seasonal variation in the flux of planktonic foraminifera time series sediment trap results from the panama basin abstract a mooring array with three automated sediment traps capable of collecting time series samples was deployed in the panama basin for one year beginning in december 1979 a series of six consecutive two month long samples was collected at each of three depths 890 2590 and 3560 m in order to examine seasonal variation in the flux of planktonic foraminifera and evaluate the contribution of foraminifera to the total carbonate flux the flux of the larger planktonic foraminifera 250 500 m and 500 1000 m is greatest during february march when upwelling is most intense in the panama basin in contrast the maximum flux of the smaller foraminifera 125 250 m is associated with a phytoplankton bloom during the summer months june through september this size dependent flux pattern appears to be a species specific effect the flux of the larger foraminifera is dominated by non spinose forms i e neogloboquadrina dutertrei and globorotalia theyeri while the flux of the smaller foraminifera consists predominantly of spinose species i e globigerinoides ruber g sacculifer and g conglobatus although the magnitude of the flux varied throughout the year the average weight of individual foraminiferal tests in different size fractions showed no seasonal variability with the exception of the june july period when there was a major coccolith bloom planktonic foraminifera greater than 125 m account for between 28 and 34 of the total carbonate flux at this location during the coccolith bloom planktonic foraminifera accounted for less than 2 of the total carbonate flux planktonic foraminifera in the 250 500 m size range are the most significant contributors to the overall particulate flux accounting for roughly 70 80 of the total foraminiferal fluxes measured at the three trap depths

proxy dependence of the temporal pattern of deglacial warming in the tropical south china sea toward resolving seasonality abstract sea surface temperatures ssts reconstructed from two globigerinoides ruber white morphotypes g ruber sensu stricto s s g ruber sensu lato s l mg ca and alkenones uk 37 from core md01 2390 from the tropical south china sea scs during the last deglaciation reveal a proxy dependent discrepancy in the temporal pattern of the deglacial warming alkenone data suggest that the deglacial warming is punctuated by a decrease in temperature between 17 and 15 ka bp corroborating previously published alkenone uk 37 sst records from the southern scs within dating uncertainties this cooling is coeval with the heinrich event 1 h1 time interval in the north atlantic region underscoring the imprints of northern hemisphere forcing on tropical scs ocean temperatures the deglacial uk 37 sst minimum is also paralleled by a maximum in g ruber morphotype specific 18o g ruber mg ca sst estimates suggest a morphotype specific record of ssts during the time interval of h1 whereas g ruber s s imply a continuous warming starting around 18 ka bp without any marked response to h1 g ruber s l based mg ca sst estimates reveal a cooling around 17 15 ka bp similar to the h1 interval cooling seen in the alkenone sst record similar proxy dependent differences in deglacial surface water warming have been recorded in the eastern equatorial pacific implying a common pattern on both sides of the tropical pacific ocean we submit that this discrepancy could be due to differences in seasonality of planktonic foraminifera g ruber morphotypes and alkenone producing algae

gisp2 oxygen isotope ratios the gisp2 oxygen isotope record with its high resolution detail yields crucial information on past climate change the glacial 18o oscillations of the gisp2 core with their very fast onsets are templates of a prototype oscillation of variable duration with an amplitude of 3 9 the halfway mark of the cold warm transition is reached in 2 years the top is reached in 50 years the 18o time gradient of the leading front is about 7 8 per 100 yr after reaching the top 18o slowly declines by 0 14 per 100 yr the duration of 18o decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones the subsequent 18o downturn during the warm cold transition has a 18o time gradient of 3 2 per 100 yr and lasts about 80 yr

a scaling approximation of equilibrium timescales for sand bed and gravel bed rivers responding to base level lowering the timescale required for a river channel to reach new equilibrium conditions following a disturbance presumably depends on channel substrate size and previous field studies have suggested that gravel bed channels should reach equilibrium more quickly than sand bed channels to estimate the relative timescale difference between sand bed and gravel bed rivers responding to base level lowering we developed a simple scaling approximation equilibrium timescales derived from these equations for sand bed rivers were found to be approximately half of those for gravel bed rivers while gravel bed channels do not need to transport as much sediment to reach equilibrium conditions as sand bed channels transport of sediment is more rapid in sand bed channels leading to quicker equilibration

