the 1979 2005 greenland ice sheet melt extent from passive microwave data using an improved version of the melt retrieval xpgr algorithm analysis of passive microwave satellite observations over the greenland ice sheet reveals a significant increase in surface melt over the period 1979 2005 since 1979 the total melt area was found to have increased by 1 22 x 10 7 km 2 an improved version of the cross polarized gradient ratio xpgr technique is used to identify the melt from the brightness temperatures the improvements in the melt retrieval xpgr algorithm as well as the surface melt acceleration are discussed with results from a coupled atmosphere snow regional climate model from 1979 to 2005 the ablation period has been increasing everywhere over the melt zone except in the regions where the model simulates an increased summer snowfall indeed more snowfall in summer decreases the liquid water content of the snowpack raises the albedo and therefore reduces the melt finally the observed melt acceleration over the greenland ice sheet is highly correlated with both greenland and global warming suggesting a continuing surface melt increase in the future

climate change greenland s ice on the scales satellite measurements of changes in earth s gravity field reveal ice loss from greenland s ice sheet over the past four years this melt has contributed to global sea level rise at an accelerating rate

about the role of westerly wind events in the possible development of an el nino in 2014 similarities between early 1997 and 2014 has prompted climate scientists to wonder if an el nino matching the 1997 el nino of the century could develop in 2014 until april 2014 the equatorial pacific exhibited positive heat content anomalies along with an eastward warm pool displacement similar to those found during the onset of strong el nino events yet in july 2014 the warm pool had retreated back to its climatological positions and equatorial temperature anomalies were much weaker than in mid 1997 dedicated oceanic simulations reveal that these weak interannual anomalies can be attributed to differences in westerly wind event wwe sequences in contrast with 1997 the lack of wwes from april to june significantly limited the growth of eastern pacific anomalies and the eastward warm pool displacement in 2014 with the absence of additional wwe activity prospects for a mature el nino in late 2014 are fading

westerly wind events in the tropical pacific 1986 95 abstract based on examination of 10 yr of 10 m winds and wind anomalies from european centre for medium range weather forecasts ecwmf analysis definitions for westerly wind events wwes of eight different types are proposed the authors construct a composite for each type of event show that a simple propagating gaussian model satisfactorily describes the evolution of zonal wind anomaly for each type of event and determine the scales of each composite event by fitting the model to each composite the authors discuss the wwes that occurred during the tropical oceans global atmosphere coupled ocean atmosphere response experiment toga coare intensive observing period iop and show the extent to which these composite events are able to reproduce the major westerly wind features of the iop the frequency of occurrence of each type of wwe for each year of this record and by calendar month are described the authors find several types of events are negatively correlated with the annual mean troup souther

holocene deceleration of the greenland ice sheet recent peripheral thinning of the greenland ice sheet is partly offset by interior thickening and is overprinted on its poorly constrained holocene evolution on the basis of the ice sheet s radiostratigraphy ice flow in its interior is slower now than the average speed over the past nine millennia generally higher holocene accumulation rates relative to modern estimates can only partially explain this millennial scale deceleration the ice sheet s dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across nares strait also contributed to this pattern thus recent interior thickening of the greenland ice sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry

growth of greenland ice sheet measurement measurements of ice sheet elevation change by satellite altimetry show that the greenland surface elevation south of 72 degrees north latitude is increasing the vertical velocity of the surface is 0 20 0 06 meters per year from measured changes in surface elevations at 5906 intersections between geosat paths in 1985 and seasat in 1978 and 0 28 0 02 meters per year from 256 694 intersections of geosat paths during a 548 day period of 1985 to 1986

continental climate gradients in north america and western eurasia before and after the closure of the central american seaway abstract the gulf stream as part of the atlantic meridional overturning circulation amoc is known as a major driver of latitudinal energy transport in the north atlantic presently causing mild winters over northwestern eurasia the intensity of the amoc throughout the neogene prior to the final closure of the central american seaway cas in the early pliocene is still poorly known but most authors assume that the circulation was considerably weaker than present here we address this issue from a continental point of view we studied the past amoc intensity by analyzing neogene continental climate patterns along north american and western eurasian transects based on a total of 317 palaeofloras thermal latitudinal gradients are reconstructed for three neogene time slices namely the middle miocene late miocene and late pliocene using the coexistence approach to obtain quantitative climate data the obtained proxy based continental temperature gradients are evaluated against data from a selection of published general circulation model gcm simulations for the three time slices studied our study suggests that shallow thermal latitudinal gradients existed in north america and western eurasia throughout the miocene but became strongly steepened in the late pliocene in both miocene time slices studied the higher latitudes were by up to 30 c warmer than present cold month mean also at times with presumed pre industrial co2 such as the late miocene in the late pliocene high latitude the temperature difference with respect to the present had decreased by up to 10 c cold month mean both mean annual temperatures and cold month means of the lower mid and low latitudes were at the present day level throughout all three time slices or even slightly below in both miocene time slices zonal temperature means at both continental transects were similar in the mid and higher latitudes however several northwest european sites reveal very mild winter condition suggesting the early existence of a probably less intense palaeo gulf stream the distinct thermal anomaly annual and cold month means today existing between north america and western eurasia appeared for the first time in the late pliocene attaining about 50 of the present day magnitude this supports the assumption that the amoc intensified after the final closure of the cas during the early pliocene the results obtained from the palaeobotanical proxies are in line with data from coeval marine archives particularly with north atlantic sea surface temperatures ssts inferred from oxygen isotopes however the proxy based thermal gradients are not well reproduced by a selection of gcm simulations due to a well known systematic underestimation of high latitude warming by gcms for the miocene and pliocene time slices

the nature of the stochastic wind forcing of enso abstractel nino southern oscillation enso is commonly viewed as a low frequency tropical mode of coupled atmosphere ocean variability energized by stochastic wind forcing despite many studies h

recent sea level contributions of the antarctic and greenland ice sheets after a century of polar exploration the past decade of satellite measurements has painted an altogether new picture of how earth s ice sheets are changing as global temperatures have risen so have rates of snowfall ice melting and glacier flow although the balance between these opposing processes has varied considerably on a regional scale data show that antarctica and greenland are each losing mass overall our best estimate of their combined imbalance is about 125 gigatons per year of ice enough to raise sea level by 0 35 millimeters per year this is only a modest contribution to the present rate of sea level rise of 3 0 millimeters per year however much of the loss from antarctica and greenland is the result of the flow of ice to the ocean from ice streams and glaciers which has accelerated over the past decade in both continents there are suspected triggers for the accelerated ice discharge surface and ocean warming respectively and over the course of the 21st century these processes could rapidly counteract the snowfall gains predicted by present coupled climate models

on the physical interpretation of the lead relation between warm water volume and the el nino southern oscillation the warm water volume wwv a proxy for the equatorial pacific heat content is the most widely used oceanic precursor of the el nino southern oscillation enso the standard interpretation of this lead relation in the context of the recharge oscillator theory is that anomalous easterlies during e g la nina favour a slow recharge of the equatorial band that will later favour a transition to el nino here we demonstrate that wwv only works as the best enso predictor during boreal spring i e during enso onset in both observations and cmip5 models at longer lead times the heat content in the western pacific wwvw is the best enso predictor as initially formulated in the recharge oscillator theory using idealised and realistic experiments with a linear continuously stratified ocean model and a comprehensive wave decomposition method we demonstrate that spring wwv mostly reflects the fast kelvin wave response to wind anomalies early in the year rather than the longer term influence of winds during the previous year wwv is hence not an adequate index of the slow recharge invoked in the recharge oscillator the wwvw evolution before spring is dominated by forced rossby waves with a smaller contribution from the western boundary reflection wwvw can be approximated from the integral of equatorial wind stress over the previous 10 months thus involving a longer term time scale than wwv main time scale 3 months we hence recommend using wwvw rather than wwv as an index for the slow recharge before the spring predictability barrier

importance of seasonal and annual layers in controlling backscatter to radar altimeters across the percolation zone of an ice sheet radar altimeters are one of the main tools for measuring elevation changes across the antarctic and greenland ice sheets and larger ice caps a ground based radar was deployed in autumn 2004 and spring 2006 in the percolation zone of the greenland ice sheet this radar is a high bandwidth system operating in the ku band the same frequency as several satellite altimeters measurements were made over an elevation range of 1795 to 2350 m along with snow pit and shallow core studies these measurements demonstrate the spatial and temporal variations in the backscatter relative strengths of surface and volume reflections change dramatically between spring and autumn and there is also high spatial variability across the percolation zone the extent of percolation will affect elevation estimates made by radar altimeters

