late pleistocene glacial evolutionary stages in the gredos mountains iberian central system during the last glacial period late pleistocene the summits of the gredos mountains iberian central system were covered by plateau glaciers and later evolved into valley and slope glaciers the evolutionary sequence of these paleoglaciers has been obtained using geomorphological indicators and taking the principal moraine as reference these moraines show the limit between the glaciation and deglaciation periods in the glaciation period g four chronologically sequenced stages have been identified the local glacial maximum or maximum ice extent gm limited retreat lr re advance ra and finally major stabilization the steady state stage of greatest glacial stability ms in the deglaciation period d six chronologically sequenced stages have been identified an absolute retreat which involved regression and stabilization d1 two major retreat periods d2 and d4 and two stabilization periods d3 and d5 finally the last stage represents the complete disappearance of the glaciers d6 this evolutionary sequence allows more precise studies of the ages assigned through numerical dating procedures and of the correlation of the paleoglaciers in the gredos mountains with those of other regions

the deglaciation of the sierra nevada southern spain abstract cosmogenic 36 cl surface exposure dating was carried out on 19 samples from glacially polished bedrock rock glaciers and moraines of four glaciated valleys with different orientations in the sierra nevada spain around pico del veleta 37 3 n 3 21 w 3398 m asl remains of moraines older than the global last glacial maximum exist but their poor preservation makes them an unreliable subject for surface exposure dating results show that the last local glacial maximum advance took place earlier than the global last glacial maximum but within the mis 2 although additional dating is needed to confirm this exposure ages from bedrock steps show a general glacier retreat in progress by 15 14 ka shortly afterwards an extensive system of rock glaciers formed at the base of the valley headwalls with the most recent rock glacier lasting until 7 ka ago from their location morphology and analysis of other proxies we interpret their origin as related to the deglaciation process and marked instability of cirque headwalls rather than to extreme periglacial conditions a recent similar occurrence was the transformation of the little ice age glacier on the north face of the pico del veleta into a rock glacier under temperate high mountain climatic conditions

structure of a frontal cyclone a diagnostic study of a mid latitude cyclone has been carried out using routinely available numerical weather prediction model products and imagery the cyclone was intense but not exceptionally so and the results are believed to have some generality the detailed description of the cyclone structure just before it began to deepen rapidly integrates concepts from a number of researchers into a common framework a dominant feature of the cyclone close to its centre was the cloud head a region of cloud with a sharp convex outer edge which formed poleward of the main polar front cloud band the cloud head was caused by two flows that entered it from the east ascending and fanning out within it one flow the cold conveyor belt brought low wet bulb potential temperature w air back into the cloud head from low levels ahead of the warm front the other flow was due to high w air that peeled off from the base of the main warm sector airflow i e part of the warm conveyor belt and travelled in the boundary layer back towards the cyclone centre first undercutting dry air that had earlier descended from the upper troposphere called a dry intrusion and then ascending at the upper boundary of the cloud head above the cold conveyor belt the transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts with a more diffuse frontal region in between frontal fracture the two sharp surface cold fronts were associated with narrow cold frontal rainbands line convection one of the line convection segments forming the southern edge of the cloud head the overrunning of the dry intrusion in the region of the frontal fracture led to a structure known as a split front in which an upper level humidity front began to run ahead of the position of the surface cold front a feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air the cold conveyor belt in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper level jet

frontogenesis and the development of secondary wave cyclones in fastex two major mechanisms of frontogenesis deformation and shear are important in frontal wave cyclone development horizontal deformation can suppress the nonlinear wave development using an analytic model bishop and thorpe have shown that large strain rates inhibit any wave slope amplification for real cases this ambient strain can be measured using the vorticity divergence attribution method developed by bishop this technique permits us to confirm the crucial role of such strain on the evolution of cases of wave development during the fronts and atlantic storm track experiment fastex horizontal shear in the presence of an along front thermal gradient is also an important mechanism of frontogenesis using an eady model joly and thorpe have shown that in cases of large along front thermal gradient frontal waves have growth rates smaller than the front itself and thus would not develop the domain independent attribution method developed by bishop is here extended to a geopotential field partition this leads via a nonlinear balance condition to the estimation of the ambient along front potential temperature gradient the role of such an along front potential temperature gradient is discussed as well as the relative contributions of the two frontogenesis mechanisms for the fastex cases

a review of glacial geomorphology and chronology in northern spain timing and regional variability during the last glacial cycle abstract in this paper we synthesize the research in glacial geomorphology and geochronology in northern spain with special attention to the evidence of local glacier maximum extent earlier than the global lgm of mis 2 18 21 ka bp more accurate models of glacier evolution have been defined based on limnogeological geochronological and geomorphological data in the pyrenees osl optically stimulated luminescence surface exposure and radiocarbon dating techniques have identified end moraines and fluvial terraces corresponding to mis 6 about 170 ka and even to mis 8 about 260 ka and also established the timing of the last local glacial maxima as prior to global lgm mis 4 ca 50 70 ka during the global lgm a smaller re advance occurred but glaciers reached different extents in the central and the eastern pyrenees in nw iberia radiocarbon and osl techniques point to local glacial maximum prior to ca 26 ka 38 ka and probably synchronous with 45 ka although some bias might have been introduced by the dating procedures this review demonstrates that in both regions the local maximum extent occurred prior to the global lgm the asynchronies between the glacial maxima chronologies in the different mountain ranges of northern spain suggest that local climate factors exert a strong control on mountain glacier dynamics

objective classification of extratropical cyclogenesis extratropical cyclones experience vastly different genesis conditions at the first point of their tracks a novel method is introduced to characterize this variability and classify genesis events by computing 30 diagnostic variables that describe the synoptic scale environment of 16029 genesis events in the northern hemisphere extratropics using era interim reanalyses from 2000 2011 these variables are referred to as precursors and include parameters characterizing upper level forcing low level baroclinicity thermodynamic stability surface fluxes and moist processes the genesis events spread over a large portion of the 30 dimensional precursor phase space and no obvious clusters occur which highlights the high variability of cyclogenesis processes and indicates that they form in a continuum rather than a few distinct categories a projection of the genesis events to the first two principle components pc of the precursor phase space allows reducing the dimensionality and introducing a meaningful segmentation of the genesis events in five classes the first two pcs are characterized by upper level forcing e g the amplitude of the upper level potential vorticity pv anomaly and low tropospheric diabatic processes e g precipitation and diabatically produced low level pv respectively the first of the five identified classes constitutes the center of the pc1 pc2 phase space and represents average conditions composites reveal that the four classes of events characterized by large positive or negative scores of pc1 and pc2 occur in distinct and strongly differing flow regimes characterized by the strength of the upper level forcing the structure of the upper level jet and the amplitude of low level moist processes and baroclinicity the four classes also have clearly differing geographical distributions many well known cyclogenesis events fall within classes characterized by strong low level moist processes with or without strong upper level forcing also discussed are the robustness of the method and the linkage to classical concepts of cyclone classifications

