correlation scales objective mapping and absolute geostrophic flow in the california current the spatial covariances of the time dependent density and geostrophic velocity fields off southern california are determined from a unique set of repeated hydrographic observations 44 cruises made by the california cooperative oceanic fisheries investigations from 1984 to 1994 the covariances and objective analysis are used to combine direct velocity observations from shipboard acoustic doppler current profiler adcp measurements made on a recent survey october 1993 with hydrographic observations the analysis reduces ageostrophic noise in the adcp velocities by smoothing and enforcing horizontal nondivergence additionally the velocities are mapped over scales that are dynamically consistent with the hydrography maximum surface geostrophic flow in the california current in october 1993 is about 35 cm s 1 50 larger than that estimated assuming a 500 m level of no motion absolute flow at 500 m is o 10 cm s 1 and indicates that the surface eddy field penetrates through the thermocline uncertainty in the geostrophic reference calculated from the adcp measurements is of o 4 cm s 1 the velocity residual objectively analyzed minus raw adcp estimates exhibits smaller correlation scales than the geostrophic flow

twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

dynamical interpolation and forecast of the evolution of mesoscale features off northern california abstract the series of cruises off northern california comprising optoma11 during two months in summer 1984 were specifically designed as an ocean prediction experiment in addition to a regional survey from cape mendocino to monterey six surveys were made of a 150 km 2 domain offshore of pt arena pt arena pt reyes during the initial phase over about ten days of optoma11 an intense speeds up to 50 cm s 1 relative to 450 m jet cyclone system propagated offshore at about 5 km day 1 the subsequent evolution over about 40 days of the streamfunction field was governed by the meandering of the jet and the associated changes in the intensity of the anticyclonic region to the north of the jet and the cyclonic region to the south from quasi geostrophic qg model hindcast experiments using the streamfunction data wind stress curl was an important forcing mechanism in the later phase of the experiment forecast in a domain extending over the continental slope were in agreement with objective anal

direct evidence for an ekman balance in the california current moored acoustic doppler current profiler velocity estimates and buoy wind observations made during a period of moderate southward winds were used to test the ekman balance at a site in the california current as in prior studies the wind driven flow was separated from the total flow by subtraction of a deep reference current the wind driven flow was shown to be in an ekman balance on daily timescales over a period of several months the mean observed transport was to the right of the wind and agreed to within 3 in magnitude and 4 in phase with the predicted ekman transport although the error bar was about 20 the mean velocity profile was a smooth spiral qualitatively similar although flatter than the theoretical ekman spiral prom the observed mean momentum balance profiles of the turbulent stress and eddy viscosity were inferred eddy viscosity estimates within the wind mixed layer were o 100 cm2 s 1

modelling the coupling of flood discharge with glacier flow during jokulhlaups we explore a mathematical model that couples together a thermomechanically evolving subglacial channel distributed cavity drainage and basal sliding along a subglacial flood path fed by a jokulhlaup lake it allows water transfer between channel and cavities and a migrating subglacial water divide or seal to form between floods notably it accounts for full coupling between the lake and subglacial drainage in terms of both discharge and pressure unlike models that neglect the pressure coupling by imposing a known history of lake discharge at the channel inlet this means that flood hydrographic evolution and its impact on glacier motion are consistently determined by our model numerical simulations for a model alpine lake yield stable limit cycles simulating repeating jokulhlaups with the channel drawing water from the cavities at a varying rate that modulates basal sliding during each flood a wave of fast sliding propagates down glacier at flood initiation followed by deceleration as the growing channel sucks water from the cavities these behaviours cannot be correctly simulated without the full coupling we show that the flood s peak discharge its initiation threshold and the magnitude of the fast sliding wave decrease with the background water supply to the cavities

lagrangian exploration of the california undercurrent 1992 95 abstract during the period 1992 95 nineteen isobaric rafos floats placed in the california undercurrent at intermediate depths 150 600 m off monterey and san francisco california reveal a region of varying width of subsurface poleward flow adjacent to the continental margin the float trajectories exhibit three patterns poleward flow in the undercurrent reversing but predominately alongshore flow adjacent to the continental margin and farther offshore anticyclonic motion accompanied by slow westward drift flow continuity of the undercurrent exists between pt reyes and at least cape mendocino with an average speed dependent on the float depth speeds were variable but common features were acceleration occurring to the south of pt arena and deceleration to the north of cape mendocino an important mechanism for floats and water to enter the ocean interior from the undercurrent is through the formation of submesoscale coherent vortices

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

patterns of upwelling and relaxation around monterey bay based on long term observations of surface currents from high frequency radar abstract a five year record of surface current observations from hf radar in the region around monterey bay are used to describe the dominant circulation patterns under upwelling and downwelling favorable wind forcing conditional averages based on local wind observations reveal more structure and intensity in the circulation patterns than do traditional temporal averages the predominant upwelling circulation pattern in the region is found to reach full strength after about three days of persistent upwelling favorable wind forcing in the opposite phase under downwelling favorable winds the circulation patterns differ between summer and winter with evidence for strong connectivity between the offshore waters and the coastline around monterey bay in summer that is not present in winter

near surface trajectories off central and southern california the near surface circulation in the santa barbara channel and off the coast of central and southern california is described based on 20 releases of drifters drogued 1 m beneath the surface from 12 sites within the channel at bimonthly intervals this description includes small scale features of the circulation which are not part of descriptions based on moored observations or of the statistics of the drifter releases the eventual fate of drifters at long time intervals compared to the residence time in the channel about 7 days is also included in the channel the trajectories document a persistent cyclonic circulation with a typical recirculation period between 3 and 5 days in the spring currents near the mainland are weaker than near the channel islands and the overall flow is toward the southeast trajectories document the possibility for water parcels to leave the channel through the interisland passes in the late fall and winter a poleward flow with velocities often exceeding 0 5 m s 1 is confined within 20 km of the mainland between these two seasons the cyclonic tendency is enhanced although most of the drifters eventually migrate westward the trajectories of drifters released at the same time from sites only 20 km apart can be remarkably different once the drifters migrate out of the channel their trajectories can be grouped into a few patterns in spring and summer drifters tend to remain in the southern california bight their trajectories often remain close over extended periods as if they were caught in convergence zones in fall the drifters often are caught in a poleward current

california undercurrent variability and eddy transport estimated from rafos float observations 1 rafos float observations collected between 1992 and 2002 were analyzed to identify the seasonal variability of circulation in four geographical boxes which extended along the central and northern california coast and were successively located farther offshore the mean pressure of the floats was 375 dbar poleward flow associated with the california undercurrent dominated the two boxes closest to shore extending from the 400 m isobath to a distance of 190 km offshore for the box closest to shore the monthly mean alongshore velocity was maximum minimum 5 4 cm s 1 7 cm s in may june february while the eddy kinetic energy eke was minimum was 33 cm2 s2 21 cm2 s2 in september december february the mean eke in the coastal region was 28 cm2 s2 increasing to 50 cm2 s2 for the region farthest offshore a distance of about 400 km for that region farthest offshore eke had a broad maximum from june to november and a minimum in april lagrangian correlation and dispersion tensors were estimated for floats that left the coastal region three different dispersive regimes of float motion were identified as ballistic transport normal diffusion and anomalous sub diffusion westward sub diffusion was induced by rossby wave like structures with a periodicity of 100 120 days

