the adiabatic pole to pole overturning circulation abstractthe adiabatic pole to pole cell of the residual overturning circulation roc is studied in a two hemisphere semienclosed basin with a zonally reentrant channel occupying the southernmost eighth of the domain three different models of increasing complexity are used a simple analytically tractable zonally averaged model a coarse resolution numerical model with parameterized eddies and an eddy resolving general circulation model two elements are found to be necessary for the existence of an adiabatic pole to pole cell 1 a thermally indirect wind driven overturning circulation in the zonally reentrant channel analogous to the deacon cell in the antarctic circumpolar current acc region and 2 a set of outcropping isopycnals shared between the channel and the semienclosed region of the northern hemisphere these points are supported by several computations varying the domain geometry the surface buoyancy distribution and the wind forcing all three models give results that are qualitat

eastern boundary contribution to the residual and meridional overturning circulations a model of the thermocline linearized around a specified stratification and the barotropic linear winddriven stommel solution is constructed the forcings are both mechanical the surface wind stress and thermodynamical the surface buoyancy boundarycondition the effects of diapycnal diffusivity and of eddy fluxesofbuoyancy parameterizedintermsofthelarge scalebuoyancygradient areincluded theeddyfluxes of buoyancy are especially important near the boundaries where they mediate the transport in and out of the narrow ageostrophic down upwelling layers the dynamics of these narrow layers can be replaced by effective boundary conditions on the geostrophically balanced flow the effective boundary conditions state that the residual flow normal to the effective coast vanishes the separate eulerian and eddy induced components may be nonzero this formulation conserves the total mass and the total buoyancy while permitting an exchangebetweentheeulerianandeddytransportofbuoyancy withinthedown upwellinglayers inturn this exchange allows buoyancy gradients along all solid boundaries including the eastern one a special focus is on the buoyancy along the eastern and western walls since east west buoyancy difference determines the meridional overturning circulation the inclusion of advection of buoyancy by the barotropic flow allows a meaningful distinction between the meridional and the residual overturning circulations while retaining the simplicity of a linear model the residual flow in both meridional and zonal directions reveals how the subsurface buoyancy distribution is established and in particular how the meridional buoyancy gradient is reversed at depth in turn the horizontal buoyancy gradient maintains stacked counterrotating cells in the meridional and residual overturning circulations quantitative scaling arguments are given for each of these cells which show how the buoyancy forcing the wind stress and the diapycnal and eddy diffusivities as well as the other imposed parameters affect the strength of the overturn

what sets the strength of the middepth stratification and overturning circulation in eddying ocean models abstract the processes maintaining stratification in the oceanic middepth between approximately 1000 and 3000 m are explored using an eddy resolving general circulation model composed of a two hemisphere semienclosed basin with a zonal reentrant channel in the southernmost eighth of the domain the middepth region lies below the wind driven main thermocline but above the diffusively driven abyssal ocean here it is argued that middepth stratification is determined primarily in the model s antarctic circumpolar current competition between mean and eddy overturning in the channel leads to steeper isotherms and thus deeper stratification throughout the basin than would exist without the channel isotherms that outcrop only in the channel are nearly horizontal in the semienclosed portion of the domain whereas isotherms that also outcrop in the northern hemisphere deviate from horizontal and are accompanied by geostrophically balanced meridional transport a northern source of deep water water with temp

eastern boundary conditions and weak solutions of the ideal thermocline equations abstract it is argued that the ideal fluid thermocline equations have weak i e nondifferentiable solutions that satisfy no mass flux boundary conditions at the east this conclusion is based on a local analysis of the eastern corner of a subtropical gyre specifically we suppose that the surface density is uniform while the density on the eastern boundary is either uniform but different from that of the surface or else is linearly stratified the surface density is injected into the interior by specified ekman pumping in the absence f dissipation the resulting solution would have a discontinuity is smoothed and becomes an internal boundary layer which separates the light fluid originating at the surface from the denser fluid which abuts the eastern boundary this solution which is of the similarity type illustrates the applicability of solutions of the ideal fluid thermocline problem with discontinuities it is these discontinuities which enable ideal fluid solutions to satisfy eastern boun

