large volcanic erup

late twentieth century simulation of arctic sea ice and ocean properties in the ccsm4 abstractto establish how well the new community climate system model version 4 ccsm4 simulates the properties of the arctic sea ice and ocean results from six ccsm4 twentieth century ensemble simulations are compared here with the available data it is found that the ccsm4 simulations capture most of the important climatological features of the arctic sea ice and ocean state well among them the sea ice thickness distribution fraction of multiyear sea ice and sea ice edge the strongest bias exists in the simulated spring to fall sea ice motion field the location of the beaufort gyre and the temperature of the deep arctic ocean below 250 m which are caused by deficiencies in the simulation of the arctic sea level pressure field and the lack of deep water formation on the arctic shelves the observed decrease in the sea ice extent and the multiyear ice cover is well captured by the ccsm4 it is important to note however that the temporal evolution of the simulated arctic sea ice cover over the

amplified north atlantic warming in the late pliocene by changes in arctic gateways under previous reconstructions of late pliocene boundary conditions climate models have failed to reproduce the warm sea surface temperatures reconstructed in the north atlantic using a reconstruction of mid piacenzian paleogeography that has the bering strait and canadian arctic archipelago straits closed however improves the simulation of the proxy indicated warm sea surface temperatures in the north atlantic in the community climate system model we find that the closure of these small arctic gateways strengthens the atlantic meridional overturning circulation by inhibiting freshwater transport from the pacific to the arctic ocean and from the arctic ocean to the labrador sea leading to warmer sea surface temperatures in the north atlantic this indicates that the state of the arctic gateways may influence the sensitivity of the north atlantic climate in complex ways and better understanding of the state of these arctic gateways for past time periods are needed

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

does ocean coupling matter for the northern extratropical response to projected arctic sea ice loss the question of whether ocean coupling matters for the extratropical northern hemisphere atmospheric response to projected late 21st century arctic sea ice loss is addressed using a series of experiments with community climate system model version 4 at 1 spatial resolution under different configurations of the ocean model component no interactive ocean thermodynamic slab ocean and full depth dynamic plus thermodynamic ocean ocean atmosphere coupling magnifies the response to arctic sea ice loss but does not change its overall structure however a slab ocean is inadequate for inferring the role of oceanic feedbacks the westerly winds along the poleward flank of the eddy driven jet weaken in response to arctic sea ice loss accompanied by a smaller magnitude strengthening on the equatorward side with largest amplitudes in winter dynamical and thermodynamic oceanic feedbacks amplify this response by approximately 50 air temperature precipitation and sea level pressure responses also show sensitivity to the degree of ocean coupling

forearc structure beneath southwestern british columbia a three dimensional tomographic velocity model 1 this paper presents a three dimensional compressional wave velocity model of the forearc crust and upper mantle and the subducting juan de fuca plate beneath southwestern british columbia and the adjoining straits of georgia and juan de fuca the velocity model was constructed through joint tomographic inversion of 50 000 first arrival times from earthquakes and active seismic sources wrangellia rocks of the accreted paleozoic and mesozoic island arc assemblage underlying southern vancouver island in the cascadia forearc are imaged at some locations with higher than average lower crustal velocities of 6 5 7 2 km s similar to observations at other island arc terranes the mafic eocene crescent terrane thrust landward beneath southern vancouver island exhibits crustal velocities in the range of 6 0 6 7 km s and is inferred to extend to a depth of more than 20 km the cenozoic olympic subduction complex an accretionary prism thrust beneath the crescent terrane in the olympic peninsula is imaged as a low velocity wedge to depths of at least 20 km three zones with velocities of 7 0 7 5 km s inferred to be mafic and or ultramafic units lie above the subducting juan de fuca plate at depths of 25 35 km the forearc upper mantle wedge beneath southeastern vancouver island and the strait of georgia exhibits low velocities of 7 2 7 5 km s inferred to correspond to 20 serpentinization of mantle peridotites and consistent with similar observations in other warm subduction zones estimated dip of the juan de fuca plate beneath southern vancouver island is 11 16 and 27 at depths of 30 40 and 50 km respectively

recent changes in arctic sea ice melt onset freezeup and melt season length 1 in order to explore changes and trends in the timing of arctic sea ice melt onset and freezeup and therefore melt season length we developed a method that obtains this information directly from satellite passive microwave data creating a consistent data set from 1979 through present we furthermore distinguish between early melt the first day of the year when melt is detected and the first day of continuous melt a similar distinction is made for the freezeup using this method we analyze trends in melt onset and freezeup for 10 different arctic regions in all regions except for the sea of okhotsk which shows a very slight and statistically insignificant positive trend 0 4 d decade 1 trends in melt onset are negative i e toward earlier melt the trends range from 1 0 d decade 1 for the bering sea to 7 3 d decade 1 for the east greenland sea except for the sea of okhotsk all areas also show a trend toward later autumn freeze onset the chukchi beaufort seas and laptev east siberian seas observe the strongest trends with 7 d decade 1 for the entire arctic the melt season length has increased by about 20 days over the last 30 years largest trends of over 10 d decade 1 are seen for hudson bay the east greenland sea the laptev east siberian seas and the chukchi beaufort seas those trends are statistically significant at the 99 level

