the role of turbulence in strong submesoscale fronts of the bay of bengal abstract the evolution of turbulence in a shallow submesoscale front is investigated using high resolution large eddy simulations les the model front has uniquely large cross front temperature and salinity differences motivated by recent observations in the bay of bengal the temperature and salinity differences are increased in a parametric study to examine their effects on the evolution of turbulence shear turbulence induced by the initially geostrophic jet causes the front to slump and an ageostrophic secondary circulation asc develops the asc is composed of two rotating currents a surface gravity current sgc with cold fresh water above a counter current cc jet with warm saline water virtual moorings on the two sides of the front are used to depict the evolution of stratification shear turbulent stresses turbulent heat and salt fluxes as well as turbulent kinetic energy tke budget terms a barrier layer with thermal inversion develops on both sides of the front while the mixed layer depth becomes shallower across the front turbulent production driven by vertical shear is the dominant source of tke on both sides of the front and horizontal shear production also contributes significantly to the tke generation the turbulent stresses heat and salt fluxes are enhanced on both sides of the front with subsurface warming in the thermal inversion layers as the cross front temperature and salinity differences increase among the cases the asc intensifies and the currents and the turbulence become stronger

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

linear instability of a mixed layer front the linear instabilities of a geostrophically balanced frontal jet are computed numerically in an effort to understand the origin of intense small scale cold core features observed in the north atlantic subtropical convergence zones during the frontal air sea interaction experiment fasinex a simple analytic continuously stratified basic state is used to represent the observed frontal structure the most unstable linear modes have e folding timescales of 2 3 days alongfront wavelengths near 70 km and significant asymmetry across the front the results provide indirect evidence that baroclinic instability of the frontal jet is a possible source of the cold core features observed during fasinex

wind driven ocean currents and ekman transport oceanographic engineering of the woods hole oceanographic institution and the massachusetts institute of technology woods hole ma 02543 oceanographers have long sought to verify the theoretical ekman transport relation which predicts that a steady wind stress acting together with the coriolis force will produce a transport of water to the right of the wind in situ measurements of wind and ocean currents provide a detailed view of this phenomenon by separating the wind driven current from the measured total current and by averaging over a long record it is found that the observed transport is consistent with theoretical ekman transport to within about 10 percent in this case the wind driven transport is strongly surface trapped with 95 percent occurring in the upper 25 meters as a result of fair summer weather

intensive surveys of the azores front 2 inferring the geostrophic and vertical velocity fields the geostrophic and vertical velocity fields are inferred using density and horizontal velocity data from three seasoar acoustic doppler current profiler surveys of the azores front the analysis is a two step procedure consisting of 1 objective analyses to reflect the observed length scales and 2 dynamical adjustments so that the density field is statically stable and the velocity field is in geostrophic balance the vertical velocity is inferred using a version of the quasi geostrophic omega equation in which the stratification is allowed to vary in the horizontal the resulting vertical velocity peaks at 2 10 4 m s 1 at 200 300 m there is a tendency for the denser water to the north of the front to be downwelled while the warmer water to the south is upwelled the implied heat flux exceeds 10 w m 2 near 100 m depth so the upper 100 m is warmed while the lower water column is cooled tending to stratify the upper ocean ageostrophic horizontal flow is toward the dense side of the front at the surface the dominant terms in the density balance are time rate of change and horizontal advection the inferred circulation cells may be an important mechanism of subduction in the upper ocean

direct observations of along isopycnal upwelling and diapycnal velocity at a shelfbreak front abstract by mapping the three dimensional density field while simultaneously tracking a subsurface isopycnal float direct observations of upwelling along a shelfbreak front were made on the southern flank of georges bank the thermohaline and bio optical fields were mapped using a towed undulating vehicle and horizontal velocity was measured with a shipboard acoustic doppler current profiler a subsurface isopycnal float capable of measuring diapycnal flow past the float was acoustically tracked from the ship the float was released near the foot of the shelfbreak front 95 100 m isobath and moved 15 km seaward as it rose from 80 to 50 m along the sloping frontal isopycnals over a 2 day deployment the float s average westward velocity was 0 09 m s 1 while a drifter drogued at 15 m released at the same location moved westward essentially alongfront at 0 18 m s 1 the float measured strong downward vertical velocities in excess of 0 02 m s 1 associated with propagation of internal tidal solibores in

