shear induced convective mixing in bottom boundary layers on slopes field data from a stratified lake demonstrate that buoyancy driven convective mixing can be an important mechanism for the formation of well mixed bottom boundary layers bbls on slopes convective turbulence is caused by differential transport of stratified water masses along the slope of a basin because the current velocity usually decreases toward the sediment water at some distance from the bottom is transported faster than is water below during upslope flow this process leads to transport of heavier water on top of lighter water and hence to unstable stratification within the bbls analogously strong bbl stratification occurs during downslope flow high frequency acoustic doppler current profiler adcp and temperature measurements in the bbls of a lake revealed the cyclic occurrence of convective turbulence driven by periodic across slope currents of internal seiching the estimated maximum buoyancy flux within the bbls was in good agreement with the highest observed dissipation rates of turbulent kinetic energy this suggests that in our specific case classical bottom shear production and the mentioned buoyancy driven convective production contributed by similar amounts to the turbulent kinetic energy

modeling semidiurnal internal tide variability in the southern california bight abstractthe regional oceanic modeling system roms is applied in a nested configuration with realistic forcing to the southern california bight scb to analyze the variability in semidiurnal internal wave generation and propagation the scb has a complex topography with supercritical slopes that generate linear internal waves at the forcing frequency the model predicts the observed barotropic and baroclinic tides reasonably well although the observed baroclinic tides feature slightly larger amplitudes the strongest semidiurnal barotropic to baroclinic energy conversion occurs on a steep sill slope of the 1900 m deep santa cruz basin this causes a forced near resonant semidiurnal poincare wave that rotates clockwise in the basin and is of the first mode along the radial azimuthal and vertical directions the associated tidal mean depth integrated energy fluxes and isotherm oscillation amplitudes in the basin reach maximum values of about 5 kw m 1 and 100 m and are strongly modulated by the sprin

anatomy of turbulence in thermally stratified lakes turbulence within the thermocline of two thermally stratified lakes lake biwa japan and lake kinneret israel was investigated using a portable flux profiler this instrument provided high resolution profiles of temperature conductivity and two components of velocity within a measuring volume of approximately 8 mm 3 each data profile was segmented into statistically stationary segments a range of properties of the turbulence including direct estimates of the vertical mass flux were then calculated for each segment it was found that turbulence in the thermocline was generally patchy but within a patch dissipation levels were relatively high the turbulent motions were found to be fine grained with small thorpe scales and a large skewness of the distribution of the displacement scale this is distinct to that found in the ocean where shear instability produces large overturns the measurement showed that the net vertical mass flux in the thermocline was negligible and less than that predicted by osborn 1980 this is explained by noting that the net buoyancy flux consisted of two opposing parts a down gradient irreversible flux due to turbulent mixing and an up gradient reversible flux due to restratification that often cancel each other within a segment the low measured net buoyancy flux within the interior of the lakes suggested that other processes such as gravitational adjustments and benthic boundary layer processes should be responsible for the basin averaged vertical transport in these lakes

statistics of microstructure patchiness in a stratified lake 1 statistics of microstructure patches in a sheared strongly stratified metalimnion of lake banyoles catalonia spain which occupied 40 of the total lake depth of 12 m are analyzed light winds 0 25 m the ratio ltp hp depends on the patch richardson and mixing reynolds numbers following the parameterization of lozovatsky and fernando 2002 analysis of the dynamics of mixing reveals that averaged vertical diffusivities ranged between 1 10 4 m2 s 1 and 5 10 5 m2 s 1 depending on the phase of the internal waves episodic wind gusts wind speed above 6 m s 1 transfer 1 6 of the wind energy to the metalimnion and 0 7 to the hypolimnion generating large microstructure patches with hp of several meters

