reconstructions of deltaic environments from holocene palynological records in the volga delta northern caspian sea new palynological and ostracod data are presented from the holocene volga delta obtained from short cores and surface samples collected in the damchik region near astrakhan russian federation in the northern caspian sea four phases of delta deposition are recognized and constrained by accelerated mass spectrometry ams radiocarbon ages palynological records show that erosive channels dunes baer hills and inter dune lakes were present during the period 11 500 8900 cal bp at the time of the mangyshlak caspian lowstand the period 8900 3770 cal bp was characterized regionally by extensive steppe vegetation with forest present at times with warmer more humid climates and with halophytic and xerophytic vegetation present at times of drought the period 3770 2080 cal bp was a time of active delta deposition with forest or woodland close to the delta indicating relatively warm and humid climates and variable caspian sea levels from 2080 cal bp to the present day aquatic pollen is frequent in highstand intervals and herbaceous pollen and fungal hyphae frequent in lowstand intervals soils and incised valley sediments are associated with the regional derbent regression and may be time equivalent with the medieval warm period fungal spores are an indicator of erosional or aeolian processes whereas fungal hyphae are associated with soil formation freshwater algae ostracods and dinocysts indicate mainly freshwater conditions during the holocene with minor brackish influences dinocysts present include spiniferites cruciformis caspidinium rugosum impagidinium caspienense and pterocysta cruciformis the latter a new record for the caspian sea the holocene volga delta is a partial analogue for the much larger oil and gas bearing mio pliocene palaeo volga delta

a model for changes in channel form at a river confluence a model for changes in river channel geometry at a confluence is deduced by substituting the hydraulic geometry relationships into the continuity equation of flow changes in the hydraulic geometry variable g can be described by g x 0 g x 1 g x 2 where the subscripts 0 1 and 2 denote the receiving stream major and minor tributaries the exponent may be a system wide parameter bar x or may be computed for individual junctions x in the first case bar x is expected to be equal to the reciprocal of the exponent of the hydraulic geometry relationship between g and water discharge in the latter case x may be found by solving the above equation iteratively or from two nomographs presented in this paper using data from the literature richards 1980 and our own measurements of widths and slopes in a miniature drainage network we show that although average morphometric changes at a junction may be adequately described by bar x the behavior of individual junctions

the evolution of el nino past and future abstract we review forecasts of the future of el nino and the southern oscillation enso a coupled instability of the ocean atmosphere system in the tropical pacific with global impacts enso in the modern world is briefly described and the physics of the enso cycle is discussed particular attention is given to the bjerknes feedback the instability mechanism which figures prominently in enso past and future our knowledge of enso in the paleoclimate record has expanded rapidly within the last 5 yr the enso cycle is present in all relevant records going back 130 kyr it was systematically weaker during the early and middle holocene and model studies indicate that this results from reduced amplification in the late summer and early fall a consequence of the altered mean climate in response to boreal summer perihelion data from corals shows substantial decadal and longer variations in the strength of the enso cycle within the past 1000 yr it is suggested that this may be due to solar and volcanic variations in solar insolation amplified by the bjerknes feedback there is some evidence that this feedback has operated in the 20th century and some model results indicate that it will hold sway in the greenhouse future but it is very far from conclusive the comprehensive general circulation models used for future climate projections leave us with an indeterminate picture of enso s future some predict more enso activity some less with the highly uncertain consensus forecast indicating little change

morphodynamics and seismostratigraphy of a deep hole at tidal channel confluence the sedimentary circulation pattern at a deep hole at a tidal channel junction in an estuary environment is examined using morphosedimentological and seismostratigraphic data the bottom and sub bottom acoustic information side scan sonar and 3 5 khz profiler obtained at a tidal channel confluence in bahia blanca estuary argentina evidences the depositional and erosive features bed load sediment transport pathways and morphological evolution of the depression this important depression in the confluence zone of the channels is characterised by the presence of steep scour faces 14 at the mouth of both channels and gentle slopes 1 5 opposite to them bed load sediment transport analysis reveals a differential circulation pattern of sand on both sides of the hole which is associated with the different sediment availability in each channel the side scan sonar information therefore indicates that there is no available sand in one of the channels whereas the other channel is characterised by the presence of a large amount of sediment coming from its inner area deposition therefore occurs on the side where sediment is mobilised as bed load by the ebb current dominance and on the face developed downstream in the latter a part from the sediment is deflected inward by the tidal flood currents which produce the deposition of sediment on the gentle side the rest of the sediment is exported from the system by ebb currents in contrast erosion occurs on the opposite side of the hole and at the mouth of each channel and it is manifested by the outcrop of old sedimentary strata the seismostratigraphic information collected in this study indicates that the depression was originally a fully erosive morphological feature a descriptive model of the morphological evolution at a channel confluence is presented in this study

interannual and decadal variability of the subsurface thermal structure in the pacific ocean 1961 90 heat content anomalies are analyzed to understand subsurface variability on both aparticular focus on the evolving basinwide patterns and oceanic connections between the extratropics and tropics various analyses indicate two distinct modes one interannual and the other decadal that involve the tropics and the north pacific subtropical gyre respectively interannual variability is associated with el nino in the tropics with a prominent see saw pattern alternately on and off the equator and in the east and west respectively the interannual cycle features a coherent propagation of subsurface signals around the tropical pacific eastward along the equator but westward off the equator at 10 15 n decadal signals are dominant in the subtropics and midlatitudes but also have a tropical component that appears to be independent of interannual variations an oceanic connection can be seen between subsurface anomalies in the midlatitudes in the subtropics and tropics on decadal time scales subsurface thermal anomalies associated with midlatitude decadal variability can propagate through the subtropics into the tropics which may modulate the intensity of interannual variability in the tropics for example in the middle and late 1970s a significant warm temperature anomaly appeared to penetrate into the western and central tropics at depth warming the tropical upper ocean and depressing the thermocline during the development of el nino therefore an extratropically preconditioned subsurface state e g an enhanced positive heat content anomaly in the western and central tropical pacific would favor a warmer sea surface temperature anomaly in the eastern equatorial pacific potentially increasing the intensity of ocean atmosphere coupling these changes in the thermocline structure and possibly in the coupling strength can further alter the very character of tropical air sea interactions this may help to explain decadal variability of el nino evolution in the tropical pacific as observed in the 1980s our subsurface variability analysis presents observational evidence for the detailed space time structure of decadal oceanic links between the extratropics and the tropics

