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Wolfe, CL, Cessi P, Cornuelle BD.  2017.  An intrinsic mode of interannual variability in the Indian Ocean. Journal of Physical Oceanography. 47:701-719.   10.1175/jpo-d-16-0177.1   AbstractWebsite

An intrinsic mode of self-sustained, interannual variability is identified in a coarse-resolution ocean model forced by an annually repeating atmospheric state. The variability has maximumloading in the Indian Ocean, with a significant projection into the South Atlantic Ocean. It is argued that this intrinsic mode is caused by baroclinic instability of the model's Leeuwin Current, which radiates out to the tropical Indian and South Atlantic Oceans as long Rossby waves at a period of 4 yr. This previously undescribed mode has a remarkably narrowband time series. However, the variability is not synchronized with the annual cycle; the phase of the oscillation varies chaotically on decadal time scales. The presence of this internal mode reduces the predictability of the ocean circulation by obscuring the response to forcing or initial condition perturbations. The signature of this mode can be seen in higher-resolution global ocean models driven by high-frequency atmospheric forcing, but altimeter and assimilation analyses do not show obvious signatures of such a mode, perhaps because of insufficient duration.

Jones, CS, Cessi P.  2016.  Interbasin transport of the meridional overturning circulation. Journal of Physical Oceanography. 46:1157-1169.   10.1175/jpo-d-15-0197.1   AbstractWebsite

The meridional overturning circulation (MOC) is studied in an idealized domain with two basins connected by a circumpolar channel in the southernmost region. Flow is forced at the surface by longitude-independent wind stress, freshwater flux, and fast temperature relaxation to prescribed profiles. The only longitudinal asymmetry is that one basin is twice as wide as the other. Two states, a preferred one with sinking in the narrow basin and an asymmetrically forced one with sinking in the wide basin, are compared. In both cases, sinking is compensated by upwelling everywhere else, including the passive basin. Despite the greater area of the wide basin, the residual overturning transport is the same regardless of the location of sinking. The two basins exchange flow at their southern edge by a geostrophic transport balanced by the difference in the depth of isopycnals at the eastern boundaries of each basin. Gnanadesikan's model for the upper branch of the MOC is extended to include two basins connected by a reentrant channel and is used to illustrate the basic properties of the flow: the layer containing the surface and intermediate water is shallower in the active basin than in the passive basin, and this difference geostrophically balances an exchange flow from the passive to the active basin. The exchange flow is larger when sinking occurs in the narrow basin. A visualization of the horizontal structure of the upper branch of the MOC shows that both the gyres and the meridional flow are important in determining the flow field.

Wolfe, CL, Cessi P.  2015.  Multiple regimes and low-frequency variability in the quasi-adiabatic overturning circulation. Journal of Physical Oceanography. 45:1690-1708.   10.1175/jpo-d-14-0095.1   AbstractWebsite

When interior mixing is weak, the ocean can support an interhemispheric overturning circulation on isopycnals that outcrop in both the Northern Hemisphere and a high-latitude southern circumpolar channel. This overturning cell participates in a salt-advection feedback that counteracts the precipitation-induced surface freshening of the northern high latitudes. The net result is an increase in the range of isopycnals shared between the two hemispheres, which strengthens the overturning circulation. However, if precipitation in the Northern Hemisphere sufficiently exceeds that in the Southern Hemisphere, the overturning cell collapses and is replaced by a cell circulating in the opposite direction, whose southern end point is equatorward of the channel. This reversed cell is shallower and weaker than its forward counterpart and is maintained diffusively. For a limited range of parameters, freshwater hysteresis occurs and multiple overturning regimes are found for the same forcing. These multiple regimes are, by definition, unstable with regard to finite-amplitude disturbances, since a sufficiently large perturbation can affect a transition from one regime to the other. Both overturning regimes show pronounced, nearly periodic thermohaline variability on multidecadal and multicentennial time scales. The multidecadal oscillation is expressed in the North Hemisphere gyre and driven by a surface thermohaline instability. The multicentennial oscillation has the character of an interhemispheric loop oscillation. These oscillations mediate transitions between overturning regimes by providing an internal source of finite-amplitude disturbances. As the diffusivity is reduced, the reverse cell becomes weaker and thus less stable to a given perturbation amplitude. This causes the width of the hysteresis to decrease with decreasing diffusivity.

Abernathey, R, Cessi P.  2014.  Topographic enhancement of eddy efficiency in baroclinic equilibration. Journal of Physical Oceanography. 44:2107-2126.   10.1175/jpo-d-14-0014.1   AbstractWebsite

The processes that determine the depth of the Southern Ocean thermocline are considered. In existing conceptual frameworks, the thermocline depth is determined by competition between the mean and eddy heat transport, with a contribution from the interaction with the stratification in the enclosed portion of the ocean. Using numerical simulations, this study examines the equilibration of an idealized circumpolar current with and without topography. The authors find that eddies are much more efficient when topography is present, leading to a shallower thermocline than in the flat case. A simple quasigeostrophic analytical model shows that the topographically induced standing wave increases the effective eddy diffusivity by increasing the local buoyancy gradients and lengthening the buoyancy contours across which the eddies transport heat. In addition to this local heat flux intensification, transient eddy heat fluxes are suppressed away from the topography, especially upstream, indicating that localized topography leads to local (absolute) baroclinic instability and its subsequent finite-amplitude equilibration, which extracts available potential energy very efficiently from the time-mean flow.

Wolfe, CL, Cessi P.  2014.  Salt feedback in the adiabatic overturning circulation. Journal of Physical Oceanography. 44:1175-1194.   10.1175/jpo-d-13-0154.1   AbstractWebsite

The adiabatic overturning circulation is the part of the meridional overturning circulation that persists in the limit of vanishing diffusivity. Two conditions are required for the existence of the adiabatic overturning circulation: a high-latitude zonally reentrant channel subject to surface westerlies and a set of outcropping isopycnals shared between the channel and the opposite hemisphere. This paper examines how different buoyancy forcing regimes, particularly freshwater flux, affect the surface buoyancy distribution and the strength of the adiabatic overturning circulation. Without freshwater forcing, salinity is uniform and buoyancy is determined by temperature only. In this case, the size of the shared isopycnal window is effectively fixed by the coupling between atmospheric and sea surface temperatures. With freshwater forcing (applied as a surface flux), the salinity, and thus the sea surface buoyancy and the size of the shared isopycnal window, is not specified by the atmospheric state alone. It is found that a salt-advection feedback leads to surface buoyancy distributions that increase the size of the isopycnal window and strengthen the adiabatic overturning circulation. The strength of the feedback is controlled by processes in high latitudes-the southern channel, where the surface salinity is determined by a balance between freshwater input from the atmosphere, salt input from upwelling deep water, and freshwater export by Ekman transport; and the Northern Hemisphere, where the overturning and wind-driven transport in the thermocline advect salty water from the subtropics, mitigating the freshening effect of the surface freshwater flux. The freshwater budget in the channel region provides an estimate of the size of the isopycnal window.

Cessi, P, Pinardi N, Lyubartsev V.  2014.  Energetics of semienclosed basins with two- layer flows at the strait. Journal of Physical Oceanography. 44:967-979.   10.1175/jpo-d-13-0129.1   AbstractWebsite

Examination of the energy budget for semienclosed seas with two-layer exchange flow at the strait shows that the energy flux at the open portion of the boundary (the strait) is proportional to the surface buoyancy flux integrated over the basin area, with the constant of proportionality given by the interface depth. When the surface buoyancy flux is positive, the energy flux is negative: these types of basins have an estuarine circulation. Antiestuarine basins have a negative surface buoyancy flux, which provides a positive energy flux, augmenting the wind work in powering the circulation. The energy budget for the semienclosed seas with vertically separated flows at the strait is examined using reanalysis products for four major semienclosed basins: the Mediterranean and Red Seas (antiestuarine) and the Black and Baltic Seas (estuarine). Important differences in the relative contribution to the energy budget of the wind work versus the surface buoyancy flux are found within basins of the same type, and these differences help explain some qualitative aspects of the basins' flow.

