topographic enhancement of eddy efficiency in baroclinic equilibration abstractthe processes that determine the depth of the southern ocean thermocline are considered in existing conceptual frameworks the thermocline depth is determined by competition between the mean and eddy heat transport with a contribution from the interaction with the stratification in the enclosed portion of the ocean using numerical simulations this study examines the equilibration of an idealized circumpolar current with and without topography the authors find that eddies are much more efficient when topography is present leading to a shallower thermocline than in the flat case a simple quasigeostrophic analytical model shows that the topographically induced standing wave increases the effective eddy diffusivity by increasing the local buoyancy gradients and lengthening the buoyancy contours across which the eddies transport heat in addition to this local heat flux intensification transient eddy heat fluxes are suppressed away from the topography especially upstream indicating that local

topographic vorticity generation submesoscale instability and vortex street formation in the gulf stream meanders and eddies are routinely observed in the gulf stream along the south atlantic bight we analyze here the instability processes that lead to the formation of submesoscale eddies on the cyclonic side of the gulf stream at the exit of the florida straits using very high resolution realistic simulations the positive relative vorticity and potential vorticity on the cyclonic side of the gulf stream are strongly intensified in the straits due to topographic drag along the continental slope the bottom drag amplifies the cyclonic shear by generating large positive vertical vorticity values within the sloped turbulent bottom boundary layer downstream from the straits the current becomes unstable to horizontal shear instability rolls up and forms a street of submesoscale vortices the vortices expand as they propagate northward along the shelf where they can generate large vertical displacements and enhance cross shelf exchanges

island wakes in deep water abstract density stratification and planetary rotation distinguish three dimensional island wakes significantly from a classical fluid dynamical flow around an obstacle a numerical model is used to study the formation and evolution of flow around an idealized island in deep water i e with vertical island sides and surface intensified stratification and upstream flow focusing on wake instability coherent vortex formation and mesoscale and submesoscale eddy activity in a baseline experiment with strong vorticity generation at the island three types of instability are evident centrifugal barotropic and baroclinic sensitivities are shown to three nondimensional parameters the reynolds number re rossby number ro and burger number bu the dependence on re is similar to the classical wake in its transition to turbulence but in contrast the island wake contains coherent eddies no matter how large the re value when re is large enough the shear layer at the island is so narrow that the ver

eddy topography interactions and the fate of the persian gulf outflow the persian gulf feeds a warm and salty outflow in the gulf of oman northern arabian sea the salt climatological distribution is relatively smooth in the gulf of oman and the signature of a slope current carrying salty waters is difficult to distinguish hundreds of kilometers past the strait of hormuz in contrast to other outflows of the world ocean this study focuses on the mechanisms involved in the spreading of persian gulf water pgw in the gulf of oman using a regional primitive equation numerical simulation the authors show that the dispersion of pgw occurs through a regime that is distinct from for example the one responsible for the mediterranean outflow dispersion the background mesoscale eddy field is energetic and participates actively to the spreading of pgw remotely formed eddies propagate into the gulf of oman and interact with the topography leading to submesoscales formation and pgw shedding eddy topography interactions are isolated in idealized simulations and reveal the formation of intense frictional boundary layers generating submesoscale coherent vortices scvs interactions take place at depths encompassing the pgw depth thus scvs trap pgw and contribute to its redistribution from the boundaries to the interior of the gulf of oman the overall efficiency of these processes is confirmed by a strong contribution of eddy salt fluxes in the interior of the basin and is quantified using particle statistics it is found to be a highly dispersive regime with an approximated eddy diffusivity of inline image

analysis of time lapse travel time and amplitude changes to assess reservoir compartmentalization fluid depletion within a compacting reservoir can lead to significant stress and strain changes and potentially severe geomechanical issues both inside and outside the reservoir we extend previous research of time lapse seismic interpretation by incorporating synthetic near offset and full offset common midpoint reflection data using anisotropic ray tracing to investigate uncertainties in time lapse seismic observations the time lapse seismic simulations use dynamic elasticity models built from hydro geomechanical simulation output and a stress dependent rock physics model the reservoir model is a conceptual two fault graben reservoir where we allow the fault fluid flow transmissibility to vary from high to low to simulate non compartmentalized and compartmentalized reservoirs respectively the results indicate time lapse seismic amplitude changes and travel time shifts can be used to qualitatively identify reservoir compartmentalization due to the high repeatability and good quality of the time lapse synthetic dataset the estimated travel time shifts and amplitude changes for near offset data match the true model subsurface changes with minimal errors a 1d velocity strain relation was used to estimate the vertical velocity change for the reservoir bottom interface by applying zero offset time shifts from both the near offset and full offset measurements for near offset data the estimated p wave velocity changes were within 10 of the true value however for full offset data time lapse attributes are quantitatively reliable using standard time lapse seismic methods when an updated velocity model is used rather than the baseline model

