el nino la nina and sahel precipitation during the middle holocene simulations with a synchronously coupled atmo sphere land ocean sea ice model indicate el nino la nina like events in the eastern tropical pacific ocean during the middle holocene 6 ka with similar intensities and frequencies as at present 0 ka july august september sahel precipitation shifts northward as expected from proxy data for present day interannual decadal variability of western sahel precipi tation is correlated with both pacific el nino la nina sea sur face temperature sst anomalies and the atlantic dipole teleconnections at 6 ka between sahel precipitation and el nino la nina sst anomalies are absent with tropical atlantic ssts asserting a dominant influence these results illustrate potential problems with using present day teleconnection patterns in interpreting past climate variability from proxy data

south atlantic multidecadal variability in the climate system model abstract strong multidecadal variability is detected in a 300 yr integration of the ncar climate system model in the south atlantic region through the application of two signal recognition techniques the multitaper method and singular spectrum analysis significant oscillations of a 25 30 yr period are found in the sea surface temperature sea level pressure and barotropic transport fields a similar scale signal is also captured in about one century long observational records a composite analysis of several model variables is performed based on the extremes of the sea surface temperature oscillation the proposed mechanism for this basin scale multidecadal signal involves changes in the intensity of the westerlies associated with variability in the southward extension of the subtropical anticyclone which drives changes in the ocean mass transport this results in variability in the intensity of the malvinas western boundary current and in the position of the brazil malvinas confluence zone anomalo

development of a sea ice model for use in zonally averaged energy balance climate models abstract a sea ice model for use in zonally averaged energy balance climate models is presented which includes the following processes surface melting basal freezing and melting lateral melting from ice flee water or growth of new ice in leads snowfall and the formation of white ice ice advection and a parameterized ice and snow thickness distribution which represents the effects of small scale dynamics the ice growth equations of hibler are solved analytically thereby permitting a gradual increase in zonal ice fraction in fall and winter both lateral and vertical melting lead to a continuous decrease of ice fraction during ice decay the correlation between ice thickness and ice thickness sensitivity to the upward heat flux at the ice base is of opposite sign seasonally and latitudinally the parameterized feedback between ice thickness and the minimum permitted lead fraction is found to be very important to the ice simulation and is a process which needs to be studied using higher resolution

pore water pressure and streambank stability results from a monitoring site on the sieve river italy to investigate the role of pore water pressures in the stability of a streambank a series of tensiometers and piezometers was installed in a bank of the sieve river tuscany italy fluvial entrainment at the bank toe was monitored by repeated cross profiling erosion pins and marked pebbles fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall evapotranspiration rising and drawdown of the river stage and variations in water table an expression was derived for the safety factor with respect to mass movement of the upper bank incorporating the failure criterion for unsaturated soils and the normal mohr coulomb criterion for saturated conditions variations in matric suction have important effects on the stability of the streambank during low flow periods the shear strength term due to the matric suction allows the bank to remain stable at a steep angle however during rainfall and flow events reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigger a mass failure without development of significant positive pore water pressures during the rising limb of high flow events the factor of safety increases as a consequence of the stabilizing confining pressure of the water in the river despite a reduction in matric suction during drawdown in the river when the suction values are still low and the confining pressure in the river decreases to zero the factor of safety falls to lower values than those experienced prior to the runoff event measurements of fluvial entrainment reveal that although the processes mechanisms and the frequency of retreat of basal and upper bank zones differ significantly the amount of retreat at the bank toe due to fluvial erosion is comparable to that of the upper portion of the bank due to mass failure copyright 1999 john wiley sons ltd

stability of composite river banks the stability of a river bank depends on the balance of forces motive and resistive associated with the most critical mechanism of failure many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size geometry and structure of the bank the engineering properties of the bank material the hydraulics of flow in the adjacent channel and climatic conditions rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt clay bank erosion occurs by fluvial entrainment of material from the lower cohesionless bank at a much higher rate than material from the upper cohesive bank this leads to undermining that produces cantilevers of cohesive material upper bank retreat takes place predominantly by the failure of these cantilevers three mechanisms of failure have been identified shear beam and tensile failure the stability of a cantilever may be analysed using static equilibrium and beam theory and dimensionless charts for cantilever stability constructed application of the charts requires only a few simple measurements of cantilever geometry and soil properties in this analysis the effects of cracks and fissures in the soil must be taken into account these cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface following mechanical failure blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue hence the rate of bank retreat is fluvially controlled even though the mechanism of failure of the upper bank is not directly fluvial in nature this cycle of bank erosion undermining cantilever failure and fluvial scour of the toe operates over several flood events and has important implications for river engineering channel changes and the movement of sediment through fluvial systems

erosion of biofilm bound fluvial sediments sediment grains in rivers are often bound together and stabilized by bacterial films experiments and mathematical models show that sediments bound by biofilms behave like a single elastic membrane that can rip catastrophically if the river flows fast enough

bank erosion processes in regulated navigable rivers vessel induced waves affect the morphology and ecology of banks and shorelines around the world in rivers used as waterways ship passages contribute to the erosion of unprotected banks but their short and long term impacts remain unclear this work investigates the effects of navigation on bank erosion along a reach of the regulated meuse river with recently renaturalized banks we apply uav sfm photogrammetry rtk gps acoustic doppler velocimetry aerial and terrestrial photography soil tests and multibeam echosounding to analyze the progression of bank retreat after riprap removal after having analyzed the effects of ship generated waves and currents floods and vegetation dynamics a process based model is proposed to estimate the long term bank retreat the results show that a terrace evolves in length and depth across the bank according to local lithology which we clustered in three types floods contribute to upper bank erosion inducing mass failures while near bank flow appears increasingly ineffective to remove the failed material due to terrace elongation vegetation growth at the upper bank toe reduces bank failure and delays erosion but its permanence is limited by terrace stability and efficiency to dissipate waves the results also indicate that long term bank retreat is controlled by deep primary waves acting like bores over the terrace understanding the underlying drivers of bank evolution can support process based management to optimize the benefits of structural and functional diversity in navigable rivers

effect of ocean gateways on the global ocean circulation in the late oligocene and early miocene 1 we investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns using a fully coupled climate model in an idealized setup we compare situations corresponding to the late oligocene the early miocene and present day the model results show the existence of a flow reversal through the panama seaway between the oligocene and miocene this flow reversal is induced by global tectonic changes related to the widening of the southern ocean passages and the closing of the tethys seaway it mainly involves the wind driven ocean circulation in agreement with previous model studies the global thermohaline circulation in the oligocene and miocene simulations is significantly different from the present day conveyor circulation as there is deepwater formation in both the north atlantic and the north pacific oceans in particular in the oligocene simulation the salinity contrast between the atlantic and pacific oceans is reduced because of water mass exchange through the low latitude connections between the two oceans

