the sensitivity of the late saalian 140 ka and lgm 21 ka eurasian ice sheets to sea surface conditions this work focuses on the late saalian 140 ka eurasian ice sheets surface mass balance smb sensitivity to changes in sea surface temperatures sst an atmospheric general circulation model agcm forced with two preexisting last glacial maximum lgm 21 ka sst reconstructions is used to compute climate at 140 and 21 ka reference glaciation contrary to the lgm the ablation almost stopped at 140 ka due to the climatic cooling effect from the large ice sheet topography late saalian sst are simulated using an agcm coupled with a mixed layer ocean compared to the lgm these 140 ka sst show an inter hemispheric asymmetry caused by the larger ice albedo feedback cooling climate the resulting late saalian ice sheet smb is smaller due to the extensive simulated sea ice reducing the precipitation in conclusion sst are important for the stability and growth of the late saalian eurasian ice sheet

the mutual interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented significant obstacles to the northern hemisphere midlatitude westerlies they must therefore have forced large changes in the atmospheric circulation and consequently also in the patterns of accumulation and melting over the ice sheets themselves a simplified threedimensional coupled ice sheet stationary wave model is developed in order to understand the ice sheet s response to the circulation changes that it induces consistent with ice age climate simulations the ice sheet topography induces an anticyclonic circulation over the ice sheet causing a slight warming over the western slopes and a stronger cooling over the remainder the modeled feedbacks significantly affect the ice sheet configuration with the most important influences being the patterns of summer temperature and the topographically induced precipitation field the time evolution of the ice sheet is also changed by the atmospheric feedbacks and the results suggest the possibility of multiple equilibrium solutions

ageostrophic geopotential fluxes in downstream and upstream development of baroclinic waves abstract with the use of a simple primitive equation model it is demonstrated that the convergence divergence of ageostrophic geopotential fluxes can be a major source sink of kinetic energy for both downstream and upstream development of baroclinic waves and can play a dominant role during the early stages of wave development it is also shown that both surface friction and effects lead to an asymmetry in the upstream versus downstream development with downstream development much stronger a total group velocity is defined based on ageostrophic fluxes and its relationship to the rate of wave packet spreading and to convective and absolute instability is discussed

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

investigating the linear and nonlinear stationary wave response to anomalous north american snow cover abstractcontinental scale snow cover represents a broad thermal forcing on monthly to intraseasonal time scales with the potential to modify local and remote atmospheric circulation a previous gcm study reported robust transient eddy responses to prescribed anomalous north american na snow cover the same set of experiments also indicated a robust upper level stationary wave response during spring but the nature of this response was not investigated until now here the authors diagnose a deep snow induced tropospheric cooling over na and hypothesize that this may represent a pathway linking snow to the stationary wave response a nonlinear stationary wave model is shown to reproduce the gcm stationary wave response to snow more accurately than a linear model and results confirm that diabatic cooling is the primary driver of the stationary wave response in particular the total nonlinear effects due to cooling and its interactions with transient eddies and orography are shown to be essential fo

the influence of continental ice atmospheric co2 and land albedo on the climate of the last glacial maximum the contributions of expanded continental ice reduced atm9spheric co2 and changes in land albedo to the maintenance of the climate of the last glacial maximum lgm are examined a series of experiments is perform ed using an atmosphere mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination the model used has been shown to produce a reasonably reajistic simulation of the reduced temperature of the lgm manabe and broccoli 1985 by comparing the results from pairs of ex periments the effects of each of these envirornnental changes can be determined expanded continental ice and reduced atmospheric co2 are found to have a substantial impact on global mean temperature the ice sheet effect is confined almost ex clusively to the northern hemisphere while lowered co2 cools both hemispheres changes in land albedo over ice free areas have only a minor thermal effect on a global basis the reduction of co2 content in the atmosphere is the primary contributor to the cooling of the southern hemisphere the model sensitivity to both the ice sheet and co2 effects is characterized by a high latitude amplification and a late autumn and early winter maximum

storm track dynamics this paper reviews the current state of observational theoretical and modeling knowledge of the midlatitude storm tracks of the northern hemisphere cool season observed storm track structures and variations form the first part of the review the climatological storm track structure is described and the seasonal interannual and interdecadal storm track variations are discussed in particular the observation that the pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales are highlighted as challenges that any comprehensive storm track theory or model has to be able to address physical processes important to storm track dynamics make up the second part of the review the roles played by baroclinic processes linear instability downstream development barotropic modulation and diabatic heating are discussed understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics and provides a context within which both observational and modeling results can be interpreted the eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks the final part of the review deals with the ability to model storm tracks the success as well as remaining problems in idealized storm track modeling which is based on a linearized dynamical system are discussed perhaps on a more pragmatic side it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure and to a certain extent the seasonal and enso related interannual variations of storm tracks in depth comparisons between observed and modeled storm track variations are still lacking

maintenance of northern summer stationary waves in a gcm abstract the maintenance of northern summer stationary waves is studied with data from a 15 year integration of the general circulation model gcm experiment performed at the geophysical fluid dynamics laboratory the model has relatively high resolution rhomboidal 30 wavenumbers 9 vertical levels and simulates the summertime stationary waves reasonably well a steady linear baroclinic model is used to understand the various forcing mechanisms for the northern summer stationary waves the linear model response to global diabatic heating is found to play a dominant role in maintaining the summertime stationary waves in the gcm especially in the subtropics this response to diabatic heating shows a baroclinic structure in the vertical with a node at about 0 5 on the other hand stationary nonlinear interaction terms are found to be largely responsible for the extratropical equivalent barotropic stationary wave features it is hypothesized that this nonlinear interaction is a result of the therm

comment on u pb and hf isotope analysis of detrital zircons from mesozoic strata of the gravina belt southeast alaska by yokelson et al 2015

volcaniclastic gravity flow deposits in the dezadeash formation jura cretaceous yukon canada implications regarding the tectonomagmatic evolution of the chitina arc in the northern cordillera of north america abstract the chitina arc in the northern cordillera of north america evolved during the accretion of the wrangellia composite terrane to the western margin of north america in the jurassic and cretaceous the dezadeash formation a 3000 m thick sequence of deepwater turbidites in southwestern yukon was deposited as a submarine fan in a backarc basin to the chitina arc only limited geochemistry of altered volcanic rocks associated with the arc are available from southern alaska however three thick volcaniclastic beds occur in the dezadeash formation these consist of fine to medium grained vitric to crystal tuffs that are interpreted as resedimented syn eruptive volcaniclastics a u pb zircon age 149 4 0 3 ma indicates they are contemporaneous with the chitina arc petrographic examination shows the volcaniclastic rocks are altered mainly by albite and locally calcite with a chemical index of alteration suggesting the rocks are relatively unweathered or have undergone only incipient weathering harker diagrams indicate that cao k 2 o and rb were mobile based on the th vs co compositional diagram the volcaniclastic rocks are classified as dacites and according to various trace element plots they are calk alkaline and adakite like in composition the volcaniclastic rocks plot in the active continental margin field on a variety of tectonic discriminant diagrams and chondrite normalized multi element plots display parallel listric shaped profiles with significant light rare earth element enrichment and minor heavy rare earth element enrichment sm nd systematics indicate the volcaniclastic rocks represent mixing of a depleted mantle source and an older crustal source these data suggest mainly slab dehydration with low pressure fractionation controlled by amphibole with possibly minor input by slab melting the chitina arc has previously been interpreted as an oceanic island arc and the continental arc signature of volcaniclastic rocks in the dezadeash formation together with a continental arc signature of altered volcanic rocks in southern alaska is attributed to arc magmas erupting through paleozoic volcanic sedimentary and plutonic rocks of the wrangellia composite terrane proxying for continental crust

storm tracks in the southern hemisphere abstract from 1979 to 1989 global european centre for medium range weather forecasts ecmwf analyses of many meteorological variables important in baroclinic storms have been bandpass filtered to highlight fluctuations on 2 to 8 day time scales in order to illuminate the relationships among the variables in baroclinic storm track regions because of the greater zonal symmetry in the southern hemisphere sh versus the northern hemisphere nh it is possible to examine these relationships over a zonal sector without being dominated by local effects associated with jet stream entrance and exit regions of the storm tracks the bandpassed variance of geopotential height is used to define the storm track and its meridional profile has a pronounced midlatitude maximum in the sh the locations of the jet stream and the variance maxima of meridional and zonal velocity components vorticity vertical motion specific humidity and temperature all bear a distinctive relationship to the storm track but the lat

