Uniform Potential Vorticity Research Articles

Geostrophic adjustment and frontogenesis in a continuously stratified fluid

The structure of geostrophic flows that are formed from smooth initial unbalanced data with zero or uniform potential vorticity has been investigated in the work. Using methods of the theory of complex-variable functions, a class of exact analytical solutions describing these flows has been constructed. It has been shown that if the amplitude of a smooth initial disturbance exceeds the critical one, the final state will contain discontinuities of frontal type. A closed system of equations for discontinuity surfaces determination in the final states has been formulated. This system includes the Margules formula as a condition at the jump. It has been established that geometric configurations of discontinuity surfaces have an universal character, i.e. they do not depend on the details of initial distributions. To describe the nonstationary wave-processes of adjustment, a numerical hydrostatic model and a simplified analytical model based on the linearized dynamic equations in the Lagrange variables have been proposed. It has been shown that the establishment of discontinuious geostrophic flows has a clearly defined wave character with the alternations of smooth and multivalued phases. At the initial rather short interval, these phases are repeated with the inertial period, i.e. a nonstationary oscillatory front is formed.

Stability of vortices in a two-layer ocean with uniform potential vorticity in the lower layer

It is well-known that oceanic vortices exist for years, whereas almost all theoretical studies indicate that they must be unstable. A rare exception is the work by Dewar & Killworth (1995), who demonstrated that a Gaussian vortex in the upper layer of a two-layer ocean becomes stable if accompanied by a weak co-rotating circulation in the lower layer. Note that this paper assumed the lower-layer circulation to have the same profile as the main (upper-layer) vortex. The present paper considers the case where the profile of the circulation in the lower layer is determined by the condition that potential vorticity (PV) there is constant – which models a vortex surrounded by water of a different origin. Given that most oceanic vortices are shed by frontal currents, such model appears to be more realistic than any ad hoc choice. The stability of vortices with uniform lower-layer PV is examined for both quasigeostrophic and ageostrophic cases, numerically and asymptotically, assuming that the upper layer of the ocean is thin. It is shown that such vortices are stable for a wide range of parameters. The effect of vortex stabilization is interpreted through representation of the unstable disturbance by two phase-locked Rossby waves, rotating around the vortex in the upper and lower layers. Then, if the lower-layer PV gradient is zero, it cannot support the corresponding wave, which inhibits the instability.

Counter-Rotating Quasigeostrophic Ellipsoidal Vortex Pair

In geophysical flows, coherent vortex structures persist for long time and their interactions dominate the dynamics of geophysical turbulence. Miyazaki et al. derived a Hamiltonian dynamical system describing the interactions of N ellipsoidal vortices, where each coherent vortex was modeled by an ellipsoid of uniform potential vorticity embedded in a locally uniform 3D shear field. In this paper, the interaction of counter-rotating vortex pair (dipole) is investigated. According to the ellipsoidal moment model, the inclination from the vertical axis approaches π/2 and one of the principal axis of the ellipsoid grows exponentially with time, if two slender vortices are placed within a critical distance initially. Direct numerical simulations, based on the Contour Advective Semi-Lagrangian algorithm, are performed in order to assess the validity of the model. The CASL computaion tells that the vortices keep the dipole structure quite robustly. They survive from tilting down by emittimg thin filaments from their top and bottom, even if they are placed very close.

The motion of a fluid ellipsoid in a general linear background flow

The study of the motion of a fluid ellipsoid has a long and fascinating history stretching back originally to Laplace in the late 18th century. Recently, this subject has been revived in the context of geophysical fluid dynamics, where it has been shown that an ellipsoid of uniform potential vorticity remains an ellipsoid in a background flow consisting of horizontal strain, vertical shear, and uniform rotation. The object of the present work is to present a simple, appealing, and practical way of investigating the motion of an ellipsoid not just in geophysical fluid dynamics but in general. The main result is that the motion of an ellipsoid may be reduced to the evolution of a symmetric, 3×3 matrix, under the action of an arbitrary 3×3 ‘flow’ matrix. The latter involves both the background flow, which must be linear in the Cartesian coordinates at the surface of the ellipsoid, and the self-induced flow, which was given by Laplace.The resulting simple dynamical system lends itself ideally to both numerical and analytical study. We illustrate a few examples and then present a theory for the evolution of a vortex within a slowly varying background flow. We show that a vortex may evolve quasi-adiabatically, that is, it stays close to an equilibrium form associated with the instantaneous background flow. The departure from equilibrium, on the other hand, is proportional to the rate of change of the background flow.

