joint inversion of source location and focal mechanism of microseismicity abstractthe seismic focal mechanism fm is an effective property to indicate source physics as well as stress and strain distribution in regional local and microscales we have developed an algorithm to jointly invert for the fm and source locations for a given velocity structure all possible combinations of source locations x y and z and fm strike dip and rake were used to compute traveltimes and polarities of waveforms correcting normal moveout times and polarities and stacking all waveforms the x y z strike dip and rake combination that gave the strongest stacking power was identified as the optimal solution compared with the traditional source scanning algorithm ssa that only scanned source locations this algorithm was thereby called the joint source scanning algorithm jssa the jssa method was tested rigorously and it was applied to a hydraulic fracturing data set our work determined several advantages against the ssa method 1 the jssa method could identify many shea

using surface waves recorded by a large mesh of three element arrays to detect and locate disparate seismic sources author posting the authors 2018 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 215 2018 942 958 doi 10 1093 gji ggy316

interannual and decadal modulations recently observed in the pacific storm track activity and east asian winter monsoon abstract interannual variability of the north pacific storm track observed over 17 recent winters is documented the local storm track activity is measured by a meridional flux of sensible heat associated with the lower tropospheric subweekly fluctuations the interannual variability in the heat flux over the northwestern nw pacific is found to be strongest in midwinter the first empirical orthogonal function of the interannual variability in midwinter captures the decadal tendency toward the enhanced storm track activity in midwinter over the nw pacific in association with the decadal weakening of the east asian winter monsoon siberian high and the aleutian low that occurred in the late 1980s the most marked signature of this enhancement is that the midwinter minimum in the storm track activity which had been apparent in the early to mid 1980s almost disappeared afterward as opposed to linear theory of baroclinic instability the enhanced activity occurred despite the weakening of the pacific j

decadal response of the kuroshio extension jet to rossby waves observation and thin jet theory abstractthis study examines interannual to decadal variability of the kuroshio extension ke jet using satellite altimeter observations from 1993 to 2010 the leading empirical orthogonal function eof mode of sea level variability in the ke region represents the meridional shift of the ke jet followed by its strength changes with a few month lag this shift of the ke jet lags atmospheric fluctuations over the eastern north pacific by about three years broad sea level anomalies slas emerge in the eastern north pacific 3 4 years before the upstream ke jet shift and propagate westward along the ke jet axis in the course of the propagation the meridional scale of the slas gradually narrows and their amplitude increases this westward propagation of slas with a speed of about 5 cm s 1 is attributed to the westward propagation of the meridional shift of the jet consistent with the thin jet theory whose importance has been suggested by previous numerical studies in addition the westward propagatin

sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier 1 large calving events at greenland s largest outlet glaciers are associated with glacial earthquakes and near instantaneous increases in glacier flow speed at some glaciers and ice streams flow is also modulated in a regular way by ocean tidal forcing at the terminus at helheim glacier analysis of geodetic data shows decimeter level periodic position variations in response to tidal forcing however we also observe transient increases of more than 100 in the glacier s responsiveness to such tidal forcing following glacial earthquake calving events the timing and amplitude of the changes correlate strongly with the step like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes the enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice bedrock interface and suggests a new means by which geodetic data may be used to infer glacier properties citation de juan j et al 2010 sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier geophys res lett 37 l12501

evidence for non tidal diurnal velocity variations of helheim glacier east greenland we have used tide gauge data from near helheim glacier east greenland and gps data acquired on the glacier to investigate the spectra of tidal forcing and flow response for both the tide gauge and gps time series we calculated amplitudes and phases for a harmonic expansion using a limited set of harmonic constituents we find that the semidiurnal constituents of the glacier flow are well modeled using a single admittance and lag with respect to the tide gauge data however diurnal variations in the glacier flow cannot be simply described using this model we find an additional signal in glacier position in phase with the s1 solar tide with some modulations at other frequencies these non tidal variations account for a peak to peak variation in glacier flow speed at a site close to the terminus of 0 7md 1 compared with a mean flow rate at this location of 22md 1 the speed variations reach their daily maximum value 6 hours after local noon we hypothesize that these additional diurnal variations are associated with peaks in lubrication of the glacier bed due to surface melting driven by diurnal solar heating

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

a barotropic inverse tidal model for the arctic ocean 1 we describe high resolution 5 km grid linear dynamics and inverse models of arctic ocean barotropic tides obtained with the osu tidal inversion software otis package the 8 constituent dynamics based model uses the latest ibcao bathymetry and open boundary forcing from the recent tpxo 6 2 global barotropic tidal solution this model performs significantly better than the present benchmark arctic tidal model 14 km grid by z kowalik and a proshutinsky as judged by comparisons with 300 coastal tide gauges the greatest improvements are found in the canadian arctic archipelago nares strait and the baffin bay and labrador sea and can be explained by the higher resolution of the new model in these topographically complex regions the new arctic inverse model assimilates coastal and benthic tide gauges and topex poseidon and ers altimetry for further improvements of the 4 dominant constituents m2 s2 k1 and o1

on the importance of midlatitude oceanic frontal zones for the mean state and dominant variability in the tropospheric circulation 1 observations indicate that midlatitude weather systems are organized into storm tracks near oceanic frontal zones with pronounced sea surface temperature sst gradients a pair of atmospheric general circulation model experiments with zonally uniform sst profiles prescribed show that their observed collocation is not fortuitous in one experiment a storm track is anchored around a midlatitude sst front that maintains near surface thermal gradients and energizes eddies westerly momentum transport by eddies produces a well defined polar front jet along the front even in winter when a subtropical jet stream intensifies in the other experiment removal of the sst front leads to a substantial weakening in eddy activity and the pfj especially in winter it also leads to a weakening of the annular mode the dominant mode of westerly jet variability and its notable structural distortion in winter though idealized our experiments suggest the importance of midlatitude oceanic fronts for the tropospheric circulation and its variability