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

orographic constraints on a modeled siberian snow tropospheric stratospheric teleconnection pathway previous modeling studies have identified a teleconnection pathway linking observation based early season siberian snow perturbations to a modulation of the winter arctic oscillation ao mode in this study the key role of orography in producing this modeled teleconnection is explicitly investigated using numerical experiments analogous to the previous studies the climatic response to the same snow perturbation is investigated under modified orographic barriers in southern and eastern siberia reducing these barriers results in a weakening of the prevailing orographically forced region of stationary wave activity centered over siberia as well as the snow forced upward wave flux anomaly that initiates the teleconnection this diminished anomaly propagates upward but does not extend into the stratosphere to weaken the polar vortex consequently poleward refraction of upper tropospheric waves and downward propagation of coupled wave mean flow anomalies which ultimately produce the negative winter ao response fail to develop thus the mountains represent an orographic constraint on the snow ao teleconnection pathway by reducing the orographic barrier the snow forced influx of wave energy remains in the troposphere and instead produces a hemispheric scale equatorward wave refraction

planetary waves kinematically forced by himalayan orography abstract an analysis is made of the planetary scale response of the atmosphere to the kinematic effects of orographic forcing by in particular the tibetan plateau himalayan mountain complex theoretical scaling arguments are used to deduce a critical mountain height hc beyond which the component of flow around will dominate that over the orography the hc is proportional to the meridional scale of the orography and depends on latitude for north south scales appropriate for the himalayas hc 1 5 km which is much less than the actual height of 3706 m when resolved with tour zonal planetary waves with the implication that the around component will dominate a steady state planetary wave model which has a full kinematic nonlinear lower boundary condition is used to simulate the response to the eastern orography whose height has been multiplied by factors ranging from 0 1 to 2 0 although the mountain configuration was fixed the locations of the simulated perturbation highs and lows change substantiall

baroclinic waves propagating from a high latitude source a time dependent primitive equation model of the southern hemisphere winter troposphere with an idealized dome shaped continent centred at 80 s has been used to assess the potential of antarctica to force stationary waves and to determine the location of the mid latitude baroclinic storm track several integrations of the model for two hundred days were made with the zonal mean fields fixed at the climatological values in the time mean of each run an anticyclonic wave response formed over the mountain and a planetary wave propagated into the mid latitudes with substantial amplitude this suggests that much of the observed stationary asymmetry in the southern mid and high latitudes may be due to antarctica the response to the antarctic forcing was different in a steady linear version of the model however interaction between the steady waves appears to be of minor importance to the time mean pattern rather e vector fields point to a substantial effect of the transients on the pattern in particular the low frequency transients appear to be important in the balance of the time mean equations near the mountain

a one dimensional model for the interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented a significant topographic obstacle to the westerly winds in northern midlatitudes this work explores how consequent changes in the atmospheric stationary wave pattern might have affected the shape and growth of the ice sheets themselves a one dimensional 1 d model is developed which permits an examination of the types and magnitudes of the feedbacks that might be expected when plausible temperature perturbations are introduced at the ice sheet margin which are proportional to the stationary wave amplitude the equilibrium shape of the ice sheet is significantly altered and depends on the sign of the perturbation the proposed feedback also affects the response of the ice sheet to time varying climate forcing the results suggest that the evolution of a continental scale ice sheet with a land based margin may be significantly determined by the changes it induces in the atmospheric circulation

the effects of orography on midlatitude northern hemisphere dry climates abstract the role of mountains in maintaining extensive midlatitude arid regions in the northern hemisphere was investigated using simulations from the gfdl global climate model with and without orography in the integration with mountains dry climates were simulated over central asia and the interior of north america in good agreement with the observed climate in contrast moist climates were simulated in the same regions in the integration without mountains during all season but summer large amplitude stationary waves occur in response to the tibetan plateau and rocky mountains the midlatitude dry regions are located upstream of the troughs of these waves where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited in summer this mechanism contributes to the dryness of interior north america as a stationary wave trough remains east of the rockies but is not effective in eurasia due to seasonal changes in the atmospheric circulation the dryness

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

the role of linear interference in northern annular mode variability associated with eurasian snow cover extent one of the outstanding questions regarding the observed relationship between october eurasian snow cover anomalies and the boreal winter northern annular mode nam is what causes the multiple week lag between positive eurasian snow cover anomalies in october and the associated peak in rossby wave activity flux from the troposphere tothe stratospherein december thisstudy exploresthe following hypothesis about thislag in order toachieve amplificationofthe waveactivity the vertically propagatingrossbywave train associated with the snow cover anomaly must reinforce the climatological stationary wave which corresponds to constructive linear interference between the anomalous wave and the climatological wave it is shown that the lag in peak waveactivityflux arisesbecause the rossby wave train associatedwiththe snowcover isin quadrature orout of phasewiththeclimatologicalstationarywavefromoctobertomid november beginninginmid novemberthe associated wave anomaly migrates into a position that is in phase with the climatological wave leading to constructive interference and anomalously positive upward wave activity fluxes until mid january climate models from the coupled model intercomparison project 3 cmip3 do not capture this behavior this linear interferenceeffectisnot onlyassociatedwith stratospheric variabilityrelatedtoeurasiansnowcoveranomalies but is a general feature of northern hemisphere troposphere stratosphere interactions and in particular dominated the negative nam events of the fall winter of 2009 10

reply to comment on u pb and hf isotope analysis of detrital zircons from mesozoic strata of the gravina belt southeast alaska by yokelson et al 2015

bokan mountain peralkaline granitic complex alexander terrane southeastern alaska evidence for early jurassic rifting prior to accretion with north america the circular bokan mountain complex bmc on southern prince of wales island southernmost alaska is a jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the alexander terrane the bmc hosts significant rare metal rare earth elements y u th zr and nb mineralization related to the last stage of bmc emplacement u pb zircon and 40ar 39ar amphibole and whole rock geochronology indicates the following sequence of intrusive activity i a paleozoic basement composed mainly of 469 4 ma granitic rocks ii intrusion of the bmc at 177 1 ma followed by rapid cooling through ca 550 c at 176 1 ma that was synchronous with mineralization associated with vertical wnw trending pegmatites felsic dikes and aegirine fluorite veins and late stage sinistral shear deformation and iii intrusion of crosscutting lamprophyre dikes at 150 ma and again at ca 105 ma the peralkaline nature of the bmc and the wnw trend of associated dikes