The shape of vortices in quasi-geostrophic turbulence

The present work discusses the most commonly occurring shape of the coherent vortical structures in rapidly rotating stably stratified turbulence, under the quasi-geostrophic approximation. In decaying turbulence, these vortices – coherent regions of the materially-invariant potential vorticity – dominate the flow evolution, and indeed the flow evolution is governed by their interactions. An analysis of several exceptionally high-resolution simulations of quasi-geostrophic turbulence is performed. The results indicate that the population of vortices exhibits a mean height-to-width aspect ratio less than unity, in fact close to 0.8.This finding is justified here by a simple model, in which vortices are taken to be ellipsoids of uniform potential vorticity. The model focuses on steady ellipsoids within a uniform background strain flow. This background flow approximates the effects of surrounding vortices in a turbulent flow on a given vortex. It is argued that the vortices which are able to withstand the highest levels of strain are those most likely to be found in the actual turbulent flow. Our calculations confirm that the optimal height-to-width aspect ratio is close to 0.8 for a wide range of background straining flows.

Evolution of a synoptic-scale vortex advecting toward a high mountain

A model-based study is undertaken of a synoptic-scale vortex’s encounter with Greenland-scale topography. The ambient setting is a barotropic westerly flow of uniform potential vorticity. This flow is both incident upon and induces an anticyclone over the topography, and it also serves to advect the vortex toward the mountain. In a simulated evolution of the flow the vortex circumnavigates poleward around the mountain but concomitantly undergoes significant dissolution on the windward side followed by a substantial reconstitution to the lee. It eventually advects away downstream as a compact vortex of reduced amplitude. This sequence of events is interpreted in terms of the combined influence of the orographic anticyclone and the self-dynamics of the vortex.

Dynamic Structure of the Kuroshio South of Kyushu in Relation to the Kuroshio Path Variations

The variation of velocity and potential vorticity (PV) of the Kuroshio at the PN line in the East China Sea and the TK line across the Tokara Strait were examined in relation to the path variations of the Kuroshio in the southern region of Japan, using quarterly data from a conductivity-temperature-depth profiler and a shipboard acoustic Doppler current profiler during 1987–97. At the PN line the Kuroshio has a single stable current core located over the continental slope and a significant maximum of PV located just onshore of the current axis in the middle part of the main pycnocline. On the other hand, the Kuroshio at the TK line has double current cores over the two gaps in the Tokara Strait; the northern core has a much larger velocity than the southern core on average during periods of the large meander of the Kuroshio, while the difference in strength between the double cores is small during the non-large-meander (NLM) period. At the TK line, PV in the middle pycnocline is variable; it is small and nearly uniform throughout the section for 40% of the total observations, while it has a significant maximum near the northern core for 30% and two maxima corresponding to the double current cores for 23%. The small, nearly uniform PV occurs predominantly during the NLM period, and is closely related to the generation of the small meander of the Kuroshio southeast of Kyushu.

Numerical Validation of Quasigeostrophic Ellipsoidal Vortex Model

In geophysical flows, coherent vortex structures persist for long time and their interactions dominate the dynamics of geophysical turbulence. Meacham et al. obtained a series of exact unsteady solution of the quasigeostrophic equation, which represents a uniform ellipsoidal vortex patch embedded in a uniform 3D shear field. Miyazaki et al. derived a Hamiltonian dynamical system describing the interactions of N ellipsoidal vortices, where each coherent vortex was modeled by an ellipsoid of uniform potential vorticity. In this paper, direct numerical simulations based on a Contour Advective Semi-Lagrangian algorithm (CASL) are performed in order to assess the validity of the Hamiltonian model. First, the instability of a tilted spheroid is investigated. A prolate spheroid becomes unstable against the third Legendre mode when the aspect ratio is less than 0.44 and the inclination angle is larger than 0.48. Weakly unstable flatter spheroidal vortices emit thin filaments from their top and bottom, whereas str...