ice front variation and tidewater behavior on helheim and kangerdlugssuaq glaciers greenland we used satellite images to examine the calving behavior of helheim and kangerdlugssuaq glaciers greenland from 2001 to 2006 a period in which they retreated and sped up these data show that many large iceberg calving episodes coincided with teleseismically detected glacial earthquakes suggesting that calving related processes are the source of the seismicity for each of several events for which we have observations the ice front calved back to a large pre existing rift these rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high which agrees well with earlier theoretical predictions in addition to the recent retreat in a period of higher temperatures analysis of several images shows that helheim retreated in the 20th century during a warmer period and then re advanced during a subsequent cooler period this apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water the cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in greenland can advance rapidly greenland s larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance

dynamics of glacier calving at the ungrounded margin of helheim glacier southeast greenland during summer 2013 we installed a network of 19 gps nodes at the ungrounded margin of helheim glacier in southeast greenland together with three cameras to study iceberg calving mechanisms the network collected data at rates up to every 7 s and was designed to be robust to loss of nodes as the glacier calved data collection covered 55 days and many nodes survived in locations right at the glacier front to the time of iceberg calving the observations included a number of significant calving events and as a consequence the glacier retreated 1 5 km the data provide real time high frequency observations in unprecedented proximity to the calving front the glacier calved by a process of buoyancy force induced crevassing in which the ice downglacier of flexion zones rotates upward because it is out of buoyant equilibrium calving then occurs back to the flexion zone this calving process provides a compelling and complete explanation for the data tracking of oblique camera images allows identification and characterisation of the flexion zones and their propagation downglacier interpretation of the gps data and camera data in combination allows us to place constraints on the height of the basal cavity that forms beneath the rotating ice downglacier of the flexion zone before calving the flexion zones are probably formed by the exploitation of basal crevasses and theoretical considerations suggest that their propagation is strongly enhanced when the glacier base is deeper than buoyant equilibrium thus this calving mechanism is likely to dominate whenever such geometry occurs and is of increasing importance in greenland

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

significance of a midlatitude sst frontal zone in the formation of a storm track and an eddy driven westerly jet abstract in a set of idealized aquaplanet experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature sst is prescribed globally as the lower boundary condition an assessment is made of the potential influence of the frontal sst gradient upon the formation of a storm track and an eddy driven midlatitude polar front jet pfj and on its robustness against changes in the intensity of a subtropical jet stj in experiments with the frontal midlatitude sst gradient as that observed in the southwestern indian ocean transient eddy activity in each of the winter and summer hemispheres is organized into a deep storm track along the sst front with an enhanced low level baroclinic growth of eddies in the winter hemisphere another storm track forms just below the intense stj core but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath the near surface westerlies are strongest near the midlatitude sst front as observe

reverse modelling for seismic event characterization summary the localization of seismic events is of utmost importance in seismology and exploration current techniques rely on the fact that the recorded event is detectable at most of the stations of a seismic network weak events not visible in the individual seismogram of the network are missed out we present an approach where no picking of events in the seismograms of the recording network is required the observed wavefield of the network is reversed in time and then considered as the boundary value for the reverse modelling assuming the correct velocity model the reversely modelled wavefield focuses on the hypocentre of the seismic event the origin time of the event is given by the time where maximum focussing is observed the spatial extent of the focus resembles the resolution power of the recorded wavefield and the acquisition this automatically provides the uncertainty in the localization with respect to the bandwidth of the recorded data the method is particularly useful for the upcoming large passive networks since no picking is required it has great potential for localizing very weak events not detectable in the individual seismogram since the reverse modelling sums the energy of all recorded traces and therefore enhances the signal to noise ratio similar to stacking in seismic exploration the method is demonstrated by 2 d and 3 d numerical case studies which show the potential of the technique events with a s n ratio smaller than 1 where the events cannot be identified in the individual seismogram of the network are localized very well by the method

spatial and temporal variations in greenland glacial earthquake activity 1993 2010 1 glacial earthquakes are anomalous earthquakes associated with large ice loss events occurring at marine terminating glaciers primarily in greenland they are detectable teleseismically and a proper understanding of the source mechanism may provide a remote sensing tool to complement glaciological observations of these large outlet glaciers we model teleseismic surface wave waveforms to obtain locations and centroid single force source parameters for 121 glacial earthquakes occurring in greenland during the period 2006 2010 we combine these results with those obtained by previous workers to analyze spatial and temporal trends in glacial earthquake occurrence over the 18 year period from 1993 2010 we also examine earthquake occurrence at six individual glaciers comparing the earthquake record to independently obtained observations of glacier change our findings confirm the inference that glacial earthquake seismogenesis occurs through the capsize of large newly calved icebergs we find a close correspondence between episodes of glacier retreat thinning and acceleration and the timing of glacial earthquakes and document the northward progression of glacial earthquakes on greenland s west coast over the 18 year observing period our results also show that glacial earthquakes occur when the termini of the source glaciers are very close to the glacier grounding line i e when the glaciers are grounded or nearly grounded