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

eastern tropical pacific hydrologic changes during the past 27 000 years from d h ratios in alkenones 1 the tropical pacific plays a central role in the climate system by providing large diabatic heating that drives the global atmospheric circulation quantifying the role of the tropics in late pleistocene climate change has been hampered by the paucity of paleoclimate records from this region and the lack of realistic transient climate model simulations covering this period here we present records of hydrogen isotope ratios dd of alkenones from the panama basin off the colombian coast that document hydrologic changes in equatorial south america and the eastern tropical pacific over the past 27 000 years a and the past 3 centuries in detail comparison of alkenone dd values with instrumental records of precipitation over the past 100 a suggests that dd can be used as a hydrologic proxy on long timescales our records indicate reduced rainfall during the last glacial period that can be explained by a southward shift of the mean position of the intertropical convergence zone and an associated reduction of pacific moisture transport into colombia precipitation increases at 17 ka in concert with sea surface temperature sst cooling in the north atlantic and the eastern tropical pacific a regional coupled model forced by negative sst anomalies in the caribbean simulates an intensification of northeasterly trade winds across central america increased evaporative cooling and a band of increased rainfall in the northeastern tropical pacific these results are consistent with the alkenone sst and dd reconstructions that suggest increasing precipitation and sst cooling at the time of heinrich event 1

grip deuterium excess reveals rapid and orbital scale changes in greenland moisture origin the northern hemisphere hydrological cycle is a key factor coupling ice sheets ocean circulation and polar amplification of climate change here we present a northern hemisphere deuterium excess profile covering one climatic cycle constructed with the use of 18 o and d greenland ice core project grip records past changes in greenland source and site temperatures are quantified with precipitation seasonality taken into account the imprint of obliquity is evidenced in the site to source temperature gradient at orbital scale at the millennial time scale grip source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events and these rapid shifts are associated with large scale changes in atmospheric circulation

seasonal changes in the isotopic composition of planktonic foraminifera collected in panama basin sediment traps abstract isotopic analyses have been made on five species of planktonic foraminifera collected in two deployments of parflux mark ii time series sediment traps in the panama basin the automated sampling system on the traps provided 4 one month samples from 29 july to 16 november 1979 and 6 two month samples from december 1979 to november 1980 the 18o values of globigerinoides ruber and globigerinoides sacculifer in this region are primarily affected by a low salinity surface layer that forms during the early winter these species each show a 1 total range in 18o the 18o values of the colder water species globorotalia menardii neogloboquadrina dutertrei and globorotalia theyeri show smaller seasonal changes in 18o the 13c values of g ruber and g sacculifer exhibit small seasonal changes 0 35 and 0 4 respectively despite large seasonal changes in surface water productivity the colder water species exhibit only slightly larger changes in 13c up to 0 55 throughout the year all colder water species exhibit minimum or near minimum 13c values during february and march which is the period of maximum upwelling and primary productivity seasonal variations in the flux of foraminifera in the water column at this location will have only a small effect on the isotopic composition of the sediment assemblage extreme values of 18o and 13c do not occur during the periods which are associated with the high flux of foraminiferal tests

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

wet phases in tropical southern africa during the last glacial period 1 a 45 000 year high resolution sedimentary record from lake masoko in tanzania shows the climate in this part of east africa to have been characterized by a short and less severe dry season during the last glacial maximum moisture and lake level proxies pollen assemblages and magnetic susceptibility indicate that rainfall in the masoko area was strongly controlled by low latitude insolation i e precessional forcing such observations contrast with the climatic patterns previously reconstructed in east africa for the last glacial indeed widespread dry conditions are generally observed and are attributed to the predominant control of high latitude glacial forcing over insolation forcing on the tropical hydrology however during the younger dryas cold event out of phase with regional insolation behaviour wetter conditions prevailed in this area suggesting that the rainfall belt over africa was probably shifted to the south bringing moisture to the southernmost tropics