Cessi, P, Wolfe CL.  2013.  Adiabatic eastern boundary currents. Journal of Physical Oceanography. 43:1127-1149.   10.1175/jpo-d-12-0211.1   AbstractWebsite

The dynamics of the eastern boundary current of a high-resolution, idealized model of oceanic circulation are analyzed and interpreted in terms of residual mean theory. In this framework, it is clear that the eastern boundary current is adiabatic and inviscid. Nevertheless, the time-averaged potential vorticity is not conserved along averaged streamlines because of the divergence of Eliassen-Palm fluxes, associated with buoyancy and momentum eddy fluxes. In particular, eddy fluxes of buoyancy completely cancel the mean downwelling or upwelling, so that there is no net diapycnal residual transport. The eddy momentum flux acts like a drag on the mean velocity, opposing the acceleration from the eddy buoyancy flux: in the potential vorticity budget this results in a balance between the divergences of eddy relative vorticity and buoyancy fluxes, which leads to a baroclinic eastern boundary current whose horizontal scale is the Rossby deformation radius and whose vertical extent depends on the eddy buoyancy transport, the Coriolis parameter, and the mean surface buoyancy distribution.

Wolfe, CL, Cessi P.  2011.  The adiabatic pole-to-pole overturning circulation. Journal of Physical Oceanography. 41:1795-1810.   10.1175/2011jpo4570.1   AbstractWebsite

The adiabatic pole-to-pole cell of the residual overturning circulation (ROC) is studied in a two-hemisphere, semienclosed basin, with a zonally reentrant channel occupying the southernmost eighth of the domain. Three different models of increasing complexity are used: a simple, analytically tractable zonally averaged model; a coarse-resolution numerical model with parameterized eddies; and an eddy-resolving general circulation model. Two elements are found to be necessary for the existence of an adiabatic pole-to-pole cell: 1) a thermally indirect, wind-driven overturning circulation in the zonally reentrant channel, analogous to the Deacon cell in the Antarctic Circumpolar Current (ACC) region, and 2) a set of outcropping isopycnals shared between the channel and the semienclosed region of the Northern Hemisphere. These points are supported by several computations varying the domain geometry, the surface buoyancy distribution, and the wind forcing. All three models give results that are qualitatively very similar, indicating that the two requirements above are general and robust. The zonally averaged model parameterizes the streamfunction associated with adiabatic buoyancy fluxes as downgradient diffusion of buoyancy thickness, with a diffusivity in the semienclosed region of the Northern Hemisphere much larger than that in the ACC region. In the simple model, the disparity in diffusivities is necessary to obtain a substantial pole-to-pole ROC. The simple model also illustrates how the geometry of the isopycnals is shaped by the interhemispheric ROC, leading to three major thermostads, which the authors identify with the major water masses of the Atlantic: that is, North Atlantic Deep Water, Antarctic Intermediate Water, and Antarctic Bottom Water.

Cessi, P, Wolfe CL, Ludka BC.  2010.  Eastern-boundary contribution to the residual and meridional overturning circulations. Journal of Physical Oceanography. 40:2075-2090.   10.1175/2010jpo4426.1   AbstractWebsite

A model of the thermocline linearized around a specified stratification and the barotropic linear wind-driven Stommel solution is constructed. The forcings are both mechanical (the surface wind stress) and thermodynamical (the surface buoyancy boundary condition). The effects of diapycnal diffusivity and of eddy fluxes of buoyancy, parameterized in terms of the large-scale buoyancy gradient, are included. The eddy fluxes of buoyancy are especially important near the boundaries where they mediate the transport in and out of the narrow ageostrophic down-/upwelling layers. The dynamics of these narrow layers can be replaced by effective boundary conditions on the geostrophically balanced flow. The effective boundary conditions state that the residual flow normal to the effective coast vanishes. The separate Eulerian and eddy-induced components may be nonzero. This formulation conserves the total mass and the total buoyancy while permitting an exchange between the Eulerian and eddy transport of buoyancy within the down-/upwelling layers. In turn, this exchange allows buoyancy gradients along all solid boundaries, including the eastern one. A special focus is on the buoyancy along the eastern and western walls since east-west buoyancy difference determines the meridional overturning circulation. The inclusion of advection of buoyancy by the barotropic flow allows a meaningful distinction between the meridional and the residual overturning circulations while retaining the simplicity of a linear model. The residual flow in both meridional and zonal directions reveals how the subsurface buoyancy distribution is established and, in particular, how the meridional buoyancy gradient is reversed at depth. In turn, the horizontal buoyancy gradient maintains stacked counterrotating cells in the meridional and residual overturning circulations. Quantitative scaling arguments are given for each of these cells, which show how the buoyancy forcing, the wind stress, and the diapycnal and eddy diffusivities, as well as the other imposed parameters, affect the strength of the overturn.

