spatial variation of radar derived basal conditions on kamb ice stream west antarctica radar profiles of bed echo intensity can survey conditions at the ice bed interface and test for the presence or absence of water however extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice we used the relationship between bed echo intensities from constant offset radar data and ice thickness to estimate depth averaged attenuation rates at several locations on and near kamb ice stream kis west antarctica we found values varying from 29 db km 1 at siple dome to 15 db km 1 in the main trunk region of kis in agreement with a previous measurement and models using these attenuation rate values we calculated the relative bed reflectivity throughout our kis surveys and found that most of the bed in the trunk has high basal reflectivities similar to those obtained in the location of boreholes that found water at the bed areas of lower bed reflectivity are limited to the sticky spot where a borehole found a dry bed and along the margins of kis these results support previous hypotheses that the basal conditions at locations like the sticky spot on kis control its stagnation and possible reactivation

eddy formation near the west coast of greenland abstract this paper extends a bracco and j pedlosky s investigation of the eddy formation mechanism in the eastern labrador sea by including a more realistic depiction of the boundary current the quasigeostrophic model consists of a meridional coastally trapped current with three vertical layers the current configuration and topographic domain are chosen to match as closely as possible the observations of the boundary current and the varying topographic slope along the west greenland coast the role played by the bottom intensified component of the boundary current on the formation of the labrador sea irminger rings is explored consistent with the earlier study a short localized bottom trapped wave is responsible for most of the perturbation energy growth however for the instability to occur in the three layer model the deepest component of the boundary current must be sufficiently strong highlighting the importance of the near bottom flow the model is able to reproduce important features o

eddies in the labrador sea as observed by profiling rafos floats and remote sensing abstract data from profiling rafos floats topex poseidon altimetry and the alongtrack scanning radiometer atsr aboard ers 1 have been used to describe the spatial and seasonal patterns of eddy variability in the labrador sea peaks in sea surface height ssh variability appear in two regions off the west greenland shelf near 61 5 n 52 w where the 3000 m isobath separates from the shelf and in the center of the basin at 58 n 52 w both locations show seasonal ranges in ssh variability of up to 40 mm with the greenland site having largest variability in january march leading the central site by 50 days a sea surface temperature image from the atsr at the greenland site shows numerous eddies both cyclonic and anticyclonic being formed by injection of west greenland current water into the labrador sea interior data from profiling rafos floats launched in 1997 as part of the labrador sea deep convection experiment are used to describe three of the west greenland current eddies in detail one of

on the use of combined geophysical methods to assess water content and water conductivity of near surface formations abstract we propose to deepen the interpretation of combined geophysical methods georadar seismic and electric to assess physical properties characterizing the near surface porous formations especially the influence of water velocity analysis of multioffset georadar data are used together with seismic methods to estimate lateral and vertical ground water fluctuations this enables us to identify transitions from non saturated to fully saturated porous layers with certainty furthermore the accurate knowledge of seismic velocities helps to estimate the porosity of the ground water formations finally we show how the radar technique may be useful in solving the problem of trade off between bulk resistivity which is deduced from electrical measurements and that of water ionic conductivity and water content these theoretical considerations are illustrated using various measurements conducted at the same test site our interpretation is compared with a few laboratory measurements on water and soil samples this study displays the impact of combined geophysical approaches for providing models of water and ionic transfers down to a depth of several metres

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

frequency response properties of forced climatic sst anomaly variability in the north atlantic abstract frequency response properties of north atlantic 5 57 n sea surface temperature anomaly tsa variability with periods of several months to 20 years are characterized using the comprehensive ocean atmosphere data set coads significant direct forcing of tsa variability by the anomalous wind field primarily through the resulting anomalous surface turbulent heat flux is observed in the westerly wind and trade wind belts to characterize properties of the large scale climatic tsa response to this forcing over the entire frequency band resolved it is necessary to consider the dual role of anomalous surface heat flux as both the dominant local forcing mechanism and the dominant damping mechanism the latter through a negative linear feedback newtonian relaxation at frequencies where wind forcing is important good agreement exists between the frequency response function estimated from data and the same function theoretically predicted by a simple stochastic forcing model where the locally fo

restratification after deep convection abstract an important yet poorly understood aspect of the water mass transformation process in the ocean is the manner in which the convected fluid once formed is accommodated and drawn into the general circulation following violent mixing in the open ocean that creates a deep homogenous body of fluid restratification of the surface 500 m layer is observed to occur rapidly sealing over the convection patch recent hydrographic casts and tomography inversions in the gulf of lions by send et al for example show that very quickly within a week or so of the cessation of cooling a stratified near surface layer develops on top of the mixed patch this restratification occurs much more rapidly than can be accounted for by air sea fluxes by analytical and numerical study the authors argue that advection by geostrophic eddies spawned by the baroclinic instability of the mixed patch is likely to be a principal mechanism by which restratification occurs a restratification timescale restrat 56r

surprising return of deep convection to the subpolar north atlantic ocean in winter 2007 2008 in the process of open ocean convection in the subpolar north atlantic ocean surface water sinks to depth as a distinct water mass the characteristics of which affect the meridional overturning circulation and oceanic heat flux in addition carbon is sequestered from the atmosphere in the process in recent years this convection has been shallow or non existent which could be construed as a consequence of a warmer climate here we document the return of deep convection to the subpolar gyre in both the labrador and irminger seas in the winter of 2007 2008 we use profiling float data from the argo programme to document deep mixing analysis of a variety of in situ satellite and reanalysis data shows that contrary to expectations the transition to a convective state took place abruptly without going through a phase of preconditioning changes in hemispheric air temperature storm tracks the flux of fresh water to the labrador sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air making the surface water sufficiently cold and dense to initiate deep convection given this complexity we conclude that it will be difficult to predict when deep mixing may occur again

