Meridional Barrier Research Articles

The impacts of the atmospheric annular mode on the AMOC and its feedback in an idealized experiment

The interdecadal variability of the atmospheric and oceanic meridional overturning circulation is studied, using a coupled model with two narrow meridional barriers representing the land and a flat bottomed Aquaplanet. Empirical orthogonal function (EOF) analysis are used in the atmospheric and oceanic meridional overturning cells, revealing the atmospheric interdecadal variability is dominated by an annular mode, in both hemispheres, which introduces in the ocean a set of patterns of variability. The most energetic EOFs in the ocean are the barotropic responses from the annular mode. The interaction between the heat anomalies, due to the barotropic response, and the thermohaline circulation of each basin leads to a resonance mechanism that feeds back to the atmospheric forcing, modulating the annular mode spectrum. Besides the barotropic response, the annular mode introduces anomalies of salinity and temperature in the subtropical Atlantic that affects its upper buoyancy. These anomalies are incorporated within the ocean circulation and advected until the areas of deep sinking in the northern Atlantic, impacting on its overturning circulation as well.

The importance of planetary rotation period for ocean heat transport.

The climate and, hence, potential habitability of a planet crucially depends on how its atmospheric and ocean circulation transports heat from warmer to cooler regions. However, previous studies of planetary climate have concentrated on modeling the dynamics of atmospheres, while dramatically simplifying the treatment of oceans, which neglects or misrepresents the effect of the ocean in the total heat transport. Even the majority of studies with a dynamic ocean have used a simple so-called aquaplanet that has no continental barriers, which is a configuration that dramatically changes the ocean dynamics. Here, the significance of the response of poleward ocean heat transport to planetary rotation period is shown with a simple meridional barrier--the simplest representation of any continental configuration. The poleward ocean heat transport increases significantly as the planetary rotation period is increased. The peak heat transport more than doubles when the rotation period is increased by a factor of ten. There are also significant changes to ocean temperature at depth, with implications for the carbon cycle. There is strong agreement between the model results and a scale analysis of the governing equations. This result highlights the importance of both planetary rotation period and the ocean circulation when considering planetary habitability.

Brief communication "Stratospheric winds, transport barriers and the 2011 Arctic ozone hole"

Abstract. The Arctic stratosphere throughout the late winter and early spring of 2011 was characterized by an unusually severe ozone loss, resulting in what has been described as an ozone hole. The 2011 ozone loss was made possible by unusually cold temperatures throughout the Arctic stratosphere. Here we consider the issue of what constitutes suitable environmental conditions for the formation and maintenance of a polar ozone hole. Our discussion focuses on the importance of the stratospheric wind field and, in particular, the importance of a high latitude zonal jet, which serves as a meridional transport barrier both prior to ozone hole formation and during the ozone hole maintenance phase. It is argued that stratospheric conditions in the boreal winter/spring of 2011 were highly unusual inasmuch as in that year Antarctic-like Lagrangian dynamics led to the formation of a boreal ozone hole.

A faunistic review of carabids (Coleoptera, Carabidae) of the Barguzin Mt. Range (Northern Baikal Area)

The list of carabids from the Barguzin Mt. Range includes 132 species belonging to 29 genera of 17 tribes. Five new species endemic or subendemic to the Barguzin Mt. Range have been discovered. The carabid fauna of the Northern Baikal Area has a high percentage of species with wide distribution in the Palaearctic. The bulk of the fauna consists of species with Circumholarctic, Transpalaearctic, Euro-Siberian, Siberian, and Asian-American ranges. Sayano-Baikalian, Southern Siberian, Baikalian, and Transbaikalian species with local distribution are less numerous than the Mongolian, Kazakhstan, Amur, and Okhotian species. Lake Baikal and mountain ranges framing it form a meridional barrier for the distribution of some species.

Continuous ultra-fine classification in a disc-stack nozzle centrifuge — effects of G-forces and disc geometry

This paper describes the use of a disc-stack nozzle centrifuge for wet classification of ultra-fine particles of calcium carbonate. The influences of G-force and disc geometry on the performance of the ultra-fine classification have been investigated. The results have demonstrated that the efficiency of the classification can be increased by (1) applying a relatively moderate G-force and (2) the use of stud spacer discs in the centrifuge. The increased extraction of the fine particles is related to the formation of the appropriate fluid velocity profile at a lower G-force and a circumferential motion without meridional barriers for the case of the stud spacer discs.