The Reddy maker

A new mechanism for the formation of high-amplitude anticyclonic eddies (lenses) from outflows emptying into the ocean at mid-depth is proposed. The essence of the new mechanism is that, in order for an inviscid outflow to exist as a continuous (uninterrupted) current, the condition g′ S/ f> α( g′ H) 1/2 [where g′ is the “reduced gravity”, S the bottom slope, f the Coriolis parameter, α a coefficient of order unity whose value depends on the outflow's potential vorticity (it is 2 for a zero potential vorticity outflow and unity for a uniform potential vorticity) and H the maximum thickness] must hold. When the above condition is not met, i.e., when g′ S/ f< α( g′ H) 1/2, the outflow can only exist as a chain of propagating lenses. Nonlinear analytical considerations leading to the above conclusion are (successfully) compared to numerical simulations which we have conducted (using a reduced gravity layer-and-a-half model). The experiments show that an outflow situated on a bottom whose (uniform) slope gradually varies in the downstream direction is continuous (i.e., is not broken into eddies) where the slope is supercritical [ g′ S/ f> α( g′ H) 1/2] and discontinuous (i.e., constitutes a chain of eddies) where the slope is subcritical [ g′ S/ f< α( g′ H) 1/2]. Hence, the eddies are generated by the gradual reduction in the bottom slope rather than by an instability process. Namely, the eddies are not formed by the breakdown of a known steady solution because such a steady solution does not exist. We note that after reaching its “balanced depth”, an outflow usually continues to (slowly) descend toward the bottom of the ocean due to frictional effects associated with an energy loss. [Note that the “balanced depth” is the depth at which the outflow has completed its initial adjustment in the sense that it has adjusted to a state where it no longer flows primarily offshore but rather propagates primarily along the isobaths. This depth needs to be distinguished from the (sometimes significantly greater) equilibrium depth corresponding to the point where the outflow's density equals the environmental density.] Most of the time, the outflow descent is accompanied by a reduction in the bottom slope S, and an entrainment which causes both a reduction in g′ and an increase in H. All of these alterations bring the outflow closer and closer to the critical condition and it is, therefore, argued that all outflows ultimately reach the critical point (unless diffusion and mixing destroy them prior to that stage). It is suggested that Reddies (i.e., isolated lenses containing Red Sea water) are formed by the above processes. Namely, we propose that the “Reddy maker” is a combination of three processes, the natural reduction in the bottom slope which the outflow senses as it approaches the bottom of the ocean, the entrainment-induced increase in the outflow's thickness, and the entrainment-induced decrease in the outflow's density. An animation of the eddy generation process can be viewed at http://doronnof.net/features.html#video (click on “Reddy maker video”).

An explicit potential-vorticity-conserving approach to modelling nonlinear internal gravity waves

This paper discusses a potential-vorticity-conserving approach to modelling nonlinear internal gravity waves in a rotating Boussinesq fluid. The focus of the work is on the pseudo-plane motion (motion in the x, z-plane), for which we present a broad range of numerical results. In this case there are two material coordinates, the density and the y-component of the velocity in the inertial frame of reference, which are related to the x and z displacements of fluid particles relative to a reference configuration. The amount of potential vorticity within a fluid region bounded by isosurfaces of these material coordinates is proportional to the area within this region, and is therefore conserved as well. Two new potentials, defined in terms of the displacements and combining the vorticity and density fields, are introduced as new dependent variables. These potentials entirely govern the dynamics of internal gravity waves for the linearized system when the basic state has uniform potential vorticity. The final system of equations consists of three prognostic equations (for the potential vorticity and the Laplacians of the two potentials) and one diagnostic equation, of Monge–Ampère type, for a third potential. This diagnostic equation arises from the nonlinear definition of potential vorticity. The ellipticity of the Monge–Ampère equation implies both inertial and static stability. In three dimensions, the three potentials form a vector, whose (three-dimensional) Laplacian is equal to the vorticity plus the gradient of the perturbation density.Numerical simulations are carried out using a novel algorithm which directly evolves the potential vorticity, in a Lagrangian manner (following fluid particles), without diffusion. We present results which emphasize the way in which potential vorticity anomalies modify the characteristics of internal gravity waves, e.g. the propagation of internal wave packets, including reflection, refraction, and amplification. We also show how potential vorticity anomalies may generate internal gravity waves, along with the subsequent ‘geostrophic adjustment’ of the flow to a ‘balanced’ wave-less state. These examples, and the straightforward extension of the theoretical and numerical approach to three dimensions, point to a direct and accurate means to elucidate the role of potential vorticity in internal gravity wave interactions. As such, this approach may help a better understanding of the observed characteristics of internal gravity waves in the oceans.