the source scanning algorithm mapping the distribution of seismic sources in time and space summary we introduce a new method which we call the source scanning algorithm ssa for imaging the distribution of seismic sources in both time and space using trial locations and origin times the method calculates the brightness function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times the spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness the greatest advantages of this method are that 1 it exploits waveform information both arrival times and relative amplitudes without the need to calculate highfrequency synthetic seismograms and 2 it requires neither pre assembled phase picking data nor any a priori assumptions about the source geometry a series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval finally we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip ets sequence in the northern cascadia subduction zone

rapid and synchronous ice dynamic changes in east greenland two major outlet glaciers in east greenland have suddenly begun to accelerate and retreat the speeds of kangerdlugssuaq and helheim remained steady during the 1990s despite progressive and substantial thinning but have abruptly increased within the last two years more than doubling ice flux to the ocean had it been an isolated example the comparable 1998 speed up of jakobshavn isbra in west greenland might have been explained simply by its chance retreat past a pinning point now that two further greenland outlets have exhibited similar behavior a common process seems likely a remarkable correspondence in the inter annual patterns of speed and ice front variation between kangerdlugssuaq and helheim implies a significant sensitivity to regional environmental factors the period of continued warming and thinning appears to have primed these glaciers for a step change in dynamics not included in current models we should expect further greenland outlet glaciers to follow suit copyright 2006 by the american geophysical union

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

comparison of migration based location and detection methods for microseismic events microseismic monitoring in the oil and gas industry commonly uses migration based methods to locate very weak microseismic events the objective of this study is to compare the most popular migration based methods on a synthetic dataset that simulates a strike slip source mechanism event with a low signal to noise ratio recorded by surface receivers vertical components the results show the significance of accounting for the known source mechanism in the event detection and location procedures for detection and location without such a correction the ability to detect weak events is reduced we show both numerically and theoretically that neglecting the source mechanism by using only absolute values of the amplitudes reduces noise suppression during stacking and consequently limits the possibility to retrieve weak microseismic events on the other hand even a simple correction to the data polarization used with otherwise ineffective methods can significantly improve detections and locations a simple stacking of the data with a polarization correction provided clear event detection and location but even better results were obtained for those data combined with methods that are based on semblance and cross correlation

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

numerical modeling of iceberg capsizing responsible for glacial earthquakes the capsizing of icebergs calved from marine terminating glaciers generate horizontal forces on the glacier front producing long period seismic signals referred to as glacial earthquakes these forces can be estimated by broadband seismic inversion but their interpretation in terms of magnitude and waveform variability is not straightforward we present a numerical model for fluid drag that can be used to study buoyancy driven iceberg capsize dynamics and the generated contact forces on a calving face using the finite element approach we investigate the sensitivity of the force to drag effects iceberg geometry calving style and initial buoyancy we show that there is no simple relationship between force amplitude and iceberg volume and similar force magnitudes can be reached for different iceberg sizes the force history and spectral content varies with the iceberg attributes the iceberg aspect ratio primarily controls the capsize dynamics the force shape and force frequency whereas the iceberg height has a stronger impact on the force magnitude iceberg hydrostatic imbalance generates contact forces with specific frequency peaks that explain the variability in glacial earthquake dominant frequency for similar icebergs top out and bottom out events have significantly different capsize dynamics leading to larger top out forces especially for thin icebergs for realistic iceberg dimensions we find contact force magnitudes that range between 5 6 1011 and 2 1014 kg m consistent with seismic observations this study provides a useful framework for interpreting glacial earthquake sources and estimating the ice mass loss from coupled analysis of seismic signals and modeling results

increasing rates of ice mass loss from the greenland and antarctic ice sheets revealed by grace 1 we use monthly measurements of time variable gravity from the grace gravity recovery and climate experiment satellite gravity mission to determine the ice mass loss for the greenland and antarctic ice sheets during the period between april 2002 and february 2009 we find that during this time period the mass loss of the ice sheets is not a constant but accelerating with time i e that the grace observations are better represented by a quadratic trend than by a linear one implying that the ice sheets contribution to sea level becomes larger with time in greenland the mass loss increased from 137 gt yr in 2002 2003 to 286 gt yr in 2007 2009 i e an acceleration of 30 11 gt yr2 in 2002 2009 in antarctica the mass loss increased from 104 gt yr in 2002 2006 to 246 gt yr in 2006 2009 i e an acceleration of 26 14 gt yr2 in 2002 2009 the observed acceleration in ice sheet mass loss helps reconcile grace ice mass estimates obtained for different time periods

role of the gulf stream and kuroshio oyashio systems in large scale atmosphere ocean interaction a review abstract ocean atmosphere interaction over the northern hemisphere western boundary current wbc regions i e the gulf stream kuroshio oyashio and their extensions is reviewed with an emphasis on their role in basin scale climate variability sst anomalies exhibit considerable variance on interannual to decadal time scales in these regions low frequency sst variability is primarily driven by basin scale wind stress curl variability via the oceanic rossby wave adjustment of the gyre scale circulation that modulates the latitude and strength of the wbc related oceanic fronts rectification of the variability by mesoscale eddies reemergence of the anomalies from the preceding winter and tropical remote forcing also play important roles in driving and maintaining the low frequency variability in these regions in the gulf stream region interaction with the deep western boundary current also likely influences the low frequency variability surface heat fluxes damp the low frequency sst anomalies ove

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

western boundary currents and frontal air sea interaction gulf stream and kuroshio extension abstract in the northern hemisphere midlatitude western boundary current wbc systems there is a complex interaction between dynamics and thermodynamics and between atmosphere and ocean their potential contribution to the climate system motivated major parallel field programs in both the north pacific kuroshio extension system study kess and the north atlantic climate variability and predictability clivar mode water dynamics experiment climode and preliminary observations and analyses from these programs highlight that complexity the gulf stream gs in the north atlantic and the kuroshio extension ke in the north pacific have broad similarities as subtropical gyre wbcs but they also have significant differences which affect the regional air sea exchange processes and their larger scale interactions the 15 yr satellite altimeter data record which provides a rich source of information is combined here with the longer historical record from in situ data to describe and compare the curr

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

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

grounding line migration and high resolution calving dynamics of jakobshavn isbrae west greenland 1 we performed three field campaigns in 2004 2007 and 2010 at the southern margin of the jakobshavn isbrae west greenland in order to infer flow velocities and their changes from photogrammetric time lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface area wide analysis of more than 3000 three dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events from 2004 to 2010 the grounding line of jakobshavn isbrae retreated 3 5 0 2 km considering previously published results the grounding line retreat amounts to 6 km since 1985 the glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m d can be reached moreover large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event after the break up the glacier slowly adjusts to the new boundary conditions within the next 4 5 days flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large scale glacier dynamics

joint location and source mechanism inversion of microseismic events benchmarking on seismicity induced by hydraulic fracturing summary seismic monitoring can greatly benefit from imaging events with a low signal to noise ratio snr as the number of the events with a low signal grows exponentially one way to detect weaker events is improvement of a snr by migration type stacking of waveforms from multiple stations we have developed a new method of location of seismic events that involves stackingofseismicphasesandamplitudesalongdiffractiontraveltimecurvestosuppressnoise and detect seismic events with a snr lower than that on individual receivers the stacking includes polarity correction based on a simultaneous seismic moment tensor inversion and detection algorithm on the stack function we applied this method to locate microseismicity induced by hydraulic fracturing first we calibrated the velocity model by locating perforation shots at known locations then we processed 3 d of data from microseismic monitoring of shale stimulation and benchmarked migration type locations of the largest events that were manually located the detected and located events induced by hydraulic fracturing in this case study are mostly shear events forming narrow bands along the maximum horizontal stress direction approximately 100 m above the injection intervals the proposed technique is fully automated and feasible for real time seismic monitoring

large scale changes in greenland outlet glacier dynamics triggered at the terminus the recent dramatic retreat of many outlet glaciers of the greenland ice sheet has raised concerns over greenland s contribution to future sea level rise simulations with a numerical ice flow model indicate that the recent rates of mass loss in greenland s outlet glaciers are transient and should not be extrapolated into the future

step wise changes in glacier flow speed coincide with calving and glacial earthquakes at helheim glacier greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

possible mechanisms for glacial earthquakes the large glacial earthquakes reported on by ekstrom et al 2003 2006 and tsai and ekstrom 2007 have previously been evaluated in terms of their seismic characteristics in this paper we attempt to take constraints such as known glacial ice properties outlet glacier size calving style and meltwater variability to construct a self consistent physical model of the glacial earthquake process since many glaciological parameters are poorly constrained we parameterize a number of important processes and estimate a wide range of possible values for some properties the range of model outputs is thus fairly large but it is still difficult to match observational constraints under most conditions we find that only a small class of models is able to satisfy the major observational constraints these models are characterized by 1 lost basal resistance coupled to viscoelastic deformation with extensive internal crevassing or with low effective elastic modulus and possibly low effective viscosity or 2 by nonequilibrium calving such as having large icebergs capsize into the glacier front although observational constraints cannot definitively rule out any of the proposed classes of mechanisms the calving model has much stronger support fortunately the various models make different predictions regarding observables that can potentially be measured in the near future

observed changes in baroclinic wave activity during the life cycles of low frequency circulation anomalies abstract anomalous baroclinic wave activity during the onset and decay of blocking patterns in the northern hemisphere winter is investigated based upon a 30 year record of 500 mb height and sea level pressure a slowly varying envelope function is defined at each gridpoint in order to represent the modulation of the amplitude of baroclinic wave activity by the low frequency circulation anomalies related to blocking the envelope function fields and the low frequency circulation fields are composited relative to the onset and decay times of four persistent dipole patterns in low frequency circulation anomalies located downstream of the climatological mean stormtracks one polarity of the anomaly patterns corresponds to dipole or modon like blocking patterns over the eastern oceans and the other to strong zonal flow patterns each of the eight composites is comprised of more than 50 persistent blocking or zonal flow episodes the most striking common feature observed during the onset of blocking over th

influence of the meridional shifts of the kuroshio and the oyashio extensions on the atmospheric circulation abstract the meridional shifts of the oyashio extension oe and of the kuroshio extension ke as derived from high resolution monthly sea surface temperature sst anomalies in 1982 2008 and historical temperature profiles in 1979 2007 respectively are shown based on lagged regression analysis to significantly influence the large scale atmospheric circulation the signals are independent from the enso teleconnections which were removed by seasonally varying asymmetric regression onto the first three principal components of the tropical pacific sst anomalies the response to the meridional shifts of the oe front is equivalent barotropic and broadly resembles the north pacific oscillation western pacific pattern in a positive phase for a northward frontal displacement the response may reach 35 m at 250 hpa for a typical oe shift a strong sensitivity since the associated sst anomaly is 0 5 k however the amplitude but not the pattern or statistical significance strongly depends on the lag and an