40ar 39ar dating of detrital grains constrains the provenance and stratigraphy of the gravina belt southeastern alaska detrital amphibole and biotite grains were extracted from five volcaniclastic sandstones of the seymour canal formation late jurassic mid cretaceous gravina belt southeastern alaska forty of these grains were dated by fusion with the 40 ar 39 ar laser microprobe ar release characteristics suggest that the smaller biotite grains may have lost 40 ar during regional tectonism and thus their mid cretaceous to late jurassic whole grain fusion dates are not reliable six additional large biotite grains were dated using laser step heating techniques results from the step heated biotite grains as integrated over the high temperature steps yield middle jurassic through early cretaceous dates ranging from 159 0 pm 14 2 to 128 7 pm 4 2 ma ages for amphibole grains are generally less precise although two thirds yielded 2 sigma errors of 10 m y the most precise amphibole grain dates cluster between 90 and 110 ma mid cretaceous indicating that the amphibole grains were deriv

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

late quaternary ice sheet history of northern eurasia the maximum limits of the eurasian ice sheets during four glaciations have been reconstructed 1 the late saalian 140 ka 2 the early weichselian 100 80 ka 3 the middle weichselian 60 50 ka and 4 the late weichselian 25 15 ka the reconstructed ice limits are based on satellite data and aerial photographs combined with geological field investigations in russia and siberia and with marine seismic and sediment core data the barents kara ice sheet got progressively smaller during each glaciation whereas the dimensions of the scandinavian ice sheet increased during the last ice age the barents kara ice sheet attained its maximum size as early as 90 80 000 years ago when the ice front reached far onto the continent a regrowth of the ice sheets occurred during the early middle weichselian culminating about 60 50 000 years ago during the late weichselian the barents kara ice sheet did not reach the mainland east of the kanin peninsula with the exception of the nw fringe of taimyr a numerical ice sheet model forced by global sea level and solar changes was run through the full weichselian glacial cycle the modeling results are roughly compatible with the geological record of ice growth but the model underpredicts the glaciations in the eurasian arctic during the early and middle weichselian one reason for this is that the climate in the eurasian arctic was not as dry then as during the late weichselian glacial maximum

contrastive analysis of karst collapses and the distribution rules in northern and southern china in northern and southern china there are two different territorial systems of karst collapse because of the differences in basic geological conditions groundwater dynamic conditions and climatic factors the karst collapses are different in northern china from those in southern china in types distribution characteristics development density scope and formation mechanism based on analysis of statistical data in bulk and analysis of basic rules of the karst collapses in china this paper systematically analyzed the basic geological conditions groundwater dynamic conditions and climatic conditions both in north and south when studied and concluded the discrepancy in features scales density regional distribution rules formation mechanism etc the contrast and rules concluded will be of important significance and have great implication value to the exploration and prevention of karst collapse planning and layout of relevant project construction

sinkhole swarms along the dead sea coast reflection of disturbance of lake and adjacent groundwater systems more than a thousand sinkholes have developed along the western coast of the dead sea since the early 1980s more than 75 of them since 1997 all occurring within a narrow strip 60 km long and 1 km wide this highly dynamic sinkhole development has accelerated in recent years to a rate of 150 200 sinkholes per year the sinkholes cluster mostly over specific sites up to 1000 m long and 200 m wide which spread parallel to the general direction of the fault system associated with the dead sea transform research employing borehole and geophysical tools reveals that the sinkhole formation results from the dissolution of an 10 000 yr old salt layer buried at a depth of 20 70 m below the surface the salt dissolution by groundwater is evidenced by direct observations in test boreholes these observations include large cavities within the salt layer and groundwater within the confined subaquifer beneath the salt layer that is undersaturated with respect to halite moreover the groundwater brine within the salt layer exhibits geochemical evidence for actual salt dissolution na cl 0 5 0 6 compared to na cl 0 25 in the dead sea brine the groundwater heads below the salt layer have the potential for upward cross layer flow and the water is actually invading the salt layer apparently along cracks and active faults the abrupt appearance of the sinkholes and their accelerated expansion thereafter reflects a change in the groundwater regime around the shrinking lake and the extreme solubility of halite in water the eastward retreat of the shoreline and the declining sea level cause an eastward migration of the fresh saline water interface as a result the salt layer which originally was saturated with dead sea water over its entire spread is gradually being invaded by fresh groundwater at its western boundary which mixes and displaces the original dead sea brine accordingly the location of the western boundary of the salt layer which dates back to the shrinkage of the former lake lisan and its transition to the current dead sea constrains the sinkhole distribution to a narrow strip along the dead sea coast the entire phenomenon can be described as a hydrological chain reaction it starts by intensive extraction of fresh water upstream of the dead sea continues with the eastward retreat of the lake shoreline which in turn modifies the groundwater regime finally triggering the formation of sinkholes

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

the basic ingredients of the north atlantic storm track part i land sea contrast and orography understanding and predicting changes in storm tracks over longer time scales is a challenging problem particularly in the north atlantic this is due in part to the complex range of forcings land sea contrast orography sea surface temperatures etc that combine to produce the structure of the storm track the impact of land sea contrast and midlatitude orography on the north atlantic storm track is investigated through a hierarchy of gcm simulations using idealized and semirealistic boundary conditions in a highresolution version of the hadley centre atmosphere model hadam3 this framework captures the largescale essence of features such as the north and south american continents eurasia and the rocky mountains enabling the results to be applied more directly to realistic modeling situations than was possible with previous idealized studies the physical processes by which the forcing mechanisms impact the large scale flow and the midlatitude storm tracks are discussed the characteristics of the north american continent are found to be very important in generating the structure of the north atlantic storm track in particular the southwest northeast tilt in the upper tropospheric jet produced by southward deflection of the westerly flow incident on the rocky mountains leads to enhanced storm development along an axis close to that of the continent s eastern coastline the approximately triangular shape of north america also enables a cold pool of air to develop in the northeast intensifying the surface temperature contrast across the eastern coastline consistent with further enhancements of baroclinicity and storm growth along the same axis

ice sheets and sea level of the last glacial maximum this paper outlines the general issues regarding ice sheets and sea level of the last glacial maximum lgm which formed the basis of an epilog project workshop papers in this special issue of quaternary science reviews provide a comprehensive assessment of these issues from the perspective of geological reconstructions of ice sheet extent records of sea level change ice sheet modelling geophysical models of glacial isostatic adjustment and geochemical proxies of ice volume this new assessment has substantially narrowed the uncertainties in the total changes in ice sheets and sea level and their proxies suggesting a net decrease in eustatic sea level at the lgm ranging from 120 to 135 m r 2001 elsevier science ltd all rights reserved

