enhanced turbulence and energy dissipation at ocean fronts the ocean surface boundary layer mediates air sea exchange in the classical paradigm and in current climate models its turbulence is driven by atmospheric forcing observations at a 1 kilometer wide front within the kuroshio current indicate that the rate of energy dissipation within the boundary layer is enhanced by one to two orders of magnitude suggesting that the front rather than the atmospheric forcing supplied the energy for the turbulence the data quantitatively support the hypothesis that winds aligned with the frontal velocity catalyzed a release of energy from the front to the turbulence the resulting boundary layer is stratified in contrast to the classically well mixed layer these effects will be strongest at the intense fronts found in the kuroshio current the gulf stream and the antarctic circumpolar current all of which are key players in the climate system

a comparison of measured and wind derived ekman transport at 11 n in the atlantic ocean abstract a comparison of measured and wind derived ageostrophic transport is presented from a zonal transect spanning the atlantic ocean along 11 n the transport per unit depth shows a striking surface maximum that decays to nearly zero at a depth of approximately 100 m we identify this flow in the upper 100 m as the ekman transport the sustained values of wind stress and the penetration depth of the ekman transport reported here are considerably greater than in previous observations which were made in conditions of light winds the transport of 12 0 5 5 106 m3 s 1 calculated from the difference of geostrophic shear and shear measured by an acoustic doppler current profiler is in good agreement with that estimated from the shipboard winds 8 8 1 9 106 m3 s 1 and from climatology 13 5 0 3 106 m3 s 1 qualitatively the horizontal distribution of the wind driven flow was best predicted by the shipboard winds the cumulative transport increased linearly over the western three fourths of t

potential vorticity structure in the north atlantic western boundary current from underwater glider observations abstractpotential vorticity structure in two segments of the north atlantic s western boundary current is examined using concurrent high resolution measurements of hydrography and velocity from gliders spray gliders occupied 40 transects across the loop current in the gulf of mexico and 11 transects across the gulf stream downstream of cape hatteras cross stream distributions of the ertel potential vorticity and its components are calculated for each transect under the assumptions that all flow is in the direction of measured vertically averaged currents and that the flow is geostrophic mean cross stream distributions of hydrographic properties potential vorticity and alongstream velocity are calculated for both the loop current and the detached gulf stream in both depth and density coordinates differences between these mean transects highlight the downstream changes in western boundary current structure as the current increases its transport downstream upper layer potential vorticity is generall

fluid dynamics of oceanic thermocline ventilation abstract a flux form of the potential vorticity pv equation is applied to study the creation and transport of potential vorticity in an ocean gyre generalized pv fluxes j vectors and the associated pv flux fines are used to map the creation by buoyancy forcing of pv in the mixed layer and its transport as fluid is subducted through the base of the mixed layer into the thermocline the pv flux lines can either close on themselves recirculation or begin and end on the boundaries ventilation idealized thermocline solutions are diagnosed using j vectors which vividly illustrate the competing process of recirculation through western boundary currants and subduction from the surface potential vorticity flux vectors are then used to quantify the flux of mass passing invisidly through a surface across which potential vorticity changes discontinuously but at which potential density and velocity are continuous such a surface might be the base of the oceanic mixed layer or in a meteorological context

effects of wind on convection in strongly and weakly baroclinic flows with application to the labrador sea abstract large buoyancy loss driving deep convection is often associated with a large wind stress that is typically omitted in simulations of convection here it is shown that this omission is not justified when overturning occurs in a horizontally inhomogeneous ocean in strongly baroclinic flows convective mixing is influenced both by the background horizontal density gradient and by the across front advection of buoyancy due to wind the former process known as slantwise convection results in deeper convection while the effect of wind depends on the relative orientation of wind with respect to the baroclinic front for the case of the labrador sea wintertime winds act to destabilize the baroclinic labrador current causing a buoyancy removal roughly one third as large as the air sea buoyancy loss simulations using a nonhydrostatic numerical model initialized and forced with observed fields from the labrador sea show how the combination of wind and lateral gradients can result in significant convec

gravitational symmetric and baroclinic instability of the ocean mixed layer abstract a hierarchy of hydrodynamical instabilities controlling the transfer of buoyancy through the oceanic mixed layer is reviewed if a resting ocean of horizontally uniform stratification is subject to spatially uniform buoyancy loss at the sea surface then gravitational instability ensues in which buoyancy is drawn from depth by upright convection but if spatial inhomogeneities in the ambient stratification or the forcing are present as always exist in nature then horizontal density gradients will be induced and within a rotation period horizontal currents in thermal wind balance with those gradients will be set up within the mixed layer there are two important consequences on the convective process upright convection will become modified by the presence of the thermal wind shear fluid parcels are exchanged not along vertical paths but rather along slanting paths in symmetric instability theoretical considerations suggest that this slantwise convection sets the potential vorticity of th