configuration of a southern ocean storm track abstractdiagnostics of ocean variability that reflect and influence local transport properties of heat and chemical species vary by an order of magnitude along the southern ocean s antarctic circumpolar current acc topographic hotspots are important regions of localized transport anomalies this study uses a primitive equation channel model to investigate the structure of eddy kinetic energy eke one measure of variability in an oceanic regime a storm track approach emphasizes the importance of stationary eddies which result from flow interactions with topography on setting eke distributions the influence of these interactions extends far downstream of the topography and impacts eke patterns through localized convergence and divergence of heat unlike for zonal averages local contributions to the stationary fluxes from terms that integrate to zero in a zonal average are important the simulations show a strong sensitivity of the zonal structure as well as the distribution and amplitude of sta

submesoscale instability and generation of mesoscale anticyclones near a separation of the california undercurrent abstractthe california undercurrent cuc flows poleward mostly along the continental slope it develops a narrow strip of large negative vertical vorticity through the turbulent boundary layer and bottom stress in several downstream locations the current separates aided by topographic curvature and flow inertia in particular near point sur ridge south of monterey bay when this happens the high vorticity strip undergoes rapid instability that appears to be mesoscale in eddy resolving simulations but is substantially submesoscale with a finer computational grid the negative relative vorticity in the cuc is larger than the background rotation f and ertel potential vorticity is negative this instigates ageostrophic centrifugal instability the submesoscale turbulence is partly unbalanced has elevated local dissipation and mixing and leads to dilution of the extreme vorticity values farther downstream the submesoscale activity abates and the remaining eddy motions exhibit an upscale organizati

the velocity and vorticity structure of the agulhas current at 32 s the full depth velocity field of the agulhas current the western boundary current of the southwest indian ocean has been directly measured using a lowered acoustic doppler current profiler ladcp fifteen combined conductivity temperature depth ctd and ladcp stations were occupied across the current at 32 s off the east coast of south africa in february march 1995 as part of the world ocean circulation experiment indian ocean program the deep velocity structure of the agulhas current was found to be very different than previously described using geostrophic estimates in particular ladcp results reveal a v shaped pattern for the level of no motion across the agulhas current and an agulhas undercurrent is observed flowing equatorward below 800 m depth directly beneath the surface core of the poleward flowing agulhas current comparisons of direct and geostrophic velocities suggest that the velocity structure of the agulhas current is essentially geostrophic below about 200 m depth where differences are generally less than estimated errors above these depths the ladcp measured shears and geostrophic shears exhibit differences and velocities can diverge shipboard adcp data support ladcp results near the surface indicating that both instruments are measuring real signals the geostrophic and ladcp measured shears are well matched away from the surface and a depth averaged fit below 200 m between them exhibits small standard deviations the geostrophic volume transport of the agulhas current as referenced to ladcp is 73 sv and is just 3 different from the direct ladcp transport estimate the most recent full depth estimate of agulhas current transport in the literature is given by toole and warren 1993 who estimate a geostrophic volume transport of 85 sv the baroclinic velocity structures from their section and from this work are similar indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved agulhas undercurrent combined ctd ladcp data are very useful for potential vorticity analysis since velocity and density measurements are simultaneous and coincident the vorticity structure of the agulhas current suggests the presence of a mixing boundary at middepths similar to features observed in the gulf stream the feature appears to be associated with a sharp upturn in isopycnals coincident with the foot of the continental slope the boundary inhibits cross stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of red sea water inshore of antarctic intermediate waters in the same density layer despite intense cross stream shears