equatorial pacific easterly wind surges and the onset of la nina events abstractthe processes responsible for the onset of la nina events have not received the same attention as those responsible for the onset of el nino events for which westerly wind events wwes in the tropical pacific have been identified as important contributors results here show that synoptic scale surface easterly wind surges ewss play an important role in the onset of la nina events akin to the role of wwes in the onset of el nino events it is found that ewss are a substantial component of zonal wind stress variance along the equatorial pacific using reanalysis wind stress fields validated against buoy measurements 340 ews events are identified between 1986 and 2012 their distributions in space time and el nino southern oscillation enso state are described about 150 ewss occur during enso neutral conditions during the months associated with la nina initiation and growth april december composites of changes in sea surface temperature anomaly ssta following these 150 events show s

envisat derived elevation changes of the greenland ice sheet and a comparison with icesat results in the accumulation area abstract we show for the first time over the greenland ice sheet that an along track method for deriving rates of elevation change can successfully be applied to envisat radar altimetry data 2002 2010 the results provide improved resolution and coverage compared to previous results obtained from cross over methods also we find that temporal changes in the elevation change rate can be derived from envisat data and show clear examples of this by generating five year running means for selected areas of the greenland ice sheet for a period between 2003 and 2009 the elevation of the ice sheets was measured by both the laser altimeter on board icesat and the radar altimeter on board envisat we compare rates of elevation change derived from icesat and envisat for this time span in which both sensors were operating we focus on the area above the equilibrium line altitude in order to specifically derive information on snow parameters a comparison of the elevation changes observed by the two sensors shows a complex pattern which can be explained regionally by model output describing the changes in both firn air content and accumulation rates

the role of subglacial water in ice sheet mass balance subglacial water can significantly affect the velocity of ice streams and outlet glaciers of ice sheets depending on the geometry and capacity of the subglacial hydrologic system increased surface melting in greenland over the coming decades may influence the ice sheet s mass balance furthermore subglacial lakes in antarctica can modulate ice velocities and act as nucleation points for new fast flowing ice streams in the coming decades significant changes in the polar regions will increase the contribution of ice sheets to global sea level rise under the ice streams and outlet glaciers that deliver ice to the oceans water and deformable wet sediments lubricate the base facilitating fast ice flow the influence of subglacial water on fast ice flow depends on the geometry and capacity of the subglacial hydrologic system water moving rapidly through a well connected system of conduits or channels will have little impact on ice sheet velocities but water injected into a spatially dispersed subglacial system may reduce the effective pressure at the base of the ice sheet and thereby trigger increased ice sheet velocities in greenland the form of the subglacial hydrologic system encountered by increasing surface melt water will determine the influence of changing atmospheric conditions on ice sheet mass balance in antarctica subglacial lakes have the capacity to both modulate velocities in ice streams and outlet glaciers and provide nucleation points for new fast ice flow tributaries climate models of ice sheet responses to global change remain incomplete without a parameterization of subglacial hydrodynamics and ice dynamics

westerly wind bursts and their relationship with intraseasonal variations and enso part ii energetics over the western and central pacific abstract the mechanism of synoptic scale eddy development in the generation of westerly wind bursts wwbs over the western central pacific and their relationship with the el nino southern oscillation enso and the madden julian oscillation mjo were examined in the wwb occurrences barotropic structures of equatorial eddy westerlies with cyclonic disturbances were found from the surface to the upper troposphere the dominant contributions to substantial eddy kinetic energy eke were the barotropic energy conversion kmke in the lower and middle tropospheres and the conversion from eddy available potential energy peke in the upper troposphere low frequency environmental westerlies centered near the equator preceded strong zonal convergence and meridional shear resulting in the substantial kmke the activation of synoptic convection also contributed to an increase in eke through peke these energies were redistributed to the lower equatorial troposphere through energy flux convergence gke the

observations of warm water volume changes in the equatorial pacific and their relationship to el nino and la nina abstract this paper describes observed changes in surface winds sea surface temperature sst and the volume of water warmer than 20 c wwv in the equatorial pacific ocean for the period 1980 99 the purpose is to test recent hypotheses about the relationship between variations in wwv and the el nino southern oscillation enso cycle the results confirm inferences based on theory models and previous empirical analyses using proxy data namely sea level that enso involves a recharge and discharge of wwv along the equator and that the cyclic nature of enso results from a disequilibrium between zonal winds and zonal mean thermocline depth the authors also find that the magnitude of enso sst anomalies is directly related to the magnitude of zonal mean wwv anomalies furthermore for a given change in equatorial wwv the corresponding warm el nino sst anomalies are larger than the corresponding cold la nina anomalies this asymmetry between the warm and cold phases of the enso cycle implies differences

gps measurements of vertical crustal motion in greenland we have analyzed 5 years of continuous global positioning system gps measurements taken at kellyville just off the western margin of the ice sheet in southern greenland a fit to the vertical component gives a negative secular uplift rate of 5 8 1 0 mm yr a negative rate i e a subsidence is consistent with archeological and historical evidence that the surrounding region has been subsiding over the last 3 kyr however it is inconsistent with estimates of the earth s continuing viscoelastic response to melting ice prior to 4 ka years ago which predict that kellyville should be uplifting rather than subsiding by 2 0 3 5 mm yr the resulting 7 8 3 6 mm yr discrepancy is too large to be the result of loading from present day changes in nearby ice we show instead that it is consistent with independent suggestions that the western ice sheet margin in this region of greenland may have advanced by 50 km during the past 3 4 kyr

a new ice thickness and bed data set for the greenland ice sheet 2 relationship between dynamics and basal topography new data sets of bed and surface elevation have been combined with balance velocities bed topography and radar reflectivity to investigate the relationship between ice flow and bed conditions it has been found that the bed topography affects the local driving stresses and initiates the onset of streaming flow in specific areas where there are large undulations this onset region only appears significantly inland in three areas the northeast greenland ice stream kangerdlugssuaq gletscher and a region in the southwest associated with abnormally low driving stresses and an apparently unnamed but active glacier bed reflectance values below the northeast ice stream suggest that basal melting could be occurring along its length and it seems likely therefore that the enhanced flow observed inland from the onset of streaming may be due in part to basal sliding

twentieth century simulation of the southern hemisphere climate in coupled models part ii sea ice conditions and variability we examine the representation of the mean state and interannual variability of antarctic sea ice in six simulations of the twentieth century from coupled models participating in the intergovernmental panel on climate change fourth assessment report the simulations exhibit a largely seasonal southern hemisphere ice cover as observed there is a considerable scatter in the monthly simulated climatological ice extent among different models but no consistent bias when compared to observations the scatter in maximum winter ice extent among different models is correlated to the strength of the climatological zonal winds suggesting that wind forced ice transport is responsible for much of this scatter observations show that the leading mode of southern hemisphere ice variability exhibits a dipole structure with anomalies of one sign in the atlantic sector associated with anomalies of the opposite sign in the pacific sector the observed ice anomalies also exhibit eastward propagation with the antarctic circumpolar current as part of the documented antarctic circumpolar wave phenomenon many of the models do simulate dipole like behavior in sea ice anomalies as the leading mode of ice variability but there is a large discrepancy in the eastward propagation of these anomalies among the different models consistent with observations the simulated antarctic dipole like variations in the ice cover are led by sea level pressure anomalies in the amundsen bellingshausen sea these are associated to different degrees in different models with both the southern annular mode and the el nino southern oscillation enso there are indications that the magnitude of the influence of enso on the southern hemisphere ice cover is related to the strength of enso events simulated by the different models