the santa monica basin tracer experiment a study of diapycnal and isopycnal mixing we have studied cross isopycnal diapycnal and lateral mixing and stirring below the sill of santa monica basin by releasing two tracers sulfur hexafluoride and perfluorodecalin as close as possible to an isopycnal surface and measuring their subsequent dispersion the target for the release was a potential temperature surface at about 790 m depth roughly 100 m above the bottom and 50 m below the sill three surveys performed immediately after about 7 weeks after and about 6 months after the release showed that the time scales for lateral stirring and mixing in the basin were between 2 and 5 months the diapycnal diffusivity for the whole period was found to be 0 29 0 06cm2 s near the injection surface where the buoyancy frequency was about 1 1 cph this estimate may include some mixing in the turbulent boundary layer near the walls of the basin our best estimate for the diapycnal diffusivity in the basin interior is 0 25 0 08 cm2 s

trapped diurnal internal tides propagating semidiurnal internal tides and mixing estimates in the california current system from sustained glider observations 2006 2012 abstract from 2006 2012 along 3 repeated cross shore transects california cooperative oceanic fisheries investigations lines 66 7 80 and 90 in the california current system 33 609 shear and 39 737 strain profiles from 66 glider missions are used to estimate mixing via finescale parameterizations from a dataset containing over 52 000 profiles elevated diffusivity estimates and energetic diurnal d1 and semidiurnal d2 internal tides are found a within 100 km of the coast on lines 66 7 and 80 and b over the santa rosa cortes ridge srcr in the southern california bight scb on line 90 while finding elevated mixing near topography and associated with internal tides is not novel the combination of resolution and extent in this ongoing data collection is unmatched in the coastal ocean to our knowledge both d1 and d2 internal tides are energy sources for mixing at these latitudes the d1 internal tide is subinertial on line 90 d1 and d2 tides are equally energetic over the srcr the main site of elevated mixing within the scb numerous sources of internal tides at the rough topography in the scb produce standing and or partially standing waves on lines 66 7 and 80 the dominant energy source below about 100 m for mixing is the d1 internal tide which has an energy density of the d2 internal tide on line 80 estimated diffusivity estimated dissipation and d1 energy density peak in summer the d1 energy density shows an increasing trend from 2006 to 2012 its amplitude and phase are mostly consistent with topographically trapped d1 internal tides traveling with the topography on their right the observed offshore decay of the diffusivity estimates is consistent with the exponential decay of a trapped wave with a mode 1 rossby radius of 20 30 km despite the variable mesoscale it is remarkable that coherent internal tidal phase is found

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

implications for ocean mixing of internal wave scattering off irregular topography abstract earlier work has suggested that internal wave reflection off sloping bottoms may cause significant diapycnal mixing in the deep ocean and may also represent an important sink of internal wave energy most theories have been limited however by the representation of the bottom as an infinite plane slope in this paper the scattering of internal waves off irregular topography is studied for a few idealized bottom shapes we pay special attention to the critical case which occurs when the bottom slope dh dx locally matches the wave ray slope s analytical solutions for bottom shapes such that dh dx s at a single point are discussed for both locally convex and concave topography and are compared with the results of specular reflection theory they lead to the important conclusion that one is more likely to observe energy enhancement at the critical frequency above locally convex rather than concave topography this suggests that energy dissipation rates associated with the breaking of internal

stress orientations from well bore breakouts in the coalinga region borehole breakouts are caused by unequal stress concentrations around a borehole resulting in shear failure of the borehole wall and creating hole elongation perpendicular to the maximum horizontal in situ stress breakouts in tertiary sedimentary rocks were identified in 10 holes in the west coalinga cantua creek and west haven oil fields in the western san joaquin valley the deeper and more reliable 1 5 to 3 7 km breakouts were oriented northwest indicating northeast compressive stress perpendicular to the major fold axes in the san joaquin valley and at a 70 90 angle to the major strike slip faults in the coast ranges this stress direction is consistent with a reverse slip component on northwest trending faults in the region and with the focal mechanisms reported for the may 1983 coalinga earthquake the existing literature on stress measurements in central and southern california indicates that on a regional scale the stress regime is transitional between thrust and strike slip faulting a hydraulic fracture test in the kettleman hills oil field at 3 km depth indicates that this stress regime exists in the upper crust near coalinga as well

a first study of sf6 as a transient tracer in the southern ocean abstract halogenated transient tracers such as the chlorofluorocarbons cfc 11 cfc 12 and cfc 113 are commonly used in oceanographic studies these compounds enter the ocean via the atmosphere and their transient atmospheric concentrations make them valuable as oceanic tracers the trends of rapidly rising atmospheric concentrations of these tracers are however broken and the oceanic signal becomes increasingly more difficult to decipher there is a need for a new transient tracer to complement the existing suit of tracers especially for recently ventilated water masses one compound that looks promising in this respect is sulphur hexafluoride sf 6 an inert gas whose atmospheric concentration is rising rapidly in this paper the use of sf 6 as a transient tracer in recently ventilated waters is discussed and the method for determination of sf 6 in seawater is described tracer data from a section in the atlantic sector of the southern ocean along 6 e from 60 s to 40 s occupied during january 1998 are presented the antarctic polar front found close to 50 s was studied with a densely sampled section down to 400 m depth and sf 6 cfc 12 ratios are used to deduce ventilation ages and dilution factors a comparison of apparent ages derived from a variety of tracers is presented together with the uncertainties in these estimates this work demonstrates that sf 6 is a useful and valuable transient tracer for waters ventilated during the last 20 years

evidence for enhanced boundary mixing in the santa monica basin transients in the heat content in santa monica and san pedro basins imply a basin wide diapycnal eddy diffusivity greater than 1 cm sup 2 s this is significantly larger than the value for sf sub 6 of 0 25 0 08 cm sup 2 s determined for the interior of santa monica basin for september 1985 to february 1986 by ledwell and watson however the exodus of sf sub 6 from santa monica basin after february 1986 by which time the tracer had mixed to the boundaries of the basin was fast enough to be consistent with a greatly enhanced vertical flux since the kinetic energy in the basin had not changed significantly it is unlikely that a temporal increase in forcing resulted in enhanced fluxes in the interior of the basin the most likely interpretation is that diapycnal fluxes in the basin are dominated by processes in the boundary regions temperature and sf sub 6 profiles from near the edges of the basin do not give conclusive evidence either for or against such enhanced mixing 20 refs 19 figs 5 tabs

evaluation of olivine spinel geothermometry as an indicator of thermal history for peridotites olivine and spinel in peridotites from the miyamori ultramafic complex and the ichinogemata crater of northeast japan show a systematic variation in the mg mg fe ratio which is correlated mainly with the grain size of spinel this correlation can be explained by a diffusion model assuming a semi infinite composite sphere under cooling or heating conditions in order to obtain absolute temperatures of thermal events the olivine spinel geothermometer is applied to pairs of spinel core and olivine core average composition the calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel in the miyamori complex the calculated temperatures decrease monotonically with decrease in grain size of spinel whereas in the ichinomegata lherzolite nodule those of spinel smaller than 0 2 mm increase as the grain size decreases and those of spinel larger than 0 2 mm remain constant regardless of further increase in grain size these observations in the light of the diffusion model suggest that the miyamori complex may have cooled from higher than 800 c to lower than 600 c and that the lherzolite nodule from the ichinomegata crater may have been in equilibrium at 900 c before it was heated above 1 100 c for less than a few days these two examples indicate that olivine spinel pairs of peridotites do not always indicate an appropriate equilibration temperature we cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity

the flushing of two deep sea basins southern california borderland a time series of water temperature salinity and nutrient measurements has been made in san pedro and san nicolas basins of the southern california borderland between i977 and 1988 these basins exhibit temporally coincident changes in temperature and nutrient content defining a period of basin flushing 1982 1984 and a period of stagnation 1984 1987 during the flushing period cooler oxygenand no rich water replaces warmer oxygenand no depleted bottom water flushing produces a decrease of 0 2 c in the average temperature of san pedro basin bottom water 790 900 m and an increase in the average oxygen content by 12 pm these values are 0 i and 3 for san nicolas basin 1 400 l 800 m the residence time of water in san pedro basin during the flushing period is 12 6 months and j t2 months in san nicolas basin a box model approach constrained by the time series data for the stagnation period was used to determine values of the effective vertical eddy diffusivities for the 790 m horizon in san pedro basin and 1 400 m horizon in san nicolas basin 1 3 0 4 and 9 9k3 4 cm sk respectively

