turbulence and mixing in a shallow shelf sea from underwater gliders the seasonal thermocline in shallow shelf seas acts as a natural barrier for boundary generated turbulence damping scalar transport to the upper regions of the water column and controlling primary production to a certain extent to better understand turbulence and mixing conditions within the thermocline two unique 12 and 17 day datasets with continuous measurements of the dissipation rate of turbulent kinetic energy e collected by autonomous underwater gliders under stratified to well mixed conditions are presented a highly intermittent e signal was observed in the stratified thermocline region which was mainly characterized by quiescent flow turbulent activity index below 7 the rate of diapycnal mixing remained relatively constant for the majority of the time with peaks of higher fluxes that were responsible for much of the increase in bottom mixed layer temperature the water column stayed predominantly strongly stratified with a bulk richardson number across the thermocline well above 2 a positive relationship between the intensity of turbulence shear and stratification was found the trend between turbulence levels and the bulk richardson number was relatively weak but suggests that e increases as the bulk richardson number approaches 1 the results also highlight the interpretation difficulties in both quantifying turbulent thermocline fluxes as well as the responsible mechanisms

forced oscillations near the critical latitude for diurnal inertial resonance abstract oscillations at or close to the inertial frequency are widely observed in shelf seas where frictional damping is weak in the vicinity of latitudes 30 n and s such motions may become significantly enhanced by a resonance in which the local inertial frequency coincides with that of diurnal forcing under these conditions regular daily variations in wind stress tend to produce large anticyclonic motions that may extend throughout the water column as shown in the analytical theory of craig here the authors examine new observations from a location close to the critical latitude on the namibian shelf in the southern hemisphere the measurements cover almost the full depth 175 m of the water column by using upward and downward looking adcps suspended in midwater the observed flow involves a steady drift to the north maximum 11 cm s 1 at 40 m but the kinetic energy budget is dominated by anticyclonic circular motions with speeds in the surface layers exceeding 40 cm s 1 at times comparably e

is the paleoproterozoic jiao liao ji belt north china craton a rift as a typical example of the paleoproterozoic crust in the eastern block of the north china craton the paleoproterozoic jiao liao ji belt consists principally of the liaohe group and its equivalents liaoji granites and mafic intrusions previous studies indicate that the evolution of the jiao liao ji belt has been mainly attributed to the opening and closing of an intracontinental rift along the eastern continental margin of the north china craton here we synthesize the paleoproterozoic magmatism sedimentation metamorphism and metallogeny against the rift model and propose a process of arc continent collision between the northern longgang and the southern nangrim blocks this conclusion is consistent with the observations including that a the 2 0 to 2 2 ga magmatism shows a typical subalkaline series rather than a bimodal distribution since the mafic rocks mostly have arc affinities and the acidic intermediate rocks belong to the calc alkaline series b the main source of the 1 9 to 2 0 ga sedimentary rocks is the paleoproterozoic arc materials indicating a fore arc or back arc basin setting c a couple of big borate deposits occur in the boron rich volcanic rocks that were formed in convergent continental margins d the north and south liaohe groups show different rock associations and metamorphic histories p t paths and e the nangrim and longgang blocks vary in lithological units geochronology and metamorphic features thus an arc continent collision tectonic scenario for the paleoproterozoic jiao liao ji belt is involved a a southward subduction in the period 2 0 2 2 ga b sedimentation during the period 1 9 2 0 ga c arc continent collision at ca 1 9 ga and d post collisional extension at 1 82 1 87 ga marking the end of the paleoproterozoic tectonothermal event

evolution of near inertial waves during an upwelling event on the new jersey inner shelf abstract a 1996 field program provided a nearly three dimensional view of the temporal evolution of near inertial motion toward the end of an upwelling event on new jersey s inner shelf the appearance of near inertial motion is marked by a rapid rise in kinetic energy at the surface that is of the same magnitude and temporal structure of the work done by the wind indicating that the inertial motions are forced by local winds the incipient near inertial motion is surface intensified and spatially coherent it has a horizontal wavelength of 300 km and a mean kinetic energy of 0 04 m2 s 2 both of which decrease to less than 100 km and 0 01 m2 s 2 within two inertial periods an energy budget suggests that the rapid decline in surface kinetic energy is primarily due to vertical propagation into the thermocline near inertial motion in the thermocline is heterogeneous suggesting interaction between near inertial motion and subinertial shears the heterogeneous nature of near inertial waves poses a practica

semidiurnal tides on the laptev sea shelf with implications for shear and vertical mixing the arctic continental shelf seas hold a globally significant source of freshwater that impacts arctic ocean stratification circulation and climate this freshwater can be injected below the surface mixed layer by intense turbulent kinetic energy dissipation events as resolved by laptev sea microstructure observations the tides provide a major source of energy that can be dissipated and hence drive diapycnal mixing in the laptev sea multiyear adcp mooring records from locations across the shelf reveal that semidiurnal tides are dominated by them2 and s2 constituents with the largest amplitudes on the outer shelf throughoutmost of the shelf tides are clockwise polarized and sheared by stratification as characteristic near the m2 critical latitude interannual variations of the tidal and shear structures on the inner shelf aremainly determined by the stratification setting lena river freshwater plume in all locations m2 tides are enhanced under sea ice and therefore changes in the seasonal ice cover may lead to changes in tides and water column structure the main conclusions of this study are that i tides play a comparatively greater role year round on the outer shelf relative to the inner shelf ii a sea ice reduction will overall decrease the predictability of the currents especially on the inner shelf and iii the freshwater distribution directly impacts diapycnal mixing by setting the vertical tidal structure these combined effects imply that future sea ice loss will increase the variability and vertical mixing of freshwater particularly on the inner shelf where the lena river first enters the laptev sea

shear at the base of the oceanic mixed layer generated by wind shear alignment observations are used to evaluate a simple theoretical model for the generation of near inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two layer structure the model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors a climatology of stratification and shear variance in drake passage is presented which shows that these assumptions are most applicable to summer fall and spring but are not highly applicable to winter temperature salinity and velocity data from a high spatial resolution cruise in drake passage show that the model does not predict all large changes in shear variance the model is most effective at predicting changes in shear squared when it arises owing to near inertial wind driven currents without requiring a rotating resonant wind stress the model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer rotary spectral and statistical analysis of an additional 242 drake passage transects from 1999 to 2011 confirmed the presence of this shear spiking mechanism particularly in summer spring and fall when stratification is stronger

the effects of tides on the water mass mixing and sea ice in the arctic ocean in this study we use a novel pan arctic sea ice ocean coupled model to examine the effects of tides on sea ice and the mixing of water masses two 30 year simulations were performed one with explicitly resolved tides and the other without any tidal dynamics we find that the tides are responsible for a 15 reduction in the volume of sea ice during the last decade and a redistribution of salinity with surface salinity in the case with tides being on average 1 0 1 8 practical salinity units psu higher than without tides the ice volume trend in the two simulations also differs 2 09 103 km3 decade without tides and 2 49 103 km3 decade with tides the latter being closer to the trend of 2 58 103 km3 decade in the piomas model which assimilates sst and ice concentration the three following mechanisms of tidal interaction appear to be significant a strong shear stresses generated by the baroclinic clockwise rotating component of tidal currents in the interior waters b thicker subsurface ice ocean and bottom boundary layers and c intensification of quasi steady vertical motions of isopycnals by 50 through enhanced bottom ekman pumping and stretching of relative vorticity over rough bottom topography the combination of these effects leads to entrainment of warm atlantic waters into the colder and fresher surface waters supporting the melting of the overlying ice

diurnal sea breezes force near inertial waves along rottnest continental shelf southwestern australia abstractobservations of upper ocean dynamics close to the critical latitude ratio of the local inertial to diurnal frequency is 0 94 from a range of platforms surface currents using hf radar moored instruments and satellite remote sensing data off southwest australia indicated the presence of energetic near inertial waves generated through the diurnal inertial resonance during the austral summer when southerly winds and land sea breeze lsb system dominated the wind regime strong counterclockwise diurnal motions amplitudes surpassing 0 3 m s 1 penetrated to 300 m depth with diurnal vertical isotherm fluctuations up to 60 m the upward phase propagation speed of 140 m day 1 deep penetration of diurnal currents below the mixed layer and the 180 phase difference between the upper and lower water column suggested that the local lsb system caused the resonant diurnal motions relative vorticity fluctuations along two cross shore transects indicated changes to the local effective coriolis freq

storms modify baroclinic energy fluxes in a seasonally stratified shelf sea inertial tidal interaction observations made near the celtic sea shelf edge are used to investigate the interaction between wind generated near inertial oscillations and the semidiurnal internal tide linear baroclinic energy fluxes within the near inertial f and semidiurnal m2 wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone over the 2 week deployment period the semidiurnal tide drove 28 48 w m 1 of energy directly on shelf little spring neap variability could be detected horizontal near inertial energy fluxes were an order of magnitude weaker but nonlinear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semidiurnal internal tide led to a 25 43 increase in positive on shelf energy flux the phase relationship between f and m2 determines whether this nonlinear interaction enhances or dampens the linear tidal component of the flux and introduces a 2 day counter clockwise beating to the energy transport two very clear contrasting regimes of a tidally and b inertially driven shear and energy flux are captured in the observations

