does flow variance affect bedload flux when the bed is dominated by grain roughness abstract previous studies have shown that spatial variance in fluid and critical shear stress caused by form roughness can increase bedload flux others have revealed that variance in flow velocity increases with relative submergence and that bed mobility is reduced at lower submergences the paper explores the link between these observations and addresses the following questions is grain roughness sufficient to cause variance in fluid shear stress and an increase in bedload flux if this variance changes with submergence does this mean that the increase is dependent on submergence and does this explain the change in mobility with submergence a simple statistical bedload model based on spatial distributions of fluid and critical shear stress has been used to explore these effects over a water worked gravel deposit estimates of spatially distributed fluid shear stress were gained from laboratory flume measurements of near bed flow velocity and a distribution of critical shear stress was simulated using a discrete particle model of the sediment distribution used in the flume the velocity data were used to describe the change in the spatial distribution of near bed velocity with relative submergence which allowed the effects of submergence on flux to also be considered the main conclusions were i spatial variance in fluid shear stress from grain roughness was not sufficient to have an appreciable effect on bedload flux over water worked gravel beds with a spatial distribution of critical shear stress ii spatial variance in critical shear stress caused by grain roughness had a much larger influence and increased bedload flux this was a similar level of increase observed in studies for conditions where form roughness was high iii spatially averaged estimates of fluid and critical shear stress should not be used to estimate bedload flux even if form roughness is low and iv a rise in relative submergence increased bedload flux this was due to changes in the spatial distribution of near bed velocity and not due to a lowering in the local flow velocity as has been suggested by previous studies

arctic ocean deep water masses in the western iceland sea the icelandic danish contribution to the international greenland sea project was devoted to a study of the distribution and transport of water masses in the western iceland sea each year in september from 1987 to 1991 temperature and salinity were observed on a station net in the area off east greenland from the denmark strait in the south to 72 30 n the data reveal that the deep water column of the western iceland sea is occupied by a number of slightly different water masses which all can be traced back to a formation in the arctic ocean it is especially noticeable that in some years almost undiluted eurasian basin deep water can be observed as far south as the denmark strait

seasonal cycle of mesoscale instability of the west spitsbergen current abstractthe west spitsbergen current wsc is a topographically steered boundary current that transports warm atlantic water northward in fram strait the 16 yr 1997 2012 current and temperature salinity measurements from moorings in the wsc at 78 50 n reveal the dynamics of mesoscale variability in the wsc and the central fram strait a strong seasonality of the fluctuations and the proposed driving mechanisms is described in winter water is advected in the wsc that has been subjected to strong atmospheric cooling in the nordic seas and as a result the stratification in the top 250 m is weak the current is also stronger than in summer and has a greater vertical shear this results in an e folding growth period for baroclinic instabilities of about half a day in winter indicating that the current has the ability to rapidly grow unstable and form eddies in summer the wsc is significantly less unstable with an e folding growth period of 2 days observations of the eddy kinetic energy eke show a p

double averaging concept for rough bed open channel and overland flows theoretical background the goal of this paper is to discuss the spatial averaging concept in environmental hydraulics and develop it further by considering transport equations for fluid momentum passive substances and suspended sediments the averaging theorems the double averaged in time and in space fluid momentum equation and advection diffusion equations for a passive substance and suspended sediments are introduced and their limitations and applications for modeling rough bed flows experimental design and data interpretation are discussed the suggested equations differ from those considered in terrestrial canopy aerodynamics and porous media hydrodynamics by accounting for roughness mobility change in roughness density in space and time and particle settling effects for the case of suspended sediments we show that the form of the double averaged equations may depend on the type of decomposition of flow variables and that this difference may have important implications for modeling we also show that the suggested methodology offers better definitions for hydraulic characteristics variables and parameters such as flow uniformity flow two dimensionality and bed shear stress

spatially averaged open channel flow over rough bed in this paper it is suggested that the double averaged in temporal and in spatial domains momentum equations should be used as a natural basis for the hydraulics of rough bed open channel flows especially with small relative submergence the relationships for the vertical distribution of the total stress for the simplest case of 2d steady uniform spatially averaged flow over a rough bed with flat free surface are derived these relationships explicitly include the form induced stresses and form drag as components of the total stress using this approach we define three types of rough bed flows 1 flow with high relative submergence 2 flow with small relative submergence and 3 flow over a partially inundated rough bed the relationships for the double averaged velocity distribution and hydraulic resistance for all three flow types are derived and compared with measurements where possible the double averaged turbulent and form induced intensities and stresses for the case of regular spherical segment type roughness show the dominant role of the double averaged turbulence stresses and form drag in momentum transfer in the near bed region

formation of denmark strait overflow water by mixing in the east greenland current abstract hydrographic data and current meter records from the east greenland current collected during theperiod 1987 to 1989 around the latitude of the central greenland sea reveal the formation of a watermass having the characteristics of denmark strait overflow water dsow the downstream southwestward transport of this newly formed water mass can be substantial amounting to roughly halfof the outflow of dsow through denmark strait as reported in the literature the formation of dsow inthe east greenland current results from isopycnic mixing of recirculated atlantic water and upper arcticintermediate water substantial formation occurs when the neccessary conditions for baroclinic instabilityare satisfied and the current variability due to mesoscale eddies is high this is subject to significantchanges from year to year

age determination of mixed water masses using cfc and oxygen data we present a new method based on a combination of optimum multiparameter analysis and cfc oxygen mixing analysis to determine the ages of water masses in regions of mixing it enables us to follow water mass movements in greater detail than with other methods which give only the combined pseudoage of a water mass mixture we define the age of a water mass as the time a water parcel needs to spread from its source region where it received its individual tracer characteristics to the point of observation the age distribution allows us to determine pathways of water masses which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes we apply the method to hydrographic data along world ocean circulation experiment section i5 in the south east indian ocean in the thermocline indian central water icw and subantarctic mode water samw meet and mix these distinct water masses have different formation mechanisms but similar temperature salinity characteristics it is shown that the convective formation of samw is a major ventilation mechanism for the lower indian thermocline in the eastern part of the south indian ocean samw dominates the oceanic thermocline and is found to be about 5 years old pure icw is present only in the thermocline of the region 48 degrees 55 degrees e with increasing age with depth confirming the subduction theory while most samw joins the equatorward gyre movement of the southeastern indian ocean some of it propagates westward through turbulent diffusive mixing reaching 55 degrees e after 15 20 years it takes icw some 25 30 years to reach 110 degrees e

bed load transport flume experiments on steep slopes experiments were conducted over uniform gravel bed materials to obtain 143 friction factor values under bed load equilibrium flow conditions in an attempt to add to the scarce data available on slopes between 1 and 9 for shields numbers between 0 08 and 0 29 analyses showed that when only flows over flat beds are considered a distinction must be made between flows with and without bed load more particularly fitting flow resistance equations indicated that the roughness parameter increases by a factor of 2 5 from clear water flow to intense bed load transport between these two states the flow resistance can be approximated by a constant for a given slope

on the hydrography of denmark strait using 111 shipboard hydrographic sections across denmark strait occupied between 1990 and 2012 we characterize the mean conditions at the sill quantify the water mass constituents and describe the dominant features of the denmark strait overflow water dsow the mean vertical sections of temperature salinity and density reveal the presence of circulation components found upstream of the sill in particular the shelfbreak east greenland current egc and the separated egc these correspond to hydrographic fronts consistent with surface intensified southward flow deeper in the water column the isopycnals slope oppositely indicative of bottom intensified flow of dsow an end member analysis indicates that the deepest part of denmark strait is dominated by arctic origin water with only small amounts of atlantic origin water on the western side of the strait the overflow water is a mixture of both constituents with a contribution from polar surface water weakly stratified boluses of dense water are present in 41 of the occupations revealing that this is a common configuration of dsow the bolus water is primarily arctic origin water and constitutes the densest portion of the overflow the boluses have become warmer and saltier over the 22 year record which can be explained by changes in end member properties and their relative contributions to bolus composition

bed load effects on hydrodynamics of rough bed open channel flows the extent to which turbulent structure is affected by bed load transport is investigated experimentally using a nonporous fixed planar bed comprising mixed sized granular sediment with a d50 of 1 95 mm three different sizes of sediment d50 0 77 1 99 and 3 96 mm were fed into the flow at two different rates 0 003 and 0 006 kg m s and subsequently transported as bed load particle image velocimetry piv was used to determine the turbulence characteristics over the fixed bed during clear water and sediment feed cases mean longitudinal flow velocities at any given depth were lower than their clear water counterparts for all but one of the mobile sediment cases the exception was with the transport of fine grains at the higher feed rate in this case longitudinal mean flow velocities increased compared to the clear water condition the coarse grains tended to augment bed roughness but fine grains saturated the troughs and interstices in the bed topography effectively causing the influence of bed irregularities to be smoothed the piv technique permitted examination of both temporal and spatial fluctuations in flow variables therefore many results are presented in terms of double averaged quantities in temporal and spatial domains in particular the form induced stress which arises from spatially averaging the reynolds averaged navier stokes equations and is analogous to the reynolds turbulent stress contributed between 15 and 35 of the total measured shear stress in the roughness layer flow around protrusive roughness elements produced a significant proportion of the turbulent kinetic energy shear production suggesting that this process is highly intermittent near rough beds

observed sources and variability of nordic seas overflow the overflows from the nordic seas maintain the deep branch of the north atlantic circulation that is an important part of the global climate system an analysis of observed ocean temperatures and salinities between 1950 and 2005 shows that the atlantic water circulating in the nordic seas is the main source for change in the overflow waters

optimum multiparameter analysis of mixing in the thermocline of the eastern indian ocean a method of water mass analysis is developed which optimizes the use of all information contained in a hydrographic data set by solving an overdetermined linear set of mixing equations it requires representation of water masses by a finite number of water types and observations from at least as many oceanographic parameters as water types the procedure to establish the water types find the correct weights for the parameters and solve the mixing equations through minimization of the residuals is described in detail the method is applied to a data set from the eastern indian ocean where indian central water and australasian mediterranean water meet to produce a strong front in the thermocline it is shown that although most of the water from the indonesian archipelago is carried westward north of 11 s contributions from the australasian mediterranean water are found as far south as 25 s particularly along the australian shelf as a result of advection with the leeuwin current

feedback between bed load transport and flow resistance in gravel and cobble bed rivers 1 to calculate bed load engineers often use flow resistance equations that provide estimates of bed shear stress in these equations on the basis of the estimate of the appropriate hydraulic radius associated with the bed only the bed roughness ks is commonly set as a constant whatever the bed load intensity however several studies have confirmed the existence of feedback mechanisms between flow resistance and bed load suggesting that a flow dependent bed roughness should be used therefore using a data set composed of 2282 flume and field experimental values this study investigated the importance of these feedback effects new flow resistance equations were proposed for three flow domains domain 1 corresponds to no bed load and a constant bed roughness ks d where d is a representative grain diameter whereas domain 3 corresponds to a high bed load transport rate over a flat bed with a constant bed roughness ks 2 6d between these two domains a transitional domain 2 was identified for which the bed roughness evolved from d to 2 6d with increasing flow conditions in this domain the darcy weisbach resistance coefficient f can be approximated using a constant for a given slope the results using this new flow resistance equation proved to be more accurate than those using equations obtained from simple fittings of logarithmic laws to mean values the data set indicates that distinguishing domains 2 and 3 is still relevant for bed load in particular the data indicate a slope dependence in domain 2 but not in domain 3 a bed load model based on the tractive force concept is proposed finally flow resistance and bed load equations were used together to calculate both shear stress and bed load from the flow discharge the slope and the grain diameter for each run of the data set efficiency tests indicate that new equations implicitly taking a feedback mechanism into account can reduce the error by a factor of 2 when compared to other equations currently in use showing that feedback between flow resistance and bed load can improve field bed load modeling

flow turbulence over fixed and weakly mobile gravel beds characteristics of turbulence structure in quasi 2d flows with static and weakly mobile gravel beds are presented three sets of measurements with acoustic doppler velocimeters in an irrigation canal were used two with subcritical bed shear stress static beds and one with the bed shear stress o close to critical oc weakly mobile bed the analyses included vertical distributions of local mean velocities turbulence intensities turbulent shear stresses velocity auto and cross spectra the quadrant method and high order velocity moments a number of properties of turbulence intensities high order moments streamwise bursting parameters and velocity spectra appeared to be similar for all three flows but some properties were different the most important one was an observed reduction in the von karman constant for the flow with weakly mobile bed comparison of these results with other studies and analogies with drag reducing flows suggest that at o oc 1 the drag on the bed for a given granula

comparing flow resistance law for fixed and mobile bed rills rills caused by run off concentration on erodible hillslopes have very irregular profiles and cross section shapes rill erosion directly depends on the hydraulics of flow in the rills which may differ greatly from hydraulics of flow in larger and regular channels in this paper a recently theoretically deduced rill flow resistance equation based on a power velocity profile was tested experimentally on plots of varying slopes ranging from 9 to 26 in which mobile and fixed bed rills were incised initially measurements of flow velocity water depth cross section area wetted perimeter and bed slope carried out in 320 reaches of mobile bed rills and in 165 reaches of fixed rills were used for calibrating the theoretical flow resistance equation then the relationship between the velocity profile parameter the channel slope and the flow froude number was separately calibrated for the mobile bed rills and for the fixed ones the measurements carried out in both conditions fixed and mobile bed rills confirmed that the darcy weisbach friction factor can be accurately estimated using the proposed theoretical approach for mobile bed rills the data were supportive of the slope independence hypothesis of velocity due to the feedback mechanism stated by govers the feedback mechanism was able to produce quasicritical flow conditions for fixed bed rills obtained by fixing the rill channel by a glue at the end of the experimental run with a mobile bed rill the slope independence of the flow velocity measurements was also detected therefore an experimental run carried out by a rill bed fixed after modelling flow action is useful to detect the feedback mechanism finally the analysis showed that for the investigated conditions the effect of sediment transport on the flow resistance law can be considered negligible respect to the grain roughness effect

recent large increases in freshwater fluxes from greenland into the north atlantic 1 freshwater fw fluxes from river runoff and precipitation minus evaporation for the pan arctic seas are relatively well documented and prescribed in ocean gcms fluxes from greenland on the other hand are generally ignored altogether despite their potential impacts on ocean circulation and marine biology here we present a reconstruction of the spatially distributed fw flux from greenland for 1958 2010 we find a modest increase into the arctic ocean during this period fluxes into the irminger basin however have increased by fifty percent 6 3 0 5 km3 yr 2 in less than twenty years this greatly exceeds previous estimates for the ice sheet as a whole the rate of increase since 1992 is 16 9 1 8 km3 yr 2 the cumulative fw anomaly since 1995 is 3200 358 km3 which is about a third of the magnitude of the great salinity anomaly gsa of the 1970s if this trend continues into the future the anomaly will exceed that of the gsa by about 2025

structure and variability of the shelfbreak east greenland current north of denmark strait abstractdata from a mooring array deployed north of denmark strait from september 2011 to august 2012 are used to investigate the structure and variability of the shelfbreak east greenland current egc the shelfbreak egc is a surface intensified current situated just offshore of the east greenland shelf break flowing southward through denmark strait this study identified two dominant spatial modes of variability within the current a pulsing mode and a meandering mode both of which were most pronounced in fall and winter a particularly energetic event in november 2011 was related to a reversal of the current for nearly a month in addition to the seasonal signal the current was associated with periods of enhanced eddy kinetic energy and increased variability on shorter time scales the data indicate that the current is for the most part barotropically stable but subject to baroclinic instability from september to march by contrast in summer the current is mainly confined to the shelf break with

ventilation of the arctic ocean estimated by a plume entrainment model constrained by cfcs intermediate and deep water formation rates in the arctic ocean are estimated using a plume entrainment model based on shelf slope processes and constrained by tracer distributions within the deep basin each plume is initiated by a fraction rj leaving the shelf break at 200 m followed by an entrainment of rj for every 150 m depth the plume descends the model is tuned by varying rj to achieve the transient tracer cfc 12 and carbon tetrachloride distribution as measured in the nansen amundsen and makarov basins during the oden 1991 expedition and the concentrations in the source waters are calculated assuming a water in 100 equilibrium with the atmosphere the formation of water entering below 500 m is computed to be 1 5 and 1 9 sv when constrained by cfc 12 and ccl4 respectively with a total uncertainty of 0 45 sv sensitivities of the model settings to the entrainment rate degree of saturation of the transient tracer in the source waters and age of the atlantic layer water are investigated processes in the arctic ocean contribute around 1 3 of the approximately 5 6 sv that flows over the scotland greenland ridge with the rest likely attributed processes in the greenland and iceland seas we thus conclude that the arctic ocean has to be included in the discussion of the sensitivity of the greenland scotland overflow to a climate change

a first study of sf6 as a transient tracer in the southern ocean abstract halogenated transient tracers such as the chlorofluorocarbons cfc 11 cfc 12 and cfc 113 are commonly used in oceanographic studies these compounds enter the ocean via the atmosphere and their transient atmospheric concentrations make them valuable as oceanic tracers the trends of rapidly rising atmospheric concentrations of these tracers are however broken and the oceanic signal becomes increasingly more difficult to decipher there is a need for a new transient tracer to complement the existing suit of tracers especially for recently ventilated water masses one compound that looks promising in this respect is sulphur hexafluoride sf 6 an inert gas whose atmospheric concentration is rising rapidly in this paper the use of sf 6 as a transient tracer in recently ventilated waters is discussed and the method for determination of sf 6 in seawater is described tracer data from a section in the atlantic sector of the southern ocean along 6 e from 60 s to 40 s occupied during january 1998 are presented the antarctic polar front found close to 50 s was studied with a densely sampled section down to 400 m depth and sf 6 cfc 12 ratios are used to deduce ventilation ages and dilution factors a comparison of apparent ages derived from a variety of tracers is presented together with the uncertainties in these estimates this work demonstrates that sf 6 is a useful and valuable transient tracer for waters ventilated during the last 20 years

hydraulic estimates of denmark strait overflow 1 upper bounds of the denmark strait deep water overflow from the nordic seas into the north atlantic are estimated using rotating hydraulic theory the calculations are made for the real bottom topography of the strait and are based on hydrographic sections surveyed during a dedicated field experiment in the area results are presented for zero as well as finite but constant potential vorticity and it is shown that the differences in outcome between these two approaches are only minor the calculated interface configurations are found to be in good agreement with those observed and the theoretically obtained transports conform with earlier estimates

least squares multiple tracer analysis of water mass composition we present an inverse method that uses measurements of the concentration of several tracer variables to find the mixture of source water types that best describes in a weighted least squares sense the composition of the water sample the solution includes two physically realistic constraints first that all sources together must sum to 100 and second that no source is present in negative amounts these constraints improve the statistical stability of the solution the method is particularly valuable in regions involving strong mixing among a relatively large number of source water types in contrast to conventional ts analysis more water types can be considered the assumption of mixing along isopycnals is unnecessary and sensitivity to errors in individual tracer measurements is reduced by averaging the influence of a larger number of tracers we demonstrate the method using data from two oceanographic regions analysis of summer data from the continental margin off vancouver island british columbia canada confirms and quantifies the importance of deep upwelling of california undercurrent water onto the continental shelf it also identifies localized outer shelf filaments of low salinity water originating from the vancouver island coastal current finally we reanalyze the north african data of tomczak and confirm his interpretation of diapycnal mixing at the frontal boundary between north atlantic central water and south atlantic central water masses

examining the physical components of boundary shear stress for water worked gravel deposits it is argued in this commentary that in order to understand better the physical mechanisms that generate boundary shear stress over water worked gravel beds flow velocity data should be re evaluated by spatial averaging the reynolds equations to produce time and space averaged double averaged momentum equations a series of laboratory experiments were conducted in which the flow velocities were measured using a piv system over two water worked gravel deposits combined with detailed data on the bed surface topography and vertical porosity the physical components of shear stress were obtained this enabled the various momentum transfer mechanisms present above within and at the interface of a porous fluvial deposit to be quantified this included the examination of the relevant contributions of temporal and spatial fluctuations in velocity and surface drag to the overall momentum transfer it is demonstrated that double averaging represents a logical framework for assessing the fluid forces responsible for sediment entrainment and for investigating intragravel flow and sediment water interface exchange mechanisms within the roughness layer in water worked gravel deposits by considering the physical components of shear stress and their relative sizes it was possible to provide a physically based explanation for existing observations of enhanced mobility of gravel sand mixtures and the transfer of solutes into porous gravel deposits this analysis reveals the importance of obtaining co located high quality spatial data on the flow field and bed surface topography in order to gain a physical understanding of the mechanisms which generate boundary shear stress copyright 2010 john wiley sons ltd

atlantic origin overflow water in the east greenland current abstractdense water masses transported southward along the east coast of greenland in the east greenland current egc form the largest contribution to the denmark strait overflow when exiting den

eddy driven recirculation of atlantic water in fram strait eddy resolving regional ocean model results in conjunction with synthetic float trajectories and observations provide new insights into the recirculation of the atlantic water aw in fram strait that significantly impacts the redistribution of oceanic heat between the nordic seas and the arctic ocean the simulations confirm the existence of a cyclonic gyre around the molloy hole near 80 n suggesting that most of the aw within the west spitsbergen current recirculates there while colder aw recirculates in a westward mean flow south of 79 n that primarily relates to the eastern rim of the greenland sea gyre the fraction of waters recirculating in the northern branch roughly doubles during winter coinciding with a seasonal increase of eddy activity along the yermak plateau slope that also facilitates subduction of aw beneath the ice edge in this area

high frequency variability in the circulation and hydrography of the denmark strait overflow from a high resolution numerical model abstractinitial results are presented from a yearlong high resolution 2 km numerical simulation covering the east greenland shelf and the iceland and irminger seas the model hydrography and circulation in the vicinity of denmark strait show good agreement with available observational datasets this study focuses on the variability of the denmark strait overflow dso by detecting and characterizing boluses and pulses which are the two dominant mesoscale features in the strait the authors estimate that the yearly mean southward volume flux of the dso is about 30 greater in the presence of boluses and pulses on average boluses pulses are 57 1 27 5 h long occur every 3 2 5 5 days and are more frequent during the summer winter boluses pulses increase decrease the overflow cross sectional area and temperatures around the overflow interface are colder warmer by about 2 6 c 1 8 c the lateral extent of the boluses is much greater than that of the pulses in both cases the along stra

flow resistance under conditions of intense gravel transport a study of flow resistance was undertaken in a channelized reach of the north fork toutle river downstream of mount st helens washington hydraulic and sediment transport data were collected in flows with velocities up to 3 m s and shear stresses up to 7 times the critical value needed for bed load transport details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance the plane gravel bed persists up to stresses 3 times critical at which point irregular bed forms appear bed forms greatly increase flow resistance and cause velocity profiles to become distorted the latter arises as an effect of flows becoming depth limited as bed form amplitude increases at very high rates of bed load transport an upper stage plane bed appeared velocity profiles measured in these flows match the law of the wall closely with the equivalent roughness being well represented by ks 3d84 of the bed load the effects noted here will be important in very large floods or in rivers that are not free to widen such as those cut into bedrock

time series of winter convection in the greenland sea 1 in spite of the lack of bottom reaching convection in the greenland sea since the 1980s convection continues to ventilate the greenland gyre down to intermediate depth the variability of this ventilation activity is determined here annually for eight winters according to a multiple criteria catalogue applied to annual summer conductivity temperature depth transects along 75 n the comparison of the ventilation depths with the meteorological forcing the ice cover and the stratification of the water column shows the decisive influence of the hydrographic structure in the upper and intermediate layers ice on the other hand is not necessary for convection to occur ice formation does not even lead to particularly deep convection a stability maximum which currently dominates the vertical structure of the water column at medium depth limits the ventilation depths effectively

mixing and convection in the greenland sea from a tracer release experiment convective vertical mixing in restricted areas of the subpolar oceans such as the greenland sea is thought to be the process responsible for forming much of the dense water of the ocean interior1 2 deep water formation varies substantially on annual and decadal timescales3 4 5 and responds to regional climate signals such as the north atlantic oscillation6 7 its variations may therefore give early warning of changes in the thermohaline circulation that may accompany climate change8 here we report direct measurements of vertical mixing by convection and by turbulence from a sulphur hexafluoride tracer release experiment in the central greenland sea gyre in summer we found rapid turbulent vertical mixing of about 1 1 cm2 s 1 in the following late winter part of the water column was mixed more vigorously by convection indicated by the rising and vertical redistribution of the tracer patch in the centre of the gyre at the same time mixing outside the gyre centre was only slightly greater than in summer the results suggest that about 10 of the water in the gyre centre was vertically transported in convective plumes which reached from the surface to at their deepest 1 200 1 400 m convection was limited to a very restricted area however and smaller volumes of water were transported to depth than previously estimated9 our results imply that it may be the rapid year round turbulent mixing rather than convection that dominates vertical mixing in the region as a whole

development of a new sea ice growth and lead parameterization a thermodynamic sea ice model that has been numerically structured to take time steps on the order of a week has been shown to be sensitive to time step size this sensitivity was caused by the extrapolation of initial ice growth rates over the long time step a new parameterization of new sea ice growth on open ocean and in leads that can be used over a large range of time step sizes at least from 0 3 to 12 days is described here in this parameterization new sea ice growth is computed as a power law function of the initial energy deficit in the ocean this power law takes into account the rapid reduction of the ice growth rate as the sea ice gets thicker and therefore reduces sensitivity to time step size tests of this parameterization show that this method does a good job of simulating the rate of new ice growth when compared to data from mawson antarctica and is relatively insensitive to the length of the time step

the jan mayen current of the greenland sea oceanographic measurements acquired by the usns bartlett during september 1989 in and north of the area of the jan mayen current jmc show that in terms of upper layer baroclinic flow about half of the jmc is a wide meander in the east greenland current egc and about half continues eastward to close the greenland gyre gg on the south this phenomenon has been charted in the past but has elicited no comment surface drifters and a numerical model corroborate this behavior at deeper depths 100 m the meander dissipates with the flow becoming more easterly a drifting float at 500 m depth confirms that the flow is relatively undeviated toward the east ultimately merging with the northward flowing norwegian atlantic current near the gg the fresh water content of the water column is low in the upper layers but high in the middepths where there is a substantial content of egc water e g in the jmc the converse is usually true the fwc of the gg computed for the period 1953 1966 was smaller by a factor of 2 in both layers than in 1989 indicating that 1989 is in another period of low salinity comparable to that of 1968 the period of the great salinity anomaly described by dickson et al 1988

sources to the east greenland current and its contribution to the denmark strait overflow data from the east greenland current in 2002 are evaluated using optimum multiparameter analysis the current is followed from north of fram strait to the denmark strait sill and the contributions of different source waters in mass fractions are deduced from the results it can be concluded that at least in spring 2002 the east greenland current was the main source for the waters found at the denmark strait sill contributing to the overflow into the north atlantic the east greenland current carried water masses from different source regions in the arctic ocean the west spitsbergen current and the greenland sea the results agree well with the known circulation of the western nordic seas but also add knowledge both to the quantification and to the mixing processes showing the importance of the locally formed greenland sea arctic intermediate water for the east greenland current and the denmark strait

meridional sea ice transport and its impact on climate sea ice is an important factor in controlling the ocean atmosphere energy exchange in polar regions and has an important impact on climate this is because sea ice insulates the relatively warm ocean from the cold winter atmosphere and has a higher surface albedo than the ocean in this study the effect of sea ice transport on the energy exchange between the atmosphere and ocean and thus on climate is examined using a coupled energy balance climate thermodynamic sea ice model transport of sea ice produces a thinning of the zonally annually average d sea ice in the poleward most zones and an extension of the ice edge equatorward thinning of the poleward most ice produces an increase in the annual sensible heat flux from the ocean to the atmosphere which produces an increase in the mean annual atmospheric temperatures sensible heat flux is decreased in the zones that contain the ice edge however warming in the poleward most zones produces an increased meridional energy convergence at the ice edge that offsets the decrease in the sensible heat flux to the atmosphere resulting in a net warming

significant role of the north icelandic jet in the formation of denmark strait overflow water the largest dense water plume feeding the lower limb of the atlantic meridional overturning circulation from the nordic seas comes from denmark strait overflow water measurements of hydrography and water velocity north of iceland and ocean model simulations indicate that a significant part of this water is supplied by the north icelandic jet

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

composition and variability of the denmark strait overflow water in a high resolution numerical model hindcast simulation the upstream sources and pathways of the denmark strait overflow water and their variability have been investigated using a high resolution model hindcast this global simulation covers the period from 1948 to 2009 and uses a fine model mesh 1 20 to resolve mesoscale features and the complex current structure north of iceland explicitly the three sources of the denmark strait overflow the shelfbreak east greenland current egc the separated egc and the north icelandic jet have been analyzed using eulerian and lagrangian diagnostics the shelfbreak egc contributes the largest fraction in terms of volume and freshwater transport to the denmark strait overflow and is the main driver of the overflow variability the north icelandic jet contributes the densest water to the denmark strait overflow and shows only small temporal transport variations during summer the net volume and freshwater transports to the south are reduced on interannual time scales these transports are highly correlated with the large scale wind stress curl around iceland and to some extent influenced by the north atlantic oscillation with enhanced southward transports during positive phases the lagrangian trajectories support the existence of a hypothesized overturning loop along the shelfbreak north of iceland where water carried by the north icelandic irminger current is transformed and feeds the north icelandic jet monitoring these two currents and the region north of the iceland shelfbreak could provide the potential to track long term changes in the denmark strait overflow and thus also the amoc

flow over rough mobile beds friction factor and vertical distribution of the longitudinal mean velocity 1 the main objective of the present study is to identify the impacts of bed mobility on the vertical profile of the mean longitudinal velocity and on resistance in flows over water worked beds of poorly sorted mixtures of sand and gravel water worked beds with sediment transport are explicitly distinguished from immobile beds with imposed sediment feed flows with different equilibrium sediment transport rates are generated in a laboratory flume the initial bed mixtures featured combinations of sand and gravel modes data collection included instantaneous velocities measured with laser doppler annemometry wall similarity in the sense of townsend 1976 is assumed the parameters of the formulae are discussed within three scenarios comprising different definitions ofu and ks combined with different conceptions of the von karman constant flow independent or flow dependent it is shown that the parameters of the formulae that express the velocity profile vary with the shields number and with the initial bed composition the variation is independent of the adopted scenario except in what concerns the formulation of hydraulic smoothening in the presence of sand sizes which depends on the definition of ks

water worked gravel beds in laboratory flumes a natural analogue an investigation has been conducted to identify the key parameters that are likely to scale laboratory sediment deposits to the field scale two types of bed formation were examined one where sediment is manually placed and screeded and the second where sediment is fed into a running flume this later technique created deposits through sequential cycles of sediment transport and deposition detailed bed surface topography measurements have been made over a screeded bed and three fed beds in addition bulk subsurface porosity and hydraulic conductivity have been measured by comparing the four beds results revealed that certain physical properties of the screeded bed were clearly different from those of the fed beds the screeded bed had a random organization of grains on both the surface and within the subsurface the fed beds exhibited greater surface and subsurface organization and complexity and had a number of properties that closely resembled those found for water worked gravel beds the surfaces were water worked and armoured and there was preferential particle orientation and direction of imbrication in the subsurface this suggested that fed beds are able to simulate in a simplified manner both the surface and subsurface properties of established gravel bed river deposits the near bed flow properties were also compared it revealed that the use of a screeded bed will typically cause an underestimation in the degree of temporal variability in the flow furthermore time averaged streamwise velocities were found to be randomly organized over the screeded bed but were organized into long streamwise flow structures over the fed beds it clearly showed that caution should be taken when comparing velocity measurements over screeded beds with water worked beds and that the formation of fed beds offers an improved way of investigating intragravel flow and sediment water interface exchange processes in gravel bed rivers at a laboratory scale copyright 2008 john wiley sons ltd

decreasing overflow from the nordic seas into the atlantic ocean through the faroe bank channel since 1950 the overflow of cold dense water from the nordic seas across the greenland scotland ridge and into the atlantic ocean is the main source for the deep water of the north atlantic ocean this flow also helps drive the inflow of warm saline surface water into the nordic seas the faroe bank channel is the deepest path across the ridge and the deep flow through this channel accounts for about one third of the total overflow previous work has demonstrated that the overflow has become warmer and less saline over time here we show using direct measurements and historical hydrographic data that the volume flux of the faroe bank channel overflow has also decreased estimating the volume flux conservatively we find a decrease by at least 20 per cent relative to 1950 if this reduction in deep flow from the nordic seas is not compensated by increased flow from other sources it implies a weakened global thermohaline circulation and reduced inflow of atlantic water to the nordic seas

optimal estimation of hydrographic station data and derived fields abstract optimal estimation is applied to contouring and analysis of hydrographic sections measured fields such as temperature and salinity and derived fields such as geostrophic velocity are decomposed into large scale and small scale components first a heavily sampled basin scale field is estimated and subtracted from the data a small scale field barely resolved by the hydrography is then found from the residuals the power and utility of the technique are illustrated by means of examples using a short section from the straits of florida and a transatlantic section along 24 n

on the nature of the mesoscale variability in denmark strait abstracttime series data from a mooring in the center of denmark strait and a collection of shipboard hydrographic sections occupied across the sill are used to elucidate the mesoscale variability of the dense overflow water in the strait two dominant reoccurring features are identified that are referred to as a bolus and a pulse a bolus is a large weakly stratified lens of overflow water associated with cyclonic rotation and a modest increase in along stream speed of 0 1 m s 1 when a bolus passes through the strait the interface height of the overflow layer increases by 60 m and the bottom temperature decreases by 0 4 c by contrast a pulse is characterized by anticyclonic rotation a strong increase in along stream speed of 0 25 m s 1 a decrease in interface height of 90 m and no significant bottom temperature signal it is estimated that on average boluses pulses pass through the strait every 3 4 5 4 days with no seasonal signal to their frequency both features have the strongest along

arctic system reanalysis improvements in topographically forced winds near greenland southern greenland is home to a number of weather systems characterized by high speed low level winds that are the result of topographic flow distortion these systems include tip jets barrier winds and katabatic flows global atmospheric reanalyses have proven to be important tools in furthering our understanding of these systems and their role in the climate system however there is evidence that their mesoscale structure may be poorly resolved in these global products here output from the regional arctic system reanalysis asrv1 30 km and asrv2 15 km grid resolutions are compared to the global era interim reanalysis era i 80 km grid resolution focusing on their ability to represent winds in the vicinity of southern greenland comparisons are made to observations from surface and upper air stations as well as from research aircraft flights during the greenland flow distortion experiment gfdex the era i reanalysis has a tendency to underestimate high wind speeds and overestimate low wind speeds which is reduced in asrv1 and nearly eliminated in asrv2 in addition there is generally a systematic reduction in the root mean square error between the observed and the reanalysis wind speeds from era i to asrv1 to asrv2 the exception being low level marine winds where the correspondence is similar in all reanalyses case studies reveal that mesoscale spatial features of the wind field are better captured in asrv2 as compared to the era i or asrv1 these results confirm that a horizontal grid size on the order of 15 km is needed to characterize the impact that greenland s topography has on the regional wind field and climate however even at this resolution there are still features of the wind field that are under resolved

turbulent and advective momentum fluxes in streams 1 the scope of the present paper is the evaluation of the exchange of momentum in natural streams to explore the contribution that turbulence and secondary currents have on the shear stress distributions within the flow and hence evaluate the overall hydraulic resistance existing methods for the evaluation of shear stresses in rivers are examined with respect to their applicability to complex flow fields and a new mathematical formulation for analyzing the shear stresses affecting the longitudinal velocity distribution is presented this new method enables one to estimate the time averaged principal shear stress and its particular components due to turbulence and secondary currents directly from three dimensional velocity data an acoustic doppler velocimeter adv was used to measure the currents and turbulence at five cross sections of two field sites over a wide range of discharges the results of the cross sectional measurements reflect the nonuniformity of the river bed roughness cross sectional form and planform limited applicability was found for methods based on the theory of two dimensional boundary layer flows shear stresses estimated using the new method emphasize the dominance of secondary currents on the total momentum flux momentum fluxes caused by secondary currents were an order of magnitude larger than momentum fluxes due to turbulence

formation of a sandy near bed transport layer from a fine grained bed under oscillatory flow bed surface coarsening was found to be an important effect for the formation of ripples and the dynamics of the boundary layer above a predominantly silt sized sediment bed median particle size equal to 26 m 20 fine sand 70 silt 10 clay under oscillatory flow with orbital velocities of 0 32 0 52 m s in a laboratory wave duct following bed liquefaction substantial winnowing of the bed surface occurred due to entrainment of finer material into suspension bed surface coarsening was quantified with micro scale visualization using a ccd charged coupled device camera under most wave orbital velocities investigated the coarse surface particles were mobilized as a near bed transport layer approximately 4 grain diameters thick the transport of these coarse sediments ultimately produced suborbital or anorbital ripples on the bed except for the highest orbital velocities considered where the bed was planar micro scale visualizations were used to construct a maximum particle velocity profile extending through the near bed transport layers using particle streak velocimetry psv these profiles had a distinctive kink in log linear space at the height of the transport layer suggesting that the near bed sediment transport reduced skin friction and contributed to the boundary roughness through extraction of momentum

a new path for the denmark strait overflow water from the iceland sea to denmark strait 1 denmark strait overflow water dsow is one of the main components of the thermohaline circulation there has been no consensus on where it is formed and by which way it is brought to the denmark strait it is shown here that it is brought to the sill by a hitherto unknown current that is traced from the sill back into the iceland sea north of iceland the transport of this current is sufficient to account for a major part of the transport of dsow as it has been measured at the sill if some entrainment of ambient water is assumed this supports theories suggesting that the iceland sea is the main source for the dsow and this has consequences for the way in which climate change affects the thermohaline circulation

evolution of the east greenland current from fram strait to denmark strait synoptic measurements from summer 2012 we present measurements from two shipboard surveys conducted in summer 2012 that sampled the rim current system around the nordic seas from fram strait to denmark strait the data reveal that along a portion of the western boundary of the nordic seas the east greenland current egc has three distinct components in addition to the well known shelfbreak branch there is an inshore branch on the continental shelf as well as a separate branch offshore of the shelfbreak the inner branch contributes significantly to the overall freshwater transport of the rim current system and the outer branch transports a substantial amount of atlantic origin water equatorward supplementing our measurements with historical hydrographic data we argue that the offshore branch is a direct recirculation of the western branch of the west spitsbergen current in fram strait the total transport of the shelfbreak egc the only branch sampled consistently in all of the sections decreased toward denmark strait the estimated average transport of dense overflow water 27 8 kg m3 and 0 c in the shelfbreak egc was 2 8 0 7 sv consistent with previous moored measurements for the three sections that crossed the entire egc system the freshwater flux relative to a salinity of 34 8 ranged from 127 13 to 81 8 msv the hydrographic data reveal that between fram strait and denmark strait the core of the atlantic origin water in the shelfbreak egc cools and freshens but changes very little in density

the impact of milankovitch solar radiation variations on sea ice and air temperature in a coupled energy balance climate sea ice model the sensitivity of thermodynamicall y varying sea ice and surface air temperature to variations in solar radiation on the 104 to 105 time scales is examined in this study model simulation results show the mean annual sea ice thickness is very sensitive to the magnitude of mid summer solar radiation during periods of high mid summer solar radiation between 115 ka b p and the present

the effect of bedload transport on mean and turbulent flow properties abstract this paper reports the results of a flume experiment that was designed to investigate the effect of bedload transport on mean and turbulent properties of the flow the experiment consisted of varying the bedload transport rate for a given hydraulic condition and of measuring the flow velocity profiles using an acoustic doppler velocimeter adv for each transport rate in order to allow for comparison bedload transport was produced by injecting gravel size particles d50 7 4 mm with a conveyer belt mounted at the upstream end of the flume the results indicate that the effect of bedload on flow characteristics is complex it is shown that bedload transport causes opposite effects on flow velocity depending on the roughness of the bed and the relative magnitude of flow and sediment transport variables a better understanding of these conflicting results is obtained from the application of an energy budget approach to the analysis of velocity data this approach demonstrates that bedload affects flow velocity by modifying the rate of dissipation of turbulent kinetic energy however the mechanisms responsible for the modification of turbulent dissipation are still unknown

optimum multiparameter analysis of the water mass structure in the western north atlantic ocean hydrographic data of temperature salinity oxygen nitrate phosphate and silicate at 81 stations with 435 samples on 3 sections between the azores the grand banks of newfoundland and the bermuda islands are used to determine the mixing of water masses by optimum multiparameter analysis over the depth range 100 1500 m the method optimally utilizes all information from our hydrographic data set by solving an overdetermined set of linear mixing equations for all parameters using the method of least squares residuals it is shown that the method gives quantitative information on the influence of the various water masses of the western north atlantic the gulf stream and the north atlantic current appear as broad bands transporting large amounts of western north atlantic central water at their warm flank western subarctic intermediate water and shelf water supplied by the labrador current and containing significant amounts of labrador current water are found on their inshore side the area of the azores front is found in the vicinity of the comer seamounts where the uniform water mass distribution of the sargasso sea changes into a more complex structure that reflects the influence of water masses originating in the labrador sea small scale structures like eddies or gulf stream rings are also detectable by this analysis method comparison with dynamic height analysis supports the circulation pattern of the north atlantic current as a continuation of the gulf stream and of the southeastward flowing azores current originating in the area of the southeast newfoundland rise

spatially representative velocity measurement over water worked gravel beds 1 recent studies have examined the spatial heterogeneity of velocity fields over natural river boundaries a key challenge is understanding how many velocity measurements are required to provide spatially representative estimates of the flow this paper describes a series of laboratory experiments in which flow velocities have been measured in a detailed spatial pattern over two water worked gravel beds these data have been utilized to deduce the minimum density of measurements required to provide representative estimates of several spatially averaged flow parameters this was coupled with an investigation into the influence of measurement density on the level of error in the estimation of these parameters empirical relationships are developed that can be used to estimate a priori the required minimum measurement density for a known precision and accuracy over macroscopically flat water worked gravel beds within a range of submergences these estimates can be based on measurements of the bed roughness length scale bed shear velocity and flow depth alone for all of the flow parameters the level of error in adopting a lower measurement density than the required minimum and its change with measurement density was especially large at the lower densities given this dependence on measurement density caution should be taken when comparing flow estimates from studies that have used similar flow and bed conditions but different measurement densities

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

hydrographic observations in denmark strait in fall 1997 and their implications for the entrainment into the overflow plume denmark strait is the most important exit for water masses formed in the arctic mediterranean sea and supplies a substantial fraction of the north atlantic deep water observations obtained from rv aranda in august september 1997 indicate that the water crossing the 620m deep sill is mainly drawn from the intermediate waters of the east greenland current the overflow plume was stratified and capped by a less saline layer as it descended beyond 2000m the presence of a low salinity lid implies that entrainment of ambient water is small and that the downstream evolution of the plume characteristics is due to mixing within the plume between the initial overflow waters low salinity but dense water from the east greenland current flowing over the shelf may cross the shelf break south of the sill and add a less dense fraction to the overflow

the arctic circumpolar boundary current abstract the main water transformations in the arctic mediterranean take place in the boundary current of atlantic water which crosses the greenland scotland ridge from the north atlantic into the eastern norwegian sea it enters and flows around the arctic ocean before it exits the arctic mediterranean as the east greenland current primarily through denmark strait on route it experiences numerous branchings and mergings by examining how the properties of this circumpolar boundary current evolve it is possible to identify and describe the processes causing the water mass transformations in the arctic mediterranean it is also possible to follow the arctic ocean deep waters as they spread into the nordic seas and eventually provide 40 of the overflow water supplying the north atlantic deep water

double averaging concept for rough bed open channel and overland flows applications the paper briefly outlines the double averaging methodology for studying environmental rough bed flows it focuses on the applications of this methodology in environmental hydraulics by providing several examples illustrating advantages of this methodology over conventional approaches examples include 1 identification of specific flow layers and flow types 2 vertical distribution of the double averaged velocity between the roughness tops and troughs 3 vertical distribution of momentum fluxes and sinks for typical roughness types due to turbulence mean flow heterogeneity secondary currents form drag and viscous drag 4 estimates of form induced dispersive stresses and evaluation of their structure using quadrant analysis and 5 closure development for mass transfer uptake processes for stream periphyton these examples illustrate the advantages of the double averaging methodology over conventional approaches as well as highlight its potential for studying flows over very rough beds hig