variability of the kuroshio extension jet recirculation gyre and mesoscale eddies on decadal time scales abstract twelve years of sea surface height ssh data from multiple satellite altimeters are used to investigate the low frequency changes and the interconnections of the kuroshio extension ke jet its southern recirculation gyre and their mesoscale eddy field the dominant signal is characterized by the steady weakening of the ke jet recirculation gyre from 1993 to 1996 followed by a gradual strengthening after 1997 during the weakening period of 1993 96 the ke path migrated southward in general and this path migration reversed in direction during the strengthening period of the ke jet and recirculation gyre after 1997 by hindcasting the ssh signals using linear vorticity dynamics it was found that weakening strengthening in the ke jet and recirculation gyre is consistent with westward propagation of negative positive ssh anomalies generating in the eastern north pacific and strengthening during their westward propagation when the ke jet and recirculation gyre were in a weak mode during 19

response of storm tracks to bimodal kuroshio path states south of japan abstracta large meridional shift of the sea surface temperature front occurs off the south coast of japan associated with transitions between the large meander and straight paths of the kuroshio most extratropical cyclones generated in winter near the kuroshio in the east china sea pass through the region where the kuroshio takes either the meander or the straight path to examine whether such cyclones change their tracks and intensities according to the two states of the path a new dataset of winter cyclone tracks derived from surface weather charts from the period 1969 70 2008 09 was produced the composite analysis of cyclone tracks with respect to the meander and straight path states reveals the following the cyclone track axis for the meander path state is located away from the south coast of japan with a dispersive tendency while that for the straight path state is attached to the south coast with a long extending feature a difference in track between these two states also occurs to the east of

iceberg meltwater fluxes dominate the freshwater budget in greenland s iceberg congested glacial fjords freshwater fluxes from the greenland ice sheet have increased over the last two decades due to increases in liquid i e surface and submarine meltwater and solid ice i e iceberg fluxes to predict potential ice sheet ocean climate feedbacks we must know the partitioning of freshwater fluxes from greenland including the conversion of icebergs to liquid i e meltwater fluxes within glacial fjords here we use repeat 0 5 m resolution satellite images from two major fjords to provide the first observation based estimates of the meltwater flux from the dense matrix of floating ice called melange we find that because of its expansive submerged area 100 km2 and rapid melt rate 0 1 0 8 m d 1 the ice melange meltwater flux can exceed that from glacier surface and submarine melting our findings suggest that iceberg melt within the fjords must be taken into account in studies of glacial fjord circulation and the impact of greenland melt on the ocean

mechanisms of enso forcing of hemispherically symmetric precipitation variability summary the patterns of precipitation anomalies forced by the el nino southern oscillation during northern hemisphere winter and spring are remarkably hemispherically symmetric and in the mid latitudes have a prominent zonally symmetric component observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically forced changes in the subtropical jets during el nino events the jets strengthen in each hemisphere and shift equatorward changes in the subtropical jet influence the transient eddy momentum fluxes and the eddy driven mean meridional circulation during el nino events eddy driven ascent in the mid latitudes of each hemisphere is accompanied by low level convergence and brings increased precipitation this changes in the transient eddy and stationary eddy moisture fluxes almost exactly cancel each other and in sum do not contribute to the zonal mean precipitation anomalies propagation of anomalous stationary waves disrupts the zonal symmetry flow around the deeper aleutian low and the eastward extension of the pacific jet stream supply the moisture for increased precipitation over the eastern north pacific and the western seaboard of the united states while transient eddy moisture convergence supplies the moisture for increased precipitation over the southern united states in each case increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes

seventeen antarctic seismic events detected by global surface waves and a possible link to calving events from satellite images 1 we detect 17 seismic events in antarctica from 1997 to 2009 by applying a surface wave detector to global seismic data we locate these events using a waveform cross correlation method and find that most occurred near the coast of antarctica and are clustered in three regions four events are on the ronne ice shelf close to the location of a 1998 calving event five events are near the vanderford glacier and eight events are near the ninnis glacier the observed rayleigh and love waves for these events have similar amplitudes and a two lobed radiation pattern matching the expected amplitude behavior of a single force source model using such a model we obtain best fitting horizontal force directions for the 14 events that have relatively better signal to noise ratios analysis of coastline changes from modis images before and after the detected events show that two events on vanderford glacier and one event near ninnis glacier are likely associated with calving events moreover the inferred force directions for the seismic events appear consistent with local ice flow directions both satellite observations and modeling results strongly suggest a link between seismic events and calving processes in the two regions however the force directions on the ronne ice shelf are aligned with observed rift propagation directions suggesting that these events may arise from rifting processes

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

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

tide modulated ice motion and seismicity of a floating glacier tongue in east antarctica we recorded the ice motion and icequakes on the floating part of langhovde glacier in east antarctica to better understand the dynamic behavior of ice shelves and floating tongues diurnal and semi diurnal variations in ice motion and seismicity were simultaneously observed at all four global navigation satellite system and three seismic stations over 2 weeks the short term along flow ice motion is explained by the elastic response of the glacier to ocean tide induced hydrostatic stress variations which decayed at a rate of 0 8 km 1 toward the grounding line we observed a large number of icequakes during mid rising and high tides that covered a broad frequency range and formed two major groups of events centered at 10 and 120 hz respectively the hourly occurrence rates were 500 events h 1 with the observed seismicity consistent with fracture due to floating tongue bending we also observed minor secondary peaks at high ice speeds which could reflect surface cracking due to stretching or basal friction our observation demonstrates that tidal modulation was the main factor to fracture the floating tongue of langhovde glacier