boundary current eddies and their role in the restratification of the labrador sea an idealized model is used to study the restratification of the labrador sea after deep convection with emphasis on the role of boundary current eddies shed near the west coast of greenland the boundary current eddies carry warm buoyant irminger current water into the labrador sea interior for a realistic end of winter state it is shown that these irminger current eddies are efficient in restratifying the convected water mass in the interior of the labrador sea in addition it is demonstrated that irminger current eddies can balance a significant portion of the atmospheric heat loss and thus play an important role for the watermass transformation in the labrador sea

integrating satellite observations with modelling basal shear stress of the filcher ronne ice streams antarctica using inverse methods constrained by recent satellite observations we have produced a comprehensive estimate of the basal shear stress beneath the filchner ronne ice streams the inversions indicate that a weak bed approx 4 20 kpa underlies much of these ice streams compared to the ross ice streams the distribution of weak subglacial till is more heterogeneous with sticky spots providing much of the resistance to flow a weak bed beneath recovery ice stream extends several hundred kilometres inland with flow along this ice stream discrepancies between thickness measurements and flux estimates suggest the existence of a deep 1400 m trough not resolved by existing maps of subglacial topography we hypothesize that the presence of this and other deep troughs is a major influence on this sector of the ice sheet that is not fully incorporated in current models of ice sheet evolution

dynamical response of the oceanic eddy field to the north atlantic oscillation a model data comparison abstract observational studies have shown that in many regions of the world ocean the eddy kinetic energy eke significantly varies on interannual time scales comparing altimeter derived eke maps for 1993 and 1996 stammer and wunsch have mentioned a significant meridional redistribution of eke in the north atlantic ocean and suggested the possible influence of the north atlantic oscillation nao cycle this hypothesis is examined using 7 yr of ocean topography experiment topex poseidon altimeter data and three resolution atlantic ocean model simulations performed over the period 1979 2000 during the french clipper experiment the subpolar subtropical meridional contrast of eke in the real ocean appears to vary on interannual time scales and the model reproduces it realistically the nao cycle forces the meridional contrast of energy input by the wind the analysis in this paper suggests that after 1993 the large amplitude of the nao cycle induces changes in the transport of the baroclinically

on the connection between dense water formation overturning and poleward heat transport in a convective basin an isopycnal two layer idealized model for a convective basin is proposed consisting of a convecting interior region and a surrounding boundary current buoyancy and wind driven parameterized eddy fluxes govern the exchange between the two to balance the interior buoyancy loss the boundary current becomes denser as it flows around the basin geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation the poleward heat transport associated with convection in the basin can thus be viewed as a result of both an overturning and a horizontal circulation when adapted to the labrador sea the model is able to reproduce the bulk features of the mean state the seasonal cycle and even the shutdown of convection from 1969 to 1972 according to the model only 40 of the poleward heat buoyancy transport of the labrador sea is associated with the overturning circulation an exact solution is presented for the linearized equations when changes in the boundary current are small numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin these results highlight how the overturning circulation is not simply related to the amount of dense water formed a speeding up of the circulation around the basin due to wind forcing for example will decrease the intensity of the overturning circulation while the dense water formation remains unvaried in general it is shown that the fraction of poleward buoyancy or heat transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors

buoyant eddies entering the labrador sea observed with gliders and altimetry abstract intense buoyant anticyclonic eddies spawned from the west greenland boundary current were observed with high resolution autonomous seaglider hydrography and satellite altimetry as they entered the labrador sea interior surveys of their internal structure establish the transport of both low salinity water in the upper ocean and warm saline irminger water at depth the observed eddies can contribute significantly to the rapid restratification of the labrador sea interior following wintertime deep convection these eddies have saline cores between 200 and 1000 m low salinity cores above 200 m and a velocity field that penetrates to at least 1000 m with 0 1000 m average speeds exceeding 40 cm s 1 their trajectory together with earlier estimates of the gyre circulation suggests why the observed region of deep convection is so small and does not occur where wintertime cooling by the atmosphere is most intense the cyclostrophic surface velocity field of the anticylones from satellite altimetry

lateral heat exchange after the labrador sea deep convection in 2008 abstractthe mechanisms through which convected water restratifies in the labrador sea are still under debate the labrador sea restratification after deep convection in the 2007 08 winter is studied with an eddy resolving numerical model the modeled mixed layer depth during wintertime resembles the argo observed mixed layer very well and the lateral heat flux during the subsequent restratification is in line with observations the irminger rings irs are reproduced with fresher caps above the 300 m depths and they are identified and tracked automatically the model underestimates both the number of irs in the convection area and the heat they carry the underestimation is most likely caused by the errors in the direction of the west greenland currents in the model which causes more irs propagating westward and only the irs originating south of 61 5 n are able to propagate southward yet with speed much slower than observed speed the model still observed three eddies propagating into the convection

pattern formation at the ocean surface sargassum distribution and the role of the eddy field positively buoyant organisms such as the macroalga sargassum and the cyanobacterium trichodesmium often form surface accumulations visible in satellite imagery that have lateral scale separation of tens of kilometers and cannot be explained by langmuir circulation here we discuss the accumulation of floating materials in the ocean in presence of meso and submesoscale activity using high resolution simulations of the ocean mesoscale in both idealized a 3 dimensional box where coherent eddies are forced by small scale winds and realistic domains western gulf of mexico where extensive concentrations of floating sargassum have been recorded in satellite images we show that the distribution of tracers at the ocean surface departs rapidly from that observed a few tens of meters below it such distribution does not resemble that observed for passive tracers in quasi geostrophic turbulence the strong divergence and convergence zones generated at the surface by ageostrophic processes in the submesoscale range are responsible for the creation of areas where the floating material accumulates floating particles are expelled from the core of mesoscale eddies and concentrate in convergence regions in patterns comparable to the ones observed through the satellite images in light of those results sargassum and or trichodesmium may provide a useful proxy to track convergence divergence processes resulting from ageostrophic processes at the ocean surface