earthquake prediction an empirical approach abstract earthquake precursor data so far accumulated in japan have shown that the larger the main shock magnitude the larger the distance between the epicenter and an observation point where a precursor is observed it is possible to empirically establish an approximate relationship between maximum detectable distance d max and main shock magnitude m the relationship being different from discipline to discipline of precursor when a number of precursors are observed we may draw circles with a radius equal to d max peculiar to each precursor discipline centered at the respective observation points on the condition that m takes on a certain value in that case the epicenter of a future earthquake should be located in the area which is common to all of the circles if m is too small some of the circles do not overlap if it is too large an epicentral area which is too wide to be realistic must be assumed in this way an approximate epicenter location and a rough value of the main shock magnitude can be assessed applying the procedure to the precursors of the izu oshima kinkai m 7 0 1978 and the niigata m 7 5 1964 japan earthquakes remarkable success is achieved probabilities of an earthquake occurring in a specified time interval can be evaluated as a function of time when a precursor is observed for such an evaluation we rely either on the log t m relationship with a prescribed value of m or on the frequency distribution of log t which is empirically obtained depending on precursor disciplines t being the precursor time when a number of precursors are observed one by one changes in synthetic probability of earthquake occurrence can be estimated according to the existing formula utsu 1977 as an example such probability changes with time are estimated making use of the precursor data for the izu oshima kinkai earthquake it turns out that the synthetic probability generally increases as time goes on when a long term precursor happens to be observed however the probability drops discontinuously being followed by a gradual rise again the temporal change in probability therefore shows considerable fluctuations this is especially so when a short interval for probability evaluation is specified because of uncertain preliminary probability involved in the evaluation and an unknown rate of false precursor signals against true ones the absolute value of probability thus evaluated may not be perfectly reliable even so the temporal change in probability may provide some clues for the actual publication of earthquake prediction

bed morphology and sedimentology at the confluence of unequal depth channels abstract spatial and temporal variations in the bed geometry and bed material size of a sand bed river confluence with unequal depth channels were monitored during a sequence of floods additionally two other sand bed confluences were surveyed to test the replicability of these observations the confluences studied here have only one avalanche face which corresponds to the front of a tributary mouth bar in the shallower channel and do not exhibit a marked scour zone three distinct morpho sedimentological zones are present 1 an area around the upstream corner of the junction where the sediments are generally finer than the mean 2 a maximum depth zone with coarser than average particles and 3 a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar the downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels the contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone

how different wind stress patterns affect the tropical subtropical circulations of the upper ocean abstract an oceanic gcm is used to investigate the response of the tropical and subtropical thermocline circulation and structure to different wind stress patterns although the subtropical winds do not affect the transport or the speed of the equatorial undercurrent significantly they do change the equatorial temperature field in the lower part of the equatorial thermocline significantly a weaker subtropical wind curl causes a cooling of the subsurface equatorial region and hence an intensification of the equatorial thermocline a weakening of the subtropical wind curl by a factor of 2 cools the equatorial lower thermocline water by 2 c

interannual variability in a tropical atmosphere ocean model influence of the basic state ocean geometry and nonlinearity abstract the behavior of a tropical coupled atmosphere ocean model is analyzed for a range of different background states and ocean geometries the model is essentially that of cane and zebiak for the tropical pacific except only temporally constant background states are considered here for realistic background states and ocean geometry the model solutions feature oscillations of period of 3 5 yr by comparing the full model solution with a linearized version of the model it is shown that the basic mechanism of the oscillation is contained within linear theory a simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere ocean system the analog model highlights the properties that produce coupled atmosphere ocean instability in the eastern ocean basin and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed negative feedback into this instability growth the growth rate of the local instabil

heartbeat of the southern oscillation explains enso climatic resonances the el ni no southern oscillation enso nonlinear oscillator phenomenon has a far reaching influence on the climate and human activities the up to 10 year quasi period cycle of the el ni no and subsequent la ni na is known to be dominated in the tropics by nonlinear physical interaction of wind with the equatorial waveguide in the pacific long term cyclic phenomena do not feature in the current theory of the enso process we update the theory by assessing low 10 years and high 10 years frequency coupling using evidence across tropical extratropical and pacific basin scales we analyze observations and model simulations with a highly accurate method called dominant frequency state analysis dfsa to provide evidence of stable enso features the observational data sets of the southern oscillation index soi north pacific index anomaly and enso sea surface temperature anomaly as well as a theoretical model all confirm the existence of long term and short term climatic cycles of the enso process with resonance frequencies of 2 5 3 8 5 12 14 61 75 180 years this fundamental result shows long term and short term signal coupling with mode locking across the dominant enso dynamics these dominant oscillation frequency dynamics defined as enso frequency states contain a stable attractor with three frequencies in resonance allowing us to coin the term heartbeat of the southern oscillation due to its characteristic shape we predict future enso states based on a stable hysteresis scenario of short term and long term enso oscillations over the next century

on playfair s law of accordant junctions geomorphologists since playfair have apparently accepted that fluvial confluences will be morphologically accordant in distinction to the hanging junctions of glaciated troughs a view reinforced by gilbert s law of declivities the issue has been largely ignored despite field instances of discordant fluvial junctions and widespread engineering practice which favours discordant confluences in non erodible materials six categories of fluvial junctions are defined accordant hanging discordant barred impeded disrupted and disturbed the study focuses principally on the first two classes and defines discordance purely morphologically in terms of relative depth of incision of the confluent thalwegs likely controls of accordance discordance are identified as flow magnitudes synchroneity of flow regimes channel boundary materials and debris calibre field investigations in malawi france west malaysia and new zealand in small 100 km2 basins on a variety of rock types are described barred junctions are relatively rare but complex in character a high proportion of confluences are distinctly discordant and there is a general continuum from accordant cases tectonic control is improbable and the principal cause identified is the degree of difference in channel forming discharges estimated by stream magnitude and channel area some evidence for lithologic control is also apparent it appears that many confluences behave as if the tributary junction is non erodible and not in the fashion predicted by the law of declivities it is argued that the observed relation is a necessary result of the general down stream changes in hydraulic geometry limited comparison with glacial trough confluences reveals a difference in degree but not in nature although the relationship between discordance and channel size is affected by cross section geometry a 10 fold difference in the cross sectional areas of channels meeting at a junction produces on average a 2 4 fold difference in mean channel depths for fluvial cases and a 4 fold difference for glacial troughs the role of varying flow regimes seems to require attention nevertheless it is maintained that playfair s law is a misleading description of the morphology of fluvial junctions which behaves fundamentally in the same fashion as glacial troughs

origin of upper ocean warming and el nino change on decadal scales in the tropical pacific ocean the cause of decadal scale variability in the tropical pacific ocean such as that marked by the 1976 77 shift in the el nino southern oscillation1 2 3 4 5 6 7 is poorly understood unravelling the mechanism of the recent decade long warming in the tropical upper ocean is a particularly important challenge given the link to el nino variability but establishing the hypothesized interannual decadal oceanic connections between middle latitudes and tropics has proved elusive8 here we present observational evidence that pacific upper ocean warming and decadal changes in the el nino southern oscillation after 1976 may originate from decadal mid latitude variability in the middle 1970s the north pacific ocean is observed to have undergone a clear phase transition a see saw subsurface temperature anomaly pattern that rotates clockwise around the subtropical gyre at middle latitudes a subsurface warm anomaly formed in the early 1970s from subducted surface waters and penetrated through the subtropics and into the tropics thus perturbing the tropical thermocline and driving the formation of a warm surface water anomaly that may have influenced el nino in the 1980s the identification of this teleconnection of extratropical thermal anomalies to the tropics through a subsurface ocean bridge may enable improved prediction of decadal scale climate variability

soil erodibility and processes of water erosion on hillslope abstract the importance of the inherent resistance of soil to erosional processes or soil erodibility is generally recognized in hillslope and fluvial geomorphology but the full implications of the dynamic soil properties that affect erodibility are seldom considered in canada a wide spectrum of soils and erosional processes has stimulated much research related to soil erodibility this paper aims to place this work in an international framework of research on water erosion processes and to identify critical emerging research questions it focuses particularly on experimental research on rill and interrill erosion using simulated rainfall and recently developed techniques that provide data at appropriate temporal and spatial scales essential for event based soil erosion prediction results show that many components of erosional response such as partitioning between rill and interrill or surface and subsurface processes threshold hydraulic conditions for rill incision rill network configuration and hillslope sediment delivery are strongly affected by spatially variable and temporally dynamic soil properties this agrees with other recent studies but contrasts markedly with long held concepts of soil credibility as an essentially constant property for any soil type properties that determine erodibility such as soil aggregation and shear strength are strongly affected by climatic factors such as rainfall distribution and frost action and show systematic seasonal variation they can also change significantly over much shorter time scales with subtle variations in soil water conditions organic composition microbiological activity age hardening and the structural effect of applied stresses property changes between and during rainstorms can dramatically affect the incidence and intensity of rill and interrill erosion and therefore both short and long term hillslope erosional response similar property changes linked to climatic conditions may also significantly influence the stability and resilience of plant species and vegetation systems full understanding of such changes is essential if current event based soil erosion models such as wepp and eurosem are to attain their full potential predictive precision the complexity of the interacting processes involved may however ultimately make stochastic modelling more effective than physically based modelling in predicting hillslope response to erodibility dynamics

flow and sediment dynamics in channel confluences confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large low gradient main channel commonly occur in nature but they have not yet been investigated measurements of the three dimensional velocity field turbulence sediment transport bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the upper rhone river switzerland the difference between the low flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone due to this bed discordance the tributary flow penetrates into the main channel mainly in the upper part of the water column whereas the main channel flow is hardly hindered by the tributary in the lower part of the water column giving rise to a two layer flow structure in the confluence zone in confluences with high supply of coarse sediment from the tributary the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity the sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near bed flow originating from the main channel the sediment transport capacity is further increased by the three dimensionality of the flow which is characterized by maximum velocities occurring near the bed and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary a conceptual model is proposed for the hydro morpho sedimentary processes and compared to existing conceptual models for confluences with different characteristics

mixing layer distortion at the confluence of channels of different depth channel confluences have received considerable recent attention in the fields of geomorphology1 3 sedimentology4 9 and hydraulic engineering10 13 where channels join rapid changes in fluid velocity turbulence intensity channel hydraulic geometry and bed geometry may occur14 17 previous models of junction dynamics have tended to assume that the confluent channels are of equal depth10 18 a condition that may be found only rarely in natural junctions more often the depths of confluent channels are different19 this discordance is commonly the result of the forma tion at the mouth of each confluent channel of bars that possess steep avalanche faces which dip into a central junction scour1 2 5 20 here we suggest that when channels of different depth join the mixing layer that forms at their confluence is distorted by interacting with a separation zone which forms in the lee of the mouth of the shallower channel this mixing layer distortion imparts a complex three dimensional flow to the junction promoting vertical fluid upwelling and significantly enhancing rapid mixing within the flow of the shallow tributary while retarding the rate of mixing across the flow of the deeper channel suspended sediments or contaminants within the deeper channel may be transferred rapidly across the flow of the shallower tributary upwelling downstream of the junction

a bayesian analysis of physical habitat changes at tributary confluences in cobble bed upland streams of the acheron river basin australia 1 tributary confluences are known to affect downstream channel morphology and have the potential to increase downstream habitat heterogeneity however few studies have explicitly investigated the effect of tributaries on physical habitat we used bayesian modelling to investigate tributary effects on channel morphology flow velocity and bed particle size distribution at eight confluences in the acheron river system in victoria australia under high flow and low flow conditions consistent tributary effects across sites and flow stages were a localized increase in channel depth at the confluence and a step increase in channel width in the postconfluence channel other tributary effects such as increased or decreased flow velocity the presence and placement of confluence bed forms e g scours pools bars and avalanche faces and altered bed particle size distributions varied between sites and flow conditions these results provide evidence of changes to the physical habitat at confluences which may affect stream biota

bedload transport in a river confluence abstract the confluence of the regulated tolten river and its tributary the unregulated allipen south of chile has proved dynamic in the last decade daily bedload measurements with a helley smith sampler bed surveys and grain size distributions of the two rivers are obtained from a field campaign that lasts 3 months in high flow season the goals are to quantify total bedload and to understand the balance between tributary and main river and the bedload distribution in space and texture the bedload transport varies 200 fold with a maximum of 5000 t day the discharge varies five fold with a maximum of 900 m 3 s two thirds of the total bedload volume are transported through the deeper area of the cross section and gravel is predominant 64 average bedload volumes in the confluence seem unbalanced in favour of the tributary main river bedload transport is predominantly at below capacity conditions while the tributary bedload transport is at capacity conditions this is deemed the main reason of inaccuracy of the bedload predictors the roles of entrainment into suspension helical flow partial transport and mobile armour are discussed

earthquake precursors in japan precursor time and detectability abstract earthquake precursor data amounting to 418 in number accumulated in japan are analyzed most data are obtained under the nation wide earthquake prediction programme the precursor time t versus main shock magnitude m relationship is different for different groups of precursors a linear relation such as log t a bm where a and b are constants seems to hold for precursor disciplines such as land deformation revealed by geodetic survey and tide gauge observation ground tilt observed by a water tube tiltmeter anomalous seismic activity and geomagnetic field change although a similar relationship seems to hold for the disciplines of earth currents radon concentration underground water etc the precursor time is substantially smaller for these disciplines than for the former disciplines the above characteristics of precursor time suggests that precursors closely connected to the dilatancy generation seem to be observed at an earlier stage of seismogenetic activity precursors probably associated with diffusion of underground water into the dilatant region seem to follow for a number of precursor disciplines such as foreshock ground tilt observed by a pendulum tiltmeter resistivity by a variometer and so on no dependence of t on m is found the relationship between m and the logarithmic epicentral distance d measured in kilometers is different from discipline to discipline relying on empirical and theoretical studies of coseismic strain step it turns out that precursors found by geodetic and geomagnetic observations seem to reflect a crastal strain of the order of 10 7 10 6 precursors obtained by a resistivity variometer seem to represent a strain of the order of 10 9 10 8 precursors of other disciplines seem to correspond to a strain of 10 8 10 7 it is clear that precursors can be detected further from the epicenter as the main shock magnitude becomes larger in some favourable cases a precursor can be observed even where the epicentral distance is several times larger than that of the epicentral area

mixing and circulation at the confluence of two rivers entering a meandering reservoir 1 a field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows ebro and segre with distinct physical chemical characteristics as they join at the upstream end of ribarroja reservoir in northern spain given the short average residence time of water in the reservoir the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam in winter inflows have similar densities 0 o 10 5 and their spatial distribution is largely determined by inertial forces and in particular by the discharge ratio downstream of the confluence both rivers flow side by side and largely unmixed over long distances in summer with 0 of o 10 3 the flow fields at the confluence are largely controlled by buoyancy forces atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms leading to localized high mixing rates mixing in general though is weak at this time of the year in fall and early winter density differences are largely controlled by conductivity differences between the incoming flows the warmer ebro water with larger thermal inertia flows beneath the colder segre water the spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong likely as a result of mixed water being denser than either of the incoming flows

a simple model of the mass exchange between the subtropical and tropical ocean abstract a simple ventilated thermocline model is used to study the subtropical tropical mass exchange it is found that the water subducted in the western subtropical gyre recirculating window tends to recirculate within the subtropical gyre while the water subducted in the eastern part exchange window tends to penetrate equatorward the exchange window expands with an increased easterly wind or basin width on the southern boundary of the subtropical gyre but shrinks with an increased wind curl within the subtropical gyre furthermore the total exchange transport increases with the easterly wind or the width of the basin on the southern boundary of the subtropical gyre but it is independent of subtropical wind the ventilation mechanism is important in supporting the exchange transport for wind with realistic strength at the southern boundary the reduction of the exchange transport is about 15 30 of the ekman transport finally relative to the exchange transport in the interior of the ocean

separation zone at open channel junctions the configuration of open channel junctions controls local sedimentary processes channel scour and sidewall erosion through its influence on the flow patterns established as confluent streams compete for limited channel capacity entry of a tributary into a mainstream ensures detachment of flow from the channel sidewall immediately downstream from the junction and flume experiments show well defined relationships between the dimensions of the separation zone and discharge from the tributary other things equal both the width and length of the separation zone increase systematically with an increase in confluence angle though it is shown that values of width are much less than predicted by recent mathematical modelling the zone grows at the expense of the proportion of the channel occupied by the immediate post confluence flow with the consequence that near bottom velocity increases by a factor of 1 3 as confluence angle is increased from 15 90 the implications for sediment entrainment are reviewed briefly

the simplest enso recharge oscillator 1 eastern pacific sea surface temperature sst and mean equatorial pacific thermocline depth are key variables in el nino southern oscillation enso a linear fit to observations leads to a remarkably simple picture enso can be represented by a classical damped oscillator with sst and thermocline depth playing the roles of momentum and position respectively an independent fit of observed relationships between western and eastern thermocline depth central wind stress and eastern pacific sst yields the same picture and supports a recharge oscillator interpretation the oscillation arises from the interaction between the recharge time of the warm pool and the time delay between east and west pacific both finite kelvin wave speed and sst dynamics contribute to the time delay including seasonality in the description we find two periods of relative instability boreal spring with a large phase progression and autumn with nearly stationary phase

impact of confluence on hydrological and morphological characters of the trunk stream a study on the manu river of north east india tributaries play a significant role in changing flow structure of the main river through their additional discharge a study was conducted at the point of convergence of the river deo with the river manu using simple geospatial technique and field work to justify the hydrological and morphological changes in downstream of the confluence point the spatio temporal change of both the river channels for 10 km length in upstream and downstream of the confluence point was considered moreover field measurements and post field work were carried out to examine the spatial variation of hydrodynamic characteristics like flow velocity depth water discharge wetted perimeter and hydraulic radius of the selected stretches sieving method was applied for grain size analysis of river bed sediment samples the result revealed that both the aggradation and degradation processes were equally active in the upstream segment of the manu river but in downstream segment aggradation exceeded the degradation activity though all the hydrodynamic variables were boosted up in downstream except flow velocity the present research highlighted that the steeper gradient of the r deo had enhanced its competency to transport medium sized grains to the r manu where fine grains were commonly found moreover increased wetted perimeter in downstream specified more friction between channel bed and its flow consequent upon reduced flow velocity with extra sediment load accumulation

upper ocean thermal variations in the north pacific during 1970 1991 abstract a newly available extensive compilation of upper ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400 m depth in the north pacific during 1970 1991 a prominent decade long perturbation in climate occurred during this time period surface waters cooled by 1 c in the central and western north pacific and warmed by about the same amount along the west coast of north america from late 1976 to 1988 comparison with data from coads suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly the vertical structure of seasonal thermal anomalies in the central north pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400 m depth individual cold events descend rapidly 100 m yr 1 superimposed upon a slower cooling 15 m yr 1 the inter

decadal variability of temperature at a depth of 400 meters in the north pacific ocean the gyre scale structure and evolution of decadal ocean thermocline variability in the north pacific is described by analyzing historical temperature data at a depth of 400 meters decadal thermocline anomalies are characterized by a distinct see saw pattern in the midlatitudes and subtropics that appears to circulate coherently in the north pacific basin with a period on the order of 20 years there are coherent spatial and temporal relationships between temperature anomalies around the subtropical gyre with phase differences at depth and at the sea surface suggesting that thermocline variability evolves independently of local sst anomalies in the subtropics subsurface anomalies can be seen to propagate all the way across the basin and thereby lead sst anomalies providing a possible phase transition mechanism for sst the basinwide anomaly pathways suggest a clockwise advection of subsurface temperature anomalies around the subtropical gyre an active role played by the mean geostrophic flow in decadal variability

structure and cycle of decadal variability of upper ocean temperature in the north pacific abstract yearly upper ocean in situ temperature anomaly data for the period 1961 90 are analyzed to reveal spatial structure and evolution of decadal variability in the north pacific ocean an eof analysis has been performed on individual temperature anomaly fields at upper ocean standard levels as well as simultaneously on the entire upper ocean data to depict the combined three dimensional structure in a coherent manner time evolution of anomaly fields is depicted by using a regression analysis the analyses detect the principal basin scale structure of decadal warm period dwp and decadal cold period dcp there is a well defined subsurface thermal anomaly pattern characterized by a prominent seesaw structure with opposite anomaly polarity between the midlatitude north pacific and the subtropical regions during a dwp a positive temperature anomaly is found in the central midlatitude upper ocean with the maximum at about 100 m depth this is accompanied by a corresponding negative anomaly in the

enso like interdecadal variability 1900 93 a number of recent studies have reported an enso like eof mode in the global sea surface temperature sst field whose time variability is marked by an abrupt change toward a warmer tropical eastern pacific and a colder extratropical central north pacific in 1976 77 the present study compares this pattern with the structure of the interannual variability associated with the enso cycle and documents its time history back to 1900 the analysis is primarily based on the leading eofs of the sst anomaly and anomaly deviation fields in various domains and the associated expansion coefficient or principal component time series which are used to construct global regression maps of sst sea level pressure slp and a number of related variables the use of anomaly deviations i e departures of local sst anomalies from the concurrent global mean sst anomaly reduces the influence of global mean sst trends upon the structure of the eofs and their expansion coefficient time series an important auxiliary time series used in this study is a southern oscillation index based on marine surface observations by means of several different analysis techniques the time variability of the leading eof of the global sst field is separated into two components one identified with the enso cycle related variability on the interannual timescale and the other a linearly independent residual comprising all the interdecadal variability in the record the two components exhibit rather similar spatial signatures in the global sst slp and wind stress fields the sst signature in the residual variability is less equatorially confined in the eastern pacific and it is relatively more prominent over the extratropical north pacific the corresponding slp signature is also stronger over the extratropical north pacific and its counterpart in the cold season 500 mb height field more closely resembles the pna pattern the amplitude time series of the enso like pattern in the residual variability reflects the above mentioned shift in 1976 77 as well as a number of other prominent features including a shift of opposite polarity during the 1940s

response of bed morphology and bed material texture to hydrological conditions at an asymmetrical stream confluence changes in bed morphology and sedimentology at stream confluences in relation to specific hydrological events have not been documented extensively moreover few if any studies have investigated the effect of net hydrological conditions on channel change at a confluence over a period of many years repeat cross section surveys and sampling of bed material at a small asymmetrical stream confluence reveal the short term effects of hydrological variability on bed morphology and spatial patterns of bed material texture survey data over a 15 year period illustrate the long term response of channel morphology at the confluence to net hydrological conditions results confirm that this confluence is a dynamic fluvial environment involving shifts in the zone of scour and repeated deposition and erosion of a bar complex at the downstream junction corner as the discharge ratio changes in response to hydrological variability in the tributary streams persistent faster runoff response times of the lateral tributary compared to the main stem have resulted in long term net erosion of the channel bank opposite this tributary the documented morphological and sedimentological changes are consistent with and can be explained by results of experimental studies and by past investigations at this confluence that have documented changes in patterns of mean flow structure and turbulence in response to variation in the discharge ratio of contributing streams

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

volcanic eruptions and wide area geophysical anomalies abstract the changes in volume linear strain and underground water level temperature observed in association with the november 21 1986 eruption of mihara volcano about 110 km southwest of tokyo japan are analyzed these changes were observed at many mainland stations several tens of kilometers distant from the volcano the so called d max method was originally invented by the writer for predicting the location and magnitude of an earthquake based on precursor data applying this technique to the geophysical anomalies observed it is concluded that the event center was located somewhere beneath the volcano and that the effective magnitude was approximately 5 8 the method relies on the empirical relation between maximum detectable distance of an earthquake precursor from the epicenter d max and the main shock magnitude m the relation is a function of the type of precursor assuming a spherical magma reservoir underneath the volcano having a specific pressure cavity radius and so on it is proved that the effective magnitude is in accord with the strain energy change associated with the pressure release at the time of eruption the model also accounts for the amount of crustal deformation actually observed it is thus concluded that the d max method is valid not only for earthquake precursors but also for analyzing geophysical anomalies associated with a volcanic eruption

turbulent flow structure at a discordant river confluence asymmetric jet dynamics with implications for channel morphology only a handful of field studies have examined turbulent flow structure at discordant confluences the dynamics of flow at such confluences have mainly been examined in the laboratory this paper reports results of a field based investigation of turbulent flow structure at a discordant river confluence these results support the hypothesis that flow at a discordant alluvial confluence with a velocity ratio greater than two exhibits jet like characteristics scaling analysis shows that the dynamics of the jet core are quite similar to those of free jets but that the complex structure of flow at the confluence imposes strong effects that can locally suppress or enhance the spreading rate of the jet this jet like behaviour of the flow has important implications for morphodynamic processes at these types of confluences the highly energetic core of the jet at this discordant confluence is displaced away from the riverbed thereby inhibiting scour however helical motion develops adjacent to the jet particularly at high flows which may promote scour numerical experiments demonstrate that that the presence or absence of a depositional wedge at the mouth of tributary can strongly influence detachment of the jet from the bed and the angle of the jet within the confluence

an experimental study of fluvial processes at asymmetrical river confluences with hyperconcentrated tributary flows abstract this paper reports findings from experimental studies of sediment transport and bed morphology at asymmetrical confluences with hyperconcentrated tributary flows in the upper yellow river the results indicate that the hyperconcentrated flow confluence can be divided into four hydraulic regions including the backwater zone above the upstream junction corner the maximum velocity area the separation flow zone and the post confluence region downstream of the junction corner the bed morphology also consists of four basic elements including the sandbar in the backwater zone the bar in the separation flow zone the thalweg for flow conveyance and sediment transport and bars in the reach downstream of the separation zone the sediment load of the hyperconcentrated flow from the tributary was the most important control on fluvial processes at such confluences the increase in deposition in the backwater zone as the sediment load increased was almost linear and the depth of sediment deposition in the backwater zone was approximately normal in distribution the validity of a conceptual model for discriminating the status of the backwater effect developed earlier from field data using the relationship between the sediment load and water volume of hyperconcentrated flows was confirmed by the experiments deposition in the reach downstream of the junction sandbar height in the backwater zone and the width and length of the separation zone bar all tended to increase as the sediment load in the tributary increased an obvious upstream directed density current occurred in the backwater zone when the sediment concentration of the hyperconcentrated flow exceeded a critical value the travel distance of the density current increased as the sediment load in the tributary increased a formula was proposed based on sediment continuity for estimating the deposition volume in the reach downstream of the junction compared with ordinary sediment laden flow confluences hyperconcentrated flow confluences have a sandbar in the backwater zone associated with an upstream directed density current that may sometimes block the main channel hyperconcentrated flow confluences have a thalweg and so are different from debris flow confluences which have a fan shaped deposit

structure and evolution of interannual variability of the tropical pacific upper ocean temperature n n yearly in situ temperature anomaly data for the period 1961 1990 are analyzed to reveal the dominant structure and evolution of interannual variability of the tropical pacific upper ocean we use multivariate empirical orthogonal function eof analyses to detect the principal three dimensional structure related to el nino there are well defined subsurface thermal patterns characterized by a prominent sea saw structure with opposite anomaly polarity in the equatorial and off equatorial tropical north pacific regions during an el nino year a positive temperature anomaly is found in the e astern and central tropical upper ocean this is accompanied by a corresponding negative anomaly at subsurface depths in the west with a maximum at 100 150 m off the equator unlike sea surface temperature whose variations are confined largely to the east with one dominant polarity temperature variations at 50 200 m depths have a dipole pattern with out of phase oscillations in the central equatorial pacific and in the western tropical north pacific a reverse pattern of these anomalies is observed during a la nina year evolution between el nino and la nina involves a significant zonal transfer of anomaly phase across the equatorial basin as well as across the off equatorial tropical north pacific showing consistent and coherent variations from west to east from subsurface to sea surface and from on the equator to off the equator this phase propagation is more evident at subsurface depths than that at the sea surface suggesting a continual movement of anomaly pattern in succession eastward along the equator and westward off the equator of the tropical north pacific with spatial inhomogeneities of zonal propagation in longitude our analyses present evidence of the manner in which temperature anomalies evolve at subsurface ocean depths in the tropical pacific thus providing an observational basis for evaluating theoretical studies and model simulations the dynamical implication of these results is also discussed

changes in channel size at river confluences with coarse bed material a field survey of thirty stream junctions from a small watershed together with data collected by miller 1958 allowed us to investigate morphometric adjustments occurring at confluences the model proposed by roy and woldenberg 1986 was slightly modified and used as a tool for morphometric analysis two parameters are necessary in order to evaluate the rate of change in channel size at a confluence the area ratio channel capacity above the confluence channel capacity below the confluence and the discharge ratio discharge of the minor tributary discharge of the major tributary our data show that total channel capacity tends to decline below most confluences a reduction in cross sectional area implies an increase in average flow velocity this interpretation is consistent with lyell s observations and with results from recent flume experiments best and reid 1984

hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material a survey of flows was conducted at a river confluence with coarse bed material bridges were installed on both tributaries at the confluence and farther downstream on the receiving stream at these stations flow velocities were measured over a dense grid for seven conditions ranging from very low flows to the bankfull stage hydraulic geometry relationships established at all four stations revealed that flow is accelerated through the confluence as stage rises at bankfull discharge average velocity is 1 6 times higher at the confluence than on either tributary flow acceleration occurs at and above intermediate flow stages and is concentrated at the centre of a linear pool located at the confluence the development of a zone of high shear stress is also associated with the cell of high flow velocity flow acceleration is dissipated at the exit of the pool where water surges over boulder ribs the acceleration is not related to the development of flow separation zones as observed by best and reid 1984 for wide junction angles nor is it explained by the reduction of the friction exerted by the banks acceleration is associated with the plan geometry of the confluence with the lateral slopes which permit water to converge and with a reduction in grain roughness at the confluence owing to the curvature of the tributary and to the acute angle of entry relative power losses through the confluence decrease with increasing stages

how different cross sectional shapes influence the separation zone of an open channel confluence abstracta key feature of a schematized open channel confluence is the separation zone that is present when the tributary flow detaches from the downstream corner of the confluence this zone of rec

time averaged flow structure in the central region of a stream confluence previous process oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel this study examines in detail the spatial evolution of the time averaged downstream velocity cross stream velocity and temperature fields between the junction apex where the flows initially meet and the entrance to the downstream channel a well defined vertically oriented mixing interface exists within this portion of the confluence suggesting that lateral mixing of the incoming flows is limited the downstream velocity field near the junction apex is characterized by two high velocity cores separated by an intervening region of low velocity or recirculating fluid in the downstream direction the high velocity cores move inwards towards the mixing interface and high velocity fluid progressively extends downwards into a zone of scour resulting in an increase in flow velocity in the centre of the confluence the cross stream velocity field is dominated by flow convergence but also includes a component associated with a consistent pattern of secondary circulation this pattern is characterized by two surface convergent helical cells one on each side of the mixing interface the helical cells appear to be the mechanism by which high momentum fluid near the surface is advected downwards into the zone of scour for transport ineffective flows the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence although the flow structure of formative events was not measured directly in this study documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows except for the fact that formative flows are not constrained by but can reshape the bed morphology the results of this investigation are consistent with and augment previous findings on time averaged flow structure in the downstream portion of the confluence 1998 john wiley sons ltd

rill network development and sediment budgets threshold conditions for rill incision are well known but few studies have examined the effect of network geometry on water and sediment fluxes within an evolving rill system this paper reports the first in a series of studies designed to identify the influence of soil properties on rill network and confluence geometry and on water and sediment fluxes in rill systems the object of this study was to prepare detailed rill network water and sediment budgets identifying important sources and sinks simulated rainfall experiments were carried out on a canadian silt loam soil in a 7 1 m 2 4 m flume on a 5 slope rill networks of varying complexity developed which were ultimately constrained by flume boundaries sediment and water fluxes and hydraulic conditions were measured within networks and at a terminal weir networks evolved by initial knickpoint incision in the lower flume as flow shear velocities reached critical levels of 4 5 5 cm s 1 followed by headward migration and tributary development microtopography determined tributary location but the timing and intensity of development were controlled by the incision and migration rate of the main channel which changed local thalwegs and base levels raising shear velocities on side slopes above critical levels sediment discharge at the weir broadly reflected rill incision intensity and transport limited conditions but as active incision moved headward the linkage became attenuated and identification of discrete erosional incidents e g local bank collapse in the signal of the weir record became very difficult detailed water and sediment budgets showed much more complex patterns of localized incision and deposition within networks strongly influenced by local changes in thalweg and in water discharge due to seepage or return flow and by the effect of confluences on hydraulic conditions results indicate the value of detailed sediment budgets in interpreting weir water and sediment flux records and the necessity of linking such measurements for erosion plots and hillslope segments to rill network characterization experimental results are consistent with a simple model of rill system evolution based on channel incision headward migration and the critical shear velocities for rill initiation copyright 2000 john wiley sons ltd

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

interdecadal variability of the pacific ocean model response to observed heat flux and wind stress anomalies variability of the pacific ocean is examined in numerical simulations with an ocean general circulation model forced by observed anomalies of surface heat flux wind stress and turbulent kinetic energy tke over the period 1970 88 the model captures the 1976 77 winter time climate shift in sea surface temperature as well as its monthly seasonal and longer term variability as evidenced in regional time series and empirical orthogonal function analyses examination of the surface mixed layer heat budget reveals that the 1976 77 shift was caused by a unique concurrance of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi month period preceding the shift in both the central pacific region where cooling occurred and the california coastal region where warming occurred in the central pacific the warm conditions preceding and the cold conditions following the shift tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux tke input into the mixed layer which deepens in the model after the shift from the early 1970s to the late 1970s and 1980s since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of the 1976 77 shift it appears that the midlatitude part of the shift was driven by the atmosphere although effects of midlatitude ocean atmosphere feedback are still possible the surface mixed layer heat budget also reveals that in the central pacific the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while horizontal advection anomalies are roughly half that size in the california coastal region in contrast where wind variability is much weaker than in the central pacific horizontal advection and vertical mixing effects on the mixed layer heat budget are only one quarter the size of typical heat flux input anomalies

indices of el nino evolution abstract to characterize the nature of el nino southern oscillation enso sea surface temperature sst anomalies in different regions of the pacific have been used an optimal characterization of both the distinct character and the evolution of each el nino or la nina event is suggested that requires at least two indices i sst anomalies in the nino 3 4 region referred to as n3 4 and ii a new index termed here the trans nino index tni which is given by the difference in normalized anomalies of sst between nino 1 2 and nino 4 regions the first index can be thought of as the mean sst throughout the equatorial pacific east of the date line and the second index is the gradient in sst across the same region consequently they are approximately orthogonal tni leads n3 4 by 3 to 12 months prior to the climate shift in 1976 77 and also follows n3 4 but with opposite sign 3 to 12 months later however after 1976 77 the sign of the tni leads and lags are reversed

the planform mobility of river channel confluences insights from analysis of remotely sensed imagery river channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment and which are locations for the preservation of thick fluvial deposits overlying a basal scour despite their importance there has been little study of the stratigraphic characteristics of river junctions or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential as a result although it is known that confluences can migrate through time models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location this is problematic for a number of reasons not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes such as confluence scour and that driven by allocyclic controls such as sea level change this paper investigates the spatial mobility of river confluences by using the 40 year record of landsat imagery to elucidate the styles rates of change and areal extent over which large river confluence scours may migrate on the basis of these observations a new classification of the types of confluence scour is proposed and applied to the amazon and ganges brahmaputra meghna gbm basins this analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally thus in the gbm basin a high sediment supply large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80 of large confluences are mobile over this 40 year window conversely this figure is 40 for the amazon basin these results highlight that i the potential areal extent of confluence scours is much greater than previously assumed with the location of some confluences on the jamuna brahmaputra river migrating over a distance of 20 times the tributary channel width ii extensive migration in the confluence location is more common than currently assumed and iii confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield

hydrodynamic characteristics and channel morphodynamics at a large asymmetrical confluence with a high sediment load main channel abstract previous studies of confluences have concentrated mainly on confluences of small streams confluences with simple geometric configurations and confluences of curving rivers however the confluences of natural rivers especially large confluences of high sediment load rivers generally have complex geometric confluent boundaries and various incoming flow and sedimentation conditions in this study several field measurements of the three dimensional velocity components water surface elevation bed topography and suspended sediment particle sizes reveal the complex hydrodynamic characteristics and bed morphology of the asymmetrical confluence of the high sediment load yellow river and its clear tributary the fen river the results show that the water surface morphology of this confluence is elevated in the stagnation zone mixing interface and the opposite side of the tributary whereas the converse is true at the downstream junction corner the geomorphologic features of the bed show a small scour hole and a long narrow scour belt under conventional hydrological conditions or a larger scour hole under hydraulic conditions when the value of the momentum flux ratio is much greater than one in contrast to the confluences of small streams and curving rivers this confluence does not exhibit continuous flow separation or helical motion but has pronounced regions such as a stagnation zone flow acceleration and deflection areas a shear layer mixing interface and a flow recovery zone the position and size of these regions vary with the momentum flux ratio the particle size of suspended sediment at the confluence is normally distributed and the changing sedimentation process corresponds to the scour and siltation state of the bed in general the particle size of the suspended sediment is small in the stagnation zone and the downstream channel near the tributary but large in the acceleration zone and shear layer these results are helpful to better understand the hydrodynamic and morphodynamics characteristics of confluences with complex geometric configurations and junction conditions

late winter generation of spiciness on subducted isopycnals the origins of density compensating anomalies of temperature and salinity spice are investigated using a model forced with the most realistic surface products available over the 40 years 1958 97 in this hindcast the largest interannual spiciness anomalies are found in the pacific ocean near the isopycnal s 0 5 25 5 where deviations as great as 1 28c and 0 6 psu are generated equatorward of winter outcropping in the eastern subtropics in both hemispheres these source regions are characterized by very unstable salinity gradients and low mean density stratification in winter two related signatures of winter mixing in the southeast pacific sep are density that is well mixed deeper than either temperature or salinity and subsurface density ratios that approach 1 all ocean basins in the model are shown to have regions with these characteristics and signatures however the resultant spiciness signals are focused on different isopycnals ranging from s 0 5 25 0 in the northeast pacific to s 0 5 26 5 in the south indian ocean a detailed examination of the sep finds that large positive anomalies are generated by diapycnal mixing across subducted isopycnals e g s 0 5 25 5 whereas negative anomalies are the result of a steady isopycnal advection moderated by vertical advection and heave there is considerable interannual variability in the strength of anomalies and in the density on which they occur historical observations are consistent with the model results but are insufficient to verify all aspects of the hindcast including the processes of anomaly generation in the sep it was not possible to relate isopycnal anomaly genesis to local surface forcing of any kind a complex scenario involving basinwide circulation of both the ocean and atmosphere especially of surface water through the subtropical evaporation zones is put forward to explain the decadal time scale evident in sep salinity anomalies on s 0 5 25 5 pacific anomalies generated on s 0 5 25 5 can be traced along mean geostrophic streamlines to the western boundary where decadal salinity variations at 78s are about 2 times as large order 60 1 psu as at 128n although there may be more variance on shallower isopycnals in the north at least portions of the s 0 5 25 5 signals appear to continue along the boundary to a convergence at the equator suggesting that the most robust sources of pacific spiciness variance coincide with equatorial exchange pathways

evidence for stronger el nino southern oscillation enso events in a mid holocene massive coral we present a 47 year long record of sea surface temperature sst derived from sr ca and u ca analysis of a massive porites coral which grew at 4150 calendar years before present b p in vanuatu southwest tropical pacific ocean mean sst is similar in both the modern instrumental record and paleorecord and both exhibit el nino southern oscillation enso frequency sst oscillations however several strong decadal frequency cooling events and a marked modulation of the seasonal sst cycle with power at both enso and decadal frequencies are observed in the paleorecord which are unprecedented in the modern record

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

flow and particle paths at a natural river confluence with coarse bed material abstract an investigation of flow and particle trajectories was conducted at a gravel bed confluence where the junction angle is gradually reduced from 60 to 20 as the tributary joins the main channel a shallow and elongated scour zone is present at the mouth of the tributary three flow stages ranging from 1 7 to 1 2 bankfull were recorded flow velocity vectors were measured from five bridges installed across the channels sediment transport paths were determined by following the progression of seeded marked particles through the confluence from may to october 1986 the results show that flow structure is responsive to stages at lower stages less than 1 3 bankfull flow velocity vectors are controlled by local bed topography and specifically by the steeper slopes around the scour zone at higher stages velocity vectors become aligned with the plan geometry of the confluence for all stages surface flow velocities are higher in the scour zone bed shear stress computed from the law of the wall for the deepest flow is higher at the bottom of the scour zone compared to its sides head and exit the trajectories of particles seeded on the tributary pass through the scour zone particles coming from the main channel displayed a dual pattern first moving laterally towards the scour zone early in the season then following paths that are consistent with the plan geometry of the confluence this shift in pattern reflects an increase in the peak discharges during the survey period these results highlight the role of stage in controlling the flow dynamics and particle paths at the confluence

a theory of interdecadal climate variability of the north pacific ocean atmosphere system abstract a linear coupled model for the atmosphere upper ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the north pacific in this model wind stress anomalies over the north pacific are related to anomalies in the meridional temperature gradient of the upper ocean the latter depends upon air sea thermodynamical feedbacks and meridional heat transport by upper ocean currents slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic rossby waves which carry out the meridional heat transport uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white noise wind stress forcing with organized spatial structure coupled dynamics produce a basin scale interdecadal oscillatory mode this mode arises from the dynamic coupling and the memory of the system residing in the slow gyre circulation ad

junction angles in drainage networks the systematics of tributary junction angles are an important link between the topology of a drainage network and its topography because the angle of junction depends on the relative slopes of the intersecting tributaries i present here a model for stream junction angle as a function of position link magnitude within the network which quantitatively predicts the observed down network increase of junction angle for tributaries joining progressively larger recipient links good agreement between model and data exists for mature drainage networks mapped at varying scales additionally the dependency of junction angle on relative tributary slopes suggests that as individual links within the network modify their slopes to evolve toward graded longitudinal profiles there exists a central tendency within the network modulated by structural and lithologie controls for evolution to an equilibrium or graded planimetric pattern extraction of process related as opposed to purely topological information from network planimetric pattern is important in terrestrial and planetary remote sensing applications