the localized role of base level lowering on channel adjustment of tributary streams in the trinity river basin downstream of livingston dam texas usa the purpose of this study is to describe and explain river channel cross sectional change in tributary streams within the 60 km scour zone of the trinity river texas downstream of livingston dam this study investigates changes within the tributary systems to base level changes triggered by incision associated with scour and channel migration within the dammed trunk stream the dam represents a precise moment and place of a system perturbation and therefore represents an opportunistic geomorphic experiment geomorphological effects of the lower trinity river tributaries were investigated through three different types of data resurveys of bridge cross sections planform change as measured from aerial photographs and field mapping of changes in channel geometry using fallen trees undercut banks and other geomorphic indicators of change while field evidence of channel change was found at sites well upstream of tributary confluences with the trinity these were not consistent with responses to lower base level downcutting and lateral changes in response to lowered base level were confined to lower tributary reaches this suggests distance decay and lag times in transmission of responses upstream and or masking of dam induced effects by other processes and controls the most dynamic locations of geomorphic change are near the confluences of the tributaries with the trinity river where the response to dam induced incision is strongly influenced by local trunk stream morphology indicating a high degree of historical and geographical contingency

assessing the roles of three eddy types in restratifying the labrador sea after deep convection abstractrestratification after deep convection is one of the key factors in determining the temporal variability of dense water formation in the labrador sea in the subsurface it is primarily governed by lateral buoyancy fluxes during early spring the roles of three different eddy types in this process are assessed using an idealized model of the labrador sea that simulates the restratification season the first eddy type warm core irminger rings is shed from the boundary current along the west coast of greenland all along the coastline the boundary current forms boundary current eddies the third type convective eddies arises directly around the convection area in the model the latter two eddy types are together responsible for replenishing 30 of the winter heat loss within 6 months irminger rings add another 45 to this number the authors results thus confirm that the presence of irminger rings is essential for a realistic amount of restratification in this area the model results are com

morphology and origin of the osbourn trough the osbourn trough is a 900 km long east west trending gravity low visible in satellite altimetry data which intersects the tonga trench at 25 30 s a recent survey collected gravity magnetic echo sounder and swath bathymetry data on three adjacent north south trending ship tracks centered on the trough the linear gravity low is 20 30 mgal less than the regional value and is accompanied by a flat lying 200 500 m deep sediment filled valley swath bathymetry images reveal several parallel east west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers including a prominent inside corner high and possible pseudo fault trace the presence of magnetic anomalies possibly anomalies 33 and 32 suggests that the seafloor here was formed after the end of the cretaceous normal superchron anomaly 34 these data support the conclusion that this trough is a spreading center which stopped spreading in late cretaceous time the existence of this feature has important implications for tectonic reconstructions in this region the osbourn trough could be part of the fossil spreading center between the pacific plate and a fragment of the phoenix plate the bellingshausen plate this would require the seafloor between the osbourn trough and the chatham rise to the south to be a remnant fragment of the bellingshausen plate this remnant may have joined to the pacific plate when the hikurangi plateau entered the gondwana subduction zone at the chatham rise possibly causing the cessation of spreading on the osbourn trough

gyre scale deep convection in the subpolar north atlantic ocean during winter 2014 2015 using argo floats we show that a major deep convective activity occurred simultaneously in the labrador sea lab south of cap farewell scf and the irminger sea irm during winter 2014 2015 convection was driven by exceptional heat loss to the atmosphere up to 50 higher than the climatological mean this is the first observation of deep convection over such a widespread area mixed layer depths exceptionally reached 1700 m in scf and 1400 m in irm the deep thermocline density gradient limited the mixed layer deepening in the labrador sea to 1800 m potential densities of deep waters were similar in the three basins 27 73 27 74 kg m 3 but warmer by 0 3 c and saltier by 0 04 in irm than in lab and scf meaning that each basin formed locally its own deep water the cold anomaly that developed recently in the north atlantic ocean favored and was enhanced by this exceptional convection

oxygen saturation surrounding deep water formation events in the labrador sea from argo o2 data deep water formation supplies oxygen rich water to the deep sea spreading throughout the ocean by means of the global thermohaline circulation models suggest that dissolved gases in newly formed deep water do not come to equilibrium with the atmosphere however direct measurements during wintertime convection are scarce and the controls over the extent of these disequilibria are poorly quantified here we show that when convection reached deeper than 800 m oxygen in the labrador sea was consistently undersaturated at 6 1 to 7 6 at the end of convection deeper convection resulted in greater undersaturation while convection ending later in the year resulted in values closer to equilibrium from which we produce a predictive relationship we use dissolved oxygen data from six profiling argo floats in the labrador sea between 2003 and 2016 allowing direct observations of wintertime convection three of the six optode oxygen sensors displayed substantial average in situ drift of 3 03 mu mol o 2 kg 1 yr 1 0 94 o 2 yr 1 which we corrected to stable deepwater oxygen values from repeat ship surveys observations of low oxygen intrusions during restratification and a simple mixing calculation demonstrate that lateral processes act to lower the oxygen inventory of the central labrador sea this suggests that the labrador sea is a net sink for atmospheric oxygen but uncertainties in parameterizing gas exchange limit our ability to quantify the net uptake our results constrain the oxygen concentration of newly formed labrador sea water and allow more precise estimates of oxygen utilization and nutrient regeneration in this water mass

further intensification of deep convection in the labrador sea in 2016 there has been a progressive deepening of winter convection in the labrador sea since 2012 with the individual profile maximum depth exceeding 1800 m since 2014 and reaching 2100 m in 2016 this increase during repeated positive phases of the winter north atlantic oscillation nao resembles that during the formation of the record depth 2500 m labrador sea water lsw class in 1987 1994 attributed to repeated positive nao forcing having provided critical preconditioning the 2012 2016 lsw class is one of the deepest and most persistent ever observed back to 1938 year round observations from profiling argo floats since 2002 complemented by annual surveys are providing novel information on the seasonal to decadal evolution of lsw such as its variable density the recent multiyear preconditioning and its 2016 density being the highest since the mid 1990s these findings should help international observation programs and numerical model studies investigating lsw influences on the subpolar north atlantic and atlantic meridional overturning circulation

boundary current eddies and their role in the restratification of the labrador sea an idealized model is used to study the restratification of the labrador sea after deep convection with emphasis on the role of boundary current eddies shed near the west coast of greenland the boundary current eddies carry warm buoyant irminger current water into the labrador sea interior for a realistic end of winter state it is shown that these irminger current eddies are efficient in restratifying the convected water mass in the interior of the labrador sea in addition it is demonstrated that irminger current eddies can balance a significant portion of the atmospheric heat loss and thus play an important role for the watermass transformation in the labrador sea

equilibrium or indeterminate where sediment budgets fail sediment mass balance and adjustment of channel form green river downstream from flaming gorge dam utah and colorado this study examines bed and bank adjustment in the 105 km reach of the green river immediately downstream from flaming gorge dam by the use of historical aerial and oblique photographs analysis of current and abandoned stream gaging records and field observations although this segment has been previously characterized as sediment deficient these data show that sediment is accumulating in all reaches and that the bed has not degraded at any location where historical data are available adjustment is occurring through a combination of deposition of post dam sediment and stabilization of pre dam deposits resulting in a 10 30 reduction in average width of the channel all post dam surfaces are colonized by woody riparian vegetation the style of channel adjustment varies between geomorphically defined reaches in canyons dominated by debris fans and gravel bedded restricted meandering reaches gravel bars have become inactive and accumulated fine sediment in the sand bedded meandering reaches existing islands have increased in size and new mid channel islands have formed in all of these types of reaches post dam deposits line the banks and sediment has accumulated in side channels that previously separated islands from the bank these findings demonstrate that sediment budgets that show a balance between inputs and outputs cannot necessarily be interpreted to indicate channel equilibrium a sediment mass balance for 150 km reach between the dam and the first long term gage indicates approximate balance of inputs and outputs for the pre and post dam periods when uncertainty in budget components is considered the mass balance is indeterminate although the green river may have been in approximate equilibrium in the pre dam period we have shown that channel width is decreasing in the post dam period the post dam deposits constitute a small but a significant component of the sediment budget upstream from the first major tributary sediment is supplied to this reach by small tributaries and to a lesser extent erosion of pre dam alluvium downstream from the study area the volume of the post dam deposits is tiny relative to the volume of sediment input from the first major tributary d 2005 elsevier b v all rights reserved

transformation of the labrador sea water in the subpolar north atlantic development spreading and decay of the thermohaline properties of two labrador sea water lsw classes are described during the development phase a specific lsw class repeatedly mixed by winter convection in the labrador sea becomes colder denser thicker and deeper once convection weakens the lsw class becomes isolated from the upper layer and starts to decay rapidly losing its volume while retaining the same density due to isopycnal mixing with the neighbouring warm saline intermediate waters a similar pattern in temperature salinity and density is seen in the other basins with different time lags from about two years in the irminger sea to ten years in the northern iceland basin and rockall trough regions the influence of lsw on the thermohaline properties of other north atlantic water masses is also discussed

decline of subpolar north atlantic circulation during the 1990s observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid to high latitude north atlantic ocean topex poseidon altimeter data show that subpolar sea surface height increased during the 1990s and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation combining the data from earlier satellites we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s direct current meter observations in the boundary current of the labrador sea support the weakening circulation trend of the 1990s and together with hydrographic data show that the mid to late 1990s decline extends deep in the water column analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations a weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation

intense oceanic uptake of oxygen during 2014 2015 winter convection in the labrador sea measurements of near surface oxygen o2 concentrations and mixed layer depth from the k1 mooring in the central labrador sea are used to calculate the change in column integrated 0 1700 m o2 content over the deep convection winter 2014 2015 during the mixed layer deepening period november 2014 to april 2015 the oxygen content increased by 24 3 3 4 mol m 2 40 higher than previous results from winters with weaker convection by estimating the contribution of respiration and lateral transport on the oxygen budget the cumulative air sea gas exchange is derived the o2 uptake of 29 1 3 8 mol m 2 driven by persistent undersaturation 5 and strong atmospheric forcing is substantially higher than predicted by standard nonbubble gas exchange parameterizations whereas most bubble resolving parameterizations predict higher uptake comparable to our results generally large but varying mixed layer depths and strong heat and momentum fluxes make the labrador sea an ideal test bed for process studies aimed at improving gas exchange parameterizations

evaluation of labrador sea water formation in a global finite element sea ice ocean model setup based on a comparison with observational data the deep water formation in the labrador sea is simulated with the finite element sea ice ocean model fesom in a regionally focused but globally covered model setup the model has a regional resolution of up to 7 km and the simulations cover the time period 1958 2009 we evaluate the capability of the model setup to reproduce a realistic deep water formation in the labrador sea two classes of modeled labrador sea water lsw the lighter upper lsw ulsw and the denser deep lsw dlsw are analyzed their layer thicknesses are compared to ulsw and dlsw layer thicknesses derived from observations in the formation region for the time interval 1988 2009 the results indicate a suitable agreement between the modeled and observational derived ulsw and dlsw layer thicknesses except for the period 2003 2007 where deviations in the modeled and observational derived layer thicknesses could be linked to discrepancies in the atmospheric forcing of the model it is shown that the model is able to reproduce four phases in the temporal evolution of the potential density temperature and salinity since the late 1980s which are known in observational data these four phases are characterized by a significantly different lsw formation the first phase from 1988 to 1990 is characterized in the model by a fast increase in the convection depth of up to 2000 m accompanied by an increased spring production of deep labrador sea water dlsw in the second phase 1991 1994 the dlsw layer thickness remains on a high level for several years while the third phase 1995 1998 features a gradual decrease in the deep ventilation and the renewal of the deep ocean layers the fourth phase from 1999 to 2009 is characterized by a slowly continuing decrease of the dlsw layer thickness on a deeper depth level by applying a composite map analysis between an index of dlsw and sea level pressure over the entire simulation period from 1958 to 2009 it is shown that a pattern which resembles the structure of the north atlantic oscillation nao is one of the main triggers for the variability of lsw formation our model results indicate that the process of dlsw formation can act as a low pass filter to the atmospheric forcing so that only persistent nao events have an effect whether ulsw or dlsw is formed based on composite maps of the thermal and haline contributions to the surface density flux we can demonstrate that the central labrador sea in the model is dominated by the thermal contributions of the surface density flux while the haline contributions are stronger over the branch of the labrador sea boundary current system lsbcs where they are dominated by the haline contributions of sea ice melting and formation our model results feature a shielding of the central labrador sea from the haline contributions by the lsbcs which only allows a minor haline interaction with the central labrador sea by lateral mixing based on the comparison of the simulated and measured lsw layer thicknesses as well as vertical profiles of potential density temperature and salinity it is shown that the fesom model is a suitable tool to study the regional dynamics of lsw formation and its impact on a global not regional restricted scale

deep convection in the labrador sea as observed by lagrangian floats abstract during the winters of 1997 and 1998 a total of 24 lagrangian floats were deployed in the labrador sea these floats were designed to match the buoyancy and compressibility of seawater they measured temperature and three dimensional position pressure for vertical position and rafos acoustic tracking for latitude and longitude as they followed water motions three dimensionally this data provides direct observation of mixed layer depth and excellent estimates of vertical velocity floats were repeatedly carried across the convecting layer by vertical velocities averaging several centimeters per second with vertical excursions of up to one kilometer in the horizontal several scales of eddy motion were resolved as was a possible float predilection toward remaining in water preconditioned for convection heat flux estimates from this data reveal entrainment and surface heat fluxes similar in magnitude the mixed layer acts as a vertical conveyor belt of temperature transporting heat from depth

elemental and major biochemical changes across an oxidation front in a relict turbidite an oxygen effect abstract the elemental and major biochemical compositions of the relict f turbidite sampled in two cores from the madeira abyssal plain were determined this fine grained distal sequence occurs at ca 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change oxygen diffused downward through sediments above this interface and in ca 10 kyr destroyed 80 of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca 140 kyr these deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates in both cores a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers the unoxidized sequences of both turbidities contained 0 93 1 02 wt organic carbon oc and 0 10 0 11 wt total nitrogen tn approximately 20 of the initial oc and 40 of the initial tn remained in the oxidized horizons with a consequent decrease in atomic c n ratio from ca 11 to ca 5 all samples gave very low yields of lignin phenols and comparable oc normalized yields of total aldoses and amino acids and indicated predominantly marine organic matter om and nonselective oxic degradation of these biochemical classes compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments except that the oxidized horizons yielded markedly elevated 3 5x percentages of nonprotein amino acids this study clearly demonstrates that prolonged exposure to oz can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone in comparison to early diagenesis however alteration of the measured biochemicals was largely nonselective such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep ocean environments

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

studies of labrador sea water formation and variability in the subpolar north atlantic in the light of international partnership and collaboration abstract labrador sea water lsw the lightest contribution to north atlantic deep water nadw and one of the most prominent water masses of the subpolar north atlantic has seen remarkable changes over the past century lsw originates in the labrador sea where it is formed through wintertime ocean convection of varying intensity depth and spatial extent formation of lsw followed by its respective injection into the mid depth circulation system is mandatory for ventilating and renewing water layers of the interior ocean indispensably important for unraveling the history of variability in formation and properties of lsw as well as for mapping its large scale spreading and export are sustained physical and chemical observations from the deep ocean these observations started at the beginning of the 20th century from occasional mostly national surveys and today constitute large scale multi national collaborative efforts including a vast arsenal of sophisticated instrumentation in a historical context we revisit major milestones over the past 100 years which have established and are constantly adding to shaping today s knowledge on lsw and present first details on the latest vintage of lsw generated during the strong winter of 2013 2014 respective argo data reveal mixed layer depths greater than 1700 m marking formation of a new cold and fresh anomaly that has spread since then over the subpolar north atlantic we further summarize the on going observational efforts in the subpolar north atlantic and present a compilation of hydrographic standard lines that serve to provide top to bottom information on nadw components

emerging impact of greenland meltwater on deepwater formation in the north atlantic ocean the greenland ice sheet has experienced increasing mass loss since the 1990s1 2 the enhanced freshwater flux due to both surface melt and outlet glacier discharge is assuming an increasingly important role in the changing freshwater budget of the subarctic atlantic3 the sustained and increasing freshwater fluxes from greenland to the surface ocean could lead to a suppression of deep winter convection in the labrador sea with potential ramifications for the strength of the atlantic meridional overturning circulation4 5 6 here we assess the impact of the increases in the freshwater fluxes reconstructed with full spatial resolution3 using a global ocean circulation model with a grid spacing fine enough to capture the small scale eddying transport processes in the subpolar north atlantic our simulations suggest that the invasion of meltwater from the west greenland shelf has initiated a gradual freshening trend at the surface of the labrador sea although the freshening is still smaller than the variability associated with the episodic great salinity anomalies the accumulation of meltwater may become large enough to progressively dampen the deep winter convection in the coming years we conclude that the freshwater anomaly has not yet had a significant impact on the atlantic meridional overturning circulation

revised history of early tertiary plate motion in the south west pacific reanalysis of early tertiary magnetic anomalies on the pacific plate south of the campbell plateau indicates that in early tertiary time there was a previously unrecognized triple junction of the pacific the antarctic and a third plate now beneath the bellingshausen sea corresponding revised parameters for pacific antarctica motion do not require early tertiary displacements within antarctica or new zealand but suggest a change in direction of the pacific plate over the hotspots of approximately the same sense and timing as the hawaiian emperor bend

nature s complex flume using a diagnostic state and transition framework to understand post restoration channel adjustment of the clark fork river montana abstract there is an imperfect symmetry between the patterns of channel evolution observed during laboratory flume experiments and those which materialize in rivers exposed to ambient environmental conditions that produce hydrogeomorphic fluxes which are more complex contingent and unpredictable one strategy to improve our understanding of short to medium term channel evolution is to study landscapes that have undergone significant disturbance and have had their biogeomorphic templates reset to a known condition in effect creating a flume in nature this study adopts a diagnostic state and transition framework to narrate and document baseline hypotheses for the potential evolutionary trajectories clark fork river near milltown montana following dam removal and remediation a 5 km stretch of the clark fork river and its adjoining floodplain were reconstructed since flow was introduced to the newly constructed channel in december 2010 complex evolutionary trajectories have been observed on the clark fork s mainstem its secondary channels and floodplain focusing particularly on the river s secondary channels this paper develops a typology of channel states that have been observed and demonstrates that multiple adjustment trajectories have materialized sometimes within the same channel a diagnostic state and transition framework offers a parsimonious strategy to quantitatively or qualitatively anticipate the influence of water sediment and ecological fluxes on channel evolution at the basin reach or segment scale it provides environmental agencies with a robust method to devise spatially explicit scenario based management plans for rivers in a variety of geomorphic settings

life of the bellingshausen plate the onset of independent bellingshausen plate movement in the south pacific is estimated at just after chron c33o this event closely follows a change in the offset sense on the transform arm of the late cretaceous pacific phoenix antarctic triple junction and accompanies the creation and or lengthening by different mechanisms of long offsets on the pacific antarctic ridge at the udintsev tharp heezen and v transforms as the triple junction stepped away from the ridge at around chron c27 the bellingshausen plate ceased to move independently and the pacific antarctic ridge inherited the set of long offset transforms shortly afterwards the gravity fabric on the flanks of much of the ridge displays evidence for a spreading rate decrease possibly related to transpression on the long offset transforms we speculate on a link between these events and regional tectonic changes around chron c27

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

recent morphological channel changes in a deltaic environment the case of the rhone river france abstract channel morphological changes were measured at two sites within the rhone delta along the main channel of the rhone river over one century results show an average erosion rate of 2 8 mm year 1 during the 20th century and decreasing to

the dependence of seafloor roughness on spreading rate bathymetric profiles taken parallel to flowlines across the mid ocean ridges of the south atlantic and south pacific strongly suggest an approximate linear relationship slope 30 50 m cm 1 yr between the rms amplitude roughness of small scale features and spreading rate this relationship is best demonstrated for spreading half rates 35 mm yr the rms roughness spreading rate dependence is most clearly defined for that component of the ridge flank relief with spatial wavelengths of less than about 25 km the rms roughnesses calculated for constant spreading rate intervals of 10 my across a single flowline profile from the south atlantic show that significant and consistent opposite flank roughness differences are typically associated with asymmetric spreading this important observation has profound implications regarding the nature of the faulting and volcanic processes acting near the ridge axis to form the small scale asymmetric roughness

the formation of labrador sea water part iii the evolution of oxygen and nutrient concentration abstract oxygen nutrient and tritium concentrations observed in the western labrador sea in march 1976 during deep convective renewal of labrador sea water are analyzed to show how a newly formed water mass obtains its characteristics common to other winter observations of deep mixed layers the oxygen concentrations are some 6 undersaturated even in the upper 20 m a gas transfer model coupled to a simple mixed layer model illustrates the difficulty of transferring sufficient oxygen across the air sea boundary to fully oxygenate the mixed layer when the mixed layer depth exceeds a few hundred meters the nutrient concentrations of the mixed layers are fairly well mixed as is consistent with the fairly narrow range of nutrient concentrations of the source waters only the tritium concentrations exhibit any structure within the mixed layer and it is argued that this is due to the much larger range of tritium concentrations in the source waters that make up the mixed layer

atmospheric forcing during active convection in the labrador sea and its impact on mixed layer depth hydrographic data from the labrador sea collected in february march 1997 together with atmospheric reanalysis fields are used to explore relationships between the air sea fluxes and the observed mixed layer depths the strongest winds and highest heat fluxes occurred in february due to the nature and tracks of the storms while greater numbers of storms occurred earlier and later in the winter the storms in february followed a more organized track extending from the gulf stream region to the irminger sea where they slowed and deepened the canonical low pressure system that drives convection is located east of the southern tip of greenland with strong westerly winds advecting cold air off the ice edge over the warm ocean the deepest mixed layers were observed in the western interior basin although the variability in mixed layer depth was greater in the eastern interior basin the overall trend in mixed layer depth through the winter in both regions of the basin was consistent with that predicted by a 1 d mixed layer model we argue that the deeper mixed layers in the west were due to the enhanced heat fluxes on that side of the basin as opposed to oceanic preconditioning this article is protected by copyright all rights reserved

antarctic tectonics constraints from an ers 1 satellite marine gravity field a high resolution gravity field of poorly charted and ice covered ocean near west antarctica from the ross sea east to the weddell sea has been derived with the use of satellite altimetry including ers 1 geodetic phase wave form data this gravity field reveals regional tectonic fabric such as gravity lineations which are the expression of fracture zones left by early 65 to 83 million years ago pacific antarctic sea floor spreading that separated the campbell plateau and new zealand continent from west antarctica these lineations constrain plate motion history and confirm the hypothesis that antarctica behaved as two distinct plates separated from each other by an extensional bellingshausen plate boundary active in the amundsen sea before about 61 million years ago

the formation of labrador sea water part i large scale processes abstract data obtained in the western labrador sea during march 1976 by hudson are analysed to show that new labrador sea water was being formed at this time on the basis of hydrographic and moored current meter data it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western labrador sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to labrador sea water to take place using a model it is demonstrated that water columns found along the western boundary of the labrador sea can be modified by cooling evaporation and mixing to form deep mixed layers with the properties of labrador sea water approximately 105 km3 of new labrador sea water was formed in 1976 an estimate that is consistent with earlier estimates of mean annual production rates this water 2 9 c 34 84 is some 0 6 c cooler and 0 06 fresher than that defined by lazier 1973 from his data collected in 1966 the variation of labrador sea water

abbot ice shelf structure of the amundsen sea continental margin and the southern boundary of the bellingshausen plate seaward of west antarctica inversion of nasa operation icebridge airborne gravity over the abbot ice shelf in west antarctica for subice bathymetry defines an extensional terrain made up of east west trending rift basins formed during the early stages of antarctica zealandia rifting extension is minor as rifting jumped north of thurston island early in the rifting process the amundsen sea embayment continental shelf west of the rifted terrain is underlain by a deeper more extensive sedimentary basin also formed during rifting between antarctica and zealandia a well defined boundary zone separates the mildly extended abbot extensional terrain from the deeper amundsen embayment shelf basin the shelf basin has an extension factor of 1 5 1 7 with 80 100 km of extension occurring across an area now 250 km wide following this extension rifting centered north of the present shelf edge and proceeded to continental rupture since then the amundsen embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence sedimentation and the advance and retreat of the west antarctic ice sheet the bellingshausen plate was located seaward of the amundsen sea margin prior to incorporation into the antarctic plate at about 62 ma during the latter part of its independent existence bellingshausen plate motion had a clockwise rotational component relative to antarctica producing convergence across the north south trending bellingshausen gravity anomaly structure at 94 w and compressive deformation on the continental slope between 94 w and 102 w farther west the relative motion was extensional along an east west trending zone occupied by the marie byrd seamounts key points abbot ice shelf is underlain by e w rift basins created at 90 ma amundsen shelf shaped by subsidence sedimentation and passage of the ice sheet bellingshausen plate boundary is located near the base of continental slope and rise

calcium carbonate saturation states in the waters of the canadian arctic archipelago and the labrador sea 1 ocean acidification is predicted to occur first in polar oceans we investigated the saturation state of waters with respect to calcite cal and aragonite arg in six sections along an arctic outflow pathway through the canadian arctic archipelago caa and into the northwestern atlantic using dissolved inorganic carbon and total alkalinity measurements from 2003 to 2005 the study area a key region connecting the arctic and the north atlantic includes smith sound barrow strait baffin bay davis strait hudson strait and the labrador sea the average arg in the arctic outflow was 1 18 0 17 in barrow strait and 1 31 0 14 in smith sound with areas where arg 1 the arctic outflow through the caa has a high content of pacific waters which have a low saturation state these waters can be traced along the western baffin bay to davis strait south of davis strait this outflow is modified by mixing with slope and offshore waters of atlantic origin and with the outflow from hudson strait despite the mixing low saturation state water can still be identified on the southern labrador shelf the aragonite saturation horizon is found at 150 m in barrow strait at 200 m in baffin bay davis strait and hudson strait and at 2300 m in the labrador sea this study provides baseline data of the saturation states for the waters of the caa and the northwest atlantic it also illustrates the downstream evolution of low saturation state arctic outflow in the northwest atlantic

the formation of labrador sea water part ii mesoscale and smaller scale processes abstract in a previous paper clarke and gascard argued that the formation of labrador sea water was taking place in a cyclonic gyre set up each winter in the western labrador sea within the gyre and at its boundaries a number of different scales of organization are believed to be important in the formation processes the longest of these scales is the mesoscale 50 km which appears to be related to topographic rossby waves generated in the labrador current and propagating offshore the next smaller scale is an eddy scale 20 km believed to arise because the mesoscale is baroclinically unstable as shown by applying a two layer model of tang this instability is believed to promote mixing by generating frontal structures and vertical motions along them thus bringing subsurface t s maxima nearer the surface then within the mesoscale and eddy scale structures intense vertical convective cells take place at scales which are probably of the order of 1 km in three dimensions these events are short liv

atmospheric conditions associated with labrador sea deep convection new insights from a case study of the 2006 07 and 2007 08 winters abstractdeep convection in the labrador sea ls resumed in the winter of 2007 08 under a moderately positive north atlantic oscillation nao state this is in sharp contrast with the previous winter with weak convection despite a similar positive nao state this disparity is explored here by analyzing reanalysis data and forced ocean simulations it is found that the difference in deep convection is primarily due to differences in large scale atmospheric conditions that are not accounted for by the conventional nao definition specifically the 2007 08 winter was characterized by an atmospheric circulation anomaly centered in the western north atlantic rather than the eastern north atlantic that the conventional nao emphasizes this anomalous circulation was also accompanied by anomalously cold conditions over northern north america the controlling influence of these atmospheric conditions on ls deep convection in the 2008 winter is confirmed by sensitivity experiments where surface forcing and or in

underlying gravel layers in a large sand bed river and their influence on downstream dam channel adjustment due to the changes in environmental factors during a river s historical development underlying gravel layers are found in many large plain rivers when the buried depth of this gravel layer is within the reach of down cutting by clear water scour after reservoir construction it may be exposed and exert a far reaching influence on channel adjustment in hanjiang river the longest tributary of the yangtze river in china down cutting has been greatly reduced and even stopped due to the total change of bed material composition the exposure of a gravel layer results in a dramatic increase in the bed s hydraulic roughness so the channel slope may increase after a decline during the period prior to the exposure of the gravel layer moreover the exposed gravel layer increases the erosion resistance of bed material making a lower relative erosion resistance of bank to bed material where the bank s erosional resistance is weak this may lead to a tendency towards channel widening

impact of hydroelectric power releases on the morphology and sedimentology of the bed of the warche river belgium the butgenbach dam on the warche river was built in 1932 in order to maintain a suf cient supply of water to the robertville reservoir situated 7 km downstream for the production of hydroelectricity during winter months releases are made almost every day from the butgenbach dam from a hydrological point of view this has resulted in signi cantly reducing the number of discharges that are higher than bankfull despite the reduction in peak discharge there is a signi cant increase in the number of ef cient discharges 0 6 bankfull the impacts of these hydrological modi cations on the bed morphology and sedimentology below the butgenbach dam have been studied and the following geomorphological modi cations have been identi ed a doubling of the width of the channel in 45 years a reduction in the number of rif es and pools an increase in the number of gravel bars and islets and an increase in bedrock outcrops in the channel moreover the nest bed particles are mobilized by the almost daily releases inducing a signi cant increase in bed material size sorting the reduction of sinuosity and the disappearance of bed differentiation and rif e pool sequences have produced a diminution of bed roughness and an increase of the competence of the river thus relatively small oods can remove the armoured layer copyright 2003 john wiley sons ltd

structure and variability of the west greenland current in summer derived from 6 repeat standard sections abstract six historical sections across the west greenland current are examined three sections have been regularly occupied since the late 1950s while the three southern ones have been taken since 1984 significant variability is observed for the freshwater core of the coastal current on the shelf with salinity varying by over 3 units between years there is also significant variability in the shape and offshore position of the main shelf break front leading to large variability in eulerian velocities significant presence of irminger water is seen during the 1960s and the 2000s being found right across the sections in recent years maximum mean transport relative to the 34 8 isohaline of 5 5 3 9 sv relative to 700 db for 1984 2005 is observed at the cape desolation section transports decrease to the north with the majority of the exchange with the interior of the labrador sea occurring between cape desolation and fylla bank inter decadal transport variability is observed at fylla bank while a decline in transports since peaks in the early 1990s is seen at cape farewell and cape desolation freshwater transport is largest at cape desolation with a mean summer transport of 60 2 20 5 msv freshwater transport increases slightly between cape farewell and cape desolation and we suggest it is related to local discharge by glaciers into juliannehaab bight as well as the melting of sea ice we also find that years of high greenland ice cap melt are consistently associated with years of high freshwater transport at cape desolation suggesting a portion of the freshwater transport of the west greenland current may be associated with melt from the greenland ice sheet finally significantly enhanced freshwater transports 33 msv at cape desolation compared to the long term mean are seen in 2008 probably a signature of the record arctic ocean ice melt and export in 2007

an experimental study of complex response in river channel adjustment downstream from a reservoir an experimental study is described which attempts to reveal the complex response process of river channel adjustment downstream from a reservoir a descriptive model has been established which identifies three adjustment stages in stage i following the change of water and sediment input the width depth ratio of river channel w d and the river channel gradient s both decrease but the river channel sinuosity r increases in stage ii because of feedback from the changing boundary conditions w d starts to increase r decreases and the rate of gradient decrease becomes very slow in stage iii equilibrium is reestablished w d r and s tend to constant values and the system recovers its stable state

hydrography of the labrador sea during active convection the hydrographic structure of the labrador sea during wintertime convection is described the cruise part of the deep convection experiment took place in february march 1997 amidst an extended period of strong forcing in an otherwise moderate winter because the water column was preconditioned by previous strong winters the limited forcing was enough to cause convection to approximately 1500 m the change in heat storage along a transbasin section relative to an occupation done the previous october gives an average heat loss that is consistent with calibrated national centers for environmental prediction surface heat fluxes over that time period 200 wm 22 deep overturning was observed both seaward of the western continental slope which was expected as well as within the western boundary current itself something that had not been directly observed previously these two geographical regions separated by roughly the 3000 m isobath produce separate water mass products the offshore water mass is the familiar cold fresh dense classical labrador sea water lsw the boundary current water mass is a somewhat warmer saltier lighter vintage of classical lsw though in the far field it would be difficult to distinguish these products the offshore product was formed within the cyclonic recirculating gyre measured by lavender et al in a region that is limited to the north most likely by an eddy flux of buoyant water from the eastern boundary current the velocity measurements taken during the cruise provide a transport estimate of the boundary current throughput and offshore recirculation finally the overall trends in stratification of the observed mixed layers are described

the interannual variability of the surface eddy kinetic energy in the labrador sea abstract the variability of the surface eddy kinetic energy eke in the labrador sea is investigated with a suite of numerical integrations using a regional ocean model simulations are performed over the period 1980 2001 and are compared to satellite observations over the last 9 years the surface eke pattern in the basin is dominated by a region along the west coast of greenland where eddies mainly anticyclonic are formed by instability of the main currents flowing over the continental slope consistent with previous idealized results here the interannual changes are linked to the shear of the incoming boundary current system imposed as boundary condition to the model domain the highly variable strength of the east greenland current at the northeast boundary derived from the simple ocean data assimilation soda reanalysis strongly influences the vortex formation in the center of the labrador sea where deep convection occurs a statistically significant portion of the modeled interannual surface eke variability is correlated with the local atmospheric forcing and both heat and wind fluxes play an important role and can be adopted as predictors at a lag of 2 3 months the arctic oscillation index can also be used as a remote indicator of the atmospheric fluxes but with lower skill than local measurements in contrast the north atlantic oscillation index does not correlate significantly with the surface eke at intraseasonal and interannual scales the analysis of altimeter data over the 1993 2001 supports the existence of this asymmetry between the regime locally forced by the atmosphere in the central basin and the regime remotely forced by the incoming boundary current along the west greenland coast those results have important implications for monitoring and predicting the surface eddy kinetic energy variability in the labrador sea

statistical analysis of lateral migration of the rio grande new mexico abstract the lateral migration rates of alluvial rivers are affected by changes in water and sediment regimes the rio grande downstream from cochiti dam exhibits spatial and temporal variability in lateral movement rates documented since 1918 a tremendous database exists that documents planform bed material size channel geometry and water and sediment regimes a statistical analysis reveals that migration rates primarily decreased with decreasing flow energy r2 0 50 p 60 the findings show that lateral movement increases with increasing flow energy and with degree of braiding

classification of geomorphological effects downstream of dams abstract the effects of dams on downstream geomorphology are reviewed and a typology is devised consisting of nine cases the classification can be seen as a further development of lane s balance between water discharge sediment load grain size and river slope depending on changes in released water flow and changes in released sediment load relative to the transport capacity of the flow it is possible to estimate resulting cross sectional geomorphology the longitudinal extent of changes and their variability with time and the tributary response to altered mainstream cross section changes are also discussed

modelling hydrographic changes in the labrador sea over the past five decades abstract inter annual to inter decadal changes of hydrographic structure and circulation in the subpolar north atlantic are studied using a coarse resolution ocean circulation model the study covers 1949 through 2001 inclusive a time mean state nudging method is applied to assimilate the observed hydrographic climatology into the model the method significantly reduces model biases in the long term mean distribution of temperature and salinity which commonly exist in coarse resolution ocean models by reducing the time mean biases we also significantly improve the model s representation of inter annual to inter decadal variations in the central labrador sea the model broadly reproduces the heat and salt variations of the labrador sea water lsw as revealed by hydrographic observations model sensitivity experiments confirm that the low frequency hydrographic changes in the central labrador sea are closely related to changes in the intensity and depth of deep convection changes in surface heat flux associated with the winter north atlantic oscillation nao index play a major role in driving the changes in t s and sea surface height ssh changes in wind stress play a secondary role in driving these changes but are important in driving the changes in the depth integrated circulation the total changes in both ssh and depth integrated circulation are almost a linear combination of the separate influences of variable buoyancy and momentum fluxes

observations of the labrador sea eddy field this paper is an observational study of small scale coherent eddies in the labrador sea a region of dense water formation thought to be of considerable importance to the north atlantic overturning circulation numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat yet their small size has hindered observational progress a large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms current meter moorings and satellite altimetry details of the current and water mass structure of individual eddy events as they are swept past by an advecting flow can then be extracted from the mooring data a transition is seen during mid 1997 with long lived boundary current eddies dominating the central labrador sea year round after this time and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time the topex poseidon altimeter covers the labrador sea with a loose net of observations through which coherent eddies can seem to appear and disappear by concentrating on locating and describing anomalous events in individual altimeter tracks a portrait of the spatial and temporal variability of the underlying eddy field can be constructed the altimeter results reveal an annual pulsation of energy and of coherent eddies originating during the late fall at a particular location in the boundary current pinpointing the time and place of the boundary current type eddy formation the interannual variability seen at the mooring is reproduced but the mooring site is found to be within a localized region of greatly enhanced eddy activity notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling these eddy observations as well as hydrographic evidence suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region

tracking labrador sea water property signals along the deep western boundary current observations of the deep western boundary current dwbc at line w on the western north atlantic continental slope southeast of cape cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the labrador sea this is most evident in the cold dense and deep class of labrador sea water dlsw that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987 1994 the arrival of this record cold fresh and low potential vorticity anomaly at line w lags its formation in the labrador sea by 3 7 years complementary observations along the path of the dwbc provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the flemish cap finally the consistency of the data with realistic advective and mixing time scales is assessed using the waugh and hall 2005 model framework the data are found to be best represented by a mean transit time of 5 years from the labrador sea to line w with a leading order role for both advection by the dwbc and mixing between the boundary flow and interior waters

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

constraints on the proposed marie byrd land bellingshausen plate boundary from seismic reflection data single channel and multichannel marine seismic data off the coast of west antarctica collected during two nathaniel b palmer cruises np92 8 and np96 2 in the vicinity of 65 s to 71 s 220 e to 250 e reveal a nnw trending graben we interpret this graben to be part of the paleodivergent plate boundary between the marie byrd land and bellingshausen plates this graben coincides with a 520 nt magnetic anomaly to the nnw and a 720 nt anomaly to the sse as well as a 20 mgal negative gravity anomaly seismic profiles subparallel to the graben 22 km ma half spreading rate reveal greater seafloor roughness to the ne where seafloor spreading was slower than to the sw 27 km ma half spreading rate these data allow the position of the marie byrd land bellingshausen plate boundary to be constrained more precisely than has previously been possible with a trend of n17 w from 68 52 s 233 65 e to 68 41 s 233 56 e the sediment filled graben has normal separation of sedimentary layers varying from 740 30 m to 580 20 m imaged in seafloor of age a33y 74 ma

the rate and pattern of bed incision and bank adjustment on the colorado river in glen canyon downstream from glen canyon dam 1956 2000 closure of glen canyon dam in 1963 transformed the colorado river by reducing the magnitude and duration of spring floods increasing the magnitude of base flows and trapping fine sediment delivered from the upper watershed these changes caused the channel downstream in glen canyon to incise armor and narrow this study synthesizes over 45 yr of channel change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations although bed lowering in lower glen canyon began when the first cofferdam was installed in 1959 most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2 6 m of sediment from the center of the channel the average grain size of bed material has increased from 0 25 mm in 1956 to over 20 mm in 1999 the magnitude of incision at riffles decreases with distance downstream from the dam while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream analysis of bed material mobility indicates that the increase in bed material grain size and reduction in reach average gradient are consistent with the transformation of an adjustable bed alluvial river to a channel with a stable bed that is rarely mobilized decreased magnitude of peak discharges in the post dam regime coupled with channel incision and the associated downward shifts of stage discharge relations have caused sandbar and terrace erosion and the transformation of previously active sand bars and gravel bars to abandoned deposits that are no longer inundated erosion has been concentrated in a few pre dam terraces that eroded rapidly for brief periods and have since stabilized the abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage discharge relations in the downstream part of the study area where riffles controlling channel elevation have not incised channel narrowing has resulted from decreased magnitude of peak discharges and minor post dam deposition these physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non native species models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit the pre dam surface and subsurface grain size and the magnitude of post dam average flows these results suggest that it is also necessary to consider 1 the possibility of variable responses among different channel elements and 2 the potential importance of exceptional flows resulting from management decisions

diachronous evolution of sea surface conditions in the labrador sea and baffin bay since the last deglaciation assessing changes in sea surface conditions due to the effects of past freshwater outflow through baffin bay and davis strait to the labrador sea hereafter referred to as the baffin bay corridor

quantifying antarctic bottom water and north atlantic deep water volumes 1 a near global census of antarctic bottom water aabw and north atlantic deep water nadw is essayed through a nonnegative least squares analysis of conservative and quasi conservative seawater properties aabw thickness generally decreases from south to north modulated by ocean bathymetry likewise nadw thickness generally decreases from north to south in the atlantic ocean nadw dominates below the thermocline of the atlantic ocean at least as far south as the subtropical gyre of the south atlantic with a lesser but still significant influence around the entire antarctic circumpolar current in the indian ocean and in the pacific ocean however in the pacific and indian oceans aabw dominates below the thermocline in addition measurable quantities of aabw reach into the abyssal north atlantic on both sides of the mid atlantic ridge the census results suggest that aabw occupies roughly twice the volume of nadw in the three main oceans and that aabw is in contact with roughly twice the area of the deep main ocean floor compared with nadw however these results are somewhat sensitive to choices of water masses their values of seawater properties and the weightings of the seawater properties used in the analysis

river channel response to climate and human induced hydrological changes case study on the meandering hernad river hungary abstract the increasing extent of human activities and climate change may have notable effects on river systems therefore a comprehensive knowledge of these effects is essential to mitigate their unfavourable consequences the aim of the present study is to analyse the hydromorphological changes caused by climate change and human impacts on the hungarian section of the hernad river and to evaluate the morphological responses of this river from a flood prevention perspective over the past 60 years considerable morphological changes were measured in the studied river sections caused by the alteration of their hydrology the main morphological change was the evolution of secondary bends on large meanders this process was driven by extrinsic factors as the morphometric changes were determined by an altered water regime caused by precipitation change and engineering works further mean discharge decreases caused the propagation of this process on smaller meanders as well meanwhile the cross sectional area of the channel also decreased primarily as a result of narrowing this altered morphology enhances the flood risk in the short run as the narrow channel is not sufficient to drain the rapid flood waves in the twenty first century climate change may cause decreasing mean discharge while also increasing the frequency of rapid floods further increasing flood risk however the projection of the geomorphological effects of future precipitation changes is very uncertain

horizontal stratification during deep convection in the labrador sea abstractdeep convection the process by which surface waters are mixed down to 1000 m or deeper forms the primary downwelling of the meridional overturning circulation in the northern hemisphere high resolution hydrographic measurements from seagliders indicate that during deep convection though water is well mixed vertically there is substantial horizontal variation in density over short distances tens of kilometers this horizontal density variability present in winter january february contains sufficient buoyancy to restratify the convecting region to observed levels 2 5 months later as estimated from argo floating platforms these results highlight the importance of small scale heterogeneities in the ocean that are typically poorly represented in climate models potentially contributing to the difficulty climate models have in representing deep convection

late cretaceous and cenozoic seafloor and oceanic basement roughness spreading rate crustal age and sediment thickness correlations abstract single channel seismic data from the south australian basin and argentine basin and bathymetry data from the flanks of the mid atlantic ridge east pacific rise and southwest indian ridge are analysed to determine the root mean square rms roughness of the seafloor and oceanic basement created at seafloor spreading rates ranging from 3 to 80 km ma half rate for these data crustal ages range from near zero to 85 ma and sediment thicknesses range from near zero to over 2 km our results are consistent with a negative correlation of basement roughness and spreading rate where roughness decreases dramatically through the slow spreading regime

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

spreading of the labrador sea water to the irminger and iceland basins 1 labrador sea water lsw is a principal convectively formed water mass of the subpolar north atlantic spna using extensive oceanographic archives we demonstrate striking changes in spna caused by massive lsw production during 1987 1994 and document recent salinification and warming imminently bringing spna to the state last time seen in the 1960s two prominent lsw classes are spreading across spna since the 1980s the first record dense deep and voluminous class has been progressively built by intense winter convection through 1987 1994 even though most of this lsw has left spna its remnants are still present there the second shallower class strengthens in 2000 over subsequent years its core becomes slightly thicker and deepens the anomalous signals acquired by these lsw classes in their formation region arrive in the irminger and iceland basins with the characteristic delays of two and five years for deeper lsw and a year and four years for shallower lsw

causes of river bed degradation river bed degradation can proceed downstream as well as upstream depending upon the cause of degradation the causes of downstream progressing degradation are primarily related to changes in independent river channel variables such as increase in water discharge decrease in size of bed material and decrease in bed material discharge the causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man made changes study of various case histories indicates that river slopes are increased by lowering a base level by decreasing the length of a river or by removal of a control point case histories also show that downstream and upstream progressing degradation can act in combination along a river system downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary

local and downstream relationships between labrador sea water volume and north atlantic meridional overturning circulation variability abstractwhile it has generally been understood that the production of labrador sea water lsw impacts the atlantic meridional overturning circulation moc this relationship has not been explored

hydrographic changes in the labrador sea 1960 2005 abstract the labrador sea has exhibited significant temperature and salinity variations over the past five decades the whole basin was extremely warm and salty between the mid 1960s and early 1970s and fresh and cold between the late 1980s and mid 1990s the full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s the freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the labrador sea sustained throughout the subsequent years it took only a decade for the whole water column to lose most of its excessive freshwater reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s if the recent tendencies persist the basin s storages of salt and heat will fairly soon likely by 2008 exceed their historic highs the main process responsible for the net cooling and freshening of the labrador sea between 1987 and 1994 was deep winter convection which during this period progressively developed to its record depths it was caused by the recurrence of severe winters during these years and in its turn produced the deepest densest and most voluminous labrador sea water lsw1987 1994 ever observed the estimated annual production of this water during the period of 1987 1994 is equivalent to the average volume flux of about 4 5 sv with some individual annual rates exceeding 7 0 sv once winter convection had lost its strength in the winter of 1994 1995 the deep lsw1987 1994 layer lost communication with the mixed layer above consequently losing its volume while gaining heat and salt from the intermediate waters outside the labrador sea while the 1000 2000 m layer was steadily becoming warmer and saltier between 1994 and 2005 the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air sea heat fluxes in the winter of 1999 2000 this change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent lsw class lsw2000 penetrating deeper than 1300 m this layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass implying that lsw2000 is likely to follow the fate of its deeper precursor lsw1987 1994 the increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid depth perimeter of the labrador sea the near bottom waters originating from the denmark strait overflow exhibit strong interannual variability featuring distinct short term basin scale events or pulses of anomalously cold and fresh water separated by warm and salty overflow modifications regardless of their sign these anomalies pass through the abyss of the labrador sea first appearing at the greenland side and then about a year later at the labrador side and in the central labrador basin the northeast atlantic deep water 2500 3200 m originating from the iceland scotland overflow water reached its historically freshest state in the 2000 2001 period and has been steadily becoming saltier since then it is argued that lsw1987 1994 significantly contributed to the freshening density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening lsw the hydrostatic adjustment to expanding lsw or both

role of resolved and parameterized eddies in the labrador sea balance of heat and buoyancy abstractdeep convection in the labrador sea is an important component of the global ocean ventilation the associated loss of heat to the atmosphere from the interior of the sea is thought to be mostly supplied by mesoscale eddies generated either remotely or as a result of convection itself processes that are not resolved by low resolution ocean climate models the authors first employ a high resolution ocean model forced with high resolution 33 km 3 h atmospheric fields to further elaborate on the role of mesoscale eddies in maintaining the balance of heat and buoyancy in the labrador sea in general agreement with previous studies it is found that eddies remove heat along the coast and supply it to the interior some of the eddies that are generated because of the barotropic instability off the west coast of greenland are recaptured by the boundary current in the region of deep convection the convergence of heat and buoyancy by eddies significantly increases with the deepening of the winter

tectonic history and new isochron chart of the south pacific we have developed an internally consistent isochron chart and a tectonic history of the south pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data highly accurate vertical deflection profiles 1 2 rad derived from 22 repeat cycles of geosat altimetry reveal subtle lineations in the gravity field associated with the south pacific fracture zones these fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas the high density of geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map the tectonic fabric was then used to derive new poles of rotation for 12 selected times in the late cretaceous and cenozoic from our reconstructions we estimated the former location of the spreading centers in order to derive a new set of isochrons interpreted unes of equal age on the ocean floor we believe that the use of new geosat altimeter data in combination with a multi plate reconstruction has led to an improvement in our understanding of south pacific tectonics

breakup and reestablishment of the armour layer in a large gravel bed river below dams the lower ebro changes in armour layer during floods under supply limited conditions are little known this paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel bed ebro river during a flooding period the study was conducted over a 28 km study reach from 2002 to 2004 the surface subsurface and bed load grain size distribution constitute the bases for the analysis of bed armouring dynamics the results indicate that the magnitude of floods controlled the degree of armouring of the river bed the initial mean armouring ratio was 2 3 with maximum values reaching 4 4 floods in the winter of 2002 2003 q8 caused the breakup of the armour layer in several sections this resulted in the erratic bed load pattern observed during the december 2002 flushing flow and in the increase in bed load transport during successive events most grain size classes were entrained and transported causing river bed incision the mean armouring ratio decreased to 1 9 in contrast during low magnitude floods in 2003 2004 q2 the coarsest fractions 64 mm did not take part in the bed load while finer particles were winnowed thus surface deposits coarsened as a result the armour layer was reestablished i e the mean armouring ratio increased to 2 3 and the supply of subsurface sediment decreased the supply and transport of bed material appear to be in balance in the river reach immediately below the dam in contrast the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream a phenomenon that progressively reduced their availability in the river bed surface hence the armour layer reworking

observations of open ocean deep convection in the labrador sea from subsurface floats abstract the occurrence and extent of deep convection in the labrador sea in winters 1996 97 and 1997 98 is investigated from measurements of over 200 neutrally buoyant subsurface profiling autonomous lagrangian circulation explorer palace and sounding oceanographic lagrangian observer solo floats in addition to providing drift velocity data and vertical profiles of temperature and salinity 55 floats are equipped with vertical current meters vcms time series of vertical velocity derived from measured pressure and vertical flow past the float and temperature are obtained from the vcm floats mixed layer depths estimated from profile measurements indicate that convection reached depths greater than 1300 m in 1997 but no deeper than 1000 m in 1998 deep mixed layers were concentrated in the western basin although a number of deep mixed layers were observed southwest of cape farewell and also north of 60 n the highest variance in vertical velocity and the lowest mean temperatures were found in t

comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel bed rivers several methods were employed in the ardennian rivers belgium to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events the use of scour chains has shown that the depth of the active layer is systematically less than the b axis of the average particle size d50 of the elements which compose the surface layer of the riffles this indicates that only a partial transport exists during low magnitude floods the bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers quantities of sediment transported during frequent floods are relatively low 0 02 t km 2 due to the armour layer which protects the subsurface material these low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with pit passive integrated transponder tags diameter 20 mm whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles the break up of the armour layer was observed only once for a decennial discharge during this event the bedload discharge increased considerably 2 t km 2 the use of sediment traps data from dredging and a helley smith sampler confirm the low bedload transport in ardennian rivers in comparison to the bedload transport in other geomorphological contexts this difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow finally the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries in the lembree river the river bed contains slag elements up to a depth of about 50 cm indicating that exceptional floods may rework the bed to a considerable depth copyright 2012 john wiley sons ltd

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated

labrador sea water property variations in the northeastern atlantic ocean 1 labrador sea water lsw property variations are analyzed using data from four hydrographic sections occupied along 20 w from iceland south toward the equator in 1988 1993 1998 and 2003 lsw between the azores biscay rise and the rockall plateau steadily becomes deeper colder fresher denser and thicker over the 15 year time series lsw apparent oxygen utilization aou remains unchanged during this time period the changes analyzed are consistent with previously published spreading rates of 8 years for lsw from the labrador sea to 20 w and emphasize the long reach of decadal lsw variability