new 3d bathymetry and sediment distribution in lake vostok implication for pre glacial origin and numerical modeling of the internal processes within the lake abstract a new distribution of water and unconsolidated sediments in subglacial lake vostok east antarctica was developed via inversion of airborne gravity data constrained by 60 seismic soundings a model was developed for host rock with a density of 2550 kg m 3 that was inferred from prior 2d modeling our 3d bathymetry model of lake vostok corresponds better with seismic data rms of 125 m than two previous models based on the same gravity dataset the good match in both water and sediment thicknesses between the gravity model and seismic measurements confirms two major facts about lake vostok 1 the lake is hosted by sedimentary rocks and 2 the bottom of the lake is covered with a layer of unconsolidated sediments that does not exceed 300 m in the southern basin and thickens almost to 400 m in the northern basin our new bathymetry model suggests much shallower water thicknesses up to twice the previous estimates in the middle and northern parts of the lake while the water layer is thicker in the southern basin numerical modeling of the internal processes in the lake reveals the relevance of our new bathymetry model to the basal mass balance a significant decrease in transport is observed in the shallower northern basin as well as a decrease of 33 in the turbulent kinetic energy however only minor differences were observed in the distribution of the calculated freezing and melting zones compared to previous models estimates for the sedimentation rates for six possible mechanisms were made possible sedimentation mechanisms are 1 fluvial and periglacial i e those that are active prior to the establishment of a large subglacial lake 2 deposition due to overlying ice sheet including melting out of the ice as well as bulldozering by the overriding ice and 3 suspended sediments from subglacial water flow including those deposited by periodical subglacial outbursts the estimates for these mechanisms show that unconsolidated sediments of the observed thickness are most consistent with a lake that existed before glaciation

pitfalls in radar diagnosis of ice sheet bed conditions lessons from englacial attenuation models 1 radar power returned from ice sheet beds has been widely accepted as an indicator of bed conditions however the bed returned power also depends on englacial attenuation which is primarily a function of ice temperature here using a one dimensional attenuation model it is demonstrated that in most cases variations in bed returned power are dominated by variations in englacial attenuation rather than bed reflectivity both accumulation rate and geothermal flux anomalies can interfere with the interpretation with the consequence analytical radar algorithms that have been widely accepted likely yield false delineations of wet dry beds more careful consideration is needed when diagnosing bed conditions spatial patterns of shallow englacial radar reflectors can be used as a proxy for accumulation rates which affect ice temperature and thus returned power i argue that it is necessary to simultaneously interpret the returned power and englacial reflector patterns to improve the bed diagnosis

laboratory experiments on eddy generation by a buoyant coastal current flowing over variable bathymetry irminger rings are warm core eddies formed off the west coast of greenland recent studies suggest that these eddies which are implicated in the rapid springtime restratification of the labrador sea are formed by an internal instability of the west greenland current wgc triggered by bathymetric variations this study seeks to explore the effect of the magnitude and downstream length scale of bathymetric variations on the stability of a simple model of the wgc in a series of laboratory experiments in which a buoyant coastal current was allowed to flow over bathymetry consisting of piecewise constant slopes of varying magnitude the currents did not form eddies over gently sloping bathymetry and only formed eddies over steep bathymetry if the current width exceeded the width of the sloping bathymetry eddying currents were immediately stabilized if they flowed onto gently sloping topography bathymetric variations that persisted only a short distance downstream perturbed the flow locally but did not lead to eddy formation eddies formed only once the downstream length of the bathymetric variations exceeded a critical scale of about 8 rossby radii these results are consistent with the observed behavior of the wgc which begins to form irminger rings after entering a region where the continental slope abruptly steepens and becomes narrower than the wgc itself in a region spanning about 20 80 rossby radii of downstream distance

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

deep convection in the irminger sea forced by the greenland tip jet open ocean deep convection one of the processes by which deep waters of the world s oceans are formed is restricted to a small number of locations for example the mediterranean and labrador seas recently the southwest irminger sea has been suggested as an additional location for open ocean deep convection the deep water formed in the irminger sea has the characteristic temperature and salinity of the water mass that fills the mid depth north atlantic ocean which had been believed to be formed entirely in the labrador basin here we show that the most likely cause of the convection in the irminger sea is a low level atmospheric jet known as the greenland tip jet which forms periodically in the lee of cape farewell greenland and is associated with elevated heat flux and strong wind stress curl using a history of tip jet events derived from meteorological land station data and a regional oceanic numerical model we demonstrate that deep convection can occur in this region when the north atlantic oscillation index is high which is consistent with observations this mechanism of convection in the irminger sea differs significantly from those known to operate in the labrador and mediterranean seas

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

sources of eddy kinetic energy in the labrador sea experiments with a suite of north atlantic general circulation models are used to examine the sources of eddy kinetic energy eke in the labrador sea a high resolution model version 112 quantitatively reproduces the observed signature a particular feature of the eke in the labrador sea is its pronounced seasonal cycle with a maximum intensity in early winter as already found in earlier studies based on altimeter data in contrast to a previously advanced hypothesis the seasonally varying eddy field is not related to a forcing by high frequency wind variations but can be explained by a seasonally modulated instability of the west greenland current wgc the main source of eke in the labrador sea is an energy transfer due to reynolds interaction work barotropic instability in a confined region near cape desolation where the wgc adjusts to a change in the topographic slope geostrophic contours tend to converge upstream of cape desolation such that the topographically guided wgc narrows as well and becomes barotropically unstable the eddies spawned from the wgc instability area dominating the eke in the interior labrador sea are predominantly anticyclonic with warm and saline cores in the upper kilometer of the water column while the few cyclones originating as well from the instability area show a more depth independent structure companion experiments with a model exhibit the strength of the wgc influenced by either changes in the wind stress or heat flux forcing as a leading factor determining seasonal to interannual changes of eke in the labrador sea

environmental seismology what can we learn from ambient noise environmental seismology consists in studying the coupling between the solid earth and the cryosphere or the hydrosphere the anthropospher in practice we monitor the modifications of the wave propagation due to environmental forcing such as temperature and hydrology using ambient seismic noise that constitute a continuous cheap and relatively reproducible source of vibrations recent developments in data processing 1 together with increasing computational power and sensor concentration have led to original observations that allow for this new field of seismology in this paper we will review how we can track and interpret tiny changes in the subsurface of the earth related to external changes from modifications of the seismic wave propagation with application to geomechanics hydrology and natural hazard 2 we will demonstrate that using ambient noise we can track thermal variations in the subsoil in buildings or in rock columns with application to damage estimation the temporal and spa

the seasonality of convective events in the labrador sea abstractmodeling deep convection is a key challenge for climate science here two simulations of the labrador sea circulation obtained with the regional ocean modeling system roms run at a horizontal resolution of 7 5 km are used to characterize the response of convection to atmospheric forcing and its seasonal variability over the period 1980 2009 the integrations compare well with the sparse observations available the modeled convection varies in three key aspects over the 30 years considered first its magnitude changes greatly at decadal scales this aspect is supported by the in situ observations second the initiation and peak of convection i e initiation and maximum shift by 2 3 weeks between strong and weak convective years third the duration of convection varies by approximately one month between strong and weak years the last two changes are associated with the variability of the time integrated surface heat fluxes over the labrador sea during winter and spring while the first resu

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

east antarctic ice stream tributary underlain by major sedimentary basin marine and rift sediments exert a fundamental control on ice stream flow in the west antarctic ice sheet and hence on its mass balance and stability in contrast most ice streams in the much larger east antarctic ice sheet are thought to be relatively stable features resting on till perhaps underlain by crystalline rock any geological controls on east antarctic ice sheet enhanced flow remain largely unknown we present aerogeophysical evidence indicating that a region of enhanced ice flow in the interior of the east antarctic ice sheet is underlain by subglacial sediments 3 km thick and that these are influencing the flow regime of the overlying ice we show that subglacial sediments are important in modulating ice dynamics not just for the west antarctic ice sheet but also for its much larger neighbor and suggest that the sedimentary basin identified here may contain information on the neogene glacial history of this part of the east antarctic ice sheet

modeling of marine seismic profiles in the t x and τ p domains in many cases comparison of real data with synthetic seismograms provides additional constraints on the velocity depth profiles obtained with simple inversion techniques obtaining a satisfactory match between the real and computed data usually requires several trials with different models but can be performed rapidly if the theoretical seismograms are themselves easily interpretable i e if the major contributions which make up the synthetic traces can be identified and separated in horizontally stratified media modeling is further simplified and is faster if the simulation techniques are implemented in the domain of the intercept time and the ray parameter p the generalized reflection and transmission matrix method is well suited for these purposes and may be used to generate synthetic seismograms in both the t x and p planes computation of plane wave seismograms is straightforward and merely corresponds to an inverse fourier transform of the overall reflectivity matrix for each ray parameter

the nature of eddy kinetic energy in the labrador sea different types of mesoscale eddies their temporal variability and impact on deep convection abstractoceanic eddies are an important component in preconditioning the central labrador sea ls for deep convection and in restratifying the convected water this study investigates the differen

winter mixed layer development in the central irminger sea the effect of strong intermittent wind events abstract the impact of the greenland tip jet on the wintertime mixed layer of the southwest irminger sea is investigated using in situ moored profiler data and a variety of atmospheric datasets the mixed layer was observed to reach 400 m in the spring of 2003 and 300 m in the spring of 2004 both of these winters were mild and characterized by a low north atlantic oscillation nao index a typical tip jet event is associated with a low pressure system that is advected by upper level steering currents into the region east of cape farewell and interacts with the high topography of southern greenland heat flux time series for the mooring site were constructed that include the enhancing influence of the tip jet events this was used to force a one dimensional mixed layer model which was able to reproduce the observed envelope of mixed layer deepening in both winters the deeper mixed layer of the first winter was largely due to a higher number of robust tip jet events which in turn was caused by the stee

time lapse monitoring of rock properties with coda wave interferometry 1 the coda of seismic waves consists of that part of the signal after the directly arriving phases in a finite medium or in one that is strongly heterogeneous the coda is dominated by waves which have repeatedly sampled the medium small changes in a medium which may have no detectable influence on the first arrivals are amplified by this repeated sampling and may thus be detectable in the coda we refer to this use of multiple sampling coda waveforms as coda wave interferometry we have exploited ultrasonic coda waves to monitor time varying rock properties in a laboratory environment we have studied the dependence of velocity on uniaxial stress in berea sandstone the temperature dependence of velocity in granite and in aluminum and the change in velocity due to an increase of water saturation in sandstone there are many other possible applications of coda wave interferometry in geophysics including dam and volcano monitoring time lapse reservoir characterization earthquake relocation and stress monitoring in mining and rock physics

seasonal and interannual variations of irminger ring formation and boundary interior heat exchange in flame abstractthe contribution of warm core anticyclones shed by the irminger current off west greenland known as irminger rings to the restratification of the upper layers of the labrador sea is investigated in the 1 12 family of linked atlantic models experiment flame model the model output covering the 1990 2004 period shows strong similarities to observations of the irminger current as well as ring observations at a mooring located offshore of the eddy formation region in 2007 09 an analysis of fluxes in the model shows that while the majority of heat exchange with the interior indeed occurs at the site of the irminger current instability the contribution of the coherent irminger rings is modest 18 heat is provided to the convective region mainly through noncoherent anomalies and enhanced local mixing by the rings facilitating further exchange between the boundary and interior the time variability of the eddy kinetic energy and the boundary to interior heat flux in the model are strongly corr

recent surface temperature trends in the interior of east antarctica from borehole firn temperature measurements and geophysical inverse methods 1 we use measured firn temperatures down to depths of 80 to 90 m at four locations in the interior of dronning maud land east antarctica to derive surface temperature histories spanning the past few decades using two different inverse methods we find that the mean surface temperatures near the ice divide the highest elevation ridge of east antarctic ice sheet have increased approximately 1 to 1 5 k within the past 50 years although the onset and rate of this warming vary by site histories at two locations nus07 5 78 65 s 35 64 e and nus07 7 82 07 s 54 89 e suggest that the majority of this warming took place in the past one or two decades slight cooling to no change was indicated at one location nus08 5 82 63 s 17 87 e off the divide near the recovery lakes region in the most recent decade inversion results indicate both cooler and warmer periods at different sites due to high interannual variability and relatively high resolution of the inverted surface temperature histories the overall results of our analysis fit a pattern of recent climate trends emerging from several sources of the antarctic temperature reconstructions there is a contrast in surface temperature trends possibly related to altitude in this part of east antarctica

intrinsic and forced interannual variability of the gulf of alaska mesoscale circulation abstract the response of the gulf of alaska goa circulation to large scale north pacific climate variability is explored using three high resolution 15 km regional ocean model ensembles over the period 1950 2004 on interannual and decadal timescales the mean circulation is strongly modulated by changes in the large scale climate forcing associated with pdo and enso intensification of the model gyre scale circulation occurs after the 1976 1977 climate shift as well as during 1965 1970 and 1993 1995 from the model dynamical budgets we find that when the goa experiences stronger southeasterly winds typical during the positive phase of the pdo and enso there is net large scale ekman convergence in the central and eastern coastal boundary the geostrophic adjustment to higher sea surface height ssh and lower isopycnals lead to stronger cyclonic gyre scale circulation the opposite situation occurs during stronger northwesterly winds negative phase of the pdo along the eastern side of the goa basin interannual changes in the surface winds also modulate the seasonal development of high amplitude anticyclonic eddies e g haida and sitka eddies large interannual eddy events during winter spring are phase locked with the seasonal cycle the initial eddy dynamics are consistent with a quasi linear rossby wave response to positive ssh anomalies forced by stronger downwelling favorable winds e g southwesterly during el nino however because of the fast growth rate of baroclinic instability and the geographical focusing associated with the coastal geometry most of the perturbation energy in the rossby wave is locally trapped until converted into large scale nonlinear coherent eddies coastally trapped waves of tropical origin may also contribute to positive ssh anomalies that lead to higher amplitude eddies however their presence does not appear essential the model ensembles which do not include the effects of equatorial coastally trapped waves capture the large haida and sitka eddy events observed during 1982 and 1997 and explain between 40 and 70 of the tidal gauges variance along the goa coast in the western side of the goa basin interannual eddy variability located south of the alaskan stream is not correlated with large scale forcing and appears to be intrinsic a comparison of the three model ensembles forced by ncep winds and a multi century long integration forced only with the seasonal cycle shows that the internal variability alone explains most of the eddy variance the asymmetry between the eddy forced regime in the eastern basin and the intrinsic regime in the western basin has important implications for predicting the goa response to climate change if future climate change results in stronger wintertime winds and increased downwelling in the eastern basin then increased mesoscale activity perhaps more or larger eddies might occur in this region conversely the changes in the western basin are not predictable based on environmental forcing eastern eddies transport important biogeochemical quantities such as iron oxygen and chlorophyll a into the gyre interior therefore having potential upscale effects on the goa high nutrient low chlorophyll region

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

boundary currents and watermass transformation in marginal seas the properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed the net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cyclonically around the basin heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current the objectives of this study are to understand better what controls the density of waters formed within the marginal sea the temperature of the outflowing waters the amount of downwelling and the mechanisms of heat transport within the marginal sea the approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system the theory compares well to a series of eddy resolving primitive equation model calculations the downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy driven region on the offshore edge of the boundary current for most high latitude regions the horizontal gyre is expected to transport more heat than does the overturning gyre

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

hydrography of the labrador sea during active convection the hydrographic structure of the labrador sea during wintertime convection is described the cruise part of the deep convection experiment took place in february march 1997 amidst an extended period of strong forcing in an otherwise moderate winter because the water column was preconditioned by previous strong winters the limited forcing was enough to cause convection to approximately 1500 m the change in heat storage along a transbasin section relative to an occupation done the previous october gives an average heat loss that is consistent with calibrated national centers for environmental prediction surface heat fluxes over that time period 200 wm 22 deep overturning was observed both seaward of the western continental slope which was expected as well as within the western boundary current itself something that had not been directly observed previously these two geographical regions separated by roughly the 3000 m isobath produce separate water mass products the offshore water mass is the familiar cold fresh dense classical labrador sea water lsw the boundary current water mass is a somewhat warmer saltier lighter vintage of classical lsw though in the far field it would be difficult to distinguish these products the offshore product was formed within the cyclonic recirculating gyre measured by lavender et al in a region that is limited to the north most likely by an eddy flux of buoyant water from the eastern boundary current the velocity measurements taken during the cruise provide a transport estimate of the boundary current throughput and offshore recirculation finally the overall trends in stratification of the observed mixed layers are described