strong inertial currents and marginal internal wave stability in the central north sea solar insolation stabilizes the water column and suppresses vertical exchange observations from the central north sea clearly show that increased heating in summer is accompanied by enhanced de stabilizing vertical current differences shear surprisingly to such extent that the equilibrium state is marginally stable under calm weather conditions the shear is predominantly generated by near inertial motions while the internal wave spectrum primarily results from nonlinear interaction between the dominating tidal and near inertial motions in terms of the associated enhanced vertical mixing across the largest vertical temperature gradients shelf seas are not different from the abyssal ocean despite the proximity to energy sources near boundaries in the former by the lack of sufficiently strong wind and tidal mixing this internal mixing is considered to be responsible for the diapycnal transport of nutrients leading to the observed increase in near surface values and triggering the late summer phytoplankton bloom

internal wave mixing in the baltic sea near inertial waves in the absence of tides 1 the dynamics of near inertial motions and their relation to mixing is investigated here with an extensive data set including turbulence and high resolution velocity observations from two cruises conducted in 2008 summer and 2010 winter in the bornholm basin of the baltic sea in the absence of tides it is found that the basin scale energetics are governed by inertial oscillations and low mode near inertial wave motions that are generated near the lateral slopes of the basin these motions are shown to be associated with persistent narrow shear bands strongly correlated with bands of enhanced dissipation rates that are the major source of mixing inside the permanent halocline of the basin in spite of different stratification near inertial wave structure and atmospheric forcing during summer and winter conditions respectively the observed dissipation rates were found to scale with local shear and stratification in a nearly identical way this scaling was different from the gregg henyey type models used for the open ocean but largely consistent with the mackinnon gregg scaling developed for the continental shelf

bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern north sea the presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column thereby inhibiting exchange to the atmosphere the bubble plume itself however generates an upward motion of fluid e g upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline measurements at site 22 4b located at 57 550n 1 380e in the uk central north sea 200 km east of the scottish mainland where gas is still being released since a blow out in 1990 have been used to identify the generation of the seasonal thermocline and thus the depth of the upper mixed layer and its breakdown in autumn data derived from two landers containing an acoustic doppler current profiler and a conductivity temperature depth recorder were used to determine the mixed layer depth and the breakdown of the thermocline mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the nearbottom temperature the adcps estimate velocity profiles in four beam directions using doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration besides that the intensity of the backscattered sound per transducer is also recorded bubbles from the nearby plume contaminate the signal during part of the tidal cycle but in bubble free periods the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima results show that the thermocline broke down between mid october and early november several weeks earlier than the breakdown of the thermocline in nearby comparable areas likely caused by bubble induced downwelling at the site the early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet like structure associated with the seasonal tidal mixing front

mixing in seasonally stratified shelf seas a shifting paradigm although continental shelf seas make up a relatively small fraction ca 7 of the world ocean s surface they are thought to contribute significantly 20 50 of the total to the open ocean carbon dioxide storage through processes collectively known as the shelf sea pump the global significance of these processes is determined by the vertical mixing which drives the net co 2 drawdown which can occur only in stratified water in this paper we focus on identifying the processes that are responsible for mixing across the thermocline in seasonally stratified shelf seas we present evidence that shear instability and internal wave breaking are largely responsible for thermocline mixing a clear development from the first order paradigm for the water column structure in continental shelf seas the levels of dissipation observed are quantitatively consistent with the observed dissipation rates of the internal tide and near inertial oscillations it is perhaps because these processes make such a small contribution to the total energy dissipated in shelf seas that they are not well represented in current state of the art numerical models of continental shelf seas the results thus present a clear challenge to oceanographic models

near inertial oscillations over the texas louisiana shelf wind induced near inertial oscillations over the texas louisiana shelf in spring and summer 1992 are described using the current and wind observations taken during the first year of the texas louisiana shelf circulation and transport processes study latex a rotary spectral analysis shows clockwise rotating energy peaks at near inertial frequencies for records from all current meter moorings after the suppression of principal tidal signals the vertical structure of near inertial oscillations is characterized by a first baroclinic mode with a near 180 phase difference between the upper mixed layer and the lower stratified layer the oscillations are intermittent with a modulation timescale of about 5 10 days they are surface intensified and have maximum values near the shelf break decaying gradually toward the coast but rapidly offshore near inertial oscillations appear to accompany a sudden change of the wind stress during frontal passages diagnostic analysis suggests that the large near inertial oscillations over the latex shelf are mainly generated by high frequency near inertial variation of the wind stress accompanying the passage of atmospheric fronts when the downward transfer of the near inertial energy to the deep stratified layer is small a simple mixed layer model forced by the observed wind stress provides a reasonable prediction of the near inertial currents in the mixed layer

near inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the arabian sea during the monsoons we present the horizontal kinetic energy ke balance of near inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15 26 n in the arabian sea during the southwest monsoon the highly sheared and stratified transition layer at the mixed layer base varies between 5 m and 35 m and correlates negatively with the wind stress results from the mixed layer near inertial ke nike balance suggest that wind energy at times can energize the transition layer and at other times is fully utilized within the mixed layer a simple two layer model is utilized to study the shear evolution in the transition layer and shown to match well with observations the shear production in this model arises from alignment of wind stress and shear although the winds are unidirectional during the monsoon the shear in the transition layer is predominantly near inertial the near inertial shear bursts in the observations show the same phasing and magnitude at near inertial frequencies as the wind shear alignment term

near inertial waves on the new england shelf the role of evolving stratification turbulent dissipation and bottom drag abstract energetic variable near inertial internal waves were observed on the springtime new england shelf as part of the coastal mixing and optics cmo project surface warming and freshwater advection tripled the average stratification during a 3 week observational period in april may 1997 the wave field was dominated by near inertial internal waves generated by passing storms wave evolution was controlled by a balance among wind stress bottom drag and turbulent dissipation as the stratification evolved the vertical structure of these near inertial waves switched from mode 1 to mode 2 with associated changes in the magnitude and location of wave shear the growth of mode 2 waves was attributable to a combination of changing wind stress forcing and a nonlinear coupling between the first and second vertical modes through quadratic bottom stress to explore both forcing mechanisms an open ocean mixed layer model is adapted to the continental shelf in this model surface wind stress and bottom stre

generation of bulk shear spikes in shallow stratified tidal seas abstract recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing the bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows remarkable variability on tidal inertial and synoptic time scales that has yet to be explained in this paper observations from the seasonally stratified northern north sea are presented for a time when the water column has a distinct two layer structure bulk shear estimates based on adcp measurements show a bulk shear vector that rotates in a clockwise direction at the local inertial period with episodes of bulk shear spikes that have an approximately twice daily period and occur in bursts that last for several days to explain this observation a simple two layer model based on layer averaging of the one dimensional momentum equation is developed

horizontal advection and dispersion in a stratified shelf sea the role of inertial oscillations abstract the role played by inertial motions in horizontal dispersion within the thermocline of a broad mid latitude shelf sea is examined through the analysis of a deliberately released dye tracer our analysis is of the horizontal and vertical evolution over 40 h of a dye tracer injected into the seasonally stratified thermocline of the celtic sea on the nw european shelf the inferred diapycnal diffusivity was 1 3 1 5 10 5 m2 s 1 and the radial horizontal diffusivities of the depth integrated dye patch ranged from 1 9 to 4 0 m2 s 1 the inferred vertical diffusivity is in agreement with microstructure based estimates and the depth integrated horizontal diffusivity is broadly in agreement with previous dye release derived estimates made over similar scales and time periods asymmetry in the horizontal evolution of the dye patch was evident we argue that mean shear dispersion was responsible for lateral elongation of the dye patch particularly between hours 23 and 35 after release during which time horizontal diffusivity along the major axis ka exceeded that along the minor axis kb by more than a factor of 10 we further show that along patch shear was predominantly a result of differential advection between a deep residual flow to the south east and an oscillating wind driven surface ekman layer in this region of strong low frequency inertial shear a time dependent model of shear dispersion young et al 1982 was able to account for the observed rate of horizontal dispersion calculated on the target isopycnal surface

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

horizontal distribution and characteristics of shear layers in the seasonal thermocline abstract high resolution velocity shear ctd and microstructure measurements were made simultaneously from the research submarine dolphin in monterey bay in october 1984 during three consecutive dives the dolphin cycled between the surface and 110 m along predetermined tracks 10 miles northwest of monterey inside the seasonal thermocline the vertical velocity shear appeared to be concentrated in layers 10 m thick extending several kilometers horizontally the thickness of the shear layers is consistent with the typical size of turbulent patches encountered in the seasonal thermocline in contrast no large shear layers were observed below a 50 m depth the depth levels at which the shear layers were observed were nearly constant throughout each dive suggesting that the shear layers with some unknown degree of intermittency might extend horizontally over several square kilometers the shear vector inside the seasonal themocline at t 25 5 rotated 360 over an inertial period but did appear to