numerical model for channel flow and morphological change studies in this paper a depth integrated 2d hydrodynamic and sediment transport model cche2d is presented it can be used to study steady and unsteady free surface flow sediment transport and morphological processes in natural rivers the efficient element method is applied to discretize the governing equations and the time marching technique is used for temporal variations the moving boundaries were treated by locating the wet and dry nodes automatically in the cases of simulating unsteady flows with changing free surface elevation in channels with irregular bed and bank topography two eddy viscosity models a depth averaged parabolic model and a depth averaged mixing length model are used as turbulent closures channel morphological changes are computed with considerations of the effects of bed slope and the secondary flow in curved channels physical model data have been used to verify this model with satisfactory results the feasibility studies of simulating morphological formation in meandering chan

bed scouring downstream of hydraulic structures under steady flow conditions experimental analysis of space and time scales and implications for mathematical modeling abstract sediment transport and bed deformation in alluvial rivers are space and time dependent the knowledge of the space and time scales can be helpful for the definition of predictive procedures of an alluvial reach response to changing boundary conditions in this work attention is paid to scouring process occurring downstream of the rigid basement of a hydraulic structure the analysis is focused on transient bed profiles which are determined under steady flow conditions by a decrease of the upstream sediment transport rate in particular the paper is aimed at improving understanding of the space and time scales required by the alluvial system to reach the equilibrium conditions for this purpose movable bed runs are carried out in a straight laboratory channel for two values of the water discharge during each run the longitudinal bed profiles are measured at different times based on experimental data spatial scale factors of the bed scouring and of the bed load transport to reach the equilibrium conditions are defined particularly it is found that the scour length varies especially during the first evolution phase and then it oscillates around of a mean value that is about 40 of the channel reach interested by the phenomenon under consideration an expression is found for the adaptation coefficient which allows the estimation of the spatial delay of the bed load transport rate using both the collected data and literature data an expression defining the time scale of scouring is deducted according to this expression the non dimensional time scale is related to shields parameter through an exponential law then by the help of a 1 d numerical model previously developed by the writer the implications for choosing the spatial and temporal resolutions on which numerical results depend significantly are shortly discussed the results show that the time and space scales which represent physical constraints of the spatial and temporal resolutions need to be properly defined in order to simulate the scouring process evolution

progress in paleosalinity overview and presentation of a new approach this paper concerns the use of stable oxygen isotope ratios 18o and stable hydrogen isotope ratios 2h or d to derive paleosalinity first i evaluate the potential and limitations of existing methods based on 18o and d separately next i propose a new theoretical framework for the combined use of 18o and d to constrain the impact of the hydrological cycle on the surface waters and to thus characterize changes in surface water salinity this new method is accompanied by an error propagation exercise to demonstrate its limitations and areas of potential improvement a feasible strategy is outlined for achieving reconstructions of paleosalinity change with an uncertainty of 1 practical salinity unit or less with the newly proposed method especially in regions with high deuterium excess values

155 000 years of west african monsoon and ocean thermal evolution a detailed reconstruction of west african monsoon hydrology over the past 155 000 years suggests a close linkage to northern high latitude climate oscillations ba ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the gulf of guinea in the eastern equatorial atlantic eea reveal centennial scale variations of riverine freshwater input that are synchronous with northern high latitude stadials and interstadials of the penultimate interglacial and the last deglaciation eea mg ca based sea surface temperatures ssts were decoupled from northern high latitude millennial scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low latitude solar insolation the onset of enhanced monsoon precipitation lags behind the changes in eea ssts by up to 7000 years during glacial interglacial transitions this study demonstrates that the stadial interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the west african monsoon dynamics through an atmospheric linkage

cambrian ordovician boundary beds at tonismagi tallinn north estonia the cambrian ordovician boundary beds at the tonismagi section in tallinn north estonia are represented by siltstones and sandstones with interlayers of clay and kerogenous argillites in this section three conodont zones cordylodus proavus c intermedius and c rotundatus angulatus were established graptolites of the rhabdinopora flabelliforme group were found in the upper part of the c proavus zone this section demonstrates that in suitable facies species of rhabdinopora may appear earlier than usually considered