the interannual variability of the surface eddy kinetic energy in the labrador sea abstract the variability of the surface eddy kinetic energy eke in the labrador sea is investigated with a suite of numerical integrations using a regional ocean model simulations are performed over the period 1980 2001 and are compared to satellite observations over the last 9 years the surface eke pattern in the basin is dominated by a region along the west coast of greenland where eddies mainly anticyclonic are formed by instability of the main currents flowing over the continental slope consistent with previous idealized results here the interannual changes are linked to the shear of the incoming boundary current system imposed as boundary condition to the model domain the highly variable strength of the east greenland current at the northeast boundary derived from the simple ocean data assimilation soda reanalysis strongly influences the vortex formation in the center of the labrador sea where deep convection occurs a statistically significant portion of the modeled interannual surface eke variability is correlated with the local atmospheric forcing and both heat and wind fluxes play an important role and can be adopted as predictors at a lag of 2 3 months the arctic oscillation index can also be used as a remote indicator of the atmospheric fluxes but with lower skill than local measurements in contrast the north atlantic oscillation index does not correlate significantly with the surface eke at intraseasonal and interannual scales the analysis of altimeter data over the 1993 2001 supports the existence of this asymmetry between the regime locally forced by the atmosphere in the central basin and the regime remotely forced by the incoming boundary current along the west greenland coast those results have important implications for monitoring and predicting the surface eddy kinetic energy variability in the labrador sea

recovery lakes east antarctica radar assessment of sub glacial water extent 1 a fast flowing tributary of recovery ice stream penetrates more than 500 km into the interior of east antarctica recent satellite based studies found surface features in the onset area of this tributary that indicate a significant subglacial hydraulic system including four large smooth basins the typical surface expression of large subglacial lakes as well as eleven smaller areas over which ice sheet surface elevations rapidly changed by discharge filling of subglacial water here we present the first ice penetrating radar evidence of subglacial conditions in this area we identified a distinct ice water interface only over a limited area within the boundaries of the investigated large smooth basins previously hypothesized to be water filled lakes the radar characteristics in some areas are similar to those of a drained lake indicating that parts of the bed are wet but not a typical lake we also find evidence for discrete water bodies outside of the lake boundaries the lines of evidence indicate that the northern most two recovery lakes have recently drained

antarctic subglacial lakes abstract antarctic subglacial lakes were first identified by robin et al 1970 after airborne radio echo sounding res investigations of the ice sheet interior recently satellite altimetry was used to measure anomalous near flat regions on the ice sheet surface that represent a manifestation of the subglacial lake beneath using res and satellite altimetry the location and extent of antarctic subglacial lakes can be identified the largest subglacial lake exists beneath vostok station and is 14 000 km2 in area the combined area of additional subglacial lakes beneath dome c is 15 000 km2 and at least 15 000 km2 of lake surface lies beneath the remainder of the ice sheet the water depth of subglacial lakes can be estimated through seismic investigations although data exist only for lake vostok and consideration of the bedrock slopes that border subglacial lakes the depths of many subglacial lakes are of the order of 10 s 100 s of metres the total volume of water held beneath the ice sheet is estimated between 4000 and 12 000 km3 to date there are six known examples of radio echo reflections from the lake floors at a depth of no more than 20 m since e m attenuation through water is related to the salinity these data indicate that subglacial water is very pure and fresh some near flat surface regions that usually occur over lakes have been observed where no lakes exist such features are may be caused by water saturated basal sediments rather than subglacial lakes finally the spatial variation in geothermal heat flux around the central regions of antarctica can be established estimated by employing a simple thermal model of the ice sheet under an assumption that the basal ice temperature above subglacial lakes is equal to the pressure melting value calculations indicate that the geothermal heat flux varies spatially over the antarctic plate between 37 and 65 mw m 2

observations of the labrador sea eddy field this paper is an observational study of small scale coherent eddies in the labrador sea a region of dense water formation thought to be of considerable importance to the north atlantic overturning circulation numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat yet their small size has hindered observational progress a large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms current meter moorings and satellite altimetry details of the current and water mass structure of individual eddy events as they are swept past by an advecting flow can then be extracted from the mooring data a transition is seen during mid 1997 with long lived boundary current eddies dominating the central labrador sea year round after this time and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time the topex poseidon altimeter covers the labrador sea with a loose net of observations through which coherent eddies can seem to appear and disappear by concentrating on locating and describing anomalous events in individual altimeter tracks a portrait of the spatial and temporal variability of the underlying eddy field can be constructed the altimeter results reveal an annual pulsation of energy and of coherent eddies originating during the late fall at a particular location in the boundary current pinpointing the time and place of the boundary current type eddy formation the interannual variability seen at the mooring is reproduced but the mooring site is found to be within a localized region of greatly enhanced eddy activity notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling these eddy observations as well as hydrographic evidence suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region

seasonal to interannual variability of the eddy field in the labrador sea from satellite altimetry sea level anomalies measured by the altimeters aboard the topex poseidon and ers satellites for the periods 1993 2001 and 1997 2001 respectively are used to investigate the eddy field in the subpolar north atlantic and in the labrador sea a quadratic correction of the obtained eddy kinetic energy eke with respect to significant wave height is applied that led to an increased correlation between moored and altimetric eke in the central labrador sea the mean eke field shows higher levels associated with the main currents and a strong seasonality in the labrador sea the annual cycle of the eke shows a propagation of west greenland current wgc eke into the central labrador sea with a mean southward propagation speed of about 3 cm s 1 while the eke maximum in the labrador current is well separated from the interior by local eke minima the interannual variability of the eke in the labrador sea shows distinct regional differences in the wgc region strong early winter maxima are found during 1993 and 1997 1999 in the central labrador sea maxima are found during march april 1993 1995 and 1997 variations in the annual cycle of the wgc eke are observed while there is a weak annual cycle in the wgc region during 1994 1996 with more continuous eke generation during 1997 2000 there is a strong seasonal cycle with maximum eke during january and particularly low eke during summer the propagation of wgc eke into the central labrador sea is enhanced during 1997 2000 leading to a long persistence of eke in the central labrador sea during 1993 1995 and 1997 the central labrador sea eke almost instantaneously increased during march april followed in the earlier years by a relatively fast destruction of the winterly generated eke

boundary circulation at the exit of the labrador sea the deep circulation and related transports of the southern labrador sea are determined from direct current observations from ship surveys and a moored current meter array the measurements covered a time span from summer 1997 to 1999 and show a well defined deep boundary current extending approximately out to the 3300 m depth contour and weak reverse currents farther offshore the flow has a strong barotropic component and significant baroclinic flow is only found in the shallow labrador current at the shelf break and associated with a deep core of denmark strait overflow water the total deep water transport below 27 74 kg m 3 was 26 5 sv sv 106 m3 s 1 comprising labrador sea water lsw gibbs fracture zone water gfzw and denmark strait overflow water dsow intraseasonal variability of the flow and transport was high ranging from 15 to 35 sv and the annual means differed by 17 a seasonal cycle is confined to the shallow labrador current in its deeper part where the mean flow is still strong no obvious seasonality could be detected the transport of the interior anticyclonic recirculation was estimated from lowered acoustic doppler current profiler stations and geostrophy yielding about 9 sv thus the net deep water outflow from the labrador sea was about 17 sv the baroclinic transport of gfzw and dsow referenced to the depth of the isopycnal 27 80 kg m 3 is only about one third of the total transport in these layers longer term variations of the total transports are not represented well by the baroclinic contribution

arctic oscillation or north atlantic oscillation abstract the definition and interpretation of the arctic oscillation ao are examined and compared with those of the north atlantic oscillation nao it is shown that the nao reflects the correlations between the surface pressure variability at its centers of action whereas this is not the case for the ao the nao pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the euro atlantic region a similar identification is found in the pacific region for the pacific north american pna pattern but no such identification is found here for the ao the ao does reflect the tendency for the zonal winds at 35 and 55 n to anticorrelate in both the atlantic and pacific regions associated with the nao and pna because climatological features in the two ocean basins are at different latitudes the zonally symmetric nature of the ao does not mean that it represents a simple modulation of the circumpolar flow an increase in the ao or nao implies stro

radar based subglacial lake classification in antarctica 1 subglacial lakes in east antarctica can be separated into four categories specified by radar reflection properties definite lakes are brighter than their surroundings by at least 2 db relatively bright and both are consistently reflective specular and have a reflection coefficient greater than 10 db absolutely bright dim lakes are relatively bright and specular but not absolutely bright indicating nonsteady ice dynamics fuzzy lakes are both relatively and absolutely bright but not specular and may indicate saturated sediments or swamps indistinct lakes are absolutely bright and specular but no brighter than their surroundings lakes themselves and the different classes of lakes are not arranged randomly throughout antarctica but are clustered around ice divides ice stream onsets and prominent bedrock troughs with each cluster demonstrating a different characteristic lake classification distribution the lake classification algorithm expands on previous studies and demonstrates a novel way to characterize ice water interactions in east antarctica

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

oceanic transport of surface meltwater from the southern greenland ice sheet meltwater runoff from the greenland ice sheet alters ocean surface salinity numerical simulations show that meltwater from southeastern greenland is transported to the labrador sea more efficiently than that from southwestern greenland

labrador sea boundary currents and the fate of the irminger sea water abstract the general circulation of the labrador sea is studied with a dataset of 53 surface drifters drogued at 15 m and several hydrographic sections done in may 1997 surface drifters indicate three distinct speed regimes fast boundary currents a slower crossover from greenland to labrador and a slow eddy dominated flow in the basin interior mean eulerian velocity maps show several recirculation cells located offshore of the main currents in addition to the cyclonic circulation of the labrador sea above the northern slope of the basin the surface drifters have two preferential paths one between the 1000 m and 2000 m isobaths and the other close to the 3000 m isobath the vertical shear estimated from ctd data supports the presence of two distinct currents around the basin one current more baroclinic flows between the 1000 m and 2000 m isobaths the other one more barotropic flows above the lower continental slope the irminger sea water carried by the boundary currents is altered as it tr

a model for seasonal changes in gps positions and seismic wave speeds due to thermoelastic and hydrologic variations it is known that gps time series contain a seasonal variation that is not due to tectonic motions and it has recently been shown that crustal seismic velocities may also vary seasonally in order to explain these changes a number of hypotheses have been given among which thermoelastic and hydrology induced stresses and strains are leading candidates unfortunately though since a general framework does not exist for understanding such seasonal variations it is currently not possible to quickly evaluate the plausibility of these hypotheses to fill this gap in the literature i generalize a two dimensional thermoelastic strain model to provide an analytic solution for the displacements and wave speed changes due to either thermoelastic stresses or hydrologic loading which consists of poroelastic stresses and purely elastic stresses the thermoelastic model assumes a periodic surface temperature and the hydrologic models similarly assume a periodic near surface water load since all three models are two dimensional and periodic they are expected to only approximate any realistic scenario but the models nonetheless provide a quantitative framework for estimating the effects of thermoelastic and hydrologic variations quantitative comparison between the models and observations is further complicated by the large uncertainty in some of the relevant parameters despite this uncertainty though i find that maximum realistic thermoelastic effects are unlikely to explain a large fraction of the observed annual variation in a typical gps displacement time series or of the observed annual variations in seismic wave speeds in southern california hydrologic loading on the other hand may be able to explain a larger fraction of both the annual variations in displacements and seismic wave speeds neither model is likely to explain all of the seismic wave speed variations inferred from observations however more definitive conclusions cannot be made until the model parameters are better constrained