altimeter derived variability of surface velocities in the california current system 2 seasonal circulation and eddy statistics abstract satellite surface height and temperature fields are used to examine the seasonal surface circulation in the california current system in spring and summer an equatorward jet develops next to the coast with an initial latitudinal structure that responds to the latitudinal distribution of equatorward alongshore winds this jet moves offshore from spring to fall and develops meanders and closed eddies that contribute eddy kinetic energy and water mass properties to the nearby deep ocean higher eke appears first in spring near the coast and moves offshore with the seasonal jet along altimeter tracks parallel to the coastline 200 and 400 km offshore the greatest eke is found in summer and fall respectively with wavelengths of approximately 300 km these are the wavelengths of the largest meanders and the distance between the largest eddies visible in altimeter snapshots of the jet at 600 800 km offshore the wavelengths are generally smaller 100 150 km similar to eddy sizes reported by others for the quiescent northeast pacific some energy with longer wavelengths arrives at the track 600 km offshore in winter and 800 km offshore in spring which we interpret as the most offshore extension of the eke generated by the jet and eddy system closer to the coast in the previous spring and summer a sequence of snapshots from october 1992 to october 1993 shows details of the evolution of the jet and eddy system over one complete annual cycle

statistical analysis of the surface circulation of the california current we use a set of mixed layer drifting buoy trajectories from the california current 20 n 40 n during 1985 1990 to obtain statistically reliable estimates of the mean currents the mean variance field and the geographically varying diffusivity integral timescales and integral space scales typical values for the diffusivity are 1 1 8 7 107 cm2 s 1 while the timescales and space scales are 2 1 7 1 days and 16 59 km respectively the variance field displays a strong westward gradient out to 125 w and diffusivity shows a tendency to decrease toward the southwest part of the domain significant anisotropy is found in the variance field near the coastal boundary and at 30 n 130 w which is the region where the subarctic and northern subtropical fronts approach the california current the antisymmetric component of the diffusivity tensor indicates that cyclonic eddies dominate the mesoscale signature of drifters in this region we seek simple parameterizations to relate the scales of motion of the random velocity field to the diffusivity by testing least squares fits to u02t and u0l where u02 is the velocity variance we found no cases for which these two hypotheses could be distinguished for the meridional component the linear regressions are not successful which suggests that the meridional departure velocities result from a flow regime that is significantly organized by for example waves or coherent structures a subset of the drifters measured temperature along their tracks and we use the resultant data to produce the first direct estimates of the horizontal eddy heat flux divergence based on lagrangian estimates in addition we separately compute the eddy diffusivity parameterization of the eddy heat flux divergence u using our diffusivity estimates and a sea surface temperature climatology the two independent terms agree well which provides a measure of reassurance about the diffusivity estimates the eddy heat flux divergence in the california current is very small 5 w m 2 and does not appear to be significant in the long term heat budget of the upper ocean in this region

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

ice sheet change detection by satellite image differencing abstract differencing of digital satellite image pairs highlights subtle changes in near identical scenes of earth surfaces using the mathematical relationships relevant to photoclinometry we examine the effectiveness of this method for the study of localized ice sheet surface topography changes using numerical experiments we then test these results by differencing images of several regions in west antarctica including some where changes have previously been identified in altimeter profiles the technique works well with coregistered images having low noise high radiometric sensitivity and near identical solar illumination geometry clouds and frosts detract from resolving surface features the etm sensor on landsat 7 ali sensor on eo 1 and modis sensor on the aqua and terra satellite platforms all have potential for detecting localized topographic changes such as shifting dunes surface inflation and deflation features associated with sub glacial lake fill drain events or grounding line changes availability and frequency of modis images favor this sensor for wide application and using it we demonstrate both qualitative identification of changes in topography and quantitative mapping of slope and elevation changes

on westward transport processes off central california revealed by rafos floats 1 westward transport processes off central california are quantified by exit time statistics for rafos floats launched at mid depths 275 375 m the random displacement model in the diffusion limit was used to interpret results the alongshore averaged westward motion of floats represented inhomogeneous eddy diffusion interspersed with short duration advective motions the across shore diffusion coefficient grew as a power law with distance from the coastline the power exponent was close to one this is evidence for offshore growth of eddy kinetic energy and may be explained through flow barotropization accompanied by a quasi geostrophic turbulent regime typical values of the float drift velocity were about 1 8 2 cm s but offshore drift velocities were as high as 4 6 cm s during the upwelling season between 150 and 250 km from shore a transition zone between nearshore and offshore was clearly identified in spring but was either very narrow or undetectable in other seasons

a view of the 1993 1994 california current based on surface drifters floats and remotely sensed data near surface drifters were deployed in the california current system during the period 1993 1994 in order to study mean and eddy currents the deployment scheme allowed a 2 year period where data density was high enough to make meaningful estimates of eddy statistics and of seasonal mean currents and variances on a 5 grid eddy variability which tended to be isotropic was higher near the coast standard deviations around 0 2 m s 1 and decreased offshore down to 0 1 m s 1 seasonal mean surface flow west of california was southward during all seasons if a davidson current existed during 1993 1995 it was probably either short lived or only found closer to the coast than our study area i e within 100 150 km of shore in some cases drifter populations within a discrete feature were high enough to allow some detailed resolution of its behavior we note a tendency for individual eddies which can last for up to 3 5 months to drift westward at rates of o 5 km day 1 not all eddies in the region are surface intensified there is clear evidence for at least the occasional presence of a thin meandering southward jet in the region that can be 1000 km or more long this jet can be identified with that found in the coastal transition zone program in association with cool filaments the jet appears to become broader and slower south of point conception we hypothesize on the basis of others results as well as ours that the eddies that are strongest at the surface result primarily from instabilities on a southward alongshore mean current and that eddies that are maximal at depth are spawned by the northward undercurrent

the large scale coastal wind field along the west coast of north america 1981 1982 we study the statistical properties of the coastal wind field along the west coast of north america for two summers 1981 and 1982 coastal ocean dynamics experiments 1 and 2 respectively and the intervening winter using measured winds and geostrophic winds calculated from fleet numerical oceanography center atmospheric pressure analyses summer wind fluctuations are driven primarily by the interaction between two relatively stationary pressure systems the north pacific subtropical high and southwest u s thermal low and by their interactions with propagating atmospheric systems to the north in particular propagating cyclones and associated fronts are often followed by a northeastward intensification of the high producing strong upwelling events along the california coast this summer event sequence occurs more frequently and is displaced farther to the south on average during summer 1981 winter wind fluctuations are primarily driven by propagating cyclones and anticyclones and they tend to have larger variance and space scales than in summer a preference for poleward equatorward propagation exists in summer winter and the largest time scales are observed in summer 1982 coastal atmospheric boundary layer processes substantially modify winds within 100 200 km of the coast consequently measured wind fluctuations are strongly polarized in the alongshore direction and have means and rms amplitudes that can vary considerably between nearby stations along the coast calculated wind fluctuations are less polarized in the alongshore direction and have alongshore correlation scales about 60 larger than those for measured winds they represent fluctuations with alongshore wavelengths of 900 km rather well but represent poorly those with smaller wavelengths and those due to coastal atmospheric boundary layer effects

a multicomponent coupled model of glacier hydrology 1 theory and synthetic examples 1 basal hydrology is acknowledged as a fundamental control on glacier dynamics especially in cases where surface meltwater reaches the bed for many glaciers at midlatitudes basal drainage is influenced by subaerial englacial and subsurface water flow one of the major shortcomings of existing basal hydrology models is the treatment of the glacier bed as an isolated system we present theoretical and computational models that couple glacier surface runoff englacial water storage and transport subglacial drainage and subsurface groundwater flow each of the four model components is represented as a two dimensional vertically integrated layer that communicates with its neighbors through water exchange governing equations are derived from the law of mass conservation and are expressed as a balance between the internal distribution of water and external sources the numerical exposition of this theory is a time dependent finite difference model that can be used to simulate glacier drainage in this paper we outline the theory and conduct simple tests using an idealized glacier geometry in the companion paper the model is tailored to trapridge glacier yukon territory canada where results are compared with measurements of subglacial water pressure

steep reverse bed slope at the grounding line of the weddell sea sector in west antarctica the bed of the west antarctic ice sheet is in places more than 1 5 km below sea level1 2 it has been suggested that a positive ice loss feedback may occur when an ice sheet s grounding line retreats across a deepening bed1 2 3 applied to the west antarctic ice sheet this process could potentially raise global sea level4 by more than 3 m hitherto attention has focussed on changes at the siple coast5 6 7 and amundsen sea embayment8 9 10 sectors of west antarctica here we present radio echo sounding information from the ice sheet s third sector the weddell sea embayment that reveals a large subglacial basin immediately upstream of the grounding line the reverse bed slope is steep with about 400 m of decline over 40 km the basin floor is smooth and flat with little small scale topography that would delay retreat indicating that it has been covered with marine sediment5 11 and was previously deglaciated upstream of the basin well defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin evidence so far suggests that the weddell sea sector of the west antarctic ice sheet has been stable but in the light of our data we propose that the region could be near a physical threshold of substantial change

satellite data empirical orthogonal functions and the 1997 1998 el nino off california abstract the features of the 1997 1998 el nino event were analyzed by empirical orthogonal functions eof statistical methods applied to the remotely sensed sea surface temperature anomalies ssta measured by avhrr radiometers anomalies of water circulation derived from sea surface height anomalies ssha measured by topex poseidon radar altimeter and meteorological information air temperature upwelling index and wind stress curl eof statistics demonstrated the features of an el nino event during the second half of 1997 and the first half of 1998 with sea level elevated along the coast and with ssha gradients indicating a retarding of both the equatorward california current and the alongshore poleward southern california countercurrent the positive sst anomaly developed first in the southern california bight and then in the zone of upwelling to the north of point conception the anomalies of upwelling index and the wind stress curl pattern also changed during the el nino event but these changes occurred later than hydrographic variations and were too weak to explain the observed changes in ssta and ssha we conclude that off central and southern california oceanic teleconnection i e the consequences of propagation northward of coastally trapped downwelling waves was responsible for the 1997 1998 el nino event

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

altimeter derived surface circulation in the large scale ne pacific gyres part 1 seasonal variability abstract the seasonal variability of sea surface height ssh and currents are defined by analysis of altimeter data in the ne pacific ocean over the region from central america to the alaska gyre the results help to clarify questions about the timing of seasonal maxima in the boundary currents as explained below the long term temporal mean of the ssh values must be removed at each spatial point to remove the temporally invariant and large signal caused by the marine geoid we refer to the resulting ssh values which contain all of the temporal variations as the residual ssh our main findings are 1 the maximum surface velocities around the boundaries of the cyclonic alaska gyre the alaska current and the alaska stream occur in winter at the same time that the equatorward california current is weakest or reversed forming the poleward davidson current the maximum surface velocities in the california current occur in summer these seasonal maxima are coincident with the large scale atmospheric wind forcing over each region 2 most of the seasonal variability occurs as strong residuals in alongshore surface currents around the boundaries of the ne pacific basin directly connecting the boundaries of the subpolar gyre the subtropical gyre and the equatorial current system 3 seasonal variability in the surface velocities of the eastward north pacific current west wind drift is weak in comparison to seasonal changes in the surface currents along the boundaries 4 there is an initial appearance next to the coast and offshore migration of seasonal highs and lows in ssh alongshore velocity and eddy kinetic energy eke in the alaska gyre similar to the previously described seasonal offshore migration in the california current 5 the seasonal development of high ssh and poleward current residuals next to the coast appear first off central america and mainland mexico in may june prior to their appearance in the southern part of the california current in july august and their eventual spread around the entire basin in november december similarly low ssh and equatorward transport residuals appear first off central america and mexico in january february before spreading farther north in spring and summer 6 the maximum values of eke occur when each of the boundary currents are maximum

exchange geometry revealed by float trajectories in the gulf stream in an effort to understand the extent to which lagrangian pathways in the gulf stream indicate fluid exchange between the stream and its surroundings trajectories of rafos floats are viewed in a frame of reference moving with the dominant zonal phase speed associated with the periodic flow in such a frame geometrical structures emerge that more clearly delineate the position of the parcel in relation to the jet core and its surroundings the basic premise of this work is that the pathways of fluid parcels in the vicinity of stagnation points as defined in the moving frame of reference are susceptible to changes in their pathways thereby facilitating fluid exchange between different regions of the flow field four representative rafos float trajectories are shown to exhibit the expected behavior in the vicinity of stagnation points to further examine the mechanism of exchange in the vicinity of these geometrical features concepts from dynamical systems theory are applied to a numerically simulated flow field the entrainment and detrainment of parcels from the jet core are explained in the context of stable and unstable manifolds and their associated lobes it is shown that the lagrangian pathways from the numerical flow and the observational trajectories exhibit a similarity based on the kinematics of a meandering flow field overall this study provides the first look at rafos float trajectories in a moving frame and provides insight as to how the temporal variability of a jet creates chaotic exchange

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

using radar sounding data to identify the distribution and sources of subglacial water application to dome c east antarctica basal radar reflectivity is the most important measurement for the detection of subglacial water however dielectric loss in the overlying ice column complicates the determination of basal reflectivity dielectric attenuation is a function of ice temperature and impurity concentration temperature distribution is a function of climate history basal heat flow and vertical strain rate all of which can be partially inferred from the structure of dated internal layers using 11 dated layers isotope records from dome c east antarctica and a model of the spatial variation of geothermal flux we calculate the vertical strain rate and accumulation rate history allowing identification of areas where the basal melt rate exceeds 1 5 mm a 1 the accumulation rate history and vertical strain rates are then used as inputs for a transient temperature model the model outputs for the present day temperature distribution are then combined with depth dependent ionic concentrations to model dielectric loss and infer basal reflectivity the resulting reflection coefficients are consistent 5 db across a variety of subglacial water bodies we also identify a high reflectivity 15 db in concordia trench and along suspected subglacial water flow routes in vincennes basin highland areas tend to have highly variable reflection coefficients near 30 db consistent with an ice bedrock interface this combined model also identifies three areas of enhanced basal melting along concordia ridge concordia subglacial lake and vincennes basin melt at concordia subglacial lake exceeds 5 mm a 1 the inferred basal melt at these locations is not possible without enhanced geothermal flux we demonstrate how radar sounding data can provide both input and verification for a self consistent model of vertical strain vertical temperature distribution and meltwater distribution

