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Dynamical Pathways of Antarctic Bottom Water in the Atlantic

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Dynamical Pathways of Antarctic Bottom Water in the Atlantic 622 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 Dynamical Pathways of Antarctic Bottom Water in the Atlantic JAMES C. STEPHENS AND DAVID P. M ARSHALL Department of Meteorology, University of Reading, Reading, United Kingdom (Manuscript received 4 November 1998, in ®nal form 17 June 1999) ABSTRACT A reduced-gravity model is developed to represent the ¯ow of Antarctic Bottom Water (AABW) over realistic bathymetry in an Atlantic domain. The dynamics are based on the steady, planetary±geostrophic, shallow-water equations, including a linear bottom friction and a uniform diapycnal upwelling through the top of the model layer. The model solutions are broadly consistent with observations of the distribution and transport of AABW. The ¯ows occur predominantly along potential vorticity contours, which are in turn broadly oriented along bathymetric contours. The characteristic weak ¯ow across potential vorticity contours of the Stommel±Arons model is present as a small addition to this stronger forced mode along potential vorticity contours. As a consequence, mass balance is maintained not by hypothesized western boundary currents as in the Stommel±Arons model, but by the interplay between topographic slope currents and interior recirculations. In particular, a transposition is found in the ¯ow of AABW from the western side of the Brazil Basin south of the equator to the western ¯ank of the Mid-Atlantic Ridge north of the equator. This is also consistent with an analytical result derived by extending the Parsons mechanism to an abyssal layer overlying arbitrary bathymetry. The authors suggest that the results provide a more convincing zero-order picture than the Stommel±Arons model for the circulation of AABW and perhaps for abyssal water masses in general. 1. Introduction mel and Arons 1960a,b) has served as a paradigm for Antarctic Bottom Water (AABW) is an abyssal water our understanding of the thermohaline circulation. In the mass, easily distinguishable in hydrography by its cold Stommel±Arons theory, uniform upwelling in the interior (u , 1.88C)1 and relatively fresh (S , 34.8 psu) sig- of a ¯at-bottomed ocean basin balances a localized source nature, which is also high in silica and low in oxygen. of dense water and drives a barotropic poleward interior In the Atlantic Ocean, AABW is comprised of Weddell ¯ow by vortex stretching; boundary currents are hypoth- Sea Deep Water, which is formed in the Southern Ocean esized to exist on the western side of ocean basins to against the Antarctic continent, and recirculating lower maintain continuity in the circulation. The upwelling in Circumpolar Deep Water, which enters the Atlantic via turn balances the downward diffusion of heat from the Drake Passage. In Fig. 1 we present a schematic of the surface ocean and maintains the permanent thermocline. circulation pathways and approximate transports de- However, one of the most interesting aspects of the duced from observations. The bathymetry of the abyssal circulation of AABW, which is not predicted by the basins and their interconnecting passages strongly in- Stommel±Arons theory, is the transposition of its ¯ow ¯uences the pathways of AABW as it ®nds its way from the western side of the Brazil Basin south of the northward into the western basins of the South Atlantic equator to the eastern side of the basin north of the and then spreads through fracture zones into the eastern equator. Here the current continues northward against basins. For reference, the bathymetric features referred the western ¯ank of the Mid-Atlantic Ridge, as repre- to in the text are represented in Fig. 2. sented in Fig. 1. The existence of such an eastern bound- ary current contradicts the Stommel±Arons theory, and a. Previous theory a number of mechanisms have been proposed to account The classical view of the abyssal circulation ®rst de- for this. One such mechanism is that of Kawase (1987), veloped by Stommel and Arons (Stommel 1958; Stom- who studied the spinup of a source-driven abyssal cir- culation numerically in a ¯at-bottomed basin straddling the equator, including a damping term on the interface 1 All temperatures referred to in this paper are potential temperatures. height of the abyssal layer to represent cross-isopycnal buoyancy mixing. In the limit of weak damping, the deep western boundary current (DWBC) generated by Corresponding author address: Dr. James C. Stephens, Geophys- ical Fluid Dynamics Laboratory, NOAA/Princeton University, Post the source in the northwest corner separates along the Of®ce Box 308, Forrestal Campus, Princeton, NJ 08542. equator, forming northward and southward ¯owing cur- E-mail: [email protected] rents along the eastern boundary. The eastern boundary q 2000 American Meteorological Society Unauthenticated | Downloaded 09/24/21 01:29 PM UTC MARCH 2000 STEPHENS AND MARSHALL 623 FIG. 1. A schematic of the major pathways and transports of AABW. Approximate transports in Sverdrups are shown in circles. The pathways connecting obervations of transport are inferred from the general concensus of obervations. The ®gure may in reality only represent the actual circulation in the very broadest sense. Shading indicates depths shallower than 4000 m (bathymetry from Row et al. 1995). currents then radiate long Rossby waves westward from where x and y are eastward and northward distances re- the eastern boundary to set up a Stommel±Arons interior spectively, f 5 2V sinu is the Coriolis parameter, y is the ¯ow. In the limit of strong damping the propagation of northward velocity, and g9 is the reduced gravity. Equation long Rossby waves into the interior is prevented and (1) was integrated and rearranged by Nof (1990) to yield this acts to ``freeze'' the solution in the state containing eastern boundary currents. Tziperman (1987) proposed 2fT 1/2 h 5 h2 2 , (2) a similar mechanism to explain the existence of the we12g9 eastern boundary Mediterranean Undercurrent. Another explanation for the equatorial transposition where T 5 # yhdxis the northward transport, and he of AABW follows from the work of Nof (1990), who and hw are the depths of the current at its offshore edge by analogy with the Parsons (1969) model of Gulf and the wall respectively. Here hw is always negative Stream separation, demonstrated that a northward-¯ow- in the Northