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

silurian turbidite provenance on the northern avalonian margin the windermere supergroup of nw england was deposited in a foreland basin on the northern margin of eastern avalonia and preserves an almost complete ashgill to poidoli sequence it was ideally positioned to record provenance changes associated with the closure of the iapetus ocean petrographic geochemical and nd isotopic parameters show distinct and concordant temporal trends the late ashgill to early llandovery is characterized by progressive hemipelagic dilution of material derived from the underlying borrowdale volcanic group from the llandovery onward petrographic and geochemical data indicate supply from a recycled orogen source input from the scandian orogen may be inferred no conclusive evidence is recorded of laurentian input into the basin by the late wenlock a unified sedimentary system probably spanned the iapetus ocean an abrupt reversal in provenance trends towards the end of the ludlow records basin inversion the occlusion of the iapetus seaway and concomitant cannibalization of older foreland basin sediments provenance data derived from upper wenlock to poidoli sediments aid the characterization of marine foreland basins

changing sediment dynamics due to natural reforestation in the dragonja catchment sw slovenia abstract article history received 18 march 2008received in revised form 20 february 2009accepted 23 february 2009keywords dragonjasloveniahillslope erosionland use changenatural reforestationwatem sedem under the in uence of socio economic changes in many regions in europe a trend of decreasing agriculturalactivity has been observed since the second world war the resulting reforestation profoundly changeswater and sediment supply to river channels deposition rates on the oodplains and erosion rates on thehillslopes we studied these changes in the 91 km 2 dragonja catchment in southwestern slovenia with the spatially distributed erosion and sediment delivery model watem sedem the hillslope sedimentdelivery to the river channel was calculated on the basis of parameters soil and precipitation parameters adem and land use measured in the eld and laboratory in 2002 and land use maps based on aerialphotographs from 1954 1975 1985 and 1994 for two independent calibrations watem sedem modelled asharp decline of 69 in total hillslope sediment delivery from 1954 to 2002 as the sub catchments rokava and upper dragonja did not reforest in the same way the sediment yieldresponse is different as well separate calculations show the same reduction 45 in sediment yield from1954 to 1975 after 1975 the sediment yield was stable in the rokava sub catchment in the upper dragonjathe trend continued to a total reduction of 76 of sediment out ow since 1954 the sources of ne sediment were determined by analysing the hysteresis of the discharge waves and thesuspended sediment texture the sediment that leaves the catchment originates from three sources hillslopes erosional bedrock banks and sedimentary riverbanks the analysis of the suspended sediment texture suggests that during a discharge wave the suspendedsediment originates predominantly from the hillslopes during low stage the sparse sediment in the watercolumn largely originates from large bedrock banks the sedimentary riverbanks are not an important sourceof suspended sediment 2009 elsevier b v all rights reserved

parameterizing tidal dissipation over rough topography the traditional model of tidal dissipation is based on a frictional bottom boundary layer in which the work done by bottom drag is proportional to a drag coefficient and the velocity cubed however away from shallow coastal regions the tidal velocities are small and the work done by the bottom boundary layer can account for only weak levels of dissipation in the deep ocean the energy flux carried by internal waves generated over rough topography dominates the energy transfer away from barotropic flow a parameterization for the internal wave drag over rough topography is included as a dissipative mechanism in a model for the barotropic tides model results suggest that the inclusion of this dissipation mechanism improves hydro dynamical models of the ocean tide it also substantially increases the amount of modeled tidal dissipation in the deep ocean bringing dissipation levels there into agreement with recent estimates from topex poseidon altimetry data

numerical modeling of the global semidiurnal tide in the present day and in the last glacial maximum 1 a hydrodynamic model incorporating a self consistent treatment of ocean self attraction and loading sal and a physically based parameterization of internal tide it drag is used to assess how accurately barotropic tides can be modeled without benefit of data and to explore tidal energetics in the last glacial maximum lgm m2 solutions computed at high resolution with present day bathymetry agree with estimates of elevations from satellite altimetry within 5 cm rms in the open ocean this accuracy and agreement with atlimetric estimates of energy dissipation are achieved only when sal and it drag are included in the model solutions are sensitive to perturbations to bathymetry and inaccuracies in available global databases probably account for much of the remaining error in modeled elevations the 100 m drop in sea level during the lgm results in significant changes in modeled m2 tides with some amplitudes in the north atlantic increasing by factors of 2 or more dissipation is also significantly changed by the drop in sea level if it drag estimated for the modern ocean is assumed dissipation increases by about 50 globally and almost triples in the deep ocean however it drag depends on ocean stratification which is poorly known for the lgm tests with modified it drag suggest that the tendency to a global increase in dissipation is a robust result but details are sensitive to stratification significant uncertainties about paleotides thus remain even in this comparatively simple case where bathymetry is well constrained

reduced mixing from the breaking of internal waves in equatorial waters in the oceans heat salt and nutrients are redistributed much more easily within water masses of uniform density than across surfaces separating waters of different densities but the magnitude and distribution of mixing across density surfaces are also important for the earth s climate as well as the concentrations of organisms most of this mixing occurs where internal waves break overturning the density stratification of the ocean and creating patches of turbulence predictions of the rate at which internal waves dissipate were confirmed earlier at mid latitudes here we present observations of temperature and velocity fluctuations in the pacific and atlantic oceans between 42 degrees n and 2 degrees s to extend that result to equatorial regions we find a strong latitude dependence of dissipation in accordance with the predictions in our observations dissipation rates and accompanying mixing across density surfaces near the equator are less than 10 of those at mid latitudes for a similar background of internal waves reduced mixing close to the equator will have to be taken into account in numerical simulations of ocean dynamics for example in climate change experiments

the impact of abyssal mixing parameterizations in an ocean general circulation model a parameterization of vertical diffusivity in ocean general circulation models has been implemented in the ocean model component of the community climate system model ccsm the parameterization represents the dynamics of the mixing in the abyssal ocean arising from the breaking of internal waves generated by the tides forcing stratified flow over rough topography this parameterization is explored over a range of parameters and compared to the more traditional ad hoc specification of the vertical diffusivity diapycnal mixing in the ocean is thought to be one of the primary controls on the meridional overturning circulation and the poleward heat transport by the ocean when compared to the traditional approach with uniform mixing the new mixing parameterization has a noticeable impact on the meridional overturning circulation while the upper limb of the meridional overturning circulation appears to be only weakly impacted by the transition to the new parameterization the deep meridional overturning circulation is significantly strengthened by the change the poleward ocean heat transport does not appear to be strongly affected by the mixing in the abyssal ocean for reasonable parameter ranges the transport of the antarctic circumpolar current through the drake passage is related to the amount of mixing in the deep ocean the new parameterization is found to be energetically consistent with the known constraints on the ocean energy budget

alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization systematic westerly biases in the northern hemisphere wintertime flow of the meteorological office 15 layer operational model and 11 layer general circulation model are described evidence that the failure to parametrize subgrid scale orographic gravity wave drag may account for such biases is presented this evidence is taken from aircraft studies surface pressure drag measurements and studies of the zonally averaged momentum budget a parametrization scheme is described in which the surface stress is proportional to the near surface wind speed and static stability and to the variance of subgrid scale orography the stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear a richardson number dependent wave breaking formulation is devised and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability it is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere warms polar regions by adiabatic descent and decelerates the mean westerlies in the troposphere the influence of the parametrization scheme on integrations of the 11 layer model is described and found to be generally beneficial in a discussion of the reasons why this problem has only recently emerged it is suggested that the satisfactory northern hemisphere winter circulations of previous coarser general circulation models were due to a compensation implied by underestimating both the surface drag and the horizontal flux of momentum by explicitly resolved large scale eddies

the age of water and ventilation timescales in a global ocean model abstract the age of water in the world ocean is studied using a passive age tracer introduced into a global ocean model additional information is derived from a transient dye tracer that tracks the time dependent spreading of surface waters into the model ocean interior of particular interest is the nature of ocean ventilation over the 10 100 yr timescale as well as the simulated age of deep and bottom water masses in the upper model levels young water is found to correspond with regions of convergence and downwelling in the surface ekman layer upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer in the deep model levels water age varies greatly between oceans with young water found in convectively active regions in the north atlantic and in the ross and weddell seas and old water found in the deep north pacific the oldest water mass mixture located at 2228 m depth in the western pacific ocean is dated at 1494 years made

possible forcing of global temperature by the oceanic tides an approximately decadal periodicity in surface air temperature is discernable in global observations from a d 1855 to 1900 and since a d 1945 but with a periodicity of only about 6 years during the intervening period changes in solar irradiance related to the sunspot cycle have been proposed to account for the former but cannot account for the latter to explain both by a single mechanism we propose that extreme oceanic tides may produce changes in sea surface temperature at repeat periods which alternate between approximately one third and one half of the lunar nodal cycle of 18 6 years these alternations recurring at nearly 90 year intervals reflect varying slight degrees of misalignment and departures from the closest approach of the earth with the moon and sun at times of extreme tide raising forces strong forcing consistent with observed temperature periodicities occurred at 9 year intervals close to perihelion solar perigee for several decades centered on a d 1881 and 1974 but at 6 year intervals for several decades centered on a d 1923 as a physical explanation for tidal forcing of temperature we propose that the dissipation of extreme tides increases vertical mixing of sea water thereby causing episodic cooling near the sea surface if this mechanism correctly explains near decadal temperature periodicities it may also apply to variability in temperature and climate on other times scales even millennial and longer

large scale mass transports water mass formation and diffusivities estimated from world ocean circulation experiment woce hydrographic data 1 a global inversion is used to estimate the oceanic mass transports and vertical diffusivities based on a subset of hydrographic data from the world ocean circulation experiment woce and java australia dynamic experiment the analysis is based on the linear inverse box model of ganachaud and wunsch 2000 that consistently combines the transoceanic sections a globally consistent solution is obtained for a depth independent adjustment to the thermal wind field the freshwater flux divergences the ekman transport and the advective and diffusive dianeutral fluxes between layers a detailed error budget permits calculation of statistical uncertainties taking into account both the nonresolved part of the solution and the systematic errors due to the temporal oceanic variability the horizontal structure of the transports for different density classes is analyzed regionally during the woce period 1985 1996 net meridional transports are estimated with 16 2 sv 106 m3 s of north atlantic deep water nadw being produced in the northern north atlantic moving southward entraining antarctic bottom water aabw and antarctic intermediate water increasing in volume transport to 23 3 sv at 30 s in the southern ocean 21 6 sv of bottom water was formed from lower circumpolar deep water which corresponds approximately to the lower nadw density range bottom water inflows nadw aabw mixture to the atlantic indian and pacific oceans are estimated at 6 1 3 11 4 and 7 2 sv respectively in the indian and pacific oceans this water returns southward at deep and intermediate levels property anomaly conservation constraints permit estimation of dianeutral diffusivities in deep layers with a global average of 3 7 0 7 cm2s 1 north of 30 s

global abyssal mixing inferred from lowered adcp shear and ctd strain profiles abstract internal wave wave interaction theories and observations support a parameterization for the turbulent dissipation rate e and eddy diffusivity k that depends on internal wave shear vz2 and strain z2 variances its latest incarnation is applied to about 3500 lowered adcp ctd profiles from the indian pacific north atlantic and southern oceans inferred diffusivities k are functions of latitude and depth ranging from 0 03 10 4 m2 s 1 within 2 of the equator to 0 4 0 5 10 4 m2 s 1 at 50 70 diffusivities k also increase with depth in tropical and subtropical waters diffusivities below 4500 m depth exhibit a peak of 0 7 10 4 m2 s 1 between 20 and 30 latitudes where semidiurnal parametric subharmonic instability is expected to be active turbulence is highly heterogeneous though the bulk of the vertically integrated dissipation e is contributed from the main pycnocline hotspots in e show some correlation with small scale bottom roughness and near bottom flow at sites where st

sensitivity of the gfdl ocean general circulation model to a parameterization of vertical diffusion abstract a coarse resolution primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability a case with constant diffusivity is compared with a case in which the diffusivity is inversely proportional to the local brunt v frequency the stability dependent parameterization of vertical diffusivity yields a poleward heat flux similar to that of a small constant diffusivity however this parameterization increases the mean temperature in the deep ocean by about 0 8 c and the strength of the meridional circulation by over 40 in addition the stability dependent diffusivity is found to increase the stratification in the deep ocean the experiments suggest that it may be possible to calibrate the rate of deep water formation of general circulation models without affecting the poleward heat transport by varying the magnitude of the vertical diffusivity below the thermocline the explicit

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

possible explanation linking 18 6 year period nodal tidal cycle with bi decadal variations of ocean and climate in the north pacific 1 bi decadal climate variation is dominant over the north pacific on inter decadal timescale however the mechanism has not been fully understood we here find that the bi decadal variations in the north pacific climate and intermediate waters possibly relate to the 18 6 year period modulation of diurnal tide in the period of strong diurnal tide tide induced diapycnal mixing makes surface salinity and density higher and the upper layer shallower along the kuril islands and the east coast of japan simple model results suggest that the coastal depth adjustment by baroclinic kelvin waves enhances the thermohaline circulation the upper layer poleward western boundary current and associated heat transport by about 0 05pw this could also explain the warmer sst in the kuroshio oyashio extension regions where positive feedback with aleutian low might amplify the bidecadal variations the 18 6 year tidal cycle hence could play a role as a basic forcing for the bi decadal ocean and climate variations

the accuracy of surface elevations in forward global barotropic and baroclinic tide models abstract this paper examines the accuracy of surface elevations in a forward global numerical model of 10 tidal constituents both one layer and two layer simulations are performed as far as the authors are aware the two layer simulations and the simulations in a companion paper deep sea research ii 51 2004 3043 represent the first published global numerical solutions for baroclinic tides self consistent forward solutions for the global tide are achieved with a convergent iteration procedure for the self attraction and loading term energies are too large and elevation accuracies are poor unless substantial abyssal drag is present reasonably accurate tidal elevations can be obtained with a spatially uniform bulk drag c d or horizontal viscosity k h but only if these are inordinately large more plausible schemes concentrate drag over rough topography the topographic drag scheme used here is based on an exact analytical solution for arbitrary small amplitude terrain and supplemented by dimensional analysis to account for drag due to flow splitting and low level turbulence as well as that due to breaking of radiating waves the scheme is augmented by a multiplicative factor tuned to minimize elevation discrepancies with respect to the topex poseidon t p constrained got99 2 model the multiplicative factor may account for undersampled small spatial scales in bathymetric datasets an optimally tuned multi constituent one layer simulation has an rms elevation discrepancy of 9 54 cm with respect to got99 2 in waters deeper than 1000 m and over latitudes covered by t p 66 n to 66 s the surface elevation discrepancy decreases to 8 90 cm 92 percent of the height variance captured in the optimally tuned two layer solution the improvement in accuracy is not due to the direct surface elevation signature of internal tides which is of small amplitude but to a shift in the barotropic tide induced by baroclinicity elevations are also more accurate in the two layer model when pelagic tide gauges are used as the benchmark and when the t p constrained tpxo6 2 model is used as a benchmark in deep waters south of 66 s for antarctic diurnal tides the improvement in forward model elevation accuracy with baroclinicity is substantial the optimal multiplicative factor in the two layer case is nearly the same as in the one layer case against initial expectations that the explicit resolution of low mode conversion would allow less parameterized drag in the optimally tuned two layer m 2 solution local values of the ratio of temporally averaged squared upper layer speed to squared lower layer speed often exceed 10

buoyancy forcing by turbulence above rough topography in the abyssal brazil basin abstract observations of turbulent dissipation above rough bathymetry in the abyssal brazil basin are presented relative to regions with smooth bathymetry dissipation is markedly enhanced above rough topography of the mid atlantic ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors furthermore mixing levels in rough areas are modulated by the spring neap tidal cycle internal waves generated by barotropic tidal flow over topography are the likely mechanism for supplying the energy needed to support the observed turbulent dissipation a model of the spatial and temporal patterns in the turbulent dissipation rate is used to constrain the diapycnal advection in an inverse calculation for the circulation in an area of rough bathymetry this inverse model uses both beta spiral and integrated forms of the advective budgets for heat mass and vorticity and contains sufficient information to resolve the full three dimensional flow the inverse model solution re

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

subtropical catastrophe significant loss of low mode tidal energy at 28 9 1 an idealized numerical study of a northward propagating internal tide reveals a dramatic loss of energy to small scale subharmonic instabilities near 28 9 n inspired by observations of the internal tide radiating northward from the hawaiian ridge a three dimensional numerical model is initialized with a northward baroclinic tidal flux of approximately 1 7 kw m after an initial spinup period energy is quickly transferred from the baroclinic tide to subharmonic motions with half the horizontal tidal wavenumber and small vertical scales through nonlinear advection of horizontal tidal velocity gradients potential oceanic implications are twofold first once a steady state has been reached the instability acts as a partial filter of northward tidal flux between 27 5 and 29 5 n in rough agreement with some altimetric tidal observations second elevated shear of the subharmonic motions suggests the potential for elevated near surface dissipation rates near the critical latitude that may be important for upper ocean mixing

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data how and where the ocean tides dissipate their energy are long standing questions1 that have consequences ranging from the history of the moon2 to the mixing of the oceans3 historically the principal sink of tidal energy has been thought to be bottom friction in shallow seas4 5 there has long been suggestive evidence6 7 however that tidal dissipation also occurs in the open ocean through the scattering by ocean bottom topography of surface tides into internal waves but estimates of the magnitude of this possible sink have varied widely3 8 9 10 11 here we use satellite altimeter data from topex poseidon to map empirically the tidal energy dissipation we show that approximately 1012 watts that is 1 tw representing 25 30 of the total dissipation occurs in the deep ocean generally near areas of rough topography of the estimated 2 tw of mixing energy required to maintain the large scale thermohaline circulation of the ocean12 one half could therefore be provided by the tides with the other half coming from action13 on the surface of the ocean

what sets the mean transport through drake passage twelve experiments with two coarse resolutions of a global ocean model using a variety of surface forcings are analyzed to address the question of what sets the mean transport through drake passage seven of the experiments do not have an active sea ice model but the remaining five do previous theories have suggested that the drake passage transport is governed by the cape horn sverdrup transport or alternatively is proportional to the square root of the meridional ekman transport at the latitude of drake passage the results presented here do not support either of these theories the drake passage transport depends quite strongly on the isopycnal diffusivity parameter in the model and less strongly on the background vertical diffusivity and horizontal viscosity parameters however when the magnitudes of these parameters are fixed the results show a very strong correlation between drake passage transport and both the strength of the meridional ekman transport at the latitude of drake passage and the thermohaline circulation off the antarctic shelf the relationships are monotonic but not linear the best estimate is that the meridional ekman transport drives 100 sv of drake passage transport while the remaining 30 sv are driven by the global thermohaline circulation

decadal climate variability is there a tidal connection abstract a possible connection between oceanic tides and climate variability arises from modulations in tidally induced vertical mixing the idea is reexamined here with emphasis on near decadal time scales occasional extreme tides caused by unusually favorable alignments of the moon and sun are unlikely to influence decadal climate since these tides are of short duration and in fact are barely larger than the typical spring tide near lunar perigee the argument by keeling and whorf in favor of extreme tides is further handicapped by an insufficiently precise catalog of extreme tides a more plausible connection between tides and near decadal climate is through harmonic beating of nearby tidal spectral lines the 18 6 yr modulation of diurnal tides is the most likely to be detectable possible evidence for this is reviewed some of the most promising candidates rely on temperature data in the vicinity of the north pacific ocean where diurnal tides are large but definitive detection is hindered by t

boundary mixing and the dynamics of three dimensional thermohaline circulations boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions the model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three dimensional dynamics of the thermohaline circulation for ky 5 20 3 1024 m2 s21 and lower the meridional overturning strength to great accuracy is proportional to meridional heat transport is proportional to the circulation 2 3 1 2 kk y y patterns resemble those from runs with uniform vertical mixing but vertical motion is entirely confined to the boundary regions near the western boundary there is upwelling everywhere near the eastern boundary there is a consistent pattern of downwelling above upwelling with downwelling reaching deeper at high latitudes this pattern is explained by convection and vertical advective diffusive balance underneath for ky 5 30 3 1024 m2 s21 and higher no steady solutions have been found the meridional overturning oscillates on a timescale of about 25 years a time averaged thermally direct overturning cell is not supported dynamically because convection extends longitudinally across the entire basin and upwelling near the western boundary does not lead to densities higher than at the eastern boundary assuming uniform upwelling in the west level isopycnals near the equator and level isopycnals along the eastern boundary south of the outcropping latitude permits the analytic determination of convection depth at the eastern wall and hence the density difference between the eastern and western walls this difference is at most one quarter the surface density difference between high and low latitudes and agrees in magnitude and latitudinal dependence with the numerical experiments scaling arguments estimate overturning strength as of the order of 10 3 106 m3 s21 and confirm the 2 3 power dependence onky the derivation also gives a dependence of overturning strength with latitude that agrees qualitatively with the numerical results the scaling for the dependence of meridional heat transport on latitude agrees well with the model results scaling for heat transport amplitude agrees less well but correctly predicts a weaker dependence on ky than maximum overturning