potential vorticity dynamics of tropical instability vortices abstracttropical instability vortices tivs in the equatorial pacific exhibit energetic horizontal and vertical circulation characterized by regions of high rossby number and low richardson number their strong anticyclonic vorticity and vertical shear can influence the broader scale circulation by driving lateral mixing and vertical exchange between the ocean surface and interior the authors use a set of nested high resolution simulations of the equatorial pacific with a finest grid size of 3 km to examine the vortex dynamics associated with tiv core water formation tiv cores are characterized by low values of the ertel potential vorticity pv as the relative vorticity is anticyclonic with magnitude comparable to the local coriolis parameter a study of the variation of pv and other scalars along lagrangian fluid parcel tracks entering the tivs shows that the low pv water in their cores is a mix of equatorial undercurrent euc water and north equatorial counter current necc water the euc water

frontogenesis subduction and cross front exchange at upper ocean fronts the subduction of water at upper ocean fronts driven by internal instabilities is investigated a simple analytic model which assumes conservation of potential density and potential vorticity is used to characterize the expected structure of subducted parcels as a function of the surface layer depth and the length scale of the parcel parcels subducted from a deep surface layer will be characterized by anomalously low potential vorticity and anticyclonic circulation while shallow surface layers result in subducted parcels with high potential vorticity and cyclonic circulation a nonlinear isopycnal primitive equation model is used to demonstrate how baroclinic instability and the resulting frontogenesis force the subduction of parcels below and across the front frontogenesis generates vertical velocities of 0 30 m d and ageostrophic cross front flows of 3 to 5 cm s the subduction is achieved by the deep ageostrophic flow which carries water from the surface layer below and across the front the subducted parcels quickly develop a circulation through balanced adjustment anticyclonic for deep surface layers and cyclonic for shallow surface layers the characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices the horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer the total permanent subduction rate is estimated to be 20 m yr for frontal subduction driven by frontogenesis although local and temporary subduction can be much larger the subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices thus forming a baroclinic dipole that propagates at approximately 3 5 cm s and is capable of transporting the anomalous water properties far from the region of formation similarities with observations in the north atlantic subtropical convergence zone and near the gulf stream are discussed

intensification of ocean fronts by down front winds abstract many ocean fronts experience strong local atmospheric forcing by down front winds that is winds blowing in the direction of the frontal jet an analytic theory and nonhydrostatic numerical simulations are used to demonstrate the mechanism by which down front winds lead to frontogenesis when a wind blows down a front cross front advection of density by ekman flow results in a destabilizing wind driven buoyancy flux wdbf equal to the product of the ekman transport with the surface lateral buoyancy gradient destabilization of the water column results in convection that is localized to the front and that has a buoyancy flux that is scaled by the wdbf mixing of buoyancy by convection and ekman pumping suction resulting from the cross front contrast in vertical vorticity of the frontal jet drive frontogenetic ageostrophic secondary circulations ascs for mixed layers with negative potential vorticity the most frontogenetic ascs select a preferred cross front width and do not translate with

seasonal thermal fronts on the northern south china sea shelf satellite measurements and three repeated field surveys seasonal thermal fronts associated with wind driven coastal downwelling upwelling in the northern south china sea are investigated using satellite measurements and three repeated fine resolution mapping surveys in winter spring and summer the results show that vigorous thermal fronts develop over the broad shelf with variable widths and intensities in different seasons which tend to be approximately aligned with the 20 100 m isobaths driven by the prevailing winter summer monsoon the band shaped fronts were observed with a magnitude exceeding 0 1 degrees c km in the subsurface and accompanied by energetic coastal downwelling upwelling due to shoreward offshore ekman transport the downward upward tilting of seasonal thermoclines across the shelf exceeds 20 m significantly contributing to the development of thermal fronts over the shelf in addition the diagnostic analysis of potential vorticity pv suggests that the summer frontal activities induced by the coastal upwelling are more stable to convection and symmetric instabilities in comparison to the winter fronts associated with downwelling favorable monsoon forcing this is primarily due to their essential differences in the upper ocean stratification and horizontal buoyancy gradients arising from wind forcing at the same time the coastal currents are substantially regulated by the seasonal winds an expected lag correlation between the velocity from mooring measurements and alongshore wind stress is detected near the frontal region these results indicate that seasonal wind forcing plays an important role in the frontal activities and coastal water transport over the shelf

the subinertial momentum balance of the north atlantic subtropical convergence zone abstract the upper ocean to forcing by pressure gradients and wind stress is examined using observations from the frontal air sea interaction experiment a moored way acquired time series of winds upper ocean currents temperatures and salinities between winter and late spring in a region of the sargasso sea known for the presence of upper ocean fronts these fronts have timescales of 10 days and dominate current variance while winds varied with the 4 day timescale of passing weather systems employing a frequency domain regression model it is found that gestrophy accounts for most of the low frequency 100 h current variance in the seasonal pycnoclino but wind forced shear becomes important nearer the surface in particular currents oriented in the typical ne sw alonfront direction display geostrophic balance while those perpendicular to them do not wind forcing can produce geostrophic currents indirectly through ekman pumping and knowledge of the geostrophic shear is required to distinguish be

regional variations in the influence of mesoscale eddies on near surface chlorophyll eddies can influence biogeochemical cycles through a variety of mechanisms including the excitation of vertical velocities and the horizontal advection of nutrients and ecosystems both around the eddy periphery by rotational currents and by the trapping of fluid and subsequent transport by the eddy in this study we present an analysis of the influence of mesoscale ocean eddies on near surface chlorophyll chl estimated from satellite measurements of ocean color the influences of horizontal advection trapping and upwelling downwelling on chl are analyzed in an eddy centric frame of reference by collocating satellite observations to eddy interiors as defined by their sea surface height signatures the influence of mesoscale eddies on chl varies regionally in most boundary current regions cyclonic eddies exhibit positive chl anomalies and anticyclonic eddies contain negative chl anomalies in the interior of the south indian ocean however the opposite occurs the various mechanisms by which eddies can influence phytoplankton communities are summarized and regions where the observed chl response to eddies is consistent with one or more of the mechanisms are discussed this study does not attempt to link the observed regional variability definitively to any particular mechanism but provides a global overview of how eddies influence chl anomalies

mesoscale and submesoscale processes in the southeast atlantic and their impact on the regional thermohaline structure the turbulent processes in the cape basin the southeasternmost gate of the atlantic ocean play a key role in the transport and mixing of upper to intermediate water masses entering the area from the indian ocean making them especially relevant for the indo atlantic transfer of heat and salt in this paper two numerical simulations at different horizontal resolutions are used to study meso and submesoscale dynamics their phenomenology their evolution and their impact on the local water masses submesoscale processes seasonally affect both the upper and intermediate layers but there are clear dynamical differences between the two layers several types of instabilities underline this spatial and temporal variability near the surface mixed layer instabilities occur during winter while mesoscale driven instabilities as the symmetric type prevail in summer the connection between these two seasonal regimes is ensured in anticyclonic eddies and within the mixed layers by charney baroclinic instabilities involved in the local formation and subduction of mode water that we have dubbed as agulhas rings mode water intermediate depths are instead characterized by mesoscale mechanisms of density compensation and lateral stirring of the tracer variance triggering a significant filamentogenesis whose vertical scales are comparable to those mentioned in previous studies this leads to a particularly efficient mixing of antarctic intermediate waters of indian and atlantic origins lagrangian estimates highlight the new and significant role of fine scale structures in setting the water masses properties of upper and lower thermocline waters materializing the indo atlantic exchange and therefore potentially affecting the global ocean circulation

inferring the subduction rate and period over the north atlantic abstract the annual rate at which mixed layer fluid is transferred into the permanent thermocline that is the annual subduction rate sann and the effective subduction period x1d4af eff is inferred from climatological data in the north atlantic from its kinematic definition sann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer geostrophic velocity fields computed from the levitus climatology assuming a level of no motion at 2 5 km are used the vertical velocity at the base of the mixed layer is deduced from observed surface ekman pumping velocities and linear vorticity balance a plausible pattern of sann is obtained with subduction rates over the subtropical gyre approaching 100 m yr twice the maximum rate of ekman pumping the subduction period x1d4af eff is found by viewing subduction as a transformation process converting mixed layer fluid into stratified thermocline fluid the effective

restratification at a california current upwelling front part i observations abstracta coordinated survey between a subsurface lagrangian float and a ship towed triaxus profiler obtained detailed measurements of a restratifying surface intensified front above 30 m within

direct observations of the ekman balance at 10 n in the pacific abstract the wind driven transport and velocity structure are estimated from direct current measurements and geostrophic shears along a transpacific section at 10 n the total velocity field is dominated by the north equatorial current and its eddies while the shear features near inertial oscillations and a wind driven current spiral the section averaged cross track ageostrophic shear can be approximated as a slab layer 30 40 m thick with a velocity of 0 05 m s 1 overlying a sheared layer in which the velocity goes to zero below 80 m the resulting zonally integrated ageostrophic mass transport is 62 10 109 kg s 1 northward similar to the estimate of 52 10 109 kg s 1 predicted by the ekman balance using winds measured from the ship climatological winds yield similar transport the zonally averaged velocity relative to the top of the thermocline forms a clockwise spiral decaying with depth as has been found at higher latitudes this mean wind driven spiral results from the diurnal cycling o

a coastal current in winter 2 wind forcing and cooling of a coastal current east of cape cod 1 the combined effect of cooling and wind driven buoyancy flux wdbf on a buoyant coastal current east of cape cod is investigated using observations and process oriented numerical modeling theoretical considerations show that with the moderately strong surface density gradients observed in the outer cape cod coastal current wdbf can substantially exceed the buoyancy loss due to cooling especially during intense winter storms evidence of deep convection associated with strong negative wdbf during downwelling favorable winds is clearly seen in the moored observations a simplified two dimensional numerical model is used to illustrate the evolution of wind and buoyancy driven cross shelf overturning circulation in response to surface cooling and episodic storm events the simulation confirms that wdbf plays an important role in driving subduction of cold surface water at the offshore surface outcrop of the coastal current font the presence of the coastal current is also shown to block onshore ekman transport as a result the downwelling circulation in a cross shore plane is predicted to have a complex multicell structure in which exchange between the inner shelf and midshelf is restricted the downwelling circulation has a major impact on the cross shelf origin of cold dense shelf waters contributing to intermediate layers of the wilkinson basin of the gulf of maine

abrupt transitions in submesoscale structure in southern drake passage glider observations and model results abstractenhanced vertical velocities associated with submesoscale motions may rapidly modify mixed layer depths and increase exchange between the mixed layer and the ocean interior these dynamics are of particular importance in the southern ocean where the ventilation of many density classes occurs here we present results from an observational field program in southern drake passage a region preconditioned for submesoscale instability due to its strong mesoscale eddy field persistent fronts strong down front winds and weak vertical stratification two gliders sampled from december 2014 through march 2015 upstream and downstream of shackleton fracture zone sfz the acquired time series of mixed layer depths and buoyancy gradients enabled calculations of potential vorticity and classifications of submesoscale instabilities the regions flanking the sfz displayed remarkably different characteristics despite similar surface forcing mixed layer depths were nearly twice as deep and horizontal buoyancy

dynamical characterization of a low oxygen submesoscale coherent vortex in the eastern north atlantic ocean a submesoscale coherent vortex scv with a low oxygen core is characterized from underwater glider and mooring observations from the eastern tropical north atlantic north of the cape verde islands the eddy crossed the mooring with its center and a 1 month time series of the scv s hydrographic and upper 100 m currents structure was obtained about 45 days after and 100 km west the scv frontal zone was surveyed in high temporal and spatial resolution using an underwater glider satellite altimetry showed the scv was formed about 7 months before at the mauritanian coast the scv was located at 80 100 m depth its diameter was 100 km and its maximum swirl velocity 0 4 m s 1 a burger number of 0 2 and a vortex rossby number 0 15 indicate a flat lens in geostrophic balance mooring and glider data show in general comparable dynamical and thermohaline structures the glider in high spatial resolution the mooring in high temporal resolution surface maps of chlorophyll concentration suggest high productivity inside and around the scv the low potential vorticity pv core of the scv is surrounded by filamentary structures sloping down at different angles from the mixed layer base and with typical width of 10 20 km and a vertical extent of 50 100 m

idealized models of slantwise convection in a baroclinic flow intermediate or deep convection in a baroclinic flow occurs along slanted paths parallel to the alongflow absolute momentum surfaces these surfaces are principally tilted due to the vertical shear in velocity but can be further modified by a nonvertical axis of rotation an inviscid lagrangian parcel model using realistic parameters is utilized to illustrate qualitatively the different scenarios resulting from the combined action of inertial and gravitational forces acting on sinking parcels of dense fluid more quantitative results are derived from a series of numerical experiments using a zonally invariant high resolution nonhydrostatic model convection occuring in a flow with tilted absolute momentum surfaces will mix properties along these slanted surfaces this implies that the fluid can retain a weak vertical stratification while overturning and also more importantly that the evolution of the convective layer cannot be described in terms of one dimensional vertical mixing the authors show for conditions typical of the labrador sea that the convective layer depth difference between that estimated by mixing vertically and one obtained allowing for slantwise mixing can be greater than 100 m slantwise convection reaches deeper because of the reduced stratification along the slanted paths an alternative slantwise mixing scheme based on the assumption of zero potential vorticity of the convected fluid is proposed

localization of deep ocean convection by a mesoscale eddy abstract observations of open ocean deep convection indicate that it is a highly localized phenomenon occurring over areas of tens of kilometers in diameter the cause of this localization has been ascribed to preconditioning the local weakening of the stable density stratification associated with upwardly domed isopycnal surfaces in a surface intensified cyclonic circulation however most numerical and laboratory studies of localized convection have prescribed the localization artificially by confining the surface buoyancy loss to a circular disk in contrast in the numerical simulations described here deep convection forced by horizontally uniform buoyancy loss is localized within a region of initially weaker stratification than its surroundings the preconditioned region is associated with a cold core cyclonic eddy in geostrophic and cyclostrophic balance as in previous studies of disk shaped cooling the localized convection region undergoes baroclinic instability at late times causing the br

enhancement in vertical fluxes at a front by mesoscale submesoscale coupling oceanic frontal instabilities are of importance for the vertical exchange of properties in the ocean submesoscale o 1 rossby number dynamics are particularly relevant for inducing the vertical and lateral flux of buoyancy and tracers in the mixed layer but how these couple with the stratified pycnocline is less clear observations show surface fronts often persist beneath the mixed layer here we use idealized three dimensional model simulations to show how surface fronts that extend deeper into the pycnocline invoke enhanced vertical fluxes through the coupling of submesoscale and mesoscale instabilities we contrast simulations in which the front is restricted to the mixed layer with those in which it extends deeper for the deeper fronts we examine the effect of density stratification on the vertical coupling our results show deep fronts can dynamically couple the mixed layer and pycnocline on time scales that increase with the peak stratification beneath the mixed layer eddies in the interior generate skew fluxes of buoyancy and tracer oriented along isopycnals thus providing an adiabatic pathway for the interior to interact with the mixed layer at fronts the vertical enhancement of tracer fluxes through the mesoscale submesoscale coupling described here is thus relevant to the vertical supply of nutrients for phytoplankton in the ocean a further implication for wind forced fronts is that the vertical structure of the stream function characterizing the exchange between the interior and the mixed layer exhibits significant qualitative differences compared to a linear combination of existing parameterizations of submesoscale eddies in the mixed layer and mesoscale eddies in the interior the discrepancies are most severe within the mixed layer suggesting a potential role for ekman layer dynamics absent in existing submesoscale parameterizations

parameterization of mixed layer eddies part i theory and diagnosis ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean in this paper a parameterization for the restratification driven by finite amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal the streamfunction is proportional to the product of the horizontal density gradient the mixed layer depth squared and the inertial period hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence in this paper the parameterization is theoretically motivated confirmed to perform well for a wide range of mixed layer depths rotation rates and vertical and horizontal stratifications it is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification two companion papers discuss the numerical implementation and the climate impacts of this parameterization

centrifugal and symmetric instability during ekman adjustment of the bottom boundary layer flow along isobaths of a sloping lower boundary generates an across isobath ekman transport in the bottom boundary layer when this ekman transport is down the slope it causes convective mixing much like a downfront wind in the surface boundary layer destroying stratification and potential vorticity in this manuscript we show how this can lead to the development of a forced centrifugal or symmetric instability regime where the potential vorticity flux generated by friction along the boundary is balanced by submesoscale instabilities that return the boundary layer potential vorticity to zero this balance provides a strong constraint on the boundary layer evolution which we use to develop theory that explains the evolution of the boundary layer thickness the rate at which the instabilities extract energy from the geostrophic flow field and the magnitude and vertical structure of the dissipation finally we show using theory and a high resolution numerical model how the presence of centrifugal or symmetric instabilities alters the time dependent ekman adjustment of the boundary layer delaying ekman buoyancy arrest and enhancing the total energy removed from the balanced flow field submesoscale instabilities of the bottom boundary layer may therefore play an important largely overlooked role in the energetics of flow over topography in the ocean

finescale vertical structure of the upwelling system off southern peru as observed from glider data abstractthe upwelling system off southern peru has been observed using an autonomous underwater vehicle a slocum glider during october november 2008 nine cross front sections have been carried out across an intense upwelling cell near 14 s during almost two months profiles of temperature salinity and fluorescence were collected at less than 1 km resolution between the surface and 200 m depth estimates of alongshore absolute geostrophic velocities were inferred from the density field and the glider drift between two surfacings in the frontal region salinity and biogeochemical fields displayed cross shore submesoscale filamentary structures throughout the mission those features presented a width of 10 20 km a vertical extent of 150 m and appeared to propagate toward the shore they were steeper than isopycnals and kept an aspect ratio close to f n the inverse of the prandtl ratio these filamentary structures may be interpreted mainly as a manifestation of submesoscale turbulence through sti

destruction of potential vorticity by winds the destruction of potential vorticity pv at ocean fronts by wind stress driven frictional forces is examined using pv flux formalism and numerical simulations when a front is forced by downfront winds that is winds blowing in the direction of the frontal jet a nonadvective frictional pv flux that is upward at the sea surface is induced the flux extracts pv out of the ocean leading to the formation of a boundary layer thicker than the ekman layer with nearly zero pv and nonzero stratification the pv reduction is not only active in the ekman layer but is transmitted through the boundary layer via secondary circulations that exchange low pv from the ekman layer with high pv from the pycnocline extraction of pv from the pycnocline by the secondary circulations results in an upward advective pv flux at the base of the boundary layer that scales with the surface nonadvective frictional pv flux and that leads to the deepening of the layer at fronts forced by both downfront winds and a destabilizing atmospheric buoyancy flux f b the critical parameter that determines whether the wind or the buoyancy flux is the dominant cause for pv

observations of near inertial internal gravity waves radiating from a frontal jet abstractshipboard adcp and towed ctd measurements are presented of a near inertial internal gravity wave radiating away from a zonal jet associated with the subtropical front in the north pacific three dimensional spatial surveys indicate persistent alternating shear layers sloping downward and equatorward from the front as a result depth integrated ageostrophic shear increases sharply equatorward of the front the layers have a vertical wavelength of about 250 m and a slope consistent with a wave of frequency 1 01f they extend at least 100 km south of the front time series confirm that the shear is associated with a downward propagating near inertial wave with frequency within 20 of f a slab mixed layer model forced with shipboard and ncep reanalysis winds suggests that wind forcing was too weak to generate the wave likewise trapping of the near inertial motions at the low vorticity edge of the front can be ruled out because of the extension of the features well south of it instead the authors

