an analytical model of the response of the meridional overturning circulation to changes in wind and buoyancy forcing abstractan analytical model of the full depth ocean stratification and meridional overturning circulation for an idealized atlantic basin with a circumpolar channel is presented the model explicitly describes the ocean response to both southern ocean winds and the global pattern and strength of prescribed surface buoyancy fluxes the construction of three layers defined by the two isopycnals of overturning extrema allows the description of circulation and stratification in both the upper and abyssal ocean the system is fully solved in the adiabatic limit to yield scales for the surface layer thickness buoyancies of each layer and overturning magnitudes the analytical model also allows scaling of the antarctic circumpolar current acc transport the veracity of the three layer framework and derived scales is confirmed by applying the analytical model to an idealized geometry eddy permitting ocean general circulation model consistent with previous results the abyssal overturning is found to scale

southern ocean deep convection in global climate models a driver for variability of subpolar gyres and drake passage transport on decadal timescales we investigate the individual and joint decadal variability of southern ocean state quantities such as the strength of the ross and weddell gyres drake passage transport and sea ice area using the national institute of water and atmospheric research uk chemistry and aerosols niwa ukca model and cmip5 models variability in these quantities is stimulated by strong deep reaching convective events in the southern ocean which produce an antarctic bottom water like water mass and affect the large scale meridional density structure in the southern ocean an increase in the near surface stratification due to freshwater forcing can be a precondition for subsequent strong convection activity the combination of enhanced gyre driven sea ice and freshwater export as well as ongoing subsurface heat accumulation lead to a time lag between changes in oceanic freshwater and heat content this causes an ongoing weakening of the stratification until sudden strong mixing events emerge and the heat is released to the atmosphere we find that strong convection reduces sea ice cover weakens the subpolar gyres increases the meridional density gradient and subsequently results in a positive drake passage transport anomaly results of available cmip5 models confirm that variability in sea ice drake passage transport and the weddell gyre strength is enhanced if models show strong open ocean convective events consistent relationships between convection sea ice drake passage transport and ross gyre strength variability are evident in most models whether or not they host open ocean convection

on the transport and angular momentum balance of channel models of the antarctic circumpolar current abstract angular momentum balances are discussed both in general as well as in the context of simple channel models of the antarctic circumpolar current acc particular emphasis is placed on the close relationship between the angular momentum balance and the meridional circulation it is found that topographic form drag is established very early in the integration whereas interfacial form drag can take much longer to develop restrictions on the geostrophic portion of the meridional circulation imposed by zonally reconnecting potential vorticity contours in the upper ocean allow derivation of an estimate for the steady state transport the estimate assumes there to be little or no circumpolar flow at great depth an assumption that stems from the belief that the band of zonally reconnecting geostrophic contours in the southern ocean does not extend to the ocean floor the predicted transport is proportional to the strength of the stratification and compares favorably with numerical results in the lite

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

eddy saturation of equilibrated circumpolar currents abstractthis study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in southern ocean wind stress and global diapycnal mixing at eddy permitting and finer resolution the sensitivity of circumpolar transport to forcing magnitude is drastically reduced at sufficiently high resolution there is little or no sensitivity of circumpolar transport to wind stress even in the limit of no wind in contrast the meridional overturning circulation continues to vary with southern ocean wind stress but with reduced sensitivity in the limit of high wind stress both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions the circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution although sensitivity always remains in contrast the overturning circulation is more sensitive to change in diapycnal

systematic variations in magmatic sulphide chemistry from mid ocean ridges back arc basins and island arcs abstract immiscible sulphide liquids preserved as magmatic sulphide globules are hosted in igneous rocks of highly variable composition formed during magmatic processes at different tectonic settings here we report on compositional in situ data of magmatic sulphides from mid ocean ridge back arc and island arc magmatic systems iron ni rich monosulphide solid solutions mss that mainly consist of pyrrhotite and pentlandite are the dominant phases in magmatic sulphide globules in subduction related magmas mss are most abundant in relatively evolved melts and characterised by low ni and high fe contents as well as low ni cu ratios 1 thus mss chemistry varies systematically in lavas from the different tectonic settings in terms of their fe ni distribution and ni cu ratio the observed mineralogical and chemical variations between mss from the different settings reflect early s saturation accompanied by olivine fractionation along mid ocean ridges compared to later stage s saturation in arc systems associated with fe ti oxide fractionation these systematics are probably related to the relatively oxidised character of subduction zone magmas opposed to more reduced mid ocean ridge melts large ni cu variations in mss from the same locality and in individual samples suggest that s saturation is a multistage or continuous process leading to inclusions of magmatic sulphides that represent different fractionation stages in the ascending magma

topographic form stress in the southern ocean state estimate we diagnose the southern ocean momentum balance in a six year eddy permitting state estimate of the southern ocean we find that 95 of the zonal momentum input via wind stress at the surface is balanced by topographic form stress across ocean ridges while the remaining 5 is balanced via bottom friction and momentum flux divergences at the northern and southern boundaries of the analysis domain while the time mean zonal wind stress field exhibits a relatively uniform spatial distribution time mean topographic form stress concentrates at shallow ridges and across the continents that lie within the antarctic circumpolar current acc latitudes nearly 40 of topographic form stress occurs across south america while the remaining 60 occurs across the major submerged ridges that underlie the acc topographic form stress can be divided into shallow and deep regimes the shallow regime contributes most of the westward form stress that serves as a momentum sink for the acc system while the deep regime consists of strong eastward and westward form stresses that largely cancel in the zonal integral the time varying form stress signal integrated longitudinally and over the acc latitudes tracks closely with the wind stress signal integrated over the same domain at zero lag 88 of the variance in the six year form stress time series can be explained by the wind stress signal suggesting that changes in the integrated wind stress signal are communicated via rapid barotropic response down to the level of bottom topography this article is protected by copyright all rights reserved

the injection of zonal momentum by buoyancy forcing in a southern ocean model abstractan overturning circulation driven by prescribed buoyancy forcing is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers the channel is designed to represent the southern ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation moc the relative contributions of wind and buoyancy forcing to the zonal circulation are examined it is found that the zonal volume transport is strongly dependent on the buoyancy forcing and that the eddy kinetic energy is primarily set by wind stress forcing the zonal momentum budget integrated over each layer is considered in the buoyancy forced wind forced and combined forcing case at equilibrium sources and sinks of momentum are balanced but the transient spinup reveals the source of momentum for the current in the buoyancy forced case the forcing creates a baroclinic shear with westward flow in the lower layer allowing topographic form stress and bottom fric

the energetics of southern ocean upwelling abstractthe ocean s meridional overturning circulation is closed by the upwelling of dense carbon rich waters to the surface of the southern ocean it has been proposed that upwelling in this region is driven by strong westerly winds implying that the intensification of southern ocean winds in recent decades may have enhanced the rate of upwelling potentially affecting the global overturning circulation however there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between southern ocean processes and the global overturning circulation in this study the sensitivity of the overturning circulation to changes in southern ocean westerly wind stress is investigated using an eddy permitting ocean sea ice model in addition to a suite of standard circulation metrics an energy analysis is used to aid dynamical interpretation of the model response increased southern ocean wind stress enhances the upper cell of the overturning circulation through creation of avail

eddy generation and jet formation via dense water outflows across the antarctic continental slope along various stretches of the antarctic margins dense antarctic bottom water aabw escapes its formation sites and descends the continental slope this export necessarily raises the isopycnals associated with lighter density classes over the continental slope resulting in density surfaces that connect the near freezing waters of the continental shelf to the much warmer circumpolar deep water cdw at middepth offshore in this article an eddy resolving process model is used to explore the possibility that aabw export enhances shoreward heat transport by creating a pathway for cdw to access the continental shelf without doing any work against buoyancy forces in the absence of a net alongshore pressure gradient the shoreward cdw transport is effected entirely by mesoscale and submesoscale eddy transfer eddies are generated partly by instabilities at the pycnocline sourcing their energy from the alongshore wind stress but primarily by instabilities at the cdw aabw interface sourcing their energy from buoyancy loss on the continental shelf this combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth cdw layer sustaining a local maximum in the eddy kinetic energy over the slope and balancing the coriolis force associated with the shoreward cdw transport the resulting slope turbulence self organizes into a series of alternating along slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets cross shore variations in the potential vorticity gradient cause the jets to drift continuously offshore suggesting that fronts observed in regions of aabw down slope flow may in fact be transient features

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

regional structure and syntectonic granite intrusion in the dalradian of the gweebarra bay area donegal 6 phases of deformation are recognized in the appin group lower dalradian metasediments around gweebarra bay the major early folds are f 2 in age including the crohy and maghery anticlines facing up to the n or ne s 2 transposes an earlier fabric s 1 which is only rarely preserved no major or minor f 1 folds are recognized and the facing of s 1 is unknown d 3 produces structures verging s on s 2 but does not produce major folds comparable with d 2 the maghery anticline previously interpreted as changing from an open upright fold in the se to overturned in the nw is shown to be overturned throughout previous interpretation involving the re folding of the maas syncline by the upright mulnamin anticline is not supported and both major folds are now interpreted as d 4 in age with the axial plane of the mulnamin anticline dipping moderately or gently s d 4 is a regional deformation phase synchronous with the intrusion of the ardara granite d 5 structures are upright or face to the n the development of a major ne sw trending sinistral shear zone adjacent to the sw end of the main donegal granite is synchronous with d 5 deformation f 5 folds rotating into sub parallelism with the shear planes

remote forcing of the antarctic circumpolar current by diapycnal mixing 1 we show that diapycnal mixing can drive a significant antarctic circumpolar current acc volume transport even when the mixing is located remotely in northern hemisphere ocean basins in the case of remote forcing the globally averaged diapycnal mixing coefficient is the important parameter this result is anticipated from theoretical arguments and demonstrated in a global ocean circulation model the impact of enhanced diapycnal mixing on the acc during glacial periods is discussed

the impact of turbulence and convection on transport in the southern ocean volume transport in the presence of fully resolved convection and turbulence is investigated in a reentrant channel model of the southern ocean the response of the meridional overturning and zonal transport to variations in wind and buoyancy forcing is quantified our simulations show two overturning cells a buoyancy driven lower cell and a wind driven upper cell the lower overturning cell is much larger in meridional extent and magnitude than the upper cell the mean component of the overturning transport is smaller than the fluctuating component in the lower overturning cell indicating that transport is dominated by eddies and or turbulent convective flow in contrast the upper cell can be primarily described as a mean flow indicating minimal eddy compensation both cells strengthen with increasing winds with the upper cell being more sensitive to increasing wind intensity than the lower cell scaling for the mean upper overturning is also derived and matches previous theories which predict a linear sensitivity of the upper cell to wind zonal transport remains insensitive to increasing wind stress suggesting that the system is eddy saturated when turbulence and eddies are resolved these results suggest that fine scale flows control zonal transport and abyssal meridional overturning in the southern ocean while wind stress drives the upper meridional overturning transport highlighting the importance of accurately characterizing turbulence and convection in large scale models

antarctic density stratification and the strength of the circumpolar current during the last glacial maximum the interaction between ocean circulation and biological processes in the southern ocean is thought to be a major control on atmospheric carbon dioxide content over glacial cycles a better understanding of stratification and circulation in the southern ocean during the last glacial maximum lgm provides information that will help us to assess these scenarios first we evaluate the link between southern ocean stratification and circulation states in a suite of climate model simulations while simulated antarctic circumpolar current acc transport varies widely 80 350 sverdrup sv it co varies with horizontal and vertical stratification and the formation of the southern deep water we then test the lgm simulations against available data from paleoceanographic proxies which can be used to assess the density stratification and acc transport south of australia the paleoceanographic data suggest a moderate increase in the southern ocean stratification and the acc strength during the lgm even with the relatively large uncertainty in the proxy based estimates extreme scenarios exhibited by some climate models with acc transports of greater than 250 sv and highly saline antarctic bottom water are highly unlikely

energetics of the southern ocean mode recently multidecadal variability in the southern ocean has been found in a strongly eddying global ocean circulation model in this paper we study the lorenz energy cycle of this so called southern ocean mode som the lorenz energy cycle analysis provides details on the energy pathways associated with the som it shows that ocean eddies and the baroclinic energy pathway together with variations in the kinetic energy input by the wind are crucial aspects of the variability it is also shown how convective mixing which is induced by the som in particular in the weddell gyre is responsible for the large scale multidecadal variability in antarctic bottom water and atlantic meridional overturning circulation

sensitivity of antarctic bottom water to changes in surface buoyancy fluxes abstractthe influence of freshwater and heat flux changes on antarctic bottom water aabw properties are investigated within a realistic bathymetry coupled ocean ice sector model of the atlantic ocean the model simulations are conducted at eddy permitting resolution where dense shelf water production dominates over open ocean convection in forming aabw freshwater and heat flux perturbations are applied independently and have contradictory surface responses with increased upper ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes aabw transport into the abyssal ocean reduces under both flux changes with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60 s through inclusion of shelf sourced aabw a process absent from most current generation climate models cooling and freshening of dense source water is facilitated via reduced on shelf off shelf exchange flow such cooling is propagated to the abyssal ocea

what sets the mean transport through drake passage twelve experiments with two coarse resolutions of a global ocean model using a variety of surface forcings are analyzed to address the question of what sets the mean transport through drake passage seven of the experiments do not have an active sea ice model but the remaining five do previous theories have suggested that the drake passage transport is governed by the cape horn sverdrup transport or alternatively is proportional to the square root of the meridional ekman transport at the latitude of drake passage the results presented here do not support either of these theories the drake passage transport depends quite strongly on the isopycnal diffusivity parameter in the model and less strongly on the background vertical diffusivity and horizontal viscosity parameters however when the magnitudes of these parameters are fixed the results show a very strong correlation between drake passage transport and both the strength of the meridional ekman transport at the latitude of drake passage and the thermohaline circulation off the antarctic shelf the relationships are monotonic but not linear the best estimate is that the meridional ekman transport drives 100 sv of drake passage transport while the remaining 30 sv are driven by the global thermohaline circulation

