on the effect of topographically enhanced mixing on the global ocean circulation abstract the strong influence of enhanced diapycnal mixing over rough topography on bottom water circulation is illustrated using results from two global ocean model experiments in the first diapycnal diffusivity is set to the observed background level of 10 5 m2 s 1 in regions not subject to shear instability convection or surface driven mixing in the second experiment mixing is enhanced above rough bottom topography to represent the dissipation of internal tides three important results are obtained first without the enhanced mixing in the abyssal ocean the deep north pacific ocean becomes essentially a stagnant basin with little bottom water circulation and very weak deep stratification allowing for the enhanced diapycnal mixing above rough bottom topography leads to increased bottom water circulation and deep stratification and a potential vorticity distribution in the north pacific that is much more realistic second the enhanced diapycnal mixing above rough topography results in a signif

vertical mixing in the ocean the thermohaline circulation of the ocean results primarily from downwelling at sites in the nordic and labrador seas and upwelling throughout the rest of the ocean the latter is often described as being due to breaking internal waves here we reconcile the difference between theoretical and observed estimates of vertical mixing in the deep ocean by presenting a revised view of the thermohaline circulation which allows for additional upwelling in the southern ocean and the separation of the north atlantic deep water cell from the antarctic bottom water cell the changes also mean that much less wind and tidal energy needs to be dissipated in the deep ocean than was originally thought

a comparison of shear and buoyancy driven planetary boundary layer flows abstract planetary boundary layer pbl flows are known to exhibit fundamental differences depending on the relative combination of wind shear and buoyancy forces these differences are not unexpected in that shear instabilities occur locally while buoyancy force sets up vigorous thermals which result in nonlocal transport of heat and momentum at the same time these two forces can act together to modify the flow field in this study four large eddy simulations less spanning the shear and buoyancy flow regimes were generated two correspond to the extreme cases of shear and buoyancy driven pbls while the other two represent intermediate pbls where both forces are important the extreme cases are used to highlight and quantify the basic differences between shear and convective pbls in 1 flow structures 2 overall statistics and 3 turbulent kinetic energy tke budget distributions results from the two intermediate les cases are used to develop and verify a velocity scaling and a tke budget mode

lenticular shale fabrics resulting from intermittent erosion of water rich muds interpreting the rock record in the light of recent flume experiments lenticular lamination is a fabric that is known from shales of all ages but its origin and paleoenvironmental significance is poorly understood we have successfully reproduced this fabric in flume experiments beds of water rich mud were eroded in a flume and yielded sub millimeter to centimeter size fragments that can be transported in bedload for distances of ten kilometers or more upon redeposition and compaction these deposits have the same textural qualities as lenticular laminated shales from the rock record although accumulation of fecal pellets or abundant burrow tubes in a shale may produce comparable fabrics upon compaction these can be distinguished from erosion produced lenticular lamination via petrographic criteria lenticular lamination in shales that is due to deposition of water rich mud fragments most likely records intermittent erosion and transport of surficial muds by currents

sedimentary record of explosive silicic volcanism in a cretaceous deep marine conglomerate succession northern antarctic peninsula lower cretaceous conglomeratic strata exposed on southern sobral peninsula were deposited on a deep marine apron in the back arc larsen basin close to its faulted boundary with the antarctic peninsula magmatic arc the succession is dominated by amalgamated beds of clast supported conglomerate which together with minor intercalated sandstones consist of varied but largely basaltic to andesitic volcanic material and clasts derived from the palaeozoic triassic meta sedimentary basement of the arc most of the volcanic clasts are thought to have been derived from lithified volcanic successions or older synvolcanic deposits rather than from sites of coeval eruption these mixed provenance strata enclose a number of intervals consisting mainly of inverse normally graded conglomerate and graded stratified pebbly sandstone in which the sand fraction is dominated by crystals and vitric grains considered to have been redeposited in the immediate aftermath of explosive silicic arc volcanism like syneruption deposits on non marine volcaniclastic aprons these intervals are more sand prone than the enclosing strata and appear to show evidence of unusually rapid aggradation plagioclase from one such interval has yielded 40ar 39ar ages concordant at 121 ma similar to those obtained from the non marine cerro negro formation deposited within the magmatic arc it is suggested that the two successions can be viewed as counterparts both recording a history of mainly basaltic to andesitic volcanism punctuated by relatively infrequent explosive silicic eruptions whereas the cerro negro formation consists mainly of syneruption deposits most of the volcaniclastic material delivered to the eruption distal deep marine apron appears to have been derived by normal degradation processes only rare silicic eruptions were capable of supplying pyroclastic material rapidly enough and in sufficient quantities to produce compositionally distinct syneruption intervals