surface vertical pv fluxes and subtropical mode water formation in an eddy resolving numerical simulation abstract subtropical mode waters are characterized by low potential vorticity pv and so the mechanisms by which pv is extracted from the ocean by air sea interaction are of great relevance to our understanding of how mode waters are formed this study analyzes those mechanisms by comparing the magnitude and spatial patterns of surface pv fluxes of diabatic and frictional origin in a high resolution numerical simulation of the north atlantic the model resolves mesoscale eddies and exhibits realism in the volume and regional distribution of subtropical mode water both in the annual mean and seasonal cycle it is found that the diabatic and mechanic fluxes of pv through the sea surface are of similar amplitude locally but their spatial structures are very different the diabatic pv flux has a large scale pattern that reflects that of air sea heat fluxes directed from the ocean to the atmosphere along and to the south of the separated gulf stream in contrast the mechanical pv flux because of its dependence on horizontal surface density gradients exhibits much smaller scales but embedded within a coherent large scale pattern when mapped over the north atlantic subtropical mode water edw outcropping region the diabatic pv flux pattern is found to be directed out of the ocean everywhere whereas the mechanical pv fluxes exhibits small scale patterns of both sign the amplitude of the diabatic pv fluxes is found to be at least one order of magnitude larger than the mechanical pv fluxes demonstrating the overwhelming importance of diabatic processes in creating mode waters finally we note that the large scale climatological patterns and magnitudes of both diabatic and mechanical pv flux mapped over the edw outcropping region are very similar to patterns obtained from coarse grained ocean state estimates that do not resolve the eddy field

mixed layer instabilities and restratification abstract the restratification of the oceanic surface mixed layer that results from lateral gradients in the surface density field is studied the lateral gradients are shown to be unstable to ageostrophic baroclinic instabilities and slump from the horizontal to the vertical these instabilities which are referred to as mixed layer instabilities mlis differ from instabilities in the ocean interior because of the weak surface stratification spatial scales are o 1 10 km and growth time scales are on the order of a day linear stability analysis and fully nonlinear simulations are used to study mlis and their impact on mixed layer restratification the main result is that mlis are a leading order process in the ml heat budget acting to constantly restratify the surface ocean climate and regional ocean models do not resolve the scales associated with mlis and are likely to underestimate the rate of ml restratification and consequently suffer from a bias in sea surface temperatures and ml depths in a

late holocene east antarctic climate trends from ice core and lake sediment proxies a high resolution record of evaporation for the last 650 years was derived from the diatom salinity signal preserved in a sediment core taken from ace lake vestfold hills 68 28 s 78 11 e antarctica the seasonal oxygen isotope signal preserved in an ice core from law dome 66 44 s 112 50 e antarctica revealed a high resolution summer temperature record for the same time period the two proxies show highly correlated behaviour despite having significantly different climatic response mechanisms and a large geographic separation the correlation observed between proxies based on such differing processes and analytical method ologies provides not only a climate record for the past 650 years that is both robust and regionally representative of coastal east antarctica but also confirmation of the utility of reconstructions using these methods

potential vorticity budgets in the north atlantic ocean abstractthis paper focuses on potential vorticity pv budgets in the north atlantic with an emphasis on the wind driven subtropical gyre since pv is the key dynamical variable of the wind driven circulation these budgets are important to understand pv is a conservative quantity on isopycnals and can only enter or exit through the boundaries like the lateral topography or the surface the latter fluxes are diagnosed and tested against the evolution of the pv content in an isopycnal layer the former are computed using the bernoulli function the essential result is found for all the tested isopycnals and the dominant feature of pv is recirculation with very little added at the surface or the boundaries density coordinates are well suited to understanding pv circulation a novel technique for computing the bernoulli function is proposed the bernoulli function is governed by a simple elliptic equation and the solutions demonstrate the dominant contribution of pv advection

rapid changes in mixed layer stratification driven by submesoscale instabilities and winds 1 submesoscale eddies generated by baroclinic instability of upper ocean fronts lead to rapid restratification of the mixed layer on a time scale of days this restratification can be opposed by a down front wind stress acting in the direction of the geostrophic velocity that drives a surface ekman flow from the dense side to the light side of the front to arrest the slumping of isopycnals a scaling diagnostic is suggested to determine whether the effect of eddies or wind dominates under different conditions using a numerical model we investigate the juxtaposition of submesoscale eddies and down front winds acting on the mixed layer by estimating the eddy induced overturning stream function in the mixed layer we separate the along and cross isopycnal fluxes of buoyancy associated with submesoscale mixed layer eddies and demonstrate the need for parameterization of the advective along isopycnal flux though the cross front transport of buoyancy induced by the down front component of the wind opposes restratification by mixed layer eddies it becomes diminished as the eddies and growth of the frontal instability disrupt alignment between the wind and frontal axis

