circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

mixing and entrainment in the red sea outflow plume part i plume structure when the salty and heavy water of the red sea exits from the strait of bab el mandeb it continues downslope into the gulf of aden mainly along two channels the 130 km long northern channel nc is topographically confined and is typically only 5 km wide in it the red sea plume shows unanticipated patterns of vertical structure turbulent mixing and entrainment above the seafloor a 25 120 m thick weakly stratified layer shows little dilution along the channel hence this bottom layer undergoes only weak entrainment in contrast a 35 285 m thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing it is thus the interface that exhibits the bulk of entrainment of the red sea plume in the nc the interfacial layer also carries most of the overall plume transport increasingly so with downstream distance the southern channel sc is wider than the nc and is accessed from the latter by a sill about 33 m above the floor of the nc entrainment into the bottom layer of the sc is diagnosed to be strong near the entry into the sc such that the near bottom density and salinity are smaller in the sc than in the nc at the same distance from bab el mandeb in comparison with winter conditions the authors encountered weaker outflow with shallower equilibration depths during the summer cruise bulk froude numbers computed for the whole plume varied within the range 0 2 1 local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment

descent and modification of the overflow plume in the denmark strait bulk properties of the denmark strait overflow dso plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described despite the presence of considerable short term variability it is found that the pathway and evolution of the plume density anomaly are remarkably steady bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction the estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays the entrainment rate increase is likely related to the increased topographic slopes in the region compounded by a decrease in interface stratification as the plume is diluted and enters a denser background

katabatic wind forcing of the terra nova bay polynya the terra nova bay polynya is a perennial winter feature in the western ross sea antarctica which occupies roughly 1000 km2 it is formed and maintained by the combined influence of persistent katabatic winds which advect newly formed bay ice eastward and the drygalski ice tongue which prevents northward drifting pack ice from entering terra nova bay existence of anomalously strong katabatic drainage along this coast is predicted by parish s 1982 simulation of wintertime airflow which reveals a pronounced confluence of surface winds upslope from the reeves glacier where the winds are further focused by local topography the simulation is strongly supported by regional sastrugi orientations average wintertime atmospheric conditions and ice sheet topography which control surface air drainage are stable on a climatic time scale therefore persistent wintertime katabatic winds should be an annual phenomenon further evidence comes from multi year landsat images which consistently show windswept snowfree areas on the reeves glacier in marked contrast to typical antarctic katabatic winds strong persistent winter winds are observed at sea level 25 km beyond the coastal slope break air probably descends as bora type winds and is likely to be significantly denser than the air at sea level conditions are not favorable for hydraulic jumps apparently typical of other katabatic regimes the horizontal density difference is maintained during airflow across the nansen ice sheet because relatively little air mass modification occurs there in contrast to situations where air moves over an ice laden ocean observations thus suggest that katabatic winds maintain their identity for some distance seaward of the coast qualitative trajectory calculations indicate that for representative geostrophic conditions this distance is on the order of the observed polynya width estimated ice freezing rates are 20 cm day 1 but wind generated waves and currents prevent ice from consolidating the blocking effect of the drygalski ice tongue is a consequence of its orientation with regard to western ross sea ice drift patterns ice tongue length controls polynya width absence of such blocking along other coasts experiencing strong katabatic outflow partly explains why similar polynyas do not form there

freshening of the ross sea during the late 20th century ocean measurements in the ross sea over the past four decades one of the longest records near antarctica reveal marked decreases in shelf water salinity and the surface salinity within the ross gyre these changes have been accompanied by atmospheric warming on ross island ocean warming at depths of 300 meters north of the continental shelf a more negative southern oscillation index and thinning of southeast pacific ice shelves the freshening appears to have resulted from a combination of factors including increased precipitation reduced sea ice production and increased melting of the west antarctic ice sheet

cold outflow from the faroe bank channel abstract the exchange of water between the norwegian sea and the north atlantic around the faroe islands was investigated in 1987 and 1988 combining ctd data with shipborne acoustic doppler current profiler adcp measurements tests were devised to assess the accuracy of the adcp measurements and the results are described it is found that estimating a reference velocity for geostrophic measurements is limited by the presence of tides of order 10 cm s 1 in the outflow of cold water from the faroe bank channel which reaches speeds of nearly 1 m s 1 the error is acceptably small but strong time dependent components are revealed the discharge of cold water is traced 75 km beyond the sill of this channel and changes in potential temperature salinity and potential vorticity on an isopycnal surface are attributed to intense mixing an effective diffusivity near 100 cm2 s 1 is derived and k h instability demonstrated as a plausible source of the turbulent energy six current meter moorings were deployed from

impact of iceberg c 19 on ross sea primary production 1 for the second time in 26 months a large iceberg c 19 calved off the face of the ross ice shelf rather than breaking up as did its predecessor b 15 the iceberg c 19 remained largely intact eventually stopping temporarily near pennell bank before moving out of the southwestern ross sea the presence of c 19 restricted advection of sea ice out of the region resulting in unusually high sea ice cover during the spring and summer consequently phytoplankton blooms in the region were diminished dramatically and primary production was reduced by over 90 relative to normal years calving events over the last two decades indicate that reduced primary productivity may be a typical consequence of large icebergs that drift through the southwestern ross sea during spring and summer

cooling and ventilating the abyssal ocean the abyssal ocean is filled with cold dense waters that sink along the antarctic continental slope and overflow sills that lie south of the nordic seas recent inte grations of chlorofluorocarbon 11 cfc measurements are similar in antarctic bottom water aabw and in lower north atlantic deep water nadw but antarctic inputs are 2 c colder than their northern counterparts this indicates comparable ventilation rates from both polar re gions and accounts for the southern ocean dominance over abyssal cooling the decadal cfc based estimates of recent ventilation are consistent with other hydrographic observa tions and with longer term radiocarbon data but not with hypotheses of a 20 th centuryslowdown in the rate of aabw formation significant variabilityis not precluded bythe available ocean measurements however and interannual to decadal changes are increasinglyevident at high latitudes

high resolution velocity and hydrographic observations of the drygalski trough gravity plume during the second cruise of the anslope project the hydrography and velocity fields within a descending gravity plume in the northwestern ross sea shelf area were sampled at high resolution the observations give insights into small scale fluctuations of the three dimensional velocity field in and around the core of a strong gravity current associated with peak velocities close to 2 m s there are two distinct layers of high acoustic backscatter at 300 khz near the seabed associated with increased velocity variance and richardson numbers near 0 25 suggesting that the high backscatter is caused by increased levels of turbulence resulting in increased acoustic bragg scattering c 2008 elsevier ltd all rights reserved

bottom water export from the western ross sea 2007 through 2010 bottom water export from the ross sea february 2007 to january 2011 exhibits seasonal and interannual variability temperature minima coupled to salinity maxima in late austral summer into the fall indicate input from high salinity shelf water hssw secondary temperature minima lacking the high salinity trait characteristic of low salinity shelf water lssw appear in the spring warmer bottom water similar to modified circumpolar deep water mcdw is observed in winter and in early summer the lssw and mcdw may be drawn from the drygalski basin as the hssw pool retreats poleward from the shelf break in response to increased winter polar easterlies allowing these less dense overlying waters to spill into the deep ocean within the benthic layer bottom salinity decreased from 2007 to 2011 by 0 007 year 1 significantly higher than regional decadal trends which we propose is a result of hssw retreat induced by strengthening polar easterlies

a recount of ross sea waters abstract oceanographic observations within the ross sea have grown dramatically in recent years both in number and quality this has prompted a parallel recount of the circulation and structure of all water masses in the southwestern continental margins of the pacific ocean a high resolution set of horizontal property distributions was combined into a new climatology which in turn is the basis of a fine volumetric s census of all ross sea water masses inshore of the shelf break 700 m isobath the ross sea volume 25 10 4 km 3 partitioning into layers of neutral density n is 25 in the top layer n 3 of antarctic surface water aasw 22 in the middle layer 28 00 kg m 3 n 3 of oceanic thermocline water 12 and modified circumpolar deep water mcdw 10 and 53 in the bottom layer n 28 27 kg m 3 of shelf water sw 1 85 c 22 precursor of the antarctic bottom water aabw offshore aasw flows westward along the outer shelf and southward along the eastern coast past cape colbeck oceanic thermocline waters 28 00 kg m 3 n 3 cross the shelf break west of 170 w and follow southward paths along banks shoaling progressively to near the sea surface of the central and western inner shelves winter buoyancy loss converts aasw and oceanic thermocline water into denser types of mcdw and sw which move cyclonically toward the sills of major troughs the census shows a continuous mode of relatively dense mcdw 28 10 kg m 3 n 3 directly linked to the circumpolar deep water cdw offshore effectively ventilating and freshening the deep ocean at levels below the salinity maximum and above aabw mcdw outflows from the ross sea shelf are traced to the antarctic circumpolar current near 155 e into the australian antarctic basin along the northern flank of the southwest pacific ridge and to near 135 w past the eastern end of the ross gyre two sources of salty sw are inferred near the coastal polynyas in the terra nova bay and western ross ice shelf areas the latter contributes to a major outflow of ice sw that reaches the shelf break along 180 vertical mixing of mcdw and sw produce msw over the shelf continuing over the sills as new aabw export of low salinity aabw s

on the total input of antarctic waters to the deep ocean a preliminary estimate from chlorofluorocarbon measurements 1 deep ocean inventories of dissolved chlorofluorocarbon 11 cfc 11 along representative sections off antarctica provide the first estimate of the overall strength of all dense water sources in the southern ocean their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the thermohaline circulation thc the bottom layer is supplied via sinking of antarctic bottom water aabw produced at a few continental shelves the middle layer receives the offshore injection of ventilated modified circumpolar deep water mcdw produced along much of the lengthy antarctic slope front the top layer is ventilated by northward export of antarctic surface water into the upper circumpolar deep water of the antarctic circumpolar current average southern ocean inputs to the upper two layers of the deep ocean for the 1970 1990 period are derived on the basis of the cfc 11 distributions along meridional sections the mean cfc 11 saturations of all water mass ingredients and the inferred mixing leading to production of dense source waters about 5 4 1 7 sv 1 sv 10 6 m 3 s 1 of near freezing shelf water ventilate the bottom layer and 4 7 1 7 sv and 3 6 1 3 sv of cold antarctic surface water ventilate the middle and top layers therefore the total contribution of ventilated southern ocean waters to the lower limb of the global thc is about 14 sv this is close to the about 17 sv estimated for north atlantic near surface sources from cfc 11 inventories their entrainment of 7 4 2 4 sv of cfc poor subsurface lower circumpolar deep water during the formation and sinking of aabw and mcdw raises the total southern ocean input to the deep ocean to about 21 sv

how iceberg calving and grounding change the circulation and hydrography in the filchner ice shelf ocean system the formation of bottom water in the southern weddell sea is strongly influenced by the flow of ice shelf water isw out of the filchner ronne ice shelf cavity the breakout of three giant icebergs in 1986 and their grounding on the shallow berkner bank modified the circulation and water mass formation in the filchner trough and the adjacent sea areas hydrographic measurements along the filchner ice shelf front carried out with rv polarstern in 1995 show significant changes in the water mass characteristics and flow patterns in the filchner trough in comparison to measurements from the early 1980s changes in the trough will affect the flow over the sill to the deep weddell abyssal plain we combine a three dimensional ocean circulation model with conductivity temperature depth and stable isotope measurements to investigate the details of the circulation in front of and beneath the filchner ice shelf we assess the impact of stranded icebergs and a more southerly ice shelf front position caused by a 1986 iceberg calving event on the circulation and observed water mass properties results indicate variations of the flow pattern in the filchner trough and on berkner bank where high salinity shelf water the feedstock for isw is produced the calving and grounding impacts illustrate the sensitivity of the ice shelf ocean system to perturbations in local bathymetric settings

satellite observed behavior of the terra nova bay polynya must frequently cross victoria land farther south cyclones sometimes behave in this fashion when cyclogenesis takes place in the victoria land trough u s navy weather service 1970 however this does not explain why winter open water persists only in terra nova bay the present comparison of this polynya s behavior with synoptic scale conditions through one winter constitutes a needed test of knapp s model for the geographic setting of the western ross sea wintertime open water has been known to exist in terra nova bay since 1912 when six men of scott s northern party were stranded on inexpressible island priestley 1914 bromwich and kurtz 1982 in the mid 1960 s knapp 1972 noted this polynya in a general survey of antarctic coastal polynyas using experimental nimbus 1 and 2 infrared photographs and gave a sequence of images illustrating its temporal variability his figure 6 zekielda 1974 probably examined it more recently using early operational infrared satellite imagery his geographic coordinates are incorrect but the profile of the drygalski ice tongue is apparent in his figure 3 szekielda noted that the polynya persisted throughout the winter and attributed its presence to marine upwelling or submarine volcanism the nature and frequency of polynya fluctuations and their relationships to synoptic scale weather were not discussed information from sunlit periods also indicates unusual conditions in terra nova bay sea ice charts from 1973 to present fleet weather facility 1975 1977 1979 naval polar oceanography center 1981 1983 consistently show open water or reduced ice concentrations from october to early march the anomaly persists in spring surrounded by concentrated pack ice in the ross sea during the summer when ice tends to remain along the victoria land coast and during fall when pack ice is rapidly forming in the open sea that the antarctic sea ice edge retreats to its most southerly point in the central and western ross sea streten and pike 1980 suggests an atypical character for this whole region prior to 1980 sea ice charts label terra nova bay as being filled with concentrated pack from mid march to early october these charts were

entrainment in bottom gravity currents over complex topography from three dimensional nonhydrostatic simulations 1 by recognizing that oceanic overflows follow the seafloor morphology which shows a self similar structure at spatial scales ranging from 100 km to 1 m the impact of topographic bumps on entrainment in gravity currents is investigated using a 3d nonhydrostatic spectral element model it is found that a bumpy surface can lead to a significant enhancement of entrainment compared to a smooth surface the change in entrainment is parameterized as a function of statistical estimates of the amplitude and wavenumber parameters of bumps with respect to the background slope

dynamics of dense water descending a continental slope abstract a reduced gravity model is used to examine the dynamics of dense water descending a continental slope the model solves for the geostrophically adjusted state before it is subjected to significant frictional decay for such bottom mounted flow it is argued that frictional torque would dominate the net vorticity balance to equalize the edge flows resulting in double velocity cores constrained by the geostrophic balance the dense water thus may settle only over a concave bottom and is sheetlike covering typically the whole slope rise as such the adjustment is characterized by a spreading rather than sinking of the layer with little descent of the upper edge but a swift downslope current propelling the lower edge through the mechanical energy balance it is found in addition that a greater density anomaly would increase the total entrainment flux to more strongly dilute the original anomaly yielding a product water that is less varied in the water mass properties model predictions compare

cascades of dense water around the world ocean dense water overflow off continental shelves cascading is one of the contributing processes of shelf deep ocean exchange and of topical interest to climate studies and nutrient fluxes dense water originating from cooling evaporation freezing and salinization on a shallow shelf spills over the shelf edge and may develop as near bottom gravity current or an intermediate depth intrusion it is difficult to observe in nature due to its intermittent character this paper provides an extensive inventory of observed cases of water cascades around the world ocean summarises their locations and individual properties and provides statistics of the identified cases the search for cascading was carried out using oceanographic databases and a literature review this study identified 61 confirmed cases world wide including 25 cases in the arctic seas 12 at mid latitudes seven in sub tropical and tropical regions and 17 off the antarctic shelves eighteen cascades had not been reported before we analyze a set of numerical parameters of dense water cascades allowing us to quantify compare and contrast the properties of water cascades the overall average density contrast between the confirmed cascades and ambient water is 0 37 kg m3 it can be as much as 2 kg m3 on some arctic shelves frequently initiated by strong cooling at the surface cascades often remain colder through the descent thus supplying the deep ocean with colder and fresher water in non dimensional variables the data from all climate zones fit well to a unique curve which represents a relationship between a cascade s internal structure and the parameters describing its forcing on average the down slope volumetric flux provided by dense water cascades is estimated as 0 05 to 0 08 sv per 100 km of shelf edge regional terms arctic antarctic north atlantic ocean barents sea mediterranean sea skagerrak tasman sea sea of okhotsk

discovery of a widespread low latitude diurnal co2 frost cycle on mars while the detection of co2 ice has only been reported outside the martian polar regions at very high elevation i e elysium olympus mons and the tharsis montes nighttime surface observations by the mars climate sounder on board the mars reconnaissance orbiter document the widespread occurrence of atmospherically corrected ground temperatures consistent with the presence of extensive carbon dioxide frost deposits in the dusty low thermal inertia units at middle low latitudes thermal infrared emissivities interpreted in conjunction with mass balance modeling suggest micrometer size co2 ice crystals forming optically thin layers never exceeding a few hundreds of microns in thickness i e 10 2 kg m 2 locally which is insufficient to generate a measurable diurnal pressure cycle 0 1 of the martian atmosphere atmospheric temperatures at middle low latitudes are not consistent with precipitation of co2 ice suggesting that condensation occurs on the surface the recurring growth and sublimation of co2 ice on martian dusty terrains may be an important process preventing soil induration and promoting dynamic phenomena soil avalanching and fluidization and regolith gardening maintaining a reservoir of micrometer size dust particles that are mobile and available for lifting the discovery of this diurnal co2 cycle represents an important step forward in our understanding of the way the martian atmosphere interacts with the surface

tides of the northwestern ross sea and their impact on dense outflows of antarctic bottom water abstract moored and depth profile measurements of hydrography and currents during the 2003 2005 antarctic slope anslope program on the shelf slope region near drygalski trough sill in the northwest ross sea have revealed an energetic benthic outflow of cold dense antarctic bottom water aabw produced near the shelf break this outflow couples dense water production processes on the continental shelf to ventilation of the global deep ocean we use time series of bottom pressure and currents on the anslope moorings and a high resolution three dimensional primitive equation numerical model to explore the hypothesis that the tidal currents near the shelf break have a significant impact on outflow volume transport and hydrographic properties tidal currents exceed 1 m s 1 at spring tide over the outer shelf and upper slope tidal advection at spring tide carries water parcels 20 km across slope during a half tidal cycle sufficient to advect water from northern drygalski trough across the sill to the shelf break or similarly from the shelf break 500 m water depth down to the 1600 m isobath a tidally rectified lagrangian mean flow of order 0 05 0 1 m s 1 over the outer shelf and upper slope accompanies the spring tide thickness and water mass properties of the benthic layer over the sill and upper slope exhibit pronounced tidal cycles due to advection of hydrographic gradients and to energetic mixing that can extend more than halfway up from the seabed to the ocean surface maximum parameterized diapycnal diffusivity in the benthic layer at spring tides approaches 1 m 2 s 1 the addition of tides has a profound effect on time averaged outflows of aabw primarily through an increase in benthic layer thickness during spring tide downslope volume fluxes of aabw over the slope averaged over a tidal cycle are 2 5 times larger at spring tide than at neap indicating a need to include the effect of tides in large scale ocean models attempting to represent shelf water contributions to the global ocean circulation

unprecedented pressure increase in deep magma reservoir triggered by lava dome collapse 1 the collapse of the soufriere hills volcano lava dome on montserrat in july 2003 is the largest such event worldwide in the historical record here we report on borehole dilatometer data recording a remarkable and unprecedented rapid 600s pressurisation of a magma chamber triggered by this surface collapse the chamber expansion is indicated by an expansive offset at the near dilatometer sites coupled with contraction at the far site by analyzing the strain data and using added constraints from experimental petrology and long term edifice deformation from gps geodesy we prefer a source centered at approximately 6 km depth below the crater for an oblate spheroid with overpressure increase of order 1 mpa and average radius 1 km pressurisation is attributed to growth of 1 3 of gas bubbles in supersaturated magma triggered by the dynamics of surface unloading recent simulations demonstrate that pressure recovery from bubble growth can exceed initial pressure drop by nearly an order of magnitude

mediterranean outflow mixing and dynamics the mediterranean sea produces a salty dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern gulf of cadiz the current accelerates to 1 3 meters per second which raises the internal froude number above 1 and is intensely turbulent through its full thickness the outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline north atlantic central water within 100 kilometers downstream the current is turned by the coriolis force until it flows nearly parallel to topography in a damped geostrophic balance the mixed mediterranean outflow continues westward slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass

overflow dynamics and bottom water formation in the western ross sea influence of tides 1 the tidal impact on overflow dynamics and bottom water production in the western ross sea is studied with the finite element ocean model which allows embedding a mesh with 0 5 km resolution in a coarse resolution 30 km setup without nesting the simulated overflow properties inside and downstream of the western ross sea are described the overflow exhibits pronounced variability at both daily and spring neap tidal time scales in the western ross sea tides increase mixing over both the outer shelf and upper slope there plume jets are shaped by tidal currents at a bathymetric bend west of the drygalski trough mouth descending rapidly and supplying the bottom water a fraction of shelf water remains over the shelf and propagates westward from the ross sea but it does not contribute significantly to bottom water formation because of energetic mixing over the upper slope compared to a simulation without tidal forcing tides with the major k1 and o1 constituents increase the outflow rate over the continental slope off cape adare by about 70 a set of sensitivity experiments show that the rate of bottom water production is not a monotonic function of the tidal currents amplitude tidal forcing with intermediate strength leads to the most efficient bottom water formation

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

antarctic bottom water production and export by tides in the ross sea 1 current measurements in the western ross sea show that antarctic bottom water is produced at the outer edge of the continental shelf by tidal stirring of antarctic surface water circumpolar deep water and shelf water current amplitudes during spring tides are greater than 1 m s sufficient to carry circumpolar deep water onto the shelf from offshore and stir at least the lower portion of the water column to a nearly homogeneous mixture the antarctic bottom water produced every day during spring tides does not appear to have a seasonal signal its export rate is estimated to be 1 95 1 85 10 6 m 3 s

energetic plumes over the western ross sea continental slope rapid descent of dense drygalski trough western ross sea antarctica shelf water over the continental slope within 100 to 250 m thick benthic plumes is described speeds of up to 1 0 m s are recorded flowing at an average angle of 35 to the isobaths entraining ambient lower circumpolar deep water en route this process is predominant in determining the concentration and placement of the shelf water injected into the deep sea as a precursor antarctic bottom water nonetheless a 4 hour duration pulse of undiluted shelf water was observed at depth 1407 m directly north of the drygalski trough moving at around 90 degrees to isobaths and at a speed of 1 4 m s thus the export of ross sea shelf water to the deep sea is accomplished within plumes descending at moderate angle to isobaths punctuated by rapid downhill cascades

notes on southern ocean hydrography sea ice and bottom water formation abstract the southern ocean circulation and sea ice distribution is briefly described the formation of extremely cold bottom water in the weddell sea and its relation to the floating ronne filchner ice shelves is discussed it is shown that a concentrated swift eroding bottom current with anomalous low 18 o ratio transports the cold and dense ice shelf water from the shelf towards large depths comments are made on possible implications of this process for the large scale deep water circulation and for the interpretation of sediment cores

western ross sea continental slope gravity currents antarctic bottom water of the world ocean is derived from dense shelf water that is carried downslope by gravity currents at specific sites along the antarctic margins data gathered by the anslope and clima programs reveal the presence of energetic gravity currents that are formed over the western continental slope of the ross sea when high salinity shelf water exits the shelf through drygalski trough joides trough immediately to the east offers an additional escape route for less saline shelf water while the glomar challenger trough still farther east is a major pathway for export of the once supercooled low salinity ice shelf water that forms under the ross ice shelf the drygalski trough gravity currents increase in thickness from 100 to 400 m on proceeding downslope from 600 m the shelf break to 1200 m upper slope sea floor depth while turning sharply to the west in response to the coriolis force during their descent the mean current pathway trends 35 downslope from isobaths benthic layer current and thickness are correlated with the bottom water salinity which exerts the primary control over the benthic layer density a 1 year time series of bottom water current and hydrographic properties obtained on the slope near the 1000 m isobath indicates episodic pulses of shelf water export through drygalski trough these cold 34 75 pulses correlate with strong downslope bottom flow extreme examples occurred during austral summer fall 2003 comprising concentrated high salinity shelf water 1 9 c 34 79 and approaching 1 5 m s 1 at descent angles as large as 60 relative to the isobaths such events were most common during november may consistent with a northward shift in position of the dense shelf water during austral summer the coldest saltiest bottom water was measured from mid april to mid may 2003 the summer fall export of high salinity shelf water observed in 2004 was less than that seen in 2003 this difference if real may reflect the influence of the large iceberg c 19 over drygalski trough until its departure in mid may 2003 when there was a marked decrease in the coldest saltiest gravity current adjacent to drygalski trough northward transport of cold saline recently ventilated antarctic bottom water observed in march 2004 off cape adare was 1 7 sv including 0 4 sv of high salinity shelf water

ice shelf water overflow and bottom water formation in the southern weddell sea 1 cold shelf waters flowing out of the filchner depression in the southern weddell sea make a significant contribution to the production of weddell sea bottom water wsbw a precursor to antarctic bottom water aabw we use all available current meter records from the region to calculate the flux of cold water 1 9 c over the sill at the northern end of the filchner depression 1 6 0 5 sv and to determine its fate the estimated fluxes and mixing rates imply a rate of wsbw formation referenced to 0 8 c of 4 3 1 4 sv we identify three pathways for the cold shelf waters to enter the deep weddell sea circulation one path involves flow constrained to follow the shelf break the other two paths are down the continental slope resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36 w and 37 w mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m probably aided by thermobaricity the plume water at this depth can flow at a speed approaching 1 m s 1 implying that the flow is occasionally supercritical we postulate that such supercriticality acts to limit mixing between the plume and its environment the transition from supercritical to slower more uniform flow is associated with very efficient mixing probably as a result of hydraulic jumps

baroclinic instability of density current along a sloping bottom and the associated transport process numerical experiments with a three dimensional nonhydrostatic ocean model have been carried out to investigate the dynamic processes of downslope density current and associated transport of dense water focusing on the effects of bottom slope for the range of slope inclination s from 0 to 0 04 bottom slope has two effects on baroclinic instability of the flow one is the stabilizing effect due to the topographic effect and to offshoreward increase in water depth the other is the destabilizing effect due to steepened inclination of isopycnal surface which increases available potential energy since the destabilizing effect over the upper slope overcomes the stabilizing one in the early stage unstable waves develop more rapidly with increasing s nonlinear energy conversion due to reynolds stresses also affects the flow stability and then the growth rate of instability increases linearly with s in this stage after the unstable waves have finite amplitudes in the mature stage the stabilizing effect of bottom slope becomes dominant especially over the lower slope thus vigorous unstable eddies are confined to the upper slope region in steep slope cases while they cover the whole slope region in gentle slope cases and this difference in the eddy field causes a drastic change in downslope transport of dense water in gentle slope cases efficiency of the transport becomes higher with increasing s since the destabilizing effect overcomes the stabilizing one as a whole on the contrary the efficiency becomes lower with increasing s in steep slope cases since the stabilizing effect surpasses the destabilizing one as a result the most effective transport is realized in the case with s 0 005 although the transport due to eddies in steep slope cases becomes weak over the lower slope it is still larger than that due to the bottom ekman current since the eddy transport occurs in the bottom layer with a thickness of 100 200 m which is 4 8 times the ekman layer thickness