increased arctic sea ice volume after anomalously low melting in 2013 changes in arctic sea ice volume are difficult to quantify five years of satellite data reveal a reduction in autumn sea ice volume in 2010 2012 but a sharp increase in 2013 and 2014 suggesting that ice volume can recover quickly

variability in sea ice thickness over the north pole from 1977 to 1990 changes in the thickness of polar sea ice have the potential to provide a signal of climate change but attempts to identify trends must take into account the range of natural variability here we present an analysis of measurements of the subsurface ice thickness draft of sea ice around the north pole made from 1977 to 1990 these data were collected during six submarine cruises in late april early may of 1977 1979 1986 1987 1988 and 1990 and represent the most extensive dataset so far for ice draft in the central arctic at the same season and location the results reveal considerable interannual variability both in mean ice draft 1 0 m and in open water extent 2 5 this variability limits the confidence that can be placed in any apparent trends observed for sea ice thickness or type since the late 1970s and illustrates the need for a reliable baseline against which to assess future trends

a model of channel response in disturbed alluvial channels dredging and straightening of alluvial channels between 1959 and 1978 in west tennessee caused a series of morphologic changes along modified reaches and tributary streams degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed levels by as much as 6 1 m following degradation reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction aggradation downstream of the area of maximum disturbance reached 0 12 m per year with the greatest rates occurring near the stream mouths the adjustment of channel geometry and phases of channel evolution are characterized by six process oriented stages of morphologic development premodified constructed degradation threshold aggradation and restabilization down cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded threshold stage channel widening continues through the aggradation stage as the slough line develops as an initial site of lower bank stability the bank profile develops three dynamic elements 1 vertical face 70 to 90 2 upper bank 25 to 50 and 3 slough line 20 to 25 alternate channel bars form during the restabilization stage and represent incipient meandering of the channel

age characteristics in a multidecadal arctic sea ice simulation 1 results from adding a tracer for age of sea ice to a sophisticated sea ice model that is widely used for climate studies are presented the consistent simulation of ice age dynamics and thermodynamics in the model shows explicitly that the loss of arctic perennial ice has accelerated in the past three decades as has been seen in satellite derived observations our model shows that the september ice age average across the northern hemisphere varies from about 5 to 8 years and the ice is much younger about 2 3 years in late winter because of the expansion of first year ice we find seasonal ice on average comprises about 5 of the total ice area in september but as much as 1 34 106 km2 survives in some years our simulated ice age in the late 1980s and early 1990s declined markedly in agreement with other studies after this period of decline the ice age began to recover but in the final years of the simulation very little young ice remains after the melt season a strong indication that the age of the pack will again decline in the future as older ice classes fail to be replenished the arctic ice pack has fluctuated between older and younger ice types over the past 30 years while ice area thickness and volume all declined over the same period with an apparent acceleration in the last decade

relationship between sea ice freeboard and draft in the arctic basin and implications for ice thickness monitoring we have confirmed our earlier finding that the probability density function pdf of ice freeboard in the arctic ocean can be converted to a pdf of ice draft by applying a simple coordinate transformation based on the measured mean draft and mean elevation this applies in each of six 50 km sections north of greenland of joint airborne laser and submarine sonar profile obtained along nearly coincident tracks from the arctic basin north of greenland and tested for this study detailed differences in the shape of the pdf can be explained on the basis of snow load and can in principle be compensated by the use of a more sophisticated freeboard dependent transformation the measured density ratio r actually mean draft mean elevation ratio for each section was found to be consistent over all sections tested despite differences in the ice regime indicating that a single value of r might be used for measurements done in this season of the year the mean value r from all six sections is 7 89 on the assumption that all six values are drawn from the same population the standard deviation is 0 55 for a single 50 km section and thus 0 22 for 300 km of track in attempting to infer ice draft from laser measured freeboard we would therefore expect an accuracy of about 28 cm in 50 km of track if mean draft is about 4 m and about 11 cm in 300 km of track these accuracies are compatible with the resolution of predictions from numerical models a simple model for the variability of r with season and with mean ice thickness gives results in reasonable agreement with observations they show that although there is a large seasonal variability due to snow load there is a stable period from november to april when the variability is chiefly dependent on the mean ice thickness alone thus in principle r can be mapped over the arctic ocean as a basis for interpreting survey data better field data are needed on the seasonal and spatial variability of three key quantities area averaged snow load mean density of first year and multiyear ice including the effect of ridging with these two ice regimes and density of near surface water

incorporation of bed texture into a channel evolution model abstract channel evolution models cem have been developed to qualitatively describe the morphological adjustments of channels undergoing incision but the grain size of the river bed material has not been addressed in existing evolution models herein bed material grain size is incorporated into an existing cem to describe the sequence of grain size changes the analysis is based on the data from bed material that were available from a 1986 sampling program in northwestern mississippi samples were taken along three sand and gravel bed channels at 300 m intervals to provide a comparable data set sampling was repeated in 1996 observed longitudinal grain size distributions were highly variable in space and time overall downstream fining trends were absent bed texture in incising channels is as dynamic as channel morphology with composition shifting from a mixture dominated by sand to one dominated by gravel or vice versa within a decade or less the modified cem predicted direction of changes in grain size in a meandering incising channel but not within two straightened incising channels most likely due to the complex influence of upstream and lateral sediment sources bed and bank erosion we suggest that over the temporal 10 years and spatial scales 10 to 20 km of this study sediment sources are the dominant factor in the development of longitudinal grain size distributions

seasonal variations of sea ice motion in the canada basin and their implications ice velocities and sea level pressure data from arctic drifting buoys for 1979 85 are used to examine the seasonal variation of sea ice drift in the canada basin a recurring reversal of the mean anticyclonic gyre is demonstrated in late summer in response to changes in atmospheric forcing the identification of this reversal of ice motion is significant for numerical modeling of arctic atmosphere ocean ice interactions

morphodynamics of a macrotidal beach abstract an intensive field investigation of hydrodynamic processes processes of sediment entrainment and suspension and morphologic change was carried out on an unprotected macrotidal beach near broome in northwestern australia the spring tide range was 9 5 m waves had heights of 0 5 1 2 m and periods of 9 13 s the beach had an overall concave upward profile with low gradient and dissipative subtidal and low tidal zones and steeper more reflective mid tidal and high tidal zones direct measurement of energy flux dissipation over the intertidal profile showed dissipation rates on the order of 1 2 w m 2 of bed and indicated an approximate balance between shoaling and dissipation of unbroken waves so as to maintain a constant wave height time averaged predictive estimates of wave work over the lunar half cycle for different points on the intertidal profile show similar dissipation rates and reveal a relatively uniform distribution of work over most of the profile but with maxima in the middle of the low tidal zone and over the lower part of the high tidal zone most of the work over the low and mid tidal zones was performed by unbroken shoaling waves rather than by surf zone processes surf zone processes only dominate over the high tidal zone the nature of the surf zone processes varied across the profile as local gradient and degree of reflectivity changed with changing tide level the growth of standing waves and infragravity surf beat oscillations as identified from spectra and cross spectra of surface elevation and currents u and v was inhibited over most of the profile however well developed secondary standing wave energy particularly at infragravity frequencies was observed over the high tidal zone at spring high tide and over the mid tidal zone at neap high tide over the low tidal and subtidal zones strong shore parallel tidal currents were subordinate only to the orbital velocities of unbroken incident waves over the subtidal zone asymmetrical tidal currents skewed toward the north attained maximum speeds of 0 5 m s 1 just after high water field measurements of suspended sediment concentration profiles under broken and unbroken waves showed very good fit to a diffusion model for wave induced sediment suspension and suggested that sediment suspension was probably attributable largely to waves northerly advection of wave suspended sediment by asymmetrical tidal currents over the subtidal and low tidal zones accounted for a net northerly longshore transport the greatest morphologic mobility of the intertidal profile occurred over the lower high tidal and upper mid tidal zones corresponding to the position of the coarsest material and secondary maximum of time averaged wave work and to a beach state intermediate between the reflective and dissipative extremes temporally the greatest mobility of the profile as a whole was observed on the short within tidal cycle time scale net changes over the longest time scale of a lunar half cycle were negligible

measurement of arctic sea ice thickness by submarine 5 years after scicex in april 2004 the royal navy submarine hms tireless became the first uk submarine to conduct environmental monitoring in the arctic ocean since 1996 as the last us scicex scientific ice expeditions cruise was in 2000 this has been the only opportunity for a civilian scientist to carry out measurement of ice draft and oceanography over a wide area of the arctic this paper presents preliminary results and compares them with similar investigations in the 1970s 90s the route of tireless covered a large area of the european sector of the arctic from 58 et o 628 w transects were carried out from the marginal ice zone in fram strait up to the north pole and along the 858 n parallel north of greenland as part of work for the european commission iris project image intensity from the advanced synthetic aperture radar instrument on the european space agency s envisat satellite has been compared with ice draft from the submarine the raw data were found to be highly variable so a moving average was applied producing a correlation of 0 79 tireless carried a full oceanographic sensor suite and expendable probes for investigation into changes in the arctic ocean the results from these show further erosion of the arctic cold halocline layer by advancing atlantic water compared to previous climatologies and fieldwork expeditions preliminary ice draft data from 858 n show deeper ice keels than those encountered by a submarine on the same route in 1987

modeling of riverbed evolution for bedload sediment mixtures a new computational methodology for fully coupled simulation of unsteady water and sediment movement in mobile bed alluvial rivers has been developed and tested the one dimensional approach treats bedload transport of nonuniform sediment mixtures with sorting and armoring effects considered spatial lag effects in nonequilibrium bedload transport are taken into account through innovative use of the recently developed notion of a loading law the new system of equations for water and sediment movement is solved in a fully coupled implicit manner using the preissmann finite difference scheme implemented to allow for either upstream or downstream induced mobile bed perturbations formal analysis of the coupled system sheds new light on boundary condition requirements for the multiple size class problem the fully coupled implicit solution has proven to be robust and stable and it respects the underlying volumetric conservation laws remarkably well as demonstrated in continuity analyses of computed bed e

north pole ice thickness and association with ice motion history 1977 1992 ice drafts measured by u s navy submarine sonar near the north pole from 1977 to 1992 were used together with arctic ocean drifting buoy data from 1979 to 1992 to investigate north pole ice thickness changes during this 15 year period a primary objective was to determine dynamical forcing mechanisms and associated time scales relevant to ice thickness variation north pole ice thickness showed much interannual variability and no systematic decrease of ice thickness was observed changes in ice thickness were closely associated with long term directional deviation of ice motion from geostrophic wind forcing and with ice convergence and divergence at the north pole during the 1 2 weeks prior to submarine measurement the geographic origin of ice within the arctic ocean was not associated with its eventual thickness at the north pole

observations of megaripples in the surf zone abstract bedform measurements were obtained under shoaling and breaking wave conditions using a pencil beam sonar in the intertidal zone at skallingen denmark measurements were made during four individual tidal cycles starting from flat bed conditions at low tide and as the tide rose and the instruments entered the surf zone megaripples appeared when the mobility number exceeded 240 which equates to the classic flat bed criterion proposed by dingler and inman 1976 the megaripples were oriented cross shore and their wavelength scaled with the wave orbital diameter wave ripples were observed when the mobility number was smaller which occurred mainly under shoaling waves since all bedforms were erased as the seabed became dry at low tide and perpendicular longshore currents were occasionally strong the megaripples were of small amplitude but their dimensions were consistent with predictions from the literature their migratory behaviour in the first few hours of existence appeared more strongly related to self organisation rather than to ambient hydrodynamics a clear gap in bedform spacing at 120 cm was identified between wave ripples and megaripples

out of the woods driftwood insights into holocene pan arctic sea ice dynamics the collation of 913 driftwood samples from across the western arctic with spatiotemporal distribution and available provenance data enabled the production of a high resolution proxy based reconstruction of holocene arctic ocean surface current and sea ice dynamics regionally bounded driftwood based sea ice reconstructions studies suggest spatiotemporally complex past arctic sea ice extent and movement however a large scale compilation of holocene arctic driftwood has not previously been developed sparse driftwood in the early holocene 8 2 cal ka b p deglacial period was followed by increased driftwood deposition in the warmer mid holocene 8 2 4 2 cal ka b p characterized by an enhanced transpolar drift tpd 7 cal ka b p leading to sea ice loss through the fram strait driftwood incursion peaks show spatial e w progression from the eurasian archipelagos to greenland and the canadian arctic archipelago suggesting a progressive shift in the orientation of the tpd on centennial millennial time scales and intermediate phases in the arctic oscillation late holocene cooling 4 2 cal ka b p is indicated by increased influx of probably north american picea via a strengthened beaufort gyre bg which enhanced sea ice recirculation starting in the western arctic and progressing eastward in recent millennia 2 cal ka b p a more variable driftwood record alternates between bg and tpd dominance on centennial time scales to further constrain a spatiotemporal reconstruction of variations in holocene ocean current and sea ice dynamics a more definitive determination of driftwood provenance is recommended to build upon the current framework such as through radiogenic isotope tracing and adna analysis

snow depth observations by adventurers traveling on arctic sea ice snow depth is a key parameter for assessing the sea ice mass budget in the arctic and for the surface energy balance at the atmosphere snow ice ocean interfaces however scientific expeditions to the high arctic ocean are rare and for large parts of the year no snow and ice data are collected in situ in most regions therefore any additional in situ observations of snow depth are of interest to the scientific community arctic adventurers and tourists are among the most frequent visitors to the arctic ocean and north pole if properly trained and carefully adhering to standard protocols they could collect valuable snow depth data from large regions here we analyse such data from four arctic basin ski traverses carried out between 1994 and 2007 individual datasets show characteristic regional differences of snow thickness which provide invaluable information for the validation of models and satellite data the observations were made applying a continuously upgraded observation guideline scheme earlier observations were based on a relatively broad view of easily observable snow and ice parameters improvements included requirements of more detailed snow thickness surveys in order to observe the spatial variability over varying sea ice surfaces in a better way possibilities for and limitations of ice thickness estimates and measurements are also discussed often assessment of ice thickness is more problematic since measurements are either time consuming or biased meaning that possibilities for collecting large datasets are limited

ice thickness in the arctic ocean the statistical reliability of experimental data the statistical reliability of ice thickness distributions derived from submarine sonar data sets of finite length is examined focusing on the standard error eh l in the mean ice draft h derived from a record of length l km this is important in assessing the significance or otherwise of quoted temporal or spatial differences in mean ice draft estimates for eh 50 are derived from four different submarine surveys each of which involved repeated sampling of apparently homogeneous ice fields values ranged from 7 18 to 15 29 of the mean for climate related comparisons of data sets from different years in the same location a value of 12 75 is recommended derived from data gathered at the north pole this value is used to reassess the significance of published data on ice draft variability quoted variability at the north pole over the 1977 1990 period is found to be not statistically significant but a difference in area averaged ice draft over the eurasian basin between 1976 and 1987 remains highly significant using hitherto unreported 1987 data from the north pole area the statistical variability of other parameters of the ice cover is examined the frequency of lead occurrence the width of leads the fractional cover of smooth ice and the frequency of keel occurrence all show standard errors of 22 24 in their mean values while mean keel draft has a 4 error all over a 50 km track length finally unreported 1987 data for an area extending from the pole to 72 n along the prime meridian and the east greenland shelf break are examined in 1 bins using eh l to assess the significance of variations in the mean a statistically significant decline in mean ice draft is observed starting at 86 n and continuing to 72 n a linear regression gives a thinning rate of 0 22 m deg the decline within the arctic basin itself is tentatively ascribed to a mixture of under ice melt and mixing of ice types since data on young ice occurrence show that divergence did not occur north of fram strait

simulating the ice thickness distribution in a coupled climate model climate simulations in a global coupled model are investigated using a dynamic thermodynamic sea ice and snow model with sophisticated thermodynamics and a subgrid scale parameterization for multiple ice thicknesses in addition to the sea ice component the model includes a full primitive equation ocean and a simple energy moisture balance atmosphere we introduce a formulation of the ice thickness distribution that is lagrangian in thickness space the method is designed to use fewer thickness categories because it adjusts to place resolution where it is needed most and it is free of diffusive effects that tend to smooth eulerian distributions experiments demonstrate that the model does reasonably well in simulating the mean arctic climate we find the climate of the arctic and northern north atlantic is sensitive to resolving the ice thickness distribution when comparing the model results to a simulation with a two level sea ice model the ice thickness distribution causes ice export through fram strait to be more variable and more strongly linked to meridional overturning in the north atlantic ocean the lagrangian formulation of the ice thickness distribution allows for the inclusion of a vertical temperature profile with relative ease compared to an eulerian method we find ice growth rates and ocean surface salinity differ in our model with a well resolved vertical temperature profile in the ice and snow and an explicit brine pocket parameterization compared to a simulation with semtner zero layer thermodynamics although these differences are important for the climate of the arctic the effects of an ice thickness distribution are more dramatic and extend into the northern north atlantic sensitivity experiments indicate that five ice thickness categories with 50 cm vertical temperature resolution capture the effects of the ice thickness distribution on the heat and freshwater exchange across the surface in the presence of sea ice in these simulations

has arctic sea ice rapidly thinned abstract reports based on submarine sonar data have suggested arctic sea ice has thinned nearly by half in recent decades such rapid thinning is a concern for detection of global change and for arctic regional impacts including atmospheric time series ocean currents and river runoff into an ocean ice snow model show that the inferred rapid thinning was unlikely the problem stems from undersampling varying winds that readily redistribute arctic ice create a recurring pattern whereby ice shifts between the central arctic and peripheral regions especially in the canadian sector timing and tracks of the submarine surveys missed this dominant mode of variability although model derived overall thinning from the 1960s to the 1990s was less than hitherto supposed there is also indication of accelerated thinning during the early mid 1990s

the decline in arctic sea ice thickness separating the spatial annual and interannual variability in a quarter century of submarine data 1 naval submarines have collected operational data of sea ice draft 93 of thickness in the arctic ocean since 1958 data from 34 u s cruises are publicly archived they span the years 1975 to 2000 are equally distributed in spring and autumn and cover roughly half the arctic ocean the data set is strong we use 2203 values of mean draft each value averaged over a nominal length of 50 km these values range from 0 to 6 m with a standard deviation of 0 99 m multiple regression is used to separate the interannual change the annual cycle and the spatial field the solution gives a climatology for ice draft as a function of space and time the residuals of the regression have a standard deviation of 0 46 m slightly more than the observational error standard deviation of 0 38 m the overall mean of the solution is 2 97 m annual mean ice draft declined from a peak of 3 42 m in 1980 to a minimum of 2 29 m in 2000 a decrease of 1 13 m 1 25 m in thickness the steepest rate of decrease is 0 08 meters per year m a in 1990 the rate slows to 0 007 m a at the end of the record the annual cycle has a maximum on 30 april and a peak to trough amplitude of 1 06 m 1 12 m in thickness the spatial contour map of the temporal mean draft varies from a minimum draft of 2 2 m near alaska to a maximum just over 4 m at the edge of the data release area 200 miles north of ellesmere island

the impact of varying atmospheric forcing on the thickness of arctic multi year sea ice 1 a 1 d thermodynamic sea ice model forced with north pole drift station observations from 1954 91 is used to study the effect of changing atmospheric forcing on multi year arctic sea ice from 1954 70 most seasons show positive trends in calculated sea ice thickness over much of the arctic a dip in calculated ice thickness takes place between 1971 77 over most of the arctic following the north pacific regime shift in 1976 1977 the period 1978 91 reveals large negative trends in calculated sea ice thickness in all seasons the results indicate that an important part of the variability and trends in arctic sea ice thickness is thermodynamically driven of the total variance in multi year sea ice thickness 10 to 20 is explained by variations in the arctic oscillation and pacific north american patterns the multi year ice thickness response to a positive wintertime arctic oscillation anomaly occurs the following summer and persists for more than a year

bank stability and channel width adjustment east fork river wyoming frequent surveys of eight cross sections located in self formed reaches of the east fork river wyoming during the 1974 snowmelt flood showed a close relation between channel morphology and scour and fill those cross sections narrower than the mean reach width filled at discharges less than bankfull and scoured at discharges greater than bankfull those cross sections wider than the mean reach width scoured at discharges less than bankfull and filled at discharges greater than bankfull the accumulation and depletion of sand sized bed material in a cross section was concentrated in the near bank parts of the stream channel and thus significantly influenced bank stability and retreat in those cross sections that scour at discharges greater than bankfull the basal bank material is eroded and the banks become undercut and unstable conversely in those cross sections that fill at discharges greater than bankfull the basal bank material is covered by the accumulated sand size material and is not eroded streambanks in these cross sections are moderately inclined and stable a resurvey in the summer of 1980 of the cross sections located in straight reaches showed that those cross sections which scoured at discharges greater than bankfull had become 2 4 feet wider whereas those cross sections which filled at discharges greater than bankfull were unchanged thus bank stability and to some extent the adjustment of stream channel width in the east fork river study reach appears to be controlled by the processes of scour and fill

frequency dependent cross shore suspended sediment transport 1 a non barred shoreface abstract the local time varying and time averaged suspended sediment transports across a marine non barred shoreface were obtained from field measurements of the near bed velocity and sediment concentration vectors using electromagnetic current meters and optical backscatterance suspended solids sensors co spectral analyses of velocity and sediment concentration revealed the contributions of waves of different frequencies to the total transport the transport attributable to quasi steady currents was determined from the product of the time averaged cross shore velocity and sediment concentration estimates of the local time integrated sediment volume flux total and net and the associated erosion or accretion were determined using depth of activity rods suspended sediment transport was associated with 1 local wind forced waves 2 swell 3 low frequency waves group bound long waves and 4 offshore directed mean currents undertow these transport components varied spatially and temporally in response to changes in the wave height to water depth ratio the local net oscillatory transport rate at wind wave and swell wave frequencies was directed onshore predominantly and increased as water depth decreased in association with wave shoaling the local mean sediment transport rate was predominantly offshore and dominated the net transport rate wherever wave induced components were weak relative to the mean currents under breaking waves the suspended sediment transport contribution associated with wind waves and swell waves was reduced as a result of dissipation the net suspended sediment transport rate exhibited a distinct vertical structure reflecting the balance at each elevation between the opposing mean and oscillatory components of transport the local time averaged total and net sediment volume flux and resulting erosion and accretion patterns support the hypothesis of a near balance in sediment flux for the complete storm event

two modes of appearance of the odden ice tongue in the greenland sea the odden ice tongue of the greenland sea normally forms locally in winter as frazil pancake ice allowing for high positive salt fluxes during freezing that leads to open ocean convection we report that another type of odden can develop usually late in the season and is composed of old ice advected through the east greenland current and by the jan mayen current this type causes the introduction of more meltwater that stabilizes the water column enhances productivity and affects the carbon cycle in the region the history of oddens is reviewed and our results indicate that from 1978 through 1997 the first type occurred almost every year except in 1984 1994 and 1995 while the second type has been observed to occur in 1987 and 1996

dynamics of large and small scale bedforms on a macrotidal shoreface under shoaling and breaking waves abstract bedform dimensions bed position changes near bed velocities and suspended sand concentrations are analyzed from measurements at a single location on a macro tidal beach in the south west of england the study was conducted in 0 5 2 25 m water depth under both swell and wind generated storm waves with both weak and strong currents present bed positions and suspended sand concentrations were measured using the 3 transducers of a multi frequency acoustic backscatter sensor with 5 mm vertical resolution two distinct bed types based on wavelength were observed each with two or more subtypes possible i small scale bedforms 20 cm which include two dimensional 5 forms and three dimensional vortex 6 forms small scale forms were dominant under non breaking conditions while the large scale forms occurred under both non breaking and breaking waves the large scale forms dominate under breaking conditions both types but particularly low steepness forms were highly mobile with maximum horizontal migration rates of 5 cm min 1 large upto 15 cm and rapid upto 3 0 cm min 1 changes in vertical bed elevation were also observed in association with the development and migration of large scale forms large scale bedforms were also highly variable spatially often being interspersed with smaller scale forms under decaying flow regimes and with areas of flat bed under increasing regimes observations suggest these forms are present even under high energy surf zone conditions wave shields 1 models for predicting ripple dimensions did not perform well in this environment this lack of agreement reflects the complex hydrodynamic regimes associated with random grouped shoaling and breaking waves together with the presence of currents often at large angles to the waves as well as the rapid rates of change in the wave forcing associated with tidal cycle oscillations in this macrotidal environment suspended sediment concentrations and transport rates are particularly sensitive to the bedforms present and also to bed position changes associated with ripple migration estimates of transport rates are subject to potentially large errors upto 30 without compensation for bed elevation changes relative to sensor position

sea ice topography of the arctic ocean in the region 70 degrees w to 25 degrees e in october 1976 a cooperative experiment was made to survey the sea ice topography in the european sector of the arctic ocean h m submarine sovereign acquired 4000 km of ice draft data by using an upward looking sonar while a canadian forces aircraft flew along the submarine s track and acquired 2200 km of ice elevation data by using a laser profilometer the two types of profile were processed in corresponding 100 km section lengths and the following statistical analyses and comparisons were made i probability density functions o f ice draft and elevation each distribution shows a peak for young ice and for undeformed multi year ice at large ice thicknesses the distributions take the analytical form of a negative exponential the mean drafts enable two distinct geographical ice regimes to be identified there is an offshore zone of very heavy pressure ridging extending up to 400 km from the coasts of ellesmere island and of north greenland with mean ice draft in the range 5 0 to 7 5 m while out in the central arctic ocean the mean ice draft is lower 3 9 5 1 m and the characteristics of the ice cover remain homogeneous over a length scale of 1000 km the transition between the two regimes is abrupt taking place in less than 25 km data from the same part of the central arctic taken in march 1971 showed a mean ice draft 0 3 m lower while data from the central beaufort sea showed a mean draft more than 0 8 m lower ii level ice distributions ice with a local gradient of less than 1 in 40 was defined as level ice and used as an indicator of the quantity and thickness distribution of undeformed i e thermodynamically grown ice in the arctic ocean the distribution has a mode at 3 0 3 1 m draft and level ice percentages are in the range 30 40 bottom side and 70 80 top side in the offshore zone and 45 55 bottom and 85 95 top in the central arctic thus about half of the arctic ice cover consists of deformed ice iii pressure ridge spacings the spacings of ridge keels fit a negative exponential distribution characteristic of randomness except at close spacings where there is a deficit of keels explained as a geometrical effect and at very large spacings where there is an excess due to the contribution of polynyas the distribution of sail spacings exhibits these two effects but also differs from a random distribution at moderate sail separations iv ridge elevations and drafts keel drafts fit a law of form p h d oc exp ah 2 d h except for an excess of keels at drafts beyond 20 m there is a positive correlation between mean keel draft and keel frequency there are 3 5 4 5 keels per kilometre with draft exceeding 9 m in the offshore zone and 2 3 in the central arctic sail elevations fit a law of form p h d h x exp a h d h with a positive correlation between mean sail elevation and sail frequency the sail elevation and ice elevation distributions can be related by assuming that all thick ice is contained in pressure ridges of triangular cross section v keel sail comparison for the 21 corresponding 100 km sections there are positive correlations between mean sail height and mean keel draft and between keel frequency and sail frequency from these it is possible to convert a sail distribution computed from a laser profile into a keel distribution enabling sea ice bulk characteristics to be derived from airborne surveys alone vi leads and polynyas a lead was defined as a continuous sequence of depth points greater than 5 m long and not exceeding 1 m draft the number density n d of leads per kilometre of width d m fits the power law n d 15 d 2 exceptionally wide leads were concentrated in the offshore zone and in the marginal ice zone close to the open water of the greenland sea

waves currents sediment flux and morphological response in a barred nearshore system abstract a shore normal array of seven bi directional electromagnetic flowmeters and nine surface piercing continuous resistance wave staffs were deployed across a multiple barred nearshore at wendake beach georgian bay canada and monitored for a complete storm cycle time integrated estimates of total itvf and net invf sediment volume flux together with bed elevation changes were determined using depth of activity rods the three bars ranging in height from 0 10 to 0 40 m accreted during the storm 0 03 m and the troughs were scoured 0 05 m sediment reactivation depths reached 0 14 m and 12 of the nearshore control volume was mobilized however the invf value for the storm was less than 1 of the control volume revealing a near balance in sediment volume in the bar system landward migration of the inner crescentic and second sinuous bars occurred in association with an alongshore migration of the bar form itself the outermost straight shore parallel bar remained fixed in location the surf zone was highly dissipative throughout the storm 3 8 10 2 192 10 2 and the wave spectrum was dominated by energy at the incident frequency spectral peaks at frequencies of the first harmonic and at one quarter that of the incident wave were associated with secondary wave generation just prior to breaking and a standing edge wave respectively the former spectral peak was within the 95 confidence band for the spectrum while the latter contributed not more than 10 to the total energy in the surface elevation spectrum even near the shoreline during the storm wave height exceeded 2 m h s and periods reached 5 s t p k orbital velocities exceeded 0 5 m s 1 u rm s and were above the threshold of motion for the medium to fine sands throughout the storm shore parallel flows in excess of 0 4 m s 1 were recorded with maxima in the troughs and minima just landward of the bar crest the rate and direction of sediment flux is best explained by the interaction of antecedent bed slopes with spatial gradients in the mean and asymmetry of the shore normal velocity field these hydrodynamic parameters represent steady flows superimposed on the dominantly oscillatory motion and assumed a characteristic spatial pattern from the storm peak through the decay period increases spatially in the magnitudes of both the mean flows and flow asymmetries cause an increasing net transport potential erosion decreases in these values spatially cause a decreasing net transport potential and thus deposition these transport potentials are increased or decreased through the gravity potential induced by the local bed slope shore parallel flow was important in explaining sediment flux and morphological change where orbital velocities mean flows and flow asymmetries were at a minimum

top bottom multisensor remote sensing of arctic sea ice the arctic sea ice cover has been studied using near simultaneous observations by passive and active synthetic aperture radar sar microwave sensors upward looking and sidescan sonars a lidar profilometer and an infrared sensor data from two aircraft and a submarine over an approximately 100 km track of central arctic sea ice were registered and analyzed to evaluate the characteristics of the ice cover and the utility of each sensor in ice studies the results of comparative and correlation analyses are as follows the probability density functions of ice draft from sonar and elevation from lidar were found to be almost identical when isostasy is taken into account this result suggests that the basic ice thickness distribution can be derived from the surface topography measurements alone reasonable correlation was found between sar backscatter and ice draft or elevation especially when scales were adjusted such that 15 20 sar pixels were averaged however surface roughness derived directly from standard deviations in the lidar elevation data was found to be poorly correlated to the sar backscatter these results indicate that the sar values are affected more by scattering from the ice than from the snow covered surface the active and passive microwave sensors are shown to generally complement each other as the two sensors are especially sensitive to different physical properties of the sea ice undeformed first year ice showed low backscatter values but high brightness temperatures while some multiyear ice showed high backscatter values and low brightness temperature however surfaces identified as multiyear ice by the passive system have a large spread in the unaveraged sar backscatter indicating limitations when using a one channel sar for ice type identification at the highest resolution also ridged ice identified by sonar and sar data covers a large range of passive microwave emissivity suggesting considerable variability in the age and salinity of this type of ice significant variations about 0 11 in the minimum emissivity of consolidated multiyear ice are observed in different regions of the arctic using the high resolution 30 m passive microwave data this suggests that regional variations in texture and scattering characteristics of multiyear ice in the arctic are present likely influenced by different histories of formation of the ice in different regions

the under ice thickness distribution of the arctic basin as recorded in 1958 and 1970 the uss nautilus ssn 571 was the first vessel to cross the arctic basin via the north pole in early august 1958 during this expedition almost continuous acoustic under ice thickness profiles were recorded in august 1970 uss queenfish ssn 651 retraced nautilus route yielding the only duplicate transect of under ice topography across the arctic basin comparisons of the statistical analysis of the under ice draft measurements obtained through use of wide beam and narrow beam and narrow beam only acoustic profilers by nautilus and queenfish respectively along the coincidental route are presented geographic areas found to have distinct under ice characteristics and ice composition are identified the under ice statistics of both cruises are considered in relation to representative field observations and modeling results of other researchers principal findings are 1 nautilus recorded generally more severe ice conditions within the canada basin than did queenfish 12 years later overall mean drafts were 3 08 and 2 39 m respectively 2 the under ice topography becomes progressively more severe when proceeding from the canadian to the eurasian side of the arctic basin 3 the canada basin may contain the most moderate under ice topography and the greatest number of open water and refrozen polynyas and leads within the central arctic basin 4 the makarov and amundsen basins and the arctic mid ocean ridge may contain some of the most severe under ice topography within the arctic basin and 5 the present study indicates an overall arctic basin mean of 3 4 open water new ice less than 30 cm in summer

energy time and channel evolution in catastrophically disturbed fluvial systems abstract two diverse fluvial systems show that with time channels adjust such that the rate of energy dissipation is minimized one fluvial system characterized by high relief and coarse grained sediment was subjected to an explosive volcanic eruption the other system characterized by low relief and fine grained sediment was subjected to dredging and straightening study of the expenditure of kinetic and potential energy components of total mechanical energy provide an energy based rationale of the interdependency between processes and forms during channel evolution spatial and temporal trends of aggradation and degradation are similar although relative amounts of aggradation in the high energy system are greatly enhanced by the deposition of large amounts of eroded bank material from upstream reaches degradation accompanied by widening is the most efficient means of energy dissipation because all components of total mechanical energy decrease with time widening dominates energy dissipation in the coarse grained system to offset increases in hydraulic depth caused by incision in the low energy fine grained system channel adjustment and energy dissipation are dominated by vertical processes because of low relative values of kinetic energy and because eroded bank sediment is transported out of the drainage basin and does not aid in downstream aggradation energy dissipation or channel recovery specific energy is shown to decrease nonlinearly with time during channel evolution and provides a measure of reductions in available energy at the channel bed data from two sites show convergence towards a minimum specific energy with time time dependent reductions in specific energy at a point act in concert with minimization of the rate of energy dissipation over a reach during channel evolution as the fluvial systems adjust to a new equilibrium

process form interactions in unstable sand bed river channels a numerical modeling approach abstract a deterministic numerical model of bed deformation and channel widening which accounts for specific mechanisms of bank erosion and collapse is used to analyze morphological and flow energy parameters in adjusting sand bed channels for a range of simulated fluvial environments the model is based on a set of conservation and process equations solved in conjunction with a set of specific initial and boundary conditions simulated channels with a range of assumed boundary material characteristics were subjected to identical disturbances imposed by reducing the supply of sediment from upstream asymptotic reductions of the rate of energy dissipation energy slope and boundary shear stress were found to be unifying characteristics of channel adjustment in all simulations morphologic responses to an identical disturbance reduction of sediment supply from upstream with identical bed material d 50 1 mm and channel gradient s b 0 001 were diverse the relative magnitude of simulated widening versus simulated bed level change was greatest for sand bank channels and least for clay bank channels stable channel dimensions were attained most rapidly for silt bank channels where simulated adjustments of channel width were similar to simulated vertical adjustments because all components of total mechanical energy flow depth or pressure head bed elevation and channel gradient or datum head and flow velocity or velocity head decrease simultaneously each simulated channel subjected to the baseline disturbance had an initial width to depth ratio f of 13 5 asymptotic values of energy dissipation rate adjusted f values and time taken to reach equilibrium after the disturbance are shown to vary as a function of simulated environmental boundary conditions the importance of channel widening in controlling and reducing the flow depth of a given discharge and average boundary shear stress is highlighted for the silt and sand bank cases channel widening causes a shift from degradation to aggradation during adjustment because of delivery of sand sized sediments from failed bank material to the simulated channel

evidence for thinning of the arctic ice cover north of greenland in may 1987 a british submarine carried out an ice profiling experiment in the arctic ocean in which the route closely approximated that of an earlier voyage in october 1976 over a zone extending more than 400 km to the north of greenland there is evidence of a significant decrease in mean ice thickness in 1987 relative to that found in 1976 this thinning amounts to a loss of volume of at least 15 over an area of 300 000 km2 the nature of the ice thickness distributions suggests that the thinning is primarily associated with the presence of a larger fraction of young and first year ice a consequence of wind driven divergence in the ice cover during the months preceding the 1987 observations such a large fluctuation in a region previously believed to possess consistently high ice thicknesses illustrates the importance of monitoring arctic ice thickness more systematically to determine whether such fluctuations fall within the limits of normal inter annual variability

flow regime and bedforms in a ridge and runnel system s e spain abstract during landward migration ridge and runnel systems are subjected to asymmetric oscillatory and or unidirectional flow regimes depending on the stage of development reached by these systems in the early stages of evolution when the ridge is situated in the upper shoreface the whole system is subjected to asymmetric oscillatory flow the runnel is under lower flow regime conditions and the ridge may be under upper or lower flow regime according to water depth and wave energy later when the ridge has migrated to a position on the foreshore the runnel is largely under a unidirectional lower flow regime while the ridge itself is under oscillatory upper flow regime when the ridge welds to berm it is largely emergent and exposed to high tide swash action under upper flow regime conditions the runnel is eventually filled with sand and transformed into a low lying area all these types grade laterally into each other one or more ridge and runnel systems can occur at the same time wave energy tide level and position of the ridge control the variations in the characteristics of the ridge and also the position of the zones of bedforms found at the upper shoreface

the distribution of nearshore bedforms and effects on sand suspension on low energy micro tidal beaches in southwestern australia abstract field measurements of bed morphologies hydrodynamics and sediment suspension were made on 15 low energy h sig two general trends in cross shore bedform patterns were identified among the low energy beaches the first trend found on beaches of finer sand median grain diameter of 0 24 mm consisted of a cross shore sequence from irregular ripples to cross ripples to parallel ephemeral ripples to plane bed in the direction of increasing bottom shear stress the second sequence found on beaches of medium sand median grain diameter 0 39 mm consisted mainly of parallel ripples with bifurcations that became increasingly organised into parallel ripples as water depth shallowed the wavelength of the cross ripples was found to increase linearly with increasing near bed orbital diameter while the wavelength of the ephemeral ripples was found to decrease slightly with increasing near bed orbital diameter suspended sediment concentrations were found to be greater over cross ripples and parallel ripples than over plane bed and ephemeral ripples even though bottom shear stresses were greater over ephemeral ripples co spectra of suspended sediment and cross shore currents revealed transport directions to be different over each of the ripple patterns

an equilibrium relationship for shear velocity and apparent roughness lenght in aeolian saltation abstract an equilibrium relationship between shear velocity and apparent surface roughness is established for aeolian saltation velocity profiles of wind over blowing sand were digitized from eleven published data sets the 106 profiles were analyzed to obtain estimates of shear velocity and apparent surface roughness lenghts these values were used to test charnock s relationship for suitability in aeolian environments the results indicate that z 0 0 0091 u 2 g modifying charnock s model to describe wind over an equilibrium saltation field yields z 0 2d 50 30 0 0252 u u t 2 g the latter result is used to demonstrate that the focal point concept proposed by bagnold stems from the increase in apparent roughness with increased shear velocity the results demonstrate an excellent example of threshold controlled equilibrium in dynamic systems

boundary roughness and bedforms in the surf zone hydrodynamical models of the nearshore system frequently assume that a single friction coefficient is sufficient to represent flow conditions at a point in the surf zone furthermore models attempting to relate bed configuration to surf zone flows have relied primarily upon the wave orbital velocity as an indicator of potential bedforms and thus as the control on boundary roughness the data presented here point out potential errors arising from either of these approaches the results of a field experiment conducted at wendake beach ontario show that at a single location in an active surf zone the darcy weisbach friction coefficient f varied by approximately 250 in this case between 0 016 and 0 041 it is also shown that existing bedform models based upon primary wave motions alone do not accurately predict conditions at this study site for a relatively constant wave orbital velocity and velocity asymmetry it is found that changes in bed roughness as a result of bedform development are reflected mainly in the vertical profile of the longshore current velocity a sequence of bedforms from oscillatory ripples through flat bed is inferred from the data and found to be supported by diver observations and preserved primary sedimentary structures

the underside of arctic sea ice imaged by sidescan sonar despite extensive submarine operations in the arctic little is yet known about the three dimensional morphology of the ice underside several studies have been done using upward looking sonar1 7 which yields a linear profile of ice draft from which the distributions of features such as pressure ridges and leads may be derived such a profile gives no information about the spatial morphology or appearance of the ice bottom surface in contrast the upper ice surface has been well documented from airborne and satellite borne sensors such as synthetic aperture radar passive microwave infrared and visual imaging here i report the first extensive set of imagery of the underside of sea ice in the arctic ocean sidescan sonar images show a distinct difference between the appearance of first year ice and of multi year ice first year ice has a smooth bottom except for a network of cracks while multi year ice is covered with an array of blisters or bulges the clear discrimination between ice types enables sidescan sonar to be used as a means of validating airborne passive microwave data

sea ice thickness distribution in the greenland sea and eurasian basin may 1987 upward sonar data from a 3400 km track in the greenland sea and the eurasian basin of the arctic ocean obtained by a british submarine in may 1987 have been statistically analyzed mean ice thicknesses in the greenland sea show significant melting with downstream distance at an average rate of 0 38 m per degree of latitude equivalent to about 2140 km3 yr 1 of freshwater input between 80 n and 75 n in the region north of fram strait between 82 n and 80 n there is remarkable consistency between mean ice drafts in 1987 and in three previous cruises 1976 1979 1985 while between greenland and the north pole there was a significant thinning in 1987 relative to 1976 in fram strait the mean ice draft shows evidence of thinning within the region of influence of a semipermanent eddy ice draft distributions show that the probability density function of deformed ice follows a negative exponential distribution and that the change of shape of the distribution with downstream distance in the greenland sea is due to preferential melting of thicker ice pressure ridge draft distributions also fit a negative exponential while their spacings fit a lognormal distribution lead width distribution follows a power law with an exponent of 1 45 for narrow leads and 2 50 for wider leads if leads are defined as having a 1 m maximum draft 1 67 and 2 76 for a 0 5 m maximum draft lead spacings follow a negative exponential distribution for moderate spacings 400 1500 m but with an excess of lead pairs at small and very large spacings

thinning of the arctic sea ice cover comparison of sea ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1 3 m in most of the deep water portion of the arctic ocean from 3 1 m in 1958 1976 to 1 8 m in the 1990s the decrease is greater in the central and eastern arctic than in the beaufort and chukchi seas preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s

stable channels with mobile gravel beds regime type equations for mobile gravel bed rivers are presented based on data obtained from 62 stable gravel bed river reaches in the united kingdom multiple regression techniques are applied to derive equations relating reach average and riffle values of width mean and maximum depth slope velocity sinuosity and riffle spacing to bankfull discharge bed and bank material characteristics valley slope bank vegetation type and an independent estimate of bankfull bed load transport rates although discharge has a dominant control on channel geometry these equations indicate that bed load discharge also has a significant influence particularly with regard to channel slope bank vegetation has a major control on width and velocity while depth velocity and slope are storngly affected by bed material size reasons for these results are considered in terms of the physical processes controlling channel adjustment the application of these regime equations is discussed and particular consideration is given to the design of sinuous channels with a pool riffle bed topograph

towards quantitative sea ice reconstructions in the northern north atlantic a combined biomarker and numerical modelling approach article i nfo organic geochemical analyses of marine surface sediments from the continental margins of east greenland and west spitsbergen provide for a biomarker based estimate of recent sea ice conditions in the northern north atlantic by means of the sea ice proxy ip25 and phytoplankton derived biomarkers e g brassicasterol and dinosterol we reconstruct sea ice and sea surface conditions respectively the combination of ip25 with a phytoplankton marker in terms of a phytoplankton marker ip25 index pip25 proves highly valuable to properly interpret the sea ice proxy signal as an under or overestimation of sea ice coverage can be circumvented a comparison of this biomarker based assessment of the sea ice distribution in the study area with 1 modern remote sensing data and 2 numerical modelling results reveal a good agreement between organic geochemical satellite and modelling observations the reasonable simulation of modern sea ice conditions by means of a regional ocean sea ice model demonstrates the feasibility to effectively integrate the complex atmospheric and oceanic circulation features as they prevail in the study area the good correlation between modelled sea ice parameters and the biomarker based estimate of sea ice coverage substantiates that linking proxy and model data occurs to be a promising concept in terms of a cross evaluation this combinatory approach may provide a first step towards quantitative sea ice reconstructions by means of ip25 future ip25 studies on marine surface sediments from the arctic realm however are recommended to extend and validate this new attempt of using ip25 in combination with a phytoplankton marker as a quantitative measure for sea ice reconstructions

arctic sea ice thickness remained constant during the 1990s the ice cover of the arctic ocean is considered to be a sensitive indicator of global climate change recent research using submarine based observations suggests that the arctic ice cover was thinner in the 1990s compared to an earlier period 1958 1979 and that it continued to decrease in thickness in the 1990s here i analyze subsurface ice thickness draft of arctic sea ice from six submarine cruises from 1991 to 1997 this extensive data set shows that there was no trend towards a thinning ice cover during the 1990s data from the north pole shows a slight increase in mean ice thickness whereas the beaufort sea shows a small decrease none of which are significant transects between the two areas from 76 n to 90 n also show near constant ice thicknesses with a general spatial decrease from the pole towards the beaufort sea combining the present results with those of an earlier study i conclude that the mean ice thickness has remained on a near constant level around the north pole from 1986 to 1997

thin ice thickness from satellite thermal imagery sea ice thickness is an important parameter in the surface energy exchange and mass balance of the polar oceans remote sensing offers the opportunity to monitor thin ice routinely we combine measurements of surface temperature from an advanced very high resolution radiometer avhrr with a thermodynamic ice growth model to estimate thin ice and its thickness distribution during the cold season in a beaufort sea area of perennial ice we find that thin ice up to a meter thick had a concentration of 18 in december 1990 and 3 in april 1991 thin ice concentrations near the ice edge in the greenland sea range from 20 to 50 these avhrr derived estimates generally compare well with thickness observations from upward looking sonar given the uncertainties in all the model inputs estimated from avhrr such as radiative and turbulent heat flux components and surface albedo we find that ice thickness is estimated with an uncertainty of roughly 50 the uncertainty in the cumulative thickness distribution is about 3 for ice less than 20 cm thick and about 9 for ice around 1 m thick these magnitudes are significantly smaller than the variations in thin ice observed by satellite in different months and regions we conclude that avhrr is capable of resolving the regional and seasonal variations in thin ice to a level of accuracy useful both for establishing a climatology and for testing ice models

low frequency passive microwave observations of sea ice in the weddell sea the microwave emission properties of first year sea ice were investigated from the r v polarstern during the antarctic winter weddell gyre project in 1989 radiometer measurements were made at 611 mhz and 10 ghz and were accompanied by video and visual observations using the theory of radiometric emission from a layered medium a method for deriving sea ice thickness from radiometer data is developed and tested the model is based on an incoherent reflection process and predicts that the emissivity of saline ice increases monotonically with increasing ice thickness until saturation occurs

further evidence of ice thinning in the arctic ocean a sea ice thickness profile obtained in september 1996 from the eurasian basin of the arctic ocean between fram strait and the north pole was compared with a profile obtained in the same region in september october 1976 a decline in mean ice draft of 43 was observed over the 20 year interval in agreement with changes observed in other parts of the arctic ocean by rothrock et al 1999

suspended sediment transport and the role of infragravity waves in a barred surf zone abstract collocated optical backscatterance suspended solids sensors and electromagnetic flowmeters were deployed at stanhope lane beach p e i canada to assess the contributions of wind waves infragravity waves and mean currents to the cross shore flux of suspended sediment across a barred surf zone the local net transport of suspended sediment magnitude and direction was highly variable both spatially and temporally with respect to the relative magnitudes of the suspended sediment transport 1 that induced by quasi steady mean currents and oscillatory currents at both incident and infragravity frequencies were almost equal in magnitude when the total surf zone was averaged over the storm event 2 waves in the near infragravity band were quantitatively more important during the waning of the storm especially where when surf persisted well after the initial decay of the offshore wave field 3 incident waves were important where when the waves were not saturated i e during the early stages of the storm at all locations and persistently throughout the storm on the seaward slope of the bar where wave saturation was limited to a very short period with respect to the direction of net sediment transport 1 that attributable to mean currents was directed offshore in the inner surf zone probably reflecting the presence of an undertow while further seaward an onshore transport occurred probably resulting from mass transport induced by surf bores or drift velocities induced by the infragravity wave boundary layer 2 that induced by oscillatory currents at incident wave frequencies was generally onshore at any one location throughout the storm event in contrast oscillatory transport at low frequencies changed direction and also varied in magnitude at any given location in part these changes reflect the existence of significant oscillatory sediment transport at discrete infragravity frequencies the standing infragravity waves consistently transported suspended sediment towards antinodes in the water surface elevation

comparison of the sea ice thickness distribution in the lincoln sea and adjacent arctic ocean in 2004 and 2005 results of helicopter borne electromagnetic measurements of total ice plus snow sea ice thickness performed in may 2004 and 2005 in the lincoln sea and adjacent arctic ocean up to 868 n are presented thickness distributions south of 848 n are dominated by multi year ice with modal thicknesses of 3 9 m in 2004 and 4 2 m in 2005 mean thicknesses 4 67 and 5 18 m respectively modal and mean snow thickness on multi year ice amounted to 0 18 and 0 30 m in 2004 and 0 28 and 0 35 m in 2005 there are also considerable amounts of 0 9 2 2 m thick first year ice modal thickness mostly representing ice formed in the recurring refrozen lincoln polynya results are in good agreement with ground based electromagnetic thickness measurements and with ice types demarcated in satellite synthetic aperture radar imagery four drifting buoys deployed in 2004 between 868 n and 84 58 n show a similar pattern of a mean southward drift of the ice pack of 83 18 km between may 2004 and april 2005 towards the coast of ellesmere island and nares strait the resulting area decrease of 26 between the buoys and the coast is larger than the observed thickness increase south of 848 n this points to the importance of shear in a narrow band along the coast and of ice export through nares strait in removing ice from the study region

equilibrium slopes and cross shore velocity asymmetries in a storm dominated barred nearshore system abstract the near bed 0 10 m elevation horizontal velocities were measured across a two barred lacustrine shoreface during a storm and reveal a spatially coherent but temporally variable pattern in the low order moments of the velocity field large cross shore velocity asymmetries indicated 1 1 spatially extensive and temporally persistent offshore mean flows up to 0 18 m s 1 across the landward slope of the linear outer bar reflecting an undertow driven by a wave induced setup 2 2 an onshore directed skewness opposing the mean flow of wind wave origin and persistent under propagating bores in the inner bar system long after this asymmetry had disappeared from the outer bar extensive sediment reactivation with near zero net sediment flux over the whole of the upper shoreface indicated a sediment prism in a steady state volumetrically over the storm cycle uniform depths of sediment reactivation with zero bed elevation change over the storm cycle revealed local slopes in a steady state in the outer linear bar however time integration of the near bed velocity field at this location indicated an offshore net flux of water while sedimentary structures indicated bedform migration and thus at least bedload almost uniformly onshore when ripples and megaripples were present the sediment transport giving rise to the local equilibrium cannot be explained therefore on the basis of the mean properties of the near bed velocity vector but requires knowledge of the time varying instantaneous sediment transport vector onshore migration of the inner bar correlated well with the large and persistent onshore skewness of the velocity distribution in the inner surf zone associated with translatory surf bores throughout the storm decay period low frequency oscillations were only significant close to the shoreline but may have influenced the development of a sinuous three dimensional inner bar form as it migrated onshore

arctic sea ice thickness characteristics in winter 2004 and 2007 from submarine sonar transects 1 a transect of the arctic ocean by the british submarine tireless in march 2007 enabled the thickness characteristics of the ice cover to be measured during the winter immediately preceding the exceptional retreat of summer 2007 in this paper we report on mean and modal drafts probability density functions of draft and the frequency and depth distribution of pressure ridges and we compare results with those from an earlier submarine cruise in winter 2004 which covered part of the same area in the region from north of fram strait to ellesmere island about 85 n 0 70 w we find no change in mean drafts between 2004 and 2007 though there is a change in ice composition with more ridging in 2007 but a lesser modal draft this agrees with the observations of younger ice being driven toward fram strait in 2007 the region north of ellesmere island continues to be a redoubt containing more thick deformed multiyear ice than any other part of the transect in the west the submarine profiled extensively under the sedna ice camp at 73 n 145 w this is in the same location as the 1976 aidjex ice camp and a sonar survey done by a u s submarine in april 1976 we found that a large decrease in mean draft had occurred 32 over 31 years and that in 2007 the sedna region contained the thinnest ice of any part of the arctic surveyed by the submarine this was a region from which the ice completely retreated during the subsequent summer of 2007

a cellular model of braided rivers a broad sheet of water flowing over non cohesive sediment typically breaks up into a network of interconnected channels called a braided stream fig 1 the dynamics of such networks are complex channels migrate laterally split rejoin and develop bars with the flow shifting unpredictably from one part of the network to another many processes are known to operate in a braided river1 3 but it is unclear which of these are essential to explain the observed dynamics we describe here a simple deterministic numerical model of water flow over a cohesionless bed that captures the main spatial and temporal features of real braided rivers the patterns arise from local scour and deposition caused by a nonlinear dependence of bedload sediment flux on water discharge although the morphology of the resulting network depends in detail on the sediment transport rule used in the model our results suggest that the only factors essential for braiding are bedload sediment transport and laterally unconstrained free surface flow

numerical simulation of widening and bed deformation of straight sand bed rivers ii model evaluation in this paper the numerical model presented in the companion paper is tested and applied assessment of model accuracy was based on two approaches first predictions of evolution of a 13 5 km reach of the south fork of the forked deer river in west tennessee were compared to observations over a 24 yr period results suggest that although the model was able to qualitatively predict trends of widening and deepening quantitative predictions were not reliable simulated widths and depths were within 15 of the corresponding observed values but observed change in these parameters at the study sites were also close to these values simulated rates of depth adjustment were within 15 of observed rates but observed rates of channel widening at the study sites were approximately three times those simulated by the model in the second approach the model was used to generate relationships between stable channel width and bank full discharge the model was able to successfully replicate the form of empirically

evaluation of arctic sea ice thickness simulated by arctic ocean model intercomparison project models six aomip model simulations are compared with estimates of sea ice thickness derived from pan arctic satellite freeboard measurements 2004 2008 airborne electromagnetic measurements 2001 2009 ice draft data from moored instruments in fram strait the greenland sea and the beaufort sea 1992 2008 and from submarines 1975 2000 drill hole data from the arctic basin laptev and east siberian marginal seas 1982 1986 and coastal stations 1998 2009 despite an assessment of six models that differ in numerical methods resolution domain forcing and boundary conditions the models generally overestimate the thickness of measured ice thinner than 2 m and underestimate the thickness of ice measured thicker than about 2 m in the regions of flat immobile land fast ice shallow siberian seas with depths less than 25 30 m the models generally overestimate both the total observed sea ice thickness and rates of september and october ice growth from observations by more than four times and more than one standard deviation respectively the models do not reproduce conditions of fast ice formation and growth instead the modeled fast ice is replaced with pack ice which drifts generates ridges of increasing ice thickness in addition to thermodynamic ice growth considering all observational data sets the better correlations and smaller differences from observations are from the ecco2 and uw models

longshore current profiles and lateral mixing across the surf zone of a barred nearshore greenwood b and sherman d j 1986 longshore current profiles and lateral mixing across the surf zone of a barred nearshore coastal eng 10 149 168 longshore currents were measured during a storm in a barred surf zone in the great lakes mean beach slope was 0 015 with three bars ranging up to 0 5 m in height comparisons with theoretical predictions revealed the effects of the bars on the horizontal structure of these currents and the degree of lateral mixing 1 perturbations in currents are localized spatially hut increase with bar relief 2 currents are lower and higher respectively in the outer and inner surf zone than expected for a plane beach the predictions of longuet higgins 1972 provide a reasonable approximation where relief is low 3 in the inner surf zone a plane beach solution suggests bars increase lateral mixing p 1 0 by increasing the velocity gradients 4 the barred slope model of ebersole and dalrymple 1981 most closely resembles the prototype 5 use of planar beach equivalents for p or n in models for barred beaches results in under prediction of the appropriate eddy viscosity

movable bed roughness in unsteady oscillatory flow a model to predict the roughness in unsteady oscillatory flows over movable noncohesive beds is presented the roughness over movable beds is shown to be a function of the boundary shear stress rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds the model partitions the roughness into two distinct contributions these two contributions are due to the form drag around individual bed forms and to the near bed sediment transport the form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress the ripples are predicted as a function of the local skin friction and a semiempirical expression is derived using standard law of the wall arguments which gives the ripple or form roughness as a function of the boundary geometry the ripple roughness is found to be proportional to the product of the ripple steepness and height favorable comparison of the form drag model with the results of bagnold s 1946 fixed ripple study is found the value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from carstens et al s 1969 experiments this value is found to be 7 or 8 grain diameters an expression is derived for the roughness associated with the maximum thickness of a near bottom sediment transporting layer consistent with owen s 1964 roughness hypothesis for saltation of uniform grains in air at large values of the boundary shear stress relative to the critical value for initial sediment motion the derived expression is similar to the results of smith and mclean s 1977 unidirectional flow approach modified for oscillatory flow the total roughness model is found to compare favorably with carstens et al s 1969 data in contrast to smith and mclean s 1977 steady flow findings the results here show that when ripples are present they account for a significant portion of the boundary roughness

seasonal variations in sea ice motion and effects on sea ice concentration in the canada basin drifting buoy data surface pressure and geostrophic wind analyses from the arctic ocean buoy program are used to examine seasonal features of the sea ice motion in the canada basin for 1979 1985 although the 7 year annual mean motion in this region is clockwise the month to month motion is highly variable in late summer to early autumn the circulation can become net anticlockwise for periods lasting at least 30 days results from a linear model demonstrate that these reversals of ice motion in the beaufort gyre are a wind driven response to persistent cyclonic activity that contrasts sharply with the predominantly anticyclonic regimes of spring late autumn and winter model predicted ice divergences of 0 5 or more per day which can occur during periods of anticlockwise ice motion are in good agreement with values calculated from optimally interpolated velocity gradient fields visible band imagery and passive microwave data confirm associated large areal reductions in ice concentration of approximately 20 data from under ice submarine sonar transects and surface pressure records prior to the study period point to frequent recurrences of these late summer to early autumn ice conditions

assimilating satellite concentration data into an arctic sea ice mass balance model 1979 1985 using satellite ice concentration data observed buoy velocities and a thermodynamic ice growth model we compute the time histories of the thickness distributions of four ice types level and ridged first year ice and level and ridged multiyear ice these histories are computed for seven regions covering the arctic ocean with a 10 day time step for 1979 through 1985 a blend of observed buoy motions and wind driven free drift velocities drives the advective and ridging parts of the model while climatological growth and melt rates determine the ice thickness the model adjusts the thickness distributions so that the modeled concentrations of open water first year ice and multiyear ice match the concentrations derived by kalman filtering of satellite passive microwave observations the major adjustment is to reduce modeled ice concentrations in summer and particularly in the peripheral seas to account for unmodeled melt the model includes an ice salinity parameterization which along with changes in ice volume gives the salt or freshwater flux from the ice our results show that the arctic ice cover consists primarily of multiyear ice 60 by area and 82 by volume and that most of this ice is ridged the average ice thickness is 2 7 m with a seasonal variation of 30 and an interannual variation of 10 the study also shows that for each cell and the arctic as a whole the changes in ice volume caused by growth minus melt approximately balance the changes caused by import minus export the annual ice export through fram strait varies from 1100 to 3000 km3 primarily because of variations in ice velocity all regions except the chukchi sea have a net ice growth a net export and a net salt input to the ocean surface the annual mean salt flux to the ocean surface from net ice production is 7 3 kg m 2 downward fluxes along the transpolar drift stream are larger than elsewhere the largest downward flux of 33 kg m 2 yr 1 in the nansen basin can be attributed to the strong divergence near fram strait a companion paper steele et al this issue uses these fluxes and ice velocities to derive the arctic ocean freshwater balance

a network scale intermediate complexity model for simulating channel evolution over years to decades abstract excessive river erosion and sedimentation threatens critical infrastructure degrades aquatic habitat and impairs water quality tools for predicting the magnitude of erosion sedimentation and channel evolution processes are needed for effective mitigation and management we present a new numerical model that simulates coupled river bed and bank erosion at the watershed scale the model uses modified versions of bagnold s sediment transport equation to simulate bed erosion and aggradation as well as a simplified bank stability and toe erosion model bstem to simulate bank erosion processes the model is mechanistic and intermediate complexity accounting for the dominant channel evolution processes while limiting data requirements we apply the model to a generic test case of channel network response following a disturbance and the results match physical understanding of channel evolution the model was also tested on two field data sets below parker dam on the lower colorado river and the north fork toutle river nftr which responded dramatically to the 1980 eruption of mount st helens it accurately predicts observed channel incision and bed material coarsening on the colorado river as well as observations for the upstream 18 km of the nftr watershed the model does not include algorithms for extensive lateral migration and avulsions and therefore did not perform well in the lower nftr where the channel migrated across a wide valley bottom rem is parsimonious and useful for simulating network scale channel change in single thread systems responding to disturbance

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