oxygen in the arabian sea sir variations in the oxigen isotope content 18 o of planktonic foraminifera from the arabian sea have been cited as evidence for stronger northeast monsoon current at the last glacial maximum lgm about 18 kyr ago than during the present day

is the north atlantic oscillation reflected in scandinavian glacier mass balance records the north atlantic oscillation nao is one of the modes of climate variability in the north atlantic region the atmospheric circulation during the winter season in this region commonly displays a strong meridional north south pressure contrast with low air pressure cyclone centred close to iceland and high air pressure anticyclone near the azores this pressure gradient drives the mean surface winds and the mid latitude winter storms from west to east across the north atlantic bringing mild moist air to northwest europe the nao index is based on the difference of normalised sea level pressures slp between ponta delgada azores and stykkisholmur iceland the slp anomalies at these stations are normalised by division of each monthly pressure by the long term 1865 1984 standard deviation interannual atmospheric climate variability in northwest europe especially over great britain and western scandinavia has during the last decades been attributed mainly to the nao causing variations in the winter weather over the northeast north atlantic and the adjacent land areas a comparison between the nao index and the winter december march precipitation between ad 1864 and 1995 in western norway shows that these are strongly linked correlation coefficient 0 77 variations in the nao index are also reflected in the mass balance records of glaciers in western scandinavia the nao index is best correlated with mass balance data from maritime glaciers in southern norway e g alfotbreen r 2 0 51 the record of holocene last ca 11 500 cal yr glacier variations of maritime glaciers in western scandinavia is thus a proxy of pre instrumental nao variations copyright 2000 john wiley sons ltd

synchronous high frequency oscillations in tropical sea surface temperatures and north atlantic deep water production during the last glacial cycle stable isotopic measurements of g sacculifer and c wuellerstorfi in a core from the western equatorial atlantic imply that there are parallel suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3 low values of g sacculifer 18o accompany high values of c wuellerstorfi 13c linking warmer sea surface temperatures ssts in the tropics with increased production of lower north atlantic deep water nadw the amplitude of the 18o oscillations is 0 6 or 2 3 c which is superimposed on a glacial interglacial amplitude of about 2 1 using the g sacculifer 18o data we calculate that surface waters were colder during stage 2 than calculated by climap 1976 1981 the longer period 2 kyr oscillations in air temperature recorded in the greenland and antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial atlantic the magnitude of these oscillations in tropical sst is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone the apparent synchroneity of equatorial sst and polar air temperature changes as well as the amplitude of the sst changes at the equator are consistent with the climate effects expected from changes in the atmosphere s greenhouse gas content h2ovapor co2 and ch4

response of the ganges dispersal system to climate change a source to sink view since the last interstade the forcing of sediment dispersal systems by climate is widely accepted but there remains disagreement on the general responses to climate change such as how resulting signals propagate downstream and ultimately how they are preserved in the stratigraphic record towards a better understanding of these issues this paper presents a comprehensive overview of ganges dispersal system behavior since the last interstade which reveals major coeval responses to changes in the south asian monsoon at 3000 km from its mountain headwaters to deep sea fan the ganges dispersal system is immense and yet shows tight coupling between source area catchment basin and coastal and marine depocenters furthermore system wide responses and downstream transfer of the signal appear to occur at least as fast as current temporal resolution of 1 2 ky this tight linkage of source to sink components is considered a function of the southwest summer monsoon s overwhelming control on regional hydrology about 80 of the ganges discharge and 95 of its sediment load are delivered to the margin during only 4 months making the system extremely sensitive to this seasonal forcing in addition the regional scale and distribution of the monsoon weather system means that changes in atmospheric circulation affect the entire drainage basin rather than local subcomponents finally despite its present intensity strength of the summer monsoon has varied significantly over the past 150 ky under orbitally driven changes in insolation and global glacial boundary conditions the resulting changes in precipitation both well above and below modern values have forced system wide responses ranging from glacier advance retreat to sediment fluxes to the margin and deep sea fan the overall conclusions are 1 that this immense dispersal system responds to multimillennial scale 104 years climate change in a system wide and largely contemporaneous manner and 2 that major sedimentary signals can be transferred rapidly from source to sink with little apparent attenuation furthermore these acute responses to climate change have produced sedimentary stratigraphic features that diverge from traditional sequence models in their nature and timing