global patterns of enso induced precipitation although there have been many analyses of el nino southern oscillation enso induced precipitation anomalies global patterns from these analyses remain incomplete here we combine recent satellite estimates of oceanic precipitation and historical rain gauge records to derive a global climatology of enso induced precipitation anomalies using empirical orthogonal function eof analyses the patterns suggest that the re arrangement of convection centers of the walker circulation during enso events induces large precipitation anomalies in the tropics while associated changes in the monsoon systems through the hadley cell over the pacific indian and atlantic oceans and their interactions with midlatitude westerlies generate coherent anomaly patterns over the extratropics our results can be used to evaluate climate models and forecast enso induced precipitation anomalies

a multiproxy assessment of the western equatorial pacific hydrography during the last 30 kyr foraminiferal species assemblages and uk37 0 1 2 c these estimates are consistent with estimates from other wpwp cores thereby suggesting that the discrepancy is due to proxy compatibility rather than differences in cores qualitity postdepositional dissolution above the lysocline might have altered the mg ca based temperature estimates in our site but this effect is insufficient to resolve discrepancies between mg ca in g ruber and the other proxies we suggest that the lower estimates obtained from mg ca in g sacculifer faunal transfer functions and uk37 0 might reflect subsurface temperature changes rather than strict surface estimates accounting for potential artefacts including dissolution and bioturbation we suggest that the glacial wpwp sst was about 2 5 0 7 c cooler than during the holocene whereas the subsurface upper thermocline temperature change was only about 1 8 0 7 c interpreting variations in d 18 osw in terms of salinity changes suggests a possibly slight decrease in surface salinity at the site of md97 2138 during the lgm though lgm freshening in md97 2138 is not robust to postdeposition dissolution effects this inferred freshening appears to be a general feature of the western equatorial pacific

oxygen isotopes upper ocean salinity and precipitation sources in the eastern tropical pacific westward transport of freshwater across the panama isthmus helps to maintain the salinity s contrast between the atlantic and pacific oceans important in global thermohaline circulation and climate relatively low sea surface salinities in the panama bight reflect high net precipitation we infer sources of this freshwater input to the region based on oxygen isotopes 18o of precipitation p and seawater sw the primary trend of the surface water sw s relationship implies a freshwater endmember 18o of 8 5 0 3 significantly lower than the mean p of rainwater we measured in the region 4 8 1 8 mixing with waters upwelled from the subsurface s 34 9 35 1 psu sw 0 3 0 4 based on limited isotope and salinity data these upwelling waters are a mixture of 60 tropical waters and 40 subtropical mode waters three secondary trends in the surface water sw s distribution point to similar freshwater sources with 18o of 2 5 1 2 4 7 0 5 and 4 8 0 6 consistent with local precipitation sources accounting for the primary surface water trend intermittent cross isthmus transport of moist air masses from the caribbean may introduce relatively 18o depleted rainfall due to a combination of recycling and orographic distillation in addition relatively infrequent but large tropical storms which may contain freshwater of both pacific and caribbean origin deliver rainfall with p of 10 to 14 the primary trend of regional surface seawater data which integrates the different sources over time suggests that about half of the total precipitation reflects local sources and half reflects a combination of cross isthmus transport and large storms the regional sw s distribution is sensitive to the relative contributions of these freshwater sources which may change under different climate regimes

on the quantification of the bed development time of alluvial meandering streams this paper concerns the quantitative evaluation of the time of bed development of alluvial meandering streams in agreement with the prevailing approach it is assumed that the stream centerline follows a sine generated curve the banks are rigid the flow is turbulent and subcritical and the flow width is much larger than the flow depth the movable bed is flat at time t 0 at t tb the bed reaches its equilibrium or developed state with the aid of dimensional and physical considerations an expression is found for the duration of bed development tb according to this expression tb is proportional to the square of the flow width b and inversely proportional to the specific volumetric bed load rate corresponding to the channel averaged flow the proportionality factor is found to be a function of the initial deflection angle 0 alone the form of this function is investigated on the basis of a series of experimental runs carried out by the writers in a sine generated channel having an intermediate