shear and strain in santa monica basin by injecting a cloud of sf6 into santa monica basin at a depth of 790 m and observing its vertical spread ledwell and watson 1991 estimate the average diapycnal diffusivity between 750 and 850 m as kp 2 5 10 5 m2 m 1 this diffusivity is higher than inferred from microstructure measurements in the open ocean thermocline for typical internal wave levels gregg and sanford 1988 to examine shear and strain in the basin we dropped expendable current profilers xcps and calculated strain from conductivity temperature and depth ctd casts shear and strain are above the gm76 model spectrum garrett and munk 1975 cairns and williams 1976 and comparisons with published parameterizations indicate more than enough shear variance to account for the high diffusivity reported by ledwell and watson thus the eddy diffusivity in santa monica basin is not at odds with lower values estimated from microstructure measurements in the open ocean but is due to an elevated internal wave field the spectral shapes however differ from typical open ocean spectra

a fluorescein tracer release experiment in the hydrothermally active crater of vailulu u volcano samoa 1 on 3 april 2001 a 20 kg point source of fluorescein dye was released 30 m above the bottom of the active summit caldera of vailulu u submarine volcano samoa vailulu u crater is 2000 m wide and at water depths of 600 1000 m with the bottom 200 m completely enclosed it thus provides an ideal site to study the hydrodynamics of an active hydrothermal system the magmatically driven hydrothermal system in the crater is currently exporting massive amounts of particulates manganese and helium the dispersal of the dye was tracked for 4 days with a fluorimeter in tow yo mode from the u s coast guard icebreaker polar sea lateral dispersion of the dye ranged from 80 to 500 m d 1 vertical dispersion had two components a diapycnal diffusivity component averaging 21 cm2 s 1 and an advective component averaging 0 025 cm s 1 these measurements constrain the mass export of water from the crater during this period to be 8 1 3 4 6 107 m3 d 1 which leads to a turnover time for water in the crater of 3 2 days coupled with temperature data from ctd profiles and mn analyses of water samples the power output from the crater is 610 100 350 mw and the manganese export flux is 240 kg d 1 the mn heat ratio of 4 7 ng j 1 is significantly lower than ratios characteristic of hot smokers and diffuse hydrothermal flows on mid ocean ridges and points to phase separation processes in this relatively shallow hydrothermal system

diapycnal mixing in the antarctic circumpolar current the vertical dispersion of a tracer released on a density surface near 1500 m depth in the antarctic circumpolar current west of drake passage indicates that the diapycnal diffusivity averaged over 1 yr and over tens of thousands of square kilometers is 1 3 6 0 2 3 10 25 m 2 s 21 diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less but still within a factor of 2 at 0 75 6 0 07 3 10 25 m 2 s 21 turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior where the energy for mixing is believed to derive from internal wave breaking indeed despite the frequent and intense atmospheric forcing experienced by the southern ocean the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean with levels elevated to only 20 50 above the garrett munk reference spectrum these results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean

mixing in a coastal environment 1 a view from dye dispersion 1 dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of new england the experiments were done in 1996 and 1997 as part of the coastal mixing and optics experiment during the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km2 to more than 50 km2 sundermeyer and ledwell 2001 diapycnal diffusivities inferred from dye dispersion range from 10 6 to 10 5 m2 s at buoyancy frequencies from 9 to 28 cycles hour diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by oakey and greenan 2004 agree closely in most cases estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by duda and rehmann 2002 and rehmann and duda 2000 are fairly consistent with the dye diffusivities the dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by mackinnon and gregg 2003 from profiling microstructure measurements obtained at the same site in august 1996 all of the measurements support the general conclusion that the diffusivity averaged over several days is seldom greater than 10 5 m2 s in the stratified waters at the site and usually not much greater than 10 6 m2 s severe storms such as a hurricane that passed over the cmo site in 1996 can dramatically increase the mixing at the site however