mesoscale to submesoscale transition in the california current system part i flow structure eddy flux and observational tests in computational simulations of an idealized subtropical eastern boundary upwelling current system similar to the california current a submesoscale transition occurs in the eddy variability as the horizontal grid scale is reduced to o 1 km this first paper in a series of three describes the transition in terms of the emergent flow structure and the associated time averaged eddy fluxes in addition to the mesoscale eddies that arise from a primary instability of the alongshore wind driven currents significant energy is transferred into submesoscale fronts and vortices in the upper ocean the submesoscale arises through surface frontogenesis growing off upwelled cold filaments that are pulled offshore and strained in between the mesoscale eddy centers in turn some submesoscale fronts become unstable and develop submesoscale meanders and fragment into roll up vortices associated with this phenomenon are a large vertical vorticity and rossby number a large vertical velocity relatively flat horizontal spectra contrary to the prevailing view of mesoscale dynamics a large vertical buoyancy flux acting to restratify the upper ocean a submesoscale energy conversion from potential to kinetic a significant spatial and temporal intermittency in the upper ocean and material exchanges between the surface boundary layer and pycnocline comparison with available observations indicates that submesoscale fronts and instabilities occur widely in the upper ocean with characteristics similar to the simulations

mesoscale and submesoscale effects on mixed layer depth in the southern ocean the authors gratefully acknowledge support from the natural environment research council awards ne j010472 1 and ne j009857 1

frontogenesis and frontal arrest of a dense filament in the oceanic surface boundary layer the evolution of upper ocean currents involves a set of complex poorly understood interactions between submesoscale turbulence e g density fronts and filaments and coherent vortices and smaller scale boundary layer turbulence here we simulate the lifecycle of a cold dense filament undergoing frontogenesis in the presence of turbulence generated by surface stress and or buoyancy loss this phenomenon is examined in large eddy simulations with resolved turbulent motions in large horizontal domains using sim 10 10 grid points steady winds are oriented in directions perpendicular or parallel to the filament axis due to turbulent vertical momentum mixing cold filaments generate a potent two celled secondary circulation in the cross filament plane that is frontogenetic sharpens the cross filament buoyancy and horizontal velocity gradients and blocks ekman buoyancy flux across the cold filament core towards the warm filament edge within less than a day the frontogenesis is arrested at a small width approx 100 text m primarily by an enhancement of the turbulence through a small submesoscale horizontal shear instability of the sharpened filament followed by a subsequent slow decay of the filament by further turbulent mixing the boundary layer turbulence is inhomogeneous and non stationary in relation to the evolving submesoscale currents and density stratification the occurrence of frontogenesis and arrest are qualitatively similar with varying stress direction or with convective cooling but the detailed evolution and flow structure differ among the cases thus submesoscale filament frontogenesis caused by boundary layer turbulence frontal arrest by frontal instability and frontal decay by forward energy cascade and turbulent mixing are generic processes in the upper ocean

submesoscale processes at shallow salinity fronts in the bay of bengal observations during the winter monsoon abstractlateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central bay of bengal during the winter monsoon are explored using high resol

frontal dynamics in a california current system shallow front 1 frontal processes and tracer structure 1 the three dimensional dynamics in a shallow front are examined using density and current data from two surveys 100 km offshore of monterey bay california survey 1 is forced by down front winds and both surveys have considerable cross front density gradients and flow curvature the maximum rossby numbers on the dense side reached maxima of 0 60 in survey 1 and 0 45 in survey 2 downwelling occurs in regions of confluence frontogenesis associated with potential vorticity pv change and thermal wind imbalance streamers of particulate matter and pv are advected southeastward by the frontal jet and downward nonlinear ekman currents advect dense water over light water in the presence of down front winds which leads to upwelling along the front and downwelling on the light side of the front at sites of active ageostrophic secondary circulation asc induced by frontogenesis or ekman effects the observed cross front ageostrophic velocity is consistent with the diagnosed vertical velocity furthermore in survey 2 ageostrophic divergence may play an important role at the curved front presumably counteracting quasi geostrophic frontogenesis due to isopycnal confluence downward frictional vertical pv flux below the surface extracts pv from the pycnocline and reinforces the frontogenetic vertical pv flux pv destruction at the surface is inferred from a low pv anomaly below the mixed layer in survey 2 since the magnitude of the frontogenetic asc is only twice the magnitude of ekman suction external forcing may have a considerable impact on the vertical heat and pv fluxes