advances in river ice hydrology river ice is present in nearly all canadian rivers for periods ranging from days to many months whether moving or stationary it interacts with the river flow in various ways resulting in multiple impacts on the economy and ecosystem and posing a major flood threat to riverside communities in the past 4 years canadian research and development efforts have been directed at a variety of problems a strong focus on ice breakup and ice jam processes resulted in improved understanding of the salient geomorphological and hydroclimatic factors enhanced modelling and prediction capabilities and development of techniques for in situ measurement of ice jam properties key contributions in the area of ecological impacts of river ice and ice jams have led not only to solid advances in knowledge but also to an appreciation of the vast scope of this subject and its numerous links to environmental science a closely related topic the flux of suspended sediment in ice laden rivers was studied for the first time in order to delineate the effects of the ice on sediment and associated contaminant loads in response to growing concern about climate change and variability several studies addressed implications to ice regime and thence to ecology and economy although not fully explored the potential impacts appear to be numerous and significant owing to the high sensitivity of river ice processes to climatic factors in the foreseeable future research is likely to continue along the above noted lines although an increased emphasis on climatic and ecological aspects is probable insights gained on the mechanisms of breakup and jamming may lead to increased modelling applications and testing of theoretical concepts

ice flood history reconstructed with tree rings from the southern boreal forest limit western quebec historic ice flood frequency and lake levels were reconstructed for the southern limit of the boreal forest in western quebec by dendrochronological analyses of ice scarred white cedar thuja occidentalis l trees growing on the shore of lake duparquet results reveal a significant increase in major ice floods since the end of the little ice age and in particular since the beginning of this century this rise was also not related to oversampling of younger scarred trees in addition time between white cedar establishment and the first recorded scar has significantly decreased in the last 200 years and this applies for trees occupying similar topographic conditions white cedar s increased exposure in the 150 years to scarring strongly suggests that lake duparquet water levels at breakup are reaching higher elevations this is supported by an approximate 100 cm rise in maximum ice scar height since the end of the little ice age years leading to abundant ice scars are those with heavy winter and

ice jam caused fluvial gullies and scour holes on northern river flood plains abstract two anomalous fluvial landforms gullies and scour holes eroded into flood plains bordering meandering and braiding river channels have not been previously reported we observed these features along the milk river in southern alberta canada and northern montana usa which has a history of frequent 50 probability of recurrence and high magnitude 12 probability of recurrence greater than bankfull ice jam floods gullies have palmate and narrow linear shapes with open ends downvalley and measure up to 208 m long 139 m wide 3 5 m deep below bankfull channel ice jams reroute river water across meander lobes and cause headward gully erosion where flow returns to the main channel erosion of the most recent gully was observed during the record 1996 ice breakup flood and ice jams scour holes bowl shaped closed depressions eroded by water vortices beneath and between grounded ice jam blocks measure up to 91 m long 22 m wide 4 5 m deep ice jam caused gullies may be precursors to the formation of u shaped oxbow lakes and multiple channels common in many northern rivers

dendroecological evidence of lake level changes during the last three centuries in subarctic quebec abstract dendroecological analysis of black spruces picea mariana mill bsp near the shore of clearwater lake in the eastern canadian subarctic yielded information on water level over the past three centuries tree positions growth froms layering tree ring patterns and ice scars provide direct evidence of a major rise in lake level from the 17th century to the present trees on the lake shore were progressively submerged by the increasing water level which reached a maximum in this century higher levels began around the mid 18th century when shoreline spruce trees started to lean because extensive wave erosion caused landward shore displacement high mortality of leaning spruce and abundant ice scar formation occurred during this century the rise in water level in inferred to have been caused by snowier winters occurring parallel to climatic warming