intensive surveys of the azores front 1 tracers and dynamics the hypothesis that fronts are sites of active subduction is examined using density temperature salinity and horizontal velocity data from a trio of surveys of the azores front done in may 1991 and march 1992 these surveys were made using a seasoar equipped with a conductivity temperature depth profiler and a shipboard acoustic doppler current profiler the potential density and potential vorticity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop this apparently subducting isopycnal has a great deal of temperature and salinity variability horizontal velocity is nearly parallel to isopycnals indicating that the time rate of change and vertical advection must be small the thermal wind balance is observed to be valid especially in the region of the largest horizontal density gradients shear at the base of the mixed layer is likely due to near inertial motions the potential vorticity is dominated by the planetary vorticity except at the front where vertical shears the tilting term become large the tilting term acts to reduce the magnitude of the potential vorticity at the front in agreement with simple theoretical models the magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline

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

reynolds stresses and secondary circulation in a stratified rotating shear flow abstract a transverse secondary circulation should result from vertical mixing of momentum in a stratified shear flow influenced by the earth s rotation a rigidly mounted acoustic doppler current profiler was deployed at the bottom of juan de fuca strait a stratified estuary on the west coast of north america for 21 days to examine this dynamical process and its variation over a spring neap tidal cycle the data demonstrate a significant fortnightly modulation of the estuarine exchange flow with stronger along channel flow less dynamic stability and much greater cross strait currents at neap tide than at spring tide reliable estimates of the reynolds stresses in the stratified shear flow away from boundaries were also obtained they were small except for several days around neap tide when they were associated with times of reduced richardson number these stresses are not related in any simple way such as with a smooth positive eddy viscosity to the shear of the mean flow but do have a pattern

the subpolar front of the japan east sea part ii inverse method for determining the frontal vertical circulation abstract an inverse method for inferring vertical velocities from high resolution hydrographic velocity surveys is formulated and applied to observations collected at the subpolar front of the japan east sea jes taken during several cold air outbreaks the method is distinct from vertical velocity inferences based on the omega equation in that the driving mechanism for the ageostrophic flow is inferred rather than assumed and hence is particularly appropriate for application to wind or buoyancy forced upper ocean currents where friction mixing inertial superinertial motions or higher order effects can contribute along with shear strain of the geostrophic flow to force vertical motions the inferred vertical circulation at the subpolar front of the jes has amplitudes o 100 m day 1 compared to the 20 m day 1 vertical velocities predicted by the omega equation time dependent near inertial motions driven by the winds and modified by the vertical vorticity of the frontal jet appear to be the primary

surface roughness changes by finescale current gradients properties at multiple azimuth view angles abstractat times high resolution images of sea surface roughness can provide stunning details of submesoscale upper ocean dynamics as interpreted transformations of short scale wind waves by horizontal current gradients are responsible for those spectacular observations those observations could prove particularly useful to validate numerical ocean models that reach increasingly high resolutions focusing on surface roughness at optical wavelengths two steps have recently been performed in that direction first it was shown in a previous paper by rascle et al that surface roughness variations not only trace surface current divergence but also other characteristics of the current gradient tensor mainly the strain in the wind direction the wind direction with respect to the current gradient thus stands out as an important interpretative parameter the second step is the purpose of the present paper where the effect of the viewing direction is investigated to this end the authors discuss pairs of

mesoscale subduction at the antarctic polar front driven by baroclinic instability abstract a study of mesoscale subduction at the antarctic polar front pf is conducted by use of hydrographic data from a high resolution quasi synoptic survey of the front the geostrophic velocity and isopycnal potential vorticity pv fields are computed and the ageostrophic flow diagnosed from the semigeostrophic omega equation it is found that the ageostrophic circulation induced by baroclinic instability counteracts the frontogenesis and frontolysis effected by the confluence and difluence respectively of the geostrophic velocity field though the sense of the ageostrophic circulation is reversed repeatedly along the front the existence of pv gradients along isopycnals leads to a net cross front bolus transport in response to a reversal of this gradient with depth a necessary condition for the onset of baroclinic instability the bolus transport is northward at the protruding temperature minimum layer that characterizes the pf and southward above this net cross front overturning circul

