sensitivity of eddy induced heat transport to diabatic forcing compensation of the poleward eddy heat transport by the heat transport of an eddy induced mean meridional overturning cell is a common feature in many eddy resolving ocean models it has been argued that this is the result of the weak thermal driving of the ocean as the actual air sea coupling is scale dependent it might be questioned whether the approximation of weak thermal driving is relevant for the oceanic eddy field in this paper the role of diabatic forcing in modifying eddy mean flow interaction is investigated emphasis has been placed on the sensitivity of the eddy induced change in heat transport to the sea surface thermal boundary condition experiments have been performed with a multilayer isopycnic primitive equation model of an idealized north atlantic subtropical and subpolar gyre for different values of the air sea coupling solutions with and without transient eddies have been compared the air sea coupling mostly affects the upper ocean thermal and velocity fields a decrease of the coupling timescale pushes the separation point of the midlatitude jet further northward and induces a tight recirculation southwest of the separated jet these effects are enhanced by the eddies in the present model there is compensation of the eddy heat transport for sea surface temperature sst relaxation times longer than 150 days a breakdown of the compensation occurs for sst relaxation times shorter than 50 days the average upper layer depth is 200 m in between is a transition regime for strong thermal driving the eddy induced change in total heat transport is of the same order as the eddy heat transport

the long term hydrographic record at bermuda abstract a long by oceanographic standards time series of hydrographic observations at bermuda was begun in 1954 and continues to the present analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer 0 200 dbar where integral timescales for salinity are 50 longer than for temperature and surface salinity changes are correlated with decadal changes in the 18 c water production temperature salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of 50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of 300 kim salinity changes in the deepest layer observed at bermuda station s 1500 2500 dbar are uncorrelated with temperature masked by measurement errors in the early years of the time series inclusion of earlier

a model study on internally generated variability in subtropical mode water formation internally generated variability in the subtropical gyre is studied as a possible mechanism for the observed interannual to decadal variability in subtropical mode water formation an isopycnic ocean model with idealized geometry and forcing which mimics the north atlantic subtropical gyre is used for this purpose the horizontal resolution is sufficiently high and the friction and diffusion sufficiently low for the flow to become barotropically and baroclinically unstable two modes of low frequency variability are found both modes consist of westward propagating thickness anomalies the anomalies have a first baroclinic modal structure with a maximum amplitude at the thermocline one mode has a timescale of 8 years and a basin wide spatial scale the other has a timescale of 4 5 years and a smaller spatial scale the modes appear to be damped when the diffusion is high in that case the 8 year mode can be excited by a spatially coherent stochastic wind stress the evolution of the modes is determined by an interaction between the mean flow and the low frequency variability itself the modes are instabilities of the mean flow determined by the basic stratification it appears that coupling to the atmosphere and a parameterization of surface mixing are necessary for the low frequency variability to appear in the mixed layer the coupling and surface mixing do not play a role in generating the modes it is concluded that these internally generated modes may play a role in the observed variability in mode water formation

heat transport by mesoscale eddies in an ocean circulation model abstract the role of mesoscale eddies in the poleward heat transport in the ocean is investigated in particular the compensation of poleward eddy heat transport by an eddy induced mean meridional circulation is examined a multilayer isopycnic primitive equation model of an idealized western north atlantic is presented to test whether compensation also occurs within an isopycnic model and when the poleward eddy heat transport becomes comparable to the mean transport also in this model configuration compensation of the poleward eddy heat transport arises it is brought about by a westward eddy heat transport in the midlatitude jet which results in a pressure drop across the basin and consequently in a modified mean meridional overturning this compensation is discussed within the framework of wave mean flow interaction it is demonstrated that compensation results from eddy mean flow interaction when the diabatic forcing is sufficiently weak rings are recaptured before their sst anomaly is modified s

does stommel s mixed layer demon work abstract stommel argued that the seasonal cycle leads to a bias in the coupling between the surface mixed layer and the main thermocline of the ocean he suggested that a demon operated that effectively only allowed fluid at the end of winter to pass from the mixed layer into the main thermocline in this study stommel s hypothesis is examined using diagnostics from a time dependent coupled mixed layer primitive equation model of the north atlantic cme the influence of the seasonal cycle on the properties of the main thermocline is investigated using two methods in the first the rate and timing of subduction into the main thermocline is diagnosed using kinematic methods from the 1 resolution cme fields in the second tracer diagnostics of the cme and idealized experiments using a date tracer identifying the timing of subduction are performed over the subtropical gyre both approaches generally support stommel s hypothesis that fluid is only transferred from the mixed layer into the main therm