Wolfe, CL, Cessi P.  2010.  What sets the strength of the middepth stratification and overturning circulation in eddying ocean models? Journal of Physical Oceanography. 40:1520-1538.   10.1175/2010jpo4393.1   AbstractWebsite

The processes maintaining stratification in the oceanic middepth (between approximately 1000 and 3000 m) are explored using an eddy-resolving general circulation model composed of a two-hemisphere, semienclosed basin with a zonal reentrant channel in the southernmost eighth of the domain. The middepth region lies below the wind-driven main thermocline but above the diffusively driven abyssal ocean. Here, it is argued that middepth stratification is determined primarily in the model's Antarctic Circumpolar Current. Competition between mean and eddy overturning in the channel leads to steeper isotherms and thus deeper stratification throughout the basin than would exist without the channel. Isotherms that outcrop only in the channel are nearly horizontal in the semienclosed portion of the domain, whereas isotherms that also outcrop in the Northern Hemisphere deviate from horizontal and are accompanied by geostrophically balanced meridional transport. A northern source of deep water (water with temperatures in the range of those in the channel) leads to the formation of a thick middepth thermostad. Changes in wind forcing over the channel influence the stratification throughout the domain. Since the middepth stratification is controlled by adiabatic dynamics in the channel, it becomes independent of the interior diffusivity kappa as kappa -> 0. The meridional overturning circulation (MOC), as diagnosed by the mean meridional volume transport, also shows a tendency to become independent of kappa as kappa -> 0, whereas the MOC diagnosed by water mass transport shows a continuing dependence on kappa as kappa -> 0. A nonlocal scaling for MOC is developed that relates the strength of the northern MOC to the depth of isotherms in the southern channel. The results of this paper compare favorably to observations of large-scale neutral density in the World Ocean.

Cessi, P, Wolfe CL.  2009.  Eddy-driven buoyancy gradients on eastern boundaries and their role in the thermocline. Journal of Physical Oceanography. 39:1595-1614.   10.1175/2009jpo4063.1   AbstractWebsite

It is demonstrated that eddy fluxes of buoyancy at the eastern and western boundaries maintain alongshore buoyancy gradients along the coast. Eddy fluxes arise near the eastern and western boundaries because on both coasts buoyancy gradients normal to the boundary are strong. The eddy fluxes are accompanied by mean vertical flows that take place in narrow boundary layers next to the coast where the geostrophic constraint is broken. These ageostrophic cells have a velocity component normal to the coast that balances the geostrophic mean velocity. It is shown that the dynamics in these thin ageostrophic boundary layers can be replaced by effective boundary conditions for the interior flow, relating the eddy flux of buoyancy at the seaward edge of the boundary layers to the buoyancy gradient along the coast. These effective boundary conditions are applied to a model of the thermocline linearized around a mean stratification and a state of rest. The linear model parameterizes the eddy fluxes of buoyancy as isopycnal diffusion. The linear model produces horizontal gradients of buoyancy along the eastern coast on a vertical scale that depends on both the vertical diffusivity and the eddy diffusivity. The buoyancy field of the linear model agrees very well with the mean state of an eddy-resolving computation. Because the east - west difference in buoyancy is related to the zonally integrated meridional velocity, the linear model successfully predicts the meridional overturning circulation.

Wolfe, CL, Cessi P.  2009.  Overturning circulation in an eddy-resolving model: the effect of the pole-to-pole temperature gradient. Journal of Physical Oceanography. 39:125-142.   10.1175/2008jpo3991.1   AbstractWebsite

The effect of the pole-to-pole surface temperature difference on the deep stratification and the strength of the global meridional overturning circulation (MOC) is examined in an eddy-resolving ocean model configured in an idealized domain roughly representing the Atlantic sector. Mesoscale eddies lead to qualitative differences in the mean stratification and the MOC compared to laminar (i.e., eddy free) models. For example, the spreading of fluid across the model's representation of the Antarctic Circumpolar Current (ACC) no longer relies on the existence of a sill in the ACC. In addition, the deep-and bottom-water masses roughly representing North Atlantic Deep Water (NADW) and Antarctic Bottom Water (ABW), respectively-are eroded by the eddies so that their zonal and meridional extents are much smaller than in the laminar case. It is found that if the north pole temperature is sufficiently warm, the formation of northern deep water is suppressed and the middepth cell is small and weak while the deep cell is large and vigorous. In contrast, if the north pole temperature is in the range of the southern channel temperatures, the middepth cell is large and strong while the deep cell has a reduced amplitude. This result is consistent with the predictions of the laminar theory of the MOC. In contrast to the laminar theory, realistically strong deep stratification is formed even if the temperature at the northern sinking site is warmer than any temperature found in the channel. Indeed, middepth stratification is actually stronger in the latter case than the former case.

Wolfe, CL, Cessi P, McClean JL, Maltrud ME.  2008.  Vertical heat transport in eddying ocean models. Geophysical Research Letters. 35   10.1029/2008gl036138   AbstractWebsite

The effect of mesoscale eddies on the vertical heat transport of the ocean is examined using two eddy-resolving numerical models. The global heat transport by the mean flow and diffusion are both downwards and are balanced by an upward eddy heat flux. Mean and eddy advective heat fluxes dominate the subpolar regions, while diffusive flux is important primarily in the subtropics. In the subtropical abyss, the mean advective heat flux is balanced by a combination of eddy and diffusive fluxes and the classical Munk-type advective-diffusive heat balance must be modified. The Munk and Wunsch (1998) expression for the vertical turbulent diffusivity over-estimates the diffusivity by as much as a factor of four near the base of the main thermocline. This implies that the mixing required to close the meridional overturning circulation determined by Munk and Wunsch (1998) may be an over-estimate due to the neglect of mesoscale eddies. Citation: Wolfe, C. L., P. Cessi, J. L. McClean, and M. E. Maltrud (2008), Vertical heat transport in eddying ocean models, Geophys. Res. Lett., 35, L23605, doi: 10.1029/2008GL036138.

Cessi, P.  2008.  An energy-constrained parameterization of eddy buoyancy flux. Journal of Physical Oceanography. 38:1807-1819.   10.1175/2007jpo3812.1   AbstractWebsite

A parameterization for eddy buoyancy fluxes for use in coarse-grid models is developed and tested against eddy-resolving simulations. The development is based on the assumption that the eddies are adiabatic (except near the surface) and the observation that the flux of buoyancy is affected by barotropic, depth-independent eddies. Like the previous parameterizations of Gent and McWilliams (GM) and Visbeck et al. (VMHS), the horizontal flux of a tracer is proportional to the local large-scale horizontal gradient of the tracer through a transfer coefficient assumed to be given by the product of a typical eddy velocity scale and a typical mixing length. The proposed parameterization differs from GM and VMHS in the selection of the eddy velocity scale, which is based on the kinetic energy balance of baroclinic eddies. The three parameterizations are compared to eddy-resolving computations in a variety of forcing configurations and for several sets of parameters. The VMHS and the energy balance parameterizations perform best in the tests considered here.

Cessi, P.  2007.  Regimes of thermocline scaling: The interaction of wind stress and surface buoyancy. Journal of Physical Oceanography. 37:2009-2021.   10.1175/jpo3103.1   AbstractWebsite

The role of the relative geometry of mechanical forcing (wind stress) and buoyancy forcing (prescribed surface temperature) in the maintenance of the main thermocline is explored. In particular, the role of the wind stress curl in enhancing or suppressing the generation of baroclinic eddies is studied in simplified domains. The dependence of key quantities, such as the depth of the thermocline and the maximum heat transport, on the external parameters such as diapycnal mixing and dissipation rate is examined. Qualitatively different regimes are found depending on the relative phase of the wind stress and surface buoyancy distribution. The most efficient arrangement for eddy generation has Ekman pumping (suction) in conjunction with high (low) surface buoyancy. This corresponds to the situation found in the midlatitudes, where the surface Ekman flow carries heat toward the warmer region (i.e., upgradient of the surface temperature). In this case, strong eddy fluxes are generated in order to counteract the upgradient heat transport by the Ekman cell. The result is a thermocline whose depth is independent of the diapycnal diffusivity. However, the competition between these opposing heat fluxes leads to a weak net heat transport, proportional to the diffusivity responsible for the diabatic forcing. This arrangement of wind stress provides a large source of available potential energy on which eddies can grow, so the mechanical energy balance for the eddies is consistent with a substantial eddy heat flux. When the same surface temperature distribution is paired with the opposite wind stress curl, the mean flow produces a sink, rather than a source, of available potential energy and eddies are suppressed. With this arrangement, typical of low latitudes and the subpolar regions, the Ekman overturning cell carries heat downgradient of the surface temperature. Thus, the net heat transport is almost entirely due to the Ekman flow and is independent of the diapycnal diffusivity. At the same time the thermocline is a thin, diffusive boundary layer. Quantitative scalings for the thermocline depth and the poleward heat transport in these two limiting cases are contrasted and successfully compared with eddy-resolving computations.

Cessi, P, Young WR, Polton JA.  2006.  Control of large-scale heat transport by small-scale mixing. Journal of Physical Oceanography. 36:1877-1894.   10.1175/jpo2947.1   AbstractWebsite

The equilibrium of an idealized flow driven at the surface by wind stress and rapid relaxation to non-uniform buoyancy is analyzed in terms of entropy production, mechanical energy balance, and heat transport. The flow is rapidly rotating, and dissipation is provided by bottom drag. Diabatic forcing is transmitted from the surface by isotropic diffusion of buoyancy. The domain is periodic so that zonal averaging provides a useful decomposition of the flow into mean and eddy components. The statistical equilibrium is characterized by quantities such as the lateral buoyancy flux and the thermocline depth; here, scaling laws are proposed for these quantities in terms of the external parameters. The scaling theory predicts relations between heat transport, thermocline depth, bottom drag, and diapycnal diffusivity, which are confirmed by numerical simulations. The authors find that the depth of the thermocline is independent of the diapycnal mixing to leading order, but depends on the bottom drag. This dependence arises because the mean stratification is due to a balance between the large-scale wind-driven heat transport and the heat transport due to baroclinic eddies. The eddies equilibrate at an amplitude that depends to leading order on the bottom drag. The net poleward heat transport is a residual between the mean and eddy heat transports. The size of this residual is determined by the details of the diapycnal diffusivity. If the diffusivity is uniform (as in laboratory experiments) then the heat transport is linearly proportional to the diffusivity. If a mixed layer is incorporated by greatly increasing the diffusivity in a thin surface layer then the net heat transport is dominated by the model mixed layer.

Gallego, B, Cessi P, McWilliams JC.  2004.  The Antarctic Circumpolar Current in equilibrium. Journal of Physical Oceanography. 34:1571-1587.   10.1175/1520-0485(2004)034<1571:taccie>;2   AbstractWebsite

A simple channel-flow model is used to examine the equilibrium upper-ocean dynamics and thermodynamics of the Antarctic Circumpolar Current (ACC). The model consists of two zonally averaged, variable-temperature layers-a surface boundary layer and a thermocline layer-separated by a turbulent interface. Weak air-sea heat flux, determined by relaxation to a prescribed atmospheric temperature, determines the leading-order temperature structure in the oceanic surface layer. The equilibrium thermal structure in the interior is mostly determined by a dominant balance between the meridional transport due to the wind-driven Eulerian mean circulation and the heat flux due to the baroclinic eddies. The resulting latitudinal temperature gradient depends on both the wind and the atmospheric temperature forcing and sustains the geostrophic zonal flow. Consideration of the next-order balance for the oceanic surface temperature results in an air-sea heat flux proportional to the magnitude of the residual flow. The residual meridional circulation ( Eulerian mean plus eddy-induced) is necessary to balance small diabatic sources and sinks of heat. Therefore, it depends on the processes of vertical diffusion, boundary layer entrainment/detrainment, and, on the polar flank, convection. In the absence of substantial lateral diffusion, the leading-order balance of weak residual circulation implies a very weak meridional heat transport across the ACC and a correspondingly weak differential heat exchange to the atmosphere. This limitation can be eased if the lateral diffusive flux of temperature in the surface layer becomes as large as the adiabatic eddy transport.

Cessi, P, Bryan K, Zhang R.  2004.  Global seiching of thermocline waters between the Atlantic and the Indian-Pacific Ocean Basins. Geophysical Research Letters. 31   10.1029/2003gl019091   AbstractWebsite

Proxy climate data from the Greenland icecap and marine deposits in the Pacific indicate that warm conditions in the North Atlantic are linked to cool conditions in the Eastern Equatorial Pacific, and vice versa. Our ocean models show that the surface branch of the overturning circulation connecting the North Atlantic to the Equatorial Pacific adjusts by exchanging thermocline water between ocean basins in response to changes in deep water formation in the northern North Atlantic. Planetary ocean waves give rise to a global oceanic seiche, such that the volume of thermocline water decreases in the Pacific-Indian Ocean while increasing in the Atlantic Ocean. We conjecture that the remotely forced changes in the thermocline of the Eastern Equatorial Pacific may trigger El Nino events. These global seiches have been previously overlooked due to the difficulty of integrating high-resolution climate models for very long time-scales.

Cessi, P, Fantini M.  2004.  The eddy-driven thermocline. Journal of Physical Oceanography. 34:2642-2658.   10.1175/jpo2657.1   AbstractWebsite

The role of baroclinic eddies in transferring thermal gradients laterally, and thus determining the stratification of the ocean, is examined. The hypothesis is that the density differences imposed at the surface by differential heating are a source of available potential energy that can be partially released by mesocale eddies with horizontal scales on the order of 100 km. Eddy fluxes balance the diapycnal mixing of heat and thus determine the vertical scale of penetration of horizontal thermal gradients (i.e., the depth of the thermocline). This conjecture is in contrast with the current thinking that the deep stratification is determined by a balance between diapycnal mixing and the large-scale thermohaline circulation. Eddy processes are analyzed in the context of a rapidly rotating primitive equation flow driven by specified surface temperature, with isotropic diffusion and viscosity. The barotropic component of the eddies is found to be responsible for most of the heat flux, and so the eddy transport is horizontal rather than isopycnal. This eddy transport takes place in the shallow surface layer where eddies, as well as the mean temperature, undergo diabatic, irreversible mixing. Scaling laws for the depth of the thermocline as a function of the external parameters are proposed. In the classical thermocline theory, the depth of the thermocline depends on the diffusivity, the rotation rate, and the imposed temperature gradient. In this study the authors find an additional dependence on the viscosity and on the domain width.

Spydell, M, Cessi P.  2003.  Baroclinic modes in a two-layer basin. Journal of Physical Oceanography. 33:610-622.   10.1175/1520-0485(2003)033<0610:bmiatl>;2   AbstractWebsite

The objective of this study is to investigate the time-dependent circulation in a closed basin where the steady circulation is included and long Rossby wave speeds are consistent with observations. Specifically, the large-scale baroclinic eigenmodes of a two-layer rectangular basin forced by surface wind stress in the limit of small dissipation are examined. Low-frequency modes with small decay rates independent of friction result when the constraint of mass conservation is enforced. The magnitude of the wind stress is found to be critical to the eigenspectrum. For all forcing magnitudes, including forcings with closed geostrophic contours, oscillatory modes with decay rates independent of friction emerge. For forcings with closed geostrophic contours, two important classes of eigenmodes with comparable decay rates emerge: purely decaying modes confined to the region of closed contours, and basin-scale oscillatory modes. The purely decaying modes also exist without the constraint of total mass conservation but their decay rates depend on the magnitude of friction to leading order.

Ferrari, R, Cessi P.  2003.  Seasonal synchronization in a chaotic ocean-atmosphere model. Journal of Climate. 16:875-881.   10.1175/1520-0442(2003)016<0875:ssiaco>;2   AbstractWebsite

The signatures of feedback between the atmosphere and the ocean are studied with a simple coupled model. The atmospheric component, based on Lorenz's 1984 model is chaotic and has intrinsic variability at all timescales. The oceanic component models the wind-driven circulation, and has intrinsic variability only in the decadal band. The phase of the cospectrum of atmospheric and oceanic temperatures is examined and it is found that in the decadal band, the oceanic signal leads the atmospheric one, while the opposite is true at shorter and longer timescales. The associated atmosphere-only model, driven by the oceanic temperature derived from a coupled run, synchronizes to the coupled run for arbitrary initial conditions. When noise is introduced in the time series of oceanic driving, episodic synchronization still occurs, but only in summer, indicating that control of the atmosphere by the oceanic variables is prevalent in this season.

Cessi, P, Otheguy P.  2003.  Oceanic teleconnections: Remote response to decadal wind forcing. Journal of Physical Oceanography. 33:1604-1617.   10.1175/2400.1   AbstractWebsite

The transhemispheric and interbasin response to time-dependent wind forcing localized in the Northern Hemisphere of a single basin is examined using the reduced-gravity shallow- water equations in domains of simple geometry. On decadal timescales, the pressure on the eastern boundary fluctuates synchronously in both hemispheres and thus communicates a signal to latitudes distant from the forcing. The signal then penetrates into the interior through westward radiation of Rossby waves. Associated with the eastern boundary pressure fluctuation is a time-dependent mass flux across the equator that, in a single basin, is balanced by a storage of mass in the unforced hemisphere. Two oceanic basins connected by a reentrant channel at the high-latitude edge of the Southern Hemisphere are then considered. Again the forcing is confined to the Northern Hemisphere of one basin only. In this geometry the time-dependent mass flux across the equator of the forced basin is not entirely balanced within the same basin, but induces a mass flux into the unforced basin, while the mass heaving within the periodic channel is negligible. This process is illustrated by considering winds oscillating at a period on the same order as the Rossby wave transit time in high latitudes. The interhemispheric and interbasin teleconnection is achieved by a combination of long Rossby waves and large-scale, low-frequency gravity waves forced by the Rossby signal. These disturbances share no characteristics of Kelvin waves; that is, they are not boundary trapped.

Cessi, P, Paparella F.  2001.  Excitation of basin modes by ocean-atmosphere coupling. Geophysical Research Letters. 28:2437-2440.   10.1029/2000gl012660   AbstractWebsite

A conceptual model of the coupling between the upper-ocean wind-driven circulation and the mid-latitude atmospheric wind-stress illustrates that large-scale basin-wide oscillations with decadal period can be excited. These oceanic modes are also found in the absence of ocean-atmosphere feedback, but they are damped. The period of the oscillation and the spatial structure of the modes are essentially the same with and without coupling. These oscillations are distinct from the coupled modes of variability arising from a delayed negative feedback between the wind-driven flow and the wind-stress. They are ocean-only linear basin modes which become sustained by ocean-atmosphere coupling.

Primeau, F, Cessi P.  2001.  Coupling between wind-driven currents and midlatitude storm tracks. Journal of Climate. 14:1243-1261.   10.1175/1520-0442(2001)014<1243:cbwdca>;2   AbstractWebsite

A model for the interaction between the midlatitude ocean gyres and the wind stress is formulated for a shallow-water, spherical hemisphere with finite thermocline displacement and the latitudinal dependence of the long Rossby wave speed. The oceanic currents create a temperature front at the midlatitude intergyre boundary that is strongest near the western part of the basin. The intergyre temperature front affects the atmospheric temperature gradient in the storm track region, increasing the eddy transport of heat and the surface westerlies. The delayed adjustment of the gyres to the wind stress causes the westerly maximum to migrate periodically in time with a decadal period. The behavior of the model in a spherical geometry is qualitatively similar to that in a quasigeostrophic setting except that here the coupled oscillation involves oceanic temperature anomalies that circulate around the subpolar gyre, whereas the quasigeostrophic calculations favor the subtropical gyre. Another difference is that here there is a linear relationship between the period of the coupled oscillation and the delay time for the adjustment of ocean gyres to changes in the wind stress. This result departs from the quasigeostrophic result, in which the advection timescale also influences the period of the decadal oscillation.

Cessi, P, Louazel S.  2001.  Decadal oceanic response to stochastic wind forcing. Journal of Physical Oceanography. 31:3020-3029.   10.1175/1520-0485(2001)031<3020:dortsw>;2   AbstractWebsite

The low-frequency linear eigenmodes of the reduced-gravity shallow-water equations with weak friction are calculated numerically and using an analytic approximation. For basins with a large variation of the Coriolis parameter, large-scale eigenmodes emerge: the eigenfrequencies are integer multiples of the frequency for the gravest mode, which, in turn, has a period given by the transit time of the slowest long Rossby wave. The e-folding decay times are comparable to the period and independent of friction. These eigenmodes are excited by stochastic wind forcing and this leads to a weak peak in the spectral response near the frequency of the least-damped eigenmode. This decadal-frequency peak is most evident on the eastern and western boundaries and in the equatorial region of the basin.