a comparison of the receiver structure beneath stations of the canadian national seismograph network three component broadband data from the recently deployed canadian national seismograph network provide a new opportunity to examine the structure of the crust and upper mantle beneath the canadian landmass receiver function analysis is an ideal method to use with this data set as it can provide constraints on the s velocity structure beneath each station of this seismograph network this analysis method is particularly useful in that it provides site specific information i e within 5 15 km of the station low velocity layers can be identified and it is possible to examine structure to upper mantle depths in this study receiver functions were computed for each of the 19 stations that made up the seismograph network in june 1994 five stations sampling a variety of tectonic environments including the appalachian orogen the canadian shield the western canadian sedimentary basin and the cascadia subduction zone were chosen for detailed modelling the results presented here are the first estimates of the s velocity structure beneath these five stations for those stations where comparisons can be made with seismic reflection and refraction results there is excellent agreement in eastern canada simple receiver functions and clear moho ps conversions at most stations indicate a relatively transparent crust and a moho depth of 40 45 km in northwestern canada moho ps phases indicate a crustal thickness of 33 38 km beneath inuvik northwest territories the moho is interpreted as two velocity steps separated in depth by 5 km and an upper mantle low velocity zone is near 47 km depth in western canada the data indicate a mid crustal low velocity zone beneath edmonton the moho of the subducting juan de fuca plate is interpreted at 52 krn depth beneath southern vancouver island several stations exhibiting complex receiver functions warrant further study they include stations at scheffewille quebec in the canadian shield deer lake newfoundland on the boundary of the grenville province and the appalachian orogen and yellowknife northwest territories at the intersection of the churchill and slave provinces and the western plains

model of the transient temperature field of the baikal rift lithosphere abstract in order to coordinate available geophysical data on the deep seated structure with observed heat flow it has to be borne in mind that the thermal field of the baikal rift zone is not in a steady state a two dimensional model of the transient temperature field of the lithosphere was constructed by a numerical method for the solution of the energy conservation equation the authors suggest that asthenospheric upwelling 250 km wide was formed on account of roof block detachment such a mechanical replacement of the lithosphere by the asthenosphere was simulated by a positive compulsory upward movement of the 1200 c isotherm with constant velocity the duration of this movement was taken as long as 30 m y which corresponds to the duration of the first slow rifting stage as soon as the asthenosphere had reached the mohorovicic discontinuity its ascending movement on a wide scale ceased because the density of the crust was less than that of the asthenosphere temperature calculations were carried out in the lithosphere above the 1200 c isotherm this model permits us to reproduce theoretically the present value of the regional heat flow and the curie depth when the time passed from the cessation of the upwelling top movement became equal to 3 5 m y the last figure corresponds to the duration of the second fast stage of rifting on the regional scale the crust of the rift zone sayan baikal domal uplift continues to be heated cooling of the crust can only occur at the segments corresponding to the local heat flow anomalies which are suggested as being produced by dikes the intrusion time of the dike beneath the southern baikal is estimated to be 3 m y

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

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

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

thinning of the arctic sea ice cover comparison of sea ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1 3 m in most of the deep water portion of the arctic ocean from 3 1 m in 1958 1976 to 1 8 m in the 1990s the decrease is greater in the central and eastern arctic than in the beaufort and chukchi seas preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s

arctic warming through the fram strait oceanic heat transport from 3 years of measurements we present estimates of volume and heat transport through fram strait for the period 1997 to 2000 from data of moored instruments full depth volume transports at 78 55 n were in the order of 10 sv both northwards and southwards with an annual mean net transport between 2 and 4 sv to the south the temperature of the northward flow of atlantic water had a strong seasonality with a minimum in winter nevertheless the northward heat transport was highest in winter caused by the winter maximum of northward volume transport during the three years of observation the heat transport in the west spitsbergen current increased from 28 to 46 tw as a result of both increased speed and temperature in contrast to the west spitsbergen current the volume and heat transport of east greenland current remained fairly constant an integration over a subsection of the east greenland current showed similar values of volume transport to that obtained by measurements in the 1980s foldvik et al 1988 the southward heat transport through modified atlantic water weakened slightly between 1997 and 1999 despite increased temperatures

an observational estimate of volume and freshwater flux leaving the arctic ocean through nares strait abstract the arctic ocean is an important link in the global hydrological cycle storing freshwater and releasing it to the north atlantic ocean in a variable fashion as pack ice and freshened seawater an unknown fraction of this return flow passes through nares strait between northern canada and greenland surveys of ocean current and salinity in nares strait were completed in the summer of 2003 high resolution data acquired by ship based acoustic doppler current profiler and via hydrographic casts revealed subtidal volume and freshwater fluxes of 0 8 0 3 sv and 25 12 msv sv 103 msv 106 m3 s 1 respectively the observations resolved the dominant spatial scale of variability the internal rossby radius of deformation ld 9 km and revealed a complex yet coherent along channel flow with a rossby number of about 0 13 close to geostrophic balance approximately one third of the total volume flux was associated with across channel slope of the sea surface and two thirds 68 with across cha

circumpolar thinning of arctic sea ice following the 2007 record ice extent minimum september 2007 marked a record minimum in sea ice extent while there have been many studies published recently describing the minimum and its causes little is known about how the ice thickness has changed in the run up to and following the summer of 2007 using satellite radar altimetry data covering the arctic ocean up to 81 5 degrees north we show that the average winter sea ice thickness anomaly after the melt season of 2007 was 0 26 m below the 2002 2003 to 2007 2008 average more strikingly the western arctic anomaly was 0 49 m below the six year mean in the winter of 2007 2008 these results show no evidence of short term preconditioning through ice thinning between 2002 and 2007 but show that after the record minimum ice extent in 2007 the average ice thickness was reduced particularly in the western arctic citation giles k a s w laxon and a l ridout 2008 circumpolar thinning of arctic sea ice following the 2007 record ice extent minimum geophys res lett 35 l22502 doi 10 1029 2008gl035710

a younger thinner arctic ice cover increased potential for rapid extensive sea ice loss 1 satellite derived estimates of sea ice age and thickness are combined to produce a proxy ice thickness record for 1982 to the present these data show that in addition to the well documented loss of perennial ice cover as a whole the amount of oldest and thickest ice within the remaining multiyear ice pack has declined significantly the oldest ice types have essentially disappeared and 58 of the multiyear ice now consists of relatively young 2 and 3 year old ice compared to 35 in the mid 1980s ice coverage in summer 2007 reached a record minimum with ice extent declining by 42 compared to conditions in the 1980s the much reduced extent of the oldest and thickest ice in combination with other factors such as ice transport that assist the ice albedo feedback by exposing more open water help explain this large and abrupt ice loss

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

implications of arctic sea ice changes for north atlantic deep convection and the meridional overturning circulation in ccsm4 cmip5 simulations 1 using ccsm4 climate simulations for 1850 2300 with four different future forcing scenarios we show that the maximum strength of the atlantic meridional overturning circulation moc decreases proportionally to the applied co2 forcing this weakening of the overturning is caused by a reduction or shut down of north atlantic na deep convection due to a surface freshening in the labrador sea the surface freshening is caused by strongly increased liquid freshwater exports from the arctic which are largely due to the decrease in the arctic sea ice cover in the strongest forcing scenario rcp8 5 the arctic becomes summer ice free by the end of the 21st century and year round ice free by the end of the 23rd century as a result of the associated freshening all na deep convection ceases by 2145 which leads to a 72 18 sv decrease of the moc strength by the end of the simulation in 2300

snow cover on arctic sea ice in observations and an earth system model spring snow depth on arctic sea ice is assessed in observations and an earth system model in both snow depth is mainly driven by the ice type on which the snow lies snow depths on multiyear ice are approximately double snow depths on first year ice in the model spring snow depths are greater on multiyear ice compared to first year ice primarily because multiyear ice begins to accumulate snow earlier in the fall but also due to snow cover survival through the summer on multiyear ice the model simulates a decline in spring snow depth driven mainly by a transition from a multiyear to first year ice regime throughout the arctic agreement in the loss of multiyear ice in the model with estimates from observations suggests that declining snow depths in observations mainly result from the loss of multiyear sea ice

a 600 ka arctic sea ice record from mendeleev ridge based on ostracodes arctic paleoceanography and sea ice history were reconstructed from epipelagic and benthic ostracodes from a sediment core hly0503 06jpc 800 m water depth located on the mendeleev ridge western arctic ocean the calcareous microfaunal record ostracodes and foraminifers covers several glacial interglacial cycles back to estimated marine isotope stage 13 mis 13 500 ka with an average sedimentation rate of 0 5 cm ka for most of the stratigraphy mis 5 13 results based on ostracode assemblages and an unusual planktic foraminiferal assemblage in mis 11 dominated by a temperate water species turborotalita egelida show that extreme interglacial warmth high surface ocean productivity and possibly open ocean convection characterized mis 11 and mis 13 400 and 500 ka respectively a major shift in western arctic ocean environments toward perennial sea ice occurred after mis 11 based on the distribution of an ice dwelling ostracode acetabulastoma arcticum spectral analyses of the ostracode assemblages indicate sea ice and mid depth ocean circulation in western arctic ocean varied primarily at precessional 22 ka and obliquity 40 ka frequencies