revisiting internal wave generation by tide topography interaction 1 to clarify the physics responsible for tidal generation of internal waves over abyssal small scale bottom topography we carry out a series of numerical experiments using a vertically two dimensional primitive equation model it is found that the stratified fluid response can be categorized into four dynamical regimes in terms of two dimensionless parameters related to tidal advection effects and buoyancy effects under parameters corresponding to the real abyssal ocean quasi steady internal waves are created over the bottom topography only during the limited time interval centered at the maximum of opposing tidal flow since the created internal waves cannot maintain the quasi steady balance with the decreasing tidal flow they then start propagating upstream this partial quasi steady response in the abyssal ocean is found to be quite similar to the response actually observed in the shallow ocean causing the widespread misinterpretation that strong tide topography interactions in the shallow ocean can generate quasi steady lee waves in the present study we clearly distinguish between the dynamical regimes corresponding to quasi steady response and partial quasi steady response to dispel this confusion about the interpretation of tide topography interactions not only for the abyssal ocean but also for the shallow ocean

tectonic implications of a petrographic and geochemical characterization of the lower to middle jurassic sailor canyon formation northern sierra nevada california the lower to middle jurassic sailor canyon formation in the northern sierra nevada of california consists of basinal volcaniclastic and tuffaceous sedimentary rocks petrographic and geochemical data from the formation provide an opportunity to assess a proposed 400 km of pre late cretaceous dextral strike slip displacement on the cryptic mojave snow lake fault a corollary to the proposed displacement is that triassic jurassic volcanic complexes exposed in eastern california were the source for sailor canyon formation sediments however geochemical data derived from volcanic rocks from this area are unlike those of the volcanic provenance characterized for the sailor canyon formation comparisons of geochemical data suggest that contemporaneous volcanic centers exposed in western nevada north of 38 n may be better candidates for the source of the volcanic material in the sailor canyon formation this interpretation implies that dextral strike slip displacement on the mojave snow lake fault may be less than 200 km a value that agrees with some interpretations of mesozoic intrabatholithic displacements in the sierra nevada

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

two dimensional simulation of orographic effects on mesoscale boundary layer convection in this paper orographic effects on mesoscale boundary layer convection are studied through a series of idealized numerical experiments it is found that hills tend to weaken convective activity at their summits under higher background winds whilst strong updraughts can be observed at the summits or slightly downwind of the peaks under light wind conditions these can be associated with so called convective cores when the background winds are strong the effects of the hill length on the results are only significant when the height of the hill reaches 500 m the combined effect of a sensible heating maximum on the hill summit and baroclinic tendencies due to the elevated heating is a tendency to produce a convective core under reasonably light wind conditions under higher wind conditions the strong ascents and descents on both sides of the hill as well as the lee wave dynamics seem to pose a more important impact on the convective features than the thermal forcing of the hill a richardson number balance has been proposed to explain these two kinds of response finally under stronger wind conditions the convective available potential energy cape is small at the summit and reaches its maximum value in the lee of the hill while under light wind conditions the air parcels at the summit have more buoyancy and the cape downwind of the top of the hill is slightly larger than that elsewhere copyright 2002 royal meteorological society

