jokulhlaups a reassessment of floodwater flow through glaciers 1 in glaciated catchments glacier generated floods jokulhlaups put human activity at risk with large sporadic jokulhlaups accounting for most flood related fatalities and damage to infrastructure in studies of jokulhlaup hydrodynamics the view predominates that floodwater travels within a distinct conduit eroded into the underside of a glacier however some jokulhlaups produce subglacial responses wholly inconsistent with the conventional theory of drainage by focusing on icelandic jokulhlaups this article reassesses how floodwater flows through glaciers it is argued that two physically separable classes of jokulhlaup exist and that not all jokulhlaups are an upward extrapolation of processes inherent in events of lesser magnitude and smaller scale the hydraulic coupling of multiple nonlinear components to the flood circuit of a glacier can induce extreme responses including pressure impulses in subglacial drainage representing such complexity in mathematical form should be the basis for upcoming research as future modeling results may help to determine the glaciological processes behind heinrich events moreover such an approach would lead to more accurate predictive models of jokulhlaup timing and intensity

calculation of ice velocities in the jakobshavn isbrae area using airborne laser altimetry abstract the ability to measure velocities of fast moving glaciers is essential to the understanding and interpretation of their dynamics and related processes this is especially important in the jakobshavn drainage basin of the greenland ice sheet where a significant portion of the ice sheet mass loss occurs and the iceberg calving rate is high in this article we present a method for using airborne laser altimetry data to determine surface velocities in portions of the jakobshavn drainage basin that have distinct elevation features by examining correlations between repeat elevation surveys the motion of these features the most distinct of which are crevasses is tracked results show that for areas with a standard deviation in elevation in excess of 1 m displacement of features can be tracked to a precision of 1 2 m and the corresponding velocities can be retrieved the maximum velocity of the jakobshavn isbrae at its calving front is found to be nearly 7 km yr 1 ice stream boundaries and flow patterns can also be observed in the derived velocity data

monitoring the daytime variations of equatorial ionospheric anomaly using ionex data and champ gps data a method which can invert the ionospheric electron content above the champ satellite is presented the daytime behavior of the equatorial ionospheric anomaly eia is monitored using the champ global positioning system data and the ionosphere map exchange format data supplied by the center for orbit determination in europe during the period at quiet geomagnetic conditions between november 7 and december 12 2003 results show that the plasma congregates and rises above the geomagnetic equator due to the daytime upward vertical drift during the early variations of eia after being lifted to higher altitudes the topside ionospheric equatorial plasma above the champ orbit over 400 km above the ground controlled mainly by the electromagnetic field starts to diffuse downward to higher latitudes along magnetic field lines and the double humped structure begins to appear in the topside ionosphere above the champ orbit however a single crest is still observed for the entire ionosphere above the ground and this single peak moves slightly to the winter hemisphere as a result of the transequatorial neutral wind with the continuous diffusion of the plasma the two crests also occur for the entire ionosphere at both sides of the geomagnetic equator

application of image cross correlation to the measurement of glacier velocity using satellite image data abstract image to image cross correlation software is applied to pairs of digital satellite images to map the velocity field of moving ice this technique uses small scale glacial surface features such as crevasse scars and snow dunes as markers on the surface of the moving ice displacements of the surface features are mapped by selecting small image areas centered on distinct features or by dividing a large area of densely featured glacial surface into a grid of areas and searching a subsequent image for matching areas using a cross correlation algorithm interpolation of the peak correlation values allows the displacements to be measured to subpixel accuracy resulting in very precise velocity measurements cross correlation is also applied to provide image coregistration in areas devoid of bedrock exposures in such areas subtle large scale topographic undulations in the ice surface related to underlying bedrock structure may be correlated by using large image areas and low pass filtered images both types of applications are demonstrated using ice stream d and ice stream e in west antarctica as test areas a high resolution map of the velocity field of the central portion of ice stream e generated by the displacement measuring technique is presented the use of cross correlation software is a significant improvement over previous manually based photogrammetric methods for velocity measurement and is far more cost effective than in situ methods in remote polar areas

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

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

location of the eastern boundary of the cocos plate during the miocene abstract the eastern equatorial pacific composed of and bordered by the cocos nazca and south american plates is tectonically complex because of large changes in configuration in the recent past the north south trending eastern boundary of the cocos plate during the miocene was apparently located in the transisthmian area of the present panama canal several sets of data topography bathymetry seismicity hot spring locations faulting patterns and gravity indicate that there is a major trans isthmian tectonic boundary between 79 and 80 w coincident with the postulated paleo plate boundary this feature which has been recognized for some 50 years by many geologists we name the canal discontinuity