patterns of sea ice retreat in the transition to a seasonally ice free arctic abstractthe patterns of sea ice retreat in the arctic ocean are investigated using two global climate models gcms that have profound differences in their large scale mean winter atmospheric circulation and sea ice drift patterns the community earth system model large ensemble cesm le presents a mean sea level pressure pattern that is in general agreement with observations for the late twentieth century the community climate system model version 4 ccsm4 exhibits a low bias in its mean sea level pressure over the arctic region with a deeper icelandic low a dynamical mechanism is presented in which large scale mean winter atmospheric circulation has significant effect on the following september sea ice extent anomaly by influencing ice divergence in specific areas a lagrangian model is used to backtrack the 80 n line from the approximate time of the melt onset to its prior positions throughout the previous winter and quantify the divergence across the pacific and eurasian sectors of the arctic i

icesat observations of arctic sea ice a first look 1 analysis of near coincident icesat and radarsat imagery shows that the retrieved elevations from the laser altimeter are sensitive to new openings containing thin ice or open water in the sea ice cover as well as to surface relief of old and first year ice the precision of the elevation estimates measured over relatively flat sea ice is 2 cm using the thickness of thin ice in recent openings to estimate sea level references we obtain the sea ice freeboard along the altimeter tracks this step is necessitated by the large uncertainties in the sea surface topography compared to that required for accurate determination of freeboard unknown snow depth introduces the largest uncertainty in the conversion of freeboard to ice thickness surface roughness is also derived for the first time from the variability of successive elevation estimates along the altimeter track overall these icesat measurements provide an unprecedented view of the arctic ocean ice cover at length scales at and above the spatial dimension of the altimeter footprint of 70 m

the influence of sea ice on ocean heat uptake in response to increasing co2 two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing co2 are investigated with community climate system model version 3 ccsm3 a deep warming below 500 m and extending down several kilometers in the southern ocean and warming in a 200 m layer just below the surface in the arctic ocean ocean heat uptake caused by sea ice retreat is isolated by running the model with the sea ice albedo reduced artificially alone this integration has a climate response with strong ocean heat uptake in the southern ocean and modest ocean heat uptake in the subsurface arctic ocean the arctic ocean warming results from enhanced ocean heat transport from the northern north atlantic at the time of co2 doubling about 1 3 of the heat transport anomaly results from advection of anomalously warm water and 2 3 results from strengthened inflow at the same time the overturning circulation is strengthened in the northern north atlantic and arctic oceans wind stress changes cannot explain the circulation changes which instead appear related to strengthened convection along the siberian shelves deep ocean warming in the southern ocean is initiated by weakened convection which is mainly a result of surface freshening through altered sea ice and ocean freshwater transport below about 500 m changes in convection reduce the vertical and meridional temperature gradients in the southern ocean which significantly reduce isopycnal diffusion of heat upward around antarctica the geometry of the sea ice cover and its influence on convection have a strong influence on ocean temperature gradients making sea ice an important player in deep ocean heat uptake in the southern ocean

one more step toward a warmer arctic this study was motivated by a strong warming signal seen in mooring based and oceanographic survey data collected in 2004 in the eurasian basin of the arctic ocean the source of this and earlier arctic ocean changes lies in interactions between polar and sub polar basins evidence suggests such changes are abrupt or pulse like taking the form of propagating anomalies that can be traced to higher latitudes for example an anomaly found in 2004 in the eastern eurasian basin took 1 5 years to propagate from the norwegian sea to the fram strait region and additional 4 5 5 years to reach the laptev sea slope while the causes of the observed changes will require further investigation our conclusions are consistent with prevailing ideas suggesting the arctic ocean is in transition towards a new warmer state

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position

reduced ice thickness in arctic transpolar drift favors rapid ice retreat 1 helicopter borne electromagnetic sea ice thickness measurements were performed over the transpolar drift in late summers of 2001 2004 and 2007 continuing ground based measurements since 1991 these show an ongoing reduction of modal and mean ice thicknesses in the region of the north pole of up to 53 and 44 respectively since 2001 a buoy derived ice age model showed that the thinning was mainly due to a regime shift from predominantly multi and second year ice in earlier years to first year ice in 2007 which had modal and mean summer thicknesses of 0 9 and 1 27 m measurements of second year ice which still persisted at the north pole in april 2007 indicate a reduction of late summer second year modal and mean ice thicknesses since 2001 of 20 and 25 to 1 65 and 1 81 m respectively the regime shift to younger and thinner ice could soon result in an ice free north pole during summer