model experiments on the evolution and stability of ice streams this is the publisher s version also available electronically from http www igsoc org annals 23

strain localisation and weakening of the lithosphere during extension we explore the sensitivity of extensional systems to the thermal structure of the lithosphere using numerical simulations that fully couple the energy momentum and continuity equations the rheology of the lithosphere is controlled by weakening processes such as shear heating that localises strain into shear zones and faults numerical models show that during extension of an initially unpatterned lithosphere structures develop spontaneously out of basic thermodynamic energy fluxes and without the imposition of ad hoc rules on strain localisation this contrasts with the classical mohr coulomb theory for brittle localisation which prescribes the angles of faults by a mathematical rule our results show that the mode of extension is sensitive to subtle changes in rheology heat flux and geometry of the system this sensitivity lies at the core of the variety and complexity observed in extensional systems localisation processes make the lithosphere weaker than previously estimated from the brace goetze quasi static approach consequently typical estimates for plate tectonic forces are capable of splitting the lithosphere under extension even without the role of active magmatism

intraseasonal oscillations in the global atmosphere part ii southern hemisphere abstract in part ii of this two part article we complete the systematic examination of oscillatory modes in the global atmosphere by studying 12 years of 500 mb geopotential heights in the southern hemisphere as in part i for the tropics and northern hemisphere extratropics the data were band pass filtered to focus on intraseasonal is phenomena and spatial eofs were obtained the leading principal components were subjected to singular spectrum analysis ssa in order to identify nonlinear is oscillations with high statistical confidence in the southern hemisphere the dominant mode has a period of 23 days with spatial patterns carried by the second and third winter eof of the is band it has a zonal wavenumber four structure the 40 day mode is second and dominated by wavenumbers three and four while a 16 day mode is too weak to separate its spatial behavior from the previous two the is dynamics in the southern hemisphere is more complex and dominated by shorter wavenumbers than the northern

modelled atmospheric response to changes in northern hemisphere snow cover the surface boundary conditions are altered in a numerical simulation of january climate by prescribing a higher and b lower than average snow extent over northern hemisphere land masses the anomalies in snow cover are shown to have quite a strong impact on the mean climatic state associated with an increase in the areal extent of the snow there is a significant reduction in temperature throughout the lower troposphere there are also large increases in sea level pressure over most land areas significant responses in the mass field are also seen at 500 hpa where reductions in atmospheric thickness lead to significant negative anomalies in the height field responses are also seen non locally over both the north pacific and north atlantic basins the impact of increased snow on cyclone tracks is also examined a reduction in cyclones is noted over both continents and over the western sectors of both ocean basins over the north atlantic basin this reduction extends across over europe significantly weakening the storm track in the north pacific cyclone density is reduced in the west while in the east there is actually a strengthening of the storm tracks there are corresponding changes in the genesis of cyclones in both of these regions the change in cyclogenesis intensity and density is demonstrated to be associated with changes in baroclinicity between the two experiments the anomalous snow boundary conditions lead to significant changes in the meridional temperature gradients over both ocean basins which impact on the baroclinic zones

foliation boudinage control on the formation of the rosebery pbzn orebody tasmania abstract the rosebery pb zn ag au massive sulphide deposit occurs within the middle cambrian mount read volcanics of western tasmania it is hosted by a tuffaceous slate lens that is surrounded by felsic volcanics the volcanic belt and its ordovician cover were deformed and metamorphosed during an early to middle devonian orogeny tabberabberan orogeny during this orogeny d 3 e w shortening was partially accommodated by reverse bedding parallel shearing within the slate unit and immediately underlying volcanic rocks this caused reactivation and intensification of an early foliation s 2 and the development of asymmetric chocolate tablet foliation boudinage structures with boudin necks parallel to the x and y directions the mineralization occurs along extensional shear planes that are associated with foliation boudinage mineralogical zoning in the orebody can be directly related to dilational bends in shears associated with foliation boudin necks it is proposed that these structures gaped during the d 3 deformation allowing infiltration of metalliferous hydrothermal fluids along shear plane and cleavage directions into the fractured wall rock progressive metasomatic replacement of the wall rock resulted in the formation of a sulphide sulphate silicate orebody the orebody is completely discordant to folds in the host rock this observation confirms a structurally controlled origin and refutes earlier interpretations of the deposit as a tightly folded chemical sediment isotope data indicate that the orebody was probaby derived from volcanogenic sulphides within the mount read volcanics the rosebery orebody is best regarded as a metamorphogenic deposit that formed by dissolution upward transport concentration and re precipitation of primary sulphides the proposed macroscopic model complements separate microstructural evidence by the author indicating a syn d 3 timing of mineralization