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

topo europe studying continental topography and deep earth surface processes in 4d abstract topo europe is a multidisciplinary international research program that addresses the interaction of processes inherent to the deep earth lithosphere mantle with surface processes erosion climate sea level which together shaped the topography of europe the objective of this programme is to assess neotectonic deformation rates and to quantify related geo risks such as earthquakes flooding landslides rock falls and volcanism research incorporating data interactive modeling focuses on the lithosphere memory and neotectonics with special attention on the thermo mechanical structure of the lithosphere mechanisms controlling large scale plate boundary and intraplate deformations anomalous subsidence and uplift and links with surface processes and topography evolution the topo europe natural laboratories in which these processes are analyzed cover a wide range of geodynamic settings these include the post collisional alpine carpathian pannonian basin system the very active aegean anatolian and apennines tyrrhenian orogens and back arc basins the caucasus levant area in the arabia europe collision zone the iberian peninsula caught up between alpine orogens the meta stable west and central european platform the stable east european platform and the seismically active and elevated scandinavian continental margin the topo europe project is a component of the international lithosphere program a european science foundation eurocores project provides funding for part of the research for a total amount of 13 5 m euro creating new research positions for 60 young researchers

tropical pacific sea surface temperature anomalies el nino and equatorial westerly wind events abstract the authors examine global statistical relationships between westerly wind events wwes and sea surface temperature anomaly ssta variability using a compositing technique for the period 1986 98 the authors describe the extent to which equatorial wwes are associated with central and eastern equatorial pacific waveguide warming and with local ssta changes under the wwe their goal is to quantify the extent to which equatorial wwes are fundamental to the onset and maintenance of warm el nino southern oscillation conditions in order to understand the effect of wwes on ssta evolution they begin by examining how ssta changes in the absence of equatorial wwes they find that ssta tends toward mean climate values in the absence of equatorial wwes whether the eastern equatorial pacific has close to normal ssta or warmer than normal ssta the two equatorial wwe types whose main surface wind anomalies are west of the date line are associated with weak local surface cooling the equatorial wwe type t

role of high and low frequency winds and wave reflection in the onset growth and termination of the 1997 1998 el nino the present study aims at studying the role played by high frequency wind variability wave reflection and easterly wind anomalies in the western pacific in the onset growth and termination phases of the 1997 1998 el nino using the trident intermediate coupled model and observations while the anomalous strength of the trade winds in 1996 favored the initiation of a warm event in 1997 via western pacific boundary rossby wave reflection the actual timing of the onset and the amplitude of the event resulted from the large march 1997 wind event once initiated high frequency westerly winds strongly contributed to the rapid growth of the warm event and to the displacement of the eastern edge of the warm pool moreover both easterly and westerly high frequency wind variability in 1997 1998 contributed to the amplitude of the event set the evolution of the warm event and potentially influenced the equatorial pacific conditions at least one year after the el nino event in addition eastern boundary reflection also significantly contributed to the amplitude and duration of the warm event whereas its termination was a combination of various factors reflection of upwelling rossby waves at the western boundary and large easterly wind anomalies observed in the western pacific from november 1997 to early 1998 these factors were sufficient to terminate the event and to switch temperature anomalies from warm to cold to conclude understanding the coupling between the high and low frequency wind variability i e studying enso as a multi scale phenomenon will certainly lead to a better comprehension of the diversity of its behavior and potentially to an improvement of its predictability

greenland ice sheet melt extent 1979 1999 analysis of melt extent on the greenland ice sheet is updated to span the time period 1979 1999 and examined along with its spatial and temporal variability using passive microwave satellite data to acquire the full record the issue of continuity between the previous passive microwave sensors smmr ssm i f 8 and ssm i f 11 and the most recent ssm i f 13 sensor is addressed the f 13 cross polarized gradient ratio melt classification threshold is determined to be 0 0154 results show that for the 21 year record a positive melt trend of nearly 1 yr is observed but this trend falls just below the 90 significance level the observed melt increase does appear to be driven by conditions in the western portion of the ice sheet rather than the east where melt appears to have decreased slightly moreover the eruption of mount pinatubo in 1991 is likely to have had some impact on the melt but not so much as previously suspected the 1992 melt anomaly is 1 7 standard deviations from the mean finally the relationship between coastal temperatures and melt extent suggests an increase in surface runoff contribution to sea level of 0 31 mm yr for a 1 c temperature rise

sensitivity of the modeled present day greenland ice sheet to climatic forcing and spin up methods and its influence on future sea level projections 1 the accuracy of the modeled present day greenland ice sheet gris is crucial for future projections of gris changes in this study we present a detailed evaluation of the modeled present day gris sensitivity to different present day climatology data sets and spin up methods and further assess the influence of the modeled present day gris on future sea level projections our study demonstrates that with present day climatology data sets derived from the regional atmospheric climate model racmo2 the modeled ice volume area and elevation of the gris agree substantially better with observations compared to simulations with present day climatology data sets from temperature parameterizations and era interim reanalysis with transient spin up the simulated rates of ice elevation changes for the 1993 2010 period are closer to observations than those with steady state spin up these results indicate that the racmo2 forcing and transient spin up may be preferable for use in the ice sheet model moreover our results reveal that the present day climatology data sets and spin up methods affect future sea level projections using transient spin up the ice sheet model estimates a range of 26 mm in projected sea level rise by 2098 under the a1b emissions scenario due to different present day climatology data sets compared to the results with transient spin up the estimated sea level rise by 2098 is reduced by 5 21 mm with steady state spin up this discrepancy is attributed mainly to differences in ice thickness and ice velocity between the modeled present day gris with transient and steady state spin ups and the effect of paleoclimatic changes with transient spin up

high ti continental tholeiites from the aznam trough northwestern saudi arabia evidence of abortive rifting in the embryonic stage of red sea opening abstract the aznam trough is a nw trending asymmetric graben on the northern saudi arabian coast of the red sea miocene or younger high ti tholeiitic basalts are exposed in the northern part of the trough these aznam basalts contain microphenocrysts of altered olivine calcic plagioclase and clinopyroxene with subordinate amounts of ilmenite and orthopyroxene in a cryptocrystalline groundmass a minor xenocryst xenolith assemblage consists of quartz sieve textured plagioclase and a variety of crustal rock fragments the aznam basalts have tio2 contents 3 0 elevated 87sr 86sr ratios 0 70486 0 70546 feo mgo ratios 2 0 and mg numbers between 47 and 50 they are compositionally and isotopically more evolved than tholeiites morb from the red sea trough minor crustal contamination of the initial basaltic magmas is shown by the elevated sr isotopic composition and enrichment in some lil components major element modeling indicates that the aznam basalts can be generated by fractionation of the major phenocryst phases out of typical red sea morb trace element distributions are mostly consistent with this conclusion with the exception of minor crustal contamination at aznam the aznam basalts are compositionally almost identical with high ti basalts formed in abortive volcanically dormant rifts in iceland apparently the same process of magmatism and rift jumping that occurred in iceland also occurred and may still be occurring in the northern red sea region as a short lived embryonic rifting that is a precursor to sea floor spreading in the red sea trough

using remotely sensed data from airs to estimate the vapor flux on the greenland ice sheet comparisons with observations and a regional climate model mass loss from the greenland ice sheet gris in recent years has been dominated by runoff from surface melt it is currently being studied extensively while little interest has been given to the smallest component of surface mass balance smb the vapor flux although poorly understood it is not negligible and could potentially play a larger role in smb in a warming climate where temperature relative humidity and precipitation changes remain uncertain here we present an innovative approach to estimate the vapor flux using the atmospheric infrared sounder airs version 6 data and a modified vapor flux model bmf13 over the gris between 2003 and 2014 one modification to the bmf13 model includes a new multiangle imaging spectroradiometer surface aerodynamic roughness product which likely produces more accurate estimates of the drag coefficient on the ice sheet when comparing airs data with gc net and programme for monitoring of the greenland ice sheet automatic weather station observations of skin temperature near surface air temperature and humidity they agree within 2 k 2 68 k and 0 34 g kg 1 largest differences occur in the ablation zone where there is significant subgrid heterogeneity overall the average vapor flux from the gris between 2003 and 2014 was found to be 14 6 3 6 gt yr 1 no statistically significant trends were found during the data record this data set is compared to the regional atmospheric climate model racmo2 3 vapor flux and bmf13 produced smaller vapor fluxes in the summer 0 05 gt d 1 and slightly more deposition in the winter 9 4 10 3 gt d 1 annually differences between bmf13 and racmo2 3 were only 30 15

progressive increase in ice loss from greenland 1 laser altimeter measurements over greenland show increasing thickening rates above 2000 m reflecting increasing snowfall in a warming climate but near coastal thinning rates have increased substantially since the mid 1990s and net mass loss more than doubled from an average of 4 50 gt yr 1 between 1993 4 and 1998 9 to 57 105 gt yr 1 between 1998 9 and 2004 this increasing trend is very similar to findings from independent mass budget studies but differs widely from ers radar altimeter results this may result from limitations associated with the large ers footprint over sloping and undulating surfaces that typify fast narrow glaciers where thinning is most pronounced