oxygen and carbon isotopic composition and biogeographic distribution of planktonic foraminifera in the indian ocean five planktonic foraminiferal assemblages were encountered along a north south transect in the indian ocean with the following dominant species globorotalia truncatulinoides in the southern subtropical assemblage globigerinoides ruber globigerinella aequilateralis and globorotalia menardii in the northern subtropical assemblage globigerinoides sacculifer and pulleniatina obliquiloculata in the tropical assemblage neogloboquadrina dutertrei and globigerina bulloides in the tropical upwelling assemblage and globigernoides conglobatus in the tropical arabian sea assemblage oxygen and carbon isotopic analyses were performed on three surface dwelling species of planktonic foraminifera in tropical and subtropical waters 22 30 c the 18o of globigerinoides ruber and g sacculifer varies with surface water temperature and the slope of the isotope temperature regression line is similar to that of epstein et al 1953 for marine molluscs however foraminiferal 18o values are significantly lighter than those of molluscs indicating isotopic disequilibrium comparison between plankton tows taken within and below the mixed layer at the same stations shows that g ruber has similar low 18o values in both sets suggesting that it secretes its shell within the mixed layer by contrast g sacculifer has slightly higher 18o values below than within the mixed layer because it continues to secrete calcite below the thermocline no significant 18o variations with shell size were found on the other hand 13c values were strongly size dependent i e the smaller the size the lighter the 13c comparison of living g ruber and g sacculifer with recent specimens from the surface sediment shows that the living foraminifera are generally lighter in 18o than the fossils supporting independent observations that the latter have secreted additional calcite during gametogenesis when the organisms settle freely from the euphotic zone for about 12 h and pass through waters lower in temperature than the epipelagic waters where the foraminifera form the bulk of their shells

continuous earlywood vessels chronologies in floodplain ring porous species can improve dendrohydrological reconstructions of spring high flows and flood levels summary plants respond to environmental stimuli through changes in growth and development characteristics of wood cells such as the cross sectional area of vessel elements hereafter referred to as vessels may store information about environmental factors present at the time of vessel differentiation the analysis of vessel characteristics therefore offers a different time resolution than annual ring width because vessels in tree rings differentiate within days to a few weeks little research has been conducted on the sensitivity of earlywood vessels in ring porous species in response to flooding the general objectives of this study were to determine the plasticity of earlywood vessel to high flows and spring flooding in floodplain black ash fraxinus nigra marsh trees and to assess the utility of developing continuous earlywood vessel chronologies in dendrohydrological reconstruction in contrast most dendrohydrological studies until now have mainly used vessel anomalies flood rings as discrete variables to identify exceptional flood events the study area is located in the boreal region of northwestern quebec vessel and ring width chronologies were generated from f nigra trees growing on the floodplain of lake duparquet spring discharge had among all hydro climatic variables the strongest impact on vessel formation and this signal was coherent spatially and in the frequency domain the mean vessel area chronology was significantly and negatively correlated to discharge and both the linearity and the strength of this association were unique in floodplain f nigra trees spring flooding promoted the formation of more abundant but smaller earlywood vessels earlywood vessels chronologies were also significantly associated with other hydrological indicators like lake duparquet s ice break up date and both ice scar frequency and height chronologies these significant relationships stress the utility of developing continuous vessels chronologies for hydrological reconstructions prior to instrumental data continuous earlywood vessel chronologies may also be useful in determining the impact of altered hydrological regime in floodplain habitat regulated by spring floods future research should involve quantifying the impact of high flows and flooding on other cell constituents and also determining the plasticity and utility of continuous anatomical series in floodplain diffuse porous species