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

active lakes of recovery ice stream east antarctica a bedrock controlled subglacial hydrological system a connected system of active subglacial lakes was revealed beneath recovery ice stream east antarctica by icesat laser altimetry here we combine repeat track analysis of icesat 2003 09 operation icebridge laser altimetry and radio echo sounding 2011 and 2012 and modis image differencing 2009 2011 to learn more about the lake activity history the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system we extend the lake activity time series to 2012 for the three lower lakes and capture twomajor lake drainages one lake underwent a large deflation between 2009 and 2011 while another lake which had been continuously filling between 2003 and 2010 started to drain after 2011 most of the active lakes are located in a 1000 km long bedrock trough under the main trunk of recovery ice stream whose base is 1500 2000 m below present day sea level the hydrologic system beneath recovery ice stream is controlled by this unusually pronounced bedrock topography in contrast to most antarctic systems studied to date which are controlled by the ice surface topography hydrologic connections among the lakes appear to be direct and responsive and we reproduce the lake activity using a simple subglacial water model we discuss potential causes of non steady hydrologic behavior in major antarctic catchments

subglacial thermal balance permits ongoing grounding line retreat along the siple coast of west antarctica changes in the discharge of west antarctic ice streams are of potential concern with respect to global sea level the six relatively thin fast flowing ross ice streams are of interest as low slope end members among antarctic ice streams extensive research has demonstrated that these rivers of ice have a history of relatively high frequency asynchronous discharge variations with evolving lateral boundaries amidst this variability a 1300 km grounding line retreat has occurred since the last glacial maximum numerical studies of ice stream d parizek and others 2002 indicate that a proposed thermal regulation mechanism clarke and marshall 1998 hulbe and macayeal 1999 tulaczyk and others 2000a b which could buffer the west antarctic ice sheet against complete collapse may be over ridden by latent heat transport within melt water from beneath inland ice extending these studies to ice stream a whillans ice stream and ice stream c suggests that further grounding line retreat contributing to sea level rise is possible thermodynamically however the efficiency of basal water distribution may be a constraint on the system because local thermal deficits promote basal freeze on especially on topographic highs observed short term variability is likely to persist

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

widespread persistent thickening of the east antarctic ice sheet by freezing from the base an international polar year aerogeophysical investigation of the high interior of east antarctica reveals widespread freeze on that drives substantial mass redistribution at the bottom of the ice sheet although the surface accumulation of snow remains the primary mechanism for ice sheet growth beneath dome a 24 of the base by area is frozen on ice in some places up to half of the ice thickness has been added from below these ice packages result from the conductive cooling of water ponded near the gamburtsev subglacial mountain ridges and the supercooling of water forced up steep valley walls persistent freeze on thickens the ice column alters basal ice rheology and fabric and upwarps the overlying ice sheet including the oldest atmospheric climate archive and drives flow behavior not captured in present models

characteristic patterns of shelf circulation at the boundary between central and southern california 1 the coastal circulation in the santa barbara channel sbc and the southern central california shelf is described in terms of three characteristic flow patterns the upwelling pattern consists of a prevailing equatorward flow at the surface and at 45 m depth except in the area immediately adjacent to the mainland coast in the sbc where the prevailing cyclonic circulation is strong enough to reverse the equatorward tendency and the flow is toward the west in the surface convergent pattern north of point conception the surface flow is equatorward while the flow at 45 m depth is poleward east of point conception along the mainland coast the flow is westward at all depths and there results a convergence at the surface between point conception and point arguello with offshore transport over a distance on the order of 100 km beneath the surface layer the direction of the flow is consistently poleward the relaxation pattern is almost the reverse of the upwelling pattern with the exception that in the sbc the cyclonic circulation is such that the flow north of the channel islands remains eastward although weak the upwelling pattern is more likely to occur in march and april after the spring transition when the winds first become upwelling favorable and while the surface pressure is uniform the surface convergent pattern tends to occur in summer when the wind is still strong and persistently upwelling favorable and the alongshore variable upwelling has build up alongshore surface pressure gradients the relaxation pattern occurs in late fall and early winter after the end of the period of persistent upwelling favorable winds

eddy properties in the california current system 1 eddy detection and tracking algorithms are applied to both satellite altimetry and a high resolution dx 5 km climatological model solution of the u s west coast to study the properties of surface and undercurrent eddies in the california current system eddy properties show remarkable similarity in space and time and even somewhat in polarity summer and fall are the most active seasons for undercurrent eddy generation while there is less seasonal variation at surface most of the eddies have radii in the range of 25 100 km sea level anomaly amplitudes of 1 4 cm and vorticity normalized by f amplitudes of 0 025 0 2 many of the eddies formed near the coast travel considerable distance westward with speeds about 2 km day consistent with the effect anticyclones and cyclones show equatorward and poleward displacements respectively long lived surface eddies show a cyclonic dominance the subsurface california undercurrent generates more long lived anticyclones than cyclones through instabilities and topographic coastline effects in contrast surface eddies and subsurface cyclones have much more widely distributed birth sites the majority of the identified eddies have lifetimes less than a season eddies extend to 800 1500 m depth and have distinctive vertical structures for cyclones and anticyclones eddies show high nonlinearity rotation speed higher than propagation speed and hence can be efficient in transporting materials offshore

observation and prediction of flotsam trajectories in the california current system based on surface drift of rafos floats abstract surface drift of eighty nine undrogued rafos floats in the california current system ccs has been studied to describe patterns of flotsam drift its seasonal variability and predictability limitations the floats were launched in the california undercurrent during 1992 2010 and were tracked by the argos system when they surfaced at the end of their subsurface missions the duration of surface trajectories varied from as short as 11 to as long as 280 days the surface drift of these floats was typically equatorward in the california current however some floats moved poleward into the subpolar gyre and others drifted westward into the north equatorial current usually observations of surface currents use drifters which are coupled to the surface layer by drogues located at 15 m depth while drogued observations are useful for studies of circulation of the upper layer of the ocean a more typical operational problem involves trying to find flotsam that has fallen off the deck of a ship or to predict the path of an abandoned vessel to better understand the behavior of these surface drifting objects observations of the surface drifts of rafos floats in the california current system were used to compare the floats motions to wind induced drift and to evaluate the drift prediction by three ocean models ocean surface current simulator oscurs global navy coastal model gncom and hybrid coordinate ocean model hycom cross calibrated multi platform ocean surface wind vector l3 0 first look analysis wind data were used to analyze the surface drift of the rafos floats the best correlation between float drift speed and wind speed was observed during summer and fall months when the wind regime over the northeast pacific is dominated by the north pacific high the predominant winds associated with this regime are steady and contribute to the surface drift in the same direction as the large scale geostrophic component of the california current evaluation of the drift prediction by three ocean models was conducted by comparing observed drifter trajectories with model simulated trajectories at 7 day time scales the model simulated trajectories were initially collocated with rafos positions and restarted every 15 days the oscurs model was the only one of three analyzed models which allowed for tuning based on the surface rafos trajectories two main parameters wind angle deflection wad and geostrophic current factor gcf were adjusted to minimize the separation between observed and modeled trajectories at shorter time scales from weeks to several months the tuned oscurs model outperformed gncom and hycom models in reproducing the observed surface drift for seasonal and longer time scales both gncom and hycom can reproduce the seasonal variability flow direction and eddy like structures in the california current system which may be useful for long term forecasts of the northeast pacific circulation