the contributions of atmosphere and ocean to north atlantic subtropical mode water volume anomalies abstract subtropical mode water stmw in the western north atlantic ocean or eighteen degree water edw as it is commonly known is formed near the gulf stream in the wintertime is dissipated by mixing and is removed by subduction the ability of edw to store and discharge large quantities of heat over periods of several years contributes to the memory of the climate system to complement the 2 year field and modeling program of the clivar mode water dynamics experiment climode with a perspective of interannual to decadal variability 1985 2007 we used a simple box model to hindcast observed edw volume anomalies in two regions one in which edw is formed and an adjacent region of subducted edw estimates of the relative contributions of heat flux anomalies vertical mixing from ekman advection mixing and circulation are examined using proxy variables derived from winds sea surface temperature hydrographic data and altimetric sea level the importance of each process is evaluated by its contribution to observed edw volume anomalies in two regions the study produced some robust conclusions 1 anomalies of formation by surface heat fluxes are clearly reflected in edw volume anomalies with some contributions by ekman advection 2 of the newly formed edw about 65 is lost by mixing and about 35 is transferred to the subducted region 3 mixing losses are well parameterized by the meandering of the nearby gs and 4 transfer and losses from the subducted region can be parameterized by the geostrophic surface flow

wind waves and fronts frictional effects in a generalized ekman model abstractocean currents in the surface boundary layer are sensitive to a variety of parameters not included in classic ekman theory including the vertical structure of eddy viscosity finite boundary layer depth baroclinic pressure gradients and surface waves these parameters can modify the horizontal and vertical flow in the near surface ocean making them of first order significance to a wide range of phenomena of broad practical and scientific import in this work an approximate green s function solution is found for a model of the frictional ocean surface boundary layer termed the generalized ekman or turbulent thermal wind balance the solution admits consideration of general more physically realistic forms of parameters than previously possible offering improved physical insight into the underlying dynamics closed form solutions are given for the wind driven flow in the presence of coriolis stokes shear a result of the surface wave field and thermal wind shear arising from a baroclinic

subduction on the northern and southern flanks of the gulf stream abstract sections of temperature salinity dissolved oxygen and velocity were made crossing the gulf stream in late january 2006 to investigate the role of frontal processes in the formation of eighteen degree water edw the subtropical mode water of the north atlantic the sections were nominally perpendicular to the stream and measured in a lagrangian frame by following a floating spar buoy drifting in the gulf stream s warm core during the survey edw was isolated from the mixed layer by the stratified seasonal pycnocline suggesting that edw was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted 64 70 w however in two of the sections the seasonal pycnocline in the core of the gulf stream was broken by an intrusion of cold fresh weakly stratified water nearly saturated in oxygen that appears to have been subducted from the surface mixed layer north of the stream the intrusion was identified in three of the sections in profi

downfront winds over buoyant coastal plumes abstractdownfront or downwelling favorable winds are commonly found over buoyant coastal plumes it is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport spatial extent and stability of the plumes in the present study the interaction of the ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets the approach makes use of an idealized numerical model and scaling theory it is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near zero average potential vorticity q for weak to moderate winds the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratificati

submesoscale instabilities in mesoscale eddies submesoscale processes have been extensively studied in observations and simulations of fronts recent idealized simulations show that submesoscale instabilities also occur in baroclinic mesoscale cyclones and anticyclones the instabilities in the anticyclone grow faster and at coarser grid resolution than in the cyclone the instabilities lead to larger restratification in the anticyclone than in the cyclone the instabilities also lead to changes in the mean azimuthal jet around the anticyclone from 2 km resolution but a similar effect only occurs in the cyclone at 0 25 km resolution a numerical passive tracer experiment shows that submesoscale instabilities lead to deeper subduction in the interior of anticyclonic than cyclonic eddies because of outcropping isopycnals extending deeper into the thermocline in anticyclones an energetic analysis suggests that both vertical shear production and vertical buoyancy fluxes are important in anticyclones but primarily vertical buoyancy fluxes occur in cyclones at these resolutions the energy sources and sinks vary azimuthally around the eddies caused by the asymmetric effects of the ekman buoyancy flux glider transects of a mesoscale anticyclone in the tasman sea show that water with low stratification and high oxygen concentrations is found in an anticyclone in a manner that may be consistent with the model predictions for submesoscale subduction in mesoscale eddies

buoyancy and wind driven convection at mixed layer density fronts in this study the influence of a geostrophically balanced horizontal density gradient on turbulent convection in the ocean is examined using numerical simulations and a theoretical scaling analysis starting with uniform horizontal and vertical buoyancy gradients convection is driven by imposing a heat loss or a destabilizing wind stress at the upper boundary and a turbulent layer soon develops for weak lateral fronts turbulent convection results in a nearly homogeneous mixed layer ml whose depth grows in time for strong fronts a turbulent layer develops but this layer is not an ml in the traditional sense because it is characterized by persistent horizontal and vertical gradients in density the turbulent layer is however nearly homogeneous in potential vorticity pv with a value near zero using the pv budget a scaling for the depth of the turbulent low pv layer and its time dependence is derived that compares well with numerical simulations two dynamical regimes are identified in a convective layer near the surface turbulence is generated by the buoyancy loss at the surface below this layer turbulence is generated by a symmetric instability of the lateral density gradient this work extends classical scalings for the depth of turbulent boundary layers to account for the ubiquitous presence of lateral density gradients in the ocean the new results indicate that a lateral density gradient in addition to the surface forcing can affect the stratification and the rate of growth of the surface boundary layer