increasing chemical weathering in the himalayan system since the last glacial maximum continental chemical weathering is central in earth s surface biogeochemical cycles as it redistributes elements across reservoirs such as the crust and the oceans however the evolution of weathering through time and its response to external forcing such as changes in climate remain poorly constrained in this work a composite sediment record from the bay of bengal is used to document the evolution of chemical weathering in the himalayan system himalayan range and indo gangetic floodplain the world largest sediment conveyor to the oceans since the last glacial maximum lgm the degree of weathering of the sediments is documented using mobile to immobile ratios such as k si and h2o si as well as detrital calcite abundance robust weathering proxies are derived by correcting the chemical composition of sediment for sorting effects that occur during transport and deposition the bay of bengal record is also further compared to the chemical composition of modern river sediments from the ganga and brahmaputra basin weathering proxies all indicate that the sediments exported by the ganga and brahmaputra rivers became increasingly weathered over the past 21 kyr whereas sr nd and major elements suggest a constant sediment provenance in the system over the last 21 kyr these changes in the degree of weathering of the sediments show that the weathering flux exported by the system to the indian ocean during the lgm was significantly lower than at present and demonstrate that chemical weathering in continental scale basins such as the ganga and brahmaputra responds to late quaternary climate changes

glacial interglacial variations in the neodymium isotope composition of seawater in the bay of bengal recorded by planktonic foraminifera this study presents neodymium isotope and elemental data for cleaned planktonic foraminifera from odp site 758 in the southernmost reaches of the bay of bengal in the north east indian ocean cleaning experiments using oxidative reductive techniques suggest that diagenetic fe mn oxyhydroxide coatings can be effectively removed and that the measured nd isotope composition reflects the composition of seawater from which the foraminiferal calcium carbonate was precipitated modern core top pulleniatina obliquiloculata and globorotalia menardii give end values of 10 12 0 16 and 10 28 0 16 respectively indistinguishable from recent direct measurements of surface seawater in this area a high resolution nd isotope record obtained from g menardii for the past 150 kyr shows systematic variations end 3 on glacial interglacial timescales the timing of those variations shows a remarkable correspondence with the global oxygen isotope record which suggests a process controlling the nd isotope composition that responds in phase with global climate cycles palaeoclimate reconstruction indicates that during the last glacial maximum changes in monsoon circulation resulted in a reduction in rainfall over the indian subcontinent and a decrease in the flux of river water delivered to the bay of bengal thus changes in the riverine input of nd a change in either flux or composition most likely caused the isotope variations although changes in dust source or local ocean circulation may have also played a role these results clearly establish a link between climate change and variations in radiogenic isotopes in the oceans and illustrate the potential of nd isotopes in foraminifera for high resolution palaeoceanographic reconstruction

late pleistocene holocene development of the tista megafan west bengal india 10 be cosmogenic and irsl age constraints abstract the himalayan proximal foreland is characterized by quaternary megafans of which the formational mechanisms remain debated the tista megafan spreads over more than 16 000 km2 from the mountain front where it is strongly incised to the confluence of the tista river with the jamuna brahmaputra river and stores sediments produced in the sikkim himalaya we propose a scenario for the late pleistocene holocene development of the tista megafan based on new 10be cosmogenic and infra red stimulated luminescence irsl age constraints and discuss the main potential controls on its evolution we suggest that two distal lobes developed successively downstream from a common proximal lobe deposition in the proximal lobe took place since at least 135 ka and incision began at 3 7 0 7 1 0 ka the western distal lobe of the megafan was deposited early in the history of the megafan when the sikkim himalaya catchment was drained by a tributary of the ganga river and was abandoned in the early holocene 10 11 ka the eastern recent

faunal distribution patterns of planktonic foraminifers in surface sediments of the low latitude pacific abstract faunal indices of sea surface temperature sst have been widely used for reconstructing past ocean variability and for verifying the results of climate modeling recent studies on the distribution patterns of modern planktonic foraminifers on surface sediments have indicated important relationships with many hydrographic variables associated with the vertical structure of upper layer oceans to investigate whether these other ocean hydrographic variables are important to the changes in the faunal distribution pattern two sets of core top faunal data from modern low latitude pacific surface sediments were compiled to compare with direct observations of sst as well as the depth of thermocline dot a primarily important variable for describing the vertical structure of upper layer oceans at low latitudes the degree of correlation between the faunal principal components defined by a q mode principal component analysis sst dot and a carbonate preservation index cpi was examined using simple and partial correlation analyses this study indicates that the effect of dot controls on faunal distributions of planktonic foraminifers in the low latitude pacific is important and interpreting the dot or other hydrographic variables of upper layer oceans from planktonic foraminifer faunal data would be more appropriate in future paleoceanographic applications

late holocene development of a floodplain along a small meandering stream northern quebec canada this paper describes the activity of a small meandering stream and the development of its floodplain during the last 4600 years calendar years bp in the northern boreal zone of quebec three trenches were excavated across the floodplain s full width and permitted the interpretation of morphosedimentary units in relation to modern analogs chronological controls within trenches was provided by the dendrochronological and radiocarbon dating of buried tree trunks from 4600 to 2900 cal bp and from 1000 to 120 cal bp the channel migrated and constructed its floodplain at very slow rates mostly because of low flow velocities vegetated streambanks and the cohesive texture of marine sediments reworked by the channel vertical accretion rates were extremely variable on the floodplain with high rates proximal to the modern channel and low rates over distal also older portions of the floodplain following a major channel shift meander cut off or avulsion around 2900 cal bp channel migration appears to have been constrained to a narrow zone adjacent to the modern channel within this constrained zone the migrating channel has reworked its own sediments leading to a marked unconformity between 2900 1000 cal bp it is thought that underlying marine sediments protrusions and perhaps the forested banks protected older alluvial sediments from being eroded during the last three millenniums our study shows that small boreal floodplains may contain in a very small area abundant and diversified archives of their evolution

subsidence control on river morphology and grain size in the ganga plain the ganga plain represents a large proportion of the current foreland basin to the himalaya the himalayan sourced waters irrigate the plain via major river networks that support approximately 10 percent of the global population however some of these rivers are also the source of devastating floods the tendency for some of these rivers to flood is directly linked to their large scale morphology in general the rivers that drain the east ganga plain have channels that are perched at a higher elevation relative to their floodplain leading to more frequent channel avulsion and flooding in contrast those further west have channels that are incised into the floodplain and are historically less prone to flooding understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the plain here we present a new basin scale approach to quantifying floodplain and channel topography that identifies areas where channels are super elevated or entrenched relative to their adjacent floodplain we explore the probable controls on these observations through an analysis of basin subsidence rates sediment grain size data and sediment supply from the main river systems that traverse the plain yamuna ganga karnali gandak and kosi rivers subsidence rates are approximated by combining basement profiles derived from seismic data with known convergence velocities results suggest a more slowly subsiding basin in the west than the east grain size fining rates are also used as a proxy for relative subsidence rates along the strike of the basin the results also indicate higher fining rates and hence subsidence rates for given sediment supply in the east by integrating these observations we propose that higher subsidence rates are responsible for a deeper basin in the east with perched low gradient river systems that are relatively insensitive to climatically driven changes in base level in contrast the lower subsidence rates in the west are associated with a higher elevation basin topography and entrenched river systems recording climatically induced lowering of river base levels during the holocene

variations of the monsoon regime during the upper quaternary evidence from carbon isotopic record of organic matter in north indian ocean sediment cores abstract the 13 c 12 c ratios and contents of organic carbon in deep sea sediment cores of the north indian ocean have been compared for the different climatic stages as determined by oxygen isotope stratigraphy using the differences in the 13 c values between marine and terrestrial organic matter the terrestrial contribution to the eastern gulf of bengal and the andaman sea has been determined for modern and glacial times the geographical distribution of the terrestrial organic carbon linked to sea surface circulation shows that during the last glacial period the ne monsoon circulation was the dominant feature the organic carbon abundance within the sediment which is related to local productivity was greater during glacial times in the areas where production depends on the ne monsoon strength coast of north india in the arabian sea and andaman sea suggesting its reinforcement

spring flood reconstruction from continuous and discrete tree ring series 1 this study proposes a method to reconstruct past spring flood discharge from continuous and discrete tree ring chronologies since both have their respective strengths and weaknesses in northern environments ring width or density series provide uninterrupted records that are indirectly linked to regional discharge through a concomitant effect of climate on tree growth and streamflow conversely discrete event chronologies constitute conspicuous records of past high water levels since they are constructed from trees that are directly damaged by the flood however the uncertainty of discrete series increases toward the past and their relationships with spring discharge are often nonlinear to take advantage of these two sources of information we introduce a new transfer model technique on the basis of generalized additive model gam theory the incorporation of discrete predictors and the evaluation of the robustness of the nonlinear relationships are assessed using a jackknife procedure we exemplify our approach in a reconstruction of may water supplies to the caniapiscau hydroelectric reservoir in northern quebec canada we used earlywood density measurements as continuous variables and ice scar dates around lake montausier in the james bay area as a discrete variable strong calibration 0 57 0 61 0 75 and validation 0 27 0 44 0 58 r2 statistics were obtained thus highlighting the usefulness of the model our reconstruction suggests that since 1965 spring floods have become more intense and variable in comparison with the last 150 years we argue that a similar procedure can be used in each case where discrete and continuous tree ring proxies are used together to reconstruct past spring floods

an experimental study on deformation and failure mechanical behavior of granite containing a single fissure under different confining pressures fissures in natural rocks play an important role in determining the strength deformability and failure behavior of rock mass however in the past triaxial compression experiments have rarely been conducted for rock materials containing three dimensional 3 d fissures and the failure mechanical behavior of fissured rocks is not well known due to the difficulty of conducting triaxial experiments on fissured rocks therefore in this research conventional triaxial compression experiments were performed to study the strength deformability and failure behavior of granite specimens with one preexisting open fissure thirty one specimens were prepared to perform conventional triaxial compression tests for intact and fissured granite first based on the experimental results the effects of the confining pressure and the fissure angle on the elastic modulus and the peak axial strain of granite specimens are analyzed second the influence of the confining pressure on the crack damage threshold and the peak strength of granite with respect to various fissure angles are evaluated for the same fissure angle the crack damage threshold and the peak strength of granite both increase with the confining pressure which is in good agreement with the linear mohr coulomb criterion with increasing fissure angle the cohesion of granite first increases and later decreases but the internal friction angle is not obviously dependent on the fissure angle third nine crack types are identified to analyze the failure characteristics of granite specimens containing a single fissure under conventional triaxial compression finally a series of x ray microcomputed tomography ct observations were conducted to analyze the internal damage mechanism of granite specimens with respect to various fissure angles reconstructed 3 d ct images indicate obvious effects of confining pressure and fissure angle on the crack system of granite specimens the study helps to elucidate the fundamental nature of rock failure under conventional triaxial compression

modulation and amplification of climatic changes in the northern hemisphere by the indian summer monsoon during the past 80 k y high frequency suborbital variations dansgaard oeschger cycles characterize the climatic history of the northern hemisphere as observed in greenland ice cores deep sea sediments of the north atlantic the californian borderland the arabian sea the south china sea and the chinese loess area paleoceanographic data from core kl126 from the bay of bengal in combination with data from the other asian monsoonal areas indicate that the feedback processes involving snow and dust of the tibetan plateau vary the summer monsoon capacity to transport moisture into central south asia and into the atmosphere we postulate that the summer monsoon initiates amplifies and terminates these cycles in the northern hemisphere

hydro climatic analysis of mechanical breakups reconstructed from tree rings necopastic watershed northern quebec canada we used a 54 year 1950 2003 ice scar chronology constructed from damaged trees to investigate relationships between large scale hydro climatic conditions and mechanical breakup magnitude in the necopastic river watershed james bay northern quebec our objectives were 1 to identify hydrologic and climatic variables that explain variations in mechanical breakup magnitude at the watershed scale 2 to organize these variables in terms of importance and 3 to construct a predictive model for ice floods in the necopastic watershed we used parametric correlation analysis to measure the degree of linear association between variables and classification trees ct coupled with a cross validation approach to construct the predictive model and to organize variables according to their importance in the necopastic watershed the type and magnitude of breakups are determined by an interaction between forces that maintain the ice cover in place and forces that tend to dislodge it in contrast with lakes early and rapidly rising floods rather than abnormally high flood discharge create conditions that favour intense mechanical breakups moreover cold and snowy spring conditions during high positive arctic oscillation ao indices delay the thermal degradation of the ice cover and preserve its mechanical properties over longer time periods

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

impacts of recurring ice jams on channel geometry and geomorphology in a small high boreal watershed article i nfo river ice jams are generally perceived as significant erosive events and are well known to impact both channel morphology and geometry however the extent of these impacts and the frequency of events required to maintain erosion induced morphologies remain unexplored in most cold region watersheds in this study we investigated downstream variations in channel width cross sectional area depth and geomorphological characteristics in a small high boreal basin we coupled these observations to dendrochronological data on ice jam frequency our results show that channels affected by ice erosion appear enlarged and present an important retreat of the upper bank such enlarged channels present a typical two level ice scoured morphology when ice jams recur more often than once every 5 years by contrast channels appear unaffected when ice jams are less frequent these results suggest that ice jams maintain ice scoured and enlarged morphologies once a minimal frequency of occurrence threshold is exceeded we therefore conclude that ice jam frequencies should be taken into account in order to better define the role of ice as a geomorphological agent in cold environments

tree ring dating of extreme lake levels at the subarctic boreal interface abstract the dates of extreme water levels of two large lakes in northern quebec have been recorded over the last century by ice scars on shoreline trees and sequences of reaction wood in shore trees tilted by wave erosion ice scar chronologies indicate high water levels in spring whereas tree tilting by waves is caused by summer high waters a major increase in both the amplitude and frequency of ice floods occurred in the 1930s no such change was indicated by the tree tilting chronologies but wave erosion occurred in exceptionally rainy years according to the modern record spring lake level rise is due to increased snowfalls since the 1930s however the absence of erosional marks in a large number of years since 1930 suggests a high frequency of low water level years resulting from dry conditions intercalary years with very large numbers of marked trees e g 1935 indicate that the interannual range of summer lake levels has increased since the 1930s increased lake flood frequency is postulated to be related to a slower expansion of arctic anticyclones favoring the passage of cyclonic air masses over the area and resulting in abundant snowfall in early winter conditions in summer are due to the rate of weakening of the anticyclones controlling the position of the arctic front in summer this position influences the path of the cyclonic air masses which control summer precipitation and consequently summer lake levels in the area

tree ring analysis of hydro climatic thresholds that trigger ice jams on the mistassini river quebec this study documents the spatiotemporal variations in the frequency and magnitude of ice jams in the mistassini river and applies that information to the identification of the hydro climatic threshold conditions associated with major events ice jams cause severe upheavals in water flow which result in flooding upstream of the ice jam front and therefore represent a significant geohazard to riparian populations to analyze the spatiotemporal variations in the magnitude frequency of ice jams the mistassini river was first divided into six different sites representing different geomorphological contexts a 50 year ice jam chronology was constructed from 85 damaged trees from all of the study sites this chronology was then coupled with hydro climatic variables to construct classification trees which helped identify the conditions and hydro climatic thresholds favourable to the triggering of ice jams in a predictive model the results indicate complex interactions between the characteristics of flow ice cover and river morphology that affect the frequency and magnitude of ice jam events on the mistassini river these factors affect the frequency and magnitude of ice jam events the triggering of extreme events seems particularly influenced by exceptional ice conditions and sites with high sinuosity and islands copyright 2015 john wiley sons ltd

microfossil evidence for changing salinity patterns in the bay of bengal over the last 20 000 years the monsson climate characterizing the northern indian ocean produces salinity gradients that are large compared with those in other open ocean areas these gradients are substantially independent of thermal and other ecologically important gradients this situation makes it possible not only to isolate the effect of salinity on sea bed distribution patterns of planktonic foraminifera but also to investigate past salinity changes along a north south transect of piston cores in the bay of bengal analysis of 152 surface sediment samples reveals that at least three factors other than salinity are important in controlling foraminiferal distribution sea bed dissolution surface productivity and surface temperature of these dissolution is the most important therefore in order to reconstruct past salinity patterns in the bay of bengal faunal indices need to be developed that isolate the effect of salinity we have developed two independent approaches the first uses ternary diagrams of the three dominant dissolution resistant species globoquadrina dutertrei globorotalia menardii pulleniatina obliquiloculata high values of g dutertrei are associated with low surface salinities the second approach uses dissolution buffered salinity transfer functions based on the total foraminiferal fauna in the 152 surface samples from the northern indian ocean applied to downcore samples these methods show the same basic historical pattern of salinity variation which is in turn consistent with independent inferences based on preliminary oxygen isotope data duplessy 1980 historical changes in the bay of bengal salinity are reconstructed along a north south transect of seven cores at 90 e the stratigraphy of the transect is controlled by oxygen isotope profiles that are similar to the generalized patterns observed in other areas over the past 20 000 yr salinity gradients are reconstructed for three time intervals 1 the last glacial maximum lgm 2 the mid termination mt and 3 the holocene during the lgm interval salinity was highger in the bay of bengal particularly in the north thus reducing the north south salinity gradient during the mt the middle of the transition from glacial to interglacial global climatic regimes northern bay of bengal surface salinity was considerably reduced while salinity to the south was generally similar to today thus the salinity gradient between 10 n and 15 n was considerably steeper than today in contrast the holocene gradient was approximately the same as the modern gradient with absolute values along the transect being slightly but uniformly higher these changes in the pattern of salinity are explained in terms of changes in the local patterns of precipitation and river run off during the lgm the salinity pattern probably results from decreased precipitation and decreased run off by the ganges brahmaputra river system during the mt decreased salinity in the northern bay of bengal is caused by increased precipitation and run off rather than by the influx of glacial meltwater the steeper mt salinity gradient in the central bay of bengal suggests increased eastward advection of equatorial surface waters by a southwest monsoon current somewhat more vigorous than today these patterns can be correlated with faunal evidence from the western arabian sea that suggest decreased upwelling during the lgm and increased upwelling during the mt this coupling of events may be associated with changes in the pattern or intensity of circulation during the southwest monsoon decreased intensity at the lgm 18 000 b p and increased intensity at the mt 10 500 to 12 500 b p

changes in glacier extent on north novaya zemlya in the twentieth century glacier retreat on north novaya zemlya for the past century was documented by registering glacier terminus positions from expedition and topographic maps and remotely sensed images recession of tidewater calving glaciers on north novaya zemlya in the first half of the twentieth century was relatively rapid 300 m yr 1 consistent with post little ice age warming documented by a 122 year instrumental record from malye karmakuly the glaciers completed 75 to 100 of the net twentieth century retreat by 1952 between 1964 and 1993 half of the studied glaciers were stable the remainder retreated modest distances of 0 75 elevated sst in the barents sea appear to reflect increased advecti