meridional overturning circulation of the deep pacific estimated assuming the vertical advective diffusive balance 1 assuming the vertical advective diffusive balance we calculate the meridional overturning circulation of the deep pacific based on the observed density field in addition to the global distribution of diapycnal diffusivity in the pycnocline bottom intensified mixing that follows a gaussian distribution with depth is taken into account when diapycnal diffusivity in the pycnocline is assumed to extend uniformly to the bottom the northward transport of the lower circumpolar deep water lcdw across the equator is only 1 sv when the bottom intensified mixing is incorporated the meridional overturning circulation increases decreases or even reverses depending on the vertical gradient of diapycnal diffusivity the northward transport of the lcdw crossing the equator becomes comparable to the estimates from previous current meter moorings as well as hydrographic surveys only when the mixing extends well above the rough ocean bottom and hence the vertical gradient of diapycnal diffusivity is small

northward abyssal transport through the samoan passage and adjacent regions a conductivity temperature depth hydrographic survey in january february 1994 and a 17 month deployment of current meter moorings from september 1992 to march 1994 were carried out to determine the volume transport water mass characteristics and diathermal fluxes of northward flowing abyssal waters in the samoan passage and adjacent regions of the south pacific ocean geostrophic calculations relative to 1 2 c potential temperature indicated northward transport of 7 8 sv in the samoan passage 1 1 sv through a gap in robbie ridge and 2 8 sv along the eastern flank of the manihiki plateau all of the total of 11 7 sv of northward geostrophic transport was in waters colder than 1 1 c the northward transport distribution was bimodal in temperature with a cold mode of 3 6 sv in the range 0 65 0 70 c occurring entirely in the samoan passage and a warm mode of 3 0 sv in the range of 0 80 0 85 c occurring mainly along the manihiki plateau within the samoan passage 7 1 sv of the northward transport was below 4000 m where the geostrophic calculation was confirmed by an equal estimate of transport from current meters during the simultaneous 3 day period the 17 month mean transport from the moored array was 6 0 sv 0 5 by using the observed temporally varying flow within the samoan passage together with the hydrographic snapshot across the region an estimate of the total mean northward transport of 10 6 sv 1 7 was obtained estimates of the flow across near bottom potential temperature surfaces indicate extraordinarily high rates of mixing with heating of the abyssal layer up to 20 w m 2 corresponding to diffusivities up to 10 1 m2 s 1

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

global mapping of diapycnal diffusivity in the deep ocean based on the results of expendable current profiler xcp surveys 1 based on the results of the extensive xcp surveys in the north pacific we first examine the empirical relationship between the estimated diapycnal diffusivity in the thermocline and the numerically predicted available energy density of the semidiurnal internal tide major energy source for diapycnal diffusivity at each location the diapycnal diffusivity is estimated by applying the parameterization of gregg 1989 to the fine scale vertical shear of horizontal velocity measured by xcp next the numerically predicted energy density of the semidiurnal internal tide at each longitude and latitude in the world s oceans is incorporated into the resulting empirical relationship to obtain the global distribution of diapycnal diffusivity in the thermocline this model predicts that strong diapycnal mixing mixing hotspot is limited to the prominent topographic features at latitudes between 20 and 30

silicic acid leakage from the southern ocean opposing effects of nutrient uptake and oceanic circulation 1 studies invoking the silicic acid leakage hypothesis to explain events of high biogenic opal deposition at low latitudes did not consider variations in formation rate of southern ocean intermediate waters comparison of 30si records from the southern ocean to records of biogenic opal accumulation from low latitude upwelling systems indicates a strong negative correlation between the silicic acid content of southern ocean surface waters and opal deposition at low latitudes during the last 80 kyr bp this implies that si leakage was somehow rendered ineffective when the silicic acid content of southern ocean waters was high comparison of opal accumulation records with records of intermediate water formation rates suggests that lower rates of intermediate water formation offset the higher silicic acid content of these water masses during glacial periods this indicates that physical processes that control intermediate water transport dominate over biological processes that set the silicic acid content of antarctic surface waters in determining the magnitude of silicic acid leakage from the southern ocean