glacial landforms and their paleoclimatic significance in sierra de guadarrama central iberian peninsula abstract this paper investigates the glacial evolution on the east face of pico de penalara 40 51 n 3 57 w 2428 m the highest elevation of sierra de guadarrama in the center of the iberian peninsula the geomorphologic stability of glacial landforms and permanence of snow cover were examined and 18 samples were selected for dating with cosmogenic isotope 36cl from moraine formations in four well differentiated phases as well as from a pre glacial periglacial blockfield and a glacial threshold near the headwall the results show the synchrony between the last glacial maximum lgm advance and most results obtained recently by cosmogenic dating in southern europe within the mis marine isotope stage 2 stadial but earlier than the global lgm i e between 25 and 19 ka bp there was slow glacial retreat with minor re advances between 19 and 16 ka and rapid deglaciation from 16 ka this paper presents initial results still pending confirmation for the minimum age of the periglacial summit block formation around 80 ka for the existence of moraine formations older than the last advance and for the existence of glaciers during the younger dryas stadial to explain the synchrony between two moraine formations at different altitudes this paper formulates the hypothesis of the secondary moraine evidenced in currently existing glaciers similar to those which existed on penalara

paleoglacial records from kavron valley ne turkey field and cosmogenic exposure dating evidence abstract kavron valley lies in the kackar mountain range of northeastern anatolia and is a north south oriented typically u shaped glacial valley consisting of a main and three tributary valleys the quaternary units that outcrop in this valley have been mapped and 22 samples have been processed for surface exposure dating using cosmogenic 10 be according to the 10 be ages the advance of the kavron paleoglacier began at least 26 0 1 2 kyr ago with the last glacial maximum lgm advance continuing until 18 3 0 9 kyr after this time the kavron paleoglacier receded although the magnitude of this recession is still unknown subsequent to this retreat the glacier most probably separated into three smaller glaciers that were restricted to the tributary valleys ifrit derebasi and mezovit paleoglaciers the main valley was definitely ice free by 15 5 0 7 kyr ago with the mezovit paleoglacier completing its recession around 15 5 0 6 kyr a late glacial advance took place around 13 0 0 8 to 11 5 0 8 kyr and little ice age moraines appear to be absent our results from the kavron valley system seem to be consistent with the lgm paleoclimate record of anatolia which has been delineated by data gathered from lowlands and lakes the deposition of red clay layers in the northwestern black sea and the deposition of heinrich 1 layers in the north atlantic

mesoscale dynamics of a deepening secondary cyclone in fastex iop16 three dimensional structure retrieved from dropsonde data the mesoscale dynamics of a secondary cyclone sampled during the field phase of the fronts and atlantic storm track experiment fastex intensive observation period iop 16 are documented using airborne dropsonde data the method proposed to recover the mesoscale analytical three dimensional 3 d dynamic fields from the dropsonde measurements is evaluated using simulated fields airborne in situ measurements and airborne doppler radar measurements these simulations and comparisons indicate that the method is able to resolve the mesoscale structures within the studied secondary cyclone both in the regions of clear air and precipitation it appears however that the most accurate description of the 3 d dynamic fields in both these regions would be obtained by combining doppler and dropsonde measurements the studied secondary cyclone is a fast moving frontal wave that developed on the trailing cold front of a parent low situated over greenland using the dropsonde derived 3 d fields the mesoscale flows involved in the organization of the secondary cyclone are described the potential vorticity and equivalent potential vorticity fields deduced from the basic analytical fields indicate that the atmosphere is roughly neutral to moist slantwise instability this configuration is known to significantly promote frontogenesis the vertical configuration of the jets and circulations within this secondary cyclone are compared with the coupling uncoupling scheme of shapiro a slight coupling is found between the thermally indirect circulation and the ageostrophic circulation the obtained coupling is however not as developed as in the ideal case described by shapiro which may explain the small upward velocities in our case airborne doppler radar and dropsonde measurements are combined so as to infer the multi scale processes involved in the mesoscale and convective scale organization of the atlantic secondary cyclones and to evaluate the new emerging theoretical interpretations of secondary cyclogenesis

geometry and kinematics of dunes during steady and unsteady flows in the calamus river nebraska usa the geometry and kinematics of river dunes were studied in a reach of the calamus river nebraska during day long surveys dune height length steepness migration rate creation and destruction were measured concurrently with bedload transport rate flow depth flow velocity and bed shear stress within a survey individual dune heights lengths and migration rates were highly variable associated with their three dimensional geometry and changes in their shape through time notwithstanding this variability there were discernible changes in mean dune height length and migration rate in response to changing discharge over several days changes in mean dune height and length lagged only slightly behind changes in discharge therefore during periods of both steady and unsteady flow mean dune lengths were quite close to equilibrium values predicted by theoretical models mean dune steepnesses were also similar to predicted equilibrium values except during high falling flows when the steepness was above that predicted variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period however the calculated rates of change of height of individual dunes vary considerably from those measured rates of dune creation and destruction were unrelated to discharge variations contrary to previous results instead creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses but not at higher bed shear stresses when suspended sediment transport is significant

on the source of midlatitude low frequency variability part i a statistical approach to persistence abstract the forcing of low frequency variability by synoptic transient traveling perturbations is investigated within a quasi geostrophic channel forced by a localized baroclinic jet spontaneously generated baroclinic perturbations grow and decay along a storm track at the end of the track a maximum of low frequency variability is obtained in agreement with atmospheric observations the structure of low frequency variability is studied with a combination of statistical methods using a multivariate red noise model as a random reference we show that the anomalies are preferentially linked with local stationary structures or long wave vacillations according to their location and their sign a systematic study of persistence is conducted with a criterion based on rms of the streamfunction variations the interesting quantity is the probability of persistence which shows a very inhomogeneous distribution in phase space and several separated maxima the composites based on these maxima exhibit the charact

the spatial distribution and evolution characteristics of north atlantic cyclones a climatology of extratropical cyclones is produced using an objective method of identifying cyclones based on gradients of 1 km height wet bulb potential temperature cyclone track and genesis density statistics are analyzed and this method is found to compare well with other cyclone identification methods the north atlantic storm track is reproduced along with the major regions of genesis cyclones are grouped according to their genesis location and the corresponding lysis regions are identified most of the cyclones that cross western europe originate in the east atlantic where the baroclinicity and the sea surface temperature gradients are weak compared to the west atlantic east atlantic cyclones also have higher 1 km height relative vorticity and lower mean sea level pressure at their genesis point than west atlantic cyclones this is consistent with the hypothesis that they are secondary cyclones developing on the trailing fronts of preexisting parent cyclones the evolution characteristics of composite west and east atlantic cyclones have been compared the ratio of their upper to lower level forcing indicates that type b cyclones are predominant in both the west and east atlantic with strong upper and lower level features among the remaining cyclones there is a higher proportion of type c cyclones in the east atlantic whereas types a and c are equally frequent in the west atlantic

life cycle simulations of shallow frontal waves and the impact of deformation strain the life cycle of shallow frontal waves and the impact of deformation strain on their development is investigated using the idealized version of the met office non hydrostatic unified model which includes the same physics and dynamics as the operational forecast model frontal wave development occurs in two stages first a deformation strain is applied to a front and a positive potential vorticity pv strip forms generated by latent heat release in the frontal updraught second as the deformation strain is reduced the pv strip breaks up into individual anomalies the circulations associated with the pv anomalies cause shallow frontal waves to form the structure of the simulated frontal waves is consistent with the conceptual model of a frontal cyclone deeper frontal waves are simulated if the stability of the atmosphere is reduced deformation strain rates of different strengths are applied to the pv strip to determine whether a deformation strain threshold exists above which frontal wave development is suppressed an objective method of frontal wave activity is defined and frontal wave development was found to be suppressed by deformation strain rates 0 4 10 5 s 1 this value compares well with observed deformation strain rate thresholds and the analytical solution for the minimum deformation strain rate needed to suppress barotropic frontal wave development the deformation strain rate threshold is dependent on the strength of the pv strip with strong pv strips able to overcome stronger deformation strain rates leading to frontal wave development than weaker pv strips copyright 2006 royal meteorological society

diagnosis of frontal instabilities over the southern ocean the development of three fronts over the southern ocean is described using seawinds on quikscat scatterometer surface winds and an attribution technique to partition the wind field in three components nondivergent and irrotational components at the scale of the front and the remaining harmonic component or environmental flow induced by the synoptic scale flow the front and the environment in which the front is embedded are analyzed separately a frontal wave is shown to develop out of the first front when the large scale alongfront stretching decreases the environmental flow becomes frontolytic and a connection with the upper levels is established in the second case the stretching remains relatively strong and no frontal wave develops the third front exhibits a developing wave but is not in a favorable configuration with the upper levels the frontal wave does not deepen significantly

orbital and sub orbital scale climate impacts on vegetation of the western mediterranean basin over the last 48 000 yr article i nfo high resolution pollen analysis of alboran sea core md95 2043 provides a 48 ka continuous vegetation record that can be directly correlated with sea surface and deep water changes the reliability of this record is supported by comparison with that of padul sierra nevada spain marine isotope stage mis 3 was characterised by fluctuations in quercus forest cover in response to dansgaard oeschger climate variability mis 2 was characterised by the dominance of semi desert vegetation despite overall dry and cold conditions during mis 2 heinrich events hes 2 and 1 were distinguished from the last glacial maximum by more intensely arid conditions taxon specific vegetation responses to a tripartite climatic structure within the hes are observed in mis 1 the bolling allerod was marked by rapid afforestation while a re expansion of semi desert environments occurred during the younger dryas the maximum development of mixed quercus forest occurred between 11 7 and 5 4 cal ka bp with forest decline since 5 4 cal ka bp on orbital timescales a long term expansion of semi desert vegetation from mis 3 into mis 2 reflects global ice volume trends while holocene arboreal decline reflects summer insolation decrease the influence of precession on the amplitude of forest development and vegetation composition is also detected

the deglaciation history of the simplon region southern swiss alps constrained by 10be exposure dating of ice molded bedrock surfaces the deglaciation history of the swiss alps after the last glacial maximum involved the decay of several ice domes and the subsequent disintegration of valley glaciers at high altitude here we use bedrock exposure dating to reconstruct the temporal and spatial pattern of ice retreat at the simplon pass altitude 2000 m located 40 km southwest of the rhone ice dome eleven 10be exposure ages from glacially polished quartz veins and ice molded bedrock surfaces cluster tightly between 13 5 0 6 ka and 15 4 0 6 ka internal errors indicating that the simplon pass depression became ice free at 14 1 0 4 ka external error of mean age this age constraint is interpreted to record the melting of the high valley glaciers in the simplon pass region during the warm bolling allerod interstadial shortly after the oldest dryas stadial two bedrock samples collected a few hundred meters above the pass depression yield older 10be ages of 17 8 0 6 ka and 18 0 0 6 ka these ages likely reflect the initial downwasting of the rhone ice dome and the termination of the ice transfluence from the ice dome across the simplon pass toward the southern foreland there the retreat of the piedmont glacier in val d ossola was roughly synchronous with the decay of the rhone ice dome in the interior of the mountain belt as shown by 10be ages of 17 7 0 9 ka and 16 1 0 6 ka for a whaleback at 500 m elevation near montecrestese in northern italy in combination with well dated paleoclimate records derived from lake sediments our new age data suggest that during the deglaciation of the european alps the decay of ice domes was approximately synchronous with the retreat of piedmont glaciers in the foreland and was followed by the melting of high altitude valley glaciers after the transition from the oldest dryas to the bolling allerod when mean annual temperatures rose rapidly by 3 c

dynamical characterization of the fastex cyclogenesis cases summary a series of diagnostics based on the automatic tracking of cyclones is applied to the 25 life cycles captured during the fastex field experiment the tracking enables a setting of the various cases into a common frame of reference that moves with the system time filtering is used to further separate the events into a basic state and perturbations information is obtained on environmental properties such as strain or baroclinicity combining these diagnostics it is shown that instability mechanisms based on the existence of potential vorticity strips or the control of these instabilities by deformation may be involved in the initial genesis of a number of cases but do not seem to characterize specific types nor to determine a systematic behaviour on the other hand it is shown that rapid deepening phases leading eastern atlantic systems to their maximum amplitude happens suddenly and at the same time as crossing the jet stream even in the absence of a clear additional upper level feature for cold air systems development means meeting a diffluent zone this is unlike the american east coast systems that are associated with an exit entrance complex another remarkable feature is the generation of a frontal waveas a re sult of the parent vorticity maximum being apparently stretched and split this has occurred in 25 of cases a number of other properties of the fastex cases are shown such as several occurrences of large

deglacial abrupt climate change in the atlantic warm pool a gulf of mexico perspective 1 during the last deglaciation greenland ice core and north atlantic sediment records exhibit multiple abrupt climate events including the younger dryas cold episode 12 9 11 7 ka however evidence for the presence of the younger dryas in the gulf of mexico gom and the relationship between gom sea surface temperature sst and high latitude climate change is less clear we present new mg ca sst records from two varieties of the planktonic foraminifer globigerinoides ruber white and pink to assess northern gom sst history from approximately 18 4 10 8 ka thirty five accelerator mass spectrometry ams 14c dates from orca basin core md02 2550 provide excellent age control and document high sedimentation rates 40 cm kyr g ruber white and pink mg ca sst data exhibit increases 4 6 0 6 c and 2 2 0 5 c respectively from at least 17 8 16 6 ka with nearly decadal resolution that are early relative to the onset of the bolling allerod interstadial moreover g ruber white sst decreases at 16 0 14 7 ka 1 0 0 5 c and 12 8 11 6 ka 2 4 0 6 c correlate to the oldest and younger dryas in greenland and cariaco basin the g ruber pink sst record which reflects differences in seasonality and or depth habitat is often not in phase with g ruber white and closely resembles antarctic air temperature records overall it appears that orca basin sst records follow antarctic air temperature early in the deglacial sequence and exhibit enhanced seasonality during greenland stadials

a classification of fastex cyclones using a height attributable quasi geostrophic vertical motion diagnostic a systematic and objective procedure is developed for applying the simple cyclone classification scheme of petterssen and smebye this method uses a height attributable solution of the quasi geostrophic equation to identify and quantify the relative importance of upper and lower tropospheric forcing and also the time trend in the horizontal spacing of the forcing at these two levels by applying this classification method to a sample of cyclogenesis events during their maximum intensification stage from the fronts and atlantic storm track experiment field experiment the type a and type b scheme of petterssen and smebye is reproduced and extended to include a type c type c consists of upper level dominated cyclones that form at high latitudes and in their initial stages resemble comma cloud type polar lows detailed examples of each of the three types are presented some cyclones can undergo more than one period of development and it is found that each development period can be classified as a different cyclogenesis category a or b these cyclones are classified as hybrid type a b there is some evidence that cyclone forecast accuracy depends on cyclone type thereby suggesting the potential for this method to be used to assign confidence levels for forecasts of cyclogenesis produced by numerical weather prediction models type b cyclones appear more difficult to predict because their development depends initially on the interaction between significant features in the upper and lower troposphere copyright 2002 royal meteorological society t d hewson s contribution is crown copyright

glacial activity and catchment dynamics in northwest greece long term river behaviour and the slackwater sediment record for the last glacial to interglacial transition woodward j c hamlin r h b macklin m g hughes p d lewin j 2008 glacial activity and catchment dynamics in northwest greece long term river behaviour and the slackwater sediment record for the last glacial to interglacial transition geomorphology 101 1 2 44 67 sponsorship uk science and engineering research council woodward the school of geography at the university of leeds hamlin and the university of cambridge hughes the uk natural environment research council gr9 2916 and ip 754 0302 funded the bulk of the analyses reported here specialissue sp iss si

effects of cloud diabatic heating on the early development of the fastex iop17 cyclone a set of numerical simulations is used to study the first development phase of the fastex cyclone a particularly well sampled frontal cyclone system that developed on the trailing front of a primary large scale baroclinic wave during intensive observation period 17 iop17 of fastex the fronts and atlantic storm track experiment the role of diabatically induced vertical circulations is investigated with a combination of diagnostic tools including a generalized omega equation and keyser s psi vector technique applied to the model outputs the full physics simulation shows a good agreement with observed data particularly the splitting of the upper level flow into a pair of jet streaks analysis of the contributions to the vertical ageostrophic transverse circulations near the jet streak pair structure shows that a positive feedback exists between the storm s dynamical configuration and the diabatic processes in the second part of the paper this interpretation is confirmed by a sensitivity analysis excluding cloud processes results suggest that after initial baroclinic growth the storm evolves towards a purely diabatic deepening regime leading to a diabatic rossby wave mechanism as proposed by recent wave cisk conditional instability of the second kind theoretical studies

frontal wave cyclogenesis in the north atlantic a climatological characterisation the ambient conditions during cyclogenesis on fronts in the north atlantic na are characterised climatologically in the era interim dataset these secondary cyclones which grow from frontal waves are typically small in scale shallow and deepen rapidly and pose a challenge to forecasters and numerical weather prediction models frontal wave development has been studied intensively in the past based on observations e g fastex real case studies or analytical models in this study frontal wave cyclogenesis is studied from a climatological perspective and is detected by a combination of two automated identification methods for the period between 1979 and 2014 a climatology of frontal wave cyclogenesis is presented horizontal composites provide an estimate of the climatological mean of conditions during frontal wave cyclogenesis which may prove useful in the assessment of the realism of proposed idealized models of frontogenesis the findings are also compared with assumptions made in earlier idealized studies of frontal waves the composites reveal coherent structures of diabatically generated low level pv in the lower troposphere neutral moist static stability at cyclogenesis and an ambient lower troposphere close to saturation the vertical pv structure in the east na is more vertically aligned than in the west na and pv anomalies from the upper and lower levels reach deeper into the mid troposphere which indicates an enhanced phase lock between the two anomalies previous studies suggest the existence of an upper limit of along frontal stretching above which frontal wave development is strongly suppressed the presented multi decadal analysis confirms its existence in a range between 0 6 and 0 8 10 5 s 1 finally a climatology of along frontal stretching for the na is presented and compared to the fraction of cyclogenesis forming on frontal waves

the campo felice late pleistocene glaciation apennines central italy study of the glacial deposits and lacustrine sediments of campo felice apennines central italy has enabled the glacial phases of the last 40 ka to be dated more precisely and has demonstrated that the maximum glacial advance did not occur in correspondence with the last global glacial maximum and with the coldest and most arid phase suggested by the pollen but in a period dated between about 33 and 27 ka characterized by a less extreme climate furthermore a glacial expansion took place also in the period prior to 35 ka correlation with the alpine glacial variations has shown that the apennine last glacial maximum occurred before that of the southern slope of the alps copyright 2012 john wiley sons ltd

identification of arid phases during the last 50 cal ka bp from the fuentillejo maar lacustrine record campo de calatrava volcanic field spain geochemical element analysis molecular analysis of organic compounds physical palynological mineralogical and sedimentary facies analysis were performed to characterise the sedimentary record in fuentillejo maar lake in the central spanish volcanic field of campo de calatrava in order to reconstruct the palaeoenvironmental and palaeoclimatic processes which controlled vegetation patterns and deposition of different sedimentary facies the upper 20 m of core fuent 1 show variations in clastic input water chemistry vegetation and organic fraction sources in the lake throughout the late pleistocene and holocene the temporal framework provided by 14c accelerator mass spectrometry dating allows assigning the sequence to the last 50 cal ka bp arid phases identified in the fuent 1 sequence are correlated to heinrich events he and to stadials of the dansgaard oeschger d o cycles siliciclastic facies with high magnetic susceptibility values high juniperus pollen content a low paq index aquatic macrophysics proxy index a decrease in the relative percentage of the n c27 and an increase in the n c31 alkanes are indicative of arid and colder climatic events related to he 2 he 1 and the younger dryas yd similar short cold and arid phases during the holocene were identified at 9 2 8 6 7 5 7 and 5 5 5 cal ka bp in dolomite mud facies the pollen data show an increase in the herbs component mainly chenopodiaceae artemisia and ephedra steppe taxa a low paq index a decrease in the relative percentage of the n c27 alkane and an increase in the n c31 alkane are also observed this facies was probably the result of lower lake levels and more saline alkaline conditions which can be interpreted as linked to arid warm periods these warm and arid phases were more frequent during marine isotope stage mis 3 and the interstadials of mis 2 he 4 he 2 he 1 and the yd in core fuent 1 were immediately followed by increases of warm steppe pollen assemblages that document rapid warming similar to the d o cycles but do not imply increasing humidity in the area fuentillejo hydrology is controlled by changes in the atmospheric and oceanic systems that operated on the north atlantic region at millennial scale during the last 50 cal ka bp copyright 2009 john wiley sons ltd

secondary cyclogenesis along an occluded front leading to damaging wind gusts windstorm kyrill january 2007 abstractwindstorm kyrill affected large parts of europe in january 2007 and caused widespread havoc and loss of life in this study the formation of a secondary cyclone kyrill ii along the occluded front of the mature cyclone kyrill and the occurrence of severe wind gusts as kyrill ii passed over germany are investigated with the help of high resolution regional climate model simulations kyrill underwent an explosive cyclogenesis south of greenland as the storm crossed poleward of an intense upper level jet stream later in its life cycle secondary cyclogenesis occurred just west of the british isles the formation of kyrill ii along the occluded front was associated i with frontolytic strain and ii with strong diabatic heating in combination with a developing upper level shortwave trough sensitivity studies with reduced latent heat release feature a similar development but a weaker secondary cyclone revealing the importance of diabatic processes during the formation of kyrill ii kyrill ii moved

the role of the environmental flow in the development of secondary frontal cyclones the impact of the environmental flow on the development of secondary frontal cyclones is investigated several case studies are examined as examples of secondary frontal cyclone events observed in the north atlantic western europe sector a simple measure of growth is defined to chart their development the vorticity attribution technique of bishop is utilized to calculate the action of the large scale environmental flow on the fronts in particular the environmental along front stretching shown to be important in theoretical models of frontal instabilities is calculated the role of the environmental deformation appears to be crucial as part of a baroclinic life cycle stretching deformation acts to build up a front but suppress along front waves if the stretching rate diminishes barotropic instabilities may then break out diagnostics are examined to try to ascertain the growth mechanisms at work in each frontal cyclone case a range of values for the commonly prescribed deformation frontogenesis and shearing frontogenesis parameters are calculated

frontal wave development by interaction between a front and a cyclone application to fastex iop 17 the way new cyclones appear along a baroclinic zone is central to the problem of the prediction of mid latitude cyclones a new barotropic mechanism for frontal wave development is proposed in the present paper the ingredients are a frontal environment characterized by horizontal wind shear and a finite amplitude initial wind perturbation orthogonal to the main deformation axis associated with the front it is found that the initial wind perturbation can be provided by a low level vortex maximum located north westward with respect to the position of the new cyclone interactions between these synoptic features are documented using local energetics the theory is only valid at the very early stage of development after a few hours the main growing phase which is mostly baroclinic will take place a wide set of experiments is performed in order to find the preferred location of the vortex at the initial time in the frame of the barotropic vorticity equation the result is found to be consistent with the fronts and atlantic storm track experiment case studied in this paper frontogenetic forcing provided by a deformation field does not inhibit the perturbation amplification it is also found that the impact of the vortex is significantly larger than potential instabilities developing along the front where the unstable feature is defined as the most unstable singular vector copyright 2004 royal meteorological society

maximum extent of late pleistocene glaciers and last deglaciation of la cerdanya mountains southeastern pyrenees abstract this paper examines glacial evolution in the la pera and malniu cirques and aranser la llosa and duran valleys in the cerdanya massifs on the south facing slopes of the eastern pyrenees a geomorphologic analysis and dating of moraine boulders glacially polished bedrock and rock glacier blocks were carried out by means of cosmogenic 36cl surface exposure dating the maximum ice advance was contemporary with the last glacial maximum at 23 ka ago and it was of greater or only slightly lesser magnitude than for previous quaternary advances the termini of glaciers remained close to maximum positions with minor advances and retreats until 18 17 ka when the glacial tongues disappeared from the valleys depending on the previous topography these glaciers left behind a single polygenic moraine in the case of confined valleys or multiple moraines next to each other in the case of flat more open areas a final glacial advance is detected during the oldest dryas close to the cirque headwalls and the glaciers finally disappeared during the bolling interstadial the glaciers were then replaced by rock glaciers whose front immediately became inactive although their activity continued near their source area until the early holocene

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

timing of glaciation in the mediterranean mountains during the last cold stage recent research in the mountains of the mediterranean has attempted to establish the timing of the maximum extent of glaciers during the last cold stage several dating methods have been utilised and new dating frameworks have emerged in key areas in several places multiple dating techniques radiocarbon optically stimulated luminescence u series applied to glacial and associ ated sediments suggest that local glacier maxima preceded the global last glacial maximum lgm ca 18 14 c ka bp 21 cal ka bp by at least several thousand years studies in spain the pyrenees the maritime alps and turkey utilising cosmogenic isotope analyses 10 be and 36 cl have yielded glacial geochronologies with maxima that correlate closely with the marine isotope record of ice volume and the global lgm in some cases the use of different geochronological techniques has led to conflicting evidence for the timing of the local last glacier maxima in the same area from a palaeoclimate perspective glacier climate reconstructions for the local last glacier maxima in the mountains of central italy northern greece and western turkey indicate that climate was wet and cold in italy and greece the local glacier maxima preceded the driest part of the last cold stage as indicated by palaeoecological records by several thousand years for western turkey however recent cosmo genic exposure ages combined with glacier modelling suggest a local last glacier maximum charac terised by cold and wet conditions coinciding with the global lgm the available evidence indicates that a dichotomy is emerging not only in our understanding of the timing of glaciation across the mediterranean but also in the palaeoclimatic interpretations derived from glacial and palaeoecolo gical records copyright 2008 john wiley sons ltd

chronology of the last glacial cycle in the european alps chronological data for glacier advances in the european alps between the last interglacial eemian and the holocene are summarised 115 to 11 ka during this time glaciers were most extensive extending tens of kilometres out onto the forelands between 30 and 18 ka that is synchronous with the global ice volume maximum of marine isotope stage mis 2 evidence for ice expanding to just past the mountain front for an earlier major glacier advance comes from swiss sites where advances have been luminescence dated to mis 5d 100 ka and mis 4 70 ka up to now no such evidence has been found in the eastern alps by 18 ka more than 80 of the late wurmian ice volume had gone subsequently glaciers readvanced reaching into the upper reaches of the main valleys during the lateglacial gschnitz stadial which likely occurred around 17 ka with final moraine stabilisation no later than 15 4 ka the link of the egesen stadial with the younger dryas climate deterioration is supported by exposure ages from four sites as well as minimum limiting radiocarbon dates from bogs within former glacier tongue areas key questions on the spatial and temporal variability of ice extents throughout the last glacial cycle have yet to be answered copyright 2008 john wiley sons ltd

environmental evolution in sierra nevada south spain since the last glaciation based on multi proxy records abstract our knowledge of the quaternary landscape evolution in the high mountain areas of the iberian peninsula has substantially improved over the last decades the sierra nevada is one of the most studied mountain ranges in southern europe regarding its environmental evolution the purpose of the present paper is to integrate and summarize all the studies focused on the reconstruction of the palaeoenvironmental history in this massif since the last glaciation research has focused both on different sedimentary records and historical sources a wide range of geomorphological sedimentological and geochronological techniques have been used to characterize the glacial periglacial wetland and lacustrine records for palaeoenvironmental purposes for the last nine centuries tens of descriptions maps and sketches describe the landscape of the summit area of the sierra nevada providing evidences of the historical environmental events based on a multi proxy approach five periods have been identified last glaciation deglaciation holocene little ice age and recent evolution recent studies have detected the maximum expansion of glaciers in the sierra nevada around 30 32 ka bp predating the global temperature minimum no data about the environmental evolution is available between 20 30 ka bp around 19 20 ka bp glaciers advanced significantly the process of deglaciation was rapid and around 14 15 ka bp the massif was almost free of ice the late glacial promoted the formation of small glaciers in the highest northern cirques and widespread active periglacial processes i e rock glaciers during the holocene there has been an alternation of colder warmer periods and changing moisture conditions periglacial processes have been generally widespread in the summit area with an increasing or decreasing activity depending on climate conditions ephemeral reappearance of small glacial cirques occurred in the highest northern cirques during the coldest and wettest phases this is the case of the little ice age as revealed by historical documents and sedimentary records since the last decades of the xix century the temperature has increased 0 93 c leading to a decrease of the intensity of periglacial processes in the high lands of the sierra nevada

the impact of deformation strain on the formation of banded clouds in idealized modelling experiments experiments are performed using an idealized version of an operational forecast model to determine the impact on banded frontal clouds of the strength of deformational forcing low level baroclinicity and model representation of convection line convection is initiated along the front and slantwise bands extend from the top of the line convection elements into the cold air this banding is attributed primarily to m adjustment the cross frontal spreading of the cold pool generated by the line convection leads to further triggering of upright convection in the cold air that feeds into these slantwise bands secondary low level bands form later in the simulations these are attributed to the release of conditional symmetric instability enhanced deformation strain leads to earlier onset of convection and more coherent line convection a stronger cold pool is generated but its speed is reduced relative to that seen in experiments with weaker deformational strain because of inhibition by the strain field enhanced low level baroclinicity leads to the generation of more inertial instability by line convection for a given capping height of convection and consequently greater strength of the slantwise circulations formed by m adjustment these conclusions are based on experiments without a convective parametrization scheme experiments using the standard or a modified scheme for this model demonstrate known problems with the use of this scheme at the awkward 4 km grid length used in these simulations copyright 2008 royal meteorological society

secondary cyclogenesis the initiation phase of a frontal wave observed over the eastern atlantic the initiation phase of a secondary frontal wave in the wake of a primary cyclogenesis event over the atlantic is documented north of the trailing surface cold front and south west of the primary cyclone an intense upper level jet generates a direct and transverse ageostrophic circulation in its entrance region evidence is shown for two conflicting roles of this circulation first stabilization of the front through frontogenetic cross frontal low level convergence and second an increase of the potential for instability of the front through diabatic e g latent heat release potential vorticity pv redistribution in the upward branch of the circulation following saturation of the primary cyclogenesis and the associated along front north eastwards shift of the upper level jet the low level branch of the ageostrophic circulation behaves like a zipper closing on the front while in the upward branch an anomalous strip of pv builds along the cold front isentropic pv maps show that parcles of stratospheric origin embedded in the downward branch of the ageostrophic circulation a tropopause fold may have ended in the vicinity of the strip of anomalous low level pv perhaps contributing to an increase in the potential for instability of the surface front using the domain independent attribution method of bishop it is shown that the frontal wave forms on a part of the cold front where first the along front stretching rate due to the environmental flow decays below the critical threshold value 0 6 10 5 s 1 prescribed by theoretical studies and second the across front frontogenetic component due to the ageostrophic circulation decays this is consistent with semi geostrophic theory where the cross frontal convergence and the stretching deformation are linked through the sawyer eliassen equation results add to other recent observational findings that the potential for instability the evolving environmental flow and the induced ageostrophic convergence each have a crucial role in differentiating between frontal waves that will grow and those that will be suppressed

frontal wave stability during moist deformation frontogenesis part i linear wave dynamics abstract it has been shown that lower tropospheric potential vorticity zones formed during moist deformation frontogenesis will support growing waves if at some time the frontogenesis ceases in this paper the ways in which these waves are affected by the frontogenetic process are identified observations show that fronts in the eastern atlantic commonly feature saturated ascent regions characterized by zero moist potential vorticity furthermore in many cases the horizontal temperature gradient in the lowest one to two kilometers of the atmosphere is rather weak these features are incorporated in an analytical archetype the dynamical implications of saturated ascent in conditions of zero moist potential vorticity are represented in the model by assuming that adiabatic temperature changes are precisely balanced by diabatic tendencies the observed small temperature gradient at low levels is represented in the model by taking it to be zero in the lowest two kilometers consequently the forcing of the

phasing of deglacial warming and laurentide ice sheet meltwater in the gulf of mexico evidence is emerging that the tropical climate system played a major role in global climate change during the last deglaciation however existing studies show that deglacial warming was asyn chronous across the tropical band complicating the identification of causal mechanisms the orca basin in the northern gulf of mexico is ideally located to record subtropical atlantic sea surface temperature sst warming in relation to meltwater input from the laurentide ice sheet paired d 18 o and mg ca data on the planktonic foraminifer globigerinoides ruber from core en32 pc6 are used to separate deglacial changes in sst and d 18 o of sea water sst as calculated from mg ca data increased by 3 8c from ca 17 2 to 15 5 ka in association with heinrich event 1 and was not in phase with greenland air temperature subtracting tem perature effects from d 18 o values in g ruber reveals two excur sions representing laurentide meltwater input to the gulf of mex ico one of 1 5 from ca 16 1 to 15 6 ka and a second major spike of 2 5 from ca 15 2 to 13 0 ka that encompassed melt water pulse 1a and peaked ca 13 8 ka during the bolling allerod conversion to salinity through the use of a laurentide meltwater end member of 225 indicates that near surface salinity de creased by 2 4 during these spikes these results suggest that gulf of mexico sst warming preceded peak laurentide ice sheet decay and the bolling allerod interval by 2 k y and that heat was retained in the subtropical atlantic during heinrich event 1 consistent with modulation of deglacial climate by thermohaline circulation

asynchroneity of maximum glacier advances in the central spanish pyrenees the deglaciation history of the escarra and lana mayor glaciers upper gallego valley central spanish pyrenees had been reconstructed on the basis of detailed geomorphological studies of glacier deposits sedimentological and palynological analyses of glacial lake sediments and an accelerator mass spectrometry ams 14c chronology based on minimum ages from glacial lake deposits the maximum extent of the pyrenean glaciers during the last glaciation was before 30 000 yr bp and pre dated the maximum advances of the scandinavian ice sheet and some alpine glaciers a later advance occurred during the coldest period around 20 000 yr bp synchronous with the maximum global ice extent but in the pyrenees it was less extensive than the previous one later there were minor advances followed by a stage of debris covered glaciers and a phase of moraine formation near cirque backwalls the deglaciation chronology of the upper gallego valley provides more examples of the general asynchroneity between mountain and continental glaciers the asynchroneity of maximum advances may be explained by different regional responses to climatic forcing and by the southern latitude of the pyrenees copyright 2002 john wiley sons ltd

the role of secondary cyclones and cyclone families for the north atlantic storm track and clustering over western europe secondary cyclones are those that form in association with a pre existing primary cyclone typically along a trailing cold front in previously studied cases they have been shown to cause extreme damage across europe particularly when multiple cyclones track over the same location in rapid succession known as cyclone clustering to determine the dynamical relationship between primary and secondary cyclones over the north atlantic a frontal identification algorithm is partnered with a cyclone identification method to objectively identify secondary cyclones in 35 extended winter periods using reanalysis data cyclones are grouped into cyclone families consisting of a single primary cyclone and one or more secondary cyclones this paper aims to quantify the differences between secondary and primary cyclones over the north atlantic and how cyclone families contribute to episodes of cyclone clustering across western europe secondary cyclones are shown to occur most frequently in the central and eastern north atlantic whereas primary cyclones are commonly found over the western north atlantic cyclone families have their strongest presence over the north atlantic ocean and contribute more than 50 of cyclones over the main north atlantic storm track a final category solo cyclones which are not associated with cyclogenesis on any connected fronts are most commonly identified over continental regions as well as the mediterranean sea primary cyclones are associated with the development of an environment that is favourable for secondary cyclone growth enhanced rossby wave breaking following primary cyclone development leads to an increase in the upper level jet speed and a decrease in low level stability secondary cyclogenesis commonly occurs in this region of anomalously low stability close to the european continent during periods of cyclone clustering secondary cyclones are responsible for approximately 50 of the total number of cyclones the increase in jet speed and decrease in static stability initiated by the primary cyclones acts to concentrate the genesis region of secondary cyclones and direct the cyclones that form along a similar track while there is an increase in the secondary cyclogenesis rate near western europe during periods of european clustering the basin wide secondary cyclogenesis rate decreases during these periods thus the large scale environment redistributes secondary cyclones during periods of clustering rather than increasing the total number of secondary cyclones

a technique for diagnosing three dimensional ageostrophic circulations in baroclinic disturbances on limited area domains abstract the methodology developed by keyser et al for representing and diagnosing three dimensional vertical circulations in baroclinic disturbances using a two dimensional vector streamfunction referred to as the psi vector is restricted to f plane channel model geometry the vertical circulation described by the psi vector consists of the irrotational or divergent part of the ageostrophic wind and the vertical velocity a key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations allowing separation of a three dimensional vertical circulation into two two dimensional components and thus objective assessment of the extent to which a three dimensional vertical circulation is oriented in a preferred direction here the methodology for determining the psi vector is modified to be suitable for real data applications the modifications consists of reformulating the diagnostic equations to apply to conformal map pr

a technique for representing three dimensional vertical circulations in baroclinic disturbances abstract the problem of representing vertical circulations in frontal zones is reexamined with the objective of devising a methodology sufficiently general to apply in situations where these circulations are no longer confined to the cross front transverse vertical plane and therefore must be viewed as fully dimensional the proposed methodology which builds upon the earlier work of hoskins and draghici and of eliassen consists of adopting a vector streamfunction that describes the vertical velocity and the horizontal irrotational flow this generalized streamfunction referred to as the psi vector may be determined uniquely from the vertical velocity field over a limited region provided that suitable lateral boundary conditions on the velocity potential for the irrotational part of the horizontal velocity can be specified a key property of the psi vector is that its projections onto arbitrary orthogonal vertical planes yield two independent vertical circulations providing an objective means for se

freshwater input and abrupt deglacial climate change in the north atlantic greenland ice core records indicate that the last deglaciation 7 21 ka was punctuated by numerous abrupt climate reversals involving temperature changes of up to 5 10oc within decades however the cause behind many of these events is uncertain a likely candidate may have been the input of deglacial meltwater from the laurentide ice sheet lis to the high latitude north atlantic which disrupted ocean circulation and triggered cooling yet the direct evidence of meltwater input for many of these event has so far remained undetected in this study we use the geochemistry paired mg ca 18 o of planktonic foraminifera from a sediment core south of iceland to reconstruct the input of freshwater to the northern north atlantic during abrupt deglacial climate change our record can be placed on the same timescale as ice cores and therefore provides a direct comparison between the timing of freshwater input and climate variability meltwater events coincide with the onset of numerous cold intervals including the older dryas 14 0 ka two events during the allerod at 13 1 and 13 6 ka the younger dryas 12 9 ka and the 8 2 ka event supporting a causal link between these abrupt climate changes and meltwater input during the bolling allerod warm interval we find that periods of warming are associated with an increased meltwater flux to the northern north atlantic which in turn induces abrupt cooling a cessation in meltwater input and eventual climate recovery this implies that feedback between climate and meltwater input produced a highly variable climate a comparison to published data sets suggests that this feedback likely included fluctuations in the southern margin of the lis causing rerouting of lis meltwater between southern and eastern drainage outlets as proposed by clark et al science 2001 v293 283 287

frontal instability generated by tropospheric potential vorticity anomalies the development of secondary disturbances on a two dimensional front is described using a linear stability analysis a two stage process is envisaged in which first strong frontogenesis produces condensation at the front this leads to the generation of a lower tropospheric zone of high potential vorticity pv lying along the front as frontogenesis weakens the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the pv anomaly or equivalently the frontal rainband thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day 1 the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of pv anomalies the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks useful insight is gained from an analysis of the energetics of the modes this shows that for narrow frontal pv anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux recent observations suggest that frontal waves are indeed characterized by such a heat flux as most frontal zones involve significant condensation and therefore are likely to exhibit a lower tropospheric pv maximum it is stressed that studies of frontal waves need to include this aspect of the frontal structure

late pleistocene and holocene glaciation in the pyrenees a critical review and new evidence from 10be exposure ages south central pyrenees a compilation and a critical assessment of the 14c data set available show that the chronology of glacial events in the pyrenees is not well constrained after reviewing the literature on glacial reconstruction we suggest a simplified subdivision of the pyrenean last glacial cycle record into last pleniglacial deglaciation and neoglacial to improve the numerical glacial chronology we provide 10be surface exposure ages for 5 glacial erosion surfaces 9 moraines and 2 erratics in the upper noguera ribagorcana valley south central pyrenees published corrected 14c data and new 10be exposure ages indicate that the major phase of moraine building recorded in this valley during the last pleniglacial probably occurred after 25 ka bp this age calls in question the generally accepted hypothesis of a very early deglaciation of the pyrenees ca 70 40 ka bp and strongly suggests that the pyrenees could have been in pleniglacial conditions during the last glacial maximum lgm however we do not exclude the possibility that the maximum glacier extent during the last glacial cycle had taken place much earlier than the lgm as indicated by some published u th ams 14c and osl data we suggest that pleniglacial conditions could have taken place during a longer 30 20 ka period than generally assumed and that the last pleniglacial could include several glacier fluctuations recorded irregularly in different valleys with a last major glacier readvance taking place around the lgm in addition the deglaciation is represented by a series of moraines deposited between ca 13 7 0 9 and 10 1 0 6 ka this moraine series indicates a highly variable climatic pattern that is partly correlated with greenland stadial 1 the younger dryas and suggests that the deglaciation could have continued into the early holocene

frontogenesis in the presence of small stability to slantwise convection abstract it is often observed that despite the existence of near neutrality to slantwise convection rainbands and snowbands can persist for long periods with narrow intense updrafts producing large quantities of precipitation in many cases this is probably due to the presence of active frontogenesis which as shown here maintains such structures in order to investigate this process we use two dimensional semigeostrophic theory to solve the deformation forced frontogenesis problem for a circumstance in which the stability to moist slantwise convection is small but positive in this case numerical simulation is necessary to determine the evolution of the front several previous studies have described the role of diabatic forcing in modifying the cross frontal circulation by use of the sawyer eliassen equation in particular analytic solutions of that equation for the case of small moist symmetric stability show that a narrow updraft should occur ahead of the maximum geostrophic frontogenesis numeri

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

glaciation in the rila mountains bulgaria during the last glacial maximum abstract relict glacial landform complexes have been studied in the mountains of the central balkan peninsula since the end of the 19th century since their existence was proved for the first time by cvijic various authors have debated the number of glacial phases as well as their age at present the problem of age determination is being solved by the application of the new methods for numerical dating this paper is a detailed study of the maximum glaciation in the rila massif which is the highest mountain in bulgaria and in the whole balkan peninsula the application of cosmogenic nuclide dating 10be on moraine samples confirmed that the largest extent of former glaciers that left moraine deposits was around the time of the last glacial maximum most probably the maximum extent occurred in two phases the first around the beginning and the second around the end of the lgm 24 23 ka bp and 18 16 ka bp respectively separated by a retreat phase during the coldest but also driest phase of the glacial stage the gis processed calculations on the basis of field evidence of the former glacier extent and the preliminary calculations of the equilibrium line altitude ela through the accumulation ablation ratio aar method show that the ela during the lgm had quite small differences throughout the rila massif from about 2150 to 2200 m a s l in the nw to about 2250 2290 m in the se despite the considerable differences in terminal moraine depositional altitudes from 1150 to 2000 m a s l this is due to both the complicated pattern in the directions of moisture supply and to the influences of local topography

diminutive frontal waves a link between fronts and cyclones abstract a number of recent publications have dealt with cyclone identification and tracking following on this paper extends the typical cyclone life cycle back in time to embrace a new feature called a diminutive frontal wave one aim is to improve predictability by extending tracks this is particularly important for small cyclonic windstorms which can often be missed in postprocessed output from operational ensemble and climate runs the recognition of diminutive waves requires a new front relative low level vorticity partition the parts are labeled frontal vorticity and disturbance vorticity and are computed respectively from front parallel and cross front low level wind components a diminutive frontal wave then lies wherever there is a local along front maximum in the disturbance vorticity computations require local coordinates these are conveniently provided at all grid points by objective front diagnostics analysis of cyclone type transitions over the north atlantic in opera

effects of a low level precursor and frontal stability on cyclogenesis during fastex iop17 the fronts and atlantic storm track experiment fastex has provided comprehensive data to document the life cycle of secondary frontal cyclones over the north atlantic and to improve the understanding of cyclogenesis mechanisms this study analyses the processes leading to the triggering of a particularly well sampled frontal cyclone that developed on the trailing cold front of a mature primary cyclone during fastex intensive observation period 17 iop17 between 16 and 20 february 1997 the case features both a classical low level vorticity strip along the primary front where the frontal cyclone develops and more unexpectedly a pre existing continental surface low further west the importance of which is revealed in the pilot study of arbogast and joly in order to study possible cyclogenetic contributions along the primary front the domain independent vorticity divergence attribution technique of bishop is used to partition the flow into contributions from the continental low and from the large scale environment results indicate that the frontal cyclone is triggered where and when along front stretching decreases below the theoretical critical threshold of bishop and thorpe it is shown that the cyclone is developing under normalized strain less than 1 in agreement with other case studies finally the role of the continental low is investigated using a series of numerical forecasts made from different initial conditions manipulated with a potential vorticity inversion tool the wind attributable to the continental low accounts for roughly 30 of the along front stretching decrease the continental low also has a frontolytic effect on the western part of the primary front where its thermal advection pattern favours cyclogenesis in general terms the frontal cyclone triggering seems to support the bishop and thorpe theoretical framework of frontal instability in low strain areas however a more complex picture arises when considering the role of the continental surface low which also acts as a low level precursor

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume