coherence of western boundary pressure at the rapid wave array boundary wave adjustments or deep western boundary current advection this study investigates the coherence between ocean bottom pressure signals at the rapid climate change programme rapid west atlantic variability experiment wave array on the western north atlantic continentalslope includingthewoodsholeoceanographicinstitutionlinew highlycoherentpressuresignals propagate southwestward along the slope at speeds in excess of 128 m s 21 consistent with expectations of barotropic kelvin like waves coherent signals are also evidenced in the smaller pressure differences relative to 1000 m depth which are expected to be associated with depth dependent basinwide meridional transport variations or an overturning circulation these signals are coherent and almost in phase for all time scales from 3 6 years down to 3 months coherence is still seen at shorter time scales for which group delay estimates are consistent with a propagationspeedofabout1 m s 21 over990 kmofcontinentalslopebutwithlargeerrorboundsonthespeed this is roughly consistent with expectations for propagation of coastally trapped waves though somewhat slower than expected a comparison with both eulerian currents and lagrangianfloat measurements shows that the coherence is inconsistent with a propagation of signals by advection except possibly on time scales longer than 6 months

modeling regional salinization of the ogallala aquifer southern high plains tx usa two extensive plumes combined area 1000 km 2 have been delineated within the ogallala aquifer in the southern high plains tx usa salinity varies within the plumes spatially and increases with depth cl ranges from 50 to 500 mg l 21 variable density flow modeling using sutra has identified three broad regions of upward cross formational flow from the underlying evaporite units the upward discharge within the modeled plume area is in the range of 10 24 10 25 m 3 day 21 and the tds concentrations are typically 3000 mg l 21 regions of increased salinity identified within the whitehorse group evaporite unit underlying the ogallala aquifer are controlled by the structure and thickness variations relative to the recharge areas distinct flow paths on the order of tens of km to 100 km in length and varying flow velocities indicate that the salinization of the ogallala aquifer has been a slow ongoing process and may represent circulation of waters recharged during pleistocene or earlier times on going pumping has had negligible impact on the salinity distribution in the ogallala aquifer although simulations indicate that the velocity distribution in the underlying units may have been affected to depths of 150 m after 30 years of pumping because the distribution of saline ground water in this region of the ogallala aquifer is heterogeneous careful areal and vertical characterization is warranted prior to any well field development q 2000 elsevier science b v all rights reserved

evidence for large scale eddy driven gyres in the north atlantic analysis of a recent climatological database for the north atlantic has detailed an extensive large scale recirculation system in the intermediate waters of the north atlantic basin the pressure fields that define this recirculation coupled with the potential vorticity fields associated with the recirculating flow provide observational evidence for basin scale eddy driven flow in the global ocean the recirculations are intimately tied to the waters carried southward by the deep western boundary current and are therefore likely to affect the distribution of climatic anomalies in the north atlantic

the key role of the western boundary in linking the amoc strength to the north south pressure gradient abstracta key idea in the study of the atlantic meridional overturning circulation amoc is that its strength is proportional to the meridional density gradient or more precisely to the strength of the meridional pressure gradient a physical basis that would indicate how to estimate the relevant meridional pressure gradient locally from the density distribution in numerical ocean models to test such an idea has been lacking however recently studies of ocean energetics have suggested that the amoc is driven by the release of available potential energy ape into kinetic energy ke and that such a conversion takes place primarily in the deep western boundary currents in this paper the authors develop an analytical description linking the western boundary current circulation below the interface separating the north atlantic deep water nadw and antarctic intermediate water aaiw to the shape of this interface the simple analytical model also shows how available potential energy is converted into

multidecadal variability of the north brazil current and its connection to the atlantic meridional overturning circulation 1 the north brazil current nbc connects the north and south atlantic and is the major pathway for the surface return flow of the atlantic meridional overturning circulation amoc here we calculate the nbc geostrophic transport time series based on 5 decades of observations near the western boundary off the coast of brazil results reveal a multidecadal nbc variability that lags labrador sea deep convection by a few years the nbc transport time series is coherent with the atlantic multidecadal oscillation in sea surface temperature which also has been widely linked to amoc fluctuations in previous modeling studies our results thus suggest that the observed multidecadal nbc transport variability is a useful indicator for amoc variations the suggested connection between the nbc and amoc is assessed in a 700 year control simulation of the geophysical fluid dynamics laboratory s cm2 1 coupled climate model the model results are in agreement with observations and further demonstrate that the variability of nbc transport is a good index for tracking amoc variations concerning the debate about whether a slowdown of amoc has already occurred under global warming the observed nbc transport time series suggests strong multidecadal variability but no significant trend

determining the strength of the deep western boundary current using the chlorofluoromethane ratio abstract the dilution of a passive tracer during deep water formation and the subsequent advection mixing in a deep boundary current is modeled with application to chlorofluoromethanes cfms in the north atlantic two different types of boundary currents are considered a uniform flow and a simple shear flow in each case the core of the flow mixes with surrounding water which continually accumulates cfms in an extreme case the coupled systems predicts that the cfm ratio in the current is unaltered from the ratio of its source water save for a time lag more realistic cases however suggests that the ratio is not a conserved quantity but is substantially altered in both the overflow basin and boundary current matching the model results to cfm data collected near the grand banks gives a predicted average core speed of 5 10 cm s 1 for the deep western boundary current and provides a constraint on the transport and diffusivity of the flow as well

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

tracing the thermohaline circulation in the western north atlantic using chlorofluorocarbons abstract measurement of chlorofluorocarbons cfcs f 11 and f 12 were obtained from vertical sections extending across the deep western boundary current dwbc along the western margin of the north atlantic ocean between 32 45 n in 1983 and 1986 these sections revealed an upper and a deep cfc maximun with highest concentrations along the continental slope the upper maximum had a potential temperature ranging from 4 2 4 6 c a core density of 1 5 34 56 and occurred at a depth of about 800m adjacent to the continental slope water with classical labrador sea water s characteristics occurs just above the cfc minimum between the upper and lower maxima and indicates that a significant volume of classical labrador sea water has not entered the region south of the grand banks for the last 3 decades the deep cfc maximum is from dense water overflowing the greenland iceland scotland ridge it occurred at about 3500m depth and had a potential temperature range of 2 02 2 18 c and a core density of 4 45 87 a source function of f 11 and f 12 for the deep maximum was estimated using temperature salinity and tracer data from previous studies the core of the dwbc defined by the deep cfc maximum was estimated to consist of roughly 2 3 gibbs fracture water and 1 3 denmark strait overflow water however 80 or more of the f 11 and f 12 signal was derived from denmark strait overflow water ages for the upper and lower cfc maxima in the dwbc were estimated from the cfc data these ages ranged from 11 12 years at the northernmost section to 18 years at the southernmost section for both features this corresponds to apparent mean current speeds of about 0 8cm s 1 for the upper maximum and 1 4cm s 1 for the deep maximum an age of 8 9 years with a corresponding apparent mean current speed of 2 2 2 5cm s 1 was also estimated for the deep cfc maximum from the observed increase in the f 11 concentration at 41 n between 1983 and 1986 the apparent mean current speeds are significantly less than current speeds measured with long term current meter moorings two reasons for this are 1 mixing between the dwbc core and adjacent water causes ages estimated from cfc data to be overestimated and hence the associated current speed to be underestimated pickart hogg and smethie 1989 and 2 water propagating along the western boundary spends some time in recirculation gyres the northern recirculation gyre south of the eastern north american continental margin is clearly apparent in the lateral distributions of f 11 and f 12 the apparent mean current speed in the northern recirculation gyre was estimated to be 1 3cm s 1 from the cfc data seaward of the dwbc north of the new england seamount chain high cfc low silica parcels of water were observed interspersed with low cfc high silica parcels of water along the bottom the high cfc low silica water is overflow water with a slightly greater component of denmark strait overflow water than contained in the dwbc deep cfc maximum the low cfc high silica water is of southern origin and has a component of antarctic bottom water

north atlantic climate response to lake agassiz drainage at coarse and ocean eddy permitting resolutions abstractthe north atlantic climate response to the catastrophic drainage of proglacial lake agassiz into the labrador sea is analyzed with coarse and ocean eddy permitting versions of a global coupled climate model the north atlantic climate response is qualitatively consistent in that a large scale cooling is simulated regardless of the model resolution or region of freshwater discharge however the magnitude and duration of the north atlantic climate response is found to be sensitive to model resolution and the location of freshwater forcing in particular the long term entrainment of freshwater along the boundary at higher resolution and its gradual partially eddy driven escape into the interior leads to low salinity anomalies persisting in the subpolar atlantic for decades longer as a result the maximum decline of the atlantic meridional overturning circulation amoc and the ocean meridional heat transport mht is amplified by about a factor of 2 at ocean eddy permitting resolution and the re

dissolution of salt on the east flank of the permian basin in the southwestern u s a abstract hydrogeologic studies prove that natural dissolution of bedded salt occurs at shallow depths in many parts of the permian basin of the southwestern u s a this is especially well documented on the east side of the basin in study areas on the cimarron river and elm fork in western oklahoma and on the red river in the southeastern part of the texas panhandle four requirements for salt dissolution are 1 a deposit of salt 2 a supply of water unsaturated with respect to nacl 3 an outlet for removal of brine and 4 energy to cause water to flow through the system the supply of fresh groundwater in the region is recharged through permeable rocks alluvium terrace deposits karstic features and fractures groundwater dissolves salt at depths of 10 250 m and the resulting brine moves laterally and upward under hydrostatic pressure through caverns fractures in disrupted rock and clastic or carbonate aquifers until it reaches the land surface where it forms salt plains and salt springs in many areas salt dissolution produces a self perpetuating cycle dissolution causes cavern development followed by collapse and subsidence of overlying rock then the resulting disrupted rock has a greater vertical permeability that allows increased water percolation and additional salt dissolution

what is the thermohaline circulation the term thermohaline circulation is widely used in the context of past and future climate change in his perspective wunsch argues that there are many different and inconsistent definitions for this term he defines ocean circulation in terms of mass fluxes which are largely driven by surface forces such as wind and tides and argues that thermohaline circulation should only refer to the separate transport of heat and salinity in the ocean

separating the influence of projected changes in air temperature and wind on patterns of sea level change and ocean heat content we present ocean model sensitivity experiments aimed at separating the influence of the projected changes in the thermal near surface air temperature and wind near surface winds forcing on the patterns of sea level and ocean heat content in the north atlantic the distribution of sea level change is more due to the thermal forcing whereas it is more due to the wind forcing in the north pacific in the southern ocean the thermal and wind forcing have a comparable influence in the ocean adjacent to antarctica the thermal forcing leads to an inflow of warmer waters on the continental shelves which is somewhat attenuated by the wind forcing the structure of the vertically integrated heat uptake is set by different processes at low and high latitudes at low latitudes it is dominated by the heat transport convergence whereas at high latitudes it represents a small residual of changes in the surface flux and advection of heat the structure of the horizontally integrated heat content tendency is set by the increase of downward heat flux by the mean circulation and comparable decrease of upward heat flux by the subgrid scale processes the upward eddy heat flux decreases and increases by almost the same magnitude in response to respectively the thermal and wind forcing regionally the surface heat loss and deep convection weaken in the labrador sea but intensify in the greenland sea in the region of sea ice retreat the enhanced heat flux anomaly in the subpolar atlantic is mainly caused by the thermal forcing

brazil malvinas frontal system as seen from 9 years of advanced very high resolution radiometer data 1 surface thermal fronts in the southwestern atlantic swa ocean are examined using 9 years 1987 1995 of advanced very high resolution radiometer data fronts are detected considering a gradient based edge detector sea surface temperature gradients are calculated from 4 km resolution 5 day composite images covering the western argentine basin south of 30 s variability in the position and intensity of the fronts from seasonal to interannual timescales is characterized in six regions including different parts of the brazil current front bcf and the subantarctic front saf shelf break brazil sb b brazil malvinas collision bm c brazil current overshoot bc o shelf break malvinas sb m malvinas return flow mrf and falkland escarpment fe fronts in the sb b sb m mrf and fe regions are controlled by the bathymetry in the bm c region the bcf and saf appear to merge as a single front this front does not present large seasonal north south excursions as previously described though it pivots seasonally around a fixed point located approximately at 39 5 s 53 5 w changing its orientation from n s in winter to nw se in summer consequently on average the front intersects the 1000 m isobath at 38 30 s in summer and north of 37 s in winter in the bc o region the bcf has a u shape centered at 53 w the intensity of the fronts in each region except in the fe region presents large seasonal variability in the sb b bm c and bc o regions the frontal intensity is highest 0 35 c km during austral winter the annual components explain 83 67 and 71 of the total variance respectively in the sb m and mrf regions the saf is most intense 0 25 c km in summer and fall the annual component of the intensity fluctuations explains 29 and 38 of the total variance respectively in the fe region the annual component of frontal intensity explains only 17 of the variability in the six regions important interannual variability is found the zapiola rise centered at 45 s 43 w appears as a gradient free region closed planetary potential vorticity contours in this area suggest it is dynamically isolated

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

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

geochemistry and hydrodynamics of deep formation brines in the palo duro and dalhart basins texas u s a abstract geological characterization of evaporite deposits as potential host rocks for radioactive waste burial must include hydrogeological investigations on both local and regional scales the palo duro and dalhart basins of texas contain candidate salt deposits which are underlain by shelf carbonates and fan delta sandstones these basins are ancient intracratonic elements exhibiting regional eastward flow in the deep brine aquifers pressures in these aquifers are subnormal however the major component of flow appears to be parallel to bedding owing to the low permeability of the overlying evaporite strata in the central part of the basin salinities computed from geophysical logs or obtained from chemical analyses indicate only small aberrations in the regional salinity profile for brines in carbonate rocks and sandstones of late pennsylvanian early permian age brines reflect reactivity with the host rock deriving salinity primarily from evaporite facies and at present apparently follow the anhydrite and calcite phase boundaries substantial outgassing of co 2 and oxidation of ferrous iron appear to have occurred during collection of the samples during wildcat drilling by industry mass transfer computer programs have been used to determine the most probable in situ brine composition additional support for the computed equilibrium state is the correlation between computed p co 2 in the brines and observed p co 2 in adjacent natural gas reservoirs

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

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

mechanisms of multidecadal atlantic meridional overturning circulation variability diagnosed in depth versus density space abstractmultidecadal variability of the atlantic meridional overturning circulation amoc is examined based on a comparison of the amoc streamfunctions in depth and in density space in a 700 yr present day control integration of the fully coupled community climate system model version 3 the commonly used depth coordinate amoc primarily exhibits the variability associated with the deep equatorward transport that follows the changes in the labrador sea deep water formation on the other hand the density based amoc emphasizes the variability associated with the subpolar gyre circulation in the upper ocean leading to the changes in the labrador sea convection combining the two representations indicates that the 20 yr periodicity of the amoc variability in the first half of the simulation is primarily due to an ocean only mode resulting from the coupling of the deep equatorward flow and the upper ocean gyre circulation near the gulf stream and north atlantic current in addition the density based amoc

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

large scale vertical and horizontal circulation in the north atlantic ocean abstract observations of large scale hydrography air sea forcing and regional circulation from numerous studies are combined by inverse methods to determine the basin scale circulation average diapycnal mixing and adjustments to air sea forcing of the north atlantic ocean dense overflows through the denmark strait and faroe bank channels are explicitly included and are associated with strong vertical and lateral circulation and mixing these processes in the far northern atlantic play a fundamental role in the meridional overturning circulation for the entire ocean accompanied by an upper cell of mode water and intermediate water circulation the two cells converge roughly at the mean depth of the midocean ridge crest the labrador sea water layer lies within this convergence south of the overflow region model derived mean diapycnal diffusivities are o 10 5 m2 s 1 or smaller at the base of the thermocline and diapycnal advection is driven primarily by air sea transformation on outcropping layers

simultaneous evolution of gyre and atlantic meridional overturning circulation anomalies as an eigenmode of the north atlantic system abstractthe authors identify an interdecadal oscillatory mode of the north atlantic atmosphere ocean system in a general circulation model gfdl cm2 1 via a linear inverse model lim the oscillation mechanism is mostly embedded in the subpolar gyre anomalous advection generates density anomalies in the eastern subpolar gyre which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later when associated anomalous advection of the opposite sign starts the other half cycle as density anomalies reach the labrador sea deep convection region atlantic meridional overturning circulation amoc anomalies are also induced both the gyre and amoc anomalies then propagate equatorward slowly following the advection of density anomalies the oscillation is further demonstrated to be more likely an ocean only mode while excited by the atmospheric forcing in particular it can be approximated as a linearly driven damped oscillator that is partly excited by the north atla

latitudinal dependence of atlantic meridional overturning circulation amoc variations 1 amoc variations are often thought to propagate with the kelvin wave speed resulting in a short time lead between high and low latitudes amoc variations however as shown in this paper using a coupled climate model gfdl cm2 1 with the existence of interior pathways of north atlantic deep water nadw from flemish cap to cape hatteras as that observed recently amoc variations estimated in density space propagate with the advection speed in this region resulting in a much longer time lead several years between subpolar and subtropical amoc variations and providing a more useful predictability the results suggest that amoc variations have significant meridional coherence in density space and monitoring amoc variations in density space at higher latitudes might reveal a stronger signal with a several year time lead

circulation and water mass modification in the brazil malvinas confluence abstract the confluence between the brazil current and the malvinas current the brazil malvinas confluence bmc in the argentine basin is characterized by a complicated thermohaline structure favoring the exchanges of mass heat and salt between the subtropical gyre and the antarctic circumpolar current acc analysis of thermohaline properties of hydrographic sections in the bmc reveals strong interactions between the acc and subtropical fronts in the subantarctic front subantarctic mode water samw antarctic intermediate water aaiw and circumpolar deep water cdw warm become saltier by 0 4 0 08 0 3 0 02 and 0 6 c 0 1 respectively in the subtropical gyre aaiw and north atlantic deep water have cooled freshened by 0 4 0 07 and 0 7 c 0 11 respectively to quantify those acc subtropical gyre interactions a box inverse model surrounding the confluence is built the model diagnoses a subduction of 16 4 sv 1 sv 106 m3 s 1 of newly formed samw and aaiw under the subtro

topographic coupling of the atlantic overturning and gyre circulations abstractthe vorticity dynamics associated with the mean and time varying gyre and overturning circulations of the atlantic ocean are examined in a realistic ocean model hindcast simulation of the late twentieth century abyssal flow interaction with sloping bottom bathymetry gives rise to the bottom pressure torque bpt term of the vertically integrated vorticity equation the dominance of this term in the closure of the barotropic gyre circulation noted in previous studies is corroborated here for both non eddy resolving and eddy resolving versions of the parallel ocean program pop model this study shows that bpt is also a dominant term in the vorticity balance of the atlantic meridional overturning circulation amoc and therefore represents a key dynamical link between the overturning and gyre streamfunctions the interannual variability of the atlantic circulation over the last several decades viewed in terms of time varying integral vorticity balances demonstrates the fundamental role played by

controls on the global distribution of contourite drifts insights from an eddy resolving ocean model abstract contourite drifts are anomalously high sediment accumulations that form due to reworking by bottom currents due to the lack of a comprehensive contourite database the link between vigorous bottom water activity and drift occurrence has yet to be demonstrated on a global scale using an eddy resolving ocean model and a new georeferenced database of 267 contourites we show that the global distribution of modern contourite drifts strongly depends on the configuration of the world s most powerful bottom currents many of which are associated with global meridional overturning circulation bathymetric obstacles frequently modify flow direction and intensity imposing additional finer scale control on drift occurrence mean bottom current speed over contourite covered areas is only slightly higher 2 2 cm s than the rest of the global ocean 1 1 cm s falling below proposed thresholds deemed necessary to re suspend and redistribute sediments 10 15 cm s however currents fluctuate more frequently and intensely over areas with drifts highlighting the role of intermittent high energy bottom current events in sediment erosion transport and subsequent drift accumulation we identify eddies as a major driver of these bottom current fluctuations and we find that simulated bottom eddy kinetic energy is over three times higher in contourite covered areas in comparison to the rest of the ocean our work supports previous hypotheses which suggest that contourite deposition predominantly occurs due to repeated acute events as opposed to continuous reworking under average intensity background flow conditions this suggests that the contourite record should be interpreted in terms of a bottom current s susceptibility to experiencing periodic high speed current events our results also highlight the potential role of upper ocean dynamics in contourite sedimentation through its direct influence on deep eddy circulation

the sensitivity of the atlantic meridional overturning circulation to freshwater forcing at eddy permitting resolutions the effect of increasing horizontal resolution is examined to assess the response of the atlantic meridional overturning circulation amoc to freshwater perturbations versions of a global climate model with horizontal resolutions ranging from 1 8 latitude 3 6 longitude to 0 2 0 4 are used to determine if the amoc response to freshwater forcing is robust to increasing resolution in the preindustrial equilibrium climate the representation of western boundary currents and meridional heat transport are improved with resolution freshwater forcings similar to the final drainage of proglacial lakes agassiz and ojibway are applied evenly over the labrador sea and exclusively along the western boundary the duration and maximum amplitude of model responses to freshwater forcing showed little sensitivity to increasing resolution an evaluation with tracers of the forcing impact on different regions of north atlantic deep water formation revealed the possibility that increases in labrador sea deep convection at higher resolution mitigate the effect of stronger boundary currents and enhanced mixing with increasing resolution there is less cooling in the subpolar west atlantic more cooling in the subpolar east atlantic and greater variability in the deep ocean response to the boundary forcing while differences exist the coarseresolution model response remains robust at finer horizontal resolutions

lagrangian observations of the deep western boundary current in the north atlantic ocean part i large scale pathways and spreading rates twenty six rafos floats were deployed in the deep western boundary current dwbc of the north atlantic ocean between the grand banks and cape hatteras in 1994 95 and tracked acoustically for up to two years half of the floats were launched in the upper chlorofluorocarbon cfc maximum associated with upper labrador sea water 800 m and the other half near the deep cfc maximum that identifies the overflow water from the nordic seas 3000 m the float observations reveal the large scale pathways of these recently ventilated water masses in the subtropics the shallow float tracks show directly that upper labrador sea water is diverted away from the western boundary and into the interior at the location where the dwbc encounters the gulf stream near 368n the crossover region consistent with previous hydrographic studies east of the crossover region only one upper labrador sea water float out of seven 15 permanently crossed to the south side of the stream in two years caught in a cold core ring formation event the other shallow floats recirculated north of the gulf stream apparently confined by the mean potential vorticity gradient aligned with the stream the deep floats closely followed the topography to the crossover region then revealed a bifurcation in fluid parcel pathways one branch continues equatorward along the western boundary and the other turns first eastward along the gulf stream path then southward the deep float pathways including the bifurcation in the crossover region can be explained in terms of the deep potential vorticity distribution comparison of the float results with results from recent modeling studies suggests that the deep flow is strongly influenced by both the depth of the main pycnocline and bottom depth the effective spreading rates of upper labrador sea water and overflow water estimated directly from the float data southward at 0 6 6 0 2 cm s21 and 1 4 6 0 4 cm s21 respectively agree well with tracer derived spreading rates mean velocities in the dwbc equatorward at 2 4 cm s21 upper labrador sea water and 4 5 cm s21 overflow water are consistent with other in situ measurements one deep float drifted almost 4000 km along the western boundary in two years revealing a fast track for the spreading of overflow water in the dwbc these observations emphasize the importance of the crossover region in the spreading and mixing of recently ventilated water masses addressed in part ii of this study

multidecadal warming and shoaling of antarctic intermediate water antarctic intermediate water aaiw is a dominant southern hemisphere water mass that spreads from its formation regions just north of the antarctic circumpolar current acc to at least 20 s in all oceans this study uses an isopycnal climatology constructed from argo conductivity temperature depth ctd profile data to define the current state of the aaiw salinity minimum its core and thence compute anomalies of aaiw core pressure potential temperature salinity and potential density since the mid 1970s from ship based ctd profiles the results are used to calculate maps of temporal property trends at the aaiw core where statistically significant strong circumpolar shoaling 30 50 dbar decade 1 warming 0 05 0 15 c decade 1 and density reductions up to 0 03 kg m 3 decade 1 are found these trends are strongest just north of the acc in the southeast pacific and atlantic oceans and decrease equatorward salinity trends are generally small with their sign varying regionally bottle data are used to extend the aaiw core potential temperature anomaly analysis back to 1925 in the atlantic and to 1960 elsewhere the modern warm aaiw core conditions appear largely unprecedented in the historical record biennially and zonally binned median aaiw core potential temperatures within each ocean basin are with the notable exception of the subtropical south atlantic in the 1950s 70s 0 2 1 c colder than modern values zonally averaged sea surface temperature anomalies around the aaiw formation latitudes in each ocean and sectoral southern annular mode indices are used to put the aaiw core property trends and variations into context

the north atlantic tritium distribution in 1972 the distribution of tritium in the north atlantic in 1972 is compared with the distribution of salinity and the first order potential voracity as mapped on six constant potential density surfaces in the north atlantic the picture presented suggests an advective transport regime which is consistent with currently developing notions of the large scale gyre structure lateral mixing along isopycnals appears to be important in the northwestern region of the subtropical gyre and vertical cross isopycnal mixing needs to be invoked only in near surface layers

variable density flow in the midcontinent basins and arches region of the united states basins in the midcontinent basins and arches region of the united states are typical of many sedimentary basins containing a diverse assemblage of lithologies that have been deformed at various stages in their geologic history and now transmit fluids of varying density on the basis of the synthesis of geologic data from more than 500 wells specific gravity data from more than 1800 samples fluid pressure data from nearly 40 deep wells and hydraulic conductivity data from core analyses of more than 35 wells the midcontinent basins and arches region is conceptualized as a layered heterogeneous hydrologically mature region containing 11 hydrostratigraphic units that have undergone various periods of deformation and comprise a sequence of aquifers and confining layers with fluid density variations ranging from freshwater to brine an interpretative threedimensional steady state variable density finite difference flow model was constructed to organize the field data and to evaluate various controls on regional flow patterns simulation results indicate that flow directions in the shallow hydrostratigraphic units are controlled by local surface elevations the deeper flow systems however show the influence of regional structural features such as the cincinnati arch that control the locations of regional groundwater divides the amount of downward cross formational flow into the older hydrostratigraphic units is greatest along the area of the cincinnati and findlay arches where the mt simon sandstone is relatively shallow and fracturing and faulting are more common spatial variations in fluid density are a significant influence on vertical cross formational flow in regions where salt bearing units are present

mechanisms underlying recent decadal changes in subpolar north atlantic ocean heat content the subpolar north atlantic spna is subject to strong decadal variability with implications for surface climate and its predictability in 2004 2005 spna decadal upper ocean and sea surface temperature trends reversed from warming during 1994 2004 to cooling over 2005 2015 this recent decadal trend reversal in spna ocean heat content ohc is studied using a physically consistent observationally constrained global ocean state estimate covering 1992 2015 the estimate s physical consistency facilitates quantitative causal attribution of ocean variations closed heat budget diagnostics reveal that the spna ohc trend reversal is the result of heat advection by midlatitude ocean circulation kinematic decompositions reveal that changes in the deep and intermediate vertical overturning circulation cannot account for the trend reversal but rather ocean heat transports by horizontal gyre circulations render the primary contributions the shift in horizontal gyre advection reflects anomalous circulation acting on the mean temperature gradients maximum covariance analysis mca reveals strong covariation between the anomalous horizontal gyre circulation and variations in the local wind stress curl suggestive of a sverdrup response results have implications for decadal predictability

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

wind forced interannual variability of the atlantic meridional overturning circulation at 26 5 n the observed atlantic meridional overturning circulation amoc at 26 5 n shows interannual variability consisting of an increase from early 2004 to late 2005 and a following downtrend which reaches a minimum in the winter of 2009 2010 these interannual amoc fluctuations are dominated by changes in the upper mid ocean geostrophic flow except during the winter of 2009 2010 when the anomalous wind driven ekman transport also has a significant contribution the physical mechanisms for the interannual changes of the amoc are proposed and evaluated in a two layer model while the ekman transport is linked to the north atlantic oscillation nao the anomalous geostrophic transport involves the oceanic adjustment to surface wind forcing in particular the intensification and weakening of the southward interior geostrophic flow is modulated by the internal rossby wave adjustment to the surface wind forcing the gulf stream on the other hand is controlled by both topographic waves along the us coast and westward propagating planetary waves our study suggests that a large part of the observed amoc interannual variability at 26 5 n can be explained by wind driven dynamics

lagrangian perspectives of deep water export from the subpolar north atlantic 1 direct observations at the grand banks have raised a quandary concerning the pathways of the lower branch of the meridional overturning circulation in contrast to moored current meters that depict an intense narrow deep western boundary current dwbc observations using different float types failed to show this continuous export path here this issue is addressed by a lagrangian analysis of synthetic particles in an eddy resolving circulation model due to intense eddy activity around the grand banks about 40 of the deep water in the dwbc is diverted into the interior spreading southward along the western flank of the mid atlantic ridge or with the eddying flow field in the basin interior imposing constraints on the vertical displacements of particles similar to those experienced by observational floats further reduces their adherence to the dwbc particularly near the southern tip of the grand banks

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

controls on the regional scale salinization of the ogallala aquifer southern high plains texas usa an extensive saline plume 250 km 2 within the regionally important unconfined aquifer in the neogene ogallala formation overlies the panhandle oil and gas field in the southern high plains texas usa relative to upgradient ogallala water the plume waters have d 18 o 6 7 to 8 8 and d d 42 to 88 values that tend to be depleted and have higher cl 150 mg l and so4 75 mg l concentrations various end member mixing models suggest that the plume composition reflects the presence of paleowaters recharged during middle to late wisconsinan time rather than salinization associated with petroleum production paleowaters probably mixed with salt dissolution zone waters from the underlying upper permian formations before discharging upward into the ogallala formation cross formational discharge is controlled primarily by the geometry of the underlying units as influenced by the amarillo uplift pinch out of the laterally adjoining confined aquifer in the triassic dockum group variations in the saturated thickness of the ogallala aquifer and the presence of potential pathways related to salt dissolution 2000 elsevier science ltd all rights reserved

kinematic elements of antarctic intermediate water in the western south atlantic the northward flowing antarctic intermediate water aaiw is a major contributor to the large scale meridional circulation of water masses in the atlantic together with bottom and thermocline water aaiw replaces north atlantic deep water that penetrates into the south atlantic from the north on the northbound propagation of aaiw from its formation area in the south western region of the argentine basin the aaiw progresses through a complex spreading pattern at the base of the main thermocline this paper presents trajectories of 75 subsurface floats seeded at aaiw depth the floats were acoustically tracked covering a period from december 1992 to october 1996 discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of aaiw in the equatorial region intermittent westward and eastward currents were observed suggesting a seasonal cycle of the aaiw flow direction at tropical latitudes just offshore the intermediate western boundary current the southward advection of an anticyclonic eddy was observed between 5 degrees s and 11 degrees s farther offshore the flow lacks an advective pattern and is governed by eddy diffusion the westward subtropical gyre return current at about 28 degrees s shows considerable stability with the mean kinetic energy to eddy kinetic energy ratio being around one farther south the eastward deeper south atlantic current is dominated by large scale meanders with particle velocities in excess of 60 cm s 1 at the brazil falkland current confluence zone a cyclonic eddy near 40 degrees s 50 degrees w seems to act as injector of freshly mixed aaiw into the subtropical gyre in general much of the mixing of the various blends of aaiw is due to the activity of mesoscale eddies which frequently reoccupy similar positions c 1999 elsevier science ltd all rights reserved

is the north atlantic in sverdrup balance abstract evidence for the widespread assumption that sverdrup balance describes the dynamics of the north atlantic subtropical gyre is reviewed critically if the balance were to hold up to the edge of the gulf stream system then there is a serious conflict with existing estimates of the net meridional flux of heat if a recirculation region near the stream is excluded then one loses numerical agreement between a geostrophic calculation of the interior mass flux and the opposite gulf stream transport our conclusion is that while the linear vorticity balance may well apply over much of the north atlantic subtropical gyre there is little evidence supporting the special version called sverdrup balance even in the relatively quiescent region east of the mid atlantic ridge to the contrary simple order of magnitude estimates suggest that it is just as likely that the flows are dominated by bottom induced vertical velocities as they are directly driven by the wind stress curl

variability in the freshwater balance of northern marguerite bay antarctic peninsula results from δ18o abstract we investigate the seasonal variability in freshwater inputs to the marguerite bay region western antarctic peninsula using a time series of oxygen isotopes in seawater from samples collected in the upper mixed layer of the ocean during 2002 and 2003 we find that meteoric water mostly in the form of glacial ice melt is the dominant freshwater source accounting for up to 5 of the near surface ocean during the austral summer sea ice melt accounts for a much smaller percentage even during the summer maximum around 1 the seasonality in meteoric water input to the ocean around 2 of the near surface ocean is not dissimilar to that of sea ice melt around 2 in 2002 and 1 in 2003 contradicting the assumption that sea ice processes dominate the seasonal evolution of the physical ocean environment close to the antarctic continent three full depth profiles of oxygen isotopes collected in successive decembers 2001 2003 indicate that around 4 m of meteoric water is present in the water column at this time of year and around 1 m of sea ice formed from this same water column the predominance of glacial melt is significant since it is known to be an important factor in the operation of the ecosystem for example by providing a source of nutrients and modifying the physical environment to control the spatial extent and magnitude of phytoplankton blooms the western antarctic peninsula is undergoing a very rapid change in climate with increasing ocean and air temperatures retreating glaciers and increases in precipitation associated with changes in atmospheric circulation as climate change continues we expect meteoric water inputs to the adjacent ocean to rise further sea ice in this sector of the antarctic has shown a climatic decrease and we expect a reduction in oceanic sea ice melt fractions if this change continues continued monitoring of the oceanic freshwater budget at the western peninsula is needed to track these changes as they occur and to better understand their ecological consequences

why western boundary currents in realistic oceans are inviscid a link between form stress and bottom pressure torques it is shown that wind stress curl is balanced by bottom pressure torque in a zonal integral over any strip wide enough to smooth out the effect of nonlinear terms typically about 3 of latitude the derivation is completely general as long as the zonal wind stress is balanced by form stress at each latitude as is known to be the case in the ocean this implies that viscous torques are not important in western boundary currents their place being taken by bottom pressure torques the prediction is confirmed in the context of a global eddy permitting numerical ocean model this link between form stress and bottom pressure torques makes it easier to consider southern ocean dynamics and subtropical gyre dynamics in the same conceptual framework with topographic interactions being important in both cases

energetic eddies at the northern edge of the antarctic circumpolar current in the southwest pacific abstract two year time series measurements of current and temperature were made at the northern edge of the antarctic circumpolar current in the southwestern pacific from 1978 to 1980 to investigate eddy heat and momentum fluxes in the circumpolar region the measurement site near 49 30 s 170 w is at a transition zone or front between subantarctic and subtropical waters in which meridional temperature and salinity gradients and hence vertical shear in eastward velocity extend down to 2 000 m depth energetic eddies are found to have typical amplitudes of 20 cm s 1 at 1 000 m depth to be vertically coherent over the depth range from 1 000 to 5 000 m to vary over temporal scales of 20 days and horizontal scales of 60 km and to propagate southeastward at about 12 cm s 1 observed eddy kinetic energy of 169 cm2 s at 1 000 m depth is larger than that at comparable depths in the gulf stream and kuroshio extension regions and in central drake passage like drake passage eddies these eddies transport a significant amount of heat poleward in the band of periods between 20 and 50 days but this band does not dominate the spectrum the way it does in drake passage energetic fluctuations with periods longer than 50 days effect an eddy heat flux whose sign is ambiguous so that the net meridional heat flux is not statistically significant eddy momentum fluxes transport eastward momentum northward away from the circumpolar current these fluxes are statistically significant but are only about one fourth of the size required to balance the eastward momentum put into the water column by the wind an estimate of the standing eddy momentum flux due to large spatial scale variations in the circumpolar current is also too small so that pressure forces against bottom topography is likely to be the mechanism by which the wind input is balanced

signal propagation related to the north atlantic overturning 1 changes of the meridional overturning circulation moc due to surface heat flux variability related to the north atlantic oscillation nao are analyzed in various ocean models i e eddying and non eddying cases a prime signature of the forcing is variability of the winter time convection in the labrador sea the associated changes in the strength of the moc near the subpolar front 45 n are closely related to the nao index leading moc anomalies by about 2 3 years in both the eddying and non eddying simulation further south the speed of the meridional signal propagation depends on model resolution with lower resolution non eddying case 4 3 resolution the moc signal propagates equatorward with a mean speed of about 0 6 cm s similar as spreading rates of passive tracer anomalies eddy permitting experiments 1 3 show a significantly faster propagation with speeds corresponding to boundary waves thus leading to an almost in phase variation of the moc transport over the subtropical to subpolar north atlantic

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

estimates of the mean circulation in the deep 2 000m layer of the eastern north atlantic a total of 131 current meter records of between 6 and 24 month duration are analysed to describe the deep flow field of the eastern north atlantic from 19 to 54 n and from the continental slope to the mid atlantic ridge mean flows are weak and may be statistically indeterminate in some records and locations but appear to indicate cyclonic circulations around the iberia and porcupine abyssal plains with a generally southward flow along the mid atlantic ridge and a deep northward slope current where measurements exist along the eastern boundary the deepest inflow to the north eastern basin that has been identified to date takes place through the discovery gap of 4 700 m sill depth at 37 25 n 15 45 w in the azores portugal ridge south of that ridge observations are sparse and no systematic circulation is yet evident these observations are discussed in relation to recent geostrophic estimates of the deep circulation

using eulerian and lagrangian approaches to investigate wind driven changes in the southern ocean abyssal circulation this study uses a global ocean eddy permitting climate model to explore the export of abyssal water from the southern ocean and its sensitivity to projected twenty first century poleward intensifying southern oceanwind stress the abyssalflow pathways and transportareinvestigated usinga combinationoflagrangian and eulerian techniques in an eulerian format the equator and poleward flows within similar abyssal density classes are increased by the wind stress changes making it difficult to explicitly diagnose changes in the abyssal export in a meridional overturning circulation framework lagrangian particle analyses are used to identify the major export pathways of southern ocean abyssal waters and reveal an increase in the number of particles exported to the subtropics from source regions around antarctica in response to the wind forcing both the lagrangian particleand eulerian analyses identify transients asplaying a key rolein the abyssal export ofwater from the southern ocean wind driven modifications to the potential energy component of the vorticity balance in the abyss are also found to impact the southern ocean barotropic circulation

possible effects of erosional changes of the topographic relief on pore pressures at depth formation fluid pressures are said to be anomalous or abnormal if they differ from hydrostatic for the depth considered explanations of the origin and maintenance of anomalies seldom include the possibility of delayed adjustment of subsurface pore pressures to erosional modifications of the topographic relief yet the present paper shows that it is entirely possible for subaerial erosion to generate pore pressure changes at great depths by cross formational energy transfer in realistic field conditions conversely pressures may survive changes in the topography and be in a transient state for hundreds to millions of years thus appearing anomalous at the time of observation according to calculations for the red earth region in alberta canada present pore pressures in the devonian immediately above the precambrian basement at a depth of approximately 1400 meters may have been induced originally by erosionally shaped topography during pliocene times 12 to 1 m y b p but be adjusting now to new boundary conditions created by the erosional exposure of the sub cretaceous unconformity a 12 adjustment may already have taken place during the last 300 thousand years leaving nearly 90 of pliocene age pressures to survive to date as slowly decaying transient pressure relicts of the geologic past in view of the possible erosional effects on pore pressures demonstrated in this paper the question of anachronous formation pressures may deserve consideration in problems related to deep basin hydrology including regional groundwater resource development migration and accumulation of hydrocarbons radioactive waste isolation dating of various geologic events and so on

the role of bottom pressure torques on the interior pathways of north atlantic deep water abstractfour versions of the same global climate model one with horizontal resolution of 1 8 3 6 and three with 0 2 0 4 are employed to evaluate the role of ocean bottom topography and viscosity on the spatial structure of the deep circulation this study is motivated by several recent observational studies that find that subsurface floats injected near the western boundary of the labrador sea most often do not continuously follow the deep western boundary current dwbc in contrast to the traditional view that the deep water formed in the north atlantic predominantly follows the dwbc it is found that with imposed large viscosity values the model reproduces the traditional view however as viscosity is reduced and the model bathymetry resolution increased much of the north atlantic deep water nadw separates from the western boundary and enters the low latitude atlantic via interior pathways distinct from the dwbc it is shown that bottom pressure torques play an important role in maintai

regional hydrodynamics of the proposed high level nuclear waste repository sites in the texas panhandle abstract bedded salt in the palo duro basin area of the texas panhandle is being evaluated as possible host rock for a high level nuclear waste repository the two potential sites are underlain by three major hydrostratigraphic units a shallow freshwater aquifer system developed in the tertiary ogallala formation and the triassic dockum group a thick shale and evaporite aquitard developed in an upper permian evaporite sequence and a deep basin brine aquifer system predominantly developed in the wolfcamp series and the pennsylvanian system water level shut in pressure and specific gravity data from inventoried wells drill stem tests and long term pumping tests were used to construct potentiometric surfaces potentiometric profiles and pressure depth diagrams water in the ogallala and dockum aquifers predominantly flows horizontally and discharges to springs and wells equipotential patterns on potentiometric profiles indicate that some water may be moving downward from the ogallala into the dockum across an intervening confining unit as well as moving downward from the dockum across the shale and evaporite aquitard and into the deep basin aquifer system this is corroborated by the slope of pressure depth data which indicate the potential for regional downflow in the shale and evaporite aquitard equipotential patterns and pressure depth data indicate predominantly horizontal flow in the deep basin aquifer system which is influenced by permeability variations related to facies changes hydraulic gradients are flatter in the more permeable shelf shelf margin and fan delta facies and steeper in the less permeable deep basin facies

the north atlantic oscillation surface current velocities and sst changes in the subpolar north atlantic changes in surface circulation in the subpolar north atlantic are documented for the recent interannual switch in the north atlantic oscillation nao index from positive values in the early 1990s to negative values in 1995 96 data from lagrangian drifters which were deployed in the north atlantic from 1992 to 1998 were used to compute the mean and varying surface currents ncep winds were used to calculate the ekman component allowing isolation of the geostrophic currents the mean ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses the northeastward flow of the north atlantic current is organized into three strong cores associated with topography along the eastern boundary in rockall trough in the iceland basin the subpolar front and on the western flank of the reykjanes ridge irminger current the last is isolated in this eulerian mean from the rest of the north atlantic current by a region of weak velocities on the east side of the reykjanes ridge the drifter results during the two different nao periods are compared with geostrophic flow changes calculated from the nasa pathfinder monthly gridded sea surface height ssh variability products and the advanced very high resolution radiometer avhrr sst data during the positive nao years the northeastward flow in the north atlantic current appeared stronger and the circulation in the cyclonic gyre in the irminger basin became more intense this was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from avhrr sst data which show that during the positive nao years with stronger westerlies the subpolar front was sharper and located farther east sst gradients intensified in the north atlantic current irminger basin and east of the shetland islands during the positive nao phase associated with stronger currents sst differences between positive and negative nao years were consistent with changes in air sea heat flux and the eastward shift of the subpolar front sst advection as diagnosed from the drifters likely acted to reduce the sst differences

lagrangian observations of the deep western boundary current in the north atlantic ocean abstract in this study the authors analyze the trajectories of 18 rafos floats launched in the deep western boundary current dwbc between the grand banks and cape hatteras to investigate the kinematics and dynamics in the region where the dwbc crosses under the gulf stream near 36 n the crossover region floats deployed in the chlorofluorocarbon cfc maximum associated with upper labrador sea water depth 800 m illustrate the entrainment process of this water mass into the gulf stream the behavior of the floats and fluid parcels in the crossover region is strongly dependent on the meandering of the gulf stream when the stream is close to its mean position fluid parcels entrained from the upper dwbc travel along the northern edge of the stream when a meander trough is present downstream of the entrainment location dwbc fluid parcels cross the gulf stream and sometimes are expelled on the south side this represents a previously unrecognized mechanism for transporting upper labrador sea w

why does the deep western boundary current leak around flemish cap abstractthe southward flowing deep limb of the atlantic meridional overturning circulation is comprised of both the deep western boundary current dwbc and interior pathways the latter are fed by

labrador sea water tracked by profiling floats from the boundary current into the open north atlantic fifteen profiling floats were injected into the deep boundary current off labrador they were ballasted to drift in the core depth of labrador sea water lsw at 1500 m depth and were deployed in two groups during march and july august 1997 initially for about three months the floats were drifting within the boundary current and the flow vectors were used to determine the mean horizontal structure of the deep labrador current which was found to be about 100 km wide with an average core speed of 18 cm s 1 north of flemish cap the boundary current encounters complicated topography around orphan knoll and there the lsw outflow splits up into different routes one obvious lsw path is eastward through the charlie gibbs fracture zone and another route is a narrow recirculation toward the central labrador sea a surprising result was that none of the floats were able to follow the boundary current southward to the grand banks area and exit into the subtropics trajectories and temperature profiles of the eastward drifting floats indicate the importance of the north atlantic current for dispersing the floats even at the level of lsw

locally and remotely forced subtropical amoc variability a matter of time scales mechanisms driving the north atlantic meridional overturning circulation amoc variability at low frequency are of central interest for accurate climate predictions although the subpolar gyre region has been identified as a preferred place for generating climate timescale signals their southward propagation remains under consideration complicating the interpretation of the observed time series provided by the rapid climate change meridional overturning circulation and heatflux array western boundary time series rapid mocha wbts program in this study we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low frequency amoc variability we analyze for this a set of four ensembles of a regional 208s 558n eddy resolving 1 128 north atlantic oceanic configuration where surface forcing and open boundary conditions are alternatively permuted from fully varying realistic to yearly repeating signals their analysis reveals the predominance of local atmospherically forced signal at interannual time scales 2 10 years whereas signals imposed by the boundaries are responsible for the decadal 10 30 years part of the spectrum due to this marked timescale separation we show that although the intergyre region exhibits peculiarities most of the subtropical amoc variability can be understood as a linear superposition of these two signals finally we find that the decadal scale boundary forced amoc variability has both northern and southern origins although the former dominates over the latter including at the site of the rapid array 26 58n

robust and nonrobust aspects of atlantic meridional overturning circulation variability and mechanisms in the community earth system model abstractrobust and nonrobust aspects of atlantic meridional overturning circulation amoc variability and mechanisms are analyzed in several 600 yr simulations with the community earth system mode

coherent surface subsurface fingerprint of the atlantic meridional overturning circulation 1 satellite altimeter data shows a weakening of the north atlantic subpolar gyre during the 1990s which is thought as an indicator of a slowdown of the atlantic meridional overturning circulation amoc however whether the recent slowing subpolar gyre is a decadal variation or a long term trend remains unclear here i show that altimeter data is highly correlated with instrumental subsurface ocean temperature data in the north atlantic and both show opposite signs between the subpolar gyre and the gulf stream path such a dipole pattern is a distinctive fingerprint of amoc variability as shown for the first time by a 1000 year coupled ocean atmosphere model simulation the results suggest that contrary to previous interpretations the recent slowdown of the subpolar gyre is a part of a multidecadal variation and suggests a strengthening of the amoc the ongoing satellite and subsurface temperature measurements could be used to monitor future amoc variations sensitively

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

surface estimates of the atlantic overturning in density space in an eddy permitting ocean model 1 a method to estimate the variability of the atlantic meridional overturning circulation amoc from surface observations is investigated using an eddy permitting ocean only model orca 025 the approach is based on the estimate of dense water formation from surface density fluxes analysis using 78 years of two repeat forcing model runs reveals that the surface forcing based estimate accounts for over 60 of the interannual amoc variability in 0 coordinates between 37 n and 51 n the analysis provides correlations between surface forced and actual overturning that exceed those obtained in an earlier analysis of a coarser resolution coupled model our results indicate that in accordance with theoretical considerations behind the method it provides a better estimate of the overturning in density coordinates than in z coordinates in subpolar latitudes by considering shorter segments of the model run it is shown that correlations are particularly enhanced by the method s ability to capture large decadal scale amoc fluctuations the inclusion of the anomalous ekman transport increases the amount of variance explained by an average 16 throughout the north atlantic and provides the greatest potential for estimating the variability of the amoc in density space between 33 n and 54 n in that latitude range 70 84 of the variance is explained and the root mean square difference is less than 1 sv when the full run is considered

a quantitative definition of global warming hiatus and 50 year prediction of global mean surface temperature abstractrecent global warming hiatus has received much attention however a robust and quantitative definition for the hiatus is still lacking recent studies by scafetta wu et al and tung and zhou showed that multidecadal variability mdv is responsible for the multidecadal accelerated warming and hiatuses in historical global mean surface temperature gmst records though mdv itself has not received sufficient attention thus far here the authors introduce four key episodes in gmst evolution according to different phases of the mdv extracted by the ensemble empirical mode decomposition method from the ensemble hadcrut4 monthly gmst time series the warming cooling hiatus and typical warming cooling periods are defined as the 95 confidence intervals for the locations of local mdv maxima minima and of their derivatives respectively since 1850 the warming hiatuses cooling hiatuses and typical warming have already occurred three times and the typical cooling has occurred twice at pr

heat flux carried by the antarctic circumpolar current mean flow 1 a stream function projection of historical hydrographic data is applied to study the heat flux problem in the antarctic circumpolar current acc the acc is defined as a circumpolar band consisting of mean streamlines passing through drake passage its mean path exhibits a globally meandering pattern the calculation of zonal heat transport shows that the acc warms along its equatorward segments south atlantic and indian ocean and cools along its poleward segment south pacific the primary heat sources for the acc system are two western boundary currents the brazil current and the agulhas current the mean baroclinic flow relative to 3000 dbar carries 0 14 pw poleward heat flux across 56 s and 0 08 pw across 60 s

oxygen isotope distribution at shallow to intermediate depths across different fronts of the southern ocean signatures of a warm core eddy abstract southern ocean an important component of the earth s climate system is changing in response to the anthropogenic climate change to understand better the dynamics of the southern ocean we documented oxygen isotopic variability and its relation to salinity especially at intermediate depths to 1000 m across different fronts at six stations in the southern ocean signatures of a warm core eddy extended from 40 to 44 s and 56 to 59 5 e it consisted of subtropical surface water stsw other water masses identified were the antarctic surface water aasw antarctic intermediate water aaiw and upper circumpolar deep water ucdw based on the salinity temperature and oxygen isotopic ratios the slope of the oxygen isotope salinity relationship indicates that the water in the warm core eddy was derived from a region dominated by evaporation precipitation while the water surrounding the eddy came from a region dominated by melting freezing the shoaling of aaiw up to water depth of 500 m was observed along the transect these new oxygen isotope data especially from the intermediate depths will also help fill existing gaps in the global seawater oxygen isotope dataset

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

on the relationship between decadal buoyancy anomalies and variability of the atlantic meridional overturning circulation abstractowing to the role of the atlantic meridional overturning circulation amoc in ocean heat transport amoc variability is thought to play a role in climate variability on a wide range of time scales this paper focuses on the potential role of the amoc in climate variability on decadal time scales coupled and ocean only general circulation models run in idealized geometries are utilized to study the relationships between decadal amoc and buoyancy variability and determine whether the amoc plays an active role in setting sea surface temperature on decadal time scales decadal amoc variability is related to changes in the buoyancy field along the western boundary according to the thermal wind relation buoyancy anomalies originate in the upper ocean of the subpolar gyre and travel westward as baroclinic rossby waves when the buoyancy anomalies strike the western boundary they are advected southward by the deep western boundary current leading to latitudinally coherent amoc variability the amoc i

oxygen isotope and salinity variations in the indian sector of the southern ocean a combined study of stable oxygen isotopes i 18o and salinity of surface ocean waters collected from the southern indian ocean during the late austral summer and early fall 25th january to 1st april 2006 helped to trace atmospheric and oceanic processes near the equator a very clear signature of the domination of evaporation precipitation process was seen while a precipitation dominated zone was located between two evaporation dominated zones south of the equator the signature of the agulhas front af and the subtropical front stf recognizable by the sharp decrease in salinity and i 18o in the surface waters were identified at 41 s and 44 s in february during the return journey march these two fronts had merged and were located at 41 s south of 45 s the slope of the i 18o salinity relationship was close to zero while north of 62 s could have some precipitation evaporation effect south of 63 s was clearly dominated by freezing melting

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

on the influence of dsow in a numerical model of the north atlantic general circulation abstract two numerical experiments regarding the north atlantic circulation are compared the two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland sea one case climatological wind stress sst and surface salinity to force the model it reproduces the atlantic circulation found in similar studies a change in surface boundary values in the iceland sea in order to improve the denmark strait overflow water properties in the second case results in major changes in circulation and hydrography the meridional overturning more than doubles and horizontal gyres are intensified recirculation cells occur north and south of the gulf stream the gulf stream separates from the north american coast in a more realistic manner and its further course is in better agreement with observations than in the reference run the subsurface temperature distribution differs by several kelvin in the subpolar and

variability in the slope water and its relation to the gulf stream path 1 sea surface temperature sst and sea surface height ssh data for 1993 2007 for the north atlantic are combined with hydrographic data at 69w to investigate the relation between the gulf stream gs position and the slope water properties sst anomalies north of the gs are correlated with changes in its path the lag of this correlation is such that positive temperature anomalies precede northward shifts of the gs and suggests that sst anomalies move westward with speeds of several cm s eof analysis of the sst and ssh fields shows that cooling and strengthening of the sw flow are in phase over the slope water which is mirrored in the vertical structure of these fields at 69w indicating larger transports in the deep western boundary current lead to southward shifts of the mean gs path this relation between the slope water and the gs path provides some predictability for the latter

gulf stream structure transport and recirculation near 68 w an analysis of the structure and transport of the gulf stream is undertaken using direct current meter observations from a 13 mooring array deployed near 68 w from june 1988 to august 1990 the analysis is based on a stream coordinate approach in which velocities are rotated into a local downstream coordinate frame and averaged according to their relative cross stream location within the current the picture so obtained represents the average synoptic structure of the gulf stream rather than the eulerian averaged structure in which the current is weakened and broadened by lateral meandering of the current and adjacent recirculations many familiar features of the gulf stream are reproduced in the analysis including an asymmetric velocity profile with larger shear on the cyclonic shoreward side of the current an offshore displacement of the velocity core with depth and a subsurface velocity maximum on the offshore side of the current westward recirculations are also seen on both sides of the gulf stream maximum downstream speeds at the axis of the gulf stream reach approximately 2 0 m s at the surface and 0 7 m s at 1000 m roughly twice the corresponding eulerian averaged values the analysis also reveals a deep extension of the gulf stream at 3500 m depth with a width of 130 km and average speeds of 3 4 cm s the transport of the gulf stream in the stream coordinate frame is 113 8 sv approximately 30 larger than the eulerian averaged transport of 88 sv on the basis of these results and other recent studies the downstream transport increase of the gulf stream and the inflow structure to the gulf stream are reconsidered it is concluded that approximately 30 sv or over half of the transport increase between cape hatteras and 68 w is fed by inflow from the northern side of the gulf stream and that this inflow is concentrated near cape hatteras and 68 w where the gulf stream flows steeply across isobaths converging from the north