petrogenesis of the early eocene i type granites in west yingjiang sw yunnan and its implication for the eastern extension of the gangdese batholiths abstract this study reports new zircon u pb and hf isotopes and whole rock elemental and sr nd isotopic data for the gneissic granite and leucogranite from the nabang metamorphic zone yingjiang area west yunnan sw china the metamorphosed granitoids crystallized during the early eocene 55 50 ma with zircons showing ehf t values from 11 to 5 3 and crustal model ages of 1 5 to 0 42 ga comparable to those of coeval i type granitoids from the gangdese batholith southern lhasa the rocks are characterized by metaluminous and weakly peraluminous hornblende bearing gneissic granites with a cnk 0 95 1 09 na2o k2o coupled with low initial sr isotopic values of 0 7049 0 7070 and high end t values from 1 1 to 7 1 the rocks were derived from crustal materials involving ancient upper crust sedimentary and juvenile mantle derived rocks together with available data from nearby regions it is proposed that the early eocene granitoids in the nabang and tengliang area can be correlated to the gangdese granitoids and represent the southeastward continuation of the magmatic arc resulting from the neotethyan subduction in southern tibet the petrogenesis of early eocene granitoids in western yunnan was probably related to the rollback of the subducting neotethyan slab that caused the remelting of the crustal materials newly modified by the underplated basaltic magma

dynamics of the atlantic meridional overturning circulation part 2 forcing by winds and buoyancy abstract recently schloesser et al 2012 explored the dynamics of the descending branch of meridional overturning circulations mocs by obtaining analytic solutions to a variable density 2 layer model vlom forced only by a surface buoyancy flux key processes involved are the poleward thickening of the upper layer along the eastern boundary due to kelvin wave adjustments the westward propagation of that coastal structure by rossby waves and their damping by mixing the resulting zonal pressure gradient causes the surface moc branch to converge into the northern basin near the eastern boundary in this paper we extend the schloesser et al 2012 study to include forcing by a zonal wind stress x y much of the paper is devoted to the derivation and analysis of analytic solutions to vlom for validation we also report corresponding numerical solutions to an ocean general circulation model ogcm solutions are obtained in a flat bottom rectangular basin confined to the northern hemisphere the buoyancy forcing relaxes upper ocean density to a prescribed profile y that increases polewards until it becomes as large as the deep ocean density at latitude y2 north of y2 then the ocean is homogeneous a 1 layer system the wind stress x drives subtropical and subpolar gyres and in our standard solution the latter extends north of y2 vertical diffusion is not included in vlom minimized in the ogcm consequently the moc is not closed by upwelling associated with interior diffusion but rather by flow through the southern boundary of the basin into a southern boundary sponge layer in the ogcm and solutions are uniquely determined by specifying the strength of that flow or the thermocline depth along the tropical eastern boundary solutions forced by x and differ markedly from those forced only by because water flows across y2 throughout the interior of the subpolar gyre not just near the eastern boundary in some of our solutions the strength of the moc s descending branch is determined entirely by this wind driven mechanism whereas in others it is also affected by rossby wave damping near the eastern boundary upwelling can occur in the interior of the subpolar gyre and in the western boundary layer providing shortcuts for the overturning circulation consequently there are different rates for the convergence of upper layer water near y 2 m n and the export of deep water south of the subpolar gyre m the latter being a better measure of large scale moc strength when western boundary upwelling occurs in our solutions m is independent of the diapycnal processes in the subpolar ocean

dynamics of downwelling in an eddy resolving convective basin the mean downwelling in an eddy resolving model of a convective basin is concentrated near the boundary where eddies are shed from the cyclonic boundary current into the interior it is suggested that the buoyancyforced downwelling in the labrador sea and the lofoten basin is similarly concentrated in analogous eddy formation regions along their eastern boundaries use of a transformed eulerian mean depiction of the density transport reveals the central role eddy fluxes play in maintaining the adiabatic nature of the flow in a nonperiodic region where heat is lost from the boundary current the vorticity balance in the downwelling region is primarily between stretching of planetary vorticity and eddy flux divergence of relative vorticity althougha narrowviscous boundarylayeris ultimately importantin closingtheregionalvorticitybudget this overall balance is similar in some ways to the diffusive viscous balance represented in previous boundary layer theories and suggests that the downwelling in convective basins may be properly represented in lowresolution climate models if eddy flux parameterizations are adiabatic identify localized regions of eddy formations and allow density to be transported far from the region of eddy formations

reconstructing the meridional overturning circulation from boundary densities and the zonal wind stress numerical models are used to test whether the meridional overturning circulation moc can be reconstructed from boundary densities and the wind stress in idealized model setups without topography the strength as well as the temporal and spatial variability of the moc cell can largely be reproduced from boundary densities and the zonal wind stress with added slopes along the meridional boundaries most of the depth averaged flow is missed and neither strength nor spatial structure of the moc is well reproduced however the temporal evolution of both moc and its estimate are similar in an eddy permitting model with realistic bottom topography the contribution of the depth averaged meridional flow to the moc is captured at some places while it is missed at others nevertheless boundary densities and the zonal wind stress allow the leading modes of the temporal and spatial moc variability to be reproduced on seasonal time scales most of the moc variability is due to the wind stress but changes in the boundary density affect the moc as well on interannual time scales the moc variability largely reflects changes in the boundary density generally the moc reconstructions are accurate when bottom velocities are small an assumption made in the reconstruction approach the results are relevant for estimates of both the modern and the past moc in the real ocean boundary densities can be obtained from measurements of temperature conductivity and pressure in the water column whereas past seawater densities have left their imprint in sea sediments

eddy driven buoyancy gradients on eastern boundaries and their role in the thermocline abstract it is demonstrated that eddy fluxes of buoyancy at the eastern and western boundaries maintain alongshore buoyancy gradients along the coast eddy fluxes arise near the eastern and western boundaries because on both coasts buoyancy gradients normal to the boundary are strong the eddy fluxes are accompanied by mean vertical flows that take place in narrow boundary layers next to the coast where the geostrophic constraint is broken these ageostrophic cells have a velocity component normal to the coast that balances the geostrophic mean velocity it is shown that the dynamics in these thin ageostrophic boundary layers can be replaced by effective boundary conditions for the interior flow relating the eddy flux of buoyancy at the seaward edge of the boundary layers to the buoyancy gradient along the coast these effective boundary conditions are applied to a model of the thermocline linearized around a mean stratification and a state of rest the linear model parameterizes the eddy fluxes of buoya

equilibrium structure and dynamics of the california current system this paper addresses the structure and dynamical mechanisms of regional and mesoscale physical variability in the subtropical northeast pacific ocean using the regional oceanic modeling system roms the model is configured with a u s west coast domain that spans the california current system ccs with a mesoscale horizontal resolution up to as fine as 3 5 km its mean seasonal forcing is by momentum heat and water fluxes at the surface and adaptive nudging to gyre scale fields at the open water boundaries its equilibrium solutions show realistic mean and seasonal states and vigorous mesoscale eddies fronts and filaments the level of eddy kinetic energy eke in the model is comparable to drifter and altimeter estimates in the solutions with sufficiently fine resolution because the model lacks nonseasonal transient forcing the authors conclude that the dominant mesoscale variability in the ccs is intrinsic rather than transiently forced the primary eddy generation mechanism is the baroclinic instability of upwelling alongshore currents there is progressive movement of meanseasonal currents and eddy energy offshore and downward into the oceanic interior in an annually recurrent cycle the associated offshore eddy heat fluxes provide the principal balance against nearshore cooling by mean ekman transport and upwelling the currents are highly nonuniform along the coast with important influences by capes and ridges in both maintaining mean standing eddies and launching transient filaments and fronts

downwelling in basins subject to buoyancy loss abstractrecent observational theoretical and modeling studies all suggest that the upper part of the downwelling limb of the thermohaline circulation is concentrated in strong currents subject to buoyancy loss near lateral boundaries this is fundamentally different from the traditional view that downwelling takes place in regions of deep convection even when resolving the buoyant boundary currents coarse resolution global circulation and climate models rely on parameterizations of poorly known turbulent mixing processes in this study the first direct measurements of downwelling occurring within a basin subject to buoyancy loss are obtained downwelling is observed near the basin s vertical wall within the buoyant boundary current flowing cyclonically around the basin although the entire basin is cooled large scale mean downwelling is absent in the basin interior laboratory rotating experiments are conducted to explicitly resolve the turbulent mixing due to convective plumes and the baroclinic ed

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

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

meridional overturning circulation stability and ocean feedbacks in a box model a box model of the inter hemispheric atlantic meridional overturning circulation is developed including a variable pycnocline depth for the tropical and subtropical regions the circulation is forced by winds over a periodic channel in the south and by freshwater forcing at the surface the model is aimed at investigating the ocean feedbacks related to perturbations in freshwater forcing from the atmosphere and to changes in freshwater transport in the ocean these feedbacks are closely connected with the stability properties of the meridional overturning circulation in particular in response to freshwater perturbations a separate box is used for representing the region north of the antarctic circumpolar current in the atlantic sector the density difference between this region and the north of the basin is then used for scaling the downwelling in the north these choices are essential for reproducing the sensitivity of the meridional overturning circulation observed in general circulation models and therefore suggest that the southernmost part of the atlantic ocean north of the drake passage is of fundamental importance for the stability of the meridional overturning circulation with this configuration the magnitude of the freshwater transport by the southern subtropical gyre strongly affects the response of the meridional overturning circulation to external forcing the role of the freshwater transport by the overturning circulation m ov as a stability indicator is discussed it is investigated under which conditions its sign at the latitude of the southern tip of africa can provide information on the existence of a second permanently shut down state of the overturning circulation in the box model m ov will be an adequate indicator of the existence of multiple equilibria only if salt advection feedback dominates over other processes in determining the response of the circulation to freshwater anomalies m ov is a perfect indicator if feedbacks other than salt advection are negligible