ekman layers and two dimensional frontogenesis in the upper ocean a two dimensional modified semi geostrophic model is used to study the evolution of oceanic fronts in the presence of vertical mixing the parameterization used for mixing is an elevated constant value of vertical viscosity and friction hence the ekman layer acts as a surrogate mixed layer with however vertical shear allowed everywhere an initial condition representative of the observed fields in frontal air sea interaction experiment fasinex is used and its modification in the presence of vertical mixing alone is investigated without external forcing an ekman layer results because of the presence of vertical geostrophic shear at the surface the maximum density gradient moves toward the dense side of the front driven by this flow convergence of ekman flow results in downward bowing of the isopycnals beneath the surface expression of the front a feature reminiscent of density sections taken during fasinex this feature is not evident when a barotropic convergence field is applied in the absence of mixing or in fully nonlinear simulations of the evolution of fronts with baroclinic instability an analytic theory suggests that the maximum density gradient will increase over time when vertical mixing alone is present in the presence of negative uniform wind stress in the direction opposite to the surface geostrophic flow the front moves toward the denser water and the jet uniformly decreases in strength in the presence of a positive uniform wind in the direction of the surface geostrophic flow the surface jet initially weakens but then strengthens again as the wind driven ekman flow opposes the frictionally driven ekman flow and the cross front density gradient increases

the subpolar front of the japan east sea part iii competing roles of frontal dynamics and atmospheric forcing in driving ageostrophic vertical circulation and subduction abstractthe effects of wind stress and surface cooling on ageostrophic vertical circulation and subduction at the subpolar front of the japan east sea are investigated using a nonhydrostatic numerical model in experiments forced by wind and or cooling ageostrophic vertical circulation is enhanced relative to the unforced case both surface cooling and wind stress intensify the circulation by enhancing frontogenesis associated with frontal meandering winds further strengthen vertical motions by generating internal gravity waves downfront winds i e oriented along the frontal jet transport surface water from the denser to lighter side of the front causing it to migrate toward the region of higher stratification and enhancing the vertical mixing at the front this induces outcropping of isopycnals from the middle of the pycnocline along which surface water is subducted hence downfront winds enhance subduction down to the middle of the pycnocline but not beneath on the other hand cooling uplifts i

diagnosis of 3d vertical circulation in the upwelling and frontal zones east of hainan island china abstractusing the generalized omega equation and cruise observations in july 2012 this study analyzes the 3d vertical circulation in the upwelling region and frontal zone east of hainan island china the results show that there is a strong frontal zone in subsurface layer along the 100 m isobath which is characterized by density gradient of o 10 4 kg m 4 and vertical eddy diffusivity of o 10 5 10 4 m2 s 1 the kinematic deformation term sdef ageostrophic advection term sadv and vertical mixing forcing term smix are calculated from the observations their distribution patterns are featured by banded structure that is alternating positive negative alongshore bands distributed in the cross shelf direction correspondingly alternating upwelling and downwelling bands appear from the coast to the deep waters the maximum downward velocity reaches 5 10 5 m s 1 within the frontal zone accompanied by the maximum upward velocity of 7 10 5 m s 1 on two sides the dynamic diagnosis indicates that sadv

two dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large scale deformation 1 ageostrophic secondary circulation asc in frontal data sets have been previously diagnosed using only adiabatic versions of omega equations this study includes the effect of vertical mixing on such diagnostics modified quasi geostrophic and semigeostrophic balance equations are examined for the existing azores front data sets and idealized fronts in the presence of diabatic vertical mixing as well as a large scale deformation field the relevant dimensionless parameter representing the relative importance of large scale deformation and vertical mixing is the inverse of the ekman number based on the deformation rate h2 ao where is large scale deformation rate h is vertical scale of the front and ao is the maximum vertical eddy viscosity when this parameter is smaller than o 102 the influence of vertical mixing may be important other relevant parameters include burger number bu and prandtl number pr the vertical velocity in ascs caused by diabatic mixing in all diagnostics solutions except for large bu cases shows good agreement with the garrett loder scaling the structure of ascs is found to be sensitive to pr the diagnostics of ascs for the azores front data set indicates that the vertical mixing intensifies the asc in the upper ocean 100 m and should not be neglected when the large scale deformation rate is o 10 6 s 1 or smaller the magnitude of vertical velocity in ascs due to mixing contributions reaches o 10 m d 1 with downwelling on the dense side and upwelling on the less dense side of azores front

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

spatial characterization of agron southern spain 1988 1989 seismic series abstract during november 1988 to april 1989 a seismic swarm of about 400 earthquakes of magnitude between 0 4 and 4 0 occurred near the village of agron andalusia southern spain the pattern of the energy release versus time clearly shows that the dataset is representative of a seismic swarm where it is not observable a typical mainshock aftershock sequence basing on the gutenberg richter relationship the catalogue is considered complete down to a magnitude of 1 we revise the dataset by selecting those digital data which were recorded by at least four stations the revised dataset is relocated using a probabilistic approach based on a bayesian formalism the epicenters are barely aligned along two main structures trending nne ssw nw se and wnw ese respectively the focii depth is confined in the 5 20 km interval application of a clustering technique based on the principal component analysis pca of the local rupture ellipsoids depicts a spatial clustering of the seismicity according to five main planes of which only one coincides with the horizontal alignment of epicenters cross correlation among waveforms allows to individuate five different groups of similar events multiplets high resolution estimate of time differences among events belonging to the same multiplet and least squares adjustment of arrival times allow for precise relative locations from which we derive the corresponding rupture planes focal mechanisms are calculated for three of the five multiplets for these cases the planes obtained from the relative locations are coincident with one of the fault plane solutions the results are consistent with those previously inferred from pca and indicate a high variability in the spatial setting of fault planes over a short distance range these observations are consistent with a fragmentation of the upper crust into small 2 4 km tectonic blocks and indicate an heterogenous stress regime with different stresses acting simultaneously in adjacent areas

diagnosis of the three dimensional circulation in mesoscale features with large rossby number several diagnoses of three dimensional circulation using density and velocity data from a high resolution upper ocean seasoar and acoustic doppler current profiler adcp survey of a cyclonic jet meander and adjacent cyclonic eddy containing high rossby number flow are compared the q vector form of the quasigeostrophic omega equation two omega equations derived from iterated geostrophic intermediate models an omega equation derived from the balance equations and a vertical velocity diagnostic using a primitive equation model in conjunction with digital filtering are used to diagnose vertical and horizontal velocity fields the results demonstrate the importance of the gradient wind balance in flow with strong curvature high rossby number horizontal velocities diagnosed from the intermediate models the iterated geostrophic models and the balance equations which include dynamics between those of quasigeostrophy and the primitive equations are significantly reduced enhanced in comparison with the geostrophic velocities in regions of strong cyclonic anticyclonic curvature consistent with gradient wind balance the intermediate model relative vorticity fields are functionally related to the geostrophic relative vorticity field anticyclonic vorticity is enhanced and cyclonic vorticity is reduced the iterated geostrophic balance equation and quasigeostrophic vertical velocity fields are similar in spatial pattern and scale but the iterated geostrophic and to a lesser degree the balance equation vertical velocity is reduced in amplitude compared with the quasigeostrophic vertical velocity this reduction is consistent with gradient wind balance and is due to a reduction in the forcing of the omega equation through the geostrophic advection of ageostrophic relative vorticity the vertical velocity diagnosed using a primitive equation model and a digital filtering technique also exhibits reduced magnitude in comparison with the quasigeostrophic field a method to diagnose the gradient wind from a given dynamic height field has been developed this technique is useful for the analysis of horizontal velocity in the presence of strong flow curvature observations of the nondivergent ageostrophic velocity field measured by the adcp compare closely with the diagnosed gradient wind ageostrophic velocity