modeling layered structure in deep pacific circulation the layered structure of the deep pacific circulation is investigated using a multilevel numerical model with idealized geometry the circulation is driven by sea surface heating and newtonian body cooling at the southwest corner of the basin the latter forcing is intended to incorporate thermohaline forcing in the southern ocean a layered structure forms in the zonally averaged meridional circulation when the reference density for the body cooling varies vertically the density balance between vertical advection and vertical diffusion holds in the zonally averaged field as well as in the interior this balance combined with the reference density for the body cooling determines the zonally averaged vertical velocity and hence the zonally averaged meridional circulation from the zonally averaged continuity equation this two dimensional mechanism for describing the layered structure for this three dimensional rotating fluid model is identical to that for a two dimensional nonrotating fluid model the increased vertical structure is accompanied by the enhancement of some higher vertical modes which brings about various changes in the three dimensional structure such as the formation of a broad diffusive deep western boundary current

preparation of oriented block samples of lignite for petrographic analysis abstract the manufacture of particulate samples for coal petrographic analysis destroys the original petrographic fabric of the coal so that fine scale millimeter to centimeter variations in petrographic composition through the seam cannot be identified the use of oriented block samples prepared from pillar samples allows the physical integrity of the coal seam to be preserved and thus studied while the preparation of polished block samples does not generally present any problems for coals of sub bituminous rank or higher the high moisture content of lignites means that the polished surface is prone to desiccation this leads to the formation of shrinkage cracks which cause distortion of the surface and render microscopic analysis under high power magnification 500 virtually impossible in order to overcome this problem samples of lignite were dehydrated in acetone and then impregnated with a low viscosity epoxy resin a thin film of immersion oil during analysis further inhibits the affects of any desiccation on the polished surface long term storage under nitrogen has been effective in preserving the high quality of the polished surface for more than four years

numerical study of an observed orogenic mesoscale convective system part 1 simulated genesis and comparison with observations abstract the interaction of topographically induced thermally and mechanically driven diurnal flow regimes in the lee of the rockies is shown to lead to the growth of a mesoscale convective system mcs an organic mcs observed during the 1977 combined south park area cumulus experiment and high plains experiment is numerically simulated with a two dimensional nonhydrostatic cloud model covering spatial scales of 1000 km in this numerical investigation meso meso and meso scales of motion are represented simultaneously as a result interesting features of cloud mesoscale interaction are predicted that cannot be represented in cloud parameterization frameworks based on the results of this simulation a six stage conceptual model of orogenic development is given

lagrangian analysis of the meridional overturning circulation in an idealized ocean basin the lagrangian ocean model is used as a tool to simulate the response of the basin scale overturning circulation to spatially variable diapycnal mixing in an idealized ocean basin the model explicitly calculates the positions velocities and tracer properties of water parcels owing to its lagrangian formulation numerical diffusion is completely eliminated and water parcel pathways and water mass ages can be quantified within the framework of the discrete advective transit time distribution to illustrate the ventilation pathways simulated trajectories were tracked backward in time from the interior ocean to the surface mixed layer where the water parcel was last in contact with the atmosphere this new diagnostic has been applied to examine the response of the meridional overturning circulation to highly localized diapycnal mixing through sensitivity experiments in particular the focus is on three simulations the first holds vertical diffusivity uniform in the second the vertical diffusivity is confined within an equatorial box and the third simulation has a diffusivity pattern based on idealized hurricane induced mixing domain integrated deep ventilation rates and heat transport are similar between the first two cases however locally enhanced mixing yields about 30 younger water mass age in the tropical thermocline due to intense localized upwelling in the third simulation a slower ventilation rate of deep waters is found to be due to the lack of abyssal mixing these results are interpreted using the classical theories of abyssal circulation highlighting the strong sensitivity of the ventilation pathways to the spatial distribution of diapycnal mixing