origin and evolution of the tengchong block southeastern margin of the tibetan plateau zircon u pb and lu hf isotopic evidence from the meta sedimentary rocks and intrusions abstract u pb ages and hf isotopic data from detrital zircons of gaoligongshan metamorphic complex and the carboniferous menghong group and igneous zircons from intrusions constrain the origin tectonic affinity east gondwana crustal evolution processes and the properties of regional high grade metamorphic rocks in the tengchong block of southeastern china three granites intruded into the tengchong block at 72 ma 114 ma and 122 ma detrital zircons range in age from archean to late ordovician for both the carboniferous menghong group and the gaoligongshan metamorphic complex analyses for these two units yield similar age clusters at 2 5 ga 1 6 ga 1 17 ga 0 95 ga and 0 65 0 5 ga as well as parallel hf isotopic distributions the protolith of the studied gaoligongshan complex in the tengchong block should deposit in the late paleozoic detrital zircon age distribution patterns of the carboniferous menghong group and the gaoligongshan complex show dominant younger grenvillian age peaks at 0 95 ga indicating the strong paleogeographic connection of the tengchong block with the indian margin the hf isotopic comparison of both detrital and igneous dated zircon shows that the tengchong block can be represented by the post archean indian continental margin after 250 ma the intensive magma events affected the region and considerable juvenile material accreted to the crust of the tengchong block

zircon u pb and hf isotope constraints from the ailao shan red river shear zone on the tectonic and crustal evolution of southwestern china abstract the ailao shan red river asrr shear zone one of the most prominent geologic strike slip shear zones in southeast asia consists of high grade metamorphic complexes that provide a rare opportunity to sample the mid crustal rocks along the western margin of the yangtze block of south china here we report combined in situ analyses of zircon u pb and lu hf isotopes of eight gneisses from the diancang shan and ailao shan segments of the asrr shear zone our zircon u pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 ma with peak ages at ca 770 350 and 240 ma suggesting three major periods of protolith formation the 350 ma zircons observed only in an orthogneiss from the diancang shan show uniform initial hf isotopic ratios marked by high and positive e hf t values from 16 to 10 this is in contrast with the other two zircon populations that are more common occurring in both the diancang shan and ailao shan and overall delineate very heterogeneous hf isotopic compositions with e hf t values ranging from 15 to 16 many zircons reveal distinctive core rim age variations zircon rims formed between ca 34 and 26 ma show significant variations in th u ratios 5 0 01 and e hf t values 14 to 10 that suggest complicated magmatic and metamorphic zircon overgrowth during the oligocene the presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies corresponding to mid crustal level p t conditions as also suggested by structural and petrologic data our data furthermore suggest that the asrr gneisses were not produced solely by shear heating during the tertiary strike slip faulting but are uplifted mid crustal basement rocks that formed essentially during two major stages of magmatism represented by the neoproterozoic kangding complex and late permian emeishan large igneous province as a result of the india asia collision these basement rocks underwent regional magmatic and metamorphic overprinting in the oligocene that based on our relevant work searle et al 2010 geosphere 6 1 23 predated the initial left lateral movement along the asrr shear zone the 350 ma protolith which requires a dominant depleted mantle input in the petrogenesis cannot be linked to any major magmatic events in south china but may be interpreted as part of the paleotethys remnants that were later incorporated into the asrr shear zone