organization of extratropical transients during el nino abstract four observed el nino southern oscillation enso events are studied to determine the mechanisms responsible for the anomalous extratropical atmospheric circulation during northern winter a parallel analysis of a gcm s response to el nino is performed in order to assess if similar mechanisms are operative in the model atmosphere the observed stationary wave anomalies over the pacific north american pna region are found to he similar during the four winters despite appreciable differences in sea surface temperatures the anomalous transient vorticity fluxes are remarkably robust over the north pacific during each event with an eastward extension of the climatological storm track leading to strong cyclonic forcing near 40 n 150 w this forcing is in phase with the seasonal mean aleutian trough anomaly suggesting the importance of eddy mean flow interactions by comparison the intersample variability of the gcm response over the pna region is found to exceed the observed inter el nino variabi

trapping of synoptic scale disturbances into the north pacific subtropical jet core in midwinter 1 over the northwestern nw pacific a subtropical jet stream develops at 30 n in midwinter as the east asian winter monsoon intensifies eddies with subweekly periods propagating through the midlatitude tropopause into the nw pacific storm track tend to be trapped into the jet core 1000 km away from a surface baroclinic zone at 40 n while lifted up by 3 km the trapping appears to weaken the interaction of the upper level eddies with the baroclinic zone thus acting against eddy amplification and thereby contributing to the midwinter minimum in the eddy activity as observed before the late 1980s since then under the reduced trapping effect by the weakened jet upper level eddies tended to propagate right above the surface baroclinic zone while strongly interacting it even in midwinter as in fall and spring this midwinter tendency acted to enhance the eddy amplification that led to the recent disappearance of the activity minimum

tidally driven ice speed variation at helheim glacier greenland observed with terrestrial radar interferometry we used a terrestrial radar interferometer tri at helheim glacier greenland in august 2013 to study the effects of tidal forcing on the terminal zone of this tidewater glacier during our study period the glacier velocity was up to 25 m d 1 our measurements show that the glacier moves out of phase with the semi diurnal tides and the densely packed melange in the fjord here detrended glacier displacement lags behind the forecasted tidal height by 8 hours the transition in phase lag between the glacier and the melange happens within a narrow 500 m zone in the fjord in front of the ice cliff the tri data also suggest that the impact of tidal forcing decreases rapidly up glacier of the terminus a flowline model suggests this pattern of velocity perturbation is consistent with weak ice flowing over a weakly nonlinear bed

analysis of glacial earthquakes in 2003 ekstrom et al reported on the detection of a new class of earthquakes that occur in glaciated regions with the vast majority being in greenland the events have a characteristic radiation pattern and lack the high frequency content typical of tectonic earthquakes it was proposed that the events correspond to large and sudden sliding motion of glaciers here we present an analysis of all 184 such events detected in greenland between 1993 and 2005 fitting the teleseismic long period surface waves to a landslide model of the source we obtain improved locations timing force amplitudes and force directions after relocation the events cluster into seven regions all of which correspond to regions of very high ice flow and most of which are named outlet glaciers these regions are daugaard jensen glacier kangerdlugssuaq glacier helheim glacier the southeast greenland glaciers the northwest greenland glaciers rinks isbrae and jakobshavn isbrae event amplitudes range from 0 1 to 2 0 10 14 kg m force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s each region has a different temporal distribution of events all glaciers are more productive in the summer but have their peak activity in different months over the study period kangerdlugssuaq has had a constant number of events each year whereas jakobshavn had most events in 1998 1999 and the number of events in helheim and the northwest greenland glaciers has increased substantially between 1993 and 2005 the size distribution of events in kangerdlugssuaq is peaked above the detection threshold suggesting that glacial earthquakes have a characteristic size

quantitative estimates of velocity sensitivity to surface melt variations at a large greenland outlet glacier this is the publisher s version also available electronically from http www ingentaconnect com

the response of the pacific storm track and atmospheric circulation to kuroshio extension variability an index of the kuroshio extension front strength is produced using a maximum covariance analysis between sea surface temperature sst and sea surface height ssh gradient observations and composites of the atmospheric state are presented during its positive and negative phases using reanalysis data 1992 2011 it is found that when the kuroshio extension is less more meandering with a stronger weaker sst front the atmospheric heat transport by transient eddies is increased in the western eastern pacific region consistent with an increase decrease in low level baroclinicity analysis of the eddy mean flow interaction shows that this zonal shift in heat transport forces anomalous barotropic flow in the eastern pacific where blocking frequency is strongly influenced the above relationships cannot be reconciled with the known response of the north pacific storm track to remote forcing from the tropical pacific nor can they be explained by the response of the ocean to atmospheric forcing via surface heat fluxes or winds rather the zonal shift in the storm track highlighted here and the associated changes in the large scale circulation are interpreted as a response to the interannual variability of the kuroshio extension front

spatial and temporal melt variability at helheim glacier east greenland and its effect on ice dynamics this is the publisher s version also available electronically from http onlinelibrary wiley com

tide deflection of campbell glacier tongue antarctica analyzed by double differential sar interferometry and finite element method abstract a glacier tongue the terminus of a glacier projecting out from the coastline and floating over the ocean experiences vertical deflection due to the fluctuation of ocean tide accurate analysis of the tidal deflection of a glacier tongue is necessary to understand the elastic properties and dynamics of glacial ice we extracted the tidal deflection of campbell glacier tongue cgt one of the fast flowing glacier tongues in east antarctica by using 120 double differential interferometric sar ddinsar images from the 16 cosmo skymed one day tandem interferometric sar image pairs obtained during 2010 and 2011 a map of tide deflection ratio map defined as the ratio of the tidal deflection over tide height was generated by pixel based linear regression between the ddinsar derived tidal deflection and the tidal variation predicted by various tide models such as tpxo7 1 fes2004 cats2008a and ross inv after the inverse barometer effect correction we also generated the maps of r 2 root mean square error rmse and intercept of the linear regression model we confirmed that the horizontal flow of the campbell glacier is steady over the whole data acquisition period due to the fact that values were close to zero over the grounded part of the glacier ross inv provided the most reliable map with the highest r 2 0 97 and the smallest rmse 0 3 cm beyond the hinge zone than other tide models the spatial variation of the tidal response of the whole cgt was clearly defined from the map the grounding line retreated by 0 3 1 5 km in comparison with that in 1996 defined by rignot et al 2011a the ice thickness of cgt was estimated to be 397 71 m from icesat surface elevation data the young s modulus of cgt was found to be 0 5 1 3 gpa 0 8 gpa at an ice thickness of 397 m by simulating the in the hinge zone using a 2 dimensional finite element method