The Squeezing of Eddies through Gaps

Abstract The islands of the Lesser Antilles present a porous meridional barrier to North Brazil Current (NBC) rings. To better understand if, when, and how a NBC ring could be squeezed into the Caribbean Sea through such a gappy barrier, the encounter of a westward drifting eddy with a porous meridional wall is considered. First an eddy encountering a single aperture is modeled. To do so, both an analytical model and a reduced-gravity primitive equations numerical model are used. This was followed by numerical experiments that explored eddy collisions with barriers containing many gaps. In all of these models, the collisions were forced by either β or a steady advection through the gap(s). Using integrated constraints, an analytical solution was constructed for a zero potential vorticity lens passing through a single gap on a β plane. The solution involves a small parameter e, the ratio of the short timescale associated with the f-plane adjustment, and the long encounter timescale due to β. It is found th...

Steady Baroclinic Flow through Ridges with Narrow Gaps*

Abstract The steady baroclinic flow in a basin containing a meridional barrier representing a midocean ridge is studied in the linear, quasigeostrophic limit of a two-layer model. Thermal damping and a simple friction provide dissipation of thickness (heat) and momentum. The ridge is pierced by two gaps in the upper layer but only a single gap in the lower layer. The flow in the model is forced by specified upwelling at the upper surface and by a specified cross-isopycnal velocity at the interface in addition to the autogenerated cross-isopycnal velocity associated with the thermal damping. The forcing may be either broad in longitude or narrowly confined. The nature of the geometry of the model ridge mixes the baroclinic and barotropic response to the forcing, and this has profound consequences for the resulting circulation. In particular, when the baroclinic interaction of the two layers is strong, the recirculation region to the east of the ridge, previously discovered in earlier barotropic models of t...

The Transparency of Ocean Barriers to Rossby Waves: The Rossby Slit Problem*

Abstract The transmission of barotropic Rossby wave energy through a meridional barrier pierced only by narrow gaps is studied with a quasigeostrophic model. The incident Rossby wave has the form of either a plane wave or a localized beam of wave energy. The application of Kelvin’s theorem on a contour girdling the separated segments of the barrier demonstrate the necessity for wave transmission and helps determine the amplitude of the transmitted wave. When the meridional scale of the incident wave is much larger than the gaps in the barrier, the transmission becomes independent of the gap width. The problem of a barrier with two or three gaps is discussed. In the former case a single bubble of wave energy issues from the region spanned by the separated barrier segment. When a third gap or slit is allowed, the structure of the transmitted wave alters depending on the symmetry of the wave field and the angle of the incidence of the ray trajectory of the incident wave.

The Transmission and Transformation of Baroclinic Rossby Waves by Topography*

Abstract The transmission of westward propagating baroclinic Rossby waves incident on a gappy meridional barrier is studied in the context of the two-layer, quasigeostrophic model. The meridional barrier models the presence of very steep topography such as the midocean ridge system or extensive island arcs. The nature of the transmission depends strongly on the nature of the gaps in the meridional barrier. If the gaps extend throughout the depth of the fluid, the Rossby waves propagate through the barrier, as a consequence of Kelvin’s theorem, with no change in vertical structure. On the other hand, if the gaps in the barrier are partial and extend only over a single layer, there is a significant transformation of the vertical structure of the wave field as it traverses the barrier. In particular, waves of baroclinic vertical structure in the model are transformed on the western side of the barrier into barotropic waves that radiate from the segment of the barrier between two such gaps. Such segments act ...

The Shadowed Island*

Abstract The circulation of fluid in a basin containing several topographic barriers is considered. Meridional barriers situated east of another barrier “shadow” the western barrier with respect to Rossby wave propagation. The steady circulation responds by reducing the flow through the gaps in the western barrier. As a consequence, the nature of the recirculation zone described in Pedlosky et al. is strongly affected by the shadowing topography. Generally, the effect of the shadowing topography is to enhance and enlarge the zone of recirculation. It is shown that the effectiveness of the shadow depends on whether the eastern barrier is a peninsula or internal “island.” In the latter case, the effect of the barrier depends strongly on the relative meridional extents of the two topographic features. In all cases, the peninsula is by far the most effective shadowing barrier in reducing the flow around the western barrier. Both analytical and laboratory models of the circulation are described. In the former ...

Rossby Normal Modes in Basins with Barriers*

Abstract The Rossby wave, linear, normal modes for a barotropic fluid in a basin on the β plane are calculated in the presence of a thin barrier, which nearly divides the basin in two. Narrow gaps allow one subbasin to communicate to the other. In the case of a meridional barrier it is shown that the modes split into two categories: either full-basin modes, which have strong expression in both subbasins, or subbasin modes, which are limited to one or the other of the subbasins. The full-basin modes have natural frequencies very close to the eigenfrequencies of the basin in the absence of a barrier, while the subbasin modes oscillate at the eigenfrequencies of the subbasins as if they were isolated. In the former case, the oscillation is accompanied by strong flow through the gaps in the barrier while, in the latter case, there is essentially no flow through the barrier. If the barrier is opened by adding additional gaps, some of the modes turn from subbasin modes to full-basin modes. If the barrier is zon...