experimental constraints on bar migration through bends implications for meander wavelength selection the bed of an initially straight channel often deforms into a series of migrating alternate bars that can cause bank erosion and the development of periodic planform curvature which is often expressed as a meandering channel trace the planform curvature in turn leads to topography similar to alternate bars but stationary with respect to the meander the migration of alternate bars over this steady topography may lead to interactions influencing meander initiation and wavelength selection we studied these interactions in flume channels with bends of various lengths and angles and having various width to depth ratios in channels whose wavelength was equal to the length of bars in straight channels nearly eight channel widths bars migrated through sine generated channels with angles up to 10 the critical bend angle was smaller for channels of longer wavelength and smaller width to depth ratio migration was nonuniform and bars temporarily stalled when in phase with the curvature induced topography leading to amplification of the topography flow against an opposing bank can lead to such a strong boundary shear stress divergence that migration of the bars is prevented because sediment that otherwise would have led to bar migration is scoured from the bar front we propose that the amplification of bar pool topography in bends where free alternate bars are stalled enhances scour of the bank which leads to selection of a meander wavelength equal to that of the alternate bars

variations in tropical convection as an amplifier of global climate change at the millennial scale the global expression of millennial scale climatic change during the glacial period and the persistence of this signal in holocene records point to atmospheric teleconnections as the mechanism propagating rapid climate variations we suggest rearrangements in the tropical convection system globally affected the concentration and location of atmospheric water vapour and modulated terrestrial and marine emissions of ch4 and n2o providing a tropical mechanism of amplifying and perpetuating millennial scale climate changes a multi proxy reconstruction reflecting various aspects of the intensity of the arabian sea summer monsoon shows strong millennial scale variability over the past 90 kyr in which low intensity is associated with a southern shift of the intertropical convergence zone itcz and an eastward shift in the equatorial convergence zone the monsoon reconstruction which is based on new data from a somali margin sediment core is supported by previously reported tropical paleoclimatic records and suggests that global scale millennial climatic variability is in part driven by modulations in the tropical hydrological cycle and tropical emissions of the greenhouse gases ch4 and n2o

high resolution greenland ice core data show abrupt climate change happens in few years the last two abrupt warmings at the onset of our present warm interglacial period interrupted by the younger dryas cooling event were investigated at high temporal resolution from the north greenland ice core project ice core the deuterium excess a proxy of greenland precipitation moisture source switched mode within 1 to 3 years over these transitions and initiated a more gradual change over 50 years of the greenland air temperature as recorded by stable water isotopes the onsets of both abrupt greenland warmings were slightly preceded by decreasing greenland dust deposition reflecting the wetting of asian deserts a northern shift of the intertropical convergence zone could be the trigger of these abrupt shifts of northern hemisphere atmospheric circulation resulting in changes of 2 to 4 kelvin in greenland moisture source temperature from one year to the next

climate impact of late quaternary equatorial pacific sea surface temperature variations magnesium calcium data from planktonic foraminifera in equatorial pacific sediment cores demonstrate that tropical pacific sea surface temperatures ssts were 2 8 0 7 c colder than the present at the last glacial maximum glacial interglacial temperature differences as great as 5 c are observed over the last 450 thousand years changes in sst coincide with changes in antarctic air temperature and precede changes in continental ice volume by about 3 thousand years suggesting that tropical cooling played a major role in driving ice age climate comparison of sst estimates from eastern and western sites indicates that the equatorial pacific zonal sst gradient was similar or somewhat larger during glacial episodes extraction of a salinity proxy from the magnesium calcium and oxygen isotope data indicates that transport of water vapor into the western pacific was enhanced during glacial episodes