stratification effects in a bottom ekman layer a stratified bottom ekman layer over a nonsloping rough surface is studied using a three dimensional unsteady large eddy simulation to examine the effects of an outer layer stratification on the boundary layer structure when the flow field is initialized with a linear temperature profile a three layer structure develops with a mixed layer near the wall separated from a uniformly stratified outer layer by a pycnocline with the free stream velocity fixed the wall stress increases slightly with the imposed stratification but the primary role of stratification is to limit the boundary layer height ekman transport is generally confined to the mixed layer which leads to larger cross stream velocities and a larger surface veering angle when the flow is stratified the rate of turning in the mixed layer is nearly independent of stratification so that when stratification is large and the boundary layer thickness is reduced the rate of veering in the pycnocline becomes very large in the pycnocline the mean shear is larger than observed in an unstratified boundary layer which is explained using a buoyancy length scale u n z this length scale leads to an explicit buoyancy related modification to the log law for the mean velocity profile a new method for deducing the wall stress based on observed mean velocity and density profiles is proposed and shows significant improvement compared to the standard profile method a streamwise jet is observed near the center of the pycnocline and the shear at the top of the jet leads to local shear instabilities and enhanced mixing in that region despite the fact that the richardson number formed using the mean density and shear profiles is larger than unity

lee wave generation rates in the deep ocean using the world s largest data set of in situ ocean current measurements combined with a high resolution topography roughness data set we use a model assisted hierarchical clustering methodology to estimate the global lee wave generation rate at the ocean floor our analysis suggests that internal wave generation contributes 0 75 0 19 tw 2 standard deviation to the oceanic energy budget but with a strong dependence on the brunt vaisala buoyancy frequency climatology used this estimate is higher than previous calculations and suggests that internal wave generation may be a much more significant contributor to the global oceanic mechanical energy budget than had previously been assumed our results imply that lee wave generation and propagation may be a dominant sink of at least half and potentially the overwhelming majority of ocean surface wind work on the geostrophic circulation

a natural example of crystal plastic deformation enhancing the incorporation of water into quartz abstract water content of quartz in and around a greenschist facies mylonitic shear zone located in the western adirondacks was analyzed by micro ftir spectroscopy the shear zone is within a pegmatitic dike which cuts across a granitic gneiss the thickness of the shear zone varies along strike from 15 cm wide and encompassing all of the pegmatite dike at its northern most exposure to 5 cm wide approximately 10 m south along strike microstructures including quartz ribbons and recrystallized grains indicate quartz and feldspar within the mylonite underwent dislocation creep infrared spectral analysis was carried out using a nicolet micro ftir on mylonitic quartz ribbons pegmatitic quartz and gneissic quartz a small aperture size 56 m by 50 m for the ir beam allowed optically clear regions of the quartz grains to be analyzed without any contribution from grain boundaries the smallest dimension of the quartz ribbons is 0 3 mm whereas the pegmatitic quartz has a grain size of 3 to 5 cm results show mylonitic quartz ribbons contain the most water 320 h 10 6 si average range of 50 to 1120 h 10 6 si pegmatite quartz contains much less water 30 h 10 6 si average range of 20 40 h 10 6 si and the gneissic quartz contained an intermediate amount 200 h 10 6 si average range of 20 to 870 h 10 6 si these data indicate that water was preferentially incorporated into the deformed quartz ribbons

generation of internal waves in the deep ocean by tides 1 a direct computation of the tidal generation of internal waves over the global ocean is presented it is based on linear wave theory and high resolution data for the bottom topography the geographical distribution of the energy flux from tides to internal waves is determined with a spatial resolution of a few kilometers the total flux over the area with a depth greater than 500 m is found to be 1 2 tw the greatest uncertainties of the computation are due to unresolved topography and to nonlinear effects caused by supercritical bottom slope

the impact of finite amplitude bottom topography on internal wave generation in the southern ocean direct observations in the southern ocean report enhanced internal wave activity and turbulence in akilometer thicklayeraboveroughbottomtopographycollocatedwiththedeep reachingfrontsoftheantarctic circumpolar current linear theory corrected for finite amplitude topography based on idealized twodimensional numerical simulations has been recently used to estimate the global distribution of internal wave generation by oceanic currents and eddies the global estimate shows that the topographic wave generation is a significant sink of energy for geostrophic flows and a source of energy for turbulent mixing in the deep ocean however comparison with recent observations from the diapycnal and isopycnal mixing experiment in the southern ocean shows that the linear theory predictions and idealized two dimensional simulations grossly overestimate the observed levelsof turbulent energy dissipation thisstudypresentstwo and three dimensional realistic topography simulations of internal lee wave generation from a steady flow interacting with topography with parameters typical of drake passage the results demonstrate that internal wave generation at threedimensional finite bottom topography is reduced compared to the two dimensional case the reduction is primarilyassociatedwithfinite amplitudebottomtopographyeffectsthatsuppressverticalmotionsandthusreduce the amplitude of the internal waves radiated from topography the implication of these results for the global leewave generation is discussed