flow perturbations a tool to study and characterize heterogeneous deformation the classification of the myriad of small scale structures that are used in tectonic analysis is presently based on their geometry which makes it difficult to discover transitions and groups amongst them an alternative would be to classify structures according to the flow type by which they form but this is difficult although most structures form by heterogeneous flow modelling studies are often focussed on bulk homogeneous flow since the mathematical treatment of heterogeneous flow is cumbersome also heterogeneous deformation patterns seem to occur in as many types as there are small scale structures this paper introduces the use of the geometry of flow perturbations to improve our understanding of heterogeneous flow and to allow classification of structures based on kinematics a flow perturbation is the deviation of a heterogeneous flow pattern from the background homogeneous flow perturbation patterns can be visualised as open or closed loops of vectors which occur in pairs rows or groups of four they usually cross cut rheological interfaces and can be subdivided into those that cross interfaces twice or four times most geological structures belong to the first group while initiating buckle folds belong to the second group q 2005 elsevier ltd all rights reserved

relationships between low frequency variability in the southern hemisphere and sea surface temperature anomalies abstract long term trends and interannual variations of circulation anomalies in the southern hemisphere are examined using the national centers for environmental prediction national center for atmospheric research reanalysis from 1949 to 1998 the changes in planetary circulation regimes are linked to global sea surface temperature anomalies sstas empirical orthogonal function eof analysis was performed on 500 hpa height anomalies the leading mode eof1 shows a strong zonal symmetry with a phase reversal between height anomalies in high and midlatitudes apart from zonal symmetry a zonal wavenumber 3 is evident with three centers located in three southern oceans in the low frequency band with fluctuations longer than 60 months eof1 is associated with the second rotated eof mode of sstas with positive loadings over three southern oceans and negative loadings in the north pacific and the north atlantic the next two modes are the pacific south american psa patterns they depict wave 3 patterns in

blocking in the southern hemisphere abstract the focus of this paper is on the frequency and spatial distributions of blocking and persistent anomalies of geopotential height over the southern hemisphere the analysis is based upon daily height fields at 1000 and 500 mb for both summer and winter histogram frequency distributions of height anomalies and maps of the skewness and kurtosis have been computed blocking events are objectively defined by requiring a large positive anomaly to exist for 5 days or more composite flow and anomaly fields for several cases are presented and examined in detail the geographical distribution of the frequency of lame amplitude 150 gpm anomalies at 500 mb that persist for only 1 3 days is very similar to that of the high frequency band 2 8 day period variances that identity the storm tracks in the southern hemisphere the primary location for blocking in the southern hemisphere is in the new zealand sector and blocking occurs through a local enhancement of the climatological split in the mean westerli

interannual variations in the southern hemisphere winter circumpolar vortex relationships with the semiannual oscillation abstract several studies of southern hemisphere circulation report changes in the late 1970s and 1980s through the mid and high latitudes that are associated with a weakening in the semiannual oscillation sao of sea level pressure and geopotential height this weakening is thought to reflect changes in the amplitude of zonal wavenumber 3 a deepening of the subantarctic trough intensification of subtropical ridging and an increase in the strength of the westerlies these conclusions suggest an equatorial expansion of the circumpolar vortex in high latitudes and a poleward contraction of the vortex in the midlatitudes the goal of this study is to examine the size and spatial variation of the southern hemisphere circumpolar vortex in winter for evidence of such change the size of the 500 hpa vortex was calculated for the mid and high latitudes in 5 longitude sectors from both the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis dataset 1

