wind driven submesoscale subduction at the north pacific subtropical front 1 upper ocean observations from the north pacific subtropical front during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss prior to generation a sharp thermohaline front was intensified by confluent flow of 1 2 10 5 s 1 relative vertical vorticity across a surface intensified along front jet on the warm side of a frontal trough was 0 5 f during the storm buoyancy loss arose due to cooling of 650 w m 2 and down front wind stress 0 5 n m 2 that generated a southward cross front ekman transport of dense water over light the resulting wind driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude the intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline they were approximately 20 m thick and horizontally elongated in the cross frontal direction the near surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline whereas the subsurface intrusions were composed of warm and saline water from the surface the apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterized by o 1 rossby and richardson numbers pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer and sloping isopycnals the intrusion properties background environmental context and forcing history are consistent with prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss possibly resulting in symmetric instability

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

rapid injection of near inertial shear into the stratified upper ocean at an antarctic circumpolar current front the impact on the upper ocean of the passage of a short intense storm over a southern ocean site in proximity to an antarctic circumpolar current front is characterized the storm causes a wind induced deepening of the mixed layer and generates an inertial current immediate post storm observations indicate a mixed layer extending to approximately 50 m depth subsequent measurements show the upper ocean to have re stratified injecting near inertial shear in stratified waters within 1 day of the storm s passage this time scale for the development of near inertial shear is one order of magnitude shorter than that predicted by the dispersion paradigm the observed rapid changes in upper ocean stratification point to the existence of an as yet undocumented efficient mechanism for injection of near inertial shear into the stratified ocean that is in turn associated with enhanced turbulence and mixing

a pv approach for dense water formation along fronts application to the northwestern mediterranean the mechanisms of dense water formation 29 0 kg m 3 at work in the baroclinic cyclonic gyre of the north western mediterranean basin are investigated through a pv budget pv potential vorticity the pv budget is diagnosed from an eddy resolving 1 36 ocean simulation driven in surface by hourly air sea fluxes provided by a non hydrostatic atmospheric model at 2 5km resolution the pv budget is controlled by the diabatic frictional and advective pv fluxes around the gyre the surface diabatic pv flux dominates the pv destruction except along the northern branch of the north current where the surface frictional pv flux is strongly negative in this region the bathymetry stabilizes the front and maintains the current northerly in the same direction as the dominant northerly wind this configuration leads to optimal wind current interactions and explains the preponderance of frictional pv destruction on diabatic pv destruction this mechanical forcing drives a cross front ageostrophic circulation which subducts surface low pv waters destroyed by wind on the dense side of the front and obducts high pv waters from the pycnocline on the light side of the front the horizontal pv advections associated with the geostrophic cyclonic gyre and turbulent entrainment at the pycnocline also contribute to the pv refueling in the frontal region the surface non advective pv flux involves energy exchanges down to 1400 w m 2 in the frontal zone this flux is 3 5 times stronger than atmospheric buoyancy flux these energy exchanges quantify the coupling effects between the surface atmospheric forcing with the oceanic frontal structures at submesoscale this article is protected by copyright all rights reserved

upstream control of the frontal jet regulating plankton production in the alboran sea western mediterranean using a coupled physical biological model we document that a 30 km wide meandering jet constitutes a major source of biological enrichment in the alboran sea western mediterranean even in the absence of wind forcing and tidal dynamics the level of enrichment is shown to vary markedly during the year depending on the upstream characteristics of the jet as it exits from the gibraltar strait when its intensity is sufficiently low and characterized by weaker cross frontal density gradients during winter spring the jet is weakly nonlinear and may not fulfill the necessary conditions for frontogenesis it then remains weakly productive in the case of stronger jet intensity 1 1 sv accompanied by stronger cross frontal density and velocity gradients within the alboran sea during summer autumn the frontal jet becomes strongly nonlinear and ageostrophic with large cross frontal vorticity changes on the order of planetary vorticity under these conditions upward vertical velocities in the range 10 50 m d 1 supply nutrients into the euphotic layer more effectively and support high level frontogenesis induced phytoplankton production on the anticyclonic side of the main jet axis the strong eddy pumping mechanism also provides a comparable level of plankton production within strongly nonlinear elongated cyclonic eddies along the outer periphery of the frontal jet the plankton biomass is advected partially by the jet along its trajectory and dispersed within the basin by mesoscale eddies and meanders