symmetric instability in the gulf stream abstract analyses of wintertime surveys of the gulf stream gs conducted as part of the clivar mode water dynamic experiment climode reveal that water with negative potential vorticity pv is commonly found within the surface boundary layer sbl of the current the lowest values of pv are found within the north wall of the gs on the isopycnal layer occupied by eighteen degree water suggesting that processes within the gs may contribute to the formation of this low pv water mass in spite of large heat loss the generation of negative pv was primarily attributable to cross front advection of dense water over light by ekman flow driven by winds with a down front component beneath a critical depth the sbl was stably stratified yet the pv remained negative due to the strong baroclinicity of the current suggesting that the flow was symmetrically unstable a large eddy simulation configured with forcing and flow parameters based on the observations confirms that the observed structure of the sbl is consistent with the dynamics of symmetric instability si forced by wind and surface cooling the simulation shows that both strong turbulence and vertical gradients in density momentum and tracers coexist in the sbl of symmetrically unstable fronts si is a shear instability that draws its energy from geostrophic flows a parameterization for the rate of kinetic energy ke extraction by si applied to the observations suggests that si could result in a net dissipation of 33 mw m 2 and 1 mw m 2 for surveys with strong and weak fronts respectively the surveys also showed signs of baroclinic instability bci in the sbl namely thermally direct vertical circulations that advect biomass and pv the vertical circulation was inferred using the omega equation and used to estimate the rate of release of available potential energy ape by bci the rate of ape release was found to be comparable in magnitude to the net dissipation associated with si this result points to an energy pathway where the gs s reservoir of ape is drained by bci converted to ke and then dissipated by si and its secondary instabilities similar dynamics are likely to be found at other strong fronts forced by winds and or cooling and could play an important role in the energy balance of the ocean circulation

submesoscale vortical wakes in the lee of topography abstractan idealized framework of steady barotropic flow past an isolated seamount in a background of constant stratification with frequency n and rotation with coriolis parameter f is used to

on eddy viscosity energy cascades and the horizontal resolution of gridded satellite altimeter products motivated by the recent interest in ocean energetics the widespread use of horizontal eddy viscosity in models and the promise of high horizontal resolution data from the planned wide swath satellite altimeter this paper explores the impacts of horizontal eddy viscosity and horizontal grid resolution on geostrophic turbulence with a particular focus on spectral kinetic energy fluxes p k computed in the isotropic wavenumber k domain the paper utilizes idealized two layer quasigeostrophic qg models realistic highresolution ocean general circulation models and present generation gridded satellite altimeter data adding horizontal eddy viscosity to the qg model results in a forward cascade at smaller scales in apparent agreement with results from present generation altimetry eddy viscosity is taken to roughly represent coupling of mesoscale eddies to internal waves or to submesoscale eddies filtering the output of either the qg or realistic models before computing p k also greatly increases the forward cascade such filtering mimics the smoothing inherent in the construction of present generation gridded altimeter data it is therefore difficult to say whether the forward cascades seen in present generation altimeter data are due to real physics represented here by eddy viscosity or to insufficient horizontal resolution the inverse cascade at larger scales remains in the models even after filtering suggesting that its existence in the models and in altimeterdata is robust however themagnitudeofthe inverse cascadeis affectedbyfiltering suggestingthat the wide swath altimeter will allow a more accurate determination of the inverse cascade at larger scales as well as providing important constraints on smaller scale dynamics