homogenization of potential vorticity in planetary gyres the mean circulation of planetary fluids tends to develop uniform potential vorticity q in regions where closed time mean streamlines or closed isolines of mean potential vorticity exist this state is established in statistically steady flows by geostrophic turbulence or by wave induced potential vorticity flux at the outer edge of the closed contours the expelled gradients of q are concentrated beyond this transition lies motionless fluid or a different flow regime in which the planetary gradient of q may be dominant the homogenized regions occur where direct forcing by external stress or heating within the closed isoline is negligible upon the potential density surface under consideration in the stably stratified ocean such regions are found at depths greater than those of direct wind induced stress or penetrative cooling in channel models of the atmosphere we again find constant q when mesoscale eddies cause the dominant potential vorticity flux in the real atmosphere the results here can apply only where internal heating is negligible the derivations given here build upon the prandtl batchelor theorem which applies to non rotating steady two dimensional flow supporting evidence is found in numerical circulation models and oceanic observations

inferring the subduction rate and period over the north atlantic abstract the annual rate at which mixed layer fluid is transferred into the permanent thermocline that is the annual subduction rate sann and the effective subduction period x1d4af eff is inferred from climatological data in the north atlantic from its kinematic definition sann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer geostrophic velocity fields computed from the levitus climatology assuming a level of no motion at 2 5 km are used the vertical velocity at the base of the mixed layer is deduced from observed surface ekman pumping velocities and linear vorticity balance a plausible pattern of sann is obtained with subduction rates over the subtropical gyre approaching 100 m yr twice the maximum rate of ekman pumping the subduction period x1d4af eff is found by viewing subduction as a transformation process converting mixed layer fluid into stratified thermocline fluid the effective

recent variability of the north atlantic thermohaline circulation inferred from surface heat and freshwater fluxes abstract an annual mean surface forced component of the north atlantic thermohaline circulation thc in density space surf is diagnosed from observed surface heat and freshwater fluxes the climatological mean of surf over 1980 97 indicates steady overturning rates for those water masses that are principally formed through surface buoyancy forcing 8 sv of greenland sea deep water gsdw 4 sv of labrador sea water lsw and 4 sv of 18 water stmw obtained for each year over 1980 97 surf reveals interannual to decadal variability in the renewal and overturning of these water masses the total surface forced overturning rate varies in the range 10 20 sv reaching peak values in 1989 90 and generally decreasing over the period 1990 97 more dramatic changes are apparent in the overturning of different water masses the overturning of stmw exhibits large amplitude interannual variability a background positive trend over 1980 97 is dominated by variance of 13 4 sv by contrast the overtur

the shikotan earthquake of october 4 1994 lithospheric earthquake a large m w 8 2 earthquake occurred off shikotan is one of the kurile is on october 4 1994 we inverted 32 body wave records to determine the rupture pattern using an iterative deconvolution method the mechanisms of the subevents were allowed to vary during rupture the source parameters obtained are the location of the initial break 43 48 n 147 40 e the centroid depth 56 km strike dip rake 49 75 125 for the total source the seismic moment m o 2 6 10 21 nm m w 8 2 source time duration t 42 s the average rupture velocity 2 5 km s we also determined the mechanism using long period love and rayleigh waves from 14 stations the solution for a finite source distributed over a depth range from 0 to 90 km is strike dip rake 54 76 129 with mo 2 3 10 21 nm in good agreement with that from body waves referring to the extent of the aftershock area and the subevent distribution we estimated the fault area s 120 60 km the average slip d 5 6m and the stress drop 11 mpa we computed synthetic waveforms as well as static displacements using either the steep or the low angle plane as the fault plane and found that the steep dip fault model fits the data better our result the mechanism large centroid depth high stress drop strongly suggests that the 1994 shikotan earthquake is a lithospheric earthquake an intra plate event that ruptures through a substantial part of the subducting oceanic lithosphere this type of lithospheric earthquake is relatively common