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

generation of submesoscale frontal eddies in the agulhas current copyright 2019 brill due to copyright restrictions the attached pdf file contains the abstract of the full text item for access to the full text item please consult the publisher s website https agupubs onlinelibrary wiley com doi full 10 1029 2019jc015229

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

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

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

the tattered curtain hypothesis revised coastal jets limit cross shelf larval transport exchange and retention of coastal waters modulate dispersal of marine larvae affecting marine ecosystem dynamics a hypothesis was put forward in the 1980s describing the coastal upwelling front as a tattered curtain that retains larvae this front was envisioned to be broken up by squirts and eddies hitting the coast under upwelling relaxation events here we revise this hypothesis by using an idealized ocean model of an eastern boundary upwelling current and an idealized particle larvae model appropriate for shelf spawning benthic species modeled larval settlement patterns were controlled by retention in the core of the upwelling jet bounded by regions of high velocity shear on the flanks of the jet squirts filaments poleward moving eddies and meanders modulated settlement patterns locally while dense packets moved equatorward within the upwelling jet correlation between settlement i e particles 20 40 d old 10 km from shore and wind was low for a lagged wind product r 0 33 and moderate for a 20 d integrated wind product r 0 62 we determined that it is not upwelling relaxation but sustained moderate upwelling that can result in a highly retentive jet that entrains larvae and acts as a barrier to cross shelf transport however the amount of retention is highly variable settlement was low after strong persistent upwelling completely tattered the jet jet cores in general should act as important retentive transport barriers across diverse coastal systems a view supported by dynamical theory modeling studies and larval recruitment observations

generation of upwelling circulation under downwelling favorable wind within bottom attached buoyant coastal currents abstracta two dimensional modeling study by moffat and lentz recently reported that downwelling favorable wind can induce cross shore upwelling circulation within a bottom attached buoyant coastal current here we extend the problem to three dimensions the driving mechanism and the sensitivity for the upwelling circulation are studied using a primitive equation ocean model and an analytical model after the initial downwelling adjustment that steepens the isopycnals and compresses the coastal current the cross shore flow can switch to steady upwelling circulation this reverse circulation coincides with a vertically well mixed water column and persists until interrupted by the arrival of river plume bulge from upstream during the upwelling phase the ageostrophic cross shore flow follows the ekman balance the sense of cross shore circulation is governed by a dimensionless parameter the shear ratio which measures the relative size of geostrophic shear and velocity shear supported by the wind in th

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 computational investigation of adsorption of organics on mineral surfaces implications for organics delivery in the early solar system abstract the adsorption of simple organic compounds onto minerals that are known to occur in the early solar nebula such as olivine spinel and water ice is examined using first principles density functional theory the calculations show that electron rich organics and organics containing cyanide amine and carboxylic functional groups can strongly bind to low index surfaces of olivine and spinel based on the surface coverage as obtained from these calculations it can be inferred that an estimated amount of 10 13 kg of organics could have been delivered to early earth via direct adsorption mechanisms thereby providing an endogenous source of planetary organics in addition adsorption of organic compounds on magnesite a carbonate phase believed to have formed via aqueous processes on asteroidal bodies is also studied the adsorption behavior of the organics is observed to be similar in both cases i e for minerals that formed during the earliest stages of nebular evolution through condensation spinel and olivine or other processes and for those that formed via hydration processes on asteroidal bodies magnesite these results suggest that direct incorporation via adsorption is an important delivery mechanism of organics to both asteroidal bodies and terrestrial planets

role of the gulf stream and kuroshio oyashio systems in large scale atmosphere ocean interaction a review abstract ocean atmosphere interaction over the northern hemisphere western boundary current wbc regions i e the gulf stream kuroshio oyashio and their extensions is reviewed with an emphasis on their role in basin scale climate variability sst anomalies exhibit considerable variance on interannual to decadal time scales in these regions low frequency sst variability is primarily driven by basin scale wind stress curl variability via the oceanic rossby wave adjustment of the gyre scale circulation that modulates the latitude and strength of the wbc related oceanic fronts rectification of the variability by mesoscale eddies reemergence of the anomalies from the preceding winter and tropical remote forcing also play important roles in driving and maintaining the low frequency variability in these regions in the gulf stream region interaction with the deep western boundary current also likely influences the low frequency variability surface heat fluxes damp the low frequency sst anomalies ove

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

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

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

why are there upwellings on the northern shelf of taiwan under northeasterly winds upwellings are observed on the northern shelf of taiwan during northeasterly winds analytical and realistic numerical models are used to explain how vertical motions are created by divergence and convergence produced by wind acting on the vorticity field of two strong jets the kuroshio and the taiwan warm current the seaward increase in cyclonic vorticity near the kuroshio s western edge favors a stronger ekman transport away from the jet producing upwelling at the shelfbreak under a northeasterly wind a similar mechanism for generating vertical motions is found across the taiwan warm current the numerical model results indicate that the vorticity effects can account for up to 30 50 of the total variation in the surface ekman transport except during summer s weak southwesterlies northeasterly wind is dominant over the east china sea suggesting that the vorticity effects may be prominent in the observed shelfbreak upwelling in nonsummer months

meridional shift of the kuroshio extension induced by response of recirculation gyre to decadal wind variations abstract the meridional shift of the kuroshio extension to decadal wind variations is examined by analyzing the hindcast simulation of a high resolution 1 12 1 12 north pacific ocean general circulation model ogcm and by conducting a series of idealized ogcm experiments the meridional shift of the kuroshio extension in the hindcast is consistent with observations and former studies using a series of sensitivity experiments in idealized settings the following possible mechanism for this shift is proposed the baroclinic rossby waves that arise in the northern half of the subtropical gyre cannot directly enter the kuroshio oyashio extension region due to the strong kuroshio extension jet when the rossby waves accompanying the negative positive sea surface height anomalies propagate westward and reach the region to the east of the recirculation gyre the difference in thermocline depth between the recirculation gyre and the region to its east becomes larger smaller resulting in stronger weaker baroclinic instabilities these stronger weaker instabilities cause a shallower deeper thermocline in the recirculation gyre to compensate for this shrinking stretching the recirculation gyre migrates southward northward leading to the southward northward shift of the kuroshio extension

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

seismic monitoring of sechilienne rockslide french alps analysis of seismic signals and their correlation with rainfalls 1 in the french alps sechilienne rockslide is one of the natural phenomena that presents one of the highest levels of risk in terms of socioeconomical outcomes this rockslide has been active for a few decades and has been instrumented since 1985 the current very active volume of this rockslide is estimated to be up to 5 million m3 located on the border of a slowly moving mass reaching 50 100 million m3 the velocity of the most active zone reached 1 4 m yr in 2008 about twice the value of 2000 a seismic network was installed on this rockslide in may 2007 to supplement the monitoring system it has now recorded several thousand events mostly rockfalls but also hundreds of local and regional earthquakes which can be distinguished and classified from their signal characteristics rockfalls and microseismicity which occur in bursts of activity are found to be weakly but significantly correlated with rainfall rockfall occurrence increases linearly with precipitation with however strong fluctuations of the numbers of rockfalls per day for the same rainfall intensity no threshold was found for rainfall triggering with even 1 mm of rain being enough to trigger rockfalls rockfall activity starts almost immediately during a rainfall episode and lasts for several days after the rainfall rain also induces strong accelerations of the rockslide movement which also start quasi instantaneously and last for about a month

intensification of the subpolar front in the sea of japan during winter cyclones the response of the subpolar front in the sea of japan also known as the east sea to winter cyclones is investigated based on quantitative analyses of gridded and satellite data sets cyclone passages affecting the sea are detected using time series of spatially averaged surface turbulent heat fluxes as the cyclones develop there are strong cold air outbreaks that produce twice the normal heat loss over the sea after removal of sea surface temperature sst seasonal trends we found that cyclone passage hence cooling mainly occurred over 3 days with maximum sst reduction of 0 4 c the greatest reduction was found along the subpolar front where frontal sharpness i e sst gradient increased by 0 1 c 100 km 1 results of a mixed layer model were consistent with both temperature and frontal sharpness and localized surface cooling along the subpolar front resulted from both horizontal heat advection and turbulent heat fluxes at the sea surface further analyses show that this localized cooling from horizontal heat advection is caused by the cross frontal ekman flow vertically averaged over the mixed layer and strong northwesterly winds associated with the cold air outbreak during cyclone passage

effects of horizontal advection on upper ocean mixing a case of frontogenesis abstract the wind stress on the ocean surface induces horizontal advection and vertical mixing two mechanisms which are capable of changing the surface water density advection and mixing can enhance or partially destroy the effects of each other frontogenesis due to convergent ekman transports forced by a wind stress field is one example a bulk model for the study of advection and mixing in such circumstances is constructed from a one dimensional model continuity of mass requires that water masses either downwell convergence or escape laterally confluence this distinction leads to a study of two extreme cases of frontogenesis each herein treated separately the model reduces to two coupled highly nonlinear prognostic equations for the mixed layer buoyancy and mixed layer depth scaling of the equations leads to the definition of a mixing parameter a nondimensional number which measures the relative importance of advection and mixing for large scale ocean frontogenesis this parameter is of th

lagrangian and eulerian characterization of two counter rotating submesoscale eddies in a western boundary current in recent decades high spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies in the surface velocity temperature and chlorophyll a fields we use a suite of high resolution data to characterize two counter rotating short lived eddies formed at the front between the warm east australian current eac and temperate coastal waters 308s eastern australia in this region submesoscale filaments and short lived eddies are dynamically generated and decay at time scales of hours to days dominant cyclonic filaments of o 1 rossby number formed along frontal jets and eddy boundaries generating localized ageostrophic circulations at the submesoscale measurements of over ocean wind direction and surface currents from high frequency radars reveal the influence of the short term small scale wind forcing on the surface circulation enhancement of the horizontal shear frontal jet destabilization and the generation and decay of the cyclonic eddy by contrast the anticyclonic eddy formation was most likely associated with eac mesoscale instability and anticyclonic vorticity lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones limiting their transport mixing between eac derived and coastal waters was increased along the frontal regions and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales the cyclonic cold core eddy entrained high chlorophyll a shelf waters on its convergent side suggesting spiral eddy cyclogenesis

mesoscale to submesoscale transition in the california current system part ii frontal processes this is the second of three papers investigating the regime transition that occurs in numerical simulations for an idealized equilibrium subtropical eastern boundary upwelling current system similar to the california current the emergent upper ocean submesoscale fronts are analyzed from phenomenological and dynamical perspectives using a combination of composite averaging and separation of distinctive subregions of the flow the initiating dynamical process for the transition is near surface frontogenesis the frontal behavior is similar to both observed meteorological surface fronts and solutions of the approximate dynamical model called surface dynamics i e uniform interior potential vorticity q and diagnostic force balance in the intensification of surface density gradients and secondary circulations in response to a mesoscale strain field however there are significant behavioral differences compared to the surface dynamics model wind stress acts on fronts through nonlinear ekman transport and creation and destruction of potential vorticity the strain induced frontogenesis is disrupted by vigorous submesoscale frontal instabilities that in turn lead to secondary frontogenesis events submesoscale vortices and excitation of even smaller scale flows intermittent submesoscale breakdown of geostrophic and gradient wind force balance occurs during the intense frontogenesis and frontal instability events

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

lagrangian sources of frontogenesis in the equatorial atlantic front estimating the processes that control the north equatorial sea surface temperature sst front on the northern edge of the cold tongue in the tropical atlantic is a key issue for understanding the dynamics of the oceanic equatorial atlantic and the west african monsoon diagnosis of the frontogenetic forcings on a realistic high resolution simulation was used to identify the processes involved in the formation and evolution of the equatorial sst front the turbulent forcing associated with the mixed layer turbulent heat flux was found to be systematically frontolytic while the dynamic forcing associated with currents was found to be frontogenetic for the equatorial sst front nevertheless the low frequency component of the turbulent forcing was frontogenetic and initiated the sst front which was then amplified and maintained by the leading dynamic forcing this forcing was mainly driven by the meridional convergence of the northern south equatorial current nsec and the guinea current which points out the essential role played by the circulation in the equatorial sst front evolution the quasi biweekly variability of the equatorial sst front and its forcings were found to be more strongly coupled to the wind energy flux wef than to the surface wind stress in fact the wef controlled the convergence divergence of the nsec and guinea current and thus the meridional component of the leading dynamic forcing the wef explains the equatorial sst front development better than the wind does because it is a coupled ocean atmosphere process

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

three dimensional structure and temporal evolution of submesoscale thermohaline intrusions in the north pacific subtropical frontal zone abstract four instances of persistent intrusive deformation of the north pacific subtropical front were tagged individually by a lagrangian float and tracked for several days each feature was mapped in three dimensions using repeat towed observations referenced to the float isohaline surface deformations in the frontal zone included sheetlike folds elongated in the alongfront direction and narrow tongues extending across the front all deformations appeared as protrusions of relatively cold and fresh water across the front no corresponding features of the opposite sign or isolated lenslike structures were observed the sheets were o 10 m thick protruded about 10 km into the warm saline side of the front and were coherent for 10 30 km along the front having about the same thickness and cross frontal extent as the sheets tongues extended less than 5 km along the front all of the intrusions persisted as long as they were followed several days to one week their structures evolved on both inertial

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

spatiotemporal variation in cross shelf exchange across the inner shelf of monterey bay california abstractcross shelf exchange resulting from wind and wave driven flows across the inner shelf has been the focus of a considerable body of work this contribution extends recent analyses to the central california coastline using 5 yr of moored current observations acoustic doppler current profiler adcp data from stations across the monterey bay two in the northern bay and one in the southern bay in water depths of 20 m showed net offshore transport throughout the year for the northern bay sites cross shelf exchange was dominated by ekman transport driven by along shelf diurnal sea breezes during the upwelling season intense stratification in the northern bay leads to very shallow observed ekman layers 5 8 m and consequently no overlap between bottom and surface ekman layers within a few hundred meters of the coast the total transport is less than predicted by theory consistent with models of shallow water ekman transport the observed transport 42 of full ekman transport is shown to b

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

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

parameterization of mixed layer eddies part ii prognosis and impact abstract the authors propose a parameterization for restratification by mixed layer eddies that develop from baroclinic instabilities of ocean fronts the parameterization is cast as an overturning streamfunction that is proportional to the product of horizontal buoyancy gradient mixed layer depth and inertial period the parameterization has remarkable skill for an extremely wide range of mixed layer depths rotation rates and vertical and horizontal stratifications in this paper a coarse resolution prognostic model of the parameterization is compared with submesoscale mixed layer eddy resolving simulations the parameterization proves accurate in predicting changes to the buoyancy the climate implications of the proposed parameterization are estimated by applying the restratification scaling to observations the mixed layer depth is estimated from climatology and the buoyancy gradients are from satellite altimetry the vertical fluxes are comparable to monthly mean air sea fluxes in large areas of

atmospheric response to a midlatitude sst front alongfront winds abstractsatellite observations and modeling studies show that midlatitude sst fronts influence the marine atmospheric boundary layer mabl and atmospheric circulation here the weather research and forecasting wrf mesoscale model is used to explore the atmospheric response to a midlatitude sst front in an idealized dry two dimensional configuration with a background wind oriented in the alongfront direction the sst front excites an alongfront wind anomaly in the free atmosphere with peak intensity just above the mabl this response is nearly quasigeostrophic in contrast to the inertia gravity wave response seen for cross front background winds the free atmosphere response increases with the background wind in contrast to previously proposed sst frontal mabl models the mabl winds are nearly in ekman balance however a cross front wind develops in the mabl as a result of friction and rotation such that the mabl cross front rossby number e 0 2 the mabl vorticity balance and scaling arguments

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

transient upwelling hot spots in the oligotrophic north pacific 1 vertical exchange between deep nutrient rich and nutrient depleted surface waters controls the physical supply of nutrients into the euphotic zone in the oligotrophic waters of the subtropical north pacific ocean sea surface temperature and ocean color satellite images are used in this study to show that submesoscale chlorophyll patches are locally intensified these patches are associated with surface temperature fronts and regions of strong stretching of the eddying flow the latter are identified by calculating the finite size lyapunov exponents of the geostrophic flow derived from the sea surface height field from altimetry surface frontogenesis and nonlinear ekman pumping nlep arise as natural candidates for the sporadic generation of large vertical velocities that could favor the formation of these features a high resolution regional numerical model simulation confirms that large vertical velocities are indeed associated with strong density fronts and gradients of relative vorticity using the q vector divergence as a proxy for frontogenetically generated vertical velocities yields good agreement with the model vertical velocities particularly in regions where they are large the pattern and magnitude of nlep suggests that it is also contributing to the strong vertical motions although nlep and model vertical velocities are not necessarily in phase surface frontogenesis and nlep provide a possible explanation for the episodic injection of nutrients at the submesoscale which are then horizontally stirred and modulated by the unstable manifolds of the flow field given that these surface intensified processes would be biogeochemically more effective when the pycnocline is shallow they could also serve as one possible explanation for the late summer chlorophyll blooms that are known to occur in the region

modulation of frontogenetic plankton production along a meandering jet by zonal wind forcing an application to the alboran sea we present a coupled physical biological modeling study to elucidate the changes in ageostrophic frontal dynamics and the frontogenetic plankton production characteristics of a meandering jet under the impacts of successive westerly easterly wind events combined with seasonal variations in the upstream transport and buoyancy flux characteristics of the jet using a case study for the alboran sea western mediterranean their nonlinear coupling is shown to result in different forms of physical and biological characteristics of the background jet structure that follows a meandering path around two anticyclonic gyres in the western and eastern basins and a cyclonic eddy in between the westerly downfront wind events broaden the jet and result in stronger cross frontal density contrast and intensify ageostrophic cross frontal secondary circulation thus they improve the frontogenetic plankton production with respect to the no wind case they also support higher production along the northern coast in response to wind induced coastal upwelling and spreading of resulting nutrient rich productive water by mesoscale stirring these features weaken gradually as the jet transport reduces in contrast stronger and longer lasting easterlies during the reduced jet transport phase weaken the currents and frontal density structure change the circular western alboran gyre to an elongated form and shift the main axis of the jet towards the southern basin then frontogenesis fails to contribute to phytoplankton production that becomes limited to the eddy pumping within cyclones apart from the frontogenetic production eddy pumping mesoscale stirring and diapycnal mixing of nutrients support intermittent and localized phytoplankton patches over the basin

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

an annual cycle of submesoscale vertical flow and restratification in the upper ocean abstractnumerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification and thus restratify the upper ocean via vertical flow howev