a tracer study of mixing in the santa cruz basin a tracer release experiment to study diapycnal and isopycnal mixing was performed in santa cruz basin in 1988 1989 approximately 1450 g of sulfur hexafluoride were injected close to an isopycnal surface at 1500 m depth in may 1988 this depth is midway between the bottom of the basin at 1920 m and the sill at 1084 m the diapycnal dispersion measured during the first few months while the tracer was still mixing to the walls of the basin yielded a diapycnal eddy diffusivity of 1 0 cm2 s 1 within a factor of 1 5 this is approximately 4 times larger than the diffusivity found by ledwell and watson 1991 in the interior of santa monica basin where the buoyancy frequency was 5 times greater the tracer was well mixed to the walls 6 months after the injection and at that time the diapycnal spread of the tracer was greater near the walls than in the interior the rate of diapycnal dispersion of the tracer increased dramatically after the tracer had mixed to the walls of the basin indicating enhanced boundary mixing an effective basin wide diapycnal diffusivity of 10 cm2 s 1 would be required to support the fluxes of tracer observed a one dimensional model for tracer and heat diffusion does not simulate the results very well however probably because of the finite mixing time between the boundary region and interior and because of the complexity of the processes in the boundary region

boundary versus internal diapycnal mixing in stratified natural waters using the fluorescent dye uranin tracer release experiments to study the contribution of bottom boundary mixing to diapycnal transport in stratified natural waters were performed in lake alpnach central switzerland during 1992 1995 a first experiment involved injecting the tracer from a point source into the center of the hypolimnion that part of the lake below the surface mixed layer an in situ fluorometer was then employed to detect the horizontal and vertical spreading of the tracer cloud allowing rates of diapycnal diffusivity to be determined as long as the tracer was confined to the interior water region the diapycnal diffusivity was relatively small however after the tracer cloud had reached the lake boundary the diapycnal diffusivity increased by approximately one order of magnitude in a second experiment the tracer was released near the sediment water interface in this case the dynamics of vertical tracer spreading were opposite during the first few hours after tracer release diapycnal diffusivities were large subsequently decreasing as the tracer cloud drifted away from the lake boundary basin wide diapycnal diffusivities calculated from heat flux measurements based on temperature profiles obtained from thermistor chains or conductivity temperature depth casts agreed well with the values obtained from the vertical tracer diffusion after horizontal homogenization the results of the tracer experiments corroborate the hypothesis that diapycnal fluxes are determined predominantly by mixing in the bottom boundary region

the baltic sea tracer release experiment 2 mixing processes 1 deep water mixing processes in the largest basin of the baltic sea gotland basin are investigated here with the help of long term moored instrumentation and turbulence microstructure measurements budgets for heat and salinity reveal a strong temporal variability in the deep water mixing coefficients that is shown to be coupled to the wind forcing down to time scales of individual wind events consistent with the results from a simultaneously conducted tracer release experiment near boundary turbulence was identified as the key processes for basin scale mixing turbulent bottom boundary layers were found to be thick and stable in the flat deep part of the basin where highest dissipation rates were observed and comparatively thin less energetic and highly variable on the slopes due to a permanent tendency for boundary layer restratification in the absence of internal tides strongly damped basin scale topographic waves with periods of a few days a deep rim current and to a smaller extent near inertial waves were identified as the main energy sources for boundary mixing these results are likely to be relevant also for other medium sized stratified basins

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

determination of diapycnal diffusion rates in the upper thermocline in the north atlantic ocean using sulfur hexafluoride 1 the apparent diapycnal diffusivity below the wind driven surface mixed layer of the ocean was determined in an anticyclonic eddy in the eastern north atlantic using sulfur hexafluoride sf6 tracer data collected in june 1998 in this tracer experiment the downward penetration of sf6 was measured for 3 weeks following the deliberate injection of sf6 in the surface mixed layer the resulting data were used to constrain the one dimensional fickian diffusion model to estimate the diapycnal diffusivity the model also includes the lateral diffusion component so that it can more accurately represent the time evolution of the sf6 concentrations along the isopycnal surface this affects the estimation of the diapycnal diffusivity for the upper thermocline immediately below the surface mixed layer we estimated the diapycnal diffusivity for the 3 week period as 0 3 0 2 cm2 s 1 at a buoyancy frequency of 8 2 cph