mesoscale variability and its seasonality in the solomon and coral seas high resolution 4 km climatologically forced ocean model validated by altimetry and glider data is used to characterize the vertical and seasonal variations of mesoscale variability in the solomon and coral seas the highest eddy kinetic energy eke in the southwest pacific is found subsurface in the gulf of papua at the depth of the low latitude western boundary current velocity core variability associated with the western boundary current especially downstream of topographic obstacles dominates the thermocline and intermediate level eke by contrast surface eke is generally enhanced in the southwest pacific with a pronounced annual cycle that has a phase difference between small scale and large scale variability large mesoscale eddies account for most of the surface eke and its annual modulation the june maximum of surface eke in the solomon sea and the december maximum in the coral sea can be accounted for by local instabilities of large scale currents small mesoscale eddies predominantly cyclonic are abundant in late winter august to september coinciding with the timing of deepest mixed layer and strongest vertical velocity they contribute to the spatially uniform surface enhanced eke over the top 100 m not associated with the western boundary current in the coral sea small mesoscale eddies are generated mostly by open ocean surface baroclinic instabilities while in the land bounded solomon sea near boundary instabilities and topographic generation are also important

gulf stream dynamics along the southeastern u s seaboard abstractthe gulf stream strongly interacts with the topography along the southeastern u s seaboard between the straits of florida and cape hatteras the dynamics of the gulf stream in this region is investigated with a set of realistic very high resolution simulations using the regional ocean modeling system roms the mean path is strongly influenced by the topography and in particular the charleston bump there are significant local pressure anomalies and topographic form stresses exerted by the bump that retard the mean flow and steer the mean current pathway seaward the topography provides through bottom pressure torque the positive input of barotropic vorticity necessary to balance the meridional transport of fluid and close the gyre scale vorticity balance the effect of the topography on the development of meanders and eddies is studied by computing energy budgets of the eddies and the mean flow the baroclinic instability is stabilized by the slope everywhere except past the bump the flow

topographic influence on submesoscale dynamics in the southern ocean topography influences the circulation in the southern ocean generating stationary meanders in the lee of topographic features triggering hot spots of mesoscale eddy kinetic energy and modifying jets and fronts however the relationship between topography and submesoscale flows with length scales of order 1 10 km has not yet been explored the first submesoscale resolving 1 80 resolution ocean model with realistic topography of the indian sector of the southern ocean is used to investigate this interaction the results show spatial inhomogeneity in submesoscale activity that is correlated with topography topographic influence is primarily indirect topography controls mesoscale flows which in turn generate submesoscale activity mesoscale eddy kinetic energy and strain rate can be used to first order to infer submesoscale vertical velocity implying a possible route to parameterise submesoscale activity in coarser resolution models

generation of submesoscale vortices a new mechanism small scale intense vortices composed of water with anomalous values of quasi conservative tracers have been observed in many oceans they are particularly numerous in the beaufort gyre of the arctic ocean possible mechanisms responsible for the generation of these features are examined conservation of temperature salinity and nutrients requires that beaufort sea vortices be created from water that flows into the arctic ocean from the pacific ocean across the broad shallow chukchi sea much of this water passes through barrow canyon near point barrow alaska observations in the canyon show the generation of vortices of approximately the same size as those observed in the beaufort sea barrow canyon is therefore a likely site for the generation of beaufort sea vortices most observed beaufort sea vortices have a central potential vorticity near zero because of a relative vorticity close to their chukchi sea source water is typically stratified with little relative vorticity so that its potential vorticity is nonzero a key step in the generation of the vortices is therefore the reduction of the potential vorticity of chukchi sea water to nearly zero it is proposed that this occurs through the action of frictional torques occurring as the chukchi sea water flows through barrow canyon rather than througji the action of vertical mixing because this flow is driven by a downstream pressure gradient it is concentrated on the right hand wall of the canyon by the coriolis force the frictional torques thus preferentially reduce the potential vorticity of the fluid anticyclonic vortices are therefore preferentially generated it is suggested that a similar scenario may play a role in the generation of submesoscale vortices containing water from the mediterranean and labrador seas

centrifugal instability and mixing in the california undercurrent abstracta regional numerical study of the california current system near monterey bay california is conducted using both hydrostatic and nonhydrostatic models frequent sighting of strong anticyclones cuddies have occurred in the area and previous studies have identified monterey bay as an apparent region of strong unbalanced flow generation here by means of a downscaling exercise a domain just downstream of point sur is analyzed and argued to be a preferred site of diapycnal mixing the scenario suggested by the simulations involves the generation of negative relative vorticity in a bottom boundary layer of the california undercurrent on the continental shelf break at point sur the current separates from the coast and moves into deep waters where it rapidly develops finite amplitude instabilities these manifest as isopycnal overturnings but in contrast to the normal kelvin helmholtz paradigm for mixing this study argues that the instability is primarily centrifugal the evidence for this com