elevation change of the southern greenland ice sheet from 1978 to 1988 interpretation the spatial pattern of elevation change estimates derived from satellite radar altimeter data for the period from 1978 to 1988 for the southern greenland ice sheet is examined as reported previously the results from 12 ice cores widely distributed in the study area indicate that much of the spatial variability in the elevation change estimates can be explained by temporal variations in accumulation most notably the areas of largest thickening and thinning east and west of the ice divide around 66 n recently experienced substantial decadal fluctuations in accumulation sufficient to explain the observed elevation change rates that span the range of 24 cm yr 1 elevation change estimates in the nw of the study area around the 2000 m elevation contour were found to be inconsistent with both short and long term changes in accumulation fluctuations examination of stratigraphy in several shallow ice cores in the nw indicates that significant melting took place in 1977 and 1978 and this was followed by many years of very little melt activity this likely resulted in a significant downward bias in the 1978 1988 elevation change estimates in the nw comparison of measured surface velocities with those calculated assuming steady state balance show very good agreement overall but there are many instances in the southeast where the steady state velocities are significantly smaller than measured surface velocities we cannot rule out the possibility that increased ice flow in lower elevation outlet glaciers has migrated farther upstream thereby causing negative long term mass imbalance in southeast greenland

acceleration of greenland ice mass loss in spring 2004 in 2001 the intergovernmental panel on climate change projected the contribution to sea level rise from the greenland ice sheet to be between 0 02 and 0 09 m from 1990 to 2100 ref 1 however recent work has suggested that the ice sheet responds more quickly to climate perturbations than previously thought particularly near the coast here we use a satellite gravity survey by the gravity recovery and climate experiment grace conducted from april 2002 to april 2006 to provide an independent estimate of the contribution of greenland ice mass loss to sea level change we detect an ice mass loss of 248 36 km3 yr 1 equivalent to a global sea level rise of 0 5 0 1 mm yr 1 the rate of ice loss increased by 250 per cent between the periods april 2002 to april 2004 and may 2004 to april 2006 almost entirely due to accelerated rates of ice loss in southern greenland the rate of mass loss in north greenland was almost constant continued monitoring will be needed to identify any future changes in the rate of ice loss in greenland

ocean freshening sea level rising sea ice melting 1 estimates of 20th century sea level rise are typically 1 5 to 2 mm y with a steric contribution of 0 5 0 2 mm y estimates of the eustatic contribution vary widely between 1 1 and 1 3 mm y we attempt an independent estimate of eustatic sea level rise based on the measured freshening of the global ocean and with attention to the contribution from melting of sea ice which affects freshening but not sea level our estimate is based on a secular decrease in global average salinity estimated by antonov et al 2002 which if assumed due entirely to run off would produce a eustatic rise of 1 8 0 7 mm y and would correspond to a run off volume of 650 cu km y measurements with upward looking sonars mounted on submarines have suggested a historical thinning of the arctic ice sheet equivalent to 525 105 cu km y allowing for some growth in antarctic sea ice a reduced figure of 430 130 cu km y is obtained allowing about 220 cu km y of run off from land sources such as glaciers this would produce a eustatic rise of only 0 6 mm y for a total of 1 1 mm y somewhat less than ipcc estimates this also has implicationsforourunderstandingofglacialretreatforatotal of 1 1 mm y index terms 1635 global change oceans 4203 4207 oceanography general arctic and antarctic oceanography 4540 oceanography physical ice mechanics and air sea ice exchange processes 4556 oceanography physical sea level variations citation wadhams p and w munk 2004

greenland 2012 melt event effects on cryosat 2 radar altimetry cryosat 2 data are used to study elevation changes over an area in the interior part of the greenland ice sheet during the extreme melt event in july 2012 the penetration of the radar signal into dry snow depends heavily on the snow stratigraphy and the rapid formation of refrozen ice layers can bias the surface elevations obtained from radar altimetry we investigate the change in cryosat 2 waveforms and elevation estimates over the melt event and interpret the findings by comparing in situ surface and snow pit observations from the north greenland eemian ice drilling project camp the investigation shows a major transition of scattering properties around the area and an apparent elevation increase of 56 26 cm is observed in reprocessed cryosat 2 data we suggest that this jump in elevation can be explained by the formation of a refrozen melt layer that raised the reflective surface introducing a positive elevation bias

influence of westerly wind events stochasticity on el nino amplitude the case of 2014 vs 2015 the weak el nino of 2014 was preceded by anomalously high equatorial pacific warm water volume wwv and strong westerly wind events wwes which typically lead to record breaking el nino like in 1997 and 2015 here we use the cnrm cm5 coupled model to investigate the causes for the stalled el nino in 2014 and the necessary conditions for extreme el ninos this model is ideally suited to study this problem because it simulates all the processes thought to be critical for the onset and development of el nino it captures el nino preconditioning by wwv the wwes characteristics and their deterministic behaviour in response to warm pool displacements our main finding is that despite their deterministic control wwes display a sufficiently strong stochastic component to explain the distinct evolutions of el nino in 2014 and 2015 a 100 member ensemble simulation initialized with early spring equatorial conditions analogous to those observed in 2014 and 2015 demonstrates that early year elevated wwv and strong wwes preclude the occurrence of a la nina but lead to el ninos that span the weak with few wwes to extreme with many wwes range sensitivity experiments confirm that numerous strong wwes shift the el nino distribution toward larger amplitudes with a particular emphasis on summer fall wwes occurrence which result in a five fold increase of the odds for an extreme el nino a long simulation further demonstrates that sustained wwes throughout the year and anomalously high wwv are necessary conditions for extreme el nino to develop in contrast we find no systematic influence of easterly wind events ewes on the el nino amplitude in our model our results demonstrate that the weak amplitude of el nino in 2014 can be explained by wwes stochastic variations without invoking ewes or remote influences from outside the tropical pacific and therefore its peak amplitude was inherently unpredictable at long lead time

greenland ice sheet mass balance distribution of increased mass loss with climate warming 2003 07 versus 1992 2002 we derive mass changes of the greenland ice sheet gis for 2003 07 from icesat laser altimetry and compare them with results for 1992 2002 from ers radar and airborne laser altimetry the gis continued to grow inland and thin at the margins during 2003 07 but surface melting and accelerated flow significantly increased the marginal thinning compared with the 1990s the net balance changed from a small loss of 7 3g t a 1 in the 1990s to 171 4g t a 1 for 2003 07 contributing 0 5 mm a 1 to recent global sea level rise we divide the derived mass changes into two components 1 from changes in melting and ice dynamics and 2 from changes in precipitation and accumulation rate we use our firn compaction model to calculate the elevation changes driven by changes in both temperature and accumulation rate and to calculate the appropriate density to convert the accumulation driven changes to mass changes increased losses from melting and ice dynamics 17 206 gt a 1 are over seven times larger than increased gains from precipitation 10 35 gt a 1 during a warming period of 2 k 10 a 1 over the gis above 2000 m elevation the rate of gain decreased from 44 to 28 gt a 1 while below 2000 m the rate of loss increased from 51 to 198 gt a 1 enhanced thinning below the equilibrium line on outlet glaciers indicates that increased melting has a significant impact on outlet glaciers as well as accelerating ice flow increased thinning at higher elevations appears to be induced by dynamic coupling to thinning at the margins on decadal timescales

recent ice sheet growth in the interior of greenland a continuous data set of greenland ice sheet altimeter height from european remote sensing satellites ers 1 and ers 2 1992 to 2003 has been analyzed an increase of 6 4 0 2 centimeters per year cm year is found in the vast interior areas above 1500 meters in contrast to previous reports of high elevation balance below 1500 meters the elevation change rate is 2 0 0 9 cm year in qualitative agreement with reported thinning in the ice sheet margins averaged over the study area the increase is 5 4 0 2 cm year or 60 cm over 11 years or 54 cm when corrected for isostatic uplift winter elevation changes are shown to be linked to the north atlantic oscillation

changes in greenland ice sheet elevation attributed primarily to snow accumulation variability the response of grounded ice sheets to a changing climate critically influences possible future changes in sea level recent satellite surveys over southern greenland show little overall elevation change at higher elevations but large spatial variability using satellite studies alone it is not possible to determine the geophysical processes responsible for the observed elevation changes and to decide if recent rates of change exceed the natural variability here we derive changes in ice sheet elevation in southern greenland for the years 1978 88 using a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation our patterns of accumulation driven elevation change agree closely with contemporaneous satellite measurements of ice sheet elevation change and we therefore attribute the changes observed in 1978 88 to variability in snow accumulation similar analyses of longer ice core records show that in this decade the greenland ice sheet exhibited typical variability at high elevations well within the long term natural variability our results indicate that a better understanding of ice sheet mass changes will require long term measurements of both surface elevation and snow accumulation

seasonal dynamic thinning at helheim glacier a b s t r a c t we investigate three annual mass balance cycles on helheim glacier in south east greenland using tandem x interferometric digital elevation models dems bedrock gps measurements and ice velocity from feature tracking the dems exhibit seasonal surface elevation cycles at elevations up to 800 m a s l with amplitudes of up to 19 m from a maximum in july to a minimum in october or november concentrated on the fast flowing areas of the glacier indicating that the elevation changes have a mostly dynamic origin by modelling the detrended bedrock loading unloading signal we estimate a mean density for the loss of 671 70 kg m 3 and calculate that total water equivalent volume loss from the active part of the glacier surface flow speeds 1m day 1 ranges from 0 5 km 3 in 2011 to 1 6 km 3

