sediment trapping characteristics of a pit trap and the helley smith sampler in a cobble gravel bed river 1 although designed for granule gravels the helley smith hs bed load sampler is often used in streams with much coarser beds to compare magnitudes and grain size distributions of sediment samples collected by a pit trap and a standard hs sampler we conducted 22 sampling events over a wide range of flows during a snowmelt freshet in a stream with a coarse gravel bed each sampling event consisted of three simultaneous measurements one made with a pit trap installed in the bed one with a hs sampler placed on the bed directly beside the pit trap hs i and one with a hs sampler placed on the downstream rim of the pit trap hs p we summed the catches of the pit trap and hs p to estimate actual bed load transport the pit trap and hs i catches were then compared to the summed sample the pit trap yielded a remarkably consistent positively skewed sigmoidal distribution of catch efficiency for all 22 measurements with near 100 efficiency for material larger than 2 8 mm the hs i sampler was more variable in its catch and trapping efficiency exhibiting low trapping efficiency for midrange material 0 71 to 16 mm but high efficiency for finer material the results cast doubt on the accuracy of bed load data sets collected by helley smith samplers in coarse gravel channels

south indian countercurrent and associated fronts a striking feature of the south indian ocean circulation is the presence of the eastward south indian countercurrent sicc that flows in a direction opposite to that predicted by the classical theories of wind driven circulation several authors suggest that the sicc resembles the subtropical countercurrents stccs observed in other oceans which are defined as narrow eastward jets on the equatorward side of subtropical gyres where the depth integrated flow is westward these jets are associated with subsurface thermal fronts at thermocline depths by the thermal wind relation however the subsurface thermal front associated with the sicc has not been described to date other studies conjecture an important role for salinity in controlling the sicc in the present work we analyze three argo based atlases and data from six hydrographic cruises to investigate whether the sicc is accompanied by permanent thermal and density fronts including salinity effects the seasonal cycle of these fronts in relation to the sicc strength is also investigated we find that the sicc is better described as composed of three distinct jets which we name the northern central and southern sicc we find that the southern sicc around 26 s has an associated thermal front at subsurface depths around 100 200 m with salinity being of secondary importance the southern branch strength is related to mode waters poleward of the front similar to a stcc like current however the sicc multiple jet structure seems to be better described as resulting from pv staircases

subantarctic mode water formation destruction and export in the eddy permitting southern ocean state estimate subantarctic mode water samw is examined using the data assimilating eddy permitting southern ocean state estimate for 2005 and 2006 surface formation due to air sea buoyancy flux is estimated using walin analysis and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30 s accounting for volume change with time water in the density range 26 5 27 1 kg m 3 that includes samw is exported northward in all three ocean sectors with a net transport of 18 2 17 1 sv 1 sv 106 m3 s 1 for years 2005 2006 air sea buoyancy fluxes form 13 2 6 8 sv diapycnal mixing removes 14 5 12 6 sv and there is a volume loss of 19 3 22 9 sv mostly occurring in the strongest samw formation locations the most vigorous samw formation is in the indian ocean by air sea buoyancy flux 9 4 10 9 sv where it is partially destroyed by diapycnal mixing 6 6 3 1 sv there is strong export to the pacific where samw is destroyed both by air sea buoyancy flux 1 1 4 6 sv and diapycnal mixing 5 6 8 4 sv in the south atlantic samw is formed by air sea buoyancy flux 5 0 0 5 sv and is destroyed by diapycnal mixing 2 3 1 1 sv peaks in air sea flux formation occur at the southeast indian and southeast pacific samws seisamws sepsamws densities formation over the broad samw circumpolar outcrop windows is largely from denser water driven by differential freshwater gain augmented or decreased by heating or cooling in the seisamw and sepsamw source regions however formation is from lighter water driven by differential heat loss

north atlantic potential vorticity and its relation to the general circulation abstract maps and sections of the large scale north atlantic potential vorticity q are presented here q is fd dz where f is the coriolis frequency the potential density and z the vertical coordinate they bear on the general circulation and on geostrophic waves instability and turbulence in many ways both eulerian and lagrangian mean circulations proceed along isostrophes q constant in a zero dissipation region in a resting fluid q varies simply as the sine of the latitude but we show here that the wind driven circulation reshapes the q field creating bowls and plateaus which allow the flow to cross latitude circles the implied nature of the western boundary current is very different than in classical frictional theory the maps show a region of uniform potential vorticity in the wind gyre 26 5 27 0 which fills the ocean between 15 37 n and 20 80 w such regions were prominent features of a circulation theory of rhines and young 1982a b at deeper levels and close to surface out

ventilation of the subtropical north pacific the shallow salinity minimum abstract the shallow salinity minimum of the subtropical north pacific is shown to be a feature of the ventilated wind driven circulation subduction of low salinity surface water in the northeastern subtropical gyre beneath higher salinity water to the south causes the salinity minimum variation of salinity along surface isopycnals causes variations in density and salinity at the minimum a model of ventilated flow is used to demonstrate how the shallow salinity minimum can arise the model is modified to account for nonzonal realistic winds it is also extended to examine the three dimensional structure of the western shadow zone the boundary between the subtropical and subpolar gyres is given by the zero of the zonal integral of ekman pumping the western shadow zone fills the subtropical gyre at the base of the ventilated layers and decreases in extent with decreasing density for parameters appropriate to the north pacific the eastern shadow zone is of very limited extent observations of salini

wave propagation in the marginal ice zone model predictions and comparisons with buoy and synthetic aperture radar data in this paper the ocean wave dispersion relation and viscous attenuation by a sea ice cover are studied for waves propagating into the marginal ice zone miz the derivation of the dispersion relation and the viscous attenuation by an ice sheet are discussed for waves under flexure and pack compression in the miz the flexure effect is important for short waves for a fixed wave period the changes in wavelength and group velocity as a function of ice thickness are significant in turn the exponential wave attenuation rate shows a rollover at short wave periods whereby the rapid increase in wave attenuation rate with decreasing wave period slows down or even turns into a decrease the labrador ice margin experiment limex conducted on the miz off the east coast of newfoundland canada in march 1987 provides us with aircraft synthetic aperture radar sar imagery wave buoy and ice property data on the basis of the wave number spectrum from the sar data and the concurrent wave frequency spectrum from the ocean buoy data and accelerometer data on the ice during limex 87 the dispersion relation has been estimated and compared with a model wave energy attenuation rates are estimated from sar data and the ice motion package data which were deployed at the ice edge and into the ice pack and compared with the model the model data comparisons are reasonably good for the ice conditions observed during limex 87 some previously reported data of wave attenuation in the miz are revisited for model comparison

the dependence of southern ocean meridional overturning on wind stress an eddy resolving numerical model of a zonal flow meant to resemble the antarctic circumpolar current is described andanalyzedusing the framework of j marshallandt radko in additionto windand buoyancy forcing at the surface the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation moc to exist at depth the strength of the residual moc is diagnosed for different strengths of surface wind stress it is found that the eddy circulation largely compensates for the changes in ekman circulation the extent of the compensation and thus the sensitivity of the moc to the winds depend on the surface boundary condition a fixed heat flux surface boundary severely limits the ability of the moc to change an interactive heat flux leads to greater sensitivity to explain the moc sensitivity to the wind strength under the interactive heat flux transformed eulerian mean theory is applied in which the eddy diffusivity plays a central role in determining the eddy response a scaling theory for the eddy diffusivity based on the mechanical energy balance is developed and tested the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress the moc sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments

c band sar observations of marginal ice zone rheology in the labrador sea synthetic aperture radar sar imagery collected during the limex 87 field program in the labrador sea is used to qualitatively interpret the deformation modes in play throughout the experiment the ice cover exhibited two distinct rheologies separated by a clear line of shear a nonlinear viscous outer regime and a quasi brittle inner regime a single constitutive relation capable of effectively modeling both is unlikely within a plastic rate independent formulation rate dependent effects are discussed in relation to favoring brittle fraction in an otherwise ductile material

homogenization of potential vorticity in planetary gyres the mean circulation of planetary fluids tends to develop uniform potential vorticity q in regions where closed time mean streamlines or closed isolines of mean potential vorticity exist this state is established in statistically steady flows by geostrophic turbulence or by wave induced potential vorticity flux at the outer edge of the closed contours the expelled gradients of q are concentrated beyond this transition lies motionless fluid or a different flow regime in which the planetary gradient of q may be dominant the homogenized regions occur where direct forcing by external stress or heating within the closed isoline is negligible upon the potential density surface under consideration in the stably stratified ocean such regions are found at depths greater than those of direct wind induced stress or penetrative cooling in channel models of the atmosphere we again find constant q when mesoscale eddies cause the dominant potential vorticity flux in the real atmosphere the results here can apply only where internal heating is negligible the derivations given here build upon the prandtl batchelor theorem which applies to non rotating steady two dimensional flow supporting evidence is found in numerical circulation models and oceanic observations

environmental reconstructions of the upper 500 m of the southern indian ocean over the last 40 ka using radiolarian protista proxies abstract in 2007 we demonstrated that radiolarians are proxies for a wide range of oceanic physico chemical properties from the surface to depths of up to 500 m below sea level in this study our results are refined and correspondence analysis ca scores derived from census counts of radiolarian subfossils from southern indian ocean core tops are correlated with the physico chemical properties of the region obtained from the 2005 world ocean database calibration and regression techniques are employed to reconstruct palaeoenvironmental conditions spanning the last 40 ka for four indian ocean cores md88 769 46 04 s 90 06 e md88 770 46 01 s 96 27 e md94 102 43 30 s 79 50 e and md94 103 45 35 s 86 31 e all from close to the southeast indian ridge for the first time reconstructions of temperature salinity dissolved oxygen and the silicate nitrate and phosphate concentrations for a range of water depths are proved possible changes of the oceanic environment and the movement of water masses over the last 40 ka as suggested by these reconstructions are discussed during marine isotope stages 2 and 3 mis 2 and mis 3 the water column at some of the core sites has similar characteristics to the waters south of the polar front today at the mis 1 mis 2 transition the development of the subantarctic mode water is apparent temperature reconstructions include evidence of the antarctic cold reversal and the holocene optimum

observation of wave refraction at an ice edge by synthetic aperture radar in this note the refraction of waves at the ice edge is studied by using aircraft synthetic aperture radar sar penetration of a dominant swell from open ocean into the ice cover was observed by sar during the labrador ice margin experiment limex conducted on the marginal ice zone miz off the east coast of newfoundland canada in march 1987 at an ice edge with a large curvature the dominant swell component disappeared locally in the sar imagery six subscenes of waves in the miz from the sar image have been processed revealing total reflection refraction and energy reduction of the ocean waves by the ice cover the observed variations of wave spectra from sar near the ice edge are consistent with the model prediction of wave refraction at the ice edge due to the change of wave dispersion relation in ice developed by liu and mollo christensen 1988

on too fast baroclinic planetary waves in the general circulation abstract recent altimetric observations of the ocean surface reveal signatures of long planetary waves near the annual frequency band comparisons of the observed wave speeds and those predicted by standard linear theory suggest that the latter is inadequate as it yields different westward speeds than those measured in the extratropical latitudes the predicted speeds are typically slower than the observations here the problem of long baroclinic wave propagation in a forced stratified ocean is considered theoretically with a view toward explaining these observations of too fast planetary waves from a quasigeostrophic analysis it is argued that baroclinic waves in a sheared environment are accelerated to the west via their interactions with both the mean advective field and the mean potential vorticity field conditions under which the ratio of actual to linear phase speeds matches the observed ratio are computed and found to be typical of the open ocean extensions of these ideas to continuously st

a comparison of sir b directional ocean wave spectra with aircraft scanning radar spectra directional ocean wave spectra derived from shuttle imaging radar b sir b l band imagery collected off the coast of southern chile on 11 and 12 october 1984 were compared with independent spectral estimates from two airborne scanning radars in sea states with significant wave heights ranging from 3 to 5 meters the sir b derived spectra at 18 and 25 off nadir yielded reasonable estimates of wavelengths directions and spectral shapes for all wave systems encountered including a purely azimuth traveling system a sir b image intensity variance spectrum containing predominantly range traveling waves closely resembles an independent aircraft estimate of the slope variance spectrum the prediction of a u s navy global spectral ocean wave model on 11 october 1984 exhibited no significant bias in dominant wave number but contained a directional bias of about 30 espect to the mean of the aircraft and spacecraft estimates

the ventilation of the world s oceans maps of the potential vorticity field abstract maps of potential vorticity q within four different density layers are presented for the north and south atlantic north and south pacific and indian oceans potential vorticity is evaluated as f 0 z a prescription that is valid for large slow scales and that allows its mapping from hydrographic data alone here f is the coriolis frequency p0 the potential density and z the vertical coordinate it is shown that the character of a layer within the thermocline of a subtropical gyre will vary greatly depending on the extent to which the layer is isolated from surface boundary conditions and forcing fluid particles recirculating within ventilated layers have their potential vorticity and other properties reset with virtually every circuit round the gyre when they pass through the outcrop zone by contrast particles within unventilated or dynamically isolated layers have time to share their properties with neighboring particles and approach a common homogenized state the north

potential vorticity distribution in the north pacific abstract vertical sections and maps of potential vorticity 1f z for the north pacific are presented on shallow isopycnals high potential vorticity is found in the tropics subpolar gyre and along the eastern boundary of the subtropical gyre all associated with ekman upwelling low potential vorticity is found in the western subtropical gyre subtropical mode water in a separate patch near the sea surface in the eastern subtropical gyre and extending around the gyre and near sea surface outcrops in the subpolar gyre the last is analogous to the subpolar mode water of the north atlantic and southern ocean meridional gradients of potential vorticity are high between the subtropical and subpolar gyres at densities which outcrop only in the subpolar gyre lateral gradients of potential vorticity are low in large regions of the subtropical gyre on these isopycnals on slightly denser isopycnals which do not outcrop in the north pacific there are large regions of low potential vorticity gradients

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

geographical variability of the first baroclinic rossby radius of deformation global 1 83 18 climatologies of the first baroclinic gravity wave phase speed c1 and the rossby radius of deformation l1 are computed from climatological average temperature and salinity profiles these new atlases are compared with previously published 5 83 58 coarse resolution maps of l1 for the northern hemisphere and the south atlantic and with a 1 83 18 fine resolution map of c1 for the tropical pacific it is concluded that the methods used in these earlier estimates yield values that are biased systematically low by 5 15 owing to seemingly minor computational errors geographical variations in the new high resolution maps of c1 and l1 are discussed in terms of a wkb approximation that elucidates the effects of earth rotation stratification and water depth on these quantities it is shown that the effects of temporal variations of the stratification can be neglected in the estimation of c1 and l1 at any particular location in the world ocean this is rationalized from consideration of the wkb approximation

analysis of flow competence in an alluvial gravel bed stream dupuyer creek montana 1 critical shear stress and unit discharge flow competence models were tested against coarse bed load data from dupuyer creek montana united states maximum particle sizes sampled dmax and d50 to d90 percentiles in the bed load grain size distribution were well correlated with both shear stress and unit discharge bed load grain sizes became coarser with increasing flow strength for the dmax curve shields dimensionless parameter for the surface d50 was estimated at 0 044 and the exponent for relative particle size di d50 was 0 59 in the unit discharge criterion the critical flow to entrain the surface d50 was poorly predicted flow competence relationships based on dmax are prone to the influence of outliers and sample mass variability the mean of the three largest particles dmax 3 is more sensitive to changes in flow strength than the d50 to d90 bed load grain sizes and may represent a good compromise

critical conditions for particle motion in coarse bed materials of nonuniform size distribution initiation of particle motion in a bed material of nonuniform size distribution may be quantified by qci qcr di dr b where qci is the critical unit discharge at which particle size di enters motion qcr is the critical condition for a reference size dr unaffected by the hiding exposure effects associated with nonuniform size distributions i and r refer to percentiles of the distribution and b varies from 0 equal mobility in entrainment of all particle sizes to 1 5 2 5 full size selective transport currently there is no generally accepted method for predicting the value of b flume and field data are therefore combined to investigate the above relationship thirty seven sets of flume data quantify the relationship between critical unit discharge and particle size for bed materials with uniform size distributions used here to approximate full size selective transport field data quantify the relationship for bed materials of nonuniform size distribution at 24 sites with b ranging from 0 15 to 1 3 intersection of the two relationships clearly demonstrates the hiding exposure effect in some but not all cases dr is close to the median size d50 the exponent has two clusters of values b 1 for sites subject to episodic rain fed floods and data collected by bedload pit trap and tracers and b 1 sites with relatively infrequent bedload transport where particle embedding and consolidation could reduce the mobility of coarser particles and for b 0 7 a looser bed structure with frequent transport events allowing hiding exposure and size selection effects to achieve their balance as yet there is no firm evidence for such a dependency on bed structure but variations in b could potentially be caused by factors outside those determining equal mobility or size selection but appearing to affect b in the same way

formation and pathways of intermediate water in the parallel ocean circulation model s southern ocean 1 the formation mechanisms and pathways of intermediate water in the southern ocean are analyzed from output of a high resolution ocean general circulation model deep winter mixed layer formation in the southern ocean is diagnosed from the model results and is found to be mostly consistent with observations diapycnal water mass transformations by air sea fluxes and internal mixing are quantified and split into mean and eddy components the diapycnal formation of the water masses that constitute the antarctic intermediate water layer in the southeast pacific is found to occur mainly in the western pacific ocean in this model in winter convection up to 900 m is found to set the potential vorticity characteristics of this layer eddy fluxes of heat and buoyancy play an important role in the formation of the intermediate waters by transferring water from the southern parts of the subtropical gyres into the antarctic circumpolar current acc and vice versa the effects of eddy fluxes are found to vary significantly along the path of the acc they are strongly concentrated in the regions near the agulhas return current in the indian ocean and the brazil malvinas confluence in the atlantic

enhanced arabian sea intermediate water flow during glacial north atlantic cold phases abstract during the last glacial period polar ice cores indicate climate asynchrony between the poles at the millennial time scale yet surface ocean circulation in large parts of the globe varied in tune with greenland temperature fluctuations suggesting that any anti phase behavior to a substantial degree must lie in the deeper global ocean circulation which is poorly understood outside the atlantic ocean here we present data from the north western indian ocean which indicate that the timing of maxima in northward extensions of glacial antarctic intermediate water gaaiw coincides with dramatically reduced thermohaline overturn in the north atlantic associated with the heinrich ice surge events he the repeated expansion of the gaaiw during hes recorded far north of the equator in the arabian sea suggests that southern hemisphere driven intermediate water mass variability forms an integral part of the inter hemisphere asynchronous climate change behavior at the millennial time scale

airborne sar imagery of ocean surface waves obtained during lewex some initial results the potential and the difficulties of using airborne synthetic aperture radar sar to study ocean surface waves are presented airborne c band sar imagery obtained with the real time sar processor on the canada centre for remote sensing ccrs convair 580 during the labrador extreme waves experiment lewex shows waves propagating through the marginal ice zone as well as through open water regions examples are chosen to illustrate both coherent and noncoherent scene motion effects coherent scene motion effects include velocity bunching acceleration defocus and coherence time limitations noncoherent scene motion effects include scanning distortion and look misregistration the slow platform velocity of airborne sar precludes replication of spaceborne observations largely due to coherent motion effects

preliminary results from woce hydrographic sections at 80 e and 32 s in the central indian ocean the hydrographic properties and circulation along sections at 80 e and 32 s in march 1995 in the indian ocean are described very briefly a halocline was well developed in the tropics a westward coastal jet of fresh bay of bengal water was present at the sea surface at sri lanka with eastward flow of saline arabian sea water below the equatorial undercurrent was well developed as were the deep equatorial jets the indonesian throughflow jet presented a large dynamic signature at 10 to 14 s coinciding with a strong front in all properties to great depth its mid depth salinity minimum is separated from that of the antarctic intermediate water the subantarctic mode water of the southeastern indian ocean imparts its high oxygen ventilation signature to the whole of the transects including the tropical portion the deepest water in the central indian basin is pooled in the center of the basin and its principal source appears to be the sill at 11 s through the ninetyeast ridge northward deep water transports across the 32 s section were similar to those observed in 1987 but the deep water was lower in oxygen and fresher than in 1987 upper ocean waters at 32 s were more saline and warmer in 1995

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

the release of 14 c depleted carbon from the deep ocean during the last deglaciation evidence from the arabian sea article i nfo during the last deglaciation the concentration of co2 in the atmosphere increased and the radiocarbon activity 14 c of the atmosphere declined in two steps corresponding in timing to heinrich stadial 1 and the younger dryas these changes have been attributed to the redistribution of 14 c depleted carbon from the deep ocean into the upper ocean and atmosphere recently reconstructions of 14 c in intermediate waters of the eastern tropical pacific have revealed pulses of very old water during the deglaciation consistent with the release of 14 c depleted carbon from the deep ocean at this time here we present reconstructions of intermediate water 14 c from the northern arabian sea near the coast of oman these reconstructions record significant aging of intermediate waters in the arabian sea during heinrich stadial 1 and to a lesser extent during the younger dryas the timing and magnitude of 14 c depletion in the arabian sea during heinrich stadial 1 is very similar to that previously observed in the eastern north pacific near baja california indicating that similar mechanisms were involved in controlling 14 c at these two sites the most parsimonious explanation of the 14 c records from the arabian sea and baja california remains the release of 14 c depleted carbon from the deep ocean by renewal of upwelling and mixing in the southern ocean these

frontal structure water masses and circulation in the crozet basin due to topographic steering by the crozet and kerguelen plateaus the antarctic circumpolar current acc core in the crozet basin area is shifted to its northernmost position in the southern ocean along the southern limb of the south indian subtropical gyre here the jet like current is embedded within a narrow band 26 5 kg m3 in the western half of the basin and heavier mode 26 7 kg m3 in its eastern half these different varieties of crozet basin mode water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone the area north of the kerguelen plateau appears to be an important source of water mass modification antarctic intermediate water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher oxygen richer colder subantarctic water energetic eddy activities observed along the frontal zone likely enhance such cross frontal exchange of water masses equatorward spreading of the modified thermocline water masses mode water antarctic intermediate water occurs as part of the anticyclonic circulation in the south indian subtropical gyre which is centered within the basin strong injection of north indian deep water into the acc south of amsterdam island is clearly evidenced for the first time this deep oxygen minimum water centered at about 3000 m can be traced to the east of madagascar from where it spreads southward through the eastern part of the crozet basin due to the influence of this water mass north atlantic deep water characteristics of the circumpolar deep water are significantly altered in kerguelen amsterdam passage our results indicate a cyclonic deep level circulation pattern in the crozet basin with a northward flow 9 sv of circumpolar deep water antarctic bottom water as a deep western boundary current and a partially compensating southward flow 3 sv of north indian deep water through the eastern part of the basin it is concluded that the crozet basin constitutes an important passage for the exchange of water masses between the southern ocean and the indian ocean

observations of watermass modification by mesoscale eddies in the subtropical frontal region of the indian ocean sector of southern ocean abstract two eddies one cyclonic and the other anticyclonic in the subtropical frontal region stf of the indian ocean sector of southern ocean so and their role on the watermass modification are described here using the hydrographic data collected during the austral summer february of 2012 and 2013 and also by using argo profiles sampled during the austral summer from 2004 to 2012 the cyclonic eddy transports cold fresh and deeper antarctic intermediate water aaiw to much shallower depths but the anticyclonic eddy pushes the warm more saline subtropical surface water stsw to deeper depths significant changes in the properties of watermasses in terms of temperature and salinity viz stsw colder by 1 c and less saline by 1 psu and subtropical mode water stmw temperature decreased by 0 5 c and salinity by 0 7 psu were noticeable in the centre of cyclonic eddy on the other hand no such changes were found in the stmw properties in the anticyclonic eddy region the observed changes in the properties of watermasses were confirmed with the argo profiles sampled from the cyclonic and anticyclonic eddy regions of the stf a striking feature observed both in the cyclonic eddy and the anticyclonic eddy was the freshening of aaiw and this is consistently evident both in the cruise based observations as well as in the argo profiles it is hypothesized that the freshening in the aaiw at a given depth in the cyclonic eddy is due to shoaling of the pycnocline causing further intrusion of fresh saline water from deeper depths whereas the corresponding freshening in the anticyclonic eddy is due to the conservation of potential vorticity

ventilation potential vorticity homogenization and the structure of the ocean circulation abstract a model for the vertical structure of the oceanic circulation is presented that combines elements of the theory of the ventilated thermocline given by luyten pedlosky and stommel with the theory of rhines and young for the wind driven circulation of an unventilated ocean our model consists of a ventilated thermocline region above an unventilated zone in which motion is limited to pools of constant potential vorticity the model is nonlinear and hence the presence of ventilation affects the dynamics of the unventilated motion and vice versa the planetary geostrophic equations are used and so the quasi geostrophic assumption of rhines and young is relaxed allowing large isopycnal excursions it is shown that the presence of ventilation generally shrinks and weakens the size and vigor of the subsurface pools of homogenized potential vorticity at the same time within those domains the strength of circulation in the ventilated zone is somewhat diminished as the subsurface layers carry a porti

flume study of the effect of relative depth on the incipient motion of coarse uniform sediments an experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0 3 m wide tilting flume the concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport the experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter a revised shields diagram relating critical stress grain reynolds number and depth to grain size ratio has been derived this has implications for the calculation of sediment transport rates design of stable alluvial channels and physical hydraulic modeling

applications of grain pivoting and sliding analyses to selective entrapment of gravel and to flow competence evaluations models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water of special interest is whether such process based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods a detailed derivation is undertaken focusing first on the fluid flow and forces at the particle level the resulting threshold equation for the particle level velocity is then modified to yield the mean entrainment stress for the flow as a whole this approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape the resulting threshold equations contain a number of coefficients e g drag and lift whose values are poorly known but can be constrained by requiring agreement with the shields curve for the threshold of grains in uniform deposits if pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment however if reasonable modifications of those coefficients are made for expected field conditions then the models yield good agreement with the data sliding models where sorting is due entirely to projection distances of the grains above the bed yield somewhat poorer agreement with the field data however the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends the existing measurements lack documentation of the mechanisms of grain movement so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding in spite of such uncertainties the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence

the climatological distribution of potential vorticity over the abyssal ocean climatological maps of the large scale potential vorticity field q along isopycnals are diagnosed for the abyssal waters over the global ocean the inferred patterns of q vary with density the basin and hemisphere at middepths the distribution of q is controlled by the background planetary vorticity gradient on deeper isopycnals there are regions where q contours deviate from latitude circles and even regions of nearly uniform q these different regimes appear to be robust features over the interior of ocean basins as the standard error is found to be relatively small there the nearly uniform q occurs in the deep waters of the north pacific and possibly in the bottom waters of the western north atlantic and north pacific the nearly uniform q has a low magnitude in each case as well as a relatively low variability for the deep waters of the north pacific this nearly uniform q signal appears to be formed when a single water mass enters the basin from a low latitude source

limex 87 ice surface characteristics implications for c band sar backscatter signatures ice surface characterization data collected in 1987 during the labrador ice margin experiment are analyzed to estimate the changes in snow and ice properties at the onset of melt surface measurements were made from an ice research vessel on several days some of which had coincident remote sensing flights at a number of locations in the marginal ice zone these data are used as input parameters in a simple scattering model to simulate the effects of variations in material properties upon c band scattering signatures

airborne synthetic aperture radar observations and simulations for waves in ice the canada centre for remote sensing cv 580 aircraft collected c band synthetic aperture radar sar data over the marginal ice zone off the east coast of newfoundland during the labrador ice margin experiment limex in march 1989 one component of the limex 89 program was the study of ocean waves penetrating the marginal ice zone in this paper we consider nearly coincidental observations of waves in ice by airborne sar and wave induced ice motion measurements we explain the wave patterns observed in the sar imagery and the corresponding sar image spectra in terms of sar wave imaging models these include the well known tilt cross section modulation linear quasi linear and nonlinear velocity bunching forward mapping models fmms and the assertion that the concept of coherence time limitation applies differently to the cases of waves in ice and open water we modify the concept of the scene coherence time to include two parts first a decorrelation time deduced from the inherent azimuth cutoff in the nonlinear velocity bunching fmm and second the intrinsic scene coherence time which is a measure of the time scale over which an open water bragg scattering patch retains its phase structure either of these coherence time scales could dominate the sar image formation process depending upon the environmental conditions the wave spectrum and the wind speed for example these two coherence time scales are independently estimated based upon a quasi linear velocity bunching fmm applied to some of the limex 89 observations observed sar image spectra and forward mapped ice motion package spectra are favorably compared

comparison of shuttle imaging radar b ocean wave image spectra with linear model predictions based on aircraft measurements during october 1984 coincident shuttle imaging radar b synthetic aperture radar sar imagery and wave measurements from airborne instrumentation were acquired the two dimensional wave spectrum was measured by both a radar ocean wave spectrometer and a surface contour radar aboard the aircraft in this paper we compare two dimensional sar image intensity variance spectra with these independent measures of ocean wave spectra to verify previously proposed models of the relationship between such sar image spectra and ocean wave spectra the results illustrate both the functional relationship between sar image spectra and ocean wave spectra and the limitations imposed on the imaging of short wavelength azimuth traveling waves

overview of limex 87 ice observations ice observations results and conclusions are summarized for the march 1987 labrador ice margin experiment limex 87 an international oceanographic study conducted in the pack ice of the grand banks area off the coast of newfoundland included are the ice extent flow size and thickness ice kinematics and rheology ice microwave properties oceanic properties under the ice and penetration of swell into the ice

on the estimation of wave slope and height varnance spectra from sar imagery a procedure is described for using synthetic aperture radar sar imagery to estimate two dimensional ocean wave slope and height variance spectra the logic underpinning the procedure is based both on the results of the numerical simulation of sar wave imagery and analytic descriptions of the sar imaging process the procedure when applied to sar imagery of waves acquired during the recent shuttle imaging radar mission sir b is shown to produce spectra that agree with independent measures of both the two dimensional slope and height variance spectra the implications of these results for future sar missions aimed at measuring ocean waves are considered

theory of synthetic aperture radar ocean imaging a marsen view this paper reviews basic synthetic aperture radar sar theory of ocean wave imaging mechanisms using both known work and recent experimental and theoretical results from the marine remote sensing marsen experiment several viewpoints that have contributed to the field are drawn together in a general analysis of the backscatter statistics of a moving sea surface a common focus for different scattering models is provided by the mean image impulse response function which is shown to be identical to the spatially varying frequency variance spectrum of the local complex reflectivity coefficient from the analysis has emerged a more complete view of the sar imaging phenomenon than has been previously available a new generalized imaging model is proposed

structure and origin of the subtropical south indian ocean countercurrent 1 the structure of the subtropical south indian ocean countercurrent sicc is revealed by altimeter derived absolute geostrophic surface velocities it is a narrow eastward flowing current between 22 and 26 s confined to planetary wave trains which propagate westward through the indian ocean multi year averaging identifies it as a well defined current between madagascar and 80 e continuing with lower intensity between 90 and 100 e it virtually coincides with the northern limit of subtropical underwater subduction geostrophic currents from hydrographic sections closely correspond to these surface patterns volume transports of the countercurrent down to 800 dbar are of order 107 m3 s 1 evidence is provided for a narrow branch of the south equatorial current sec approaching madagascar near 18 s and feeding the southern east madagascar current emc which appears to continue westward around the southern tip of madagascar it then partially retroflects and nourishes the sicc

resolution of the ocean wave propagation direction in sar imagery there is an inherent 180 degrees ambiguity in the derived wave propagation direction when using conventional spectral analysis techniques on standard synthetic aperture radar sar image products three different techniques are successfully used to resolve this ambiguity in propagation direction using a single pass of airborne sar data the fact that the sar is characterized by a large time bandwidth product is used to advantage a sequence of individual looks extracted from the doppler spectrum represents images of the scene collected at a sequence of discretely delayed intervals of time the techniques utilized include cross correlation based motion analysis of a pair of looks phase weighting based upon a pair of looks and the ocean wave dispersion relation and a three dimensional spectral analysis the phase weighting technique is also demonstrated for a seasat sar scene

northern boundary currents and adjacent recirculations off southwestern australia full depth hydrographic and velocity measurements along 115 e south of australia whp section i9s reveal an active northern boundary current regime within 200 300 km of the australian continental shelf and an offshore recirculation regime within the south australian basin in the upper ocean an oft described westward flowing northern boundary current the flinders current extends through the thermocline and the salinity minimum strata of antarctic intermediate water with an opposing eastward recirculation to its south the new data shows that beneath the flinders current an eastward flowing deep northern boundary current transports water of subtropical indian ocean origin supplied by southward deep flow along the western continental slope of australia in the perth basin between these northern boundary currents and the antarctic circumpolar current thermocline and deep geostrophic shears indicate weak recirculations eastward at thermocline levels and westward in the deep water lowered acoustic doppler current profiler data reveal stronger organized recirculations to both the flinders current and the deep northern boundary current with the westward deep recirculation exceeding the eastward transport of the deep northern boundary current and thus representing a net supply of deep water to the perth basin

stability of armour layers in rivers the importance of both the overpassing and the embedding of river bed particles is demonstrated by experiments when a single layer thick armouring layer of uniform d d1 size particles on a bed of uniform finer d d2 size particles is broken at one location the stability of the remainder of the armour layer depends on the tendency of the d d1 particles to embed into the surface of d d2 particles if a d d1 size particle is seated in the exposed position on the surface of the d d2 sized bed sediment it may by virtue of its exposure to the flow be preferentially entrained and it may then overpass the finer bed sediment alternatively the d d1 particle may be too large for overpassing and it will become embedded armour layers composed of particles which embed tend to be more stable

synthesizing the gulf stream thermal structure from xbt data abstract thirty six xbt temperature profiles have been used in a parametric model introduced by hendry to model the gulf stream s thermal structure at 65 w between 200 and 1200 dbar with an rms residual error of 0 56 c velocity has been computed geostrophically relative to 1200 dbar and has been included in calculating potential vorticity analytically from the model the resulting potential vorticity section for 65 w has been compared with the analogous result from hendry s parametric model at 59 w as well as observed potential vorticity sections from 68 to 55 w there is a significant feature in the potential vorticity structure at 65 w not found at 59 w namely a relative minimum in potential vorticity along isopycnals centered at the gulf stream s axis and 350 dbar the modeled potential vorticity sections are consistent with the observation including the downstream disappearance of this feature the dynamical implications of these results are briefly discussed

deep upwelling and diffusivity in the southern central indian basin transport of the deepest water westward through a gap at 28 s in the ninetyeast ridge between the central indian basin and the west australia basin is calculated from hydrographic data collected as part of woce hydrographic program section i8n zero reference velocity levels at mid depth were chosen through consideration of water masses the small transport of 1 0 sv westward of water denser than 4 45 92 kg m 3 through the gap must all upwell in the southern central indian basin of this 0 7 sv upwells between the central and western sill sections that is close to the sill itself using the areas covered by the isopycnal we calculate an average vertical velocity of 3 3 10 3 cm s 1 close to the sill and of 4 2 10 4 cm s 1 west of the sill associated average vertical diffusivities are 105 cm s 1 close to the sill and 13 cm s 1 west of the sill in this bottom layer

eddy subduction and the vertical transport streamfunction subduction the transport of fluid across the base of mixed layer exchanges water masses and tracers between the ocean surface and interior eddies can affect subduction in a variety of ways first eddies shoal the mixed layer by restratifying water columns through baroclinic instabilities second eddies induce an isopycnic transport that leads to the entrainment of warm waters and subduction of cold waters which effectively counters the wind driven overturning circulation in this study the authors use an idealized model to examine these two mechanisms by which eddies influence subduction and to discuss how eddy subduction may be better approximated using the concept of vertical transport streamfunction than the conventional meridional transport streamfunction

sar imaging of waves in water and ice evidence for velocity bunching synthetic aperture radar sar images collected over the arctic marginal ice zone show gravity wave patterns in both the open water and the ice diffuse wave patterns are visible in the water at near range small incidence angles while more distinct wave patterns are visible in the ice across the entire swath the wave patterns in the ice appear as bright lines rather than sinusoidal intensity variations additionally the images show a periodic displacement of the ice water boundary apparently due to doppler shift effects associated with the gravity wave orbital motions these observations are interpreted as evidence for the velocity bunching effect and also illustrate the effects of random scatterer motions in the open water

three dimensional isoneutral potential vorticity structure in the indian ocean the three dimensional isoneutral potential vorticity structure of the indian ocean is examined using world ocean circulation experiment and national oceanic and atmospheric administration conductivity temperature depth data and historical bottle data the distribution of the potential vorticity is set by the indian ocean s source waters and their circulation inside the basin the lower thermocline has a high potential vorticity signal extending westward from northwest of australia and a low signal from the subantarctic mode water in the south the antarctic intermediate water inflow creates patches of high potential vorticity at intermediate depths in the southern indian ocean below which the field becomes dominated by planetary vorticity indicating a weaker meridional circulation and weaker potential vorticity sources wind driven gyre depths have lower potential vorticity gradients primarily due to same source waters homogenization and western shadow zones are not observed the effect dominates the effect of the somali current and the red sea water on the potential vorticity distribution isopleths tilt strongly away from latitude lines in the deep and abyssal waters as the circumpolar deep water fills the basins in deep western boundary currents indicating a strong meridional circulation north of the antarctic circumpolar current the lower gradient intermediate layer surrounded vertically by layers with higher meridional potential vorticity gradients in the subtropical indian ocean suggests that rossby waves will travel 1 3 times faster than standard theory predicts to the south several pools of homogenized potential vorticity appear in the upper 2000 m of the southern ocean where gyres previously have been identified south of australia the abyssal potential vorticity structure is set by a combination of the antarctic circumpolar current and the bathymetry

introduction studies of ocean wave spectra from the shuttle imaging radar b experiment imagery of ocean surface waves obtained from synthetic aperture radar sar provides a unique two dimensional fine resolution view of the ocean wave field which has been long thought to be of considerable value for studying air sea inter action wave generation and propagation and wave current refraction wave spectra from sar have also been thought to be extremely useful for updating and verifying global wave forecast models such promise prompted a sar to be placed on seasat an oceanographic satellite which operated in a rather glorious mode for only a little over 3 months in 1978 this imagery has been well studied and indeed has demonstrated the feasibility of acquiring useful directional wave spectra from space e g vesecky and stewart 1982

an exact analytic representation of a regular or interferometric sar image of ocean swell the problem of obtaining quantitative data on spatial ocean wave spectra from the images of the ocean surface by either regular sar or along track interferometric sar insar is studied the dominant mechanism which allows imaging of ocean waves by sar insar is the so called velocity bunching this mechanism is essentially nonlinear the theoretical analysis of sar insar imagery of the ocean surface due to velocity bunching is performed and nonlinear solutions of the sar insar images of monochromatic waves and of the spectra of these images are obtained analytic expressions are presented which allow for the accurate simulation both sar and insar images of waves with arbitrary lengths heights and propagation directions it is demonstrated that a monochromatic wave expands in the sar insar images into an infinite number of harmonics in addition to the nonlinearity parameters of sar which is related to the velocity bunching mechanism it is shown that for complex insar the degree of nonlinearity depends also on separation time between the two antennas the results of the present study indicate that in addition to the prevailing practice to consider the phase component of the insar image an analysis of the imaginary part of the complex insar map of the ocean surface may provide some supplementary information beneficial in particular for rough sea

water mass and biogeochemical variability in the kerguelen sector of the southern ocean a machine learning approach for a mixing hot spot author s rosso isabella mazloff matthew r talley lynne d purkey sarah g freeman natalie m maze guillaume

the zonal dimension of the indian ocean meridional overturning circulation abstract the three dimensional structure of the meridional overturning circulation moc in the deep indian ocean is investigated with an eddy permitting ocean model the amplitude of the modeled deep indian ocean moc is 5 6 sv 1 sv 106 m3 s 1 a broadly realistic but somewhat weak overturning although the model parameterization of diapycnal mixing is inaccurate the model s short spinup allows the effective diapycnal velocity the sum of model drift and the explicitly modeled diapycnal velocity to resemble the true real ocean diapycnal velocity for this reason the model is able to recover the broad zonal asymmetry in the turbulent buoyancy flux that is suggested by observations the model features a substantial deep depth reversing zonal circulation of nearly 50 of the moc the existence of this circulation brought about by the zonally asymmetric distribution of diapycnal mixing implies a much slower ventilation of the deep indian ocean by a factor of 5 6 than would be in place without zon

when does bedload transport begin in steep boulder bed streams the paper presents the results of field measurements of critical conditions for bedload motion conducted in the rio cordon a steep boulder bed stream in the italian alps under conditions of high reynolds numbers and low relative submergence poorly explored before two methods have been used to determine threshold of motion the displacement of marked clasts and the flow competence approach which uses the largest grain size diameter transported by each flood event the high values of confirm the great relevance of non bedload effective shear stresses in step pool streams given by the additional form drag associated with this morphology relative submergence effects on the dimensionless critical shear stress have been quantified by considering the relative submergence ratio rh d84 and the major effect of relative size on the mobility of each particle in steep widely graded bed mixtures has been evaluated finally the dimensionless critical unit discharge has also been adopted in the regression equations as the critical hydraulic parameter because it may represent an easier parameter to use than the critical shear stress for steep rough mountain rivers copyright 2006 john wiley sons ltd

on the origins of deep and bottom waters of the indian ocean the characteristics of the deep and bottom waters of the indian ocean when illustrated on potential density anomaly surfaces indicate that the waters enter from both the atlantic and pacific oceans the paths of spreading are constrained by the complex topography and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments especially in the northern indian ocean the weddell sea contributes to the densest waters found in the western basins and the ross sea and adelie coast to the densest waters found in the eastern basins both dense water varieties are altered by and incorporated in the less dense water above initially water carried by the circumpolar current then water from the north atlantic and finally by deep water whose characteristics are derived in the northern indian ocean contact with the sediments increases the silica content of the bottom water in the southern ocean in the northern indian ocean the sediments alter the silica of the water at the bottom and together with enhanced salinity from diffusion of saline overflows from the marginal seas above imprint unique markers to the deep water that flows back to the south at middepths the series of ridges between madagascar and australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern indian ocean characteristics southward within the circumpolar current low oxygen deep water from the pacific is carried across the atlantic and into the indian ocean south of africa part flows around the cyclonic weddell sea gyre and part extends across the southern ocean water from another pacific source can be seen near 2000 m extending westward from the tasman sea south of aus tralia and across the indian ocean and perhaps to the aghulas current region southeast of africa in this study we show some maps of deep and bottom water characteristics of the indian ocean which may be used in a qualitative sense to infer pathways of spreading of water below a depth of about 2000 m the indian ocean has numerous ridges and basins figure 1 that severely restrict possible pathways of deep and bottom wa ter flow in the following we will begin by looking at char acteristics at the bottom of the ocean to reveal the path ways of spreading of the densest bottom waters from basin to basin then a series of six potential density anomaly surfaces at progressively shallower depths from about 4000 m up to about 2000 m will be shown to see what character istics influence the deep waters and how pathways of flow are constrained by the bathymetry of the indian ocean the choice of the specific density levels table 1 was based primarily on various property extrema shown by the 18os atlantis ii cruise 93 colored sections that accompany warren s 1981 detailed discussion of the major features of the central indian ocean between madagascar and aus tralia the sections approximately midway between antarctica and the northern indian ocean boundaries are well positioned to show features originating from both the north and the south as well as some brought into the indi an ocean by the antarctic circumpolar current the full ocean area isopycnal maps provide essential evidence on the origins and fates of features seen in the sections and

large and small scale spatial evolution of digitally processed ocean wave spectra from seasat synthetic aperture radar precision digitally processed and corrected synthetic aperture radar sar image spectra along a large portion of seasat orbit 1339 september 28 1978 have revealed a number of unique potentials but also some substantial limitations of spaceborne sar for global ocean wave monitoring sar appears to be capable of monitoring the spatially evolving ocean wave number spectrum with high statistical reliability the large scale few hundred kilometer spatial evolution of the spectrum is generally consistent with the location in time and space of wave generating sources even when multiple wave systems are present on a more local scale of just a few tens of kilometers the nearshore wave number spectrum responds to local depth changes but is too noisy to respond reliably to local current changes the seasat sar was severely restricted in its ability to sense azimuthtraveling along track waves in high sea states azimuth waves corresponding to ocean wavelengths shorter than about 200 m were either undetected or severely attenuated in the higher sea states hs 2 m azimuth waves shorter than 100 m are not detectable in even the lowest sea states moreover there is a tendency for attenuated wave systems to be shifted toward the range direction this effect will significantly impede studies of spatial evolution of seasat sar spectra without some additional correction strategy

airborne sar observations of ocean surface waves penetrating floating ice the results are presented for a new and improved procedure for estimating the synthetic aperture radar sar image spectrum of ocean waves this procedure the spectral sum method involves summing individual image spectra derived from each of the looks of a multilook set an automatic registration of the per look spectral information is achieved accounting for subimage look misregistration due to the wave propagation between looks spectral sum processing is compared with traditional look sum processing as a function of the radar slant range spectral sum processing is applied to sar imagery of waves penetrating the marginal ice zone

a simulation for spaceborne sar imagery of a distributed moving scene a computer simulation that is designed to represent aspects of spaceborn synthetic aperture radar sar imagery of the ocean surface is presented the simulation is unique in that a scatterer density per resolution cell is explicitly included thus allowing the incorporation of various scattering natures from purely specular to purely diffuse the simulation may be applied to ocean surface wave imaging since velocity bunching and scene coherence times are also included certain assumptions inherent in the velocity bunching formulation limit the applicability of the simulation in its present form to spaceborn sar systems only two experiments based on this simulation are considered 1 the effect of varying the target density and 2 the effect of the mean scene coherence time in the imaging of a ocean swell system

the incidence and nature of bedload transport during flood flows in coarse grained alluvial channels a continuous record reveals that the incidence of bedload in a coarse grained river channel changes from flood to flood long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood wave but when floods follow each other closely the bed material is comparatively loose and offers less resistance to entrainment in this case substantial amounts of bedload are generated on the rising limb this is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season this produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse grained channels besides this the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds of final motion this adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory bedload is shown to be characterized by a series of pulses with a mean periodicity of 1 7 hours in the absence of migrating bedforms it is speculated that this well documented pattern reflects the passage of kinematic waves of particles in a slow moving traction carpet the general pattern of bedload including pulsations is shown to occur more or less synchronously at different points across the stream channel

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small

dynamics of the oceanic general circulation the potential vorticity field the general circulation of the oceans is determined by vorticity temperature and salinity the theory of large scale circulation puts a strong constraint on the fields of potential vorticity q new calculations and observations of the north atlantic and north pacific using the field of potential vorticity as a point of comparison reveal strong similarities in particular the observations can be modelled by gyres around which geostrophic contours deform to follow the circulation and within which q is uniform the effects of injected waters with relatively low potential vorticity are significant