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

estimation of rock physics properties from seismic attributes part 1 strategy and sensitivity analysis abstractthe quantitative estimation of rock physics properties is of great importance in any reservoir characterization we have studied the sensitivity of such poroelastic rock physics properties to various seismic viscoelastic attributes velocities quality factors and density because we considered a generalized dynamic poroelastic model our analysis was applicable to most kinds of rocks over a wide range of frequencies the viscoelastic attributes computed by poroelastic forward modeling were used as input to a semiglobal optimization inversion code to estimate poroelastic properties porosity solid frame moduli fluid phase properties and saturation the sensitivity studies that we used showed that it was best to consider an inversion system with enough input data to obtain accurate estimates however simultaneous inversion for the whole set of poroelastic parameters was problematic due to the large number of parameters and their trade off consequently we restricted the sensitivity tests to

improved extended reconstruction of sst 1854 1997 abstract an improved sst reconstruction for the 1854 1997 period is developed compared to the version 1 analysis in the western tropical pacific the tropical atlantic and indian oceans more variance is resolved in the new analysis this improved analysis also uses sea ice concentrations to improve the high latitude sst analysis and a modified historical bias correction for the 1939 41 period in addition the new analysis includes an improved error estimate analysis uncertainty is largest in the nineteenth century and during the two world wars due to sparse sampling the near global average sst in the new analysis is consistent with the version 1 reconstruction the 95 confidence uncertainty for the near global average is 0 4 c or more in the nineteenth century near 0 2 c for the first half of the twentieth century and 0 1 c or less after 1950

mesoscale eddies in the labrador sea and their contribution to convection and restratification abstract the cycle of open ocean deep convection in the labrador sea is studied in a realistic high resolution 4 km regional model embedded in a coarser north atlantic setup this configuration allows the simultaneous generation and evolution of three different eddy types that are distinguished by their source region generation mechanism and dynamics very energetic irminger rings irs are generated by barotropic instability of the west greenland and irminger currents wgc ic off cape desolation and are characterized by a warm salty subsurface core they densely populate the basin north of 58 n where their eddy kinetic energy eke matches the signal observed by satellite altimetry significant levels of eke are also found offshore of the west greenland and labrador coasts where boundary current eddies bces are spawned by weakly energetic instabilities all along the boundary current system bcs baroclinic instability of the steep isopycnal slopes that result from a deep convective over

hydrographic changes in the labrador sea 1960 2005 abstract the labrador sea has exhibited significant temperature and salinity variations over the past five decades the whole basin was extremely warm and salty between the mid 1960s and early 1970s and fresh and cold between the late 1980s and mid 1990s the full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s the freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the labrador sea sustained throughout the subsequent years it took only a decade for the whole water column to lose most of its excessive freshwater reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s if the recent tendencies persist the basin s storages of salt and heat will fairly soon likely by 2008 exceed their historic highs the main process responsible for the net cooling and freshening of the labrador sea between 1987 and 1994 was deep winter convection which during this period progressively developed to its record depths it was caused by the recurrence of severe winters during these years and in its turn produced the deepest densest and most voluminous labrador sea water lsw1987 1994 ever observed the estimated annual production of this water during the period of 1987 1994 is equivalent to the average volume flux of about 4 5 sv with some individual annual rates exceeding 7 0 sv once winter convection had lost its strength in the winter of 1994 1995 the deep lsw1987 1994 layer lost communication with the mixed layer above consequently losing its volume while gaining heat and salt from the intermediate waters outside the labrador sea while the 1000 2000 m layer was steadily becoming warmer and saltier between 1994 and 2005 the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air sea heat fluxes in the winter of 1999 2000 this change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent lsw class lsw2000 penetrating deeper than 1300 m this layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass implying that lsw2000 is likely to follow the fate of its deeper precursor lsw1987 1994 the increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid depth perimeter of the labrador sea the near bottom waters originating from the denmark strait overflow exhibit strong interannual variability featuring distinct short term basin scale events or pulses of anomalously cold and fresh water separated by warm and salty overflow modifications regardless of their sign these anomalies pass through the abyss of the labrador sea first appearing at the greenland side and then about a year later at the labrador side and in the central labrador basin the northeast atlantic deep water 2500 3200 m originating from the iceland scotland overflow water reached its historically freshest state in the 2000 2001 period and has been steadily becoming saltier since then it is argued that lsw1987 1994 significantly contributed to the freshening density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening lsw the hydrostatic adjustment to expanding lsw or both

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

role of resolved and parameterized eddies in the labrador sea balance of heat and buoyancy abstractdeep convection in the labrador sea is an important component of the global ocean ventilation the associated loss of heat to the atmosphere from the interior of the sea is thought to be mostly supplied by mesoscale eddies generated either remotely or as a result of convection itself processes that are not resolved by low resolution ocean climate models the authors first employ a high resolution ocean model forced with high resolution 33 km 3 h atmospheric fields to further elaborate on the role of mesoscale eddies in maintaining the balance of heat and buoyancy in the labrador sea in general agreement with previous studies it is found that eddies remove heat along the coast and supply it to the interior some of the eddies that are generated because of the barotropic instability off the west coast of greenland are recaptured by the boundary current in the region of deep convection the convergence of heat and buoyancy by eddies significantly increases with the deepening of the winter

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

the use of geophysical prospecting for imaging active faults in the roer graben belgium as part of a paleoseismological investigation along the bree fault scarp western border of the roer graben various geophysical methods electrical profiling electromagnetic em profiling refraction seismic tests electrical tomography ground penetrating radar gpr and high resolution reflection seismic profiles were used to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters these geophysical investigations in parallel with geomorphological and geological analyses helped in the decision to locate trench excavations exposing the fault surfaces the results could then be checked with the observations in four trenches excavated across the scarp geophysical methods pointed out anomalies at all sites of the fault position the contrast of physical properties electrical resistivity and permittivity seismic velocity observed between the two fault blocks is a result of a differences in the lithology of the juxtaposed soil layers and of a change