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

greenland ice sheet increased coastal thinning repeated laser altimeter surveys and modelled snowfall summer melt show average ice loss from greenland between 1997 and 2003 was 80 12 km3 yr 1 compared to about 60 km3 yr 1 for 1993 4 1998 9 half of the increase was from higher summer melting with the rest caused by velocities of some glaciers exceeding those needed to balance upstream snow accumulation velocities of one large glacier almost doubled between 1997 and 2003 resulting in net loss from its drainage basin by about 20 km3 of ice between 2002 and 2003 copyright 2004 by the american geophysical union

merging and analysis of elevation time series over greenland ice sheet from satellite radar altimetry spatial temporal variability and changes of greenland ice sheet elevation from 1992 to 2008 are analyzed from merged ers 1 ers 2 and envisat satellite radar altimeter data a methodology for determining intersatellite biases was developed and applied in order to merge measurements from these different satellites and to create continuous and consistent time series intersatellite biases of elevation and backscatter coefficient have shown to be significantly affected by the bias between measurements in ascending and descending orbits adjustment of elevation time series for its dependence on backscatter coefficient and other waveform parameters performed in this paper substantially reduced the amplitude of elevation seasonal variations and locally corrected elevation change rate estimates by up to several centimeters per year it was found that the correction depends not only on the variations in the waveform parameters but also on the temporal variations of the correlation gradients which represent the sensitivity of the elevation change to the change in the waveform parameters an elevation change rate of 2 8 0 2 cm year from 1992 to 2008 over 76 of the greenland ice sheet area was found increases in surface elevation from 1995 observed over the high elevation regions of greenland were followed by an elevation decrease from 2006 for the whole period of 1992 2008 the elevation increase is 4 0 0 2 cm year over 87 of the area above 1500 m in contrast over 38 of the low elevation areas below 1500 m the rate of elevation change is 7 0 1 0 cm year and the surface elevation decrease that started from 2000 has continued

hydrologic response of the greenland ice sheet the role of oceanographic warming the response of the greenland ice sheet to ongoing climate change remains an area of great uncertainty with most previous studies having concentrated on the contribution of the atmosphere to the ice mass balance signature here we systematically assess for the first time the influence of oceanographic changes on the ice sheet the first part of this assessment involves a statistical analysis and interpretation of the relative changes and variations in sea surface temperatures ssts and air temperatures around greenland for the period 1870 2007 this analysis is based on hadisst1 and reynolds oi v2 sst analyses in situ sst and deeper ocean temperature series surface air temperature records for key points located around the greenland coast and examination of atmospheric pressure and geopotential height from ncep ncar reanalysis second we carried out a novel sensitivity experiment in which ssts were perturbed as input to a regional climate model and document the resulting effects on simulated greenland climate and surface mass balance we conclude that sea surface ocean temperature forcing is not sufficient to strongly influence precipitation snow accumulation and melt runoff of the ice sheet additional evidence from meteorological reanalysis suggests that high greenland melt anomalies of summer 2007 are likely to have been primarily forced by anomalous advection of warm air masses over the ice sheet and to have therefore had a more remote atmospheric origin however there is a striking correspondence between ocean warming and dramatic accelerations and retreats of key greenland outlet glaciers in both southeast and southwest greenland during the late 1990s and early 2000s copyright 2008 john wiley sons ltd

modulation of westerly wind bursts by sea surface temperature a semistochastic feedback for enso abstract westerly wind bursts wwbs in the equatorial pacific are known to play a significant role in the development of el nino events they have typically been treated as a purely stochastic external forcing of enso recent observations however show that wwb characteristics depend upon the large scale sst field the consequences of such a wwb modulation by sst are examined using an ocean general circulation model coupled to a statistical atmosphere model i e a hybrid coupled model an explicit wwb component is added to the model with guidance from a 23 yr observational record the wwb parameterization scheme is constructed such that the likelihood of wwb occurrence increases as the western pacific warm pool extends a semistochastic formulation which has both deterministic and stochastic elements the location of the wwbs is parameterized to migrate with the edge of the warm pool it is found that modulation of wwbs by sst strongly affects the characteristics of enso in particular coupled fe

mass balance of higher elevation parts of the greenland ice sheet satellite radar and aircraft laser altimeter data and a volume budget comparison of total snow accumulation with total ice discharge give three independent estimates of the recent mass balance of parts of the greenland ice sheet above 2000 m elevation results show the entire region on average to be in balance to within 10 mm yr 1 with very low rates of regional thickness change dh dt in the northeast but high rates with large spatial variability in the south only the volume budget estimates show significant thinning in the northwest but thinning in this area is also inferred from local measurements of ice vertical velocities down boreholes south of latitude 68 n there has been rapid thickening west of the ice divide with equally rapid thinning in the southeast but with large differences between the three estimates of dh dt throughout this region the radar data apply to the period 1978 1988 the laser data are for 1993 1994 1998 1999 and the volume budget calculations represent conditions over at least the last few decades consequently many of the differences between results could be caused by temporal changes particularly in snow accumulation rates that occurred since the 1970s however taken with other information our results suggest long term thickening in the southwest and possibly quite recent onset of rapid thinning in the southeast

estimation of steric sea level variations from combined grace and jason 1 data abstract we estimate the mean steric sea level variations over the 60 s 60 n oceanic domain for the recent period from august 2002 to april 2006 by combining sea level data from jason 1 altimetry with time variable gravity data from grace the observed global mean sea level change from satellite altimetry results in total from steric plus ocean mass change as grace measurements averaged over the ocean represents the ocean mass change component only the difference between grace and altimetry observations provides an estimate of the mean steric sea level two different sets of grace geoid solutions the grgs eigen gl04 and the gfz eigen grace04s products have been used each grace data set ranges over approximately 3 yr or more august 2002 april 2006 for the grgs geoids and february 2003 february 2006 for the gfz geoids we first focus on the seasonal variations the two grace data sets agree very well in terms of mean annual mass variation both in amplitude approximately 7 3 mm equivalent sea level and phase maximum on october 14 for both time spans jason 1 sea level minus grace ocean mass has an annual amplitude of approximately 5 8 mm with a maximum on march 11 the latter signal compares well with the steric annual sea level estimated from the world ocean 2004 climatology and the ishii et al m ishii m kimoto k sakamoto and s i iwasaki steric sea level changes estimated from historical ocean subsurface temperature and salinity analyses journal of oceanography 62 2 2006 155 170 ocean temperature data we also examine the interannual fluctuations of the jason 1 minus grace sea level the two resulting steric sea level time series based on the two grace data sets agree well the inferred steric sea level curve exhibits an increasing trend during the last 3 5 yr august 2002 april 2006 of the same order of magnitude as the 1993 2003 steric sea level trend computed with in situ hydrographic data however over the last 3 5 yr we note a strong discrepancy between altimetry minus grace and in situ based steric sea level trend the latter exhibiting a negative slope the cause for such a discrepancy is yet unknown but may be related to inadequate sampling of in situ ocean temperature measurements