distribution of fine scale shear in the deep waters of the north pacific obtained using expendable current profilers 1 recent numerical experiments hibiya et al 1996 1998 2002 predicted that the energy cascade across the internal wave spectrum down to small dissipation scales was under strong control by parametric subharmonic instabilities that transfer energy from the low vertical wave number double inertial frequency wave band to a high vertical wave number near inertial frequency wave band to test whether or not the numerically predicted energy cascade process is actually dominant in the real deep ocean we deployed a total of 106 expendable current profilers over a large area in the north pacific to examine the spatial distribution of high vertical wave number vertical wavelength 25 m shear at midlatitudes significant enhancement of the 25 m vertical shear was found over prominent generation regions of semidiurnal internal tides such as the hawaiian ridge and the izu ogasawara ridge where the semidiurnal tidal frequency exceeds twice the local inertial frequency at high latitudes in contrast no significant enhancement of the 25 m vertical shear was found to occur even over another prominent generation region of semidiurnal internal tides the aleutian ridge where the semidiurnal tidal frequency is less than twice the local inertial frequency we find that the spatial distribution of the intensity of the 25 m vertical shear correlates very well with that of the low vertical wave number double inertial frequency internal wave energy numerically predicted by nagasawa et al 2000 and niwa and hibiya 2001a 2001b this is the first in situ evidence for the dominant role of parametric subharmonic instability in transferring deep ocean internal wave energy down to small dissipation scales this study provides a theoretical framework for future attempts to determine the large scale structure of mixing over the world s oceans

a sedimentary facies model for glacial age sediments in baldwin lake southern california abstract a combined sedimentological and high resolution petrographic analysis was conducted on a glacial age 20 000 65 000 cal yr bp sediment core from baldwin lake southern california the results of this research represent the most complete glacial age terrestrial climate record from southern california to date these results are used to characterize the different sediment types and to investigate the difference in depositional processes and environments between the core s three predominant sediment types massive semi laminated and laminated sediments massive sediments are commonly associated with a blocky texture and or desiccation cracks are organic poor have high magnetic susceptibility values and are coarser grained thin sections from massive sediments reveal a homogenous sediment fabric sub centimeter scale laminated and centimeter scale semi laminated sediments are generally organic rich have low magnetic susceptibility values and are finer grained thin sections from laminated and semi laminated sediments reveal diffuse sub millimeter to millimeter scale laminae this combination of sedimentological and high resolution petrographic data enabled us to characterize four sediment facies each related to specific depositional processes and environments 1 a playa lake 2 a perennial shallow lake 3 an intermediate lake with variable lake level and 4 a perennial deep lake at centennial to millennial timescales lower lake levels are represented by deposition of massive to semi laminated sediments in a playa to a perennial shallow lake environment at similar timescales higher lake levels are recorded by semi laminated to laminated sediments deposited in an intermediate lake to a perennial deep lake environment these results provide an additional sedimentological study for comparison to similar arid environment basins and for comparison to existing regional paleoclimatic reconstructions

latitudinal dependence of diapycnal diffusivity in the thermocline estimated using a finescale parameterization 1 the spatial distribution of diapycnal diffusivity in the thermocline over a large area in the interior of the north pacific is obtained by incorporating the finescale vertical shear of horizontal velocity measured by expendable current profilers into the empirical formula proposed by gregg 1989 it is clearly shown that diapycnal diffusivity in the north pacific strongly depends on the latitude for example the estimated diapycnal diffusivities near the hawaiian ridge and the izu ogasawara ridge exceed 10 4 m2 s 1 whereas those near the aleutian ridge and the emperor seamounts are of the order of 0 1 10 4 m2 s 1 even though the available semidiurnal internal tidal energy is similar among all of these prominent topographic features the observed results are consistent with the numerical prediction that the cascade of internal tidal energy down to dissipation scales is dominated by parametric subharmonic instability which is expected to occur only below 30 n