detrital record of the gravina arc southeastern alaska petrology and provenance of seymour canal formation sandstones sandstones of the upper jurassic to mid cretaceous seymour canal formation northern gravina belt southeastern alaska are volcaniclastic wackes they exhibit a variably developed solution cleavage and zeolite to low greenschist grade metamorphic assemblages point counts of 37 lithofeldspathic sandstones spanning the unit9s stratigraphic range yield framework modes typical of detritus shed from undissected arc sources q 4 f 40 l 56 qm 3 f 40 lt 57 qm 8 p 91 k 1 qp 3 lvm 91 lsm 6 with only minor detritus characteristic of older terranes or continental margin sources sandstones of the dominant thinly bedded turbidite lithology were derived almost exclusively from the contemporaneous gravina arc in contrast sandstones of channelized conglomeratic facies are richer in quartz sedimentary metasedimentary and plutonic detritus derived from basement terrane s the compositional heterogeneity and grain size of conglomeratic facies are consistent with derivation from local source areas and we interpret them to reflect minor unroofing of the alexander terrane and the superposed gravina arc mostly during early cretaceous time a distinctive although volumetrically minor clinopyroxene rich sandstone petrofacies signals a change in modal expression of source area volcanism during the cretaceous and it probably records initial eruption of pyroxene rich basalts of the mid cretaceous douglas island volcanics supracrustal records preserved in the nutzotin mountains sequence dezadeash flysch and gravina sequence are also dominated by shallow level arc detritus although coeval supracrustal units on the insular terrane of british columbia reflect deeper incision of arc basement possibly due to along strike variation in arc geometries or tectonics of the jura cretaceous insular terrane

nonlinear orographically forced planetary waves abstract traditionally stationary wave models have been linearized about a zonal mean flow and the response calculated to various fixed orographic and thermal forcings in this paper it is shown that the inclusion of nonlinear interactions can significantly modify the solution for orographic forcing this arises primarily from the improved boundary condition that allows the flow to be deflected around the mountain as well as over it in this context a useful conceptual model is obtained by linearization based on the smallness of the latitudinal extent of a mountain the nonlinear model is considerably less sensitive to the zonal mean surface flow and in some instances the perturbation amplitude decreases with increasing surface flow the nonlinear hemispheric solutions for full orography and wintertime basic state are shown for both the northern and southern hemispheres they suggest that the direct effect of orographic forcing alone accounts for less than one half of the observed time mean asymmetries

relation between eurasian snow cover snow depth and the indian summer monsoon an observational study abstract satellite derived snow cover data for 22 yr and snow depth data for 9 yr over eurasia have been analyzed to reexamine the possible relation of snow with the indian summer monsoon in contrast to the previous studies that use snow cover averaged over all of eurasia as a single number the frequency of occurrence of snow at each grid point over eurasia is correlated with the indian summer monsoon rainfall thus specific geographical regions over eurasia that are responsible for the well known inverse relationship between eurasian snow cover and indian monsoon rainfall are delineated it is found somewhat surprisingly that western eurasia is the only geographical region for which a significant inverse correlation exists between winter snow cover and subsequent summer monsoon rainfall however composites for high and low snow cover over eurasia show spatially homogeneous large scale patterns of snow cover and surface temperature anomalies winters of high and low snow cover for eurasia are found t

variability of the observed midlatitude storm tracks in relation to low frequency changes in the circulation pattern abstract the principal modes of month to month variability of the wintertime storm tracks over the north pacific and north atlantic are identified by empirical orthogonal function analysis of the root mean square statistics of bandpass 2 5 6 day filtered geopotential height data for 19 yr one of the two leading modes depicts fluctuations in the level of synoptic scale activity without any noticeable spatial displacement of the storm track axes whereas the other mode is associated with meridional shifts of the storm tracks from their time averaged positions higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions it is demonstrated that the leading storm track modes are linked to some of the best known monthly averaged teleconnection patterns the dipolar western pacific and western atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans whereas the m

stochastic modelling of coalbed methane resources a case study in southeast qinshui basin china this paper presents a stochastic analysis of coalbed methane cbm resources for a coal seam in southeast qinshui basin china log and laboratory data are used to predict the coal thickness coal ash content and coal gas content using reservoir modelling the distributions of the karst collapse column kcc coal seam thickness coal quality and coal gas content are generated the convergent interpolation and sequential gaussian simulation methods are used to model the surface and structure of the coal seam the structural models are determined by the surface structure and coal seam thickness based on the structural models the coal and kcc are converted to two facies and their distributions are obtained using object modelling the coal density distributions are simulated based on each facies model using the sequential gaussian simulation finally different realisations are used to study cbm resources the results show that the heterogeneities in coal seam thickness and coal quality lead to significant uncertainty in estimating cbm resources a model with lower heterogeneity gives a greater cbm resource the distributions of kcc coal seam thickness coal quality and gas content are the main sources for uncertainty in cbm resource estimation the density variogram and top structure contribute less to the uncertainty in cbm resources

understanding the seasonality of orographically forced stationary waves interaction between mechanical and thermal forcing abstract idealized simulations of the atmosphere s stationary response to the rockies tibetan plateau and the greenland ice sheet are made using a nonlinear quasigeostrophic model and are compared to observations observational data indicate low level heating cooling occurs above the rockies and tibet in the summer winter low level cooling is found above greenland in both seasons the atmosphere responds to both diabatic heating termed thermal forcing and low level flow being obstructed by the mountain s presence termed mechanical forcing the response to thermal and mechanical forcing together can be very different from the response to either forcing individually the presence of modest low level heating or cooling 1 5 k day 1 causes significant changes to the mechanical forcing and thereby to the stationary wave response for example while the nonlinear response to mechanical forcing and low level heating is characterized by a cyclone over the orography the response to mechanical forc

last glacial maximum temperatures over the north atlantic europe and western siberia a comparison between pmip models margo sea surface temperatures and pollen based reconstructions abstract evaluating the ability of models to simulate climates different from the modern one is important for climate prediction here we present a first comparison between results from simulations of the last glacial maximum climate and continental and surface ocean reconstructions for the north atlantic europe and western siberia the simulations include prescribed sea surface temperature sst and slab ocean atmospheric general circulation model runs performed within the pmip1 project and atmosphere ocean fully coupled runs performed after pmip1 and within the pmip2 project the surface ocean reconstructions are from the margo project continental reconstructions are based on pollen data over the north atlantic most models simulate the strengthening of the sst meridional gradient suggested by the reconstructions but most do not reproduce the lgm control sst anomaly at the right location nor with the right amplitude over western siberia the model results are much improved when a new ice sheet reconstruction ice 5g is used to force the models the main discrepancy remains for western europe winter temperatures for which lgm control anomalies are significantly underestimated by all models all models indicate that this region during the lgm experienced significantly higher interannual variability in coldest month temperatures compared to the control runs this increased variability could have conspired to bias the apparently extremely cold pollen based temperature reconstructions

spatial variability and trends in observed snow depth over north america 1 this study uses a gridded dataset of daily u s and canadian surface observations from 1960 2000 to study regional spatial and temporal variability and trends in snow depth across north america analysis shows minimal change in north american snow depth through january with regions of decreasing snow depths beginning in late january these regional decreases grow in intensity and extent through march and into april implying an earlier onset of spring melt the region showing the greatest decreases in snow depth occurs in central canada along a line from the yukon territory in northwestern canada to the great lakes region the regional decreases in spring snow depth across central canada are likely a result of more rapid melt of shallower winter snowpacks evident through shallower snow cover 2 10 cm during may and october and a decrease in extent of deeper snowpacks 40cm through march and april

geologically and geomorphologically constrained numerical model of laurentide ice sheet inception and build up abstract the locations shapes and masses of evolving ice sheets during interglacial to glacial transitions remain elusive but need to be clarified for identification of the climatic feedbacks that amplified low amplitude insolation changes to become the major global climatic shift of the last glacial maximum the best way to explore probable spatial patterns of ice sheet inception and build up is through ice sheet modelling and validating model output against geomorphological and geological data we have modelled the 120 55 kyr bp evolution of the laurentide ice sheet focusing on the locations outline flow patterns and basal thermal regimes of its precursor ice centres for this we employed a time dependent mass balance driven finite element model forced with grip ice core oxygen isotope values calibrated to temperature the model achieves a good fit to relict ice flow systems in keewatin the interior plains and the hudson bay lowland thus our model indicates that the first coherent ice centre in north america was situated over baffin island and the melville and boothia peninsulas and expanded onto mainland canada at 90 75 kyr bp a previously unknown 200 km long moraine zone near the thlewiaza river keewatin may demarcate the marginal position of this early central arctic ice sheet cordilleran and keewatin ice merged and the hudson bay lowland was inundated from the east from quebec by 65 kyr bp

role of the tibetan plateau thermal forcing in the summer climate patterns over subtropical asia the mechanism of the tibetan plateau tp thermal forcing in influencing the summer climate patterns over subtropical asia is investigated by means of ncep ncar reanalysis diagnosis results show that since the tp is a huge elevated heating source with the strongest heating in the surface layers in summer the thermal adaptation results in a shallow cyclonic circulation near the surface and a deep anticyclonic circulation above it according to the steady barotropic vorticity equation for large scales airflow must converge in the lower layers and diverge in the higher layers over the eastern side of the tp however the western side of the tp is characterized by a reversed structure i e divergence in lower layers but convergence in higher layers hence pumping and sucking processes bring in upward and downward movement over the east and west sides of the tp respectively such a circulation is embedded in the large scale circulation that is forced by the eurasian continental heating because the tp together with iran plateau are located at the central and eastern parts of the continent and because the orography induced circulation is in phase with the continental scale circulation the role of the tp thermal forcing is to intensify the east asian monsoon to its east and the dry and hot desert climate in mid asia to its west the summertime thermal forcing of the rockies and andes can generate similar circulations along the two subtopics as the tp does since they are located near the western coasts but the lower troposphere poleward flow that is induced by orographic thermal forcing does not coincide with the poleward flows over the eastern coastal region that is induced by continental heating and the monsoon rainfall in north and south america is not as strong as in east asia however the equatorward flow and the associated subsidence induced by the two mountain ranges along the western coasts of both north and south america are in phase with those induced by continental heating these contribute to the formation of the stable low stratus clouds and strong long wave radiative cooling over the eastern subtropical pacific regions just off the western coast of the continent

circulation response to eurasian versus north american anomalous snow scenarios in the northern hemisphere with an agcm coupled to a slab ocean model abstractthe difference between snow covered and snow free conditions is the most climatically significant natural seasonal change the land surface can experience most gcm studies investigating snow atmosphere interactions have focused on impacts of eurasian snow anomalies caused by the magnitude of snow mass while north american snow has been shown to have a weaker relationship with downstream climate experiment design of recent snow atmosphere interactions studies has been limited to atmosphere only models with sea surface temperature sst and sea ice extent represented as boundary conditions the authors explore the circulation response to anomalous snow scenarios for both north america and eurasia using a slab ocean model surface response include significant sst cooling directly downstream of each individual forcing region in addition to upstream centers of remote cooling under maximum snow conditions atmospheric response to anomalous snow conditions is consistent through multiple levels in th

northern winter stationary waves theory and modeling a review is provided of stationary wave theory the theory for the deviations from zonal symmetry of the climate to help focus the discussion the authors concentrate exclusively on northern winter several theoretical issues including the external rossby wave dispersion relation and vertical structure critical latitude absorption the nonlinear response to orography and the interaction of forced wave trains with preexisting zonal asymmetries are chosen for discussion while simultaneously presenting a decomposition of the wintertime stationary wave field using a nonlinear steady state model

the impact of topographically forced stationary waves on local ice sheet climate a linear two level atmospheric model is employed to study the influence of ice sheet topography on atmospheric stationary waves in particular the stationary wave induced temperature anomaly is co

impact of the north american ice sheet orography on the last glacial maximum eddies and snowfall the present work evaluates the influence of the north american ice sheet orography on the last glacial maximum lgm 21 000 years ago atmospheric circulation and snowfall in the northern mid latitudes focusing on the north atlantic sector three atmospheric general circulation model experiments are analysed a control and an lgm climate simulation and an lgm run with flat ice sheets over northern north america this ice sheet orography affects lee cyclogenesis over north america and forces differences in stationary waves and therefore in the baroclinicity of the mean flow as a result the atlantic storm track is reinforced in the flat ice sheet experiment compared to the lgm one this in turn has a profound impact on snowfall over northern europe implying a coupling between the two ice sheets

submarine fan deposystems and tectonics of a late cretaceous forearc basin along an accretionary convergent plate boundary maccoll ridge formation wrangell mountains alaska analysis of upper cretaceous sedimentary and volcanic strata in the wrangell mountains of south central alaska provides an opportunity to study the tectonics depositional systems and provenance of a forearc basin that developed along an accretionary convergent plate boundary new data from the 1150 m thick maccoll ridge formation indicate that deposition occurred during the campanian on a coarse grained submarine fan that was derived from an uplifted allochthonous terrane exposed in the hanging wall of a fault system that separated the forearc basin from the subduction complex new age controls include palynoflora indicative of a late middle to late campanian age and compatible radiometric age determinations of volcanic vitric crystal tuffs near the top of the formation which have 40ar 39ar isochron ages of 79 4 0 7 and 77 9 2 1 ma sedimentological and paleontological data show that sedimentation occurred on the inner portions of a sand and gravel rich submarine fan system evidence for this inte

modeling and observations of martian stationary waves abstract a linear coordinate model is used in conjunction with a mars gcm northern hemisphere winter zonal mean circulation to study the orographically forced stationary disturbances on mars the disturbances are compared to their counterparts on earth and to the bright streak data from the mariner 9 and viking images diabatic heating on mars is predominantly radiative and inversion near the surface causes mountains to cool and depressions to heat the atmosphere due to the short radiative timescales and large meridional surface pressure gradient the atmosphere responds to radiative forcing by changing its temperature and pressure gradient the strong meridional flow near the equator and short radiative timescales combine to make responses to mechanical and thermal forcing of the same order in the tropics and in parts of the northern midlatitudes in the subtropics small zonal and meridional flows allow the response to radiative forcing to dominate all these factors contribute to making the martian

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

on the three dimensional propagation of stationary waves abstract a locally applicable nonzonally averaged conservation relation is derived for quasi geostrophic stationary waves on a zonal flow a generalization of the eliassen palm relation the flux which appears in this relation constitutes it is argued a useful diagnostic of the three dimensional propagation of stationary wave activity this is illustrated by application to a simple theoretical model of a forced rossby wave train and to a northern hemisphere winter climatology results of the latter procedure suggest that the major forcing of the stationary wave field derives from the orographic effects of the tibetan plateau and from nonorographic effects diabatic heating and or interaction with transient eddies in the western north atlantic and north pacific oceans and siberia no evidence is found in the data for wave trains of tropical origin forcing by the orographic effects of the rocky mountains seems to be of secondary importance

the last interglacial ocean the final effort of the climap project was a study of the last interglaciation a time of minimum ice volume some 122 000 yr ago coincident with the substage 5e oxygen isotopic minimum based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean last interglacial sea surface temperatures sst were compared with those today there are small sst departures in the mid latitude north atlantic warmer and the gulf of mexico cooler the eastern boundary currents of the south atlantic and pacific oceans are marked by large sst anomalies in individual cores but their interpretations are precluded by no analog problems and by discordancies among estimates from different biotic groups in general the last interglacial ocean was not significantly different from the modern ocean the relative sequencing of ice decay versus oceanic warming on the stage 6 5 oxygen isotopic transition and of ice growth versus oceanic cooling on the stage 5e 5d transition was also studied in most of the southern hemisphere the oceanic response marked by the biotic census counts preceded led the global ice volume response marked by the oxygen isotope signal by several thousand years the reverse pattern is evident in the north atlantic ocean and the gulf of mexico where the oceanic response lagged that of global ice volume by several thousand years as a result the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years these regional lead lag relationships agree with those observed on other transitions and in long term phase relationships they cannot be explained simply as artifacts of bioturbational translations of the original signals

modeling the water masses of the atlantic ocean at the last glacial maximum 1 we produced gridded monthly sea surface boundary conditions for the atlantic ocean at the last glacial maximum lgm based on the sea surface temperature reconstruction of the glamap project we used an ocean general circulation model ogcm subject to these sea surface boundary conditions and a corresponding wind stress field from an atmospheric general circulation model to study the differences in the distribution of the main water masses between the lgm and the present our global ogcm is characterized by high vertical resolution low vertical diffusion and isopycnal mixing and hence allows for a realistic representation of the hydrology and circulation of the modern atlantic ocean according to a series of lgm experiments with an increasing sea surface salinity anomaly in the weddell sea the ventilated thermocline was colder than today by 2 3 c in the north atlantic ocean and in the experiment with the largest anomaly 1 0 beyond the global anomaly by 4 5 c in the south atlantic ocean its depth was reduced by 50 m on average most notably in the tropics in the north atlantic ocean the outcrop locations of the thermocline isopycnal surfaces migrated southward by 5 10 and the ventilation increased in the south atlantic ocean the mixed layer and thermocline water masses were dominated by cold water originating from drake passage and the import of warm water from the indian ocean was reduced to about 4 sv or 40 of its modern value antarctic intermediate water was colder by 3 4 c and could be traced as far as 10 n the meridional overturning rates of north atlantic deep water nadw and antarctic bottom water aabw in the atlantic ocean were similar to those of the present day experiment 9 10 sv and 4 sv respectively however nadw cooled by 2 5 c and aabw by 1 c aabw was near the freezing point of seawater at the surface and the saltiest water mass in the atlantic ocean even saltier than nadw we show that the differences between the lgm and the present day experiments can be traced back to the changes in the subpolar and interhemispheric sea surface density gradients

linear simulation of the stationary eddies in a gcm part ii the mountain model abstract the validity of linear stationary wave theory in accounting for the zonal asymmetries of the winter averaged tropospheric circulation obtained in a general circulation model gcm is ascertained the steady linear primitive equation model used towards this end has the same vertical and zonal resolution as the spectral gcm but is finite differenced in the meridional direction it is linearized about a zonally symmetric basic state and forced by topography and 3 dimensional diabatic heating and transient flux convergence fields all of which are taken from the gcm as in part i in which we studied a gcm with a flat lower boundary we obtained the best correspondence between the gcm and the linear solutions when strong rayleigh friction is included in the linear model not only near the surface but in the interior of the tropical troposphere as well there is sufficient quantitative correspondence between the gcm and the linear solution to justify decomposing the linear simulation into parts for

relative impacts of siberian and north american snow anomalies on the winter arctic oscillation 1 numerical model mean climatic response to realistic land surface snow forcings is evaluated for two different forcing regions siberia and north america the atmospheric teleconnection pathway and negative winter ao mode response produced by the siberia forcing described by the authors in previous studies is not produced by the comparable extent north america forcing it is shown that the combination of a large snow forced local diabatic heating anomaly over a region of substantial stationary wave activity is required to produce strong upward wave activity flux anomalies which initiate the teleconnection pathway these features are unique to siberia making it a critical region for reproducing the snow winter ao statistical relationship evident in the observational record

effects of continental scale snow albedo anomalies on the wintertime arctic oscillation the ncar cam3 gcm with prescribed satellite based snow albedo sa is used to investigate the remote effects of snow cover on northern hemisphere nh winter climate a pair of 100 ensemble member experiments are integrated through the autumn winter season with prescribed high and low sa over eurasia ea similar to other non cam gcm studies using prescribed snow mass anomalous ea snow albedo produces a wave activity pulse that propagates into the stratosphere culminating in a negative phase arctic oscillation ao like surface response this occurs for idealized but representative sa anomalies as well as for more realistic sa anomalies similar experiments over north america na and the entire nh are also performed unlike prior studies anomalous na snow yields a significant ao signal here the local na surface cooling elicits a transient eddy response which propagates downstream to eurasia resulting in significant but short lived cooling and upward propagating wave activity over siberia a negative ao like response develops primarily confined to the stratosphere upper troposphere which eventually gives way to a tropospheric ao like response of the opposite phase due to equatorward wave refraction and wave divergence reanalysis data support this na snow positive ao response snow forcing experiments for the whole nh however yield a weakened ao signal this is due to a muted wavenumber 1 and 2 response caused by destructive interference between the background stationary wave and the corresponding rossby wave response which results in negligible wave activity reaching the stratosphere

mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group the age relations geochemistry and sedimentology of the rocky ridge formation of the skeena group are used to test competing tectonic reconstructions for the mid cretaceous canadian cordillera as well as the timing and location of the accretion of the insular superterrane pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the bowser basin in the early albian to early cenomanian single crystal fusion and step heating 40ar 39ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded middle cenomanian ages of 94 3 0 4 95 6 1 6 and 95 0 1 6 ma vesicular basalt flows interbedded with crystal rich tuff breccias contain evidence for hot emplacement as pyroclastic flows individual eruptive centers are identified by their proximal facies paleoflow indicators within the lava flows paleoflow indicators within interbedded volcaniclastic fluvial deposits geochemical differences and geographic isolation of volcanic deposits major and trace element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics the basalts of the northern rocky ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative nb ta anomalies whereas the basalts of the southern tahtsa lake volcanic center show depletion to slight enrichment of light rare earth elements slight enrichment of large ion lithophile elements with minimal negative nb ta anomalies the geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension the presence of deeper marine facies to the west and the lack of a western sediment source in the skeena group indicate that the technically active insular superterrane was not west of the study area during mid cretaceous time thus we reconsider the omineca belt as the main axis of a mid cretaceous continental arc placing the intermontane superterrane in the intraarc to forearc position with the rocky ridge volcanics erupted along the forearc side of the omineca arc coeval regional strike slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for rocky ridge intraarc extension an electronic supplement of tables a1 a2 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 95tc03496 mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group kari n bassett and karen l kleinspehn diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

overview of karst geo environments and karst water resources in north and south china the karst geological system in north china is different from that in the south due to differences in basic geological conditions and environment the karst evolutional pattern and karst water resources among other things are also different in the two regions this paper based on on site fieldwork and analysis of an extensive database of karst in china presents a systematic and contrastive analysis of karst geological environment and karst water resources in north versus south china highlights the differences between the two regions in basic karst geological conditions groundwater dynamics and karst water resources and concludes with the economic environmental and engineering implications of these differences these findings will be helpful for the strategic planning and decision making processes associated with exploitation of karst geological resources and with prevention and control of karst geological hazards

forced planetary waves in the northern hemisphere winter wave coupled orographic and thermal forcings abstract a more complete and new formulation of the orographic forcing and new thermal forcings are included in a steady state model of the northern hemisphere planetary waves when both forcings are included the simulation produces excellent results which are compared in detail with observations in particular the siberian high the tropospheric east asian trough and subtropical tropospheric east asian jet stream maxima are well reproduced even though the forcing is primarily extratropical in origin the modes uses a lower boundary condition in which the orographic forcing is determined by the effects of the total flow not just the zonal mean basic state consequently the net orographic forcing changes when thermal forcing is added and the tow solution is not equal to the linear sum of the solutions with orographic and thermal forcings separately the thermally induced orographic forcing is found to be very significant and in the troposphere there is strong interaction between the two forcings with

mechanisms setting the strength of orographic rossby waves across a wide range of climates in a moist idealized gcm abstractorographic stationary rossby waves are an important influence on the large scale circulation of the atmosphere especially in northern hemisphere winter changes in stationary waves with gl

the role of orography in the dynamics of storm tracks abstract this paper investigates the role of the northern hemispheric orography in the maintenance of the winter storm tracks with a dry balance three layer hemispheric model first the zonal winter mean flow in 1982 83 together with the corresponding orographically induced model planetary wave field is shown to have comparable linear instability properties as the total observed winter mean flow then the nonlinear version of this model is shown to be capable of simulating the pacific and atlantic storm tracks with considerable realism the position of the pacific storm track is slightly off and the intensity of the two model storm tracks is about 70 of the observed while the localized baroclinic regions tend to be neutralized by the eddies in the model they are sufficiently reestablished by the interaction between the slowly varying zonal flow and the tibetan plateau and the rockies to maintain the storm tracks moist dynamics is therefore not believed to be essential for the existence of storm tr

environment and climate in southwestern switzerland during the last termination 15 10 ka bp abstract various records from sites ranging in altitude between 372 and 2290 m along a nw se transect through sw switzerland were compiled and correlated within well defined chrono and biostratigraphic units deglaciation of lowland areas is estimated at ca 15 000 14 000 bp minimum age while the lower part of the rhone valley and the jura mountains may have become ice free later a significant climatic warming is clearly indicated by several palaeoecological records at ca 12 600 bp the compiled data give no clear evidence of a cooler phase between 12 200 12 000 bp oxygen isotope data point to a slight and progressive decrease in the 18 o values between ca 12 500 11 000 bp which is followed by a marked drop at 11 000 bp a very distinct change in lithology rock glacier development and in mollusc and vegetation records characterises many low and high altitude sites ca 300 years later at 10 700 bp between 11 000 and 10 000 bp oxygen isotope records seem to respond more rapidly to the climatic shifts than the biostratigraphical records possible explanations for these time lags are discussed our compilation shows that further multidisciplinary research in key sites would provide more precise palaeoclimatological information both qualitatively and quantitatively higher time resolution and quantitative estimates of climatic parameters are needed to evaluate in detail the mechanisms and consequences of these rapid climatic changes in sw switzerland

sensitivity of the lln climate model to the astronomical and co2 forcings over the last 200 ky the louvain la neuve climate model there referred to as the lln 2 d model has been used extensively to simulate the northern hemisphere ice volume under both the insolation and co2 forcings the period analysed here covers the last 200 ky first sensitivity analyses to constant co2 concentration were performed the model was accordingly forced by insolation changes only the co2 concentration being kept constant to respectively 210 250 and 290 ppmv results show that the simulated ice volume variations are comparable to the geological reconstructions only when the co2 concentration is low 210 ppmv and that the sensitivity of the simulated northern hemisphere ice volume to co2 is not constant through time second three co2 reconstructions were used to force the lln 2 d model in addition to insolation results show 1 a better agreement with the specmap oxygen isotope time series in particular as far as the amplitude of the signal is concerned and 2 that the simulated northern hemisphere ice volume is not very sensitive to the slight differences between these three reconstructions

orographically forced planetary waves in the northern hemisphere winter steady state model with wave coupled lower boundary formulation abstract a planetary wave model has been developed in which the orographic forcing at the lower boundary arising from the kinematically induced vertical motion is due to the total flow impinging on the mountains rather than just the zonal mean basic state component of the flow over the mountains used in previous models consequently the effects of the vertical motions produced by the eddies at the lower boundary are included and are found to be as large if not larger than the zonal mean component the model remains linear mathematically but all the planetary waves become coupled through the lower boundary condition lbc and the model wave equations have to be solved for simultaneously a contrast is drawn between the wave coupled solutions and the solutions using the traditional lower boundary formulation in which the planetary waves are decoupled the model is symmetric about the equator and uses the linear balance set of equations on the sphere with full spherical geometry and spherical harmonic f

modeled climate state and dynamic responses to anomalous north american snow cover abstract the radiative and thermal properties of widespread snow cover anomalies have the potential to modulate local and remote climate over monthly to seasonal time scales in this study physical and dynamical links between anomalous north american snow conditions and northern hemisphere climate are examined a pair of 40 member ensemble agcm experiments is run with prescribed high and low snow forcings over north america during the course of an entire year ey the difference between the two ensemble averages reflects the climatic response to sustained ey snow forcing local surface responses over the snow forcing occur in all seasons and a significant remote surface temperature response occurs over eurasia during spring a hemispheric scale transient eddy response to ey forcing also occurs which propagates downstream from the forcing region to eurasia and then reaches a maximum in extent and amplitude in spring the evolution of this transient eddy response is indicative of considerable downstr

transient eddy forcing of the time mean flow as identified by geopotential tendencies abstract the forcing of the time mean flow by transient eddies is examined within the framework of a quasi geostrophic equation relating the geopotential tendency to the convergence of transient eddy transports of heat and vorticity the forcing functions of this equation are computed using observed circulation statistics for the wintertime northern hemisphere and solutions are sought for the three dimensional structure of geopotential and temperature tendencies associated with eddies of different time scales in general the geopotential tendencies associated with vorticity fluxes are of the same sign within a given atmospheric column whereas the polarity of the geopotential tendencies associated with heat fluxes in the lower troposphere is opposite to that in the upper troposphere the geostrophic wind tendencies associated with synoptic scale eddies with periods between 2 5 and 6 days are strongest in the vicinity of the oceanic storm tracks the enhanced poleward heat transports by active disturbanc

the structure and maintenance of stationary waves in the winter northern hemisphere abstract previous studies of extratropical stationary waves in the winter northern hemisphere nh often focused on effects of orography and land ocean thermal contrast on the formation structure and maintenance of these waves in contrast research attention to tropical stationary waves was attracted by the summer monsoon circulations and the enso related climate variability consequently the structure and basic dynamics of tropical stationary waves and the relationship of these waves with those in mid high latitudes have long been neglected thus the following several distinct features of observed winter nh stationary waves have not been explained 1 an abrupt change in the longitudinal phase across 30 n 2 a transition from the vertical phase reversal of tropical stationary waves to the vertically westward tilt of extratropical stationary waves and 3 a longitudinally quarter phase relationship between stationary waves and east west circulations and a reversal of this relationship across 30 n

the effect of mesoscale mountain over the east indochina peninsula on downstream summer rainfall over east asia abstractthe mesoscale mountain over the east indochina peninsula named annam cordillera plays a key role in shaping the south china sea scs summer climate in both the atmosphere and the ocean however its effect is not limited to the scs ensemble simulations using a high resolution regional atmospheric model with or without the mountain reveals that the annam cordillera has a significant impact on regional climate as far as 3000 km over south and east china and western northwest pacific wnp by blocking lifting the warm and moist air from the bay of bengal the annam cordillera forces upward motion and precipitation on the windward side and subsidence on the leeward side and a low level southwesterly jet to the southeast tip of the indochina peninsula over the scs the latter gives rise to coastal upwelling and cold sea surface temperature sst filaments in the western scs reducing surface sensible and latent heat fluxes and thus suppressing convection over the scs heating associated with the

sensitivity of climate to late cenozoic uplift in southern asia and the american west numerical experiments experiments with general circulation models with no mountains half mountains and full modern mountains show the sensitivity of atmospheric circulation patterns to progressive amounts of uplift the experiments were motivated by geologic evidence for large kilometer scale uplift in the late cenozoic in southern asia and the american west particularly within the last 10 m y in january the amplitude of the mid latitude upper tropospheric planetary waves increased with uplift and the low level winds were progressively blocked or diverted around the topographic features in july the progressive uplift caused monsoonlike circulations to develop in the vicinity of the colorado and tibetan plateaus atmospheric heating rates midtroposphere vertical motion and upper tropospheric planetary wave amplitudes varied approximately linearly with progressive uplift these results suggest that geologically recent kilometer scale uplift may have had climatic consequences comparable in magnitude and pattern to those of earlier stages of uplift comparison of the simulated climatic effects of uplift with the geologic evidence for late cenozoic climatic change especially for the last 10 m y is given in accompanying papers

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

interactions between stationary waves and ice sheets linear versus nonlinear atmospheric response this study examines the mutual interaction between topographically forced atmospheric stationary waves and continental scale ice sheets using a thermomechanical ice sheet model coupled to a linear

vegetation dynamics amplifies precessional forcing 1 the astronomical theory of climate variations predicts that the climatic precession which changes the seasonal distance between earth and sun does not affect the annual mean irradiation at any given latitude however previous modeling studies suggest that during interglacials the interaction between atmosphere vegetation and ocean can transform the seasonal forcing by precession into an annual mean global signal here we show that this result can be generalized a distinct precessional signal emerges in a climate system model over many precessional cycles while neither the atmosphere ocean nor the atmosphere vegetation model are able to produce a large amplitude of global temperature in the precessional band only the mutual amplification of biogeophysical feedback and sea ice albedo feedback allows a strong amplification of the precessional signal

linear stationary wave simulations of the time mean climatological flow abstract a spectral coordinate model linearized about a zonal mean flow is used to investigate the maintenance of the december february climatological stationary waves the model is generally run with 15 levels in the vertical and a t31 truncation the sum of the response to orography diabatic heating and transient eddy flux convergences specified from data gives a generally good northern hemisphere simulation at 200 mb with some deterioration in the lower troposphere the same is true for the southern hemisphere but only when the forcing terms from recent years are used the response is broken down into that associated with particular orographic features tropical and midlatitude heating and transient vorticity and heat flux convergence all of these are shown to be significant in particular orography accounts for approximately 30 of the total 200 mb planetary wave response in midlatitudes the diabatic heating maxima in the two storm tracks are important this heating is related to latent and se

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

topography climate and ice masses a review topography acts as a filter between regional climate and the consequent response of a glacier or ice sheet it influences the mass and energy inputs and modifies the ice dynamics the evolution of the ice sheet surface and over longer time scales of the bed topography modulates the climatic forcing until we have a better understanding of these topographic linkages the use of geophysical signals from former ice masses to determine palaeoclimates must be regarded as fundamentally flawed

glacial climate instability in the northeast pacific ocean recent climate records from greenland ice cores1 2 and north atlantic sediments3a 5 have challenged the long held notion that pleistocene climate fluctuates between two relatively stable states glacials and interglacials it has been appreciated for some time that the transitions from one state to another are not smooth6 but the new records indicate that the glacial and interglacial periods themselves appear to be punctuated by significant climate variability several short interstadial events punctuated the last glacial period for example but it has not been clear whether this climate instability is a global phenomenon or is peculiar to the north atlantic region here we present climate proxy records from sediment cores from the eastern margin of the north pacific ocean which indicate that climate in this region was also highly unstable during the last glaciation our observations suggest that glacial climate instability throughout the northern hemisphere might be linked to rapid changes in the size of the laurentide ice sheet and associated changes in atmospheric circulation

stationary waves of the ice age climate abstract a linearized steady state primitive equation model is used to simulate the climatological zonal asymmetries stationary eddies in the wind and temperature fields of the 18 000 ybp climate during winter we compare these results with the eddies simulated in the ice age experiments of broccoli and manabe who used climap boundary conditions and reduced atmospheric co2 in an atmospheric general circulation model gcm coupled with a static mixed layer ocean model the agreement between the models is good indicating that the linear model can be used to evaluate the relative influences of orography diabatic heating and transient eddy heat and momentum transports in generating stationary waves we find that orographic forcing dominates in the ice age climate the mechanical influence of the continental ice sheets on the atmosphere is responsible for most of the changes between the present day and ice age stationary eddies this concept of the ice age climate is complicated by the sensitivity of t

the role of linear interference in the annular mode response to extratropical surface forcing abstract the classical problem of predicting the atmospheric circulation response to extratropical surface forcing is revisited in the context of the observed connection between autumnal snow cover anomalies over siberia and wintertime anomalies of the northern annular mode nam previous work has shown that in general circulation model gcm simulations in which autumnal siberian snow forcing is prescribed a vertically propagating rossby wave train is generated that propagates into the stratosphere drives dynamical stratospheric warming and induces a negative nam response that couples to the troposphere important questions remain regarding the dynamics of the response to this surface cooling it is shown that previously unexplained aspects of the evolution of the response in a comprehensive gcm can be explained by examining the time evolution of the phasing and hence the linear interference between the rossby wave response and the background climatological stationary wave when the wave response a

subsidence and sinkhole hazard assessment in the southern dead sea area jordan the dead sea area is increasingly facing serious subsidence and sinkhole hazards on march 22 2000 the dyke of a two month old major salt evaporation pond located along the shore of the lisan peninsula jordan collapsed over almost two kilometers the pond was set up over unstable new lands that have been progressively emerging during the last three decades in one hour 56 millions m 3 of brine poured out into the northern natural part of the sea here we present data suggesting that the drop of the water level in conjunction with the particular tectonic setting of this area is at least one of the factors that led to the disaster we focused our study over the northern part of the lisan peninsula and ghor al haditha which are two places undergoing the most intense deformations along the jordanian dead sea coast we used the results of a static high precision gravimetric survey to detect subsurface cavities in ghor al haditha we analyzed a interferometric digital terrain model of the recent emerged platform of the lisan peninsula and interpreted radar differential interferograms contemporary with gravity measurements for the peninsula we discuss the possibilities to detect assess and monitor areas prone to collapse on the jordanian side of the southern dead sea coast

large scale redistributions of atmospheric mass abstract seven years of daily global surface pressure ps analyses derived from european centre for medium range forecasts ecmwf data are examined to describe more fully interhemispheric mass exchanges and intraseasonal variability extreme events in hemispheric mean ps are determined and composited grid point differences show that hemispheric anomalies are mainly determined by pressures in the north pacific western north atlantic northern asia and the southern hemisphere sh circumpolar trough seasonal differences in the composites indicate that the regional anomalies occur farther poleward in the winter hemisphere and the tropical anomalies tend to have the same sign as that of the summer hemispheric mean anomaly long lasting localized extreme ps anomalies are identified in 18 significant events of hemispheric mass imbalance and are found to be highly favored when the hemispheric mean departs significantly from normal the result implies that regionally persistent anomalies are related to