meltwater flux and runoff modeling in the ablation area of jakobshavn isbrae west greenland the temporal variability in surface snow and glacier melt flux and runoff were investigated for the ablation area of lakobshavn isbrae west greenland high resolution meteorological observations both on and outside the greenland ice sheet gris were used as model input realistic descriptions of snow accumulation snow and glacier ice melt and runoff are essential to understand trends in ice sheet surface properties and processes snowmodel a physically based spatially distributed meteorological and snow evolution modeling system was used to simulate the temporal variability of lakobshavn isbrre accumulation and ablation processes for 2000 01 2006 07 winter snow depth observations and modis satellite derived summer melt observations were used for model validation of accumulation and ablation simulations agreed well with observed values simulated annual surface melt varied from as low as 3 83 x 10 sup 9 m sup 3 2001 02 to as high as 8 64 x 10 sup 9 m sup 3 2004 05 modeled surface melt occurred at elevations reaching 1 870 m a s l for 2004 05 while the equilibrium line altitude ela fluctuated from 990 to 1 210 m a s l during the simulation period the snowmodel meltwater retention and refreezing routines considerably reduce the amount of meltwater available as ice sheet runoff without these routines the lakobshavn surface runoff would be overestimated by anmore average of 80 from september october through may june no runoff events were simulated the modeled interannual runoff variability varied from 1 81 x 10 sup 9 m sup 3 2001 02 to 5 21 x 10 sup 9 m sup 3 2004 05 yielding a cumulative runoff at the jakobshavn glacier terminus of approx 2 25 m w eq to approx 4 5 m w eq respectively the average modeled lakobshavn runoff of approx 3 4 km sup 3 y sup 1 was merged with previous estimates of jakobshavn ice discharge to quantify the freshwater flux to illulissat icefiord for both runoff and ice discharge the average trends are similar indicating increasing insignificant influx of freshwater to the illulissat icefiord for the period 2000 01 2006 07 this study suggests that surface runoff forms a minor part of the overall jakobshavn freshwater flux to the fiord around 7 approx 3 4 km sup 3 y sup 1 of the average annual freshwater flux of approx 51 0 km sup 3 y sup 1 originates from the surface runoff less

the role of lateral ocean physics in the upper ocean thermal balance of a coupled ocean atmosphere gcm the sensitivity of the upper ocean thermal balance of an ocean atmosphere coupled gcm to lateral ocean physics is assessed three 40 year simulations are performed using horizontal mixing isopycnal mixing and isopycnal mixing plus eddy induced advection the thermal adjustment of the coupled system is quite different between the simulations confirming the major role of ocean mixing on the heat balance of climate the initial adjustment phase of the upper ocean sst is used to diagnose the physical mechanisms involved in each parametrisation when the lateral ocean physics is modified significant changes of sst are seen mainly in the southern ocean a heat budget of the annual mixed layer defined as the bowl shows that these changes are due to a modified heat transfer between the bowl and the ocean interior this modified heat intake of the ocean interior is directly due to the modified lateral ocean physics in isopycnal diffusion this heat exchange especially marked at mid latitudes is both due to an increased effective surface of diffusion and to the sign of the isopycnal gradients of temperature at the base of the bowl as this gradient is proportional to the isopycnal gradient of salinity this confirms the strong role of salinity in the thermal balance of the coupled system the eddy induced advection also leads to increased exchanges between the bowl and the ocean interior this is both due to the shape of the bowl and again to the existence of a salinity structure the lateral ocean physics is shown to be a significant contributor to the exchanges between the diabatic and the adiabatic parts of the ocean

implications of changing scattering properties on greenland ice sheet volume change from cryosat 2 altimetry abstract long term observations of surface elevation change of the greenland ice sheet gris is of utmost importance when assessing the state of the ice sheet satellite radar altimetry offers a long time series of data over the gris starting with ers 1 in 1991 esa s cryosat 2 mission launched in 2010 provides an invaluable radar altimetry dataset for monitoring the current changes of the ice sheets due to its dense spatial and temporal coverage of these areas here we investigate the effects of including different parameters which describe the shape of the return radar waveform waveform parameters in the elevation change algorithm to correct for temporal changes in the ratio between surface and volume scatter in cryosat 2 observations we present elevation and volume changes for the greenland ice sheet in the period from 2010 until 2014 the waveform parameters considered here are the backscatter coefficient and the leading edge width which are both available in the esa cryosat 2 level 2i data product investigations into relocation of radar reflection points are also included inter comparison of the cryosat 2 derived elevation changes with those derived from operation icebridge laser data suggests waveform parameters to be applicable for correcting for changes in volume scattering the best results in the synthetic aperture radar interferometric mode area of the gris are found when applying only the backscatter correction whereas the best result in the low resolution mode area is obtained by only applying a leading edge width correction using this approach to correct for the scattering properties a volume loss of 292 38 km 3 yr 1 is found for the gris for the time span november 2010 until november 2014 the inclusion of waveform parameter corrections and improved relocation for the gris helps to reconcile the satellite derived elevation changes with those observed by operation icebridge however the bias of temporal changes in the scattering horizons of cryosat 2 is not entirely removed and suggests that future improvements could be made by including climate data and or additional waveform parameters to make additional corrections in the cryosat 2 radar altimetry

the march 1997 westerly wind event and the onset of the 1997 98 el nino understanding the role of the atmospheric response in a previous study the effect of the march 1997 westerly wind event wwe on the evolution of the tropical pacific ocean was studied using an ocean general circulation model gcm the response was characterized by i a cooling of the far western pacific 0 88c ii a rapid eastward displacement of the warm pool 2000 km in a month and iii a weak warming of the central eastern pacific along the path of the oceanic kelvin wave excited by the wwe 0 58c in this study the atmospheric response to these aspects of the sea surface temperature sst response are investigated using an atmospheric gcm forced with the sst anomalies from the ocean only experiments the results have demonstrated that the three aspects of the sst anomaly field generated by the wwe

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

semiempirical and process based global sea level projections we review the two main approaches to estimating sea level rise over the coming century physically plausible models of reduced complexity that exploit statistical relationships between sea level and climate forcing and more complex physics based models of the separate elements of the sea level budget previously estimates of future sea level rise from semiempirical models were considerably larger than those from process based models however we show that the most recent estimates of sea level rise by 2100 using both methods have converged but largely through increased contributions and uncertainties in process based model estimates of ice sheets mass loss hence we focus in this paper on ice sheet flow as this has the largest potential to contribute to sea level rise progress has been made in ice dynamics ice stream flow grounding line migration and integration of ice sheet models with high resolution climate models calving physics remains an important and difficult modeling issue mountain glaciers numbering hundreds of thousands must be modeled by extensive statistical extrapolation from a much smaller calibration data set rugged topography creates problems in process based mass balance simulations forced by regional climate models with resolutions 10 100 times larger than the glaciers semiempirical models balance increasing numbers of parameters with the choice of noise model for the observations to avoid overfitting the highly autocorrelated sea level data all models face difficulty in separating out non climate driven sea level rise e g groundwater extraction and long term disequilibria in the present day cryosphere sea level system

remote sensing of greenland ice sheet using multispectral near infrared and visible radiances 1 we present the physical basis of and validate a new remote sensing algorithm that utilizes reflected visible and near infrared radiation to discriminate between dry and wet snow when applied to the moderate resolution imaging spectroradiometer modis satellite data our discrimination algorithm has the potential to retrieve melting regions of the ice sheet at a spatial resolution of 0 25 km 2 over three orders of magnitude higher than the resolution of current microwave methods the method should be useful for long term monitoring of the melt area of the greenland ice sheet especially regions close to ice sheet margins and of the outflow glaciers our analysis of modis retrievals of the western portion of the greenland ice sheet over the period 2000 to 2006 indicates significant interannual variability with a maximum melt extent in 2005 collocated in situ meteorological data reveal a high correlation 0 80 between the modis melt day area and the average summer temperature our analysis suggests that it is the magnitude of the summer temperature that dominates the melting not the variability of the length of the melting season furthermore we find that the melt day area increases by about 3 8 for each 0 1 k increase in the average surface air summer temperature we combine this empirical relationship with historic temperature data to infer that the melt day area of the western part of the ice sheet doubled between the mid 1990s and mid 2000s and that the largest ice sheet surface melting probably occurred between 1920s and 1930s concurrent with the warming in that period

the melt anomaly of 2002 on the greenland ice sheet from active and passive microwave satellite observations 1 active and passive microwave satellite data are used to map snowmelt extent and duration on the greenland ice sheet the passive microwave pm data reveal the extreme melt extent of 690 000 km2 in 2002 as compared with an average extent of 455 000 km2 from 1979 2003 a statistical analysis of the melt time series affirms an increased melt extent earlier in the melt season the quikscat qscat or am analysis for 1999 to 2004 confirms the extreme melt situation in 2002 qscat data show a significant increase in melt season length over several areas in 2003 ssm i and qscat melt data reveal that the later detects melt earlier and is more sensitive to surface melt qscat detects an earlier stage of melt and ssm i detects a later stage of melt the ssm i xgpr melt extent is approximately confined to the qscat melt areas experiencing 2 weeks or more of melting time

estimation of volume change rates of greenland s ice sheet from icesat data using overlapping footprints abstract data from the icesat glas laser altimetry mission is used to obtain an estimate of the volume change of greenland s ice sheet over the time span of february 2003 to april 2007 a novel processing strategy is developed and applied it uses approximately 1 million icesat elevation differences at geometrically overlapping footprints of both crossing and repeated tracks the data are edited using quality flags defined by the icesat glas science team as well as other additional criteria in order to reduce the influence of surface slope we propose a correction based on the icesat glas laser altimetry digital elevation model three slightly different processing strategies to convert the observed temporal elevation differences to elevation volume changes are compared for 6 different drainage systems further divided into regions above and below 2000 m in elevation the final chosen strategy includes the correction for surface slopes but does not include the removal of outlying elevation changes for the region above 2000 m a positive elevation change rate of 2 cm year is obtained which corresponds to a volume change rate of 21 km3 year for the region below 2000 m the estimated elevation change rate is 24 cm year which corresponds to a volume loss of 168 km3 year in general the obtained results are in agreement with trends discovered by other authors that were also derived from laser altimetry nevertheless the estimation obtained in this study suggests a more negative trend than those obtained previously the differences can be explained by differences in the sampling of the region below 2000 m and to a certain extent by different time spans of the datasets used a representative sampling of coastal areas is identified as the most critical issue for an accurate estimation of volume change rates in greenland