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

submesoscale impacts on horizontal and vertical transport in the gulf of mexico 1 impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions 1 and 5 km the focus is the northwestern gulf of mexico where the loop current eddies are surrounded by smaller vortices by increasing the horizontal resolution the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically and with them the vertical velocities inside the coherent eddies and at their peripheries increased vertical velocities for increasing resolution are associated to near inertial motions and they are not limited to the mixed layer but are found at all depths horizontal velocities on the contrary are similar lagrangian isobaric tracers are deployed close to the surface and at 100 m and three dimensional neutrally buoyant particles are released close to the surface at the base of the mixed layer and at 100 m the modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution close to the ocean surface however convergence zones generated by submesoscale ageostrophic motions and resolved at 1 km resolution influence the details of the tracer distributions vertical dispersion increases by several folds for increasing resolution at all depths explored with the largest differences found close to the surface therefore submesoscales processes play a fundamental role in driving vertical transport in eddy dominated flows both within and below the mixed layer for times comparable to the eulerian time scale

ageostrophic secondary circulation in a subtropical intrathermocline eddy abstractvertical motions play a key role in the enhancement of primary production within mesoscale eddies through the introduction of nutrients into the euphotic layer however the details of the vertical velocity field w driving these enhancements remain under discussion for the first time the mesoscale w associated with an intrathermocline eddy is computed and analyzed using in situ high resolution three dimensional 3d fields of density and horizontal velocity by resolving a generalized omega equation valid for high rossby numbers in the seasonal pycnocline the diagnosed w reveals a multipolar structure with upwelling and downwelling cells located at the eddy periphery in the main pycnocline w is characterized by a dipolar structure with downwelling velocities upstream of the propagation path and upwelling velocities downstream maximum values of w reach 6 4 m day 1 an observed enhancement of chlorophyll a at the eddy periphery coincides with the location of the upwelling and downwelling cells a

upwelling and convergence in the middle atlantic bight shelfbreak front convergent and upwelling circulation within the shelfbreak front in the middle atlantic eight are detected using a dye tracer injected into the bottom boundary layer at the foot of the front from the three day displacement and dispersion of two dye injections within the front we infer lagrangian isopycnal diapycnal velocities and diffusivities of 2 x 10 2 m s 4 x 10 6 m s and 9 m 2 s 6 x 10 6 m 2 s these results substantiate model predictions of chapman and lentz 1994 and previous dye tracer observations by houghton 1997

ageostrophic secondary circulation at a submesoscale front and the formation of gravity currents abstractlarge eddy simulations are performed to investigate the development of the ageostrophic secondary circulation asc and associated transport in a submesoscale front based on the observatio

vorticity and vertical circulation at an ocean front abstract density and velocity data with 4 km horizontal resolution from a survey of a front during fasinex frontal air sea interaction experiment are combined to describe the structure of the top 300 m of the ocean the geostrophic velocity field is derived and is used to examine the relative importance of stratification relative vorticity and twisting terms in ertel s potential vorticity q tenfold isopycnic changes in q are found across a horizontal scale of only 10 km these changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer the ageostrophic velocity field is quantified by solution of the omega equation and vertical velocities of up to 40 m day 1 are found small 40 km surface trapped top 200 m eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer

eddy induced mixed layer shallowing and mixed layer thermocline exchange the effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high resolution model of an unstable front a spring scenario is considered with a mixed layer maintained against surface warming by wind mixing tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front giving a mixed layer as shallow as 10 m in some places compared with the equilibrium monin obukhov depth of 30 m away from the front variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field vertical velocities associated with the growing instabilities push tongues of a ventilation tracer initially only present within the surface layer down to depths of up to 100 m after 24 days a quarter of this ventilation tracer has been deposited in the thermocline conversely the vertical motions pull nutrients from the thermocline up into the mixed layer giving enhanced surface values along the front in the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of o 3 10 10 4m2s 1