an investigation of internal mixing in a seasonally stratified shelf sea the shelf sea seasonal thermocline is a critical interface within the marine environment separating the euphotic zone from nutrient rich deep water fluxes across the thermocline therefore represent a key biogeochemical pathway in this paper we quantify the rate of mixing across the seasonal thermocline for a location in the celtic sea and investigate the processes responsible for driving thermocline fluxes profiles of the rate of dissipation of turbulent kinetic energy e show enhanced dissipation within the thermocline region similar to 6 x 10 5 w m 3 the diffusivity implied by these measurements is similar to 0 5 cm 2 s 1 similar to previous shelf sea studies and is sufficient to explain the observed warming of the deep water suggesting that vertical mixing is the dominant control on water column structure two potential sources of mixing energy are identified the internal tide and near inertial waves the mechanism of energy transfer from the candidate mixing mechanisms to turbulence is not clear thermocline dissipation rates were found to have no richardson number dependence but scaled positively with n 2 and s 2 in agreement with a previous turbulence parameterization application of this model to our data does a good job of capturing the mean characteristics of the observed heating flux across the thermocline although none of the short term fluctuations in mixing were reproduced

a review of oceanographic and meteorological controls on the north sea circulation and hydrodynamics with a view to the fate of north sea methane from well site 22 4b and other seabed sources abstract the north sea hydrodynamics are key to the redistribution of methane released at the 22 4b site located at 57 55 n 1 38 e in the uk central north sea 200 km east of the scottish mainland this review summarizes the current state of knowledge on the north sea circulation stratification and variability therein and briefly discusses the potential consequences for the distribution and fate of methane released from site 22 4b or other seabed sources astronomical tidal waves follow a counter clockwise path and tide topography interaction generates a residual circulation in the same direction wind stress forcing can enhance reduce or even reverse this circulation variations in the strength of the fair isle current fic are important the fic enters the north sea between the orkneys and shetland follows approximately the 100 m isobath passes along site 22 4b and ends up in the norwegian trench the north atlantic oscillation nao also causes variability a positive negative nao index is associated with stronger weaker than normal westerly winds nao situations strengthen the circulation in the north sea whereas it weakens during nao conditions and is directed northeastward high positive correlations exist between the sst at site 22 4b and the nao index climate change can have a long term effect on the hydrodynamics of the north sea seasonal stratification has potentially the most important imprint on methane derived from the well 22 4b site summertime heating stratifies the northern part of the north sea in autumn loss of heat to the atmosphere causes the stratification to breakdown until tides and storms mix the entire water column during the period of stratification the bulk of dissolved methane released from site 22 4b gets trapped below the thermocline the loss of methane to the atmosphere thus becomes a function of the relative time scales of transport and horizontal and vertical mixing processes versus the time scale of microbial degradation oxidation in the water column

variability and mixing of the filchner overflow plume on the continental slope weddell sea abstracta large fraction of antarctic bottom water is produced in the weddell sea through mixing between the cold and dense shelf water masses and the warm and saline off shelf water we present o

ocean nutrient pathways associated with the passage of a storm storms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open north atlantic ocean in this study we use an interdisciplinary dataset collected in the region to quantify nutrient supply by two pathways associated with a storm event entrainment of nutrients during a period of high wind forcing and subsequent shear spiking at the pycnocline due to interactions of storm generated inertial currents with wind the post storm increase in surface layer nitrate by 20 mmol m 2 was predominantly driven by the first pathway nutrient intrusion during the storm alignment of post storm inertial currents and surface wind stress caused shear instabilities at the ocean pycnocline forming the second pathway for nutrient transport into the euphotic zone during the alignment period pulses of high turbulent nitrate flux through the pycnocline up to 1 mmol m 2 day 1 approximately 25 times higher than the background flux were detected however the impact of the post storm supply was an order of magnitude lower than during the storm due to the short duration of the pulses cumulatively the storm passage was equivalent to 2 5 5 of the nitrate supplied by winter convection and had a significant effect compared to previously reported sub mesoscale dynamics in the region as storms occur frequently they can form an important component in local nutrient budgets

thermocline mixing in summer stratified continental shelf seas 1 a key process in the shelf sea pumping of co2 to the open ocean is vertical mixing in the summer stratified zones of continental shelf seas here we present measurements of profiles of the rate of dissipation of turbulent kinetic energy from which vertical mixing is inferred for several locations in the summer stratified zones of the nw european shelf seas at both sites we find significant thermocline mixing with 20 of the observed turbulent dissipation taking place within the thermocline we then present evidence to demonstrate that the thermocline mixing is a result of shear instability within the thermocline this is contrary to the conventional view that vertical mixing in shelf seas is largely driven by boundary shear stresses