prediction of 2 d bed topography in rivers mathematical models are being developed at the delft hydraulics laboratory for the computation of time dependent two dimensional bed deformation in alluvial rivers a most promising verification of the predictive capacity of one such model has been obtained for the waal river in the netherlands this model was developed for rivers with dominant bed load transport without significant grain sorting effects and with approximately constant width also some results of computations with variable boundary conditions are given which simulate the effect of the river regime and the extraction and return of cooling water on the bed topography

modeling bed changes in meandering rivers using triangular finite elements a two dimensional depth averaged model was used for the simulation of scour and deposition in sand bed meandering channels with fixed banks the model employs unstructured meshes based on triangular elements and incorporates the effects of curvature induced helical flow and transverse bed slope in the direction of bed load sediment transport the model was tested using experimental data from a well known laboratory curved channel and a full scale meandering river the numerical results agreed well with observed data demonstrating that the model can reproduce the main features of bed profiles along meandering rivers such as the formation of point bars and pools

itcz rather than enso signature for abrupt climate changes across the tropical pacific latitudinal movements of the intertropical convergence zone itcz analogous to its present day seasonal shifts and el nino southern oscillation enso type variability both potentially impacted rainfall changes at the millennial timescale during the last glacial period in this study we compare tropical pacific sedimentary records of paleoprecipitation to decipher which climate mechanism was responsible for the past rainfall changes we find that latitudinal movements of the itcz are consistent with the observed rainfall patterns challenging the enso hypothesis for explaining the rapid rainfall changes at low latitudes the itcz related mechanism appears to reflect large scale atmospheric rearrangements over the tropical belt with a pronounced heinrich dansgaard oeschger signature this observation is coherent with the simulated tropical rainfall anomalies induced by a weakening of the atlantic thermohaline circulation in modeling experiments c 2009 university of washington published by elsevier inc all rights reserved

the amplitude and phasing of climate change during the last deglaciation in the sulu sea western equatorial pacific 1 variations in tropical sea surface temperature patterns and the phasing relative to climate change in higher latitudes provide insight into the mechanisms of climate change on both orbital and shorter time scales here we present well dated high resolution records of planktonic foraminiferal 18o and mg ca based sst spanning the last deglaciation from the sulu sea located in the western equatorial pacific the results indicate that the last glacial maximum was 2 3 0 5 c cooler than present in the sulu sea with a concomitant decrease in sea surface salinity the similarity between variations in surface salinity in the sulu sea the western and eastern equatorial pacific and the greenland ice core record suggests that the observed changes in salinity reflect large scale rearrangement of atmospheric patterns which were coherent and synchronous throughout the northern hemisphere the results suggest that the glacial equatorial pacific climate was strongly influenced by both tropical and extra tropical forcing although it is not clear whether interannual enso variability is a good analogue of glacial interglacial climate change

mechanisms for millennial scale global synchronization during the last glacial period global climate during the last glacial period was punctuated by abrupt warmings and occasional pulses of freshwater into the north atlantic that disrupted deepwater production these massive freshwater pulses known as heinrich events arose in part from instabilities within the laurentide ice sheet paleoevidence from the north atlantic suggests that these events altered the production of deep water and changed downstream climate throughout the northern hemisphere in the tropical western pacific sea surface temperatures and salinity varied together with ocean and climate changes at high latitudes here we present results from coupled modeling experiments that shed light on a possible dynamical link between the north atlantic ocean and the western tropical pacific this link involves a global oceanic standing wave pattern brought about by millennial scale glacial density variations in the north atlantic atmospheric teleconnections triggered by meridional sea surface temperature gradients and local air sea interactions furthermore our modeling results are compared with hydrological records from the cariaco basin the indian ocean the sulu sea and northern australia

millennial scale trends in west pacific warm pool hydrology since the last glacial maximum models and palaeoclimate data suggest that the tropical pacific climate system plays a key part in the mechanisms underlying orbital scale and abrupt climate change atmospheric convection over the western tropical pacific is a major source of heat and moisture to extratropical regions and may therefore influence the global climate response to a variety of forcing factors the response of tropical pacific convection to changes in global climate boundary conditions abrupt climate changes and radiative forcing remains uncertain however here we present three absolutely dated oxygen isotope records from stalagmites in northern borneo that reflect changes in west pacific warm pool hydrology over the past 27 000 years our results suggest that convection over the western tropical pacific weakened 18 000 20 000 years ago as tropical pacific and antarctic temperatures began to rise during the early stages of deglaciation convective activity as inferred from oxygen isotopes reached a minimum during heinrich event 1 ref 10 when the atlantic meridional overturning circulation was weak pointing to feedbacks between the strength of the overturning circulation and tropical pacific hydrology there is no evidence of the younger dryas event in the stalagmite records however suggesting that different mechanisms operated during these two abrupt deglacial climate events during the holocene epoch convective activity appears to track changes in spring and autumn insolation highlighting the sensitivity of tropical pacific convection to external radiative forcing together these findings demonstrate that the tropical pacific hydrological cycle is sensitive to high latitude climate processes in both hemispheres as well as to external radiative forcing and that it may have a central role in abrupt climate change events

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

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

rate of growth and other features of the temporal development of pool bar complexes in meandering streams despite substantial research on various aspects of bed deformation in meandering streams no systematic effort has yet been made to establish the timescales of the process additionally in past experiments as a rule the bed was not monitored throughout the development which has left undocumented essential features of the growth of meandering pool bar complexes in this paper an effort is made to penetrate some of these aspects more specifically the five laboratory runs presented by the writers in their 2009 paper in this journal are revisited with the following two objectives 1 to reveal the rate of growth of the resulting meandering pool bar complexes and 2 to document the extent if any of upstream or downstream migration of developing pool bar complexes in examples of sufficiently wide streams in which the bed deformation is primarily because of the convective behavior of flow the runs were conducted in a 70 sine generated channel having rigid banks in each run the bed was allowed to

time scale for local scour at bridge piers the temporal development of clear water local scour depth at cylindrical bridge piers in uniform sand beds is considered new data are presented and used to quantify the influence of flow duration on the depth of local scour an equilibrium time scale t is defined the data show that both t and the equilibrium scour depth dse are subject to similar influences of flow and sediment parameters as might be expected because they are inherently interdependent a method is given for determination of the time for development of dse for a given pier sediment and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole the results show that the scour depth after 10 of the time to equilibrium is between about 50 and 80 of the equilibrium scour depth depending on the approach flow velocity of local scour d se is rapidly attained in live bed conditions but rather more slowly in clear water conditions fig 1 clear water scour occurs for mean flow velocities up to the threshold velocity for bed sediment entrainment i e v vc while live bed scour occurs for v vc the maximum equilibrium scour depth dse max occurs at v vc in armored cobble or cohesive sediment bed streams multiple flood events may be required before the maximum clear water scour is reached this may take many years the equilibrium scour depth in live bed con ditions fluctuates due to the effects of bed form migration the dashed lines in fig 1 represent the temporal average scour depth under live bed conditions the diagram also shows the time taken te for the equilibrium scour depth to develop the equilibrium time te is the focus of this paper it increases rapidly with flow velocity under clear water conditions but then decreases rapidly for live bed scour existing equations for depth of local scour at bridge piers give the equilibrium depth and are therefore conservative re garding temporal effects for the live bed conditions that typ ically pertain in floods equilibrium scour depths are appro priate however where clear water scour conditions exist the equilibrium depth of scour may be overly conservative peak flood flows may last only a number of hours or a few days in the field and short floods have insufficient time to generate equilibrium depths for example bridge piers situated on the floodplain may be wet for periods of less than one day during a flood typically clear water conditions pertain at such sites the actual scour may be only a small fraction of the equilib rium scour depth which could take weeks to fully develop johnson and mccuen 1991 developed an analytical model to simulate the temporal process of local scour at piers the model was applied to a hypothetical bridge pier using a gen erated sequence of flood flows over a 75 year period the in dividual storms being of 24 h duration at the end of the 75 year period the scour depth was still increasing

synchronous tropical south china sea sst change and greenland warming during deglaciation the tropical ocean plays a major role in global climate it is therefore crucial to establish the precise phase between tropical and high latitude climate variability during past abrupt climate events in order to gain insight into the mechanisms of global climate change here we present alkenone sea surface temperature sst records from the tropical south china sea that show an abrupt temperature increase of at least 1 c at the end of the last glacial period within the recognized dating uncertainties this sst increase is synchronous with the bolling warming observed at 14 6 thousand years ago in the greenland ice sheet project 2 ice core

possible evidence for wet heinrich phases in tropical ne australia the lynch s crater deposit unarguably one of the most significant paleoclimatological discoveries of the last two decades has been that of abrupt climate events dansgaard oeschger cycles and heinrich events most evidence for these events has originated from the high latitude northern hemisphere with few records documenting the response of the low latitude southern hemisphere here we present new data from lynch s crater a unique terrestrial record from ne australia that may show evidence for southward propagations of the intertropical convergence zone itcz during abrupt climate perturbations as a result of alteration of the low latitude air masses proxies for precipitation wetness indicate enhanced rainfall in the region during heinrich events h events 1 3 and the 8 2 ka northern hemisphere cold event a fully coupled atmosphere ocean climate model simulating a 1 sv freshwater influx to the north atlantic ocean produces a scenario which agrees with the climate changes shown by the lynch s crater record the model shows precipitation anomalies that include a southward migration of the itcz and a zonal shift in mid latitude storm tracks over the southern hemisphere equatorial region these data indicate large scale shifts of the austral summer itcz position that is known to control monsoonal precipitation in ne australia this terrestrial record from australia may demonstrate the involvement of the tropical western pacific ocean in itcz migrations during abrupt climate events of the last glacial period defining such past migrations offers insight into the importance and role of the equatorial region in global climate dynamics

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

seawater isotope constraints on tropical hydrology during the holocene 1 paleoceanographic data from the low latitude pacific ocean provides evidence of changes in the freshwater budget and redistribution of freshwater within the basin during the holocene reconstructed holocene seawater 18o changes compare favorably to differences predicted between climate simulations for the middle holocene mh and for the pre industrial late holocene lh the model simulations demonstrate that changes in the tropical hydrologic cycle affect the relationship between 18osw and surface salinity and allow for the first time quantitative estimates of western pacific salinity change during the holocene the simulations suggest that during the mh the mean salinity of the pacific was higher because less water vapor was transported from the atlantic ocean and more was transported to the indian ocean the salinity of the western pacific was enhanced further due both to the greater advection of salt to the region by ocean currents and to an increase in continental precipitation at the expense of maritime precipitation the latter a consequence of the stronger asian summer monsoon

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

east asian monsoon forcing of suborbital variability in the sulu sea during marine isotope stage 3 link to northern hemisphere climate 1 we have generated a new high resolution record of variations in planktonic foraminiferal oxygen isotopes 18o and mg ca from a sediment core images 97 2141 in the sulu sea located in the philippine archipelago of western tropical pacific this record reveals distinct suborbital scale 18o changes most notably during marine isotope stage 3 mis3 30 000 to 60 000 years b p the amplitudes of these 18o fluctuations 0 4 to 0 7 exceed that which can be attributed to sea level changes and must be due to changes in sea surface conditions in the same interval variations in planktonic foraminifera mg ca suggest that suborbital surface ocean temperature variations of 1 to 1 5 c in the sulu sea were not in phase with 18o combined this evidence indicates that the mis3 millennial 18o events in the sulu sea were primarily the result of changes in surface water salinity which today is directly related to the east asian monsoon eam and its influence on the balance between surface water contributions from the south china sea and western pacific warm pool wpwp within dating uncertainties the mis3 sulu sea 18o suborbital variability indicates that times of fresher surface conditions in the sulu sea coincide with similar conditions in the wpwp stott et al 2002 and also with intensifications of the summer eam as recorded in the u th dated chinese hulu cave speleothem 18o record wang et al 2001 and thus by inference with interstadials in the greenland ice core records combined these results indicate that pronounced suborbital variability in the summer eam and intertropical convergence zone itcz during mis3 was tightly coupled with climate conditions in the northern high latitudes