Equatorial Pacific Subsurface Countercurrents: A Model–Data Comparison in Stream Coordinates*

Abstract An isopycnal stream-coordinate analysis of velocity, transport, and potential vorticity (PV), recently applied to observations of the subsurface countercurrents (SCCs) in the equatorial Pacific Ocean, is applied here to the SCCs in a numerical general ocean circulation model, run by the Japan Marine Science and Technology Center (JAMSTEC). Each observed SCC core separates regions of nearly uniform potential vorticity: low on the equatorward side, high on the poleward side. Similar low-PV pools are found in the model, but the high-PV region poleward of the southern SCC is missing. The potential vorticity gradient in each core is weaker in the model than in observations, and relative vorticity plays only a minor role in the model. Its unusually high vertical resolution, with 55 levels, together with its weak lateral dissipation may be key factors in the JAMSTEC model's ability to simulate SCCs.

The Unusual Dynamics of Northern Dark Spots on Neptune

Hubble Space Telescope (HST) and ground-based observations of Neptune from 1991 to 2000 show that Neptune's northern Great Dark Spots (NGDS) remained remarkably stable in latitude and longitudinal drift rate, in marked contrast to the 1989 southern Great Dark Spot (GDS), which moved continuously equatorward during 1989 and dissipated unseen during 1990. NGDS-32, discovered in October 1994 HST images, (H. B. Hammel et al., 1995, Science 268, 1740–1742), stayed at ∼32°N from 1994 through at least 1996, and possibly through 2000. The second northern GDS (NGDS-15), discovered in August 1996 HST images, (L. A. Sromovsky et al. 2001, Icarus 146, 459–488), appears to have existed as early as 8 March 1996 and remained near 15°N for the 16 months over which it was observed. NGDS-32 had a very uniform longitudinal drift rate averaging −36.28±0.04°/day from 10 October 1994 to 2 November 1995, and −35.84±0.02°/day from 1 September 1995 through 24 November 1995. A single circulation feature certainly exists during each of the first two periods, though it is not certain that it is the same feature. It is probable, but less certain, that only a single circulation feature was tracked during the 1996–1998 period, during which positions are consistent with a modulated drift rate averaging −35.401±0.001°/day, but with a peak-to-peak modulation of 1.5°/day with an ∼760-day period. If NDS-32 varied its drift rate in accord with the local latitudinal shear in the zonal wind, then all its drift-rate changes might be due to only ∼0.4° of latitudinal motion. The movement of NGDS-15 is also not consistent with a uniform longitudinal drift rate, but the nature of its variation cannot be estimated from the limited set of observations. The relatively stable latitudinal positions of both northern dark spots are not consistent with current numerical model calculations treating them as anticyclonic vortices in a region of uniform potential vorticity gradient (R. P. Lebeau and T. E. Dowling 1998, Icarus 132, 239–265). Possible explanations include unresolved latitudinal structure in the zonal wind background or unaccounted-for variations in vertical stability structure.

Quasigeostrophic Ellipsoidal Vortex Model

An ellipsoidal vortex model is developed in order to investigate the dynamics of vortex interactions in a quasigeostrophic turbulence. Each coherent vortex is modeled by an ellipsoid of uniform potential vorticity. It is assumed that each vortex remains to be an ellipsoid, although its principal axes and inclination angles change under the influence of the strain field induced by other vortices. The dynamics of ellipsoidal vortices are shown to be a Hamiltonian system of finite degree of freedom extracted from the partial differential equations that describe quasigeostrophic vortex dynamics. The center of vorticity and the angular momentum are conserved, besides the total energy and Casimirs of the system, such as the vortex height and the vortex volume. Chaotic motions are observed even in a two-body system.

Mechanism of balanced flow and frontogenesis

The final balanced state of an initial unbalanced flow is discussed with the same method as Vallis (1992). For the two-dimensional, inviscid, rotating and nonlinear model, the final state of the flow depends on the initial conditions. If the initial potential vortcity of the flow is non-uniform, the final state is not necessarily geostrophic. However, for the zero and uniform potential vorticity flow, the final state will satisfy the thermal wind relation when the length scale of the initial disturbance is large enough. Otherwise, discontinuity will occur in the geostrophic solution. In this case, the final balanced state will not be geostrophic any longer and an extended momentum coordinate is introduced to overcome the mult-value problem.

Eddy Mixing of Potential Vorticity versus Thickness in an Isopycnic Ocean Model

Parameterizations of the eddy-induced velocity that advects tracers in addition to the Eulerian mean flow are traditionally expressed as a downgradient Fickian diffusion of either isopycnal layer thickness or large-scale potential vorticity (PV). There is an ongoing debate on which of the two closures is better and how the spatial dependence of the eddy diffusivity should look like. To increase the physical reasoning on which these closures are based, the authors present a systematic assessment of eddy fluxes of thickness and PV and their relation to mean-flow gradients in an isopycnic eddy-resolving model of an idealized double-gyre circulation in a flat bottom, closed basin. The simulated flow features strong nonlinearities, such as tight inertial recirculations, a meandering midlatitude jet, pools of homogenized PV, and regions of weak flow where β/h dominates the PV gradient. It is found that the zonally averaged eddy flux of thickness scales better with the zonally averaged meridional thickness gradient than the eddy flux of PV with the PV gradient. The reason for this is that the two-scale approximation, which is often invoked to derive a balance between the downgradient eddy flux of PV and enstrophy dissipation, does not hold. It is obscured by advection of perturbation enstrophy, which is multisigned and weakly related to mean-flow gradients. On the other hand, forcing by vertical motions, which enters the balance between the downgradient eddy flux of thickness and dissipation in most cases, acts to dissipate thickness variance. It is dominated by the conversion from potential to kinetic energy and the subsequent downgradient transport of thickness. Also, advection of perturbation thickness variance tends to be more single-signed than advection of perturbation enstrophy, forcing the eddy flux of thickness to be more often down the mean gradient. As a result, in the present configuration a downgradient diffusive closure for thickness seems more appropriate to simulate the divergent eddy fluxes than a downgradient diffusive closure for PV, especially in dynamically active regions where the eddy fluxes are large and in regions of nearly uniform PV.

Flow structure and seasonalityin the Hebridean slope current

The intensity, structure and variability of the slope current have been determined from 16 months of observations with Acoustic Doppler Current Profilers (ADCP) and conventional current meters on a cross-slope section at the Hebridean shelf edge during the Shelf Edge Study (SES) programme. After removal of the tidal signals, the mean flow over the upper slope is found to be closely parallel to the topography with speeds of ≈ 20 cm·s –1. The flow extends down to a depth of 500 m and is predominantly barotropic, especially in winter when the flow is practically uniform between 350 m and the surface. In summer, there is a significant baroclinic component with a pronounced maximum in current at a depth of about 200 m but more than 80% of the kinetic energy is in the barotropic component. Flow in the core of the current is highly persistent with the Neumann’s steadiness St > 0.8 in summer. In winter the flow is generally more energetic and variable and extends onto the adjacent shelf. The cross-slope profile of sea surface elevation, computed from the mean barotropic currents, shows a consistent relation to seabed topography through the seasonal cycle. Long-term averages of the cross-slope components are generally small (≈ 2 cm·s –1) with some indication of persistent down-slope flow in the bottom Ekman layer. Measurements with shipboard ADCP on sections at intervals along the slope show a high degree of continuity in the structure of the flow. The core of the flow appears to be related to a weak positive salinity anomaly and a depression of the 9.5 °C isotherm near the shelf, but there is no strong correlation between the core of the slope-current and the core of the salinity anomaly. It is proposed that this may be due to differences in the cross-stream diffusion of salt and momentum which have different boundary conditions at the slope. The observed cross-stream structure of the current supports the hypothesis that JEBAR is the principal forcing mechanism but the result cannot be regarded as conclusive since a uniform potential vorticity model of the flow produces a similar cross-sectional structure of the current.

Stationary Wave Accumulation and the Generation of Low-Frequency Variability on Zonally Varying Flows

Abstract A potent mechanism for the generation of low-frequency atmospheric variability on vortex basic states consisting of a single potential vorticity jump, or contour, separating two regions of uniform equivalent barotropic potential vorticity is described. Such basic states represent in a simple manner the potential vorticity distribution of the extratropical upper troposphere. It is shown that the group velocity for stationary waves propagating on such states can vanish for realistic zonal variations in the basic-state flow along the vortex edge, leading to local exponential disturbance growth due to the accumulation of wave action. Further, pseudo-energy stability criteria are derived that suggest that exponentially growing global disturbances are possible for sufficiently strong zonal variations in the flow along the vortex edge. These predictions are examined using linear and nonlinear initial value problem calculations. For wavenumber-1 flow variations in the basic-state zonal flow along the vor...

Hydraulic adjustment to an obstacle in a rotating channel

In order to gain insight into the hydraulics of rotating-channel flow, a set of initial-value problems analogous to Long's towing experiments is considered. Specifically, we calculate the adjustment caused by the introduction of a stationary obstacle into a steady, single-layer flow in a rotating channel of infinite length. Using the semigeostrophic approximation and the assumption of uniform potential vorticity, we predict the critical obstacle height above which upstream influence occurs. This height is a function of the initial Froude number, the ratio of the channel width to an appropriately defined Rossby radius of deformation, and a third parameter governing how the initial volume flux in sidewall boundary layers is partitioned. (In all cases, the latter is held to a fixed value specifying zero flow in the right-hand (facing downstream) boundary layer.) The temporal development of the flow according to the full, two-dimensional shallow water equations is calculated numerically, revealing numerous interesting features such as upstream-propagating shocks and separated rarefying intrusions, downstream hydraulic jumps in both depth and stream width, flow separation, and two types of recirculations. The semigeostrophic prediction of the critical obstacle height proves accurate for relatively narrow channels and moderately accurate for wide channels. Significantly, we find that contact with the left-hand wall (facing downstream) is crucial to most of the interesting and important features. For example, no instances are found of hydraulic control of flow that is separated from the left-hand wall at the sill, despite the fact that such states have been predicted by previous semigeostrophic theories. The calculations result in a series of regime diagrams that should be very helpful for investigators who wish to gain insight into rotating, hydraulically driven flow.

Quasigeostrophic Ellipsoidal Vortices in a Two-Dimensional Strain Field

Vortex motions in a stably stratified rotating fluid are considered theoretically, based on the quasigeostrophic approximation. A class of exact staionary solution is obtained, which represents an ellipsoidal volume with uniform potential vorticity Q 0 embedded in a two-dimensional uniform strain field \({\hat e}\) with uniform background vorticity \(2{\hat \gamma}\). In a pure strain field (\({\hat \gamma}=0\)), stationary solutions are allowed for \(|{\hat e}/Q_0| \) about -0.1 in a simple shear flow (\({\hat \gamma}={\hat e}\)). We study the stability of these exact solutions against infinitesimal disturbances. In a pure strain field, highly elongated ellipsoids are shown to be unstable to Lame-modes whose order m is higher than 2. In a simple shear flow, a highly elongated ellipsoid whose major axis is perpendicular to the flow direction, is unstable, whereas any ellipsoidal vortex seems to be stable, if the major axis is pa...

Interior Pycnocline Flow from the Subtropical to the Equatorial Pacific Ocean*

Interior circulation pathways from the subtropics to the equator are markedly different in the Northern and Southern Hemispheres of the Pacific Ocean. In the North Pacific the pycnocline shoals and strengthens dramatically under the intertropical convergence zone, separating the North Equatorial Current from the North Equatorial Countercurrent. While the high potential vorticity between these currents would intuitively seem to inhibit meridional water-property exchange between the subtopics and the equator, transient tracer analyses and some modeling studies have suggested an interior pathway from the subtropics to the equator in the pycnocline of the central North Pacific. This study delineates this pathway and estimates an upper bound for its magnitude at 5 (±1) × 109 kg s−1. In contrast, the southern branch of the South Equatorial Current clearly brings pycnocline water estimated at 15 (±1) × 109 kg s−1 from the southern subtropics directly to the equator in the South Pacific through an interior region of low and relatively uniform potential vorticity. In both hemispheres, these interior pathways extend downward as far as the lightest waters of the equatorial pycnostad. The subsurface countercurrents flanking the pycnostad form the equatorward limbs of tropical subsurface cyclonic gyres. These deep gyres are consistent with the absence of interior ventilation of the equator from the subtropics below the pycnocline. Measured and derived fields from a hydrographic climatology are presented on neutral surfaces and in meridional–vertical sections to show that salinity, potential vorticity, and acceleration potential are all consonant with these arguments. The vertical extent of interior communication is also in agreement with the transient tracer results.