reconciling the stommel box model with the stommel arons model a possible role for southern hemisphere wind forcing abstract in the stommel box model the strength of the overturning circulation is parameterized in terms of the density and hence the pressure difference between the two boxes straub has pointed out that this parameterization is not consistent with the stommel arons model for the abyssal circulation in particular the zonally averaged density field implied by the stommel arons model is unrelated to the strength or the direction of the meridional overturning circulation here the inconsistency is examined using the abyssal circulation model of kawase and a variant to include the effect of southern hemisphere wind forcing the important parameter is r the ratio of two timescales the timescale for a perturbation to the density field to propagate by either wave or advective processes from a high latitude source to the equator and the timescale for the dissipation of a perturbation to the density field by diapycnal mixing if the model is forced only by a deep water source in the northern basin it is

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

global adjustment of the thermocline in response to deepwater formation the global adjustment of the thermocline in response to deepwater formation is studied in a single mode model on a beta plane the signal is carried from ocean to ocean by kelvin waves which travel equatorward along western boundaries eastward across the equator poleward at the eastern boundaries and then eastward around the southern tip of continents into the next ocean basin the interior is filled by rossby waves emanating from eastern boundaries stronger weaker deepwater formation induces an upward downward motion of the main thermocline in the world oceans the adjustment is completed on centennial time scales

a damped decadal oscillation in the north atlantic climate system a simple stochastic atmosphere model is coupled to a realistic model of the north atlantic ocean a north south sst dipole with its zero line centered along the subpolar front influences the atmosphere model which in turn forces the ocean model by surface fluxes related to the north atlantic oscillation the coupled system exhibits a damped decadal oscillation associated with the adjustment through the ocean model to the changing surface forcing the oscillation consists of a fast wind driven positive feedback of the ocean and a delayed negative feedback orchestrated by overturning circulation anomalies the positive feedback turns out to be necessary to distinguish the coupled oscillation from that in a model without any influence from the ocean to the atmosphere using a novel diagnosing technique it is possible to rule out the importance of baroclinic wave processes for determining the period of the oscillation and to show the important role played by anomalous geostrophic advection in sustaining the oscillation

adjustment of the ocean under buoyancy forces part i the role of kelvin waves abstract the early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model the initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the north atlantic the main adjustment in the time examined here the first few months is by coastal kelvin waves the first baroclinic mode dominates so the behavior proves to be very similar to that in a shallow water model also the dynamics is locally close to that for an f plane but account must be taken of the way f varies with latitude the essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin especially at the level where the first mode has the maximum effect this study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations namely in a model with too coarse a horizontal resolution and an artif

the role of horizontal boundaries in parameter sensitivity and decadal scale variability of coarse resolution ocean general circulation models abstract coarse resolution f plane and plane frictional geostrophic models are used to study the response to restored surface buoyancies and fixed surface buoyancy fluxes with restored surface buoyancies it is found that the overturning and meridional buoyancy transport generally follow the scaling from thermal wind and vertical advective diffusive balance when maximum midbasin rather than maximum overall streamfunction is used as the metric the sensitivity of overturning magnitude to vertical diffusivity agrees quite closely with that of a two dimensional rayleigh frictional model that follows an analogous scaling this measure of meridional overturning as well as the meridional buoyancy transport also roughly follow the predicted f 1 3 scaling and are relatively insensitive to variations in and horizontal viscosity the sensitivity experiments indicate that the coarse resolution model overturning and thermodynamic structure are well characterized by the adjustment of a uniformly rotating viscous

a zonally averaged ocean model for the thermohaline circulation part i model development and flow dynamics abstract a two dimensional latitude depth ocean model is developed on the basis of the zonally averaged balance equations of mass momentum heat and salt its purpose is to investigate the dynamics and variability of the buoyancy forced thermohaline circulation for the time scales of interest an annually averaged model is selected and the momentum balance is taken to be diagnostic the east west pressure gradient which arises upon zonally averaging the momentum equations is parameterized in terms of the meridional pressure gradient the thermohaline circulation is driven by mixed surface boundary conditions i e temperatures are relaxed to prescribed values while the salt flux is held constant the dynamics of the flow is investigated in hemispheric and global geometries for both short and long time integrations the latter extending over many thousands of years as has been noted by previous investigators it is possible to perturb a steady state such that a diffusively dominated regime results

the regulation of northern overturning by southern hemisphere winds abstract coarse resolution models are used to study the interaction of zonal wind stress at the latitude of drake passage and the production of northern deep water a nearly linear response to wind stress at the tip of south america is seen in northern deep water production in both one and two basin configurations a comparison of steady state circulations forced by different wind stress strengths over the southern hemisphere westerlies shows the changes in circulation away from the southern ocean largely confined to western boundary currents the change in circulation accompanying stronger winds is similar to that predicted by the southern upwelling northern sinking couplet discussed by stommel transient studies are conducted in which wind stress is increased over the region of the southern hemisphere westerlies in an ocean initially stratified and at rest forced only by the southern hemisphere westerlies the development of the flow is similar to that observed by kawase in a two layer model barocli

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

vorticity dynamics and zonally averaged ocean circulation models abstract diagnostic equations relating the zonally averaged overturning circulation to north south density variations are derived and used to determine a new closure scheme for use in zonally averaged ocean models the presentation clarifies the dynamical link between vorticity dissipation in the western boundary layer and the meridional overturning circulation and provides insight into the relevance and limitations of zonally averaged models the new closure scheme is substantially different from that introduced by wright and stocker in particular it includes nonlocal effects which are potentially important the model formulation results in a solution with three free parameters a parameter representing the damping associated with meridional velocity gradients has very little effect on the solution and could be set to zero the other two are used to tune the model to agree with results of a general circulation model good agreement is achieved for values consistent with expectations the closure schem

the diabatic deacon cell abstract southern ocean air sea fluxes from the coads dataset are examined for compatibility between buoyancy gain and northward ekman transport an analysis in density classes points to an upwelling of upper circumpolar deep water and subsequent buoyancy gain from air sea exchange as water flows northward in the ekman layer the upper circumpolar deep water layer is too shallow to admit southward geostrophic flow along topography and an eddy mass flux mechanism for southward transport in this layer to replenish the upwelling is advanced based on observed large scale potential vorticity gradients estimates of the strength and vertical structure of the meridional flow are given using repeated hydrographic sections and a new climatology

a linear stratified ocean model of the equatorial undercurrent a linear stratified ocean model is used to study the wind driven response of the equatorial ocean the model is an extension of the lighthill 1969 model that allows the diffusion of heat and momentum into the deeper ocean and so can develop non trivial steady solutions to retain the ability to expand solutions into sums of vertical normal modes mixing coefficients must be inversely proportional to the square of the back ground vaisala frequency the model is also similar to the earlier homogeneous ocean model of stommel i960 he extended ekman dynamics to the equator by allowing his model to generate a barotropic pressure field the present model differs in that the presence of stratification allows the generation of a baroclinic pressure field as well the most important result of this paper is that linear theory can produce a realistic equatorial current structure the model undercurrent has a reasonable width and depth scale there is westward flow both above and below the undercurrent the meridional circulation conforms to the classical picture suggested by cromwell 1953 unlike the stommel solution the response here is less sensitive to variations of parameters ocean boundaries are not necessary for the existence of the undercurrent but are necessary for the existence of the deeper equatorial intermediate current the radiation of equatorially trapped rossby and kelvin waves is essential to the development of a realistic undercurrent because the system supports the existence of these waves low order vertical modes can very nearly adjust to sverdrup balance defined below which in a bounded ocean and for winds without curl is a state of rest as a result higher order vertical modes are much more visible in the total solution this property accounts for the surface trapping and narrow width scale of the equatorial currents the high order modes tend to be in yoshida balance defined below and generate the characteristic meridional circulation pattern associated with equatorial ekman pumping

the arrival of recently formed labrador sea water in the deep western boundary current at 26 5 n the deep western boundary current dwbc of the north atlantic is a principal conduit between the formation region for labrador sea water lsw and the oceanic interior to the south time series 1985 1997 of hydrographic properties obtained in the dwbc at 26 5 n show that prior to 1994 temperature salinity and transient tracer properties within the lsw density range showed little indication of recently formed parcels properties characteristic of a newer version of lsw cooler fresher and higher tracer concentrations were observed beginning in 1994 and continuing through 1997 longer time series of temperature and salinity developed from a regional data base show both the 1994 and a 1980 1981 event in the abaco region both events are consistent with anomalies in the labrador sea that occurred some 10 years earlier the 10 year transit time from the labrador sea to 26 5 n is less than the 18 year transit time inferred from earlier studies

interdecadal variability and oceanic thermohaline adjustment changes in the strength of the thermohaline overturning circulation are associated by geostrophy with changes in the east west pressure difference across an ocean basin the tropical polar density contrast and the east west pressure difference are connected by an adjustment process in flat bottomed ocean models the adjustment is associated with viscous baroclinic kelvin wave propagation weak high latitude stratification leads to the adjustment having an interdecadal timescale we reexamine model interdecadal oscillations in the context of the adjustment process for both constant flux and mixed surface boundary conditions under constant surface flux interdecadal oscillations are associated with the passage of a viscous kelvin wave around the model domain we carry out experiments suppressing wave propagation along each of the model boundaries suppressing wave propagation along either the tropical or eastern boundary does not eliminate the oscillation but increases both its period and amplitude suppressing wave propagation along either the polar or the western boundary eliminates the oscillation our results suggest the oscillations can be self sustained by perturbations to the western boundary current arising from the southward boundary wave propagation mixed boundary condition oscillations are characterized by the eastward cross basin movement of salinity dominated density anomalies and the westward return of these anomalies along the northern boundary we suggest the latter is associated with viscous kelvin wave propagation under both types of boundary conditions the strength of the thermohaline overturning and the tropical polar density contrast vary out of phase we show how the phase relationship is related to the boundary wave propagation box models and zonally averaged models assume that the east west and north south pressure gradients vary in phase and are proportional to one another we suggest this assumption is valid only on timescales long compared to the adjustment timescale the importance of boundary regions indicates an urgent need to examine the robustness of interdecadal variability in models as the resolution is increased and as the representation of the coastal shelf slope wave guide is improved

multiple equilibria of an asymmetric two basin ocean model abstract an ocean general circulation model is used to examine the role of model geometry and surface buoyancy and wind stress forcing in the asymmetry of the global thermohaline circulation the model domain is a highly idealized atlantic and pacific linked by a circumpolar ocean in the south and the integrations are performed under mixed boundary conditions diagnosed from spinups under various temperature and salinity profiles constructed from the present day climatology the model exhibits a tendency to favor either a conveyor type circulation with sinking in the northern north atlantic and upwelling in the north pacific or a southern sinking state with deep sinking in the antarctic only this bias is not dictated solely by the hydrological cycle nor apparently by the greater northern extension of the atlantic basin but presumably by the overall asymmetry of the geometry equilibria with northern sinking in both basins can appear however when the winds in the southern ocean are reduced or th

dynamics of the atlantic meridional overturning circulation and southern ocean in an ocean model of intermediate complexity abstract a steady state variable density 2 layer ocean model vlom is used to investigate basic dynamics of the atlantic meridional overturning circulation and southern ocean the domain consists of idealized rectangular representations of the atlantic southern and pacific oceans the model equations represent the depth averaged layer 1 response except for one solution in which they represent the depth integrated flow over both layers to allow for overturning water can cross the bottom of layer 1 at the velocity w e w d w m w n the three parts representing interior diffusion w d that increases the layer 1 thickness h throughout the basin mixed layer entrainment w m that ensures h is never less than a minimum value h m and diapycnal cooling processes external to the basin w n that adjust h to h n for most solutions horizontal mixing has the form of rayleigh damping with coefficient which we interpret to result from baroclinic instability through the closure v f 2 p where p 1 2 g h 2 is the depth integrated pressure gradient g is the reduced gravity coefficient and is a mixing coefficient with this interpretation the layer 1 flow corresponds to the sum of the eulerian mean and eddy mean v transport widths that is the residual circulation finally layer 1 temperature cools polewards in response to a surface heat flux q and the cooling can be strong enough in the southern ocean for g 0 south of a latitude y 0 in which case layer 1 vanishes and the model reduces to a single layer 2 solutions are obtained both numerically and analytically the analytic approach splits fields into interior and boundary layer parts from which a coupled set of integral constraints can be derived the set allows properties of the circulation upwelling driven transport out of the southern ocean m downwelling transport in the north atlantic transport of the antarctic circumpolar current and stratification atlantic thermocline depth and the latitudes y and y 0 where h thins to h m or layer 1 vanishes in the southern ocean to be evaluated in terms of model forcings southern ocean wind strength a q entrainment due to w d processes v in the southern ocean northern sinking upwelling within the atlantic subpolar gyre and to the presence of the pacific ocean a hierarchy of solutions is reported in which forcings and processes are individually introduced the complete solution set includes a wide variety of solution types with m 0 and m 0 with and without wind forcing with without and for two parameterizations of northern boundary sinking that represent cooling external to and within the north atlantic for a wide range of and a and for different closures novel aspects of the model and solutions include the following use of vlom which allows q forcing to be introduced realistically the aforementioned closure which allows v to be determined when layer 1 represents both the surface mixed layer h h m and the depth of subsurface isopycnals h h m latitude y where layer 1 outcrops in the southern ocean being internally determined rather than externally specified and a boundary layer based on gill s 1968 solution that smoothly connects the southern and atlantic ocean responses across the latitude of the southern tip of south america finally some solutions in the set are comparable to solutions to idealized ocean general circulation models ogcms in these cases our solutions provide insight into the underlying dynamics of the ogcm solutions for example pointing toward processes that may be involved in eddy saturation and compensation

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

a simple dynamical model of the warm water branch of the middepth meridional overturning cell a reduced gravity model is presented of the warm water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection the model describes the balance between production of warm water by ekman advection across the circumpolar current dissipation of warm water by eddy fluxes southward across the current and the net production or dissipation of warm water by diabatic processes north of the current the results emphasize the role of the eastern boundary condition in setting the thermocline structure northofthe current andthe nonlinear interactions betweenwind forcing eddyfluxes and diabatic mixing which together control the structure and amplitude of the model meridional overturning circulation solutionsareshowntoexistinwhichthenorthwardekmantransportacrossthecircumpolarcurrent is completely compensated by southward eddy fluxes and the meridional overturning north of the current is entirely driven by diabatic forcing and interior upwelling through the base of the layer other solutions are shown to exist in which the interior upwelling into the warm layer at midlatitudes is negligible and the meridional overturning circulation consists of a continuous cell that carried the fluid delivered by the northward ekman transport across the circumpolar current through midlatitudes to the northern hemisphere subpolar gyre where it cools and returns todepth the results emphasize that the coupled elements ofwind driving eddy fluxes and diabatic processes are inextricably intertwined in the middepth meridional overturning circulation

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

modeling erosion of ice rich permafrost bluffs along the alaskan beaufort sea coast the arctic climate is changing inducing accelerating retreat of ice rich permafrost coastal bluffs along alaska s beaufort sea coast erosion rates have increased roughly threefold from 6 8 to 19 m yr 1 since 1955 while the sea ice free season has increased roughly twofold from 45 to 100 days since 1979 we develop a numerical model of bluff retreat to assess the relative roles of the length of sea ice free season sea level water temperature nearshore wavefield and permafrost temperature in controlling erosion rates in this setting the model captures the processes of erosion observed in short term monitoring experiments along the beaufort sea coast including evolution of melt notches topple of ice wedge bounded blocks and degradation of these blocks model results agree with time lapse imagery of bluff evolution and time series of ocean based instrumentation erosion is highly episodic with 40 of erosion is accomplished during less than 5 of the sea ice free season among the formulations of the submarine erosion rate we assessed we advocate those that employ both water temperature and nearshore wavefield as high water levels are a prerequisite for erosion any future changes that increase the frequency with which water levels exceed the base of the bluffs will increase rates of coastal erosion the certain increases in sea level and potential changes in storminess will both contribute to this effect as water temperature also influences erosion rates any further expansion of the sea ice free season into the midsummer period of greatest insolation is likely to result in an additional increase in coastal retreat rates

on the influence of dsow in a numerical model of the north atlantic general circulation abstract two numerical experiments regarding the north atlantic circulation are compared the two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland sea one case climatological wind stress sst and surface salinity to force the model it reproduces the atlantic circulation found in similar studies a change in surface boundary values in the iceland sea in order to improve the denmark strait overflow water properties in the second case results in major changes in circulation and hydrography the meridional overturning more than doubles and horizontal gyres are intensified recirculation cells occur north and south of the gulf stream the gulf stream separates from the north american coast in a more realistic manner and its further course is in better agreement with observations than in the reference run the subsurface temperature distribution differs by several kelvin in the subpolar and

response of circulation and heat transport in the north atlantic to changes in thermohaline forcing in northern latitudes a model study to avoid an explicit simulation of the overflows across the greenland scotland ridge many models of the large scale ocean circulation seek to include the net effect of the inflowing dense water masses by restoring temperature and salinity near the ridge to observed conditions in this paper the authors examine the effect of different datasets for the northern restoring condition in two versions eddy resolving and non eddy resolving of the model of the north and equatorial atlantic that has been developed in recent years as a community modeling effort for woce it is shown that the use of smoothed climatological fields of temperature and salinity south of the denmark strait leads to strong deficiencies in the simulation of the deep flow field in the basin a switch to actual hydrographic data from the denmark strait ignites a rapid dynamic response throughout the north atlantic affecting the transport and vertical structure of the deep western boundary current and by virtue of the jebar efffect the transport of the horizontal gyres meridional overturning and northward heat transport too weak in the cases with climatological boundary conditions increase to more realistic levels in the subtropical north atlantic the initial response to switches in the high latitude thermohaline forcing is mediated by fast waves along the westurn boundary leading to changes in the deep western boundary current in low latitudes after about two years in the non eddy resolving cast the initial timescale depends on the horizontal grid spacing of the model in the high resolution case the first signal reaches the equator in a few months the adjustment to a new dynamic quasi equilibrium involves kelvin waves along the equator and rossby wave in the interior and is attained in less than two decades throughout the north atlantic it is suggested that these fast dynamic adjustment processes could play an important role in possible fluctuations of the thermohaline circulation or transitions between different equilibrium states of the coupled ocean atmosphere system and may have determined the timescale of the observed climatic transitions before and during the last deglaciation