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David Marshall and John Marshall

Abstract

It is shown that subtle changes in the velocity profile across the seaward extension of midlatitude jets, such as the Gulf Stream, can lead to dramatic changes in the zonal-penetration scale. In particular, if α = dq/dψ > 0, where q is the absolute vorticity and ψ is a streamfunction for the geostrophic flow, then the jet tends to penetrate across to the eastern boundary; conversely if α < 0, the jet turns back on itself creating a tight recirculation on the scale of order |α|−frac12;. This behavior is demonstrated in a quasigeostrophic ocean model in which a jet profile is prescribed as an inflow condition at the western margin of a half-basin, and radiation conditions along the remainder of the western boundary allow the injected fluid to escape. Jet inflows with both vertical and horizontal structure are considered in one and one-half-, two-, and three-layer models.

Finally, the implications of our study for numerical simulations of ocean gyres, which frequently show sensitivity of jet penetration to horizontal and vertical resolution and to choice of boundary conditions, are discussed. In particular, it is demonstrated that poor resolution of the horizontal jet structure may lead to a dramatic reduction in penetration.

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David Marshall and John Marshall

Abstract

The thermodynamic processes attendant on the transfer of fluid between a surface mixed layer and a stratified thermocline beneath are discussed. For a parcel of fluid in the mixed layer to pass into the stratified thermocline—to subduct—it must be stratified by buoyancy input; this buoyancy can be supplied by local air–sea exchange and/or by lateral advective processes.

A series of experiments is described in which a mixed layer, coupled to an ideal-fluid thermocline, undergoes differing seasonal cycles: in one limit the mixed layer is held fixed in a steady, winter configuration; in the other the mixed layer is, more realistically, shallow over most of the year and deepens briefly in late winter. It is shown that the annual subduction rate S ann depends, to first order, only on late winter mixed layer properties. However the annual-mean air–sea buoyancy exchange is sensitive to the details of the seasonal cycle and becomes vanishingly small as the effective subduction period shortens. In this limit the buoyancy is provided through advective processes in the Ekman layer.

The authors conclude that in ocean models that do not explicitly represent a seasonal cycle it is necessary to parameterize the process through a prescription of the winter mixed layer density and depth. The buoyancy forcing diagnosed from such models must be interpreted as the combined contribution of the annual air–sea exchange and lateral advectivc processes in the summer Ekman layer.

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Christophe Herbaut and John Marshall

Abstract

Idealized nonhydrostatic numerical calculations that resolve both plumes and geostrophic eddies are used to mimic isobaric float observations taken in the Labrador Sea Deep Convection Experiment and study mechanisms of buoyancy transport through mixed layers. The plumes and eddies are generated in a periodic channel, initialized with a vertical profile of temperature, and cooled by surface heat loss varying across the channel. Probability density functions and time series of vertical velocity and temperature computed from the floats are interpreted in terms of the kinematics of plumes and geostrophic eddies, and their role in buoyancy transport. Estimates from Eulerian time series from the numerical model suggest that geostrophic eddies and plumes have a comparable contribution to vertical heat flux.

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Ross Tulloch and John Marshall

Abstract

Multidecadal variability in the Atlantic meridional overturning circulation (AMOC) of the ocean is diagnosed in the NCAR Community Climate System Model, version 3 (CCSM3), and the GFDL Coupled Model (CM2.1). Common diagnostic approaches are applied to draw out similarities and differences between the two models. An index of AMOC variability is defined, and the manner in which key variables covary with it is determined. In both models the following is found. (i) AMOC variability is associated with upper-ocean (top 1 km) density anomalies (dominated by temperature) on the western margin of the basin in the region of the Mann eddy with a period of about 20 years. These anomalies modulate the trajectory and strength of the North Atlantic Current. The importance of the western margin is a direct consequence of the thermal wind relation and is independent of the mechanisms that create those density anomalies. (ii) Density anomalies in this key region are part of a larger-scale pattern that propagates around the subpolar gyre and acts as a “pacemaker” of AMOC variability. (iii) The observed variability is consistent with the primary driving mechanism being stochastic wind curl forcing, with Labrador Sea convection playing a secondary role. Also, “toy models” of delayed oscillator form are fitted to power spectra of key variables and are used to infer “quality factors” (Q-factors), which characterize the bandwidth relative to the center frequency and hence AMOC predictability horizons. The two models studied here have Q-factors of around 2, suggesting that prediction is possible out to about two cycles, which is likely larger than the real AMOC.

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Helen Jones and John Marshall

Abstract

The intensity and scale of the geostrophically adjusted end state of the convective overturning of a homogeneous rotating ocean of depth H at a latitude where the Coriolis parameter is f, induced by surface buoyancy loss of magnitude B 0, are studied by numerical experiment. The experiments are related to observations and laboratory studies of open-ocean deep convection. A numerical model based on the nonhydrostatic Boussinesq equations is used. The grid spacing of the model is small enough that gross aspects of convective plumes themselves can be resolved, yet the domain of integration is sufficiently large to permit study of the influence of plumes on the large scale and geostrophic adjustment of the convected water.

Numerical simulations suggest that cooling at the sea surface is offset by buoyancy drawn from depth through the agency of convective plumes. These plumes efficiently mix the water column to generate a dense chimney of fluid, which subsequently breaks up through the mechanism of baroclinic instability to form spinning cones of convectively modified water that have a well-defined and predictable scale.

A measure of the importance of rotation on the convective process is provided by a natural Rossby number introduced by Maxworthy and Narimousa:
i1520-0485-23-6-1009-eq1
where l rot = (B 0/f 3)1/2 is the length scale that marks the transition from three-dimensional, thermally driven turbulence to quasi-two-dimensional, rotationally dominated motions. Here u rot=(B 0/f)1/2 is the velocity of a particle gyrating in inertia] circles of radius l rot.

In the parameter regime typical of open-ocean deep convection, we find that Ro* ≲ 1; rotation influences the intensity and scale of both plumes and cones. In particular, the scale, intensity, buoyancy excess, and generation rate of the cones of geostrophically adjusted fluid, which result from the breakup of the chimney, are found to depend in a predictable way on this single nondimensional number, formed from the external parameters f, B 0, and H.

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Jason Goodman and John Marshall

Abstract

An analytical model of the mutual interaction of the middle-latitude atmosphere and ocean is formulated and studied. The model is found to support coupled modes in which oceanic baroclinic Rossby waves of decadal period grow through positive coupled feedback between the thermal forcing of the atmosphere induced by SST anomalies and the resulting wind stress forcing of the ocean. Growth only occurs if the atmospheric response to thermal forcing is equivalent barotropic, with a particular phase relationship with the underlying SST anomalies. The dependence of the growth rate and structure of the modes on the nature of the assumed physics of air–sea interaction is explored, and their possible relation to observed phenomena discussed.

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Timour Radko and John Marshall

Abstract

A simple theory is developed for the large-scale three-dimensional structure of the Antarctic Circumpolar Current and the upper cell of its overturning circulation. The model is based on a perturbation expansion about the zonal-average residual-mean model developed previously by Marshall and Radko. The problem is solved using the method of characteristics for idealized patterns of wind and buoyancy forcing constructed from observations. The equilibrium solutions found represent a balance between the Eulerian meridional overturning, eddy-induced circulation, and downstream advection by the mean flow. Depth and stratification of the model thermocline increase in the Atlantic–Indian Oceans sector where the mean wind stress is large. Residual circulation in the model is characterized by intensification of the overturning circulation in the Atlantic–Indian sector and reduction in strength in the Pacific Ocean region. Predicted three-dimensional patterns of stratification and residual circulation in the interior of the ACC are compared with observations.

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Guillaume Maze and John Marshall

Abstract

Analyzed fields of ocean circulation and the flux form of the potential vorticity equation are used to map the creation and subsequent circulation of low potential vorticity waters known as subtropical mode water (STMW) in the North Atlantic. Novel mapping techniques are applied to (i) render the seasonal cycle and annual-mean mixed layer vertical flux of potential vorticity (PV) through outcrops and (ii) visualize the extraction of PV from the mode water layer in winter, over and to the south of the Gulf Stream. Both buoyancy loss and wind forcing contribute to the extraction of PV, but the authors find that the former greatly exceeds the latter. The subsequent path of STMW is also mapped using Bernoulli contours on isopycnal surfaces.

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John Marshall and George Nurser

Abstract

No abstract available.

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John C. Marshall

Abstract

The combined problem of determining the ocean circulation and improving the geoid from satellite altimetry is formulated and studied. Minimum variance estimation is used to form optimum estimates of the ocean topography and the geoid. These estimates are a function of the altimeter observations, prior knowledge of the ocean circulation and prior knowledge of the geoid. Particular emphasis is placed on the use of a dynamical ocean model as a source of a priori oceanographic information capable of discriminating between geoid errors and ocean topography. The technique is illustrated in a simulation study of Gulf Stream variability, in which an ocean topography, degraded by noise representing the uncertainty in a gravimetric geoid, is reconstructed by assimilation into an ocean model. At the same time an improved estimate of the geoid is made.

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