neogene volcanism at the front of the central mexican volcanic belt basaltic andesites to dacites with contemporaneous shoshonites and high tio2 lava as part of the continuing study of the young mexican volcanic belt designed to document the ages and types of volcanism from the gulf of california to the valley of mexico a 50 km wide segment of the central part of the belt has been mapped and five types of lava have been found pliocene 3 77 ma shoshonite lava flows k 2 o 2 wt sio 2 52 58 wt form eroded plateaus more than 50 km behind the present volcanic front but in the past 1 m y shoshonites have erupted closer to the volcanic front 300 km from the middle america trench the shoshonite lava type is the most enriched in ba sr and zr of the suite and plagioclase phenocrysts are absent presumably because of high contents of dissolved water 3 5 wt quaternary shield volcanoes and several cinder cones with small volume lava flows are composed of high tio 2 lavas 1 2 wt which have 51 57 wt sio 2 9 ppm and zr 160 230 ppm pliocene high tio 2 lava is found within the eroded plateaus located 50 km behind the present volcanic front quaternary basaltic andesite 52 57 wt sio 2 with up to 10 wt mgo is found at the volcanic front along with more siliceous andesite 57 63 wt sio 2 representatives of the siliceous andesites 57 63 wt sio 2 are free of plagioclase phenocrysts low in al 2 o 3 15 7 wt but rich in mgo 5 wt experiments reported elsewhere suggest that the magmas contained 3 7 wt dissolved water this andesite erupted along a normal fault between 0 3 and 0 005 ma and it is associated with dacite similarly lacking plagioclase phenocrysts but having comparatively abundant pyroxene other dacite in the zitacuaro area is richly porphyritic having plagioclase and hornblende this dacite forms clusters of steep sided domes or widespread pyroclastic deposits and the latest eruptions dated by radiocarbon and k ar occurred between 0 03 0 05 ma estimates of the volume of magma erupted in the zitacuaro valle de bravo region in the past 1 m y 1 8 km 3 m y 1 km 1 show that this area is somewhat less productive per 1 km of arc than those to the west 3 5 km 3 m y 1 km 1 in the michoacan guanajuato volcanic field mgvf a large volume of dacite has erupted in the zitacuaro valle de bravo region which is rare in the mgvf the cone density in the zitacuaro valle de bravo region 2 1 100 km 2 is only slightly lower than the 2 6 cones 100 km 2 found in the mgvf

changes in the critical erosion velocity for sediment colonized by biofilm in flowing water the incipient motion of sediment can be affected by the presence of microbial biofilm growth this article documents a series of flume experiments using non uniform sediments in which sediment entrainment was investigated for cases where the sediment was immersed in deionized water so that no biofilm developed and for cases where a bio sediment was cultivated by placing the sediment in a mixture of natural water and nutrient solution differences in entrainment and the velocity at incipient motion were measured over an eight week period as biofilm grew it was found that the incipient motion phenomena were quite distinct between the two kinds of sediment sediment with biofilm was more stable and over time incipient velocity increased to a threshold level before declining biofilm development is clearly an important control on the stability of sediments especially in eutrophic water bodies two incipient velocity formulas were derived for sliding and rolling conditions film water theory was utilized to describe the cohesive force between sediment particles and the adhesive force generated by biofilm was introduced into the formula derivation the time variation characteristics of biofilm strength and the features of the substrate were also taken into consideration such analyses can help to predict sediment transport changes due to biofilm presence in nutrient rich water bodies

a mechanism of decadal variability of the sea ice volume in the northern hemisphere a long term simulation performed with a coarse resolution global atmosphere ocean sea ice model displays strong decadal variability of the sea ice volume in the northern hemisphere with a significant peak at about 15 18 years this model results from the coupling of ecbilt a spectral t21 3 level quasi geostrophic atmospheric model and clio a sea ice ocean general circulation model first the mechanism underlying the variability of ice volume in the model was studied by performing correlation analyses between the simulated variables in a second step a series of additional sensitivity experiments was performed in order to illustrate the role of specific physical processes this as allowed us to identify a feedback loop in the ice ocean system which proceeds as follows an increase in at tic sea ice volume induces an increase in the salinity there this salinity anomaly is transported to the greenland sea where it promotes convective activity this warms up the surface oceanic layer and the atmosphere in winter and induces a decrease of the ice volume completing half a cycle the changes in ice volume are driven by a geopotential height pattern characterised by centres of action of opposite signs over greenland and the barents kara central arctic area thermodynamic feedback between the ice and the atmosphere appear also to be very important for the persistence of the oscillation the dynamical response of the atmosphere to sea ice and temperature anomalies at surface plays a smaller role

the asian australian monsoon and el nino southern oscillation in the ncar climate system model features associated with the asian australian monsoon system and el nino southern oscillation enso are described in the national center for atmospheric research ncar global coupled climate system model csm simulation characteristics are compared with a version of the atmospheric component of the csm the ncar ccm3 run with time evolving ssts from 1950 to 1994 and with observations the csm is shown to represent most major features of the monsoon system in terms of mean climatology interannual variability and connections to the tropical pacific this includes a representation of the southern oscillation links between strong asian australian monsoons and associated negative sst anomalies in the eastern equatorial pacific the equatorial sst gradient across the pacific in the csm is shown to be similar to the observed with somewhat cooler mean ssts across the entire pacific by about 18 28c the seasonal cycle of ssts in the eastern equatorial pacific has the characteristic signature seen in the observations of relatively warmer ssts propagating westward in the first half of the year followed by the reestablishment of the cold tongue with relatively colder ssts propagating westward in the second half of the year like other global coupled models the propagation is similar to the observed but with the establishment of the relatively warmer water in the first half of the year occurring about 1 2 months later than observed the seasonal cycle of precipitation in the tropical eastern pacific is also similar to other global coupled models in that there is a tendency for a stronger than observed double itcz year round particularly in northern spring but with a well reproduced annual maximum of itcz strength north of the equator in the second half of the year time series of area averaged ssts for the nino3 region in the eastern equatorial pacific show that the csm is producing about 60 of the amplitude of the observed variability in that region consistent with most other global coupled models global correlations between nino3 time series global surface temperatures and sea level pressure slp show that the csm qualitatively reproduces the major spatial patterns associated with the southern oscillation lower slp in the central and eastern tropical pacific when nino3 ssts are relatively warmer and higher slp over the far western pacific and indian oceans with colder water in the northwest and southwest pacific indices of asian australian monsoon strength are negatively correlated with nino3 ssts as in the observations spectra of time series of indian monsoon australian monsoon and nino3 sst indices from the csm show amplitude peaks in the southern oscillation and tropospheric biennial oscillation frequencies 3 6 yr and about 2 3 yr respectively as observed lag correlations between the nino3 sst index and upper ocean heat content along the equator show eastward propagation of heat content anomalies with a phase speed of about 0 3 m s21 compared to observed values of roughly 0 2 m s21 composites of el nino la nina events in the csm show similar seasonal evolution to composites of observed events with warming cooling of greater than several tenths of a degree beginning early in northern spring of year 0 and diminishing around northern spring of year 11 but with a secondary resurgence in the csm events later in northern spring of year 11 the csm also shows the largest amplitude enso sst and low level wind anomalies in the western tropical pacific with enhanced interannual variability of ssts extending northeastward and southeastward toward the subtropics compared to largest interannual sst variability in the central and eastern tropical pacific in the observations

effects of rheology and ice thickness distribution in a dynamic thermodynamic sea ice model realistic treatment of sea ice processes in general circulation models is needed to simulate properly global climate and climate change scenarios as new sea ice treatments become available it is necessary to evaluate them in terms of their accuracy and computational time here several dynamic ice models are compared using both a 2 category and 28 category ice thickness distribution simulations are conducted under normal wind forcing as well as under increased and decreased wind speeds it is found that the lack of a shear strength parameterization in the cavitating fluid rheology produces significantly different results in both ice thickness and ice velocity than those produced by an elliptical rheology furthermore use of a 28 category ice thickness distribution amplifies differences in the responses of the various models while the choice of dynamic model is governed by requirements of accuracy and implementation it appears that in terms of both parameterization of physical properties and computational time the elliptical rheology is well suited for inclusion in a gcm

last glacial maximum in south america paleoclimate proxies and model results 1 observations in south america based on paleoenvironmental data for the last glacial maximum lgm that shows a generally lower temperatures and reduced precipitation of the region are compared to simulation results from the paleoclimate version of the national center for atmospheric research coupled climate system model analyses of the lgm wind simulation for the tropical atlantic show that the convergence zone does not extend all the way into south american continent during the austral summer this would have prevented moisture inflow into the adjacent continental area of equatorial northeastern brazil interpretations of paleoclimate proxy records from this region are consistent with this scenario in the subtropics lgm westerlies were weaker and data show characteristics of more humid conditions at higher latitudes model simulations suggest intensification of the westerlies which in combination with colder sea surface temperatures would imply less moisture influx into the south american continent from the atlantic sector low lake levels in high southern latitudes and general aridity would support the model findings

river width adjustment i processes and mechanisms in 1993 a task committee tc of the asce was established to study the hydraulics bank mechanics and modeling of width adjustment in alluvial channels the work of the tc in reviewing width adjustment processes and mechanisms is reported in this paper a companion paper presents the findings of the tc with regard to width adjustment modeling this paper first establishes the geomorphic context within which width adjustments occur and it demonstrates that width adjustment may take place over a wide range of scales in time and space in the past engineering analyses of channel width have tended to concentrate on prediction of the equilibrium width for stable channels most commonly the regime extremal hypothesis and rational mechanistic approaches are used and these are reviewed herein more recently attention has switched to channels that are adjusting their morphology either due to natural instability or in response to changes in watershed land use river regulation or channel engineering chara

the swash of solitary waves on a plane beach flow evolution bed shear stress and run up the swash of solitary waves on a plane beach is studied using large scale experiments ten wave cases are examined which range from non breaking waves to plunging breakers the focus of this study is on the influence of breaker type on flow evolution spatiotemporal variations of bed shear stresses and run up measurements are made of the local water depths flow velocities and bed shear stresses using a shear plate sensor at various locations in the swash zone the bed shear stress is significant near the tip of the swash during uprush and in the shallow flow during the later stages of downrush in between the flow evolution is dominated by gravity and follows an explicit solution to the nonlinear shallow water equations i e the flow due to a dam break on a slope the controlling scale of the flow evolution is the initial velocity of the shoreline immediately following waveform collapse which can be predicted by measurements of wave height prior to breaking but also shows an additional dependence on breaker type the maximum onshore directed bed shear stress increases significantly onshore of the stillwater shoreline for non breaking waves and onshore of the waveform collapse point for breaking waves a new normalization for the bed shear stress which uses the initial shoreline velocity is presented under this normalization the variation of the maximum magnitudes of the bed shear stress with distance along the beach which is normalized using the run up follows the same trend for different breaker types for the uprush the maximum dimensionless bed shear stress is approximately 0 01 whereas for the downrush it is approximately 0 002

global monsoons in the mid holocene and oceanic feedback the response of the six major summer monsoon systems the north american monsoon the northern africa monsoon the asia monsoon the northern australasian monsoon the south america monsoon and the southern africa monsoon to mid holocene orbital forcing has been investigated using a coupled ocean atmosphere general circulation model foam with the focus on the distinct roles of the direct insolation forcing and oceanic feedback the simulation result is also found to compare well with the ncar csm the direct effects of the change in insolation produce an enhancement of the northern hemisphere monsoons and a reduction of the southern hemisphere monsoons ocean feedbacks produce a further enhancement of the northern africa monsoon and the north american monsoon however ocean feedbacks appear to weaken the asia monsoon although the overall effect direct insolation forcing plus ocean feedback remains a strengthened monsoon the impact of ocean feedbacks on the south american and southern african monsoons is relatively small and therefore these regions especially the south america experienced a reduced monsoon regime compared to present however there is a strong ocean feedback on the northern australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region a new synthesis is made for mid holocene paleoenvironmental records and is compared with the model simulations overall model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations

an estimate of the mean field of arctic sea ice motion the observational basis for our understanding of sea ice motion comes from the trajectories of particular ice floes these ice floes may have been marked by a manned research station or more recently by an automatic data buoy since 1893 almost 100 station years of drift data have been recorded using the principles of optimal linear estimation these data are used to obtain a quantitative estimate of the field of mean motion

a climatology of sea ice embayments in the cosmonaut sea antarctica 1 the cosmonaut sea off the coast of antarctica is characterized by highly variable sea ice conditions as the sea ice expands during the austral fall and winter embayments open bodies of water surrounded on three sides by sea ice as well as polynyas often form using sea ice concentration data provided by satellite based passive microwave instruments from 1979 to 2004 we characterize these embayment events by their size and temporal evolution the data suggests a striking periodicity in the occurrence of these embayments with the exception of a quiet period between 1984 and 1987 the large embayment events seem to appear approximately every three years in the same location and persist for anywhere between mid june to mid august these results indicate there is predictability in the occurrence of embayment events adding important insight into the characterization of sea ice variability in this region

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

a self limiting bank erosion mechanism inferring temporal variations in bank form and skin drag from high resolution topographic data fluvial bank erosion rates are often quantified by assuming that the erosion rate is a function of the excess above a critical threshold boundary shear stress applied by the flow research has shown that the form roughness induced by natural topographic bank features such as slumps spurs and embayments is the dominant component of the spatially averaged total shear stress meaning that form roughness provides an important control on bank erosion rates however measuring the relative components of the total shear stress for a natural system is not straightforward in this paper we employ the method of kean and smith 2006a b to partition the form and skin drag components of river bank roughness using a time series 2005 2011 of high resolution topographic surveys of an eroding bank of the cecina river in central italy this method approximates the form drag component of the roughness along a longitudinal bank profile as a series of user defined gaussian curves the extracted metrics are used in conjunction with an estimate of the outer region flow velocity to partition the form and skin drag components of the total boundary shear stress according to the kean and smith analytical solution the relative magnitude of the form and skin shear stress at each survey date is analysed alongside dems of difference to reveal that intense episodes of erosion are followed by periods of quiescence we show that this is due to the protection offered by increased form drag roughness following erosion we conceptualise the dynamic feedbacks that exist between river discharge bank erosion processes and bank form roughness into a simple model of the self limiting nature of river bank erosion

oceanic wave dynamics and interdecadal variability in a climate system model western boundary disturbances detected in the first multicentury integration of the national center for atmospheric research climate system model are analyzed and the role they play in the model interdecadal variability is investigated the boundary signals propagate southwestward along the coast of north america at depth they reach the equator and propagate along the equator but with decreasing amplitude away from the western boundary their southwestward propagation is associated with the evolution of the western boundary current system the gulf stream system in the upper part of the water column and the deep western boundary current at depth from weaker than average to stronger than average conditions thus the boundary disturbances seem to play an important role in the model thermohaline circulation variability at interdecadal timescales they show similarities with the boundary waves found in ocean only models exhibiting interdecadal variability and their role in the model s response to changes in the rate of sinking appears to be in agreement with the theory of the spin up of the deep ocean circulation the main result of this study is that the similarities with ocean only results and theories support the idea that interdecadal timescales of variability originate from the ocean dynamics

sea ice response to wind forcing from amip models abstract the arctic surface circulation simulated by atmospheric general circulation models is assessed in the context of driving sea ice motion a sea ice model is forced by geostrophic winds from eight models participating in the first atmospheric model intercomparison project amip1 and the results are compared to simulations with the sea ice model forced by observed winds the mean sea level pressure in the amip models is generally too high over the arctic ocean except in the beaufort and chukchi seas where it is too low this pattern creates anomalous winds that tend to transport too much ice away from the coast of greenland and the canadian archipalego and into the east siberian sea producing a pattern of ice thickness in the arctic that is rotated by roughly 180 relative to what is expected based on observations amip winds also drive too little ice transport through fram strait and too much transport east of svalbard by way of the barents sea these errors in ice thickness and transport inf

tropical pacific variability in the ncar climate system model a 300 yr simulation with the ncar climate system model csm version 1 captured only 60 of the observed enso signal and exaggerated the interannual variability of sst in the western tropical pacific here a simulation with a new version of the csm which significantly improves the spatial and temporal patterns of tropical pacific variability is described maximum sst variability is shifted to the central and eastern pacific a better simulation of the equatorial pacific thermocline structure results in nino 3 and nino 4 statistics com parable to the observed estimates for the last century the evolution of sst and subsurface temperature anomalies is in excellent agreement with observed events the majority of events evolve as a standing mode with weak sst anomalies occurring in the northern spring in the eastern tropical pacific and maximum anomalies covering the eastern tropical pacific ocean to the date line by the following northern winter at the same time subsurface temperature anomalies spread eastward and upward along the tropical thermocline the delayed oscillator and wyrtki s buildup hypothesis are consistent with aspects of the csm simulation on the equator a westerly wind stress anomaly in the central pacific forces off equatorial upwelling anomalies which propagate westward reaching the western boundary about one half year later this upwelling signal then propagates eastward along the equator arriving 2 months before cooling in the eastern pacific basin the tropical pacific atmospheric response to warm oceanic events also agrees with observational analyses with a negative southern oscillation pattern in sea level pressure wind stress anomalies and low level convergence to the west of the maximum sst anomalies and enhanced deep convection and precipitation in the central and eastern tropical pacific

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

sea ice and polar climate in the ncar csm the climate system model csm consists of atmosphere ocean land and sea ice components linked by a flux coupler which computes fluxes of energy and momentum between components the sea ice component consists of a thermodynamic formulation for ice snow and leads within the ice pack and ice dynamics using the cavitating fluid ice rheology which allows for the compressive strength of ice but ignores shear viscosity the results of a 300 yr climate simulation are presented with the focus on sea ice and the atmospheric forcing over sea ice in the polar regions the atmospheric model results are compared to analyses from the european centre for medium range weather forecasts and other observational sources the sea ice concentrations and velocities are compared to satellite observational data the atmospheric sea level pressure slp in csm exhibits a high in the central arctic displaced poleward from the observed beaufort high the southern hemisphere slp over sea ice is generally 5 mb lower than observed air temperatures over sea ice in both hemispheres exhibit cold biases of 2 4 k the precipitationminus evaporation fields in both hemispheres are greatly improved over those from earlier versions of the atmospheric gcm the simulated ice covered area is close to observations in the southern hemisphere but too large in the northern hemisphere the ice concentration fields show that the ice cover is too extensive in the north pacific and subarctic north atlantic oceans the interannual variability of the ice area is similar to observations in both hemispheres the ice thickness pattern in the antarctic is realistic but generally too thin away from the continent the maximum thickness in the arctic occurs against the bering strait not against the canadian archipelago as observed the ice velocities are stronger than observed in both hemispheres with a consistently greater turning angle to the left in the southern hemisphere they produce a northward ice transport in the southern hemisphere that is 3 4 times the satellite derived value sensitivity tests with the sea ice component show that both the pattern of wind forcing in csm and the air ice drag parameter used contribute to the biases in thickness drift speeds and transport plans for further development of the ice model to incorporate a viscousplastic ice rheology are presented in spite of the biases of the sea ice simulation the 300 yr climate simulation exhibits only a small degree of drift in the surface climate without the use of flux adjustment this suggests a robust stability in the simulated climate in the presence of significant variability

sensitivity of sea ice to physical parameterizations in the giss global climate model 1 the giss coupled model is used to investigate the sensitivity of sea ice to each of the following parameterizations 1 two sea ice dynamics cf cavitating fluid vp viscous plastic 2 the specification of oceanic isopycnal mixing coefficients in the gent and mcwillams isopyncal mixing gm and 3 the wajsowicz viscosity diffusion wv the large scale sea ice properties are highly sensitive to sea ice dynamics with the inclusion of resistance to shear stress vp captures the major observed sea ice drift features and improves the simulations of sea ice concentrations thickness and export through fram strait relative to cf gm significantly improves the simulation of vertical temperature distributions in the southern ocean although it leads to a dramatic reduction of antarctic sea ice cover the reduced oceanic isopycnal mixing coefficients lead to arctic sea ice that tends to be less and thinner in almost the entire arctic except in the north pacific and labrador sea while antarctic sea ice that extends more equatorward throughout the circumpolar regions the responses of sea ice to wv show an enlargement and thickening of sea ice in the arctic within the ice packs around the antarctic and a reduction and thinning of sea ice in the northern weddell and ross seas on the basis of these experiments two composite experiments with the best parameterizations are investigated the atmospheric responses associated with sea ice changes are discussed while improvements are seen there are still many unrealistic aspects that will require further improvements to sea ice and ocean components

on the improvement of sea ice models for climate simulations the sea ice model intercomparison project experiments with dynamic thermodynamic sea ice models indicate a strong dependence of the net freezing rate sea ice transport and variability on dynamic model parameters although current dynamic thermodynamic sea ice models show relatively good agreement with observations an optimization seems to be necessary especially for the parameterizations of dynamic processes presently only a few coupled climate models use dynamic thermodynamic sea ice models in order to promote by means of coordinated numerical experiments the development of an optimal sea ice model for climate research the sea ice ocean modelling panel of the arctic climate system study acsys a project of the world climate research programme has initiated the sea ice model intercomparison project simip the first results from this model hierarchy approach are presented

influence of present day and glacial surface conditions on the antarctic oscillation southern annular mode 1 based upon coupled climate simulations driven by present day and glacial boundary conditions we demonstrate that the internal variability associated with the pronounced south polar climate anomalies that characterized the glacial climate regime was largely confined to the winter season in particular the intensity and spatial structure of the antarctic oscillation southern annular mode is found to display a strong seasonal cycle in the summer season we demonstrate that this mode of internal climate variability is extremely robust in response to a change from modern to glacial conditions in that it seems not to depend upon the latitudinal position of the baroclinic zone nor on the strength of the mean atmospheric zonal circulation in winter however and under glacial conditions this mode is characterized by a significantly modified atmospheric circulation that differs dramatically in terms of its internal variability from that characteristic of modern conditions

periodic waves in the surf zone analysis of experimental data abstract the paper discusses observed characteristics of some basic integral parameters for regular waves essential for the determination of the energy flux the radiation stress and the energy dissipation throughout the surf zone the paper further indicates a connection to the parameters at the breaking point and hence a connection to the wave shoaling area prior to the waves breaking on a beach the variation of the integral mainly time averaged wave parameters within the surf zone is in general given as a parametric description calling for further theoretical work in this field

numerically predicting seepage gradient forces and erosion sensitivity to soil hydraulic properties summary research has suggested that streambank seepage can be an important mechanism of bank instability however limited information is available on the level of soil characterization necessary to accurately predict seepage gradient forces and erosion the objective of this research was to quantify the expected range of predicted seepage gradients for various degrees of site characterization uncertainty analysis on seepage gradient predictions was performed relative to variability in soil hydraulic properties a two dimensional unsaturated saturated groundwater flow model was used to simulate a homogeneous soil layer for sand and loamy sand soils packed at various bulk densities qb a pedotransfer function rosetta designed to estimate unsaturated hydraulic properties from surrogate soil data i e texture and bulk density was used to derive the saturated hydraulic conductivity ks and water retention parameters for various levels of site information i e only textural class percent sand silt and clay ssc ssc and qb and ssc qb and ks statistical distributions were derived for each soil hydraulic parameter and monte carlo simulations were performed to generate distributions of maximum seepage gradient the deviation in predicted seepage gradient was calculated using assumed baseline conditions ranges in predicted soil hydraulic parameters and maximum seepage gradients were considerably reduced when using ssc as compared to soil texture therefore at a minimum soil samples should be taken for particle size analysis for qb between 1450 and 1500 kg m 3

coupled simulations of fluvial erosion and mass wasting for cohesive river banks the erosion of sediment from riverbanks affects a range of physical and ecological issues bank retreat often involves combinations of fluvial erosion and mass wasting and in recent years bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models in related work finite element seepage analyses have also been used to account for the influence of pore water pressure in controlling the onset of masswasting this paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion finite element seepage and limit equilibrium stability models are for the first time fully coupled application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where i there is no fluvial erosion and the bank geometry profile remains constant throughout ii there is no fluvial erosion but the bank profile is deformed by simulated mass wasting and iii the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass wasting the results are limited in scope to the specific conditions encountered at the study site but they nevertheless demonstrate the significant role that fluvial erosion plays in steepening the bank profile or creating overhangs thereby triggering mass wasting however feedbacks between the various processes also lead to unexpected outcomes specifically fluvial erosion also affects bank stability indirectly as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank thereby modulating the evolution of the pore water pressure field consequently the frequency magnitude and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included

quasi stationary waves in the southern hemisphere an examination of their simulation by the ncar climate system model with and without an interactive ocean abstract the three primary quasi stationary waves in the geopotential height field of the southern hemisphere as simulated by the national center for atmospheric research ncar climate system model csm1 and the community climate model version 3 ccm3 are examined and compared with the ncar national centers for environmental prediction reanalyses fourier analysis is used to decompose the geopotential heights into its zonal harmonic components both models are able to simulate the mean and zonal asymmetry of the geopotential heights however the csm1 simulates the interannual variability considerably better than the ccm3 the amplitude and phase of wave 1 are well simulated by the models particularly in the subantarctic region the models are also able to reproduce the interannual variation in phase and amplitude of wave 1 the success of the simulation is attributed to the models ability to simulate well the important features of the geopotential height and temperature distributions the models

bank and near bank processes in an incised channel abstract gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion the interaction of these forces control streambank mechanics hydraulic forces exerted by flowing water on in situ bank toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower bank profiles seepage forces exerted on in situ bank material by groundwater downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength data from a study site on goodwin creek ms usa clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore water pressures negative pore water pressures matric suction have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear a stable bank can be transformed into an unstable bank during periods of prolonged rainfall through 1 increase in soil bulk unit specific weight 2 decrease or complete loss of matric suction and therefore apparent cohesion 3 generation of positive pore water pressures and therefore reduction or loss of frictional strength 4 entrainment of in situ and failed material at the bank toe and 5 loss of confining pressure during recession of stormflow hydrographs relatively small frequent flows during the winter have the ability to erode failed bank materials maintain oversteepened unstable bank surfaces and promote prolonged periods of bank retreat channel migration and high yields of fine grained sediment confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe which steepens the bank for example more than 2 m of bank retreat occurred during the study period at the research site on goodwin creek northern mississippi the loss of matric suction negative pore pressures due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability apparent cohesion friction angle soil bulk unit weight and moisture content were measured in situ matric suction was measured continuously in situ with a series of five pressure transducer tensiometers a bank failure algorithm which combines the mohr coulomb approach for saturated conditions and the fredlund modification for unsaturated conditions was developed for layered cohesive streambanks the resulting equation has been used successfully to investigate the role of matric suction positive pore water pressures and confining pressure for layered streambanks composed of cohesive materials

what sets the mean transport through drake passage twelve experiments with two coarse resolutions of a global ocean model using a variety of surface forcings are analyzed to address the question of what sets the mean transport through drake passage seven of the experiments do not have an active sea ice model but the remaining five do previous theories have suggested that the drake passage transport is governed by the cape horn sverdrup transport or alternatively is proportional to the square root of the meridional ekman transport at the latitude of drake passage the results presented here do not support either of these theories the drake passage transport depends quite strongly on the isopycnal diffusivity parameter in the model and less strongly on the background vertical diffusivity and horizontal viscosity parameters however when the magnitudes of these parameters are fixed the results show a very strong correlation between drake passage transport and both the strength of the meridional ekman transport at the latitude of drake passage and the thermohaline circulation off the antarctic shelf the relationships are monotonic but not linear the best estimate is that the meridional ekman transport drives 100 sv of drake passage transport while the remaining 30 sv are driven by the global thermohaline circulation

decadal variability in high northern latitudes as simulated by an intermediate complexity climate model a 2500 year integration has been performed with a global coupled atmospheric sea ice ocean model of intermediate complexity with the main objective of studying the climate variability in polar regions on decadal time scales and longer the atmospheric component is the ecbilt model a spectral t21 three level quasi geostrophic model that includes a representation of horizontal and vertical beat transfers as well as of the hydrological cycle ecbilt is coupled to the clio model which consists of a primitive equation free surface ocean general circulation model and a dynamic thermodynamic sea ice model comparison of model results with observations shows that the ecbilt clio model is able to reproduce reasonably well the climate of the high northern latitudes the dominant mode of coupled variability between the atmospheric circulation and sea ice cover in the simulation consists of an annular mode for geopotential height at 800 hpa and of a dipole between the barents and labrador seas for the sea ice concentration which are similar to observed patterns of variability in addition the simulation displays strong decadal variability in the sea ice volume with a significant peak at about 18 years positive volume anomalies are caused by 1 a decrease in ice export through fram strait associated with more anticyclonic winds at high latitudes 2 modifications in the freezing melting rates in the arctic due to lower air temperature and higher surface albedo and 3 a weaker heat flux at the ice base in the barents and kara seas caused by a lower inflow of warm atlantic water opposite anomalies occur during the volume decrease phase of the oscillation

the atmospheric response to realistic arctic sea ice anomalies in an agcm during winter abstract the influence of realistic arctic sea ice anomalies on the atmosphere during winter is investigated with version 3 6 of the community climate model ccm3 6 model experiments are performed for the winters with the most 1982 83 and least 1995 96 arctic ice coverage during 1979 99 when ice concentration estimates were available from satellites the experiments consist of 50 member ensembles using large ensembles proved critical to distinguish the signal from noise the local response to ice anomalies over the subpolar seas of both the atlantic and pacific is robust and generally shallow with large upward surface heat fluxes 100 w m 2 near surface warming enhanced precipitation and below normal sea level pressure where sea ice receded and the reverse where the ice expanded the large scale response to reduced enhanced ice extent to the east west of greenland during 1982 83 resembles the negative phase of the arctic oscillation north atlantic oscillation ao nao with a ridge over t

atmospheric circulation and its effect on arctic sea ice in ccsm3 simulations at medium and high resolution the simulation of arctic sea ice and surface winds changes significantly when community climate system model version 3 ccsm3 resolution is increased from t42 2 8 to t85 1 4 at t42 resolution arctic sea ice is too thick off the siberian coast and too thin along the canadian coast both of these biases are reduced at t85 resolution the most prominent surface wind difference is the erroneous north polar summer anticyclone present at t42 but absent at t85 an offline sea ice model is used to study the effect of the surface winds on sea ice thickness in this model the surface wind stress is prescribed alternately from reanalysis and the t42 and t85 simulations in the offline model ccsm3 surface wind biases have a dramatic effect on sea ice distribution with reanalysis surface winds annual mean ice thickness is greatest along the canadian coast but with ccsm3 winds thickness is greater on the siberian side a significant difference between the two ccsm3 forced offline simulations is the thickness of the ice along the canadian archipelago where the t85 winds produce thicker ice than their t42 counterparts seasonal forcing experiments with ccsm3 winds during spring and summer and reanalysis winds in fall and winter relate the canadian thickness difference to spring and summer surface wind differences these experiments also show that the ice buildup on the siberian coast at both resolutions is related to the fall and winter surface winds the arctic atmospheric circulation is examined further through comparisons of the winter sea level pressure slp and eddy geopotential height at both resolutions the simulated beaufort high is quite weak weaker at higher resolution eddy heights show that the wintertime beaufort high in reanalysis has a barotropic vertical structure in contrast high ccsm3 slp in arctic winter is found in association with cold lower tropospheric temperatures and a baroclinic vertical structure in reanalysis the summertime arctic surface circulation is dominated by a polar cyclone which is accompanied by surface inflow and a deep ferrel cell north of the traditional polar cell the arctic ferrel cell is accompanied by a northward flux of zonal momentum and a polar lobe of the zonal mean jet these features do not appear in the ccsm3 simulations at either resolution

deciphering the role of southern gateways and carbon dioxide on the onset of the antarctic circumpolar current 1 growth of antarctic ice sheet during the cenozoic 34 million years ago appears as a potential tipping point in the long term cooling trend that began 50 ma ago for decades the onset of the antarctic circumpolar current acc following the opening of the drake passage and of the tasman seaway has been suggested as the main driver of the continental scale antarctic glaciation however recent modeling works emphasized that the eocene oligocene atmospheric carbon dioxide co2 lowering could be the primary forcing of the antarctic glaciation questioning the acc theory here we investigate the response of the acc to changes in co2concentrations occurring from the late eocene to the late oligocene we used a fully coupled atmosphere ocean model foam with a mid oligocene geography we find that the opening of southern oceanic gateways does not trigger the onset of the acc for co2typical of the late eocene 840 ppm a cooler background climatic state such as the one prevalent at the end of the oligocene is required to simulate a well developed acc in this cold configuration the intensified sea ice development around antarctica and the resulting brine formation lead to a strong latitudinal density gradient in the southern ocean favoring the compensation of the ekman transport and consequently the acc our results imply that the acc has acted as a feedback rather than as a driver of the global cooling

identifying the effects of parameter uncertainty on the reliability of riverbank stability modelling bank retreat is a key process in fluvial dynamics affecting a wide range of physical ecological and socioeconomic issues in the fluvial environment to predict the undesirable effects of bank retreat and to inform effective measures to prevent it a wide range of bank stability models have been presented in the literature these models typically express bank stability by defining a factor of safety as the ratio of driving and resisting forces acting on the incipient failure block these forces are affected by a range of controlling factors that include such aspects as the bank profile bank height and angle the geotechnical properties of the bank materials as well as the hydrological status of the riverbanks in this paper we evaluate the extent to which uncertainties in the parameterization of these controlling factors feed through to influence the reliability of the resulting bank stability estimate this is achieved by employing a simple model of riverbank stability with respect to planar failure which is the most common type of bank stability model in a series of sensitivity tests and monte carlo analyses to identify for each model parameter the range of values that induce significant changes in the simulated factor of safety these identified parameter value ranges are compared to empirically derived parameter uncertainties to determine whether they are likely to confound the reliability of the resulting bank stability calculations our results show that parameter uncertainties are typically high enough that the likelihood of generating unreliable predictions is typically very high 80 for predictions requiring a precision of 15 because parameter uncertainties are derived primarily from the natural variability of the parameters rather than measurement errors much more careful attention should be paid to field sampling strategies such that the parameter uncertainties and consequent prediction unreliabilities can be quantified more robustly

length scales and statistical characteristics of outer bank roughness for large elongate meander bends the influence of bank material properties floodplain vegetation and flow inundation this paper explores the length scales and statistical characteristics of form roughness along the outer banks of two elongate bends on a large meandering river through investigation of topographic variability of the bank face the analysis also examines how roughness varies over the vertical height of the banks and when the banks are exposed subaerially and inundated during flood stage detailed data on the topography of the outer banks were obtained subaerially using terrestrial lidar during low flow conditions and subaqueously using multibeam echo sounding mbes during near bankfull conditions the contributions of various length scales of topographic irregularity to roughness for subaerial conditions were evaluated for different elevation contours on the bank faces using hilbert huang transform hht spectral analysis statistical characteristics for discrete areas on the bank faces were determined by calculating the root mean square of normal distances from a tin surface results of the hht analysis show that the characteristics of roughness along bank faces composed primarily of non cohesive sediment and eroding into cropland vary with bank elevation and exhibit a dominant range of roughness length scales 15 50 m on the other hand bank faces composed predominantly of cohesive material and carved into a forested floodplain have relatively uniform topographic roughness characteristics over the vertical extent of the bank face and do not exhibit a dominant roughness length scale or range of length scales additionally comparison between local surface roughness for subaerial versus subaqueous conditions shows that roughness decreases considerably when the banks are submerged most likely because of the removal of vegetation and eradication of small scale erosional features in non cohesive bank materials by flow along the bank face thus roughness appears to be linked to the hydraulic conditions affecting the bank at least relative to conditions that develop when banks are exposed subaerially

near bank impacts of river stage control a gently sloping subaqueous bench is the topographical phenomenon resulting from the retention of pools to allow navigation in controlled stage streams bench formation has been observed at many sites on the ohio illinois and kanawha rivers they are the result of bank failure and erosion processes including seepage induced erosion localized failure of undermined layers reworking of failed soils by waves and erosion by current forces benches are prevalent along alluvial banks composed of layered soils and are covered by a shallow water zone that is ecologically significant this article summarizes the research to date on the bench phenomenon and draws inferences for future work in bank protection and development of riverine ecosystems

turbulent open channel flows with variable depth across the channel the flow of water in straight open channels with prismatic complex cross sections is considered lateral distributions of depth mean velocity and boundary shear stress are derived theoretically for channels of any shape provided that the boundary geometry can be discretized into linear elements the analytical model includes the effects of bed generated turbulence lateral shear turbulence and secondary flows experimental data from the science and engineering research council serc flood channel facility are used to illustrate the relative importance of these three effects on internal shear stresses new experimental evidence concerning the spatial distribution of reynolds stresses yx and zx is presented for the particular case of compound or two stage channels in such channels the vertical distributions of zx are shown to be highly nonlinear in the regions of strongest lateral shear and the depth averaged values of yx are shown to be significantly different from the depth mean apparent shear stresses the importance of secondary flows in the lateral shear layer region is therefore established the influence of both reynolds stresses and secondary flows on eddy viscosity values is quantified a numerical study is undertaken of the lateral distributions of local friction factor and dimensionless eddy viscosity the results of this study are then used in the analytical model to reproduce lateral distributions of depth mean velocity and boundary shear stress in a two stage channel the work will be of interest to engineers engaged in flood channel hydraulics and overbank flow in particular

evaluation of coupled ocean atmosphere simulations of the mid holocene using palaeovegetation data from the northern hemisphere extratropics we have used the biome4 biogeography biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean atmosphere general circulation models to mid holocene changes in orbital forcing in the mid to high latitudes of the northern hemisphere all the models produce a a northward shift of the northern limit of boreal forest in response to simulated summer warming in high latitudes the northward shift is markedly asymmetric with larger shifts in eurasia than in north america b an expansion of xerophytic vegetation in mid continental north america and eurasia in response to increased temperatures during the growing season c a northward expansion of temperate forests in eastern north america in response to simulated winter warming the northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in north america are supported by palaeovegetation data the expansion of xerophytic vegetation in mid continental north america is consistent with palaeodata although the extent may be over estimated the simulated expansion of xerophytic vegetation in eurasia is not supported by the data analysis of an asynchronous coupling of one model to an equilibrium vegetation model suggests vegetation feedback exacerbates this mid continental drying and produces conditions more unlike the observations not all features of the simulations are robust some models produce winter warming over europe while others produce winter cooling as a result some models show a northward shift of temperate forests consistent with though less marked than the expansion shown by data and others produce a reduction in temperate forests elucidation of the cause of such differences is a focus of the current phase of the palaeoclimate modelling intercomparison project

intra swash hydrodynamics and sediment flux for dambreak swash on coarse grained beaches the paper reports on dambreak type swash experiments in which intra swash hydrodynamics and sediment flux are measured for swash on a coarse sand beach and a gravel beach flow velocity and depth are measured using piv and lif respectively the intra swash sediment flux is measured using sediment traps comparison of measured hydrodynamics with the immobile permeable bed experiments of kikkert et al 2013 indicates that bed mobility impacts on the swash hydrodynamics reducing the maximum run up by approximately 8 for both beaches compared to the maximum run up on the corresponding immobile beach the measured intra swash sediment flux at a given location is characterised by high flux at the moment of bore arrival followed by rapid decay during uprush becoming zero at some time before flow reversal for the gravel beach the backwash sediment flux is negligibly small while for the sand beach the backwash flux increases slowly as the flow accelerates down the beach and peaks at about the time of maximum backwash velocity intra swash sediment flux calculated using the meyer peter and muller bed load transport formula with measured hydrodynamics as input and bed shear stress estimated using both the swart and colebrook formulae is within a factor 2 of the measured intra swash flux the agreement between the calculated and measured flux is better for the sand beach than for the gravel beach and better for uprush than for backwash for the sand beach there is good agreement between calculated and measured total uprush and total backwash sediment volumes the agreement is less good for the gravel beach for which calculated and measured uprush volumes show a similar trend but the calculated backwash volumes over estimate the negligible volumes observed in the experiments

satellite derived maps of arctic and antarctic sea ice motion 1988 to 1994 standard image processing methods applied to 85 5 ghz special sensor microwave imager ssm i data provide previously unseen details about sea ice motion in the arctic and southern oceans means calculated from daily ice motions for 1988 1994 confirm the basic circulation patterns established from drifting stations and buoys but extend these observations to illustrate less well known transport patterns in the arctic coastal zones and sub arctic seas in the antarctic the gridded motion fields show a nearly continuous westward transport along the antarctic coast with well defined regions of exchange between this east wind drift and the antarctic circumpolar current this detailed view of mean ice motion in the southern ocean is unique and presents a comprehensive overview not previously available for antarctic ice motions

an experimental investigation on mass failures occurring in a riverbank composed of sandy gravel abstract this paper reports the results and interpretation of laboratory experiments carried out on a model of a relatively coarse sandy gravel riverbank with the aim of investigating the basic processes and possible factors of instability after a series of initial tests three main experiments were carried out in a glass walled tank where a bank model was built with bank angles varying from 75 to 90 bank height of 70 cm and same sediment mixture 60 gravel 40 sand but with the addition of 1 of cement in the third experiment only during the experiments the bank was subject to a given hydrograph associated with a static oscillation of the water level and corresponding variations in pore water pressures were measured results show the occurrence of a large variety of processes including erosion and failures because of loss of matric suction cantilever slab and slide failures and granular flows during the ascending phase of the hydrograph bank instability was related to a superimposition of initial geometric factors and progressive reduction of apparent cohesion during the rising water stage apparent cohesion was sufficient to maintain a stable bank in loose material but only for low bank height and or slopes whereas a limited percentage of cement is able to explain a markedly different response in terms of stability and mechanisms of failure

a simulation of the last glacial maximum climate using the ncar ccsm the national center for atmospheric research community climate system model ncar ccsm is used in a coupled atmosphere ocean sea ice simulation of the last glacial maximum lgm around 21 000 years ago climate in the tropics the simulation shows a moderate cooling of 3 c over land and 2 c in the ocean in zonal average this cooling is about 1 c cooler than the climap sea surface temperatures ssts but consistent with recent estimates of both land and sea surface temperature changes subtropical waters are cooled by 2 2 5 c also in agreement with recent estimates the simulated oceanic thermohaline circulation at the lgm is not only shallower but also weaker than the modern with a migration of deep water formation site in the north atlantic as suggested by the paleoceanographic evidences the simulated northward flow of antarctic bottom water aabw is enhanced these deep circulation changes are attributable to the increased surface density flux in the southern ocean caused by sea ice expansion at the lgm both the gulf stream and the kuroshio are intensified due to the overall increase of wind stress over the subtropical oceans the intensified zonal wind stress and southward shift of its maximum in the southern ocean effectively enhances the transport of the antarctic circumpolar current acc by more than 50 simulated ssts are lowered by up to 8 c in the midlatitudes simulated conditions in the north atlantic are warmer and with less sea ice than indicated by climap again in agreement with more recent estimates the increased meridional sst gradient at the lgm results in an enhanced hadley circulation and increased midlatitude storm track precipitation the increased baroclinic storm activity also intensifies the meridional atmospheric heat transport a sensitivity experiment shows that about half of the simulated tropical cooling at the lgm originates from reduced atmospheric concentrations of greenhouse gases

riverbank stability analysis i theory in this paper a slope stability analysis for steep banks is used in conjunction with a method to calculate lateral erosion distance to predict bank stability response to lateral erosion or bed degradation the failure plane angle failure block width and volume of failed material per unit channel length may be calculated for the critical case these parameters define the bank geometry following failure and form the starting point for subsequent analyses the calculation procedure is illustrated by a worked example following mass failure slump debris accumulates at the bank toe the debris is removed by lateral erosion prior to further oversteepening or degradation generating further mass failures any process based model for channel width adjustment must account for the combined effects of lateral erosion and mass instability in producing bank instability the approach adopted here represents a marked improvement over earlier work which does not account for changes in bank geometry due to lateral er

polar climate instability and climate teleconnections from the arctic to the midlatitudes and tropics abstract it is generally accepted that the ocean thermohaline circulation plays a key role in polar climate stability and rapid climate change recently reported analyses of the impact of anomalous freshwater outflows from the north american continent onto either the north atlantic or arctic oceans demonstrate that in either case a clear reduction in the atlantic meridional overturning circulation accompanied by an increase in sea ice extent is predicted the results also reconcile proxy inferred younger dryas greenland temperature variations the aim of the present work is to provide a detailed investigation of the pathways along which the signal associated with overturning circulation anomalies propagates into both the midlatitudes and the tropics and the effect such teleconnections have on the tropical ocean atmosphere system the authors consider both the impact of substantial slowing of the overturning circulation due to freshwater forcing of the north atlantic as well as its recovery after the a