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

a baroclinic mechanism for the eddy feedback on the zonal index a baroclinic mechanism for the positive eddy feedback on the zonal index is proposed the author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies the transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation but over time the action of surface drag leads to enhanced baroclinicity at these latitudes the steady state response has positive baroclinicity at the latitude of eddy generation if the eddies propagate away from this latitude before they dissipate this reinforcement of the low level baroclinicity provides a positive feedback if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low level baroclinicity the proposed mechanism explains observed and modeled features of zonal index variations the frequency and drag dependence of eddy feedback the bandedness in latitude of zonal wind variations and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

ice flow modulated by tides at up to annual periods at rutford ice stream west antarctica 1 for over 2 years we have collected gps data 40 km upstream of the grounding line on rutford ice stream west antarctica here we examine deviations from mean downstream flow although the record is incomplete during winter there is clear modulation of flow at semi diurnal diurnal two weekly semi annual and annual ocean tidal frequencies this is the first observation of ice stream flow variations over such a long time period and such a wide range of frequencies the ice stream flows fastest at equinoxes when there are two semi diurnal tides of equal magnitude and slowest at solstices when one of the semi diurnal tides has lower amplitude the sensitivity of the downstream flow is greatest to the long period forcing which suggests that ice stream velocity may be affected by future changes in sea level if so this effect would provide a feedback whereby rising sea levels could increase ice stream velocity and hence discharge

the shape propagation and mean flow interaction of large scale weather systems abstract for a zonal average operator the eliassen palm flux provides a diagnostic of both eddy behavior and the feedback of the eddies onto the mean flow this paper addresses the diagnosis problem for other averaging operators and in particular for time averaging which has proved in recent years such a powerful means of viewing the three dimensional tropospheric flow the horizontal velocity correlation tensor gives a measure of the characteristic horizontal eddy shape at a point it also implies the direction of the group velocity relative to the mean flow in cases where such a concept is valid however the major emphasis here is on the mean flow feedback of eddies in this respect the eddy vorticity flux is determined by derivatives of the components of the anisotropic part of the tensor making a reasonable approximation allows the eddy vorticity flux convergence to be written in terms of e v 2 u 2 u v a simple interpretation of the mean flow feedback of eddies is then possible a sligh

north pacific decadal variability in the community climate system model version 2 abstract north pacific decadal oceanic and atmospheric variability is examined from a 650 yr control integration of the community climate system model version 2 the dominant pattern of winter sea surface temperature sst variability is similar to the observed pacific decadal oscillation with maximum amplitude along the kuroshio extension sst anomalies in this region exhibit significant spectral peaks at approximately 16 and 40 yr lateral geostrophic heat flux divergence caused by a meridional shift of the kuroshio extension forced by basin scale wind stress curl anomalies 3 5 yr earlier is responsible for the decadal sst variability local surface heat flux and ekman heat flux divergence act as a damping and positive feedback respectively a simple linear rossby wave model is invoked to explicitly demonstrate the link between the wind stress curl forcing and decadal variability in the kuroshio extension the rossby wave model not only successfully reproduces the two decadal spectral peaks but a

on the existence of storm tracks abstract given that middle latitude weather systems transport heat in a manner such as to weaken the baroclinicity that is thought to be crucial to their growth it is perhaps surprising that concentrated regions of such eddy activity i e storm tracks are found in the northern hemisphere winter the existence and possible self maintenance of storm tracks is investigated using a linear stationary wave model with storm track region forcings taken from data averaged over a number of winters it is found that the direct thermal effect of the eddies does indeed act against the existence of the storm track their vorticity fluxes lead to some reduction of this effect it is argued that the mean diabatic heating in the storm track region is an indirect eddy effect this heating is found to maintain the mean maximum in baroclinicity in the region further the mean low level flow induced by the eddy effects is such as to enhance the warm western oceanic boundary currents that are crucial to the existence of t

large and rapid melt induced velocity changes in the ablation zone of the greenland ice sheet continuous global positioning system observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the greenland ice sheet within days ice velocity reacts to increased meltwater production and increases by a factor of 4 such a response is much stronger and much faster than previously reported over a longer period of 17 years annual ice velocities have decreased slightly which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input which results in a more or less constant ice flux over the years the positive feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades

tidal triggering of low frequency earthquakes near parkfield california implications for fault mechanics within the brittle ductile transition 1 studies of nonvolcanic tremor nvt have established the significant impact of small stress perturbations on nvt generation here we analyze the influence of the solid earth and ocean tides on a catalog of 550 000 low frequency earthquakes lfes distributed along a 150 km section of the san andreas fault centered at parkfield lfe families are identified in the nvt data on the basis of waveform similarity and are thought to represent small effectively co located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km we calculate the sensitivity of each of these 88 lfe families to the tidally induced right lateral shear stress rlss fault normal stress fns and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity lfe occurrence is most strongly modulated by fluctuations in shear stress with the majority of families demonstrating a correlation with rlss at the 99 confidence level or above producing the observed lfe rate modulation in response to shear stress perturbations requires low effective stress in the lfe source region there are substantial lateral and vertical variations in tidal shear stress sensitivity which we interpret to reflect spatial variation in source region properties such as friction and pore fluid pressure additionally we find that highly episodic shallow lfe families are generally less correlated with tidal stresses than their deeper continuously active counterparts the majority of families have weaker or insignificant correlation with positive tensile fns two groups of families demonstrate a stronger correlation with fault normal tension to the north and with compression to the south of parkfield the families that correlate with fault normal clamping coincide with a releasing right bend in the surface fault trace and the lfe locations suggesting that the san andreas remains localized and contiguous down to near the base of the crust the deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth synthesizing our observations with those of other lfe hosting localities will help to develop a comprehensive understanding of transient fault slip below the seismogenic zone by providing constraints on parameters in physical models of slow slip and lfes

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

dependence of the climatological axial latitudes of the tropospheric westerlies and storm tracks on the latitude of an extratropical oceanic front 1 major storm tracks where migratory cyclones and anticyclones recurrently develop are observed around midlatitude oceanic frontal zones with strong meridional gradient of sea surface temperature sst a set of atmospheric general circulation model experiments is performed with zonally uniform sst prescribed at the model lower boundary the latitudinal sst profile for each hemisphere is characterized by a single front the frontal latitude is varied systematically from one experiment to another while the intensity of the frontal gradient is kept unchanged though idealized the experiments reveal a climatological tendency for a low level storm track to be organized along or slightly poleward of the sst front if located in the subtropics or midlatitudes as a surface manifestation of an eddy driven polar front jet pfj surface westerly axis tends to form on the poleward flank of the front this anchoring effect of the sst front is also hinted at upper levels but the climatological positions of the storm track and pfj are less sensitive to the frontal latitude for the sst front at subpolar latitude the joint primary axes of the upper level storm track and pfj form in midlatitudes away from the sst front their positions correspond to their counterpart simulated with a particular sst profile from which frontal gradient has been removed suggesting that the anchoring effect of a subpolar sst front on the storm track and pfj is overshadowed by atmospheric internal dynamics namely the self maintenance mechanism of a midlatitude storm track and pfj through their interactions

seasonal evolutions of atmospheric response to decadal sst anomalies in the north pacific subarctic frontal zone observations and a coupled model simulation abstractpotential impacts of pronounced decadal scale variations in the north pacific sea surface temperature sst that tend to be confined to the subarctic frontal zone safz upon seasonally varying atmospheric states are investigated by using 48 yr observational data and a 120 yr simulation with an ocean atmosphere coupled general circulation model cgcm sst fields based on in situ observations and the ocean component of the cgcm have horizontal resolutions of 2 0 and 0 5 respectively which can reasonably resolve frontal sst gradient across the safz both the observations and cgcm simulation provide a consistent picture between sst anomalies in the safz yielded by its decadal scale meridional displacement and their association with atmospheric anomalies correlated with sst anomalies persistent in the safz from fall to winter a coherent decadal scale signal in the wintertime atmospheric circulation over the north pacific starts emerging in november and develops into an equivalent barotropic an

glacier fjord and seismic response to recent large calving events jakobshavn isbrae greenland 1 the recent loss of jakobshavn isbrae s extensive floating ice tongue has been accompanied by a change in near terminus behavior calving currently occurs primarily in summer from a grounded terminus involves the detachment and overturning of several icebergs within 30 60 min and produces long lasting and far reaching ocean waves and seismic signals including glacial earthquakes calving also increases near terminus glacier velocities by 3 but does not cause episodic rapid glacier slip thereby contradicting the originally proposed glacial earthquake mechanism we propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving

non linear glacier response to calving events jakobshavn isbrae greenland jakobshavn isbrae a tidewater glacier that produces some of greenland s largest icebergs and highest speeds reached record high flow rates in 2012 joughin and others 2014 we use terrestrial radar interferometric observations from august 2012 to characterize the events that led to record high flow we find that the highest speeds occurred in response to a small calving retreat while several larger calving events produced negligible changes in glacier speed this non linear response to calving events suggests the terminus was close to flotation and therefore highly sensitive to terminus position our observations indicate that a glacier s response to calving is a consequence of two competing feedbacks 1 an increase in strain rates that leads to dynamic thinning and faster flow thereby promoting destabilization and 2 an increase in flow rates that advects thick ice toward the terminus and promotes restabilization the competition between these feedbacks depends on temporal and spatial variations in the glacier s proximity to flotation this study highlights the importance of dynamic thinning and advective processes on tidewater glacier stability and further suggests the latter may be limiting the current retreat due to the thick ice that occupies jakobshavn isbrae s retrograde bed

glacier dynamics near the calving front of bowdoin glacier northwestern greenland to better understand recent rapid recession of marine terminating glaciers in greenland we performed satellite and field observations near the calving front of bowdoin glacier a 3 km wide outlet glacier in northwestern greenland satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for 20 years ice radar measurements showed that the glacier front is grounded but very close to the floating condition these results in combination with the results of ocean depth soundings suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat presumably glacier thinning due to atmospheric and or ocean warming triggered the initial retreat in situ measurements showed complex short term ice speed variations which were correlated with air temperature precipitation and ocean tides ice speed quickly responded to temperature rise and a heavy rain event indicating rapid drainage of surface water to the bed semi diurnal speed peaks coincided with low tides suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance these observations demonstrate that the dynamics of bowdoin glacier are sensitive to small perturbations occurring near the calving front