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

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

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

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

timing of recent accelerations of pine island glacier antarctica 1 we have used interferometric synthetic aperture radar insar data and sequential landsat imagery to identify and temporally constrain two acceleration events on pine island glacier pig these two events are separated by a period of at least seven years 1987 1994 the change in discharge between two flux gates indicates that the majority of the increase in discharge associated with the second acceleration originates well inland 80 km from the grounding line an analysis indicates that changes in driving stress consistent with observed thinning rates are sufficient in magnitude to explain much of the acceleration

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

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

icesat antarctic elevation data preliminary precision and accuracy assessment 1 since first light on february 20th 2003 nasa s ice cloud and land elevation satellite icesat has derived surface elevations from 86 n to 86 s latitude these unique altimetry data have been acquired in a series of observation periods in repeated track patterns using all three geoscience laser altimeter system glas lasers here we focus on antarctic ice sheet elevation data that were obtained in 2003 2004 we present preliminary precision and accuracy assessments of selected elevation data and discuss factors impacting elevation change detection we show that for low slope and clear sky conditions the precision of gla12 laser 2a release 21 data is 2 1 cm and the relative accuracy of icesat elevations is 14 cm based on crossover differences

2001 2009 elevation and mass losses in the larsen a and b embayments antarctic peninsula we investigate the elevation and mass balance response of tributary glaciers following the loss of the larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively our study uses modis imagery to track ice extent and aster and spot5 digital elevation models dems plus atm and icesat laser altimetry to track elevation changes spanning the period 2001 09 the measured larsen b tributary glaciers hektoria green evans punchbowl jorum and crane lost up to 160 m in elevation during 2001 06 and thinning continued into 2009 elevation changes were small for the more southerly flask and leppard glaciers which are still constrained by a larsen b ice shelf remnant in the northern embayment continued thinning of 3 m a 1 on drygalski glacier 14 years after the larsen a ice shelf disintegrated suggests that mass losses for the exposed larsen b tributaries will continue for years into the future grounded ice volume losses exceed 13 km 3 for crane glacier and 30 km 3 for the hektoria green evans glaciers the combined mean loss rate for 2001 06 is at least 11 2 gt a 1 our values differ significantly from published mass budget based estimates for these embayments but are a reasonable fraction of grace derived rates for the region 40 gt a 1

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

spot5 hrs digital elevation models and the monitoring of glacier elevation changes in north west canada and south east alaska abstract monitoring the response of land ice to climate change requires accurate and repeatable topographic surveys the spot5 hrs high resolution stereoscopic instrument covers up to 120 km by 600 km in a single pass and has the potential to accurately map the poorly known topography of most glaciers and ice caps the acquisition of a large hrs archive over ice covered regions is planned by the french space agency cnes and spotimage france during the 2007 2008 international polar year ipy here we report on the accuracy and value of hrs digital elevation model dem over ice and snow surfaces a dem is generated by combining tools available from cnes with the pci orthoengine se software using hrs images acquired in may 2004 over south east alaska usa and northern british columbia canada the dem is evaluated through comparison with shuttle radar topographic mission srtm dem and icesat data on and around the glaciers a horizontal shift of 50 m is found between the hrs and srtm dems and is attributed to errors in the srtm dem over ice free areas hrs elevations are 7 m higher than those of srtm with a standard deviation of 25 m for the difference between the two dems the 7 m difference is partly attributed to the differential penetration of the electromagnetic waves visible for hrs microwave for srtm in snow and vegetation we also report on the application of sequential dems srtm dem in february 2000 and hrs dem in may 2004 for the monitoring of glacier elevation changes we map the topographic changes induced by a surge of one tributary of ferris glacier maximum surface lowering of 42 10 m and rising of 77 10 m are observed in the 4 years time interval thinning rates up to 10 2 5 m yr are observed at low altitudes and confirm the ongoing wastage of glaciers in south east alaska

the triggering of sub glacial lake drainage during the rapid glacier drawdown crane glacier antarctic peninsula ice surface altimetry from icesat 1 and nasa aircraft altimeter overflights spanning 2002 09 indicate that a region of lower crane glacier antarctic peninsula shows an unusual temporal pattern of elevation loss a period of very rapid drawdown 91 m a 1 between september 2004 and september 2005 bounded by periods of large but more moderate rates 23 m a 1 until september 2004 12 m a 1 after september 2005 the region of increased drawdown is 4 5 km 2 2 km based on satellite aster and spot 5 stereo image digital elevation model dem differencing spanning the event in a later differential dem the anomalous drawdown feature is not seen bathymetry in crane glacier fjord reveals a series of flat lying formerly subglacial deeps interpreted as lake sediment basins we conclude that the elevation change feature resulted from drainage of a small deep subglacial lake we infer that the drainage event was induced by hydraulic forcing of subglacial water past a downstream obstruction however only a fraction of crane glacier s increase in flow speed that occurred near the time of lake drainage derived from image feature tracking appears to be directly attributable to the event instead retreat of the ice front off a subglacial ridge 6 km downstream of the lake is likely the dominant cause of renewed fast flow and more negative mass balance in the subsequent 4 years

