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Richard Schopp

Abstract

An interpretation in terms of planetary waves is proposed, which sheds light on the dynamics underlying the large-scale cross-gyre geostrophic flow recently developed in a two-layer ventilated thermocline model.

The cross-gyre communication flow is the result of an arrested nondispersive baroclinic Rossby wave in the presence of zonal Sverdrup transport along the line of vanishing Ekman pumping. A baroclinic adjustment is described in which a resting ocean settles to a steady communicating solution.

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Richard Schopp

Abstract

A simple Sverdrup-type two-layer model that allows the outcropping of isopycnals is forced by wind stress, is completed with a frictional western boundary layer, and is investigated along the zero wind-stress curl line separating the subpolar gyre from the subtropical gyre. The study focuses on the different cross-gyre flow patterns. Intermediate length-scale dynamics, which is able to take the dispersion of Rossby waves and the steepening of isopycnals into account, is used to analyze the evolution of these cross-gyre currents. In particular, these transients show that the solution, which exhibits an arrested Rossby wave, is unstable in the western part of the basin. Nevertheless, this solution is able to evolve to other more stable solutions present in the dynamics: one in which there is an exchange of water masses between gyres and another one in which both gyres are independent. The first one has a deep (upper) slow northward (southward) flow in midoceanic regions and a strong western deep (upper) southward (northward) boundary current. This current system could well help to account for some of the transport in the western boundary undercurrent observed in the North Atlantic Ocean, and therefore the theory presented could indicate that the undercurrent and cross-gyre flow might have wind-driven components. The second stable solution, in which exchange is not allowed, would be rather representative of the North Pacific Ocean.

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Richard Schopp
and
Michel Arhan

Abstract

A mechanism is proposed, based on the assumption of ventilation, to explain the middepth northward flow observed in the North Atlantic. The main feature of the solution is that the outcropping line of an intermediate layer at high latitudes is uniquely specified by the condition that the middepth waters of the subtropical gyre may be “sucked up” by the positive Ekman pumping of the subpolar gyre.

This model is consistent with several characteristic features of the circulation in that region such as its vertical structure, the greater northward extension of the warm waters in the eastern ocean and the shape of the large scale Mediterranean water plume. In a variation of the model, the observed isopycnal slopes along the eastern coast are included in the analysis; this implies the existence of eastern boundary layers.

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Peter B. Rhines
and
Richard Schopp

Abstract

Simulations of the wind-driven Ocean circulation, carded out with an eddy-resolving quasi-geostrophic numerical model, and symmetric, idealized wind forcing have a large-scale structure that is predicted wen by the steady nonlinear theory of Rhines and Young. The sharp jet and inertial recirculation am often confined weft inside the region of closed hyperbolic characteristics, defined by that theory, and hence do not affect the Sverdrup-dynamics part of the gyre. The characteristics make possible simple predictions about the development of the circulation, including time dependence and eddy stirring. By tilting the line of vanishing Ekman pumping away from the east-west orientation (as it is tilted in the North Atlantic, and less so the North Pacific), we explore a family of circulations. As the tilt of the wind held is increased, characteristics originating at the eastern boundary begin to thread through the energetic region occupied by the free jet. Then, extensive new branches of eddy-driven flow occur, reaching poleward into the subpolar gyre.

Lagrangian float trajectories are shown and Lagrangian mean circulation and diffusivity discussed. A Peclet number measuring the relative strengths of advection and eddy mixing of potential vorticity is defined and mapped. Its value, typically 3 to 5, suggests the dominant nature of mesoscale eddy mixing in the ocean. Stirring by mesoscale eddies arises in midocean from baroclinic instability. It leads to a loss of 'memory” of quasi-conserved properties over typically 300 to 1000 km. Eddies are essential to the transport of potential vorticity from subpolar to subtropical regions, across the limiting characteristic, finally determining the structure of the recirculating subtropical gyre.

Permanent tongues of potential vorticity invade the subtropics from the subpolar gyre, entering where the characteristics of the theory form a stagnation point.

The experiments exhibit several features of the observed circulation of the ocean. With increasing tilt of the winds we find: decreasing total energy of the circulation; great decrease in the length of the eastward-flowing free jet; increased concentration of the circulation in the upper ocean where it wore closely resembles the simple Sverdrup transport function, with broad regions of eastward flow., increased production of cutoff rings near the western boundary (rather than just at the eastern end of the jet, as with symmetric winds); shrinkage of the north-south extent of the subtropical gyre at the 300–1000 m level yet increase in its consent extent (so that it reaches 5000 km northeastward, to the eastern boundary); and displacement of the boundary current separation point poleward of the line of vanishing Ekman pumping. The subpolar gyre shrinks in size. The simulations help one to understand the differences between, on the one hand, the North Atlantic Ocean, with its very confined middepth circulation and NE-SW strike of the 1000-m potential vorticity contours, and a relative small region of penetration of the concentrated Gulf Stream jet into the interior, and, on the other hand, the North Pacific, where the subtropical anticyclone penetrates much deeper and the Kuroshio jet penetrates a greater distance eastward.

A review of relevant observations of the North Atlantic is given, particularly to show that the regime of the model is realistic; as one moves toward the “quiet” parts of midocean, the ratio of eddy to mean kinetic energy actually rises, suggesting that eddy mixing cannot be neglected there.

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Bach Lien Hua
,
Frédéric Marin
, and
Richard Schopp

Abstract

A fully three-dimensional primitive equation simulation is performed to “reunite” the local equatorial dynamics of the subsurface countercurrents (SCCs) and thermostad with the large-scale tropical ventilated ocean dynamics. It captures (i) the main characteristics of the equatorial thermostad, the SCCs' location and their eastward evolution, and the potential vorticity budget with its equatorial homogenization to zero values and (ii) the large-scale meridional shoaling of the thermocline equatorward. It supports the idea that the two-dimensional Hadley cell mechanism proposed by Marin et al. is a candidate able to operate in a fully three-dimensional ocean. The main difference between the 2D Hadley cell mechanism and the oceanic 3D case is that for the 3D case the large-scale meridional velocity at zeroth order is geostrophic, while the cell mechanism is a next-order, small-scale mechanism. A detailed budget of the zonal momentum equation is provided for the ageostrophic dynamics at work in the SCCs. The mean meridional advection and the Coriolis term dominate, discounting the possibility that lateral eddies play a major role for the SCCs' creation. A 3½-layer idealized ventilation model, calibrated to the three-dimensional simulation parameters, is able not only to capture the tropical density structure, but also to isolate the main controlling factors leading to the triggering of the equatorial secondary cells with its associated jet and thermostad, namely, the shoaling of the equatorial thermocline because of low potential vorticity injection at distant subduction latitudes. It is also shown that equatorial recirculation gyres play a quantitative role that may be of the same order of magnitude as ventilation from higher latitudes.

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Frédéric Marin
,
Richard Schopp
, and
Bach Lien Hua

Abstract

Sensitivity tests are performed to assess the respective influences of the large-scale ventilation and of the near-equatorial winds on the dynamics of the the subsurface countercurrents (SCCs) and thermostad. They show that the intensity of the inertial jets is a function of the potential vorticity (PV) values at subduction and that stronger jets are favored by low PV injection, forced in the authors' framework either by a deep mixed layer at subduction and/or by an injection of PV at lower latitudes. Such circumstances lead to a strong meridional shoaling of the thermocline near the equator. The resulting inertial jets occur at about 3°N in the western part of the basin and are the poleward limit of a near-0 PV region and of an equatorial thermostad. A necessary condition for the existence of inertial jets is that the equatorial wind fetch is large enough, otherwise only weak time-mean eastward currents are produced by a nonlinear rectification of instability waves farther away from the equator. The presence of a North Equatorial Countercurrent does not constitute a barrier for equatorward motions within the lower thermocline, and inertial jets are still controlled by the meridional slope of the SSCs' layer setup through the establishment of tropical PV pools predicted by ventilation theory.

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Frédéric Marin
,
Bach Lien Hua
, and
Richard Schopp

Abstract

From a numerical simulation of the Atlantic Ocean, Jochum and Malanotte-Rizzoli provide evidence that the equatorial subsurface countercurrents can be triggered by tropical instability waves through eddy–mean flow interactions in a low-Rossby-number regime. Adapting the transformed Eulerian mean formalism to a shoaling jet, they propose eddy heat fluxes to be the driving mechanism for the subsurface countercurrents. Here it is shown that such a formalism relying on the existence of a residual meridional streamfunction cannot be applied to a shoaling jet, so that the eddy heat fluxes term in the zonal momentum equation cannot be rigorously justified. Moreover, the role of the zonal pressure gradient that was dropped in their study needs to be reassessed. Despite this mathematical questioning of Jochum and Malanotte-Rizzoli’s framework, the authors agree with them that eddy heat fluxes may contribute to the dynamics of the subsurface countercurrents.

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Mark D. Fruman
,
Bach Lien Hua
, and
Richard Schopp

Abstract

Depth-dependent barotropic instability of short mixed Rossby–gravity (MRG) waves is proposed as a mechanism for the formation of equatorial zonal jets. High-resolution primitive equation simulations show that a single MRG wave of very short zonal wavelength and small to moderate amplitude is unstable and leads to the development of a largely barotropic, zonally symmetric flow, featuring a westward jet at the equator and extra-equatorial jets alternating in direction with latitude. At higher but still moderate amplitude, westward flow still prevails at the equator at depths of maximum horizontal velocity amplitude in the initial wave, but the long-term equilibrated state can also feature eastward “superrotating” jets at the equator near the depths of zero horizontal velocity in the initial wave. The formation of the superrotating jets in the simulations is found to be sensitive to the inclusion of the nontraditional Coriolis force in the equations of motion. A linear theory is used to demonstrate the existence of exponentially growing horizontally nondivergent perturbations with a dominant zonally symmetric zonal velocity component. An argument for the sense and positioning of the jets relative to the equator is given in terms of inertial instability and the meridional mixing of planetary vorticity by the small zonal-scale components of the linearly unstable modes. In the long time evolution of the flow, if the amplitude of the westward equatorial jet becomes too great, zonally symmetric inertial instability limits the growth of the jets, and inertial adjustment leads to the homogenization of potential vorticity in latitude and depth around the equator.

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Daniele Iudicone
,
Keith B. Rodgers
,
Richard Schopp
, and
Gurvan Madec

Abstract

Antarctic Intermediate Water (AAIW) occupies the intermediate horizon of most of the world oceans. Formed in the Southern Ocean, it is characterized by a relative salinity minimum. With a new, denser in situ National Oceanographic Data Center dataset, the authors have reanalyzed the export characteristics of AAIW from the Southern Ocean into the South Pacific Ocean. These new data show that part of the AAIW is exported from the subpolar frontal region by the large-scale circulation through an exchange window of 10° width situated east of 90°W in the southeast corner of the Pacific basin. This suggests the origin of this water to be in the Antarctic Circumpolar Current. A set of numerical modeling experiments has been used to reproduce these observed features and to demonstrate that the dynamics of the exchange window is controlled by the basin-scale meridional pressure gradient. The exchange of AAIW between the Southern and Pacific Oceans must therefore be understood in the context of the large basin-scale dynamical balance rather than simply local effects.

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Thomas Meunier
,
Claire Ménesguen
,
Richard Schopp
, and
Sylvie Le Gentil

Abstract

The dynamics of the formation of layering surrounding meddy-like vortex lenses is investigated using primitive equation (PE), quasigeostrophic (QG), and tracer advection models. Recent in situ data inside a meddy confirmed the formation of highly density-compensated layers in temperature and salinity at the periphery of the vortex core. Very high-resolution PE modeling of an idealized meddy showed the formation of realistic layers even in the absence of double-diffusive processes. The strong density compensation observed in the PE model, in good agreement with in situ data, suggests that stirring might be a leading process in the generation of layering. Passive tracer experiments confirmed that the vertical variance cascade in the periphery of the vortex core is triggered by the vertical shear of the azimuthal velocity, resulting in the generation of thin layers. The time evolution of this process down to scales of O(10) m is quantified, and a simple scaling is proposed and shown to describe precisely the thinning down of the layers as a function of the initial tracer column’s horizontal width and the vertical shear of the azimuthal velocity. Nonlinear QG simulations were performed and analyzed for comparison with the work of Hua et al. A step-by-step interpretation of these results on the evolution of layering is proposed in the context of tracer stirring.

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