a multilayer model of the thermohaline and wind driven ocean circulation abstract a hybrid multilayer model for the oceanic general circulation is formulated and tested the model includes a mixed layer at the surface which is specified by eulerian coordinates and three moving layers below which are specified by quasi lagrangian isopycnal coordinates initial tests have been carried out with a 22 22 horizontal grid mesh covering a subtropical subpolar basin 6000 6000 km2 the numerical results demonstrate a strong interaction between the wind driven and the thermally driven circulations including outcropping of the lower isopycnal layers a gulf stream like interior boundary current and convection which produces mode water and abyssal water the model provides insight into the potential vorticity balance and its relation to both the wind driven and thermohaline components of the circulation which has not been previously available from eulerian numerical models or analytical models based on the assumption of an ideal fluid thermocline

the trophic structure of the biota of the peterborough member oxford clay formation jurassic uk the peterborough member of the oxford clay formation is organic rich and contains an abundance of well preserved vertebrate and invertebrate fossils a high nutrient input supported a diverse biota phytoplankton was exceptionally abundant in the surface water and formed the basis for an intricate food web in both surface and bottom waters top predators include some of the largest known mesozoic marine reptiles a giant teleost fish was analogous to modern filter feeding whales and sharks benthic faunas depended on organic matter sinking from surface waters and two parallel food webs may have existed trophic partitioning allowed the higher level predators to become diverse especially the plesiosaurs productivity was high in the surface waters probably high in the mid water column and high on the sea floor at times although benthic diversity may have been reduced due to substrate consistency and or dysoxia bacterial activity within the sediment was also intense the contribution of organic material to the sea floor was high but heterotrophic reworking probably reduced the abundance considerably during early diagenesis a decrease in organic carbon content in the upper callovian and lower oxfordian parts of the oxford clay formation parallels a decreasing abundance of vertebrate fossils

north atlantic modeling of low frequency variability in mode water formation abstract the generation of interannual and near decadal variability in the formation of mode waters in the western north atlantic is investigated in the realistic framework of an isopycnic coordinate ocean model forced with atmospheric data from 1946 to 1988 at bermuda the model reproduces quite well the observed potential vorticity and isopycnal depth anomalies associated with the subtropical mode water stmw heat storage and preconditioning of the convective activity are found to be the important factors for the generation of stmw variability with persistence of cold warm conditions associated with anomalous heat loss gain over the western subtropics being more significant for the generation of the simulated variability than are strong anomalous events in isolated years in the labrador sea the model captures the phase and order of magnitude of the observed near decadal variability in the convective activity if not its maximum amplitude the simulated potential vorticity anomalies are as o

eddy subduction in a model of the subtropical gyre abstract subduction is the process by which fluid transfers from the mixed layer to the interior of the ocean south of the gulf stream extension 18 mode water is formed in a region of high subduction rates in this region there is high mesoscale eddy activity in the present study the role of eddies in modifying the large scale subduction into mode water is investigated an eddy resolving isopycnic ocean model with mixed layer physics included and coupled to an atmospheric anomaly model is used for this purpose the geometry and forcing of the model are idealized annual mean subduction rates into mode water up to 200 m yr 1 are found south of the gulf stream extension the eddy contribution to the annual subduction is estimated by comparing the annual mean subduction rates obtained from monthly mean quantities with the total subduction rates i e eddy plus mean contributions the latter are determined by integrating the detrainment rates over the period when fluid is irreversibly detrained eddy

mixed layer thermocline interaction in a three dimensional isopycnic coordinate model abstract the annual buildup and obliteration of the seasonal thermocline and the associated ventilation of the permanent thermocline in a wind and thermally driven ocean basin are simulated numerically the model developed for this purpose is a combination of a single layer model of the oceanic mixed layer based on a simple closure of the turbulence kinetic energy equation and a three dimensional isopycnic coordinate model of the stratified oceanic interior the joint model set in a rectangular ocean basin is forced by annually varying wind stress and radiative plus turbulent heal fluxes approximating zonnaly averaged conditions over the north atlantic special emphasis is placed on the description of the mixed layer detrainment process which requires distributing mixed layer water of continuously variable density among constant density interior layers the truncation errors associated with this process are found to be numerically tolerable the quasi lagrangian character of the model s vertical coo

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

does atmospheric cooling drive the gulf stream recirculation abstract a two layer model for the recirculation is studied initially a narrow jet of the upper layer moves eastward with the lower layer remaining stagnant att 0 cold air flows over the narrow front region all the moving water in the upper layer sinks to the lower layer with the momentum vertically well mixed within the lower layer thus cooling creates an unbalanced eastward jet in the second layer and an unbalanced pressure field at a vertical density front after the geostrophic adjustment a high pressure center south of the front and a low pressure center north of the front are established these pressure centers drive a much stronger barotropic eastward current slightly north of the pressure center and slow westward return flow in the far field both south and north of the front thus cooling over a narrow stream can intensify an eastward jet and create recirculation gyres both north and south of the stream

ventilation of the north atlantic and north pacific subduction versus obduction abstract ventilation in the north atlantic and north pacific is examined by analyzing the levitus climatological data and the hellerman and rosenstein wind stress data ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes subduction and obduction subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above veatilation in the north atlantic and north pacific can be clarified into four physically different regions the subductive region the obductive region the ambiductive region where both subduction and obduction take place and the insulated region where neither subduction nor obduction occurs although the total subduction rates in these two oceans are comparable the total obduction rates are considera

thermocline forced by varying ekman pumping part ii annual and decadal ekman pumping abstract thermocline variability forced by zonally uniform ekman pumping with annual to decadal periods is investigated both analytical and numerical solutions are obtained by the method of characteristics as found in part i there is little thermocline variability in the ventilated zone or pool zone in contrast strong variability may exist in the shadow zone for annual forcings nonlinearity is negligible however the linear solution is influenced substantially by the basic state thermocline structure as a result local responses dominate for a shallow interface while remote rossby waves dominate for a deep interface under a strong decadal forcing nonlinearity may become important the time mean thermocline in the shadow zone is shallower than the steady thermocline under the mean ekman pumping particularly in the western part of a shadow zone where the mean deviation may reach the order often meters this shallower mean thermocline is caused by the nonlinear rossby wave

mode water variability in a model of the subtropical gyre response to anomalous forcing abstract the response of mode water formation to typical atmospheric forcing anomalies is studied as a possible mechanism for generating the observed interannual to decadal variability in mode water an isopycnal model of the north atlantic subtropical gyre coupled to a mixed layer model is used for this purpose geometry and forcing are idealized the control run shows that mode water is a well ventilated water mass formation rates up to 200 m yr 1 are found at the outcrop of the mode water layer in a series of experiments the sensitivity to the position of the anomalous forcing and to the timescale of the forcing is examined the anomalous forcing has a dipole pattern that mimics the spatial structure of the north atlantic oscillation anomalous cooling induces a positive thickness anomaly in the mode water layer at the center of the gyre and a negative anomaly at the eastern side of the gyre the response to anomalous heat flux forcing appears to be sensitive to the position of the forcing anomaly

an eddy resolving numerical model of the ventilated thermocline time dependence abstract a primitive equation eddy resolving numerical model is used to study the inherent time scales of variability in the subtropical ocean assuming temporally constant surface forcing three primary scales arise mesoscale variability of roughly 50 day period zonally elongated barotropic bands of 1 1 year period and basin scale undulations of approximately 4 year period the latter are identified as first baroclinic mode rossby waves associated with bursts of ventilation at isopycnal outcrops as a result the equatorward transport required by sverdrup theory in the subtropics occurs not as a broad sluggish drift throughout the interior but as a succession of more intense flows that slowly propagate westward the zonally elongated bands agree in characteristics to those predicted from theory of homogeneous turbulence eddy energy that is generated by baroclinic instability leaks from baroclinic to barotropic mode and thereafter due to rotational constraints seeks low zonal wavenumbers the gr

eddy induced heat transport in a coupled ocean atmospheric anomaly model the midlatitude meridional heat transport in the ocean can be partitioned into a transport by the mean flow and an eddy transport this heat transport has been studied in several ocean only models surprisingly it was found that eddy resolving and coarse resolution models have similar total heat transport this is caused by a compensation mechanism in which poleward eddy transport is counterbalanced by an eddy induced meridional circulation recently it was shown that this compensation depends on details of the atmospheric forcing and in fact only occurs for weak thermal coupling where thermal coupling is defined as the rate of change of the surface heat flux with respect to the sea surface temperature sst the thermal coupling varies with the spatial scale of the sst anomaly to study the actual strength of this coupling on the eddy length scale the authors have coupled an isopycnic ocean model with embedded mixed layer to an atmospheric anomaly model by comparing coarse resolution and eddy resolving simulations it is found that 1 the thermal coupling is strong on the eddy length scale and 2 the aforementioned compensation does not occur consequences for the temperature boundary condition in ocean only models the gent and mcwilliams eddy parameterization and climate modeling are discussed