pattern of retreat and disintegration of larsen b ice shelf antarctic peninsula the retreat of the larsen b ice shelf antarctic peninsula and the collapse of its northern section are analyzed using satellite images acquired between january 1995 and may 2003 over 1 week during march 2002 after a period of steady retreat since 1995 2300 km 2 of the ice shelf broke up into many small icebergs this rapid collapse occurred at the end of an exceptionally warm summer and after a multi year period of decreasing surface net mass balance ice thinning flow acceleration and widening of rifts the ice shelf area decreased from 11 512 km 2 in january 1995 to 3463 km 2 in march 2002 and 2667 km 2 in april 2003 ers synthetic aperture radar sar images were used to identify ice shelf zones with different surface morphology which generated icebergs of different sizes and shapes the pattern of retreat and break up similar to that of larsen a in 1995 suggests that fracturing enhanced by abundant surface melt played a key role in addition the recent changes of grounded and residual floating ice north of larsen b are analyzed by means of envisat advanced synthetic aperture radar asar images up to summer 2003 showing significant loss of grounded ice upstream of those ice shelf sections which disintegrated in 1995 and 2002

fast tidewater glaciers some iceberg calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets no temperate tidewater glaciers are known to have floating termini but many polar and subpolar tidewater glaciers do the fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed which may approach zero and the longitudinal back stress on the terminus the terminus boundary condition the calving relation is imperfectly known yet is vital to the dynamics of these glaciers calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested the calving relations for floating termini are virtually unknown this together with the imperfect understanding of basal sliding inhibits confidence in our understanding of the stability of these glaciers columbia glacier alaska is an instructive example because observations have been made on the major changes in its geometry calving rate and dynamics that have occurred in less than 10 years the calving flux has increased more rapidly than the glacier flux causing thinning and retreat as a result the ice velocity has increased markedly the short term velocity changes relate to changes in back pressure ice recession tidal changes and the flux of water injected to the bed these results have relevance to the mechanisms of basal sliding glacier surges and the stability of marine ice streams

the gamburtsev mountains and the origin and early evolution of the antarctic ice sheet the manner in which the antarctic ice sheet first formed at a time of rapid global climate change about 34 million years ago has been the subject of considerable speculation several lines of evidence point to central antarctic mountain ranges as the site of the initial ice sheet but our knowledge of the present ice sheet topography is highly restricted in the very zone where ice sheet initiation is most likely along the subglacial gamburtsev mountain range at the centre of the present ice sheet now a detailed survey and analysis of the morphology of the gamburtsev mountains has been performed based on an ice penetrating radar study conducted over two seasons the data reveal a mountain landscape initially incised by rivers then over deepened by ice movement the topography beneath up to 3 000 metres of ice resembles an exploded version of classic alpine valley topography and seems to have developed more than 34 million years ago when mean summer temperatures were about 3 c the nature of initial glaciation on antarctica about 34 million years ago is a mystery results from an intensive radar survey now show that the gamburtsev mountains at dome a at the centre of the present ice sheet were initially incised by rivers and then subsequently overdeepened by ice movement this is suggestive of topographical development before 34 million years ago when mean summer temperatures were about 3 c ice sheet development in antarctica was a result of significant and rapid global climate change about 34 million years ago1 ice sheet and climate modelling suggest reductions in atmospheric carbon dioxide less than three times the pre industrial level of 280 parts per million by volume that in conjunction with the development of the antarctic circumpolar current led to cooling and glaciation paced by changes in earth s orbit2 based on the present subglacial topography numerical models point to ice sheet genesis on mountain massifs of antarctica including the gamburtsev mountains at dome a the centre of the present ice sheet2 3 our lack of knowledge of the present day topography of the gamburtsev mountains4 means however that the nature of early glaciation and subsequent development of a continental sized ice sheet are uncertain here we present radar information about the base of the ice at dome a revealing classic alpine topography with pre existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 c this landscape is likely to have developed during the initial phases of antarctic glaciation according to antarctic climate history estimated from offshore sediment records the gamburtsev mountains are probably older than 34 million years and were the main centre for ice sheet growth moreover the landscape has most probably been preserved beneath the present ice sheet for around 14 million years