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

connected subglacial lake activity on lower mercer and whillans ice streams west antarctica 2003 2008 we examine patterns of localized surface elevation change in lower mercer and whillans ice streams west antarctica which we interpret as subglacial water movement through a system of lakes and channels we detect and measure the lake activity using repeat track laser altimetry from icesat and image differencing from modis image pairs a hydrostatic potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes our analysis shows that within these regimes some of the subglacial lakes appear to be linked with drainage events in one reservoir causing filling and follow on drainage in adjacent lakes we also observe changes near ice raft a in lower whillans ice stream and interpret them as evidence of subglacial water and other changes at the bed the study provides quantitative information about the properties of this complex subglacial hydrologic system and a relatively unstudied component of ice sheet mass balance subglacial drainage across the grounding line

upper ocean water mass characteristics of the california current summer 1993 abstract two large high resolution upper ocean surveys of the california current region were conducted in the summer of 1993 temperature and salinity were measured from a seasoar vehicle and velocity was measured by a ship borne acoustic doppler current profiler both surveys extended from the continental margin to 128 w and consisted of zonal sections 28 km apart the first survey 7 28 june extended from 39 5 n to 36 25 n and the second 16 august to 1 september extended from 39 0 n to 37 0 n the current field was fairly simple in june showing an equatorward surface jet with only one gentle meander a nearly continuous poleward california undercurrent near the continental margin and two anticyclonic subsurface eddies offshore of the jet by august meanders in the surface jet had strengthened and eddies dominated the flow field large scale trends in the water mass characteristics spiciness along three isopycnal surfaces 26 4 26 0 and 25 6 kg m 3 show that temperature and salinity tend to increase towards the equator and towards shore consistent with poleward advection along the continental margin local minima in spiciness tend to coincide with the equatorward surface jet local maxima in spiciness are associated with subsurface anticyclonic eddies which have a core depth of about 150 m and seem to originate in the california undercurrent over the continental slope at local latitudes we identified several different anticyclonic eddies each with different water mass characteristics all of the eddies we observed had diameters larger than the baroclinic radius of deformation water mass characteristics and migration rates are consistent with formation over the continental margin within the preceding 6 8 months

sea surface temperature fronts in the california current system from geostationary satellite observations 1 sea surface temperature sst fronts are determined for the 2001 2004 time period from geostationary operational environmental satellites goes data in the california current system ccs the probability of detecting a sst front at an individual pixel location in the ccs is presented as a bi monthly climatology fronts clearly indicate the seasonal evolution of coastal upwelling as well as meanders and filaments that are often linked with irregularities in coastline geometry winter is characterized by low frontal activity along the entire coast fronts first appear close to the coast during spring particularly south of cape blanco where upwelling favorable winds are already persistent the area of high frontal activity continues to increase during summer especially between monterey bay and cape blanco extending more than 300 km from the coast the region with high frontal activity widens at 2 6 km day 1 off northern baja california a band with persistent fronts is found close to the coast year round but there is no evidence of a seasonal widening of the area of higher activity during fall the weakening of upwelling favorable winds leads to a gradual decrease in frontal activity an empirical orthogonal function eof decomposition reveals the development of sst fronts associated with seasonal upwelling for locations north of monterey bay with less summer intensification to the south the first appearance of fronts close to the coast during spring and the occurrence of the fronts offshore later in the season are represented by additional statistically significant eof modes

eddy induced sea surface temperature gradients in eastern boundary current systems many processes are known to influence the distribution of sea surface temperature sst fronts in eastern boundary current systems ebcs including wind forcing mesoscale activity flow instabilities and flow topography interactions here we used satellite observations to quantify the relative importance of one of these processes mesoscale eddy activity on the distribution of sst gradients in ebcs eddies are characterized by large sst and sst gradient anomalies in all ebcs at the end of the upwelling season eddy induced sst gradients are dominant components of the full sst gradient offshore of 300 km from the coast especially in regions of high eddy activity comparisons between eddy induced and full sst gradients in the california current system indicate that the offshore migration of mesoscale eddies plays a significant role on the seasonal widening of the region of high frontal activity sst gradients associated with anticyclonic eddies are largest within 400 km from shore while the signature of cyclones can extend for up to 700 km from the coast

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

identification and control of subglacial water networks under dome a antarctica 1 subglacial water in continental antarctica forms by melting of basal ice due to geothermal or frictional heating subglacial networks transport the water from melting areas and can facilitate sliding by the ice sheet over its bed subglacial water flow is driven mainly by gradients in overburden pressure and bed elevation we identify small median 850 m water bodies within the gamburtsev subglacial mountains in east antarctica organized into long 20 103 km coherent drainage networks using a dense 5 km grid of airborne radar data the individual water bodies are smaller on average than the water bodies contained in existing inventories of antarctic subglacial water and most are smaller than the mean ice thickness of 2 5 km reflecting a focusing of basal water by rugged topography the water system in the gamburtsev subglacial mountains reoccupies a system of alpine overdeepenings created by valley glaciers in the early growth phase of the east antarctic ice sheet the networks follow valley floors either uphill or downhill depending on the gradient of the ice sheet surface in cases where the networks follow valley floors uphill they terminate in or near plumes of freeze on ice indicating source to sink transport within the basal hydrologic system because the ice surface determines drainage direction within the bed constrained network the system is bed routed but surface directed along flow variability in the structure of the freeze on plumes suggests variability in the networks on long 10s of ka timescales possibly indicating changes in the basal thermal state