growth and reworking of the early precambrian continental crust in the north china craton constraints from zircon hf isotopes abstract we synthesize more than 2600 hf isotope data on the archean paleoproterozoic zircons from the north china craton ncc recalculation of the data based on single stage and two stage hf model ages of the eastern block of the ncc shows peak ages of 3902 13 ma and 3978 18 ma respectively and also small peaks at 3 5 4 0 ga the majority of zircon ehf t values are positive suggesting the possibility of the crust and the mantle differentiation at ca 3 9 4 0 ga in the eastern block of the ncc most magmatic zircons from the whole of ncc have their hf model age range of 2 4 2 9 ga and the single stage model ages is cluster at 2698 4 ma whereas the two stage model ages concentrate at 2714 5 ma implying that the protoliths were juvenile crustal rocks the most prominent peak at 2 7 ga indicates that this period marks the most important stage of the crust mantle differentiation and crust formation of the ncc the widespread 2 5 ga rocks in the ncc and the absence of the 2 5 ga peaks in hf model ages are consistent with the partial melting and reworking of the juvenile rocks at 2 5 ga furthermore the 2 5 1 7 ga zircon hf isotope features are also related to the reworking of the crustal rocks our results from the integration of a large database suggest that the eastern block and the trans north china orogen have undergone similar crust mantle differentiation and magmatism leading to the conclusion that the essential cratonization of the north china took place at the end of neoarchean

seasonal overturning circulation in the red sea 2 winter circulation author posting 0american geophysical union 2014 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 119 2014 2263 2289 doi 10 1002 2013jc009331

mesoscale eddy buoyancy flux and eddy induced circulation in eastern boundary currents abstracta dynamical interpretation is made of the mesoscale eddy buoyancy fluxes in the eastern boundary currents off california and peru chile based on regional equilibrium simulations the eddy fluxes are primarily shoreward and upward across a swath several hundred kilometers wide in the upper ocean as such they serve to balance mean offshore air sea heating and coastal upwelling in the stratified interior the eddy fluxes are consistent with the adiabatic hypothesis associated with a mean eddy induced velocity advecting mean buoyancy and tracers furthermore with a suitable gauge choice the horizontal fluxes are almost entirely aligned with the mean horizontal buoyancy gradient consistent with the advective parameterization scheme of gent and mcwilliams the associated diffusivity is surface intensified matching the vertical stratification profile the fluxes span the across shore band of high eddy energy but their alongshore structure is unresolved because of sampling limitations in the sur

monitoring gulf stream transport by radar altimeter and inverted echo sounders abstract profiles of the gulf stream sea surface height ssh from the geosat altimeter were compared with thermocline depth measurements from inverted echo sounders ies for a period of about 19 months the correlation between ssh anomalies and thermocline depth anomalies at about 68 w was 0 92 to distinguish between agreement in the location of the gulf stream jet and agreement in the strength of the jet separate comparisons were made for jet location and the ssh difference across the gulf stream based on an error function fit to each geosat and ies profile the correlation between the jet centers from the two types of measurements was 0 97 and the correlation between the amplitudes of the error functions was 0 76 although the altimeter measures surface geostrophic velocity the high degree of vertical correlation in the gulf stream velocity profiles allows the altimeter to monitor transport fluctuations as well as changes in jet position based on this assessment of the accuracy of the altimeter

boundary mixing and the dynamics of three dimensional thermohaline circulations boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions the model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three dimensional dynamics of the thermohaline circulation for ky 5 20 3 1024 m2 s21 and lower the meridional overturning strength to great accuracy is proportional to meridional heat transport is proportional to the circulation 2 3 1 2 kk y y patterns resemble those from runs with uniform vertical mixing but vertical motion is entirely confined to the boundary regions near the western boundary there is upwelling everywhere near the eastern boundary there is a consistent pattern of downwelling above upwelling with downwelling reaching deeper at high latitudes this pattern is explained by convection and vertical advective diffusive balance underneath for ky 5 30 3 1024 m2 s21 and higher no steady solutions have been found the meridional overturning oscillates on a timescale of about 25 years a time averaged thermally direct overturning cell is not supported dynamically because convection extends longitudinally across the entire basin and upwelling near the western boundary does not lead to densities higher than at the eastern boundary assuming uniform upwelling in the west level isopycnals near the equator and level isopycnals along the eastern boundary south of the outcropping latitude permits the analytic determination of convection depth at the eastern wall and hence the density difference between the eastern and western walls this difference is at most one quarter the surface density difference between high and low latitudes and agrees in magnitude and latitudinal dependence with the numerical experiments scaling arguments estimate overturning strength as of the order of 10 3 106 m3 s21 and confirm the 2 3 power dependence onky the derivation also gives a dependence of overturning strength with latitude that agrees qualitatively with the numerical results the scaling for the dependence of meridional heat transport on latitude agrees well with the model results scaling for heat transport amplitude agrees less well but correctly predicts a weaker dependence on ky than maximum overturning