simulations of low level convergence lines over north eastern australia we describe high resolution numerical model simulations of low level convergence lines over north eastern australia using the pennsylvania state university national center for atmospheric research mesoscale model mm5 the simulations are for selected events that were documented during the gulf lines experiment held in september october 2002 the calculations provide further insights into the dynamics of the convergence lines and the mechanisms involved in their formation in particular they show two clearly distinct convergence lines one that corresponds to the morning glory and one which corresponds to the north australian cloud line the former originates from the east coast sea breeze over cape york peninsula south of about 14 s while the latter originates from the east coast sea breeze north of this latitude they support also a recently proposed conceptual model for the generation of southerly morning glories and show for the first time the separation of a bore like disturbance following the collision of a nocturnal cold front to the south of the inland trough with a sea breeze front to the north of the trough moreover they show the progressive transition of the east coast sea breeze front and the inland cold front from gravity current like flows to bore like disturbances overnight to form north easterly and southerly morning glories respectively copyright 2006 royal meteorological society

identification of coherent links between interannual sedimentary structures and daily meteorological observations in arctic proglacial lacustrine varves potentials and limitations proglacial lacustrine sediments from high arctic lake r 76 17 9 n 90 59 3 w unofficial name are shown to be annually laminated varved and contain a variety of subannual structures the formation of the subannual structures and overall varve was controlled by a combination of meteorologic temperature and rainfall and geomorphic factors using a training set of the ten thickest varves in the 38 year sedimentary record a heuristic model was developed to link subannual structures with regional meteorological conditions within the training set significant correlations were shown between subannual structure thickness and the magnitude of the corresponding melt event defined as a period of continuously positive temperature however these correlations deteriorated as the varves progressively thinned and several varves exhibited no relationship between their subannual structures and respective meteorological conditions grain size analyses showed that the thin varves were significantly finer than the thick varves and are inferred to reflect changed sediment inflow patterns that altered deposition and reduced the fidelity of the model despite these complexities this study identified the potential to produce long term subannual reconstructions of weather conditions model results revealed the limitations of simple varve meteorology relationships as well as identified necessary environmental and sampling conditions required to produce a more robust model for future applications

eastward propagating undular bores over cape york peninsula abstract the existence of eastward propagating bores over cape york peninsula is shown from data obtained during the gulf lines experiment glex conducted during september and october 2002 and from numerical modeling experiments the disturbances were detected regularly at two stations observable on 24 days during the 40 day experiment the passage of a typical disturbance exhibits a sudden increase in pressure of around 1 hpa often accompanied by undulations and a change in the wind speed and direction from an easterly to a westerly flow disturbances were not observed during days of strong easterly flow a two dimensional nonhydrostatic mesoscale model is used to examine the formation of these disturbances it is shown that the west coast sea breeze is shallow and does not penetrate far inland because of the opposing low level easterly flow in contrast the east coast sea breeze is deeper and is less stable because it is modified by daytime convective mixing as it crosses the peninsula as the east

holocene delevelling of devon island arctic canada implications for ice sheet geometry and crustal response the raised beaches and deltas of devon island contain an abundance of dateable materials a large set of radiocarbon dates 228 154 of which are new are used to construct relative sea level curves and isobase maps for the island the best materials for this purpose are driftwood logs 61 dates and bowhead whale bones 74 dates from raised beaches and mollusc shells from marine limit deltas 20 dates or from altitudes close to marine limit 14 dates during the last glacial maximum the island is thought to have lain beneath the southeastern flank of the innuitian ice sheet the relative sea level history is congruent with that inferred ice configuration the island spans half the ice sheet width relative sea level curves are of simple exponential form except near the glacial limit where an early holocene emergence proceeded to a middle holocene lowstand below present sea level which was followed by submergence attending the passage of the crustal forebulge the response times of relative sea lev

last glacial maximum and deglaciation of devon island arctic canada support for an innuitian ice sheet abstract the extent and origin of quarternary glaciers in the queen elizabeth islands of arctic canada especially during the last glacial maximum lgm has been debated for well over a century no consensus has yet emerged and the spectrum of interpretations within the last decade is the broadest ever the glacial geology of devon island strongly supports the late wisconsinan innuitian ice sheet hypothesis during lgm the southeastern part of the queen elizabeth islands was covered by an ice sheet flowing from a centre or a divide centrally located in the archipelago another ice divide extended from the centre across devon island flow from the innuitian centre and convergent flow from devon island and probably also from cornwallis and bathurst islands sustained an ice stream in wellington channel ice was locally advancing to its lgm limit about 23 ka bp recession in the vicinity of the present coastline was underway by 10 ka bp and final ice remnants west of the present devon ice cap vanished about 8 ka bp the island bears the same two part landscape zonation as do other islands and peninsulas in the region these reflect a cold based core and warm based periphery at lgm

regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the canadian arctic stable pb isotope profiles in dated lake sediment cores were used to gauge the relative amounts and possible sources of anthropogenic pb deposited from the atmosphere in different regions of the canadian arctic a distinct north south difference was found in four high arctic lakes i e north of 66 n in this study recent pb isotopic shifts or concentration increases attributable to anthropogenic pb were negligible the maximum possible contribution from anthropogenic pb was 0 to 19 of acid leachable pb in the 1980s or 1990s in contrast two lakes in the hudson bay region displayed significantly lower pb isotope ratios and threefold to fivefold increases of pb concentrations in modern sediments corresponding to anthropogenic pb inputs of at least 72 to 91 of leachable pb eurasian urban and industrial pb is known to dominate the high arctic atmosphere a possible explanation for its negligible influence on northern lake sediments is that atmospheric pb deposition at northern latitudes is reduced compared with southern regions and is small compared to local geological inputs 210pb deposition declines with increasing latitude apparently because of declining precipitation rates stable pb deposition may be similarly affected meteorological considerations and variations in the post 1900 pb isotopic trajectories indicated that the predominant anthropogenic pb source region in nw hudson bay was eurasia while in se hudson bay it was canada and the united states with a minor eurasian component

travelling waves and bores in the lower atmosphere the morning glory and related phenomena abstract observations of solitary wave and travelling bore like disturbances in the lower atmosphere are reviewed together with relevant theoretical laboratory and numerical studies concerning the structure propagation and genesis of such disturbances primary emphasis is given to a discussion of the morning glory phenomenon in northern australia the example par excellence of these types of disturbance and the best documented observationally to date observations of related phenomena elsewhere are reviewed briefly while such occurrences appear to be less frequent in any single location than in the southern gulf of carpentaria region of northern australia reported occurrences are widespread and there is mounting evidence that such disturbances are more common than is generally supposed the evidence suggests that whenever a suitable wave guide exists for the propagation of disturbances the latter can be generated as the natural response of quite general forcing mechanisms examples being the collision of two sea breezes from opposite directions on a peninsula the interaction between a thunderstorm outflow and a sea breeze front or simply the transformation of a thunderstorm or squall line outflow as it moves into a low level stable layer because of the large vertical displacements they produce the larger amplitude disturbances typically with surface pressure jumps of 1 mb or more must be considered as potential triggers for deep convection if the atmosphere is sufficiently conditionally unstable and the large low altitude wind shears associated with them present a serious hazard to low flying aircraft

recent warming in a 500 year palaeotemperature record from varved sediments upper soper lake baffin island canada laminated sediments from upper soper lake on southern baffin island provide a new 500 year record of temperature change in the arctic radiometric dating using 210pb and pu shows that the light and dark coloured laminae couplets are annually deposited varves dark laminae thickness is strongly correlated to average june temperature from kimmirut r 0 82 reflecting the influence of temperature on snowmelt and fluxes of runoff and suspended sediment this relationship allowed the construction of a palaeotemperature record that documents large amplitude interannual to decadal variability superimposed on distinct century scale trends including 2 c average warming and maximum temperatures during the 1900s similar patterns of change are seen in individual and regionally averaged palaeotemperature records from around the circum arctic upper soper lake records temperatures rates of change and variance during the twentieth century that are all anomal ously high within the context of the last 500 years and outside the observed range of natural variability comparisons of upper soper lake and arctic average palaeotemperature to proxy records of hypothesized forcing mechanisms suggest that the recent warming trend is mostly due to anthropogenic emissions of atmos pheric greenhouse gases the magnitude of the warming and decade scale variability throughout the records however indicate that natural forcing mechanisms such as changing solar irradiance and volcanic activity as well as positive feedbacks within the arctic environment also play an important role

holocene climate change in arctic canada and greenland abstract this synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the holocene in arctic canada and greenland our synthesis includes 47 records from a recently published database of highly resolved holocene paleoclimate time series from the arctic sundqvist et al 2014 we analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records mostly from datasets that did not fit the selection criteria for the arctic holocene database combined we review evidence from a variety of proxy archives including glaciers ice cores and glacial geomorphology lake sediments peat sequences and coastal and deep marine sediments the temperature sensitive records indicate more consistent and earlier holocene warmth in the north and east and a more diffuse and later holocene thermal maximum in the south and west principal components analysis reveals two dominant holocene trends one with early holocene warmth followed by cooling in the middle holocene the other with a broader period of warmth in the middle holocene followed by cooling in the late holocene the temperature decrease from the warmest to the coolest portions of the holocene is 3 0 1 0 c on average n 11 sites the greenland ice sheet retracted to its minimum extent between 5 and 3 ka consistent with many sites from around greenland depicting a switch from warm to cool conditions around that time the spatial pattern of temperature change through the holocene was likely driven by the decrease in northern latitude summer insolation through the holocene the varied influence of waning ice sheets in the early holocene and the variable influx of atlantic water into the study region

a record of climate over the last millennium based on varved lake sediments from the canadian high arctic a varved sediment record that extends back over the last millennium was recovered from lower murray lake northern ellesmere island nunavut canada 81 20 n 69 30 w flatbed scan images and back scattered electron images were analysed to provide varve thickness and other quantitative sedimentary indices on an annual basis in many studies of lakes from the high arctic varve thickness is a good proxy for summer temperature and we interpret the lower murray lake varves in this way on that basis the lower murray lake varve thickness record suggests that summer temperatures in recent decades were among the warmest of the last millennium comparable with conditions that last occurred in the early twelfth and late thirteenth centuries but estimates based on the sediment accumulation rate do not show such a recent increase the coldest conditions of the little ice age were experienced from ad 1700 to the mid nineteenth century when extensive ice cover on the lake led to widespread anoxic conditions in the deepest parts of the lake basin an overall decline in median grain size over the last 1000 years indicates a reduction in the energy available to transport sediment to the lake many of these features of the record are also observed in other palaeoclimatic records from the north american arctic the very recent appearance of the diatom campylodiscus which was not observed through out the record of the last millennium suggests that a new threshold in the ontogenetic development of the lake has now been passed

signal and noise in four ice core records from the agassiz ice cap ellesmere island canada details of the last millennium for stable isotopes melt and solid conductivity four ice cores and two deep pit auger sequences from the top of the agassiz ice cap have been variously analysed for o18 ecm solid conductivity and ice melt layer stratigraphy the high resolution data are presented on time scales covering about the last 1000 years the 8 time series are compared and the noise examined in terms of snow drifting and wind scouring of winter snow from the higher exposed parts of flow lines the highly scoured 8 records reflect a mixture of summer biased conditions and the amount of winter scoured snow sites away from the ridges and domes are not scoured the signal part of the scoured and unscoured sites is obtained by stacking averaging normalized series the stacked unscoured series of 8 and accumulation are compared to similar stacked series from central greenland for the last century of annual averages the comparison is surprisingly good suggesting that at these resolutions low noise 8 series have some coherence over fairly large distances the percentage melt

hydrological and sediment yield response to summer rainfall in a small high arctic watershed in cold regions the response and related antecedent mechanisms that produce flood flows from rainfall events have received limited study in 2007 a small watershed at cape bounty melville island nunavut was studied in detail during the melt season two rainfall events on june 30 and july 22 totalling 9 2 and 10 8 mm respectively represented significant contributions to seasonal discharge and sediment transport in a year with a low winter snowpack the precipitation events elevated discharge and suspended sediment concentrations to twice the magnitude of the nival melt and generated the only measurable downstream lacustrine turbidity current of the season in two days rainfall runoff transported 35 of the seasonal suspended sediment load in contrast to 29 transported over the nival freshet the magnitude and intensity of the rain events were not unusual in this setting but the rainfall response was substantial in comparison with equivalent past events exceptional temperatures of july 2007 generated early deep permafrost thaw and ground ice melt the resultant increase in soil moisture amplified the subsequent rainfall runoff and sediment transport response these results demonstrate the importance of antecedent moisture conditions and the role of permafrost active layer development as an important factor in the rainfall runoff and sediment transport response to precipitation events copyright 2009 john wiley sons ltd

evolution and vertical structure of an undular bore observed on 20 june 2015 during pecan abstractthis study documents the evolution of an impressive largely undular bore triggered by an mcs generated density current on 20 june 2015 observed as part of the plains elevated convection at night pecan experiment the university of wyoming king air with profiling nadir and zenith viewing lidars sampled the south bound bore from the time the first bore wave emerged from the nocturnal convective cold pool and where updrafts over 10 m s 1 and turbulence in the wave s wake were encountered through the early dissipative stage in which the leading wave began to lose amplitude and speed through most of the bore s life cycle its second wave had a higher or equal amplitude relative to the leading wave striking roll clouds formed in wave crests and wave energy was detected to about 5 km agl the upstream environment indicates a negative scorer parameter region due to flow reversal at midlevels providing a wave trapping mechanism the observed bore strength of 2 4 2 9 and speed of 15 16 m s 1 agree

north australian cloud lines abstract a satellite classification and climatology of propagating mesoscale cloud fines in northern australia is presented these cloud fines range from long narrow lines of shallow convection to extensive deep convective squall lines with mesoscale stratiform rain areas these lines are the predominant weather feature during periods of easterly flow over the australian tropics

low level convergence lines over northeastern australia part i the north australian cloud line observations of dry season north australian cloud lines nacls that form in the gulf of carpentaria region of northern australia and the sea breeze circulations that initiate them are described the observations were made during the 2002 gulf lines experiment glex and include measurements made by an instrumented research aircraft the observations are compared with numerical simulations made from a two dimensional cloud scale model particular emphasis is placed on the interaction between the east coast and west coast sea breezes near the west coast of cape york peninsula the sea breezes are highly asymmetric due to the low level easterly synoptic flow over the peninsula the west coast sea breeze is well defined with a sharp leading edge since the opposing flow limits its inland penetration keeping it close to its source of cold air in contrast the east coast sea breeze is poorly defined since it is aided by the easterly flow and becomes highly modified by daytime convective mixing as it crosses over the peninsula both the observations and the numerical model show that in the early morning hours the mature nacl forms at the leading edge of a gravity current the numerical model simulations show that this gravity current arises as a westward moving land breeze from cape york peninsula convergence at the leading edge of this land breeze is accompanied by ascent which when strong enough produces cloud observations show that the decay of the nacl is associated with a decline in the low level convergence and a weakening of the ascent

hydrochemical and sedimentary responses of paired high arctic watersheds to unusual climate and permafrost disturbance cape bounty melville island canada high arctic river responses to changing hydroclimatic and landscape processes are poorly understood in non glacierized basins snowmelt and rainfall generate river discharge which provides first order control over fluxes further factors include the seasonality of precipitation seasonal active layer development and permafrost disturbance these controls were evaluated in terms of sedimentary and biogeochemical fluxes from paired catchments at cape bounty melville island nunavut during 2006 2009 results indicate that the source of runoff can be more important than the amount of runoff for sediment solutes and organic yields although the snowmelt period is typically the most important time for these yields heavy late summer precipitation events can create disproportionately large yields rainfall increases yields because it hydrologically connects areas otherwise isolated inorganic solute yields from late summer rainfall are higher because the thick active layer maximizes hydrologic interactions with mineral soils and generates high solute concentrations results also indicate that while the catchments are broadly similar subtle topographic differences result in important inter catchment differences in runoff and suspended and dissolved loads the east watershed which had less extensive permafrost disturbance consistently had higher concentrations of dissolved solids these higher dissolved fluxes cannot therefore be explained by thermokarst features but rather by deeper active layer development due to a greater proportion of south facing slopes although warm temperatures in 2007 led to extensive active layer disturbance in the west watershed because the disturbances were largely hydrologically disconnected the total disturbed area was small and inter annual variability in discharge was high there was no detectable response in dissolved loads to disturbances sediment availability increased after 2007 but yields have largely returned to pre disturbance levels results indicate that seasonality and frequency magnitude characteristics of projected increases in precipitation must be considered along with active layer changes to predict the fluvial sedimentary and biogeochemical response to regional climate change copyright 2011 john wiley sons ltd

holocene climate change influences on trace metal and organic matter geochemistry in the sediments of an arctic lake over 7 000 years abstract this study employed a 6 48 m long dated sediment core to characterize the amount of variation in background trace metal concentrations and develop an understanding of the role of climatic influences on sediment inorganic and organic geochemistry in a high arctic lake over 7000 years of the holocene element geochemistry reflected varying detrital contributions from two geological sources within the catchment carbonates dolomites and limestones which provided the dominant major element by weight i e ca and carbonaceous mudrocks primarily shales which may have contributed most of the other trace and minor elements the presence of eroded shale hosted organic matter om in sediments was confirmed by reflectance measurements on reworked vitrinitic macerals which had similar values to palaeozoic carbonaceous rocks in the watershed bimodal distribution of the reflectance data suggested that two sources of vitrinitic macerals were present from different formations variable dilution of shale sourced elements by carbonates related to climate temperature influenced rates of dissolution and erosion was the dominant process controlling inorganic geochemical composition rockeval pyrolysis of bulk sediment om revealed the unusual finding of two distinct sources of sediment kerogen which alternated in importance during the holocene a baseline state of eroded shale hosted om which was probably always present but which dominated sediment om when autochthonous algal production was minimal and an enhanced algal state which dominated when limnological conditions favored higher autochthonous productivity periods with more frequent examples of the enhanced algal state occurred during the mid and late holocene coincident with periods of relatively high summer air temperatures in the region this study provides evidence that climate particularly air temperature influenced sediment inorganic and organic geochemical compositions in this lake through its effect on catchment geology erosion rates and aquatic primary productivity it shows the value of studying very long periods of sediment accumulation as a background context for recent sediment metal concentrations

a 750 yr record of autumn snowfall and temperature variability and winter storminess recorded in the varved sediments of bear lake devon island arctic canada abstract the varve record from high arctic proglacial bear lake reveals a regionally coherent hydroclimatic signal as well as complexities due to changing hydroclimatic and limnologic conditions varve formation is strongly dependent on underflows that exhibit variability in strength during the past 750 yr periods with reduced underflow sedimentation and accumulation rates fail to produce varves in the distal part of the lake isolated coarse silt and sand grains occur in 80 of the varves and are interpreted to be niveo aeolian in origin coarse 500 m sand grains deposited on the lake ice by strong winter winds are notably less common since a d 1850 likely due to reduced storminess regression of the varve thickness record with meteorological records indicates high correlations with autumn september and october temperatures and total monthly snowfall these correlations are best at times when underflow activity is sufficiently strong to produce varves throughout the lake the close association with warmer temperatures and snow bearing synoptic systems moving north in baffin bay suggests that the primary climate signal in the varves is varying autumn snow pack that controls nival discharge in the following year the similarity between the other records of melt season temperature and sea ice cover and the bear lake record suggests that summer and autumn conditions were generally similar across the baffin bay region through much of the last millennium

characteristics of sedimentary varve chronologies a review abstract we constructed a varve data base vdb to study the worldwide distribution of varved sediment records and the fidelity of their varve chronologies clusters of lakes with varved sediments exist in central north america in the canadian arctic and in northern and central europe among the 108 sites included in the vdb varved records are typically 200 500 cm long and cover a period of 1000 2000 years their varve chronologies are often based on counting of annual layers from fresh sediment surfaces and photographs or from epoxy embedded sediment blocks and thin sections the vdb indicates that chronological errors associated with counting sedimentary varves fall generally between 1 and 3 but only 57 of published records are providing quantitative error estimations for their chronologies with a careful documentation of varves and by applying radiometric dating methods to surface sediments as well as using historical events as time markers there is the potential of having centennial long varved records with a very precise age depth control however it is unrealistic to expect errors significantly below 1 from several millennia long sections we found no indication that varve chronologies would have been substantially more accurate and precise in some parts of the world than in others or statistically dependent on varve thickness or temporal extension of a varved sequence according to the vdb close to 90 of the published varve chronologies have been cross checked with some independent dating methods

temporal patterns of suspended sediment yield following moderate to extreme hydrological events recorded in varved lacustrine sediments a 487 year annually laminated varved sediment record from nicolay lake cornwall island in the canadian high arctic was evaluated to determine the impact that years with high sediment yields had on sediment yields in subsequent years all of the 40 largest years showed evidence for increased sediment yield in the subsequent 10 30 years the positive anomalies in lagging years were approximately scaled according to the size of the initiating year although many intermediate years 25 to 100 year recurrence showed weak or variable responses the smallest events considered 10 to 25 year recurrence showed a consistent but low amplitude response additionally the 10 year events revealed frequent negative sediment yield anomalies in the preceding decade this behaviour was interpreted as a frequent sediment activation cycle initiated by the modest year and leading to sediment yield hysteresis lasting 15 25 years the largest years greater than 50 year recurrence showed consistently above average sediment yields in the preceding decade in part due to the frequent occurrence of moderate q10 years it is hypothesized that temporary storage of sediment and previous initiation of erosion sites resulted in extraordinary sediment yields during intense summer rainfall events this study demonstrates the potential use of varved lake sediment records to improve our understanding of long term sediment dynamics these records present an opportunity to further develop and test sediment dynamic and routing models to gain insight into the interaction of time and space in fluvial and sediment delivery processes copyright 2002 john wiley sons ltd

biolaminated sedimentation in a high arctic freshwater lake a laminated sequence of lacustrine sediment from an arctic coastal lake was examined for potential chronological and successional information the laminae are microbially induced sedimentary structures the unlithified precursor of stromatolites and are previously unreported in the arctic the inferred annual sequence was interpreted to contain a basal clastic unit overlain with successive cyanobacterial and extracellular polymeric substance units the complex succession of laminae and inconclusive dating provide a challenge for identifying the chronological nature of the sedimentary structures markov chain and time series analyses suggest that a plausible quasi annual sequence can be identified to provide a context for palaeo environmental reconstruction recognition of sedimentary structure in recent biolaminated sediments offers a first step towards the palaeo environmental evaluation of the geomicrobiological sequence

paleoclimate studies of minerogenic sediments using annually resolved textural parameters 1 we obtained quantitative multivariate data from each varve of a minerogenic lacustrine sequence from the canadian high arctic using an image analysis technique applied to thin sections the information on each varve from the uppermost core section was compared with a 35 yr meteorological dataset snowmelt intensity which is an index reflecting the energy available for sediment transport correlates well with the median grain size measured for each varve as well as with the weight of the 10 20 and 20 60 m fractions the proportion of fine silt also correlates with low intensity summer precipitation this methodology allows us to decipher the climatic control on sedimentary processes and yield a new perspective for constructing models which link climate to sediments that contain few biological remains this model can then be used to infer paleoclimate with annual resolution from the downcore grain size analysis

the life cycle of an undular bore and its interaction with a shallow intense cold front abstract the evolution of an undular bore and its associated wind shift spawned by the passage of a shallow surface cold front over the southern great plains of the united states is examined using surface and remote sensing observations along with output from a high resolution numerical model simulation observations show that a separation between the wind shift and thermodynamic properties of the front was induced by the formation of a bore over south central kansas around 0200 utc 29 november 2006 by the time the front bore complex passed through lamont oklahoma approximately 4 h later the bore had reached its maximum intensity and its associated wind shift preceded the trailing baroclinic zone by 20 min within several hours the bore decayed and a cold frontal passage characterized by a wind shift coincident with thermodynamic properties was observed at okmulgee oklahoma thus a substantial transformation in both the structural and dynamical characteristics of the bore as well as its relations

holocene sediments from a coastal lake on northern devon island nunavut canada sediment cores from lake dv09 northern devon island nunavut canada 75 34 34 n 89 18 55 w were studied to reconstruct the lake ontogeny through analysis and interpretation of the sediment stratigraphy the lake was uplifted from marine inundation 7600 cal bp after a millennium of rapid sediment accumulation which coincided with the holocene thermal maximum in the region accumulation rates decreased over the past 6000 years as the arctic became colder this resulted in the deposition of very fine laminae that were interpreted as varves the uppermost laminated sediments provided a 1600 year history of annual sediment transport and deposition into the lake during periods of warmer temperatures such as between 6000 and 7500 cal bp and during the medieval climate anomaly 950 1300 ce ce christian era hydroclimatic and permafrost slope processes increased sedimentation rates into the basin

a nonclassical cold front observed during cops 91 frontal structure and the process of severe storm initiation abstract this case study addresses the issue of gravity current and bore development at surface cold fronts and the role of these phenomena in the generation of severe frontal convection the event investigated occurred on 27 april 1991 during the cooperative oklahoma profiler studies 1991 field project the development of a bore from a gravity current like structure along a cold front the subsequent propagation of the bore ahead of the front on a low level inversion and the process of severe thunderstorm development along the front are revealed by a dense network of remote sensing and other special observations evidence for the gravity current and bore is strengthened by comparisons made between the synthesized observations and theory the bore developed after a nocturnal inversion which acted as a waveguide had become established the bore and gravity current were both evident as fine lines in the radar reflectivity displays a microscale envelope of enhanced water vapor with an embedded roll cl

testing the fidelity of sediment chronology comparison of varve and paleomagnetic results from holocene lake sediments from central finland abstract a careful construction of independent varve and paleomagnetic chronologies from three varved lake sediment records from central finland resulted in detailed and continuous age depth profiles cumulative varve calculation errors were estimated to be 3 for lake alimmainen savijarvi 10 295 340 varve years and 1 for lakes nautajarvi 9898 97 varve years and korttajarvi 9590 103 varve years paleomagnetic secular variation psv curves demonstrate good consistency between these sites and within a margin of varve chronology error they show similar declination and inclination features commonly described in the uk and sweden the timing of psv patterns from finnish varved sites agree well with uk curves between the present and 2000 bc and in pre 6000 bc deposits and with swedish data between the present and 5000 bc combined inclination records from three independently dated finnish varved sites possess an opportunity for accurate paleomagnetic comparisons within the margin of varve chronology errors and throughout the last 10 000 years

trace metal profiles in the varved sediment of an arctic lake abstract varved annually laminated sediments offer a rare and physically undisturbed archive of past trace metal deposition and limnological conditions here a high resolution 1 300 year record of metal accumulation is presented from a varved lake sediment on devon island in the canadian high arctic down core concentration profiles of cd cu and zn were positively correlated p

solar activity expressed in a modern varve thickness sequence annually laminated sediments varves form in particular depositional settings e g where seasonal climate produces fluctuations in runoff volume variations in runoff affect the amount and type

multi proxy record of postglacial environmental change south central melville island northwest territories canada abstract a sediment core from lake bc01 75 10 945 n 111 55 181 w 225 m asl on south central melville island nwt canada provides the first continuous postglacial environmental record for the region fossil pollen results indicate that the postglacial landscape was dominated by poaceae and salix typical of a high arctic plant community whereas the arctic herb oxyria underwent a gradual increase during the late holocene pollen based climate reconstructions suggests the presence of a cold and dry period 12 000 cal yr bp possibly representing the younger dryas followed by warmer and wetter conditions from 11 000 to 5000 cal yr bp likely reflective of the holocene thermal maximum the climate then underwent a gradual cooling and drying from 5000 cal yr bp to the present suggesting a late holocene neoglacial cooling diatom preservation was poor prior to 5000 cal yr bp when conditions were warmest suggesting that diatom dissolution may in part be climatically controlled diatom concentrations were highest 4500 cal yr bp but then decreased substantially by 3500 cal yr bp and remained low before recovering slightly in the 20th century an abrupt warming occurred during the past 70 yr at the site although the magnitude of this warming did not exceed that of the early holocene

holocene climatic records from agassiz ice cap ellesmere island nwt canada four ice cores from the top of the agassiz ice cap and down a flow line have been variously analysed for 8 18o ecm solid conductivity and ice melt layer stratigraphy stratigraphic correlation

amino acid racemization and radiocarbon dating of interstadial marine strata oxygen isotope stage 3 gulf st vincent south australia abstract concordant results for the extent of racemization of the amino acids aspartic acid leucine and valine and isoleucine epimerization in several mollusc genera support radiocarbon data that indicate an interstadial age oxygen isotope stage 3 45 000 to 30 000 yr b p for a sequence of mollusc rich carbonate muds in gulf st vincent south australia a numeric age of 46 500 9100 yr b p was obtained on marine molluscs from these sediments using a model based on apparent parabolic racemization kinetics in a global context the sequence represents one of the few well documented occurrences of marine strata of this age from a tectonically quiescent setting a sea level as high as 22 m is indicated for this interval amino acid racemization provides an important method for validating conventional finite radiocarbon ages close to the practical limits of radiocarbon dating and in studies of reworking

the generation of the morning glory abstract a high resolution cloud model is used to explore in detail the generation of the morning glory a low level nonlinear atmospheric internal wave observed on the southwestern side of cape york peninsula australia the model is two dimensional and nonhydrostatic and simulates an east west cross section of the southern part of cape york peninsula at a horizontal resolution of 200 m most of the numerical experiments are initialized at sunrise with a 5 m s 1 easterly flow and a sounding taken upstream from the peninsula the sea breezes that develop over cape york peninsula are highly asymmetric with the east coast sea breeze being both deeper and warmer than its western counterpart when the sea breezes meet the east coast sea breeze rides over that from the west coast and in the process produces a series of waves that propagate on the west coast sea breeze the model calculations show that when the phase speed of these waves matches the westward propagation speed of the east coast sea breeze the

structure and evolution of north australian cloud lines observed during amex phase i abstract north australian clouds lines are distinctive squall line phenomena that occur in easterly flow across northern australia three basic types have been identified ranging from a long narrow line of convective clouds type 1 to a severe squall line type 3 in this paper we examine a group of type 1 lines which occurred during the first phase of the australian monsoon experiment amex the lines occurred in an ambient easterly flow with a distinct maximum near 850 hpa most of the lines developed on the western side of deep convective cells along the sea breeze front in a manner that had substantial similarities to the african squall line development described by bolton the resolvable structure resembled a shallow version of the moncrieff miller squall line

paleoenvironments of the canadian high arctic derived from pollen and plant macrofossils problems and potentials abstract analyses of peat sections lake sediments and ice cores provide information about late quaternary arctic environments palynomorphs and plant macrofossils from each of these three types of sediments record different aspects of the environment with particular spatial and temporal scales of resolution in the arctic the limits to the interpretation of past environments are particularly significant problems of low pollen concentrations long distance transport stratigraphic inversions and contamination by fossils from older deposits are more serious in this region due to the biogeographic context that characterizes these high latitudes and influences the particular ecological and geomorphological processes however recent work has shown that past environments can be reconstructed from evidence preserved in high arctic sediments and ice cores if these problems are taken into account before their interpretation

modern and holocene pollen assemblages from some small arctic lakes on somerset island nwt canada abstract modern pollen samples from 15 lakes along a north south transect on western somerset island nwt canada show a decrease in pollen concentrations from the high arctic to the mid arctic zone but there are few differences in the pollen percentages between these sites long distance transport accounts for up to 50 of the pollen in these lake sediments cores from two lakes show few changes in the percentages of important pollen types except for an initial period before 6000 yr b p of increased salix the pollen concentration of lake rs36 from the mid arctic is twice that of lake rs29 from the high arctic and at both sites the concentrations decreased during the past 6000 yr b p this suggests a climatic deterioration during the past 6000 yr which has caused a decrease in the abundance of plants on the landscape

autumn snowfall and hydroclimatic variability during the past millennium inferred from the varved sediments of meromictic lake a northern ellesmere island canada abstract we examined the hydroclimatic signal in a record of annual lamina varve thickness from high arctic lake a ellesmere island 83 00 00 n 75 30 00 w in this unglacierized catchment nival melt is the dominant source for meltwater and transport of sediment to the lake and autumn snowfall is highly influential on varve thickness through the amount of snow available for melt in the following year for the period during which climatic data are available varve thickness in lake a was significantly correlated r 0 50 p r 0 39 p

holocene climate and vegetation change on victoria island western canadian arctic abstract a detailed pollen record from victoria island provides the first quantitative holocene climate reconstruction from the western canadian arctic the pollen percentage data indicate that arctic herbs increased over the holocene in response to long term cooling the influx of locally and regionally derived pollen grains varies throughout the core and tracks several major changes observed in the biogenic silica record from arolik lake alaska and the gisp2 ice core suggesting that climate change closely controlled arctic plant productivity using modern analogue and transfer function techniques we generated quantitative reconstructions of mean july temperature and total annual precipitation for the past 10 000 years to place recent climate changes within the context of holocene climate variability the quantitative reconstructions indicate that july temperature cooled by 1 1 5 c during the holocene the pollen based reconstructions record an increase in temperature of 0 5 c over the last 100 years and the pollen percentage and influx data indicate impacts of recent warming on the regional vegetation

the formation of internal bores in the atmosphere a laboratory model summary recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low level temperature inversions it has been speculated that such disturbances are generated by some type of gravity current such as cold fronts sea breeze fronts and thunderstorm outflows interacting with an existing temperature inversion in this paper we describe a systematic laboratory study in a water channel of internal bores and their generation by the movement of gravity currents through a two layer model of the atmosphere we compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development