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

dynamic distributed drainage implied by the flow evolution of the 1996 1998 adventure trench subglacial lake discharge abstract the transport of subglacial water beneath the east antarctic ice sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities and ultimately global climate periodic outbursts are a critical mechanism in this process recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km 3 traveling some 260 km along the ice bed interface of the adventure subglacial trench between 1996 and 1998 wingham et al 2006 rapid discharge connects antarctic subglacial lakes nature 440 1033 1036 using radar echo sounding data from the adventure subglacial trench region in conjunction with the previously reported satellite observations along with some basic modeling we calculate a mass budget and infer a flow mechanism for the 1996 1998 event the volume released from the source lake exceeded the volume received by the destination lakes by 1 1 km 3 this discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward the downstream release of water from the destination lakes continued until at least 2003 several years after the 1998 cessation of surface subsidence at the source lake by 2003 a total of 1 5 km 3 or nearly 75 of the water released by the source lake had traveled downstream from the destination lakes the temporal evolution of discharge from the outlet can be simulated with the classic ice walled semicircular channel model if and only if the retreat of the source lake shoreline is taken into account further downstream the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable along these reaches mechanical lifting of the ice roof and or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge a distributed system is also consistent with the 3 month delay between water release at the source lake and water arrival at the destination lake observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system ultimately the presence of large subglacial lakes along the flow path of the 1996 1998 adventure subglacial trench flow path delayed the arrival of water to points downstream by approximately 12 months

lagrangian observations of the along slope path of the california undercurrent abstract the flow of the california undercurrent cuc along the continental slope and its interaction with deeper offshore waters is studied using trajectories of 53 isobaric rafos the floats were launched off central california and data collected between 1993 and 2009 the mean pressure for the float measurements was 382 dbar the trajectories were broken into 126 segments of continuous alongshore drift over the continental slope poleward drift occurred 88 of the time the mean length of the poleward equatorward drift segments was 262 km 98 km and the exit time for these distances was 32 3 2 3 days 16 1 2 2 days five poleward drift segments exceeded 500 km in length the longest segment stretched from central california to central oregon a distance of 966 km locations where floats left or joined the along slope flow occurred at most latitudes between 35 5 n to 42 5 n here the mean rate at which floats left joined along slope drift was 5 3 3 5 per 50 km of coastline each year locations favored for leaving the undercurrent included the northern edge of the gulf of the farallones 37 5 n and between cape mendocino and cape blanco 41 5 42 n a location favored for joining the undercurrent was the northern edge of the santa lucia bank 35 5 n no floats were observed to join the alongshore flow immediately north of cape mendocino 40 5 41 n longer 290 day float missions often included offshore excursions into the northeastern pacific ocean thirty one westward excursions of the undercurrent were observed with a mean duration of 88 days and a mean offshore distance of 199 km the character of the excursions varied but included offshore and onshore transport in jets cyclonic and anticyclonic eddies and interaction with cold filaments the longest excursion lasted 587 days and extended 1007 km to the west

seasonal cycles of currents temperatures winds and sea level over the northeast pacific continental shelf 35 n to 48 n seasonal cycles of coastal wind stress adjusted sea level asl shelf currents and water temperatures off the west coast of north america 35 n to 48 n are estimated by fitting annual and semiannual harmonics to data from 1981 1983 longer records 9 34 years of monthly asl indicate that these two harmonics adequately represent the long term monthly average seasonal cycle and that the current measurement period is long enough to estimate the seasonal cycles we characterize the differences between fall winter and spring summer as follows for fall winter monthly mean winds north of 35 n are northward for 3 6 months longer in the north than in the south south of 35 n the mean winds are near zero or weakly southward monthly mean alongshore currents are northward over midshelf and shelf break at all locations sampled at depths of 35 m and deeper and are associated with high coastal sea levels and relatively warm water temperatures for spring summer monthly mean wind stresses are southward at all latitudes for 3 6 months longer in the south than in the north sea levels are low and water temperatures are relatively cool monthly mean currents at 35 m depth over the shelf are southward for 1 6 months longer at the shelf break than over midshelf and longer in the north than in the south while the deeper currents are less southward or northward the magnitudes of the seasonal cycles of all variables are maximum between approximately 38 n and 43 n generally decreasing slightly to the north and greatly to the south at each location the seasonal cycle of the alongshore current from 35 m depth at midshelf leads the sea level slightly and both lead the wind stress and temperatures by 1 2 months the seasonal cycles of all variables show a south to north progression south leads north by 1 2 months at 48 n annual mean currents at 50 m depth over the shelf break oppose the annual mean wind northward wind and southward current similarly at 35 n annual mean currents at 35 m depth over both midshelf and shelf break are opposed to the annual mean wind southward wind and northward current from 35 n to 43 n both summer and winter regimes are dominated by strongly fluctuating currents

fracture propagation propensity in relation to snow slab avalanche release validating the propagation saw test 1 the propagation saw test pst was designed to assess the fracture propagation propensity of weak snowpack layers in relation to snow slab avalanche release its predictions were tested against independent field observations of weak layer fracture initiation and slope scale fracture propagation e g avalanche release a total of 170 tests were performed at 23 sites approximately 76 of tests correctly predicted the observed slope scale fracture propagation or lack thereof however 20 of tests predicted that propagation would not occur at sites that had recently propagated fractures the predictive accuracy of the dataset improves if only test columns approximately 1 0 m long are selected critical fracture energy release rate calculations show that the lowest values are found in snowpacks where fractures initiated but did not propagate this suggests that physical descriptions of propagation propensity in weak snowpack layers should include a sustainability term

near surface mean circulation and kinetic energy in the central north atlantic from drifter data an analysis of a large drifting buoy data set is presented the objective is to obtain a self contained description of the properties of the near surface circulation drogue depth 100 m in the central north atlantic ocean independent of hydrographic data a necessary preanalysis step was the removal of all data from undrogued buoys from the data set the physical parameters of the circulation were deduced by averaging the remaining data in 2 3 boxes the minimum amount of data which is necessary to get statistically stable results was determined by an empirical quality criterion all important mean currents in the investigation area are reproduced by the near surface mean velocity field a separation of the mean velocity field into a nondivergent and an irrational part shows that the flow field is almost nondivergent the distribution of eddy kinetic energy is concentrated along the mean currents and provides the largest part of the total kinetic energy but there are regional variations energy from inertial movements dominates the high frequency part of the kinetic energy its distribution is very patchy the analysis of the reynolds stress terms shows an energy transfer from the eddy field to the mean circulation in the vicinity of the north atlantic current

modeling 5 years of subglacial lake activity in the macayeal ice stream antarctica catchment through assimilation of icesat laser altimetry subglacial lakes beneath antarctica s fast moving ice streams are known to undergo 1 km 3 volume changes on annual timescales focusing on the macayeal ice stream macis lake system we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from ice cloud and land elevation satellite icesat laser altimetry we construct a steady state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the icesat derived filling or draining rates the modeled volume change rates of five large subglacial lakes in the downstream portion of macis are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered however the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream overall we show that for this fast flowing ice stream most subglacial lakes receive 90 of their water from distant distributed sources throughout the catchment and we confirm that water is transported from regions of net basal melt to regions of net basal freezing our study provides a geophysically based means of validating subglacial water models in antarctica and is a potential way to parameterize subglacial lake discharge events in large scale ice sheet models where adequate data are available

spatial and temporal characteristics of the mesoscale circulation of the california current from eddy resolving moored and shipboard measurements moored observations of currents and temperatures made in the upper 600 rn on eddy resolving scales over a 2 year period are used to examine the spatial and temporal characteristics of the california current mesoscale circulation the observations were made at three principal longitudes 124ow 126ow and 128ow in the vicinity of point arena they bracket the 600 km wide band of high mesoscale variability found along the eastern boundary of the north pacific at all locations the mesoscale variability was larger than the mean flow and the spatial modes of variability as determined from empirical orthogonal function analysis consisted of an alongshore mode a cross shore mode and a rotational mode observations made near the continental slope 124ow were dominated by the poleward flowing california undercurrent with mesoscale eddies and meanders superposed the nearshore eddy kinetic energy peaked in a band centered around 60 days observations made at 128ow near the offshore boundary between the energetic mesoscale band and the eddy desert of the northeast pacific were characterized by small means fewer eddy events and a peak in eddy kinetic energy at 120 180 days the good horizontal resolution of the current meter arrays allowed us to estimate the relative vorticity horizontal divergence and rossby number and therefore to evaluate the relative strength and occurrence of anticyclones and cyclones we found the mesoscale eddy field to be strongly nonlinear with rossby numbers ranging from 0 1 to 0 5 all of the eddies observed at the offshore site were nonlinear deep warm anticyclones shipboard hydrography revealed the origin of one of these anticyclones to be the california undercurrent and this eddy retained its strong anomalies after several months and several hundred kilometers of propagation despite the lower incidence of eddies as one moves west from the coast the eddies that we observed offshore provide evidence for propagation and transport of properties from the coast to the central north pacific across the california current system

on subinertial flow in submarine canyons effect of geometry shelf break canyons on the west coast of canada and the united states have been observed to be regions of enhanced upwelling during southward currents compared to the surrounding shelf break most shelf break canyons from oregon north cross only part of the continental shelf cutting from the shelf break toward the coast but end on the continental shelf well below the mixed layer juan de fuca canyon on the other hand cuts the continental shelf from the slope to and actually continues into the strait of juan de fuca this difference in geometry has a very strong effect on the subinertial flow around the canyon model canyon shapes which include convergent bathymetric contours are constructed and motivated for juan de fuca canyon and a typical shelf break canyon geostrophic analytic solutions show that the in canyon flow in juan de fuca canyon is generated by first order geostrophic dynamics whereas in the majority of canyons of which astoria is an example in canyon flow is generated by higher order effects this difference is postulated to lead to the observed very deep upwelling over juan de fuca canyon compared to more moderate episodic upwelling over astoria canyon

seasonal variability of alongshore geostrophic velocity off central california seasonal variability of alongshore geostrophic velocity relative to 500 dbar is examined from 23 years of hydrographic data along two sections off central california one off point sur and the other off point conception the seasonal cycles are determined by least square fits of the gappy data records to harmonics with annual and semiannual periods attention is focused on variability over the outer continental slope geostrophic flow in the upper 100 m along both sections is coherent and predominantly annual with equatorward flow from february to september and poleward flow from october to january the flow deeper than 100 m is distinctly different along the two sections the nearshore deep flow is predominantly semiannual off point conception notably different from surface flow and poleward all year with maxima in december and june this semiannual variability at depth is a prevalent feature of the california current system the nearshore deep flow off point sur is unusual in that it is predominantly annual with maximum poleward flow in december and weak equatorward flow from march to may the seasonal variations over the continental slope compare favorably with existing models of eastern boundary current systems in two respects 1 the surface flow leads the predominantly annual wind forcing in this region by about one month and 2 the deep poleward counterflow is coherent with the local poleward barotropic pressure gradient at both locations with a phase lag of approximately 2 months however contrary to the results of existing dynamical models the semiannual poleward pressure gradient and undercurrent off point conception appear to be unrelated to the wind forcing which is predominantly annual in this region these semiannual variations are highly coupled to the semiannual flow throughout the water column inside the southern california bight suggesting that they may be topographically generated

synthesizing multiple remote sensing techniques for subglacial hydrologic mapping application to a lake system beneath macayeal ice stream west antarctica we present an analysis of the active hydrologic system of macayeal ice stream macis west antarctica from a synthesis of multiple remote sensing techniques satellite laser altimetry satellite image differencing and hydrologic potential mapping using a satellite derived dem and a bedrock dem from airborne radio echo sounding combining these techniques augments the information provided by each one individually and allows us to develop a protocol for studying subglacial hydrologic systems in a holistic manner our study reveals five large active subglacial lakes under macis the largest of which undergoes volume changes of at least 1 0 km 3 we discuss the hydrologic properties of this system and present evidence for links between the lakes at least three of the lakes are co located with sticky spots i e regions of high local basal shear stress we also find evidence for surface elevation changes due to ice dynamic effects not just water movement caused by changes in basal resistance lastly we show that satellite radar altimetry is of limited use for monitoring lake activity on fast flowing ice streams with surfaces that undulate on 10 km length scales

subtidal currents over the central california slope evidence for offshore veering of the undercurrent and for direct wind driven slope currents abstract in february 1991 an array of six current meter moorings was deployed for one year across the central california outer shelf and slope the main line of the array extended 30 km offshore of the shelf break out to water depths of 1400 m a more sparsely instrumented line displaced 30 km to the northwest extended 14 km offshore though shorter the northern line spanned similar water depths because the gradient of the topography steepened in the northern region a poleward flow pattern typical of the california undercurrent was seen across both lines in the array over most of the year the poleward flow was surface intensified in general the portion of the undercurrent that crossed the southern line had larger amplitudes and penetrated more deeply into the water column than the portion that crossed the northern line transport over the year ranged from 0 to 2 5 sverdrups sv poleward across the southern line 0 to 1 sv poleward across the northern line we suggest the difference in transport was caused by topographic constraints which tended to force the poleward flow offshore of the northern measurement sites the slope of the topography steepened too abruptly to allow the poleward flow to follow isobaths when currents were strong when current velocities lessened a more coherent flow pattern was seen across both lines in the array in general the poleward flow patterns in the undercurrent were not affected by local winds or by the local alongshore pressure gradient nor was a strong seasonal pattern evident rather unexpectedly a small but statistically significant fraction of the current variance over the mid and outer slope was driven by the surface wind stress an alongshelf wind stress caused currents to flow along the slope parallel to the wind field down to depths of 400 m below the surface and out to distances of 2 rossby radii past the shelf break the transfer functions were weak 3 4 cm s per dyn cm 2 but comparable to wind driven current amplitudes of 4 6 cm s per unit wind stress over the middle shelf equatorward alongshelf winds also caused water from 200 300 m over the slope to upwell onto the shelf as the surface water moved offshore

a numerical study of the seasonal variability in the circulation off central chile the princeton ocean model is used to study the coastal ocean circulation off central chile between 30 and 45 s and its seasonal transitions in response to monthly mean climatological wind stress and surface heat flux the model reproduces many of the dominant circulation features including an equatorward coastal current coastal upwelling and a poleward undercurrent to get the correct seasonal variability in the undercurrent it was necessary to prescribe the inflow of the undercurrent on the northern model boundary meanders and eddies form throughout the simulation period seaward of the coastal current in response to baroclinic instability of the flow the flow associated with the meanders shows upwelling in the shoreward flow and downwelling in the seaward flow this upwelling downwelling pattern is explained by conservation of relative vorticity in the flow the upwelling velocity associated with the meanders corresponds to a vertical migration of 15 30 m over a typical timescale of the meanders of 1 2 months it is argued that this upwelling may be important for maintaining observed high phytoplankton biomass and primary production seaward of the coastal zone directly affected by wind driven upwelling

alongshore currents and mesoscale variability near the shelf edge off northwestern australia 1 shelf break conditions and alongshore flow off northwestern australia are studied during the strongly evaporative conditions of austral winter 2003 present results along with those of previous authors confirm that a poleward fresh leeuwin current core is normally found near the shelf break salinity increases alongshore toward the southwest although there is no obvious shelf break front there is a persistent offshore upward tilting of isopycnals in the depth range of 100 150 m repeated mesoscale surveys were made at the shelf edge during 8 days when the shelf break flow was equatorward waters offshore of the shelf break contain a rich baroclinic ageostrophic eddy field with typically 10 km length scales and the eddy patterns in the upper 60 m are uncorrelated with those below 90 m the two depth horizons yield energy transfers from mean to eddy potential energy and appear to represent distinct finite amplitude instabilities on upper ocean and 180 m mean velocity cores respectively a linear stability model supports the existence of the two vertically separated instability types

hydrographic observations along the code central line off northern california 1981 abstract repeated ctd observations were made along a single section spanning the continental shelf off northern california at 38 40 n during 1981 the section consisted of nine standard stations between 1 and 46 km from the shore in water depths between 40 and 1700 m the shelf break was at a depth of 150 m about 25 km from shore the section was occupied 17 times between 13 april and 3 august and twice in december and a similar section had been occupied in february during the april august period winds were strong and persistently favorable for upwelling isotherms isohalines and isopycnals in the upper 200 in sloped persistently upward toward the coast and coldest saltiest and densest surface waters almost always occurred at the most inshore nation variations in wind strength caused changes in the surface layer over the entire shelf stronger winds were associated with lower temperatures and higher salinities subsurface temperatures increased gradually between april and august we were unable to

why is it hard to predict the future of ice sheets ice sheet retreat hypotheses of instability could be missing important processes that limit the rate or extent of retreat or conversely ice sheet behavior is strongly influenced by processes at its margin and base observations of rapid changes at these boundaries are helping modelers to improve predictions of future changes

high sensitivity of subglacial hydrological pathways in antarctica to small ice sheet changes 1 the flow of water beneath the antarctic ice sheet has recently been shown to involve drainage from and exchange between subglacial lakes with flow routing over distances of 100km as subglacial water is known to be important to ice sheet dynamics changes to the flow routing of basal water may yield an ice sheet response here we show that the directions of subglacial hydrological flowpaths are highly sensitive to the ice sheet surface topography at certain sites in antarctica surface elevation changes of between 5 and 15 m are found to be sufficient to produce significant changes to the subglacial water flowpaths which affect the potential drainage routes of large subglacial lakes we identify the reasons for this high sensitivity and recognise several locations in east antarctica where changes to the drainage regime could have important effects on large scale basal water distribution

variability of the near surface eddy kinetic energy in the california current based on altimetric drifter and moored current data low pass filtered velocities obtained from world ocean circulation experiment woce surface drifters deployed in the california current off northern california during 1993 1995 have been compared with surface geostrophic velocity estimates made along subtracks of the topex poseidon altimeter and with moored acoustic doppler current profiler adcp data to obtain absolute geostrophic velocities a mean sea surface height ssh field was estimated using the mean drifter velocities and historical hydrographic data and was added to the altimetric ssh anomalies the correlation between collocated drifter and altimetric velocities is 0 73 significant at the 95 level the component of the drifter velocity which was uncorrelated with the altimetric velocity was correlated with the wind in the ekman transport sense monthly averages of eddy kinetic energy eke estimated using all drifter and altimeter data within the domain 124 132 w 33 40 5 n show energy levels for the drifters that are about 13 greater than those for the altimeter drifter altimeter and adcp measurements all exhibit similar seasonal cycles in eke with the altimeter data reaching maximum values of about 0 03 m 2 s 2 in late summer fall wavenumber spectra of the altimeter velocity indicate that the velocity fluctuations were dominated by features with wavelengths of 240 370 km while the adcp data suggest that the temporal scales of these fluctuations are at least several months between 36 and 40 5 n the region of monthly maximum eke migrates westward to about 128 w on a seasonal timescale this region of maximum eke coincides with the maximum zonal ssh gradient with increased eke associated with increased southward flow a simple model shows that much of the seasonal cycle of the ssh anomalies can be produced by linear processes forced by the curl of the wind stress although the model cannot explain the offshore movement of the front

structure of an eddy measured with drifters drifters are used to measure the structure of an eddy on western bank scotian shelf over a 5 day period a regression method that uses both the temporal and spatial structure of flow field observations is presented and used to analyze this eddy the eddy has vorticity 1 9 10 5 s 1 with divergence stretching deformation and shearing deformation smaller by more than an order of magnitude the eddy is centered on the crest of western bank the motion residual to the eddy had a slab like part that moved all points on a plane in a circle without rotation this latter type of motion can cause substantial error when vorticity and other eddy properties are estimated using methods that operate on a single drifter track the eulerian statistics of residual motion show correlations greater than 0 5 are restricted to spatial separations less than 2 km and temporal lags less than 3 h but motion directed along the vector separating two velocity measurements has a significant eulerian correlation for timescales of more than 40 hours and length scales of order 5 km transverse motion is positively correlated over lesser scales much cluster distortion results from subtidal frequency motion that has spatial scales that are seldom coherent over drifter separation scales greater than 2 3 km the long lagrangian timescale is inconsistent with a description of horizontal mixing in terms of an eddy diffusion coefficient rather we might picture drifters being dispersed by slowly evolving narrow streams of current that have larger spatial scales along their axis than across their axis

a separating coastal upwelling jet at cape blanco oregon and its connection to the california current system the coastal upwelling region near cape blanco oregon 433n o the west coast of the united states was studied using a towed ctd on seasoar a shipboard acoustic doppler current pro ler adcp satellite sea surface temperature maps and surface drifters during august 1995 the equatorward upwelling jet was inshore of the shelfbreak north of cape blanco meandered gently o shore around the cape and then veered sharply o shore just to the south of the cape analysis of vertical sections of density velocity and spicinessa con rmed the separation of an upwelling jet and front the jet was followed from over the shelfbreak north of the cape to 100 km o shore downstream of the cape the separating jet originates from the coastal upwelling jet leaving the shelf but is augmented by an o shore branching of the poleward undercurrent by combining information from adcp streamfunction maps with analysis of hydrographic data including distributions of the tracer like quantity spicinessa a conceptual model of the three dimensional circulation near the cape emerges a mid shelf upwelling jet encounters a coastal promontory then turns o shore stretching and deepening as it crosses the continental margin the jet then turns back shoreward cyclonically where the deepened equatorward ow encounters the top of the poleward undercurrent owing along the continental slope this causes a portion of the undercurrent to turn o shore to join and strengthen the equatorward transport in the separated jet throughout the separation process the coastal upwelling front and jet are continuous robust features separation of a coastal upwelling jet is an important mechanism for cross shelf transport from the coast to the deep ocean cape blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current characteristic of the california current system

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream