thermohaline variability the effects of horizontal resolution and diffusion abstract an idealized coupled ocean atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the thermohaline circulation a series of experiments ranging from 4 to 0 25 resolution with appropriate horizontal viscosities and diffusivities in each case is performed for both coupled and ocean only models spontaneous internal variability primarily on the decadal timescale is found to exist in the higher resolution cases with the exception of one of the restoring experiments the decadal oscillation whose period varies slightly between cases is described as an advective convective mechanism that is thermally driven and linked to the value of the horizontal diffusivity utilized in the model increasing the diffusivity in the high resolution cases presented in this paper is enough to destroy the variability whereas decreasing the diffusivity in the moderately coarse resolution cases is capable of inducing decadal scale variability as the resol

interdecadal variability of the thermohaline circulation in box ocean models forced by fixed surface fluxes intrinsic modes of decadal variability are analyzed using box geometry ocean models forced by constant surface fluxes an extensive parameter sensitivity analysis of the oscillatory behavior is carried out with respect to the spherical cartesian geometry the b effect the coriolis parameter the parameterization of momentum dissipation and associated boundary conditions the vertical and horizontal diffusivities the convective adjustment algorithm the horizontal and vertical model resolution the forcing amplitude and the basin width the oscillations stand out as a robust geostrophic feature whose amplitude is mainly controlled by the horizontal diffusivity unsuccessful attempts to reproduce the variability in zonally averaged 2d models suggest that the 3d adjustment processes are necessary however the b effect is not necessary for decadal variability to occur and therefore classical rossby waves play no fundamental role in the mechanism various experiments with different geometry and forcing are conducted and do not support the necessity of viscous numerical boundary waves or any boundary in sustaining the oscillations the models show two types of oscillatory behavior 1 temperature anomalies propagating geostrophically westward in the eastward jet northern part of the basin and inducing an opposite anomaly in their wake and 2 stationary temperature anomalies in the northwest quarter that respond to the western boundary current transport changes and reinforce geostrophically this change until the opposite temperature anomaly built on the east finally reverses the meridional overturning anomaly the analysis of the transition from steady to oscillatory states using heat fluxes diagnosed at the equilibrium under restoring boundary conditions and the comparison of the variability under various forcing fields suggest that the oscillations are triggered in the regions of strongest surface cooling finally a simple box model analogy is proposed that captures the crucial phase shift between meridional overturning and north south density gradient anomalies on these decadal timescales

low frequency interaction between horizontal and overturning gyres in the ocean 1 low frequency variability of the horizontal circulation in an idealized eddy permitting numerical model drives the dominant mode of low frequency variability in the meridional overturning circulation this coupling takes place through the influence of lateral advection in the cyclonic high latitude boundary current on the mixed layer depth along the boundary the mean and low frequency variability of the meridional overturning circulation are well predicted by a diagnostic estimate that assumes the downwelling is controlled by the thermal wind shear within the mixed layer along the boundary which is in turn determined by a simple balance between lateral advection and surface cooling the more general result is the demonstration that the mean and low frequency variability of the meridional overturning streamfunction are controlled by the baroclinic pressure gradient within the mixed layer along the boundary which may be influenced by numerous factors such as low frequency variability in lateral advection wind stress surface buoyancy fluxes or ice melt and freshwater runoff

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

snowmelt runoff pathways in a boreal forest hillslope the role of pipe throughflow abstract on an instrumented slope of the lac laflamme basin we observed that during snowmelt season 1 the unsaturated matrix flow seems to be a negligible component of the downslope flow through the soil horizons 2 the groundwater flow from an aquifer in till a few meters thick is the major pathway for rain melt inputs and 3 when groundwater levels are high a turbulent pipe throughflow occurs at the mineral and organic soils interface gauging one of the subsurface streams showed that although its discharge is correlated and synchronized with nearby groundwater levels its hydrograph often reflected that of rain melt input according to our calculations pipe throughflow does not carry more than one fifth of the seasonal rain melt input during short periods hourly basis however it may become the main source of water input to the lake the water delivered by pipe throughflow forms a distinct 0 5 1 m thick acidic layer ph 4 4 4 8 that invades the lake just beneath the lake ice cover and is susceptible to affect the fish spawning ground

gas exchange and co2 flux in the tropical atlantic ocean determined from 222rn and pco2 measurements measurements of sup 222 rn vertical profiles and pco sub 2 in the surface water and the atmosphere were made simultaneously in the tropical atlantic ocean as part of the tto tas program the gas exchange rate or piston velocity was determined from the sup 222 rn profiles and the delta pco sub 2 between the surface ocean and the atmosphere was determined from the pco sub 2 measurements the net flux of co sub 2 across the sea air interface was calculated from these two data sets the piston velocity ranged from 1 4 to 6 9 m d and was correlated with wind speed the slope of piston velocity versus wind speed was estimated to be between 0 3 and 1 1 m d m s the delta pco sub 2 ranged from minus 35 mu atm at 15 degree n 55 degree w to 64 mu atm at 5 degree s 28 degree w with the zero delta pco sub 2 isopleth located at about 10 degree n the high delta pco sub 2 values can be explained by lateral advection of surface water from the east with heating and biological consumption of co sub 2 and alkalinity during transit the net flux of co sub 2 was into the ocean north of 10 degree n latitude with values reaching a maximum of 1 4 mol m sup minus 2 yr sup minus 1 at 15 degree n 50 degree w south of 10 degree n more the net flux was out of the ocean reaching a maximum value of 2 7 mol m sup minus 2 yr sup minus 1 at 8 degree s 28 degree w the average net flux from 10 degree n to 10 degree s was 1 3 mol m sup minus 2 yr sup minus 1 out of the ocean which is equivalent to 0 15 gigatons of carbon per year if the flux determined applied throughout the year 56 refs 16 figs 4 tabs ss less

oceanic control of multidecadal variability in an idealized coupled gcm idealized ocean models are known to develop intrinsic multidecadal oscillations of the meridional overturning circulation moc here we explore the role of ocean atmosphere interactions on this low frequency variability we use a coupled ocean atmosphere model set up in a flat bottom aquaplanet geometry with two meridional boundaries the model is run at three different horizontal resolutions 4 2 and 1 in both the ocean and atmosphere at all resolutions the moc exhibits spontaneous variability on multidecadal timescales in the range 30 40 years associated with the propagation of large scale baroclinic rossby waves across the atlantic like basin the unstable region of growth of these waves through the long wave limit of baroclinic instability shifts from the eastern boundary at coarse resolution to the western boundary at higher resolution increasing the horizontal resolution enhances both intrinsic atmospheric variability and ocean atmosphere interactions in particular the simulated atmospheric annular mode becomes significantly correlated to the moc variability at 1 resolution an ocean only simulation conducted for this specific case underscores the disruptive but not essential influence of air sea interactions on the low frequency variability this study demonstrates that an atmospheric annular mode leading moc changes by about 2 years as found at 1 resolution does not imply that the low frequency variability originates from air sea interactions

upper ocean singular vectors of the north atlantic climate with implications for linear predictability and variability the limits of predictability of the meridional overturning circulation moc and upper ocean temperatures due to errors in ocean initial conditions and model parametrizations are investigated in an idealized configuration of an ocean general circulation model gcm singular vectors optimal perturbations are calculated using the gcm its tangent linear and adjoint models to determine an upper bound on the predictability of north atlantic climate the maximum growth time scales of moc and upper ocean temperature anomalies excited by the singular vectors are 18 5 and 13 years respectively and in part explained by the westward propagation of upper ocean anomalies against the mean flow as a result of the linear interference of non orthogonal eigenmodes of the non normal dynamics the ocean dynamics are found to actively participate in the significant growth of the anomalies an initial density perturbation of merely 0 02 kg m 3 is found to lead to a 1 7 sv moc anomaly after 18 5 years in addition northern hemisphere upper ocean temperature perturbations can be amplified by a factor of 2 after 13 years the growth of upper ocean temperature and moc anomalies is slower and weaker when excited by the upper ocean singular vectors than when the deep ocean is perturbed this leads to the conclusion that predictability experiments perturbing only the atmospheric initial state may overestimate the predictability time interestingly optimal moc and upper ocean temperature excitations are only weakly correlated thus limiting the utility of sst observations to infer moc variability the excitation of anomalies in this model might have a crucial impact on the variability and predictability of atlantic climate the limit of predictability of the moc is found to be different from that of the upper ocean heat content emphasizing that errors in ocean initial conditions will affect various measures differently and such uncertainties should be carefully considered in decadal prediction experiments copyright c

steady state analysis of unsaturated flow above a shallow water table aquifer drained by ditches the one dimensional pressure head profile above a fixed water table was studied for different steady infiltration rates as shown in previous studies when the infiltration rate qin is less than the soil saturated hydraulic conductivity ks this profile can be divided into two parts 1 from the water table surface z0 to an elevation z the pressure head varies from 0 to a value h such as k h qin 2 above the elevation z the pressure head is constant and equal to h above the water table the zone where the pressure head is variable has been called transition zone its height is shown to be highly variable and to depend on soil properties as well as on the infiltration rate this transition zone is not the capillary fringe as defined by gillham gillham r w 1984 the capillary fringe and its effect on water table response j hydrol 67 307 324 numerical experiments performed with hydrus 2d for the case of a water table drained by parallel ditches have shown that the height of the transition zone is similar in the one dimensional profile and in the two dimensional system as long as the local slope of the water table is small this result is important since in a two dimensional system the transition zone is the place where all the horizontal component of the unsaturated flow occurs the ratio of the horizontal component of the unsaturated flow vs the total horizontal flow in both the unsaturated and saturated zones has been computed for a given soil this ratio decreases as the infiltration rate increases for a given infiltration rate the soil with the thinnest transition zone transfers the largest amount of water above the water table

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

role of the gulf stream and kuroshio oyashio systems in large scale atmosphere ocean interaction a review abstract ocean atmosphere interaction over the northern hemisphere western boundary current wbc regions i e the gulf stream kuroshio oyashio and their extensions is reviewed with an emphasis on their role in basin scale climate variability sst anomalies exhibit considerable variance on interannual to decadal time scales in these regions low frequency sst variability is primarily driven by basin scale wind stress curl variability via the oceanic rossby wave adjustment of the gyre scale circulation that modulates the latitude and strength of the wbc related oceanic fronts rectification of the variability by mesoscale eddies reemergence of the anomalies from the preceding winter and tropical remote forcing also play important roles in driving and maintaining the low frequency variability in these regions in the gulf stream region interaction with the deep western boundary current also likely influences the low frequency variability surface heat fluxes damp the low frequency sst anomalies ove

water table shapes and drain flow rates in shallow drainage systems abstract a relationship between drain flow rate elevation and shape of the water table and recharge intensity in shallow drainage systems is developed from an analytical spatial integration of the boussinesq equation in transient conditions drain flow rates are shown to be the sum of three terms the first one is proportional to the maximum water table elevation and more generally to the steady state flow rate at the same water table elevation the second one is a fraction of the recharge rate of the water table depending on the water table shape the third one accounts for possible changes in water storage in the water table due to its shape changes drain flow rates in shallow drainage systems are shown to be fully predicted by one unique variable that is a function of a combination of the hydraulic conductivity k the drainable porosity f and the drain spacing 2l namely k f2l2 this variable also determines the dynamics of changes in the water table shape and in turn the respective parts of the three terms in the equation it is shown that drainage systems with values of 1 m 1 h 1 respond very fast to recharge events water table shape changes occur very quickly so that the third component of the drain flow can be neglected in that case the equation results in a simplified analytical relationship the reliability of the complete and simplified equations to predict drain flow rates in transient conditions is discussed in relation to in situ measurements

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

the role of oceanic heat transport and wind stress forcing in abrupt millennial scale climate transitions the last glacial period was punctuated by rapid climate shifts known as dansgaard oeschger events with strong imprint in the north atlantic sector suggesting that they were linked with the atlantic meridional overturning circulation here an idealized single hemisphere three dimensional ocean atmosphere sea ice coupled model is used to explore the possible origin of the instability driving these abrupt events and to provide a plausible explanation for the relative stability of the holocene focusing on the physics of noise free millennial oscillations under steady external solar forcing it was shown that cold climates become unstable that is exhibit abrupt millennial scale transitions for significantly lower freshwater fluxes than warm climates in agreement with previous studies making use of zonally averaged coupled models this fundamental difference is a direct consequence of the weaker stratification of the glacial ocean mainly caused by upper ocean cooling using a two hemisphere configuration of a coupled climate model of intermediate complexity it is shown that this result is robust to the added presence of a bottom water mass of southern origin the analysis reveals that under particular conditions a pronounced interdecadal variability develops during warm interstadials while the nature of the instability driving the millennial oscillations is identical to that found in ocean models under mixed boundary conditions these interstadial interdecadal oscillations share the same characteristics as those previously found in ocean models forced by fixed surface fluxes the wind stress forcing is shown to profoundly affect both the properties and bifurcation structure of thermohaline millennial oscillations across a wide range of variation of freshwater forcing in particular it is shown that the wind stress forcing favors the maintenance of thermally direct meridional overturning circulations during the cold stadial phases of dansgaard oeschger cycles

interdecadal variability of the meridional overturning circulation as an ocean internal mode the meridional overturning circulation moc in the coupled echam5 mpiom exhibits variability at periods of near 30 years and near 60 years the 30 year variability referred to as interdecadal variability idv exist in an ocean model driven by climatological atmospheric forcing suggesting that it is maintained by ocean dynamics the 60 year variability the multidecadal variability mdv is only observed in the fully coupled model and therefore is interpreted as an atmosphere ocean coupled mode the coexistence of the 30 year idv and the 60 year mdv provides a possible explanation for the widespread time scales observed in climate variables further analyses of the climatologically forced ocean model shows that the idv is related to the interplay between the horizontal temperature dominated density gradients and the ocean circulation temperature anomalies move along the cyclonic subpolar gyre leading to fluctuations in horizontal density gradients and the subsequent weakening and strengthening of the moc this result is consistent with that from less complex models indicating the robustness of the idv we further show that along the north atlantic current path the sea surface temperature anomalies are determined by the slow lsw advection at the intermediate depth

baroclinic instability an oceanic wavemaker for interdecadal variability numerical simulations of coarse resolution idealized ocean basins under constant surface heat flux are analyzed to show that the interdecadal oscillations that emerge naturally in such configurations are driven by baroclinic instability of the mean state and damped by horizontal diffusion when the surface heat fluxes are diagnosed from a spinup in which surface temperatures are strongly restored to apparent atmospheric temperatures the most unstable regions diagnosed by large downgradient eddy heat fluxes are located in the basin northwest corner where the surface heat losses are largest the long wave limit of the baroclinic instability of idealized mean flows in a three layer model with vertical shears as observed in the gcms demonstrates that growth rates of order one cycle per year can be produced locally large enough to amplify thermal anomalies in the face of lateral diffusion the proposed instability mechanism that favors surface intensified perturbations also explains the lack of oscillations if the restoring to a surface climatology is too strong to assess whether this instability process of oceanic origin is robust enough to cause interdecadal variability of coupled ocean atmosphere models a four box ocean atmosphere model is constructed given the large heat capacity of the ocean as compared to the atmosphere the dynamical system that governs the model evolution is reduced to only two degrees of freedom the oceanic overturning thermohaline circulation and the interior north south temperature gradient the authors show that when the baroclinic instability growth rate exceeds the overall dissipation caused by turbulent eddy diffusion in the atmosphere and ocean and infrared back radiation the dynamical system undergoes a hopf bifurcation and interdecadal oscillations emerge through a limit cycle

the damping effect of bottom topography on internal decadal scale oscillations of the thermohaline circulation by comparing the response of flat and bowl shaped basins to fixed heat fluxes of various magnitudes it is determined that coastal topography has a considerable damping influence upon internal decadal oscillations of the thermohaline circulation it is proposed that this is because the adjustment of baroclinic currents to the no normal flow boundary condition at weakly stratified coasts is aided in the topography case by the generation of substantial barotropic flow

the internal generation of the atlantic ocean interdecadal variability abstractnumerical simulations of a realistic ocean general circulation model forced by prescribed surface fluxes are used to study the origin and structure of intrinsic interdecadal variability of

interdecadal variability in a hybrid coupled ocean atmosphere model abstract a hybrid coupled ocean atmosphere model is used to investigate low frequency variability in the climate system the model s atmospheric component is a budyko sellers north two dimensional energy balance model the oceanic component is a simplified general circulation model the coupled model is confined to an idealized rectangular north atlantic basin in the present model version the ocean density depends exclusively on temperature an interdecadal oscillation with a period of 40 50 years is found in the hybrid coupled model when model parameters are within the climatological range even though density does not depend on salinity this interdecadal oscillation is characterized by a pair of vortices of opposite signs that grow and decay in quadrature with each other in the ocean s upper layer their centers follow each other anticlockwise through the northwestern quadrant of the model domain the interdecadal oscillation s physical mechanism resembles that of the interdecadal oscillation anal

pipe flow in james bay coastal wetlands pipes are common along the banks of streams that drain the wetlands of the james bay coast the primary source of water to the pipes is overland flow which responds very quickly to rainfall events pipe flow therefore becomes insignificant shortly after the termination of surface runoff an assessment of the relative contributions of pipe flow and darcy s groundwater flow to a small coastal creek shows that whereas groundwater flow contributed less than 1 to summer stormflow pipes provided about 10 of stream runoff

response of a simplified oceanic general circulation model to idealized nao like stochastic forcing abstract the response of a simplified model of the subtropical and subpolar gyres to a mechanical stochastic forcing that mimics the north atlantic oscillation nao is studied an internal damped mode of oscillation with a 22 24 yr period is found with a maximum amplitude in the northwestern part of the basin subpolar gyre and intergyre area its mechanism is as follows in response to the wind stress forcing anomalous currents appear between 35 and 40 n they create temperature anomalies near the western coast which are in turn advected by the mean current into the subpolar gyre and in the convection area these temperature anomalies affect the thermohaline circulation which produces new temperature anomalies near 35 40 n and thus appears as a key component of this damped periodic mode wind stress forcing acts as efficiently as heat flux at exciting this oscillation which can persist if external forcing is strong enough

influences of tropical extratropical interaction on the multidecadal amoc variability in the ncep climate forecast system we have examined the mechanisms of a multidecadal oscillation of the atlantic meridional overturning circulation amoc in a 335 year simulation of the climate forecast system cfs the climate prediction model developed at the national centers for environmental prediction ncep both the mean and seasonal cycle of the amoc in the cfs are generally consistent with observation based estimates with a maximum northward volume transport of 16 sv 106 m3 s near 35 n at 1 2 km the annual mean amoc shows an intermittent quasi 30 year oscillation its dominant structure includes a deep anomalous overturning cell referred to as the anomalous amoc with amplitude of 0 6 sv near 35 n and an anomalous subtropical cell stc of shallow overturning spanning across the equator the mechanism for the oscillation includes a positive feedback between the anomalous amoc and surface wind stress anomalies in mid latitudes and a negative feedback between the anomalous stc and amoc a strong amoc is associated with warm sea surface temperature anomaly ssta centered near 45 n which generates an anticyclonic easterly surface wind anomaly this anticyclonic wind anomaly enhances the regional downwelling and reinforces the anomalous amoc in the mean time a wind evaporation sst wes feedback extends the warm ssta to the tropics and induces a cyclonic wind stress anomaly there which drives a tropical upwelling and weakens the stc north of the equator the stc anomaly in turn drives a cold upper ocean heat content anomaly hca in the northern tropical atlantic and weakens the meridional heat transport from the tropics to the mid latitude through an anomalous southward western boundary current the anomalous stc transports cold hca from the subtropics to the mid latitudes weakening the mid latitude deep overturning

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

multidecadal variability of the overturning circulation in presence of eddy turbulence at low resolution idealized ocean circulation models forced by prescribed differential surface heat fluxes show spontaneous multidecadal variability depending critically on eddy diffusivity coefficients the existence of this critical threshold in the range of observational estimates legitimates some doubt on the relevance of such intrinsic oscillations in the real ocean through a series of numerical simulations with increasing resolution up to eddy resolving ones 10 km and various diapycnal diffusivities this multidecadal variability proves a generic ubiquitous feature at least in model versions with a flat bottom the mean circulation largely changes in the process of refining the horizontal grid along with the associated implicit viscosity and diffusivity and the spatial structure of the variability is largely modified but there is no clear influence of the resolution on the main oscillation period the interdecadal variability appears even more robust to low vertical diffusivity and overturning when mesoscale eddies are resolved the mechanism previously proposed for these oscillations involving westward propagating baroclinically unstable rossby waves in the subpolar region and its feedback on the mean circulation appears unaffected by mesoscale turbulence and is simply displaced following the polar front

interdecadal variability in a hybrid coupled ocean atmosphere sea ice model interdecadal climate variability in an idealized coupled ocean atmosphere sea ice model is studied the ocean component is a fully three dimensional primitive equation model and the atmospheric component is a two dimensional 2d energy balance model of budyko sellers north type while sea ice is represented by a 2d thermodynamic model in a wide range of parameters the model climatology resembles certain aspects of observed climate two types of interdecadal variability are found the first one is characterized by northward propagating upper ocean temperature anomalies in the northwestern part of the ocean basin and a westward propagating wavelike temperature pattern at depth the other type has larger scale temperature anomalies that propagate westward in both the upper and deep ocean along the sea ice edge both types of oscillations have been found previously in similar models that do not include sea ice therefore the oscillation mechanism does not depend on sea ice feedbacks nor is it modified very much by the inclusion of sea ice for some parameter values the interdecadal oscillations are self sustained while for others they are damped stochastic forcing experiments show that in the latter case significant interdecadal signals can still be identified in the time series of oceanic heat transport the periods of these signals however do not closely match those identified in a stability analysis of the deterministic model when linearized about its steady state the authors show that linearization around the actual climatology of the stochastically forced integrations provides a better match for some of the modes that were poorly explained when linearizing about the deterministic model s steady state the main difference between the two basic states is in the distribution of climatological convective depth which is affected strongly by intermittent atmospheric forcing

analytical solutions for two dimensional groundwater flow with subsurface drainage tiles summary the tile drainage problem in an unconfined aquifer was investigated a mathematical model was established that describes two dimensional groundwater flow in an unconfined aquifer near a river with a linearized boussinesq equation time dependent sources and a sloped tile analytical solutions for groundwater level and discharge were derived and used to compare hydrologic conditions in a system with and without tile natural drainage we found that the spatial and temporal variations of groundwater level and discharge were significantly altered by the presence of drainage tile in an aquifer with tile drainage the groundwater level was lower and total groundwater discharge to the river increased compared to an aquifer with no tile application of the solutions to a synthetic case demonstrates that the analytical solutions derived can be used to quantify effect of tiles on nitrate loads in the baseflow of a river and assess the effectiveness of various conservation practices

the different nature of the interdecadal variability of the thermohaline circulation under mixed and flux boundary conditions the differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse resolution ocean model in an idealized flat bottom single hemisphere basin objective features are determined that allow one type of oscillation to be distinguished versus the other first by performing a linear stability analysis of the steady state obtained under restoring boundary conditions it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises respectively from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state based on the budgets of density variance it is shown next that the two types of oscillations have different energy sources under the constant flux boundary condition the thermal mode the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability whereas under mixed boundary conditions the salinity mode a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation furthermore the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance in addition the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure the authors propose that these differences will allow one to identify which type of oscillation if any is at play in the more exhaustive climate models

vertical structure of midlatitude mesoscale instabilities abstract instability processes in frontal jet regions outside the western boundary currents are examined for their ability to produce mesoscale variability comparable to that observed in the open ocean the important of sufficient numerical resolution for a correct modeling of these processes is considered using a local plane quasi geostrophic multilevel model with periodic horizontal boundary conditions the combined effects of special density and current profiles typical for the midlatitude eastern north atlantic are studied the linearized approach yields a set of vertical shear modes with different vertical structure depending on the zonal wavelength of the perturbation some of these shear modes are unstable the mesoscale range 50 500 km reveals two different types a surface intensified shear mode and a deep sea intensified one the growth rates of the former are usually largely exceeding those of the latter due to the larger velocities and shears their exponential amplification time scale ra

influence of slope on subsurface drainage of hillsides hillside drainage was investigated using numerical solutions to the two dimensional form of the richards equation for combined unsaturated and saturated flow the objectives of the study were to examine the influence of slope on drain flow rates and water table depths in the center regions of sloping hillsides solutions were obtained for slopes 135 m long containing parallel drains spaced 30 m apart and located l m below the surface in 3 and 7 m deep profiles drainage under steady infiltration and for transient conditions was simulated on slopes of up to 0 40 m m using two soils small slopes 0 15 m m were found to have little effect on drain flow rates and water table depths in the center of the region indicating that drainage theory developed for flat land can be used for interior parallel drains on these slopes

theoretical investigation of the atlantic multidecadal oscillation a weakly damped mode of variability corresponding to the oceanic signature of the atlantic multidecadal oscillation amo was found through the linear stability analysis of a realistic ocean general circulation model a simpletwo level model wasproposed torationalize bothits period and damping rate this model isextended heretothree levelstoinvestigate how the modecan draw energy fromthe mean flow as found in various ocean and coupled models a linear stability analysis in this three level model shows that the positive growth rate of the oscillatory mode depends on the zonally averaged isopycnal slope this mode corresponds to a westward propagation of density anomalies in the pycnocline typical of large scale baroclinic rossby waves the most unstablemodecorrespondstothelargestscaleone atleastforlowisopycnalslope themodecanbedescribed in four phases composing a full oscillation cycle 1 basin scale warming of the north atlantic amo positive phase 2 decrease in upper ocean poleward transport hence a reduction of the atlantic meridional overturning circulation amoc 3 basin scale cooling negative amo and 4 amoc intensification a criterion is developed to test in oceanic datasets or numerical models whether this multidecadal oscillation is an unstable oceanic internal mode of variability or if it is stable and externally forced consistent with the classical theory of baroclinic instability this criterion depends on the vertical structure of the mode if the upper pycnoclinesignatureisinadvanceofthedeeper pycnoclinesignaturewith respecttothewestwardpropagation the mode is unstable and could be described as an oceanic internal mode of variability

large scale baroclinic instability of the mean oceanic circulation a local approach abstractlarge scale baroclinic instability is investigated as a potential source of rossby waves and large scale variability in the ocean this baroclinic instability is first reviewed in a 2 5 layer model as already noticed by several authors the instability arises in westward surface mean flow when the phase velocities of the two vertical modes are made equal by mean flow influence this large scale instability is stronger at low latitudes and thus is likely to happen in the westward return flow of the subtropical gyres further investigations with a continuous stratification quasigeostrophic model show that the instability is stronger where the mean flow projects negatively on the second baroclinic mode imposing positive vertical modes at the surface the linear stability calculation is then performed on argo derived mean flow along with mean stratification data the results show that the unstable regions are situated at low latitudes in every oceanic basin in western boundary currents and in som

modelling investigation of water partitioning at a semiarid ponderosa pine hillslope the effects of vegetation root distribution on near surface water partitioning can be two fold on the one hand the roots facilitate deep percolation by root induced macropore flow on the other hand they reduce the potential for deep percolation by root water uptake processes whether the roots impede or facilitate deep percolation depends on various conditions including climate soil and vegetation characteristics this paper examines the effects of root distribution on deep percolation into the underlying permeable bedrock for a given soil profile and climate condition using hydrus modelling the simulations were based on previously field experiments on a semiarid ponderosa pine pinus ponderosa hillslope an equivalent single continuum model for simulating root macropore flow on hillslopes is presented with root macropore hydraulic parameterization estimated based on observed root distribution the sensitivity analysis results indicate that the root macropore effect dominates saturated soil water flow in low conductivity soils kmatrix below 10 7 m s while it is insignificant in soils with a kmatrix larger than 10 5 m s consistent with observations in this and other studies at the ponderosa pine site the model with simple rootmacropore parameterization reasonably well reproduces soil moisture distribution and some major runoff events the results indicate that the clay rich soil layer without root induced macropores acts as an impeding layer for potential groundwater recharge this impeding layer results in a bedrock percolation of less than 1 of the annual precipitation without this impeding layer percolation into the underlying permeable bedrock could be as much as 20 of the annual precipitation this suggests that at a surface with low permeability soil overlying permeable bedrock the root penetration depth in the soil is critical condition for whether or not significant percolation occurs copyright 2010 john wiley sons ltd

identification of the mechanism of interdecadal variability in the north atlantic ocean the aim of this paper is to identify the physical mechanism of interdecadal variability in simulations of the north atlantic ocean circulation with the modular ocean model of the geophysical fluid dynamics laboratory to that end a hierarchy of increasingly complex model configurations is used the variability in the simplest case that of viscous purely thermally driven flows in a flat bottom ocean basin with a box shaped geometry is shown to be caused by an internal interdecadal mode the westward propagation of temperature anomalies and the phase difference between the anomalous zonal and meridional overturning that characterize the interdecadal mode are used as fingerprints of the physical mechanism of the variability in this way the variability can be followed toward a less viscous regime in which the effects of continental geometry and bottom topography are also included it is shown that although quantitative aspects of the variability like period and spatial pattern are changing the physical mechanism of the interdecadal variability in the more complex simulations can be attributed to the same processes as in the simplest model configuration

a two level wind and buoyancy driven thermocline model abstract a simple two level model is designed to simulate the thermocline equations applicable for large scale steady oceanic flow the model serves two functions first it replaces problems with the interpretation of slablike dynamics e g luyten et al 1983 by using continuously horizontally varying buoyancy but at the cost of reducing the vertical resolution drastically the equations used are geostrophy plus a small linear drag to close a stommel like western boundary layer mass conservation and buoyancy conservation with a small but necessary horizontal diffusion inclusion of vertical diffusion has little effect the ocean is driven by an ekman layer whose functions are to provide a given surface input of mass through ekman pumping and buoyancy through a specified buoyancy in the ekman layer i e to maintain the same boundary conditions as in classical thermocline studies sidewall conditions are not well understood and are almost certainly over specified in this formulation s

field scale modeling of subsurface tile drained soils using an equivalent medium approach research conducted for the last 35 years has shown that subsurface drainage has a significant impact on hydrology and contaminant transport this can be observed at the field scale and also at the watershed scale impacts are always associated with modifying otherwise natural flow paths most computer model representations of drainage have been drawn at the field scale these models require relatively precise data that are usually unavailable when simulating hydrology and water quality in large watersheds we believe that in this case drainage representation should be simplified and yet closely match observations as a first step towards incorporating drainage systems into large scale hydrological models we propose an equivalent representation of drains buried in a soil profile by using a homogeneous anisotropic porous medium without drains this representation is based on a self consistent approach and on geometrical considerations simplification is such that calculating the equivalent hydraulic conductivity requires only information on the main length and spacing of the tile drains and not on their precise location this approach also provides a much simpler discretisation of the domain because of the absence of internal boundary conditions on the drainage pipes compared to other methods that have simplified drainage representation in existing watershed models it requires no parameter fitting two alternatives to the method are presented in the first one the soil profile equipped with the actual drain pipes is represented by an equivalent horizontally layered system with no pipes in the second the layered system has been replaced with an equivalent homogeneous profile the efficiency of these approaches was tested against a classical representation of tile drains using the swms 3d code which solves the richards equation for a typical drained plot configuration the equivalent medium approach appears to give satisfying results for global water outflow and mean water table elevation

on the robustness of the interdecadal modes of the thermohaline circulation ocean models in box geometry forced by constant surface fluxes of density have been found to spontaneously generate interdecadal oscillations of the thermohaline circulation this paper analyzes the sensitivity of these oscillations to various physical effects including the presence of mesoscale turbulence various thermal surface boundary conditions and the presence of wind forcing or bottom topography the role of unstable long baroclinic waves is also reexamined in an attempt to understand the oscillation period in idealized geometry it is found that the low frequency variability of the thermohaline circulation under quasi constant surface fluxes is a robust feature of the large scale circulation it is not strongly affected by energetic mesoscale turbulence the oscillation period is relatively invariant with respect to varying resolution and momentum and tracer horizontal mixing coefficients although it loses some regularity as shorter and longer periods of variability emerge when the mesoscale activity increases in strength with smaller mixing coefficients the oscillations are also retained as the ocean model is coupled to an interactive atmospheric energy balance model the thermohaline modes are robust to a range of exchange coefficients that widens with the amplitude of the mean circulation the presence of an additional wind forced component generally weakens the oscillation and depending on the relative strength of thermodynamic and dynamic forcings the oscillation may be completely killed a simple interpretation is given highlighting the role of upward ekman pumping in damping density anomalies finally the interaction of these baroclinic modes with bottom topography depends strongly on the relative directions of the mean topographic features and the mean currents and baroclinic waves but usually results in a damping influence

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte

sensitivity of stream aquifer seepage to spatial variability of the saturated hydraulic conductivity of the aquifer abstract in this paper the sensitivities of stream aquifer seepage flow and the state of connection or disconnection between stream and aquifer to spatial variability in the aquifer saturated hydraulic conductivity are studied with the variably saturated modelling programme swms 2d using a monte carlo technique a simple stream aquifer flow system is used with a rectangular stream with a clogging layer on its perimeter partially penetrating an alluvial aquifer a 2d nearest neighbour method is used to generate fields of log normally distributed saturated conductivity for the alluvial aquifer for a specific set of degree of heterogeneity and the nearest neighbour autoregressive parameters seven different cases are studied five of these investigate the effect of the nearest neighbour parameters and three cases investigate the effect of aquifer water table level the results show that the nearest neighbour parameters and the aquifer degree of heterogeneity significantly affect the modelling of stream seepage pressure heads and the state of connection disconnection between stream and aquifer particularly when the water table is some distance below the streambed moreover comparisons of these results with a purely deterministic model show good agreement when the stream and the aquifer are connected or disconnected but with a high water table however there are significant differences between the deterministic and stochastic models when the water table is well below the streambed