turbulent mixing and exchange with interior waters on sloping boundaries abstractmicrostructure measurements along the axes of monterey and soquel submarine canyons reveal 200 300 m thick well stratified turbulent near bottom layers with average turbulent kinetic energy dissipation rates e 4 10 8 w kg 1 and eddy diffusivities k 16 10 4 m2 s 1 assuming mixing efficiency 0 2 to at least thalweg depths of 1200 m turbulent dissipation rates are an order of magnitude lower in overlying waters whereas buoyancy frequencies are only 25 higher well mixed bottom boundary layer thicknesses hn are an order of magnitude thinner than the stratified turbulent layer hn he because well stratified turbulent layers are commonly observed above slopes arguments that mixing efficiency should be reduced on sloping boundaries do not hold in cases of energetic internal wave generation or interaction with topography an advective diffusive balance is used to infer velocities and transports predicting horizontal upslope flows of 10 50 m day 1 extrapolating this estimate globa

a laboratory model of vertical ocean circulation driven by mixing a model of deep ocean circulation driven by turbulent mixing is produced in a long rectangular laboratory tank the salinity difference is substituted for the thermal difference between tropical and polar regions freshwater gently flows in at the top of one end dense water enters at the same rate at the top of the other end and an overflow in the middle removes the same amount of surface water as is pumped in mixing is provided by a rod extending from top to bottom of the tank and traveling back and forth at constant speed with reynolds numbers 500 a stratified upper layer thermocline deepens from the mixing and spreads across the entire tank simultaneously a turbulent plume deep ocean overflow from a dense water source descends through the layer and supplies bottom water which spreads over the entire tank floor and rises into the upper layer to arrest the upper layer deepening data are taken over a wide range of parameters and compared to scaling theory energetic considerations and simple models of turbulently mixed fluid there is approximate agreement with a simple theory for reynolds number 1000 in experiments with a tank depth less than the thermocline depth a simple argument shows that mixing and plume potential energy flux rates are equal in magnitude and it is suggested that the same is approximately true for the ocean

integrating samplers for the deep sea two new types of mechanical integrating samplers have been tried in the deep sea one driven by a constant force spring the other by a spring driven hydraulic system the samplers are triggered simultaneously with the niskin bottle to which they are attached and fill in about 90 min they are useful for sampling poorly mixed tracers and have been used successfully in tracer release experiments at 800 m depth in santa monica basin in 1985 and to depths of 1900 m in santa cruz basin in 1988

an isopycnal view of near boundary mixing and associated flows abstract the effectiveness of mixing near a sloping boundary is reduced not just by the ratio of the stratification there to that in the interior but by the square of this ratio this result has previously been derived and interpreted by invoking the upgradient vertical buoyancy flux associated with secondary circulation as this opposes the downgradient vertical diffusive flux here it is shown that the result may be derived very simply by considering the diffusive flux across an isopycnal rather than a horizontal surface the reduction in the buoyancy flux comes from the increased isopycnal spacing and the reduced distance to the boundary derivation of the secondary circulation in the isopycnal diapycnal framework is more subtle however as it involves allowing for the curvilinear nature of the coordinates

dispersion of a tracer in the deep gulf of mexico a 25 km streak of cf3sf5 was released on an isopycnal surface approximately 1100 m deep and 150 m above the bottom along the continental slope of the northern gulf of mexico to study stirring and mixing of a passive tracer the location and depth of the release were near those of the deep hydrocarbon plume resulting from the 2010 deepwater horizon oil well rupture the tracer was sampled between 5 and 12 days after release and again 4 and 12 months after release the tracer moved along the slope at first but gradually moved into the interior of the gulf diapycnal spreading of the patch during the first 4 months was much faster than it was between 4 and 12 months indicating that mixing was greatly enhanced over the slope the rate of lateral homogenization of the tracer was much greater than observed in similar experiments in the open ocean again possibly enhanced near the slope maximum concentrations found in the surveys had fallen by factors of 104 107 and 108 at 1 week 4 months and 12 months respectively compared with those estimated for the initial tracer streak a regional ocean model was used to simulate the tracer field and help interpret its dispersion and temporal evolution model data comparisons show that the model simulation was able to replicate statistics of the observed tracer distribution that would be important in assessing the impact of oil releases in the middepth gulf