Effects of partial meridional barriers on the Antarctic Circumpolar Current—Wind-driven barotropic model

Abstract The structure and transport of the Antarctic Circumpolar Current are examined by means of a simple barotropic model. We investigate the effects of two partial meridional barriers, corresponding to the South American peninsula and the island arc to the east. The transport is given by the ratio of the pressure difference produced by wind stress to the resistance resulting from bottom friction. In the limiting case with no friction, the pressure difference is determined by the magnitude of the wind stress at the latitudes of the ends of the two meridional barriers, and the resistance is proportional to the difference between the Coriolis parameters at the same sites. The transport predicted by the model is reasonable as compared with the observed value. This suggests that the mechanism discussed in this paper can be applied to the real ocean. We also find the relationship between the transport and the topographic drag on the meridional barriers. The time change of the transport is determined by the drag on the meridional barriers, the bottom stress, and the rate at which momentum is supplied by wind stress. The transport lags wind stress by several days.

Effects of partial meridional barriers on the Antarctic Circumpolar Current—Buoyancy-driven three-layer model

The transport and vertical structure of the Antarctic Circumpolar Current (ACC) are examined, especially the component of the current driven by buoyancy, by using a three-layer model. We investigate the effects of the South American peninsula, the island arc to the east, and the Macquarie ridge, which are modeled as partial meridional barriers overlapping meridionally each other. We found that the buoyancy-driven component is given as a function of the transport out of the Weddell Sea (S W ) and the sum of the transports into the North Atlantic (S A ) and the North Pacific (S P ) out of the Southern Ocean. The buoyancy-driven current flows westward, ifS W andS A +S P are positive. The transport depends on the value ofS W more thanS A +S P by one order of magnitude within a realistic range of parameters. The most predominant term in the transport equation is inversely proportional to the difference between the Coriolis parameters at the tips of the partial meridional barriers. Thus, the magnitude of the transport strongly depends on the overlapping length of the meridional barriers. The eastward current of the ACC is driven by the predominant eastward wind stress in the Southern Ocean, although a part of the wind-driven component is canceled by the westward buoyancy-driven component. The vertical structure of the ACC is found to be attributed to the surface wind-driven circulation and the deep and bottom buoyancy-driven circulation.

Stratified Abyssal Flow in the Presence of Fractured Ridges

Abstract A linear, two-layer model for abyssal flow driven by a specified upwelling into the thermocline is studied when the ocean basin is partially blocked by a meridional barrier in the lower layer. The barrier is meant to represent a midoceanic ridge, and the space between the termination points of the ridge and the northern and southern basin boundaries represents the gaps that pierce most ridge systems. It is found that the deep flow is only partially blocked by the barrier. A substantial flow threads through the gaps in the form of zonal jets. The portion of the flow that is blocked by the ridge produces a recirculating, cyclonic gyre east of the ridge, and its latitudinal extent is typically smaller than the unbroken interval of the ridge. The overall structure of the recirculation and the degree of blocking are related to robust integral conditions constraining the circulation around the ridge. Although weaker, a similar recirculation is often found in the unblocked layer above the ridge as an up...

On steady, frictional wind-driven ocean circulation with topography in a circumpolar basin

Abstract An analytical boundary layer solution is developed for barotropic, steady, frictional, wind-driven flow over a meridionally aligned top-hat ridge in a circumpolar basin on a local β-plane. Classical Sverdrup dynamics cannot hold in a basin with no meridional barriers. With a top-hat ridge, frictional boundary layers exist adjacent to the depth discontinuities which can support along- and cross-ridge transport. The boundary layers thus provide a mechanism for transporting fluid across latitudes of zero wind stress curl. The relevance of this simple model to the dynamics of the Southern Ocean is discussed.

Transient Response of a Global Ocean-Atmosphere Model to a Doubling of Atmospheric Carbon Dioxide

Abstract The transient response of climate to an instantaneous increase in the atmospheric concentration of carbon dioxide has been investigated by a general circulation model of the coupled ocean-atmosphere-land system with global geography and annual mean insulation. An equilibrium climate of the coupled model is perturbed by an abrupt doubling of the atmospheric carbon dioxide. The evolution of the model climate during the 60-year period after the doubling is compared with the result from a control integration of the model without the doubling. The increase of surface air temperature in middle and high latitudes is slower in the Southern Hemisphere than the Northern Hemisphere The large thermal inertia of the ocean-dominated hemisphere is partly responsible for this difference. The effective thermal inertia of the oceans becomes particularly large in high southern latitudes. Owing to the absence of meridional barriers at the latitudes of the Drake Passage. a wind-driven. deep cell meridional circulatio...

Effects of Centrifuge Shape on the Separation of a Mixture

Abstract This paper examines the effect of centrifuge shape on the separation time for a two-phase mixture in a centrifuge with a meridional barrier. The work is based on the results of Greenspan and Ungarish. Both the inner radius of the centrifuge and the shape of the lids affect the separation time. It was found, through numerical experiments, that altering the shapes of the lids yields more enhancement than increasing the inner radius. Also, centrifuges with parallel, linear, sloped lids give better results than those with lids having slight or moderate curves superimposed on the linear lids. The curvature appears to interfere with the Boycott effect. However, a sawtooth pattern superimposed on the linear lids is an improvement over the linear lids.

On the enhancement of centrifugal separation

We consider the two-phase flow of a suspension in a rotating cylinder with inclined endplates for which inertial and viscous effects are small. It is shown that, when the Coriolis force is dominant, flow in the core is essentially unaffected by geometry. If a fluid particle can make a complete circuit about the rotation axis, the sedimentation velocity cannot be augmented by geometrical effects as it can in gravitational settling. However, with the insertion of a complete meridional barrier to block movement around the centre, separation becomes more sensitive to the shape of the container walls. In this case, behaviour similar to that in a gravitational field is possible once again.

A Model of the Equatorial Sea Surface Temperature Field and Associated Circulation Dynamics

Abstract A linear, continuously stratified analytical model of the equatorial ocean is developed and used to simulate the major features of the equatorial sea surface temperature (SST) field and current system. The model is bounded by meridional barriers to flow in the east and west and is forced by an idealized southeast trade wind field. The steady state response is studied here, although the model is time periodic and capable of simulating the annual cycle or a more general time dependence. The surface heat flux forcing is set to zero so that the contribution of the equatorial upwelling field in maintaining the cool equatorial surface water can be isolated. Many of the observed features of the equatorial SST field and current system are simulated. The model generates cool SST anomalies an the equator in the eastern half of the basin. The cool water core is located to the south of the equator in the easternmost regions of the basin, with the coldest water along the eastern boundary. The latitudinal and ...

The stability of buoyancy-driven coastal currents : Griffiths, R.W. and P.F. Linden, 1981. Dynam. Atmos. Oceans, 5(4):281–306

Griffiths, R.W. and Linden, P.F., 1981. The stabil i ty of buoyancy-dr iven coastal currents. Dyn. Atmos . Oceans, 5: 281--306. Buoyancy-dr iven boundary currents were generated in the labora tory by releasing buoyan t fluid f rom a source adjacent to a vertical boundary in a rota t ing container . The boundary removed the Coriolis force parallel to it, a l lowing the buoyan t fluid to spread in a current along the boundary . Use of a cylindrical boundary and a line source that released fluid un i formly around the c i rcumference enabled an ax isymmetr ic (zonal) current to be produced. With the con t inuous release of fluid f rom the source, the current grew in width and depth unti l it became unstable to non-ax isymmetr ic disturbances. The wavelength and phase velocit ies o f the disturbances were consistent wi th a mode l of barocl inic instabil i ty of two-layer f low when fr ict ional dissipation due to Ekman layers is included. However , when the current only occupied a small f ract ion of the total depth, baro t ropic processes were also thought to be impor tan t , with the growing waves gaining energy f rom the hor izonta l shear. In o ther exper iments , gravity currents were produced by a poin t source adjacent to ei ther a zonal (circular) or a meridional (radial) vertical boundary . The currents were also observed to become unstable to the same upst ream breaking waves as those on the cont inuous zonal current . Finally, some compar isons are made with oceanic coastal currents. 1. I N T R O D U C T I O N Buoyancy-driven coastal currents occur in many parts of the world oceans. A particularly striking example is the East Greenland Current in which cold, fresh polar water flows southward from the Arctic Ocean along the east coast of Greenland. It occupies a wedge-shaped region some 200 km in width, with the maximum depth (200 m) at the coast. The primary driving force appears to be the density difference between the polar water and the water of the Norwegian Sea. Where land masses are not present, the southward spreading of the less dense polar water is inhibited by the effects of the Earth's rotation. Southward motion induces an east--west flow to conserve angular momentum, and this flow, in turn, produces a north--south Coriolis force which opposes the buoyancy force. Consequently, further southward motion is inhibited. However, at a meridional barrier such as the 0 3 7 7 0 2 6 5 / 8 1 / 0 0 0 0 0 0 0 0 / $ 0 2 . 5 0 © 1981 Elsevier Scient if ic Publishing Company