observations of seasonal variation in the ekman layer abstract the seasonal variation in the ekman layer is examined using observations from the long term upper ocean study lotus including surface meteorology current and temperature the near surface current is found to be coherent with the wind at low frequencies periods greater than one day using a wind relative averaging method the authors find the observed near surface volume transport to be consistent with ekman transport the mean current profile has a spiral shape that is flat compared to the classic ekman spiral in that it rotates less with depth than does the ekman spiral the mean current e folds over 12 m in the summer and 25 m in the winter with a decay in amplitude that implies eddy viscosities of 60 10 4 and 250 10 4 m2 s 1 respectively diurnal cycling is the dominant mode of variability in the summer and it determines the vertical structure of the spiral in the winter diurnal cycling is almost nonexistent due to greatly reduced solar insolation and stronger winds a persisten

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

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

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

kuroshio meanders in the east china sea abstract an array of seven inverted echo sounders was moored along and across the kuroshio in the east china sea for more than one year the data from this array show evidence of energetic meanders with periods of 7 11 and 16 days the respective phase velocities of these meanders are 28 20 and 17 km day 1 downstream the 7 and 16 day waves are intermittent but the 11 day waves are present throughout the deployment the instability responsible for these waves is investigated with a spectral numerical model applied to a background state representing the kuroshio in this region the fastest growing instability from the model has e folding growth time of 2 days period of 12 days and phase velocity of 18 km day 1 downstream it appears to be a close representation of the 11 day wave seen in the observational data such a model has been previously used to represent meanders in the gulf stream at similar latitudes off the east coast of the united states the kuroshio meanders have approximately half the

enhancement of fronts by vertical mixing microstructure observations near upwelled fronts indicate considerable variation in the structure of vertical mixing across the frontal region observations of cool filaments off northern california indicate that within the cool dense core of filaments the raised pycnocline inhibits the penetration to middepths of surface generated mixing the microstructure profiles are used to estimate the available wind energy for mixing as a function of pycnocline or mixed layer depth a greater portion of energy input at the surface is available for entrainment of dense fluid through the pycnocline and into the surface mixed layer where the pycnocline is shallow hence surface forced mixing may cause a more rapid increase in mixed layer density within the cool filament than outside the filament resulting in an enhanced horizontal density gradient in the mixed layer assuming the flow adjusts towards geostrophy the enhanced horizontal density gradient at the front could result in an accelerated mixed layer in the direction of the preexisting geostrophic flow proportions relating the gain in potential energy to the wind energy vary with pycnocline depth and differ by as much as an order of magnitude from the findings of denman and miyake 1973 and davis et al 1981 horizontal variability of pycnocline erosion may not be properly taken into account in some models and should more realistically be parameterized by including dependence on pycnocline depth during spring and early summer april july the northerly winds along the california coast induce coastal upwelling bringing anomalously cool st 2oc saline 5s 2 0 water to the surface raising the pycnocline over the continental shelf satellite images have revealed large cool features extending offshore beyond the shelf break often hundreds of kilometers seaward of the upwelling region e g bernstein et al 1977 ikeda and emery 1984 flament et al 1985 these anomalous features are believed to play a crucial role in the exchange of properties across the coastal transition zone the region between the nearshore upwelling area and the offshore ocean mooers and robinson 1984 these features were the focus of the coastal transition zone ctz experiment the fronts observed were located west of point arena california seaward of the continental shelf during upwelling conditions isopycnals dome over the shelf and the southward flowing california current develops along the continental shelf break huyer 1983 brink 1983 the strong current found along the sloping isopycnals is not confined to the shelf slope region and often features resembling meanders squirts jets and eddies form on the western edge of the upwelled front bernstein et al 1977 the cause of these features and the resulting influences that they impose on the local ocean are not understood it is believed these features modify the exchange of water between the nearshore upwelling region and the offshore ocean direct transport increased mesoscale eddy formation turbulent diffusion and intrusions near the frontal region are some of the possible mechanisms by which these variable structures enhance diffusion across the transition zone observations of micro scale processes indicate that the rate of surface induced mixing may vary with proximity to the front and that the horizontal density gradients thus established may act to accelerate the mixed layer at the front a series of detailed transects across a meander indi