circumpolar response of southern ocean eddy activity to a change in the southern annular mode 1 analysis of satellite altimeter data reveals anomalously high eddy kinetic energy eke in the antarctic circumpolar current acc during the period 2000 2002 around 2 3 years earlier 1998 the circumpolar eastward wind stress as quantified by the southern annular mode sam showed a significant positive peak and we have shown previously that the acc peaked around 1998 in response an eddy resolving ocean model is used to investigate the delay between wind forcing and the eddy response and demonstrates that the lag is due to the time taken to influence the deep circulation of the acc winds over the southern ocean have shown a strong climatic increase over the past few decades if this increase in winds is also reflected as an increase in eddy activity as our analysis suggests it might then the increased poleward heat flux may have played a significant role in the observed warming of the southern ocean citation meredith m p and a m hogg 2006 circumpolar response of southern ocean eddy activity to a change in the southern annular mode geophys res lett 33 l16608 doi 10 1029 2006gl026499

role of residual overturning for the sensitivity of southern ocean isopycnal slopes to changes in wind forcing abstractthe antarctic circumpolar current plays a central role in the ventilation of heat and carbon in the global ocean in particular the isopycnal slopes determine where each water mass outcrop

reshaping the antarctic circumpolar current via antarctic bottom water export abstractzonal momentum input into the antarctic circumpolar current acc by westerly winds is ultimately removed via topographic form stress induced by large bathymetric features that obstruct the path of the current these bathymetric features also support the export of antarctic bottom water aabw across the acc via deep geostrophically balanced northward flows these deep geostrophic currents modify the topographic form stress implying that changes in aabw export will alter the ocean bottom pressure and require a rearrangement of the acc in order to preserve its zonal momentum balance a conceptual model of the acc momentum balance is used to derive a relationship between the volume export of aabw and the shape of the sea surface across the acc s standing meanders this prediction is tested using an idealized eddy resolving acc antarctic shelf channel model that includes both the upper and lower cells of the southern ocean meridional overturning circulation using two different topographic configu

analysis of three dimensional strain modified uniform distributions andalusite fabrics from a granite aureole a method of analysing finite strain from an initial random distribution of lines or planes is presented the shape of the strain ellipsoid is mapped to the eigenvalue ratios of an orientation matrix the method is used to interpret the andalusite fabrics from the aureole of the ardara granite county donegal ireland the high flattening strains determined in the aureole are accompanied by oblate strains in the granite margins and support the concept of deformation produced by expansion of the pluton

the force balance of the southern ocean meridional overturning circulation the southern ocean so limb of the meridional overturning circulation moc is characterized by three vertically stacked cells each with a transport of about 10 sv sv 10 6 m 3 s 1 the buoyancy transport in the so is dominated by the upper and middle moc cells with the middle cell accounting for most of the buoyancy transport across the antarctic circumpolar current a southern ocean state estimate for the years 2005 and 2006 with 1 6 resolution is used to determine the forces balancing this moc diagnosing the zonal momentum budget in density space allows an exact determination of the adiabatic and diapycnal components balancing the thickness weighted residual meridional transport it is found that to lowest order the transport consists of an eddy component a directly wind driven component and a component in balance with mean pressure gradients nonvanishing time mean pressure gradients arise because isopycnal layers intersect topography or the surface in a circumpolar integral leading to a largely geostrophic moc even in the latitude band of drake passage it is the geostrophic water mass transport in the surface layer where isopycnals outcrop that accomplishes the poleward buoyancy transport

a method for the determination of the initial shapes of deformed xenoliths in granitoids abstract measuring strain from deformed xenoliths is problematic due mainly to the large initial shape variations of these markers a method is described which allows mean initial shape to be determined for a number of xenolith populations by displaying their logarithmic ranges log rfmax log rfmax on a range diagram the diagram contains a check on validity and allows bad samples to be recognised xenolith data from a deformed granite in northwest ireland is analysed using the method

isopycnal momentum budget of the antarctic circumpolar current in the fine resolution antarctic model abstract the momentum budget of the antarctic circumpolar current acc is analyzed using data from the fine resolution antarctic model fram using density as a vertical coordinate since density is approximately conserved on streamlines this steady budget is balanced volume heat and salt budgets are also computed within density layers although these remain time dependent the leading order momentum balance is of approximate equality of the form drag on the top and bottom surfaces of each undulating density layer but because density layers outcrop at the surface this does not imply that the surface wind stress is simply transferred downward by form drag without change of amplitude restricting attention to the net form drag on a layer this is found to be balanced by coriolis force and if the layer outcrops the amount of wind stress put into the layer where it outcrops between 40 and 60 of the density layers at any latitude outcrop somewhere at the surface so that wind stress can be moved d