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

calibrating a surface mass balance model for austfonna ice cap svalbard austfonna 8120 km 2 is by far the largest ice mass in the svalbard archipelago there is considerable uncertainty about its current state of balance and its possible response to climate change over the 2004 05 period we collected continuous meteorological data series from the ice cap performed mass balance measurements using a network of stakes distributed across the ice cap and mapped the distribution of snow accumulation using ground penetrating radar along several profile lines these data are used to drive and test a model of the surface mass balance the spatial accumulation pattern was derived from the snow depth profiles using regression techniques and ablation was calculated using a temperature index approach model parameters were calibrated using the available field data parameter calibration was complicated by the fact that different parameter combinations yield equally acceptable matches to the stake data while the resulting calculated net mass balance differs considerably testing model results against multiple criteria is an efficient method to cope with non uniqueness in doing so a range of different data and observations was compared to several different aspects of the model results we find a systematic underestimation of net balance for parameter combinations that predict observed ice ablation which suggests that refreezing processes play an important role to represent these effects in the model a simple pmax approach was included in its formulation used as a diagnostic tool the model suggests that the surface mass balance for the period 29 april 2004 to 23 april 2005 was negative 318 mm w e

compensating changes in the penetration depth of pulse limited radar altimetry over the greenland ice sheet changes in firn properties affect the shape of pulse limited radar altimeter echoes acquired over the polar ice sheets we apply a waveform deconvolution model to cryosat 2 low resolution mode echoes to determine the depth distribution of radar backscattering across the greenland ice sheet the deconvolution allows us to calculate the relative contributions of surface and volume scattering and the effective penetration depth of the radar echoes into the snowpack the most prominent signal is associated with the extreme surface melting of summer 2012 which resulted in a shift of the dominant radar scattering horizon toward the snow surface in the accumulation zone at locations above 2000 m the average penetration depth in july 2012 prior to the melt event was 3 79 1 12 m following the melt event there was an abrupt reduction in the average penetration depth across the same region to 1 45 0 94 m the average penetration depth then gradually increased to 3 28 1 13 m by the end of 2017 as fresh snow accumulated on the ice sheet surface although the variation in penetration is evident in surface height estimates derived from the cryosat 2 echoes the magnitude of the effect is reduced by waveform retracking using airborne laser altimeter data recorded over the same time period we show that the penetration variation can be compensated effectively by incorporating the deconvolution penetration depth into the surface height retrieval

ice sheet mass balance assessment attribution and prognosis contrary to prior expectations that warming would cause mass addition averaged over the greenland and antarctic ice sheets and over the next century the ice sheets appear to be losing mass at least partly in response to recent warming with warming projected for the future additional mass loss appears more likely than not

effects of westerly wind bursts on el nino a new perspective daily observations from 1971 to 2010 reveal that every el nino during this period was accompanied by congregated westerly wind bursts suggesting a close relationship of these bursts with both cold tongue and warm pool el nino events with the addition of burst like multiplicative noise to an intermediate ocean atmosphere coupled model it is shown that westerly wind bursts by generating eastward equatorial surface currents and downwelling kelvin waves could be responsible for the existence of the warm pool el nino and for the irregularity and extremes of the cold tongue el nino whether these bursts give rise to one type of el nino or the other depends on the timing of their occurrence relative to the phase of the recharge discharge cycle of the equatorial upper ocean heat content

enso asymmetry in cmip5 models abstractthe el nino la nina asymmetry is evaluated in 14 coupled models from phase 5 of the coupled model intercomparison project cmip5 the results show that an underestimate of enso asymmetry a common problem noted in cmip3 models remains a common problem in cmip5 coupled models the weaker enso asymmetry in the models primarily results from a weaker sst warm anomaly over the eastern pacific and a westward shift of the center of the anomaly in contrast sst anomalies for the la nina phase are close to observations corresponding atmospheric model intercomparison project amip runs are analyzed to understand the causes of the underestimate of enso asymmetry in coupled models the analysis reveals that during the warm phase precipitation anomalies are weaker over the eastern pacific and westerly wind anomalies are confined more to the west in most models the time mean zonal winds are stronger over the equatorial central and eastern pacific for most models wind forced ocean gcm experiments suggest

a comparison of recent elevation change estimates of the devon ice cap as measured by the icesat and envisat satellite altimeters we have used surface elevation measurements acquired by the ice cloud and land elevation satellite geoscience laser altimeter system glas and envisat radar altimeter 2 ra 2 satellite altimeters to assess the elevation change of the 13 700 km2 devon ice cap dic in arctic canada between 2002 and 2008 we present algorithms for the retrieval of elevation change rates over ice caps using data acquired from these satellites a comparison of glas elevation data to those acquired by the ra 2 shows reasonable agreement between the two instruments the root mean square elevation change difference was 56 cm and the correlation coefficient between the two data sets was 0 68 using only ra 2 elevation measurements which are spatially and temporally more continuous we determined the elevation change rate of the areas of the dic where the surface geometry allows the ra 2 retracker to maintain lock this includes most of the dic excluding large parts of the eastern half of the ice cap the elevation change rate was found to be insignificant given a statistical estimate of the measurement error 0 09 0 29 m a we also present an assessment of the regional variations of the dic elevation change including a significant 0 71 0 49 m a elevation change rate of the 1980 km2 western arm furthermore we present evidence of a localized 2 m drop in the surface elevation of the south croker bay glacier during summer 2007 this drop is apparent within both satellite data sets and we interpret this signal to reflect a sudden speedup of the glacier

triggering of el nino by westerly wind events in a coupled general circulation model two ten members ensemble experiments using a coupled ocean atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event wwe when the tropical pacific has initial conditions favourable to the development of a warm event in the reference ensemble cref no wind perturbation is introduced whereas a strong westerly wind event anomaly is introduced in boreal winter over the western pacific in the perturbed ensemble cwwe our results demonstrate that an intense wwe is capable of establishing the conditions under which a strong el nino event can occur first it generates a strong downwelling kelvin wave that generates a positive sea surface temperature sst anomaly in the central eastern pacific amplified through a coupled ocean atmosphere interaction this anomaly can be as large as 2 5 c 60 days after the wwe secondly this wwe also initiates an eastward displacement of the warm pool that promotes the occurrence of subsequent wwes in the following months these events reinforce the initial warming through the generation of additional kelvin waves and generate intense surface jets at the eastern edge of the warm pool that act to further shift warm waters eastward the use of a ten members ensemble however reveals substantial differences in the coupled response to a wwe whereas four members of cwwe ensemble develop into intense el nino warming as described above four others display a moderate warming and two remains in neutral conditions this diversity between the members appears to be due to the internal atmospheric variability during and following the inserted wwe in the four moderate warm cases the warm pool is initially shifted eastward following the inserted wwe but the subsequent weak wwe activity when compared to the strong warming cases prevents to further shift the warm pool eastwards the seasonal strengthening of trade winds in june july can therefore act to shift warm waters back into the western pacific reducing the central eastern pacific warming this strong sensitivity of the coupled response to wwes may therefore limit the predictability of el nino events as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead

radar response of firn exposed to seasonal percolation validation using cores and fdtd modeling we use ground penetrating radars gprs firn cores and electromagnetic finite difference time domain fdtd numerical modeling to characterize the gpr response to a frozen high arctic firn pack as a result of extensive summertime percolation the firn pack comprises a high fraction of ice layers lenses and vertical glands we show that the gpr response on the firn pack mainly depends on the following 1 the thickness of the ice layers 2 the distance between layers 3 the layer roughness and 4 the presence or absence of elliptical ice lenses using 3 d fdtd modeling we show that the gpr is not sensitive to typical ice glands which implies that the gpr underestimates firn heterogeneity such that firn stratigraphy in percolation and wet snow zones could be incorrectly interpreted as being better preserved than it actually is we find that thin ice layers 0 05 m or multiple thin ice layers give a strong response thicker ice layers typically give a weaker backscatter per unit area mainly due to the lack of interference of the reflections from the upper and lower interfaces but are due to their continuity easily trackable ice layers with a thickness comparable to the gpr wavelength give 180deg phase shifted upper and lower reflections and are in general separated by a band of low gpr response due to the lack of permittivity contrast within the ice layers despite the ice lenses relatively short horizontal correlation length as inferred from cores bands of high amplitude clutter caused by these features can be traced over several kilometers in gpr profiles

the extreme el nino of 2015 2016 the role of westerly and easterly wind bursts and preconditioning by the failed 2014 event at the beginning of 2015 as one year earlier in 2014 the scientific community anticipated that el nino conditions could develop in the tropical pacific by year end such projections were related to the occurrence of westerly wind bursts during winter spring of each year that generated strong downwelling kelvin waves indicative of an emerging el nino however the event s progression quickly stalled in 2014 but actively continued in 2015 leading to an extreme warm event comparable to 1997 or 1982 here we compare climate evolution during these two years using satellite observations and numerical simulations we show that during 2014 el nino development was interrupted mid year by an exceptionally strong easterly wind burst whereas during the second year it continued through the summer further we show that the failed 2014 event created favorable conditions for el nino development during the next year as it kept ocean heat content recharged and the western pacific warm pool extended eastward subsequently the winter spring westerly wind bursts in 2015 were followed by a series of state dependent westerly bursts as part of a strong positive bjerknes feedback analogue simulations with a coupled gcm wherein we superimpose the observed sequences of westerly and easterly wind bursts support these conclusions stressing the role of the failed 2014 event in preconditioning the ocean atmosphere system for the development of an extreme el nino in our simulations the probability of an extreme event following early year westerly wind bursts increases from 14 to nearly 60 due to this preconditioning thus the interplay between westerly and easterly wind bursts shapes el nino development and diversity

westerly wind bursts enso s tail rather than the dog westerly wind bursts wwbs in the equatorial pacific occur during the development of most el nino events and are believed to be a major factor in enso s dynamics because of their short time scale wwbs are normally considered part of a stochastic forcing of enso completely external to the interannual enso variability recent observational studies however suggest that the occurrence and characteristics of wwbs may depend to some extent on the state of enso components implying that wwbs which force enso are modulated by enso itself satellite and in situ observations are used here to show that wwbs are significantly more likely to occur when the warm pool is extended eastward based on these observations wwbs are added to an intermediate complexity coupled ocean atmosphere enso model the representation of wwbs is idealized such that their occurrence is modulated by the warm pool extent the resulting model run is compared with a run in which the wwbs are stochastically applied the modulation of wwbs by enso results in an enhancement of the slow frequency component of the wwbs this causes the amplitude of enso events forced by modulated wwbs to be twice as large as the amplitude of enso events forced by stochastic wwbs with the same amplitude and average frequency based on this result it is suggested that the modulation of wwbs by the equatorial pacific sst is a critical element of enso s dynamics and that wwbs should not be regarded as purely stochastic forcing in the paradigm proposed here wwbs are still an important aspect of enso s dynamics but they are treated as being partially stochastic and partially affected by the large scale enso dynamics rather than being completely external to enso it is further shown that wwb modulation by the large scale equatorial sst field is roughly equivalent to an increase in the ocean atmosphere coupling strength making the coupled equatorial pacific effectively self sustained

recent elevation changes of svalbard glaciers derived from icesat laser altimetry abstract we have tested three methods for estimating 2003 2008 elevation changes of svalbard glaciers from multi temporal icesat laser altimetry a linear interpolation of crossover points between ascending and descending tracks b projection of near repeat tracks onto common locations using digital elevation models dems and c least squares fitting of rigid planes to segments of repeat track data assuming a constant elevation change rate the two repeat track methods yield similar results and compare well to the more accurate but sparsely sampled crossover points most glacier regions in svalbard have experienced low elevation thinning combined with high elevation balance or thickening during 2003 2008 the geodetic mass balance excluding calving front retreat or advance of svalbard s 34 600 km2 glaciers is estimated to be 4 3 1 4 gt y 1 corresponding to an area averaged water equivalent w e balance of 0 12 0 04 m w e y 1 the largest ice losses have occurred in the west and south while northeastern spitsbergen and the austfonna ice cap have gained mass winter and summer elevation changes derived from the same methods indicate that the spatial gradient in mass balance is mainly due to a larger summer season thinning in the west and the south than in the northeast our findings are consistent with in situ mass balance measurements from the same period confirming that repeat track satellite altimetry can be a valuable tool for monitoring short term elevation changes of arctic glaciers

pacific meridional mode and el nino southern oscillation 1 we present intriguing evidence that the majority of el nino events over the past four decades are preceded by a distinctive sea surface warming and southwesterly wind anomaly in the vicinity of the intertropical convergence zone itcz during the boreal spring this phenomenon known as the meridional mode mm is shown to be intrinsic to the thermodynamic coupling between the atmosphere and ocean the mm effectively acts as a conduit through which the extratropical atmosphere influences enso modeling results further suggest that the mm plays a vital role in the seasonal phase locking behavior of enso the findings provide a new perspective for understanding the important role of thermodynamic ocean atmosphere feedback in enso and may have profound implications for enso prediction particularly the unresolved issue of the spring predictability barrier citation chang p l zhang r saravanan d j vimont j c h chiang l ji h seidel and m k tippett 2007 pacific meridional mode and el nino southern oscillation geophys res lett 34 l16608 doi 10 1029 2007gl030302

precipitation over greenland and its relation to the north atlantic oscillation the equation method based on an equivalent isobaric geopotential height in coordinates has been used to retrieve the precipitation over greenland this approach is designed to accurately represent the topographic effects of the greenland ice sheet on atmospheric motion and precipitation the 11 year mean precipitation from 1985 to 1996 over all of greenland is 376 mm yr 1 which is close to the long term mean precipitation of 346 mm yr 1 estimated from glaciological data the precipitation over all of greenland shows that the largest value in 1986 is 472 mm yr 1 and the smallest value in 1995 is 309 mm yr 1 the major interannual variability of the atmospheric circulation in the north atlantic can be represented by the variation of the north atlantic oscillation nao index which is most pronounced during winter it is found that if the nao index increases the total precipitation over greenland decreases and vice versa the correlation coefficient between these two series for 1985 1995 is 0 75 the mean precipitation over southern greenland where the majority of precipitation falls is more closely related to the nao index in winter and their correlation coefficient is 0 80 this relationship can be understood from the composite maps of sea level pressure and greenland precipitation for the high and low index months during months of high nao index values the icelandic low is strong during months of low nao index values the monthly mean low is located to the southwest of greenland over the labrador sea precipitation amounts over the southeast coast of greenland are about 100 mm larger during the low nao index months than the high nao index months precipitation over all of greenland during the low nao index months is higher there are significant downward trends in annual precipitation from 1985 1995 for all of greenland and its southern and central west coastal regions amounting to about 3 per year

mass balance of polar ice sheets recent advances in the determination of the mass balance of polar ice sheets show that the greenland ice sheet is losing mass by near coastal thinning and that the west antarctic ice sheet with thickening in the west and thinning in the north is probably thinning overall the mass imbalance of the east antarctic ice sheet is likely to be small but even its sign cannot yet be determined large sectors of ice in southeast greenland the amundsen sea embayment of west antarctica and the antarctic peninsula are changing quite rapidly as a result of processes not yet understood

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century

role of non linear oceanic processes in the response to westerly wind events new implications for the 1997 el nino onset in march 1997 a strong westerly wind event wwe occurred in the western equatorial pacific prior to the 1997 1998 el nino event it produced downwelling kelvin waves that interacted non linearly with the surface temperature salinity and zonal current fronts located at the eastern edge of the warm fresh pool eewp this non linear interaction locally increased zonal currents by a factor of three compared to a theoretical linear response and advected the eewp at an unexpected rate 1m s to which the ocean atmosphere coupled system may have been responding rapidly to trigger el nino conditions