indices of the southern hemisphere zonal wind abstract the time and spatial variation of the southern hemisphere 500 hpa zonal wind has been examined using spectral analysis bandpass filtering and empirical orthogonal function eof analysis applied to a 15 year sample of australian hemispheric analyses spectral analysis of the entire 15 year record of the 500 hpa zonal mean westerly wind shows significant peaks only on the seasonal and interannual time scales the mean segmented spectra for each season failed to show any additional significant peaks at higher frequencies or any significant differences between seasons the primary pattern of variation revealed by eof analysis shows similar behavior on synoptic to seasonal time scales with compensating departures occurring at intervals of 20 latitude the first eof has opposing peaks near 40 and 60 s and shows an equivalent barotropic pattern when correlated with individual grid point values at 500 hpa and mean sea level pressure correlations of the zonal pattern with regional anomalies reveal

the signature of a blocking episode on the general circulation in the southern hemisphere abstract two major blocking events took place over the south pacific ocean east of new zealand 23 july 4 august and 19 27 august 1979 the configuration of the overall flow including the split in the westerlies the location of the blocking highs and the wavenumber breakdown were very similar the tendency for blocking highs to reform in the same location in close sequence is referred to as a blocking episode the blocking events are analyzed in detail and the imprint they left on the general circulation statistics for the winter as a whole are presented since the blocking episode mostly prevented eddies from moving through the region the variance of meridional velocity is small nevertheless owing to the sequence of blocks the variance of geopotential height is a maximum this signature on the general circulation is in marked contrast to that in storm track regions where the variance of geopotential height and meridional wind are both a maximum a global scale redistribution of mass was associated

late acadian alleghenian transpressional deformation evidence from asymmetric boudinage in the casco bay area coastal maine abstract asymmetric competent layer and foliation boudinage in the casco bay area of coastal maine formed during regional f 2 hinge parallel elongation and distributed dextral strike slip movement it consists of backward rotated oblique shear fracture boudinage and forward rotated extension fracture boudinage initially symmetric quartz filled extension fractures were intermittently developed as boudin partings orthogonal to the prominent ductile stretching lineation due to fault related fluctuations in fluid pressure and tectonic stresses modification by concurrent non coaxial deformation related to dextral strike slip movement resulted in the clockwise rotation sinistral slip and vein parallel elongation reflected in the forward rotated asymmetric boudinage discordant planar pegmatite veins and variably oriented boudinaged pegmatite sheets common in these exposures underwent a similar evolution the development of this asymmetric boudinage is attributed to progressive brittle ductile non coaxial deformation related to orogen parallel dextral transpression along the norumbega nonesuch river strike slip fault system during the later stages of the acadian orogeny and subsequent alleghenian deformation

ice atmosphere interactions during sea ice advance and retreat in the western antarctic peninsula region 1 the seasonal evolution of sea ice extent concentration and drift in the western antarctic peninsula wap region along with regional atmospheric synoptic variability are described for a winter period 1992 when sea ice advance and retreat were both anomalously early daily time series of winds opening and closing of the sea ice cover and volume fluxes in and out of the wap region indicate that synoptic variability in meridional winds determines whether the ice edge advances or retreats on daily to weekly timescales the importance of this finding is that the dynamics as opposed to the thermodynamics initiate and thereby dominate in the production of ice edge anomalies further the abrupt mid winter shift from persistently positive to negative ice edge anomalies indicates that there was a distinct change in the regional atmospheric circulation the ice atmosphere interactions in the winter of 1992 are then compared to those in the winter of 1990 when sea ice advance and retreat were both late instead of early but again the mid winter shift was abrupt the comparison highlights possible circumpolar and tropical polar atmospheric linkages in the south pacific that involve changes in the amplitude phase of the semi annual oscillation sao which in turn appear

palaeocirculation across new zealand during the last glacial maximum at 21 ka abstract what circulation pattern drove southern alps glacial advances at 21 ka late 20th century glacial advances in new zealand are commonly attributed to a dual precipitation increase and cooler than normal temperatures associated with enhanced westerly flow that occur under synoptic pressure patterns termed zonal regimes kidson 2000 but was the circulation pattern that supported major southern alps glacial advances during the global lgm similar to the modern analog here a regional climate regime classification rcrc time slice was used to infer past circulation for new zealand during the lgm at 21 ka palaeoclimate information that supported the construction of the 21 ka time slice was derived from the nz intimate climate event stratigraphy ces one new auckland maar proxy record and additional low resolution data sourced from the literature the terrestrial evidence at 21 ka implicates several possibilities for past circulation depending on how interpretations for some proxies are made the interpretation considered most tenable for the lgm based on the agreement between terrestrial evidence marine reconstructions and palaeoclimate model results is an anticyclonic zonal circulation regime characterized by increased influences from blocking highs over the south island during winter and an increase in zonal and trough synoptic types with southerly to westerly quarter wind flow during summer these seasonal circulation traits would have generated lower mean annual temperatures cooler than normal summer temperatures and overall lower mean annual precipitation for new zealand particularly in the western south island at 21 ka the anticyclonic zonal time slice reconstruction presented in this study has different spatial traits than the late 20th century and the early little ice age signatures suggesting more than one type of regional circulation pattern can drive southern alps glacial activity this finding lends support to the hypothesis that temperature over precipitation change is more important as the primary modulator of southern alps ice advances the rcrc approach also demonstrates some subtle advantages of integrating multi proxy data within a palaeocirculation context for new zealand notably because this reconstruction technique enables direct comparisons to coarsely resolved palaeoclimate model outputs that do not have downscaled information

baroclinicity meridional temperature gradients and the southern semiannual oscillation abstract it has long been known that a strong half yearly oscillation exists in surface pressure at high southern latitudes there are two minima during the year the one occurring in october being more intense than that in march the semiannual oscillation sao has been related to the different surface types and inertia at 50 and 65 s these conspire to produce a semiannual wave in the meridional temperature gradient between those two latitudes the timing of the maxima for these is as above but the stronger is in march however if the arguments of van loon are applied literally then the seasonal evolution of temperature gradient should follow a comparable seasonal evolution of baroclinicity and surface pressure a full calculation of baroclinicity shows that the seasonal evolution of static stability combined with the twice annual maxima of meridional temperature gradient produces a larger peak of baroclinicity in october consistent with the sao in surface pressure

a reexamination of the mechanism of the semiannual oscillation in the southern hemisphere the local pressure changes associated with the twice annual contraction intensification and expansion weakening of the circumpolar trough of low pressure around antarctica termed the semiannual oscillation sao was the dominant signal in the annual cycle at mid and high southern latitudes before 1979 the mechanism as shown by van loon 1967 arises from diverent response to the surface heat budget over the polar continent and the midlatitude ocean it has subsequently been shown that in most years since 1979 the sao has weakened considerably evidence is presented here from surface temperature data 500 mb temperatures from a station pair and zonal mean 500 mb temperatures from the ncar ncep reanalyses to show that a warming trend since 1979 has not been evenly distributed through the year at each latitude thus an anomalous change in the temperature gradient between 50 s and 65 s with peaks in roughly may and november has modulated the mechanism that produces the sao with its peaks in march and september consequently the magnitude of the sao has decreased in the more recent period

on the position of southern hemisphere westerlies at the last glacial maximum an outline of agcm simulation results and evaluation of their implications abstract the paper evaluates the claim of a poleward displacement of the southern hemisphere mid latitude westerly circulation at the last glacial maximum lgm the claim was initially based on the interpretation of the late quaternary paleoenvironmental records of southern south america and drew on conceptual paleo circulation reconstructions and the results of early agcm simulations a guide to likely lgm changes in the position of the southern hemisphere westerlies can in principle be obtained through a modelling of associated storm tracks a detailed analysis of the position of the southern hemisphere storm tracks at the lgm was undertaken using the ugamp gcm the results of the experiment suggest that a general poleward displacement of the zonal maximum of the southern hemisphere westerlies occurred at the lgm but this is a generalization and in detail the changes in the westerly circulation are more far ranging with significant regional differences

finite element modelling of the structure and evolution of the south kenya rift east africa abstract a finite element fe model was used in this study to investigate the intraplate rifting process in south kenya east africa the rifting model shows that the important factors influencing the amount of shoulder uplift and rift subsidence include the horizontal deviatoric extensional stresses the viscosity of the lower crust and the dimension and density contrast of the low velocity upper mantle anomaly thus for instance a reduced lower crust viscosity as well as an increased extensional far field stress favours the subsidence of the rift basin in turn for a reduced lower crust viscosity the magnitude of the applied extensional stress should be reduced in order to end up with the same topography of the rifted area as can be obtained with a more viscous lower crust combined with a relatively high extensional far field stress in addition it also became obvious that the uplift of the crust mantle boundary beneath the rift depends predominantly on the size and density contrast of the upper mantle anomaly an increased density contrast between the upper mantle anomaly and normal mantle results in increased buoyancy forces thus producing an increased uplift of the crust mantle boundary in turn in order to avoid too large values for the surface topography a reduced lower crust viscosity in the order of 1021 pa s is required models without an upper mantle low density anomaly have resulted in crustal thickening beneath the rift and not in crustal thinning in accordance with the geological field evidence we have modelled the evolution of the south kenya rift in two stages in the first stage from 16 to 10 ma ago a broad regional uplift of a few hundred metres produced by a relatively small but broad low density upper mantle anomaly was accompanied by an insignificant amount of faulting in the second stage from 10 ma ago to the present day significant faulting accompanied by the buoyancy effects of a larger low density low velocity upper mantle anomaly concentrated more beneath the rift itself have quite successfully reproduced the rift subsidence shoulder topography and crust mantle boundary uplift observed at the present day

large scale foliation boudinage in gneisses abstract large scale extensional structures in gneisses symmetric and asymmetric foliation boudinage c type shears are common in crustal scale shear zones and may constitute shear criteria as well as being indicators of high strain

ascent dynamics of large phreatomagmatic eruption clouds the role of microphysics 1 we examine the impact of abundant surface water interaction on the development of volcanic clouds from large scale 108 kg s 1 magma phreatomagmatic eruptions presenting the first 2 d large eddy simulations of wet volcanic plumes that incorporate the effects of microphysics the active tracer high resolution atmospheric model was forced with field derived inputs from an exceptionally large phreatomagmatic eruption the 27 ka oruanui supereruption taupo volcano new zealand surface water contents were varied from 0 to 40 wt for eruptions with equivalent magma eruption rates of 1 3 108 and 1 1 109 kg s 1 our findings confirm that increased surface water has a pronounced impact on column stability leading to transitional column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents pdcs contrary to the suggestion of previous workers however abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds owing to vigorous microphysics assisted lofting of pdcs the simulated behavior and ash cloud dimensions provide a close match to field evidence from the oruanui case study cloud heights from the collapsing eruption columns also show a notable sensitivity to changes in the altitude of the tropopause while ambient humidity primarily impacts the abundance of airborne hydrometeors particularly ice associated with the volcanic clouds general relationships between eruption style meteorological conditions and the resulting vertical profiles of volcanic emissions outlined here could also be adapted for use in operational volcanic ash forecasting and deposit reconstruction techniques already in existence

intraseasonal variations in the southern hemisphere circulation abstract intraseasonal variations in the southern hemisphere atmospheric circulation have been examined through the application of a 10 50 day bandpass filter to daily ecmwf analyses for 1980 88 variations on this time scale contribute more than 40 of the daily variance in 500 hpa geopotential over much of the middle and high latitudes of the southern hemisphere eof analysis of the unnormalized variance for all seasons shows that 49 of this variance can be explained by zonal wave trains centered on the south pacific and southern atlantic indian oceans a high latitude mode of global extent and a wavenumber 3 pattern at midlatitudes these modes are essentially equivalent barotropic but slope westward with height so that the patterns at 100 hpa typically lag those at 1000 hpa by around 1o longitude there appears to be little interaction with the low latitude circulation all of the leading modes propagate eastward but the most consistent movement is shown by the south pacific wave train represented

modeling evidence for enhanced el nino southern oscillation amplitude during the last glacial maximum we present a numerical eigenmode analysis of an intermediate el nin o southern oscillation enso model which is driven by present day observed background conditions as well as by simulated background conditions for the last glacial maximum lgm about 21 000 years ago the background conditions are obtained from two lgm simulations which were performed with the national center for atmospheric research climate system model csm1 4 and an earth system model of intermediate complexity ecbilt clio our analysis clearly shows that the leading present day unstable recharge discharge mode changes its stability as well as its frequency during lgm conditions simulated lgm background conditions were favorable to support large amplitude self sustained interannual enso variations in the tropical pacific our analysis indicates that off equatorial climate conditions as well as a shoaling of the thermocline play a crucial role in amplifying the lgm enso mode

interdecadal variation of the southern hemisphere circulation abstract previous studies examining the seasonal contrast and spatial structure of the interdecadal variation in the southern hemisphere sh atmospheric circulation and the relationship between this interdecadal variation and cyclone activity have been extended using the sh data generated by the australian bureau of meteorology for 1972 92 the major findings of this study are the following in addition to the deepening of the circumpolar trough and the increase of midlatitude sea level pressure slp in winter a zonal wavenumber 3 pattern appears in the high middle latitudes during summer the interdecadal change is dominated by three latitudinal zones of slp anomalies with negative values in low and high latitudes and positive values in midlatitudes a zonal wavenumber 3 pattern that develops during the winter persists into early spring while the three latitudinal slp anomaly zones that emerge in late spring and early summer continue on through the summer an enhancement of cyclone activity is assoc

decadal variation of the annual cycle in the australian dataset abstract during the 1970s and 1980s elements of the atmospheric circulation over the southern hemisphere changed markedly from their state in the two previous decades the change was evident in the zonal asymmetry especially in the zonal harmonic wave 3 at higher latitudes and in the semiannual wave in pressure and wind the semiannual wave changed in the same sense in all three southern oceans but by the largest amount in the south pacific ocean the second harmonic in a 12 month series by which we describe the semiannual wave dominates the shape of the long term mean annual curve of sea level pressure in mid and high latitudes this harmonic weakened during the period and its midlatitude peak disappeared in the south pacific ocean as part of the low frequency changes during the 18 years from 1972 to 1988 the central pressure in the subantarctic trough fell the trough moved northward and the subtropical ridge moved south the meridional movement of the trough and the ridge and the concurrent

late quaternary hydroclimatology of a hyper arid andean watershed climate change floods and hydrologic responses to the el nino southern oscillation in the atacama desert abstract although certain characteristics of the el nino southern oscillation enso are well known on contemporary timescales less is known about the magnitude frequency relationships of this atmospheric phenomenon on longer timescales or its relationship to widespread flooding especially in its core zone along the sub tropical andes where la nina or el nino episodes control regional hydroclimatology using a combination of stratigraphic evidence geochronologic dating 14c and osl stable isotope analyses and water geochemistry along the rio moquegua in the northern fringes of the atacama desert we assemble a paleoflood chronology for mainstem and tributary sections for the past ca 20 ka and ascertain the variation in enso frequency and magnitude because of the inherent watershed structure and regional hydroclimatology mid valley tributaries of the rio moquegua only flood during el nino episodes and thus provide an important proxy of extreme el ninos while mainstem stratigraphy records both la nina and el nino episodes el nino floods appear to have been pronounced during the late pleistocene and up to at least the younger dryas 12 000 cal yr bp while stratigraphic evidence of large el nino floods is lacking in tributary systems during the mid holocene flood stratigraphy in a 2 ka 7 m high terrace along the mainstem indicates an increased frequency and magnitude of large floods between ca 700 and 1610 ad as compared to the period from ca 160 bce to 700 ad with mega ninos occurring ca 1330 ad and ca 1650 ad water geochemistry and radiocarbon dating indicate that at least two major aquifers exist with wells and springs in the mid valley dating to 710 and 3100 14c yr bp respectively while water from a spring in the headwaters dates to 10 320 14c yr bp this range in dates suggests that groundwater flow in the mid valley is neither fossil water nor exclusively recharged from local precipitation while the older date for headwater sections suggests a more fossil groundwater source and lack of contemporary recharge the 18o similarity between groundwater in the mid valley and the regional meteoric water line mwl suggests a pacific moisture source and the role of el nino precipitation in recharging local aquifers thus el ninos and pacific sourced moisture are an important component of the regional hydroclimatology and the variation in enso frequency and intensity has significant social and hydrologic repercussions in these hyper arid settings

barrier holes and trench application to reduce blast induced vibration in seyitomer coal mine in this research experimental applications have been performed to reduce blast induced vibrations in open pit mines for this purpose artificial discontinuity zones such as barrier holes and trench were opened in a dragline panel of seyitomer lignite enterprise kutahya turkey peak particle velocities in front of and behind them were measured by seismographs in this way their effect against vibration was observed and compared barrier holes were opened 24 m deep through 3 parallel lines and aligned at 1 m spacing on the other hand the trench had 8 m depth and 4 m width during the research 209 measurements were taken belonging to 105 explosions twenty eight explosions were carried out in the barrier holes experiments and 77 explosions were carried out in the trench experiments a decrease in vibration of 14 3 18 5 was obtained behind the barrier holes while a decrease of 24 8 58 1 was provided by the trench

northern hemisphere storm tracks in present day and last glacial maximum climate simulations a comparison of the european pmip models abstract extratropical weather systems are an essential feature of the midlatitude climate and global circulation at the last glacial maximum lgm the formation of regions of high transient activity referred to as storm tracks is strongly affected by the presence of large ice sheets over northern america and scandinavia and by differences in sea surface temperature sst distributions in the framework of the palaeoclimate modelling intercomparison project simulations of the lgm climate have been run with a wide range of atmospheric general circulation models agcms using the same set of boundary conditions allowing a valuable comparison between simulations of a climate very different from the present one in this study the authors focus on the storm track representation in the models and its relationship with the surface temperatures the mean flow and the precipitation storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three dimensional

mechanisms of basal ice formation in polar glaciers an evaluation of the apron entrainment model 1 previous studies of polar glaciers have argued that basal ice can form when these glaciers override and entrain ice marginal aprons that accumulate adjacent to steep ice cliffs to test this idea we have studied the morphology structure composition and deformation of the apron and basal ice at the terminus of victoria upper glacier in the mcmurdo dry valleys which are located on the western coast of the ross sea at 77 si n southern victoria land antarctica our results show that the apron has two structural elements an inner element that consists of strongly foliated ice that has a steep up glacier dip and an outer element that lacks a consistent foliation and has a down glacier slope parallel dip although strain measurements show that the entire apron is deforming the inner element is characterized by high strain rates whereas relatively low rates of strain characterize the outer part of the apron co isotopic analyses of the ice together with analysis of solute chemistry and sedimentary characteristics show that the apron is compositionally different from the basal ice our observations show that aprons may become deformed and partially entrained by advancing glaciers however such an ice marginal process does not provide a satisfactory explanation for the origin of basal ice observed at the ice margin our interpretation of the origin of basal ice is that it is formed by subglacial processes which are likely to include deformation and entrainment of subglacial permafrost

a coupled model study of the last glacial maximum was part of the north atlantic relatively warm a coupled ocean atmosphere general circulation model is used to simulate the climates of today and the last glacial maximum lgm the model which does not require artificial flux adjustments produces a pattern of cooling at the lgm that is broadly consistent with the findings from simpler models and palaeoclimatic data however changes to the ocean circulation produce anomalously warm lgm surface conditions over parts of the north atlantic seemingly at odds with palaeoceanographic data the thermohaline circulation is intensified for several centuries as is the northward heat transport in the atlantic equatorward of 55 n but this may be a transient result mechanisms that lead to this response are discussed

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

features of the zonal mean circulation in the southern hemisphere during the last glacial maximum 1 characteristics of the zonal mean circulation of the southern hemisphere during the last glacial maximum lgm are investigated using a high resolution global atmospheric climate model set up under paleoclimate modeling intercomparison project glacial conditions the output is compared with the output from a control simulation accounting for the biases in the model differences between the control and lgm circulation are ascribed to the climate of the lgm during the lgm the mean zonal winds were slightly stronger at midlatitudes and weaker at high latitudes implying a small equatorward shift of the polar trough in the zonal mean the amplitude of the wave number 3 pattern of the general circulation was slightly larger particularly during the austral winter and there was a slight shift to shorter wavelengths in the planetary waves meridional heat transport at midlatitudes was reduced which was mainly the result of an increase in the occurrence and strength of southerly winds however meridional heat transport by the transient eddies had increased slightly north of 60 s

two dimensional numerical modeling of fracturing and shear band development in glacier fronts in this contribution we present a two dimensional numerical model of a deforming glacier front the model is based on a hybrid lattice spring network approach where particles in the model can deform in a volume conservative visco elastic manner but at the same time they can be compressed elastically and fracture by discrete failure we restrict ourselves to a simple setting where the glacier sits on a frictionless slope that dips with 5 10 the ice block is fixed on one side and has a free surface on the other the glacier varies in viscosity and can flow at the base whereas it is brittle at the top results show that the head of the glacier is unstable failure happens as a combination of extension fractures crevasses at the top surface of the glacier and shear fractures that are dipping toward the glacier head once the shear fractures intersect with the free side wall of the glacier a triangular ice block is carving from the glacier head during successive flow of the glacier the failure is stepping backwards into the glacier and large shear planes develop that connect the sliding ice at the base with crevasses at the top variations of overall viscosity of the glacier indicate that higher viscosities and thus a more brittle glacier lead to larger spacing of shear surfaces and thus to larger ice blocks that are carving from the head of the glacier in addition the geometry of the deformation structures within the glacier does not vary significantly with the height of the ice indicating that larger glaciers carve larger blocks a higher tilt of the ground surface however leads to tighter spacing of shear surfaces and a more pronounced crevasse development this indicates that glacier heads that lie on steeper slopes will carve smaller blocks than glacier heads that lie on shallower slopes failure and carving of ice from the model glaciers is a combination of early developing closely spaced extension fractures crevasses and later developing wider spaced and more localized shear fractures or shear zones

variability of the atmospheric circulation in the southern hemisphere during the last glacial maximum 1 a high resolution atmospheric climate model has been used to investigate short term variability of the atmospheric circulation in the southern hemisphere during the last glacial maximum lgm the model was forced at the lower boundary by sea surface temperatures ssts and sea ice fields from a lower resolution coupled model simulation of the lgm although the semiannual oscillation sao in the control simulation showed a different annual cycle than in the national centers for environmental prediction ncep reanalysis the lgm sao was similar to that of today albeit slightly weaker with the onset of the second peak delayed by a month nearly one third of the interannual variability of monthly mean 500 hpa height anomalies was accounted for by the southern annular mode empirical orthogonal function analysis showed that the next two most important modes looked similar to those associated with el nino southern oscillation enso however correlations of the associated time series with those of the coupled model s tropical sea surface ssts were much smaller than for the observed present day enso correlations

possible enso signals prior to the last glacial maximum during the last deglaciation and the early holocene from new zealand annually laminated lake sediments from a maar crater in auckland new zealand were analyzed spectrally for three time slices just prior to the last glacial maximum lgm during the deglaciation and during the early holocene strong spectral power in 2 7 year 9 30 year and a centennial 80 120 year timescale prior to the lgm and during two brief intervals in the deglaciation contrasts with virtually no spectral power during the rest of the deglaciation and during the early holocene the antarctic cold reversal acr appears to be the younger deglacial event the results indicate that el nino southern oscillation enso and possibly the inter decadal pacific oscillation ipo operated in a modern mode prior to the lgm pulsed on and off during the deglaciation and were suppressed or absent from northern new zealand during the early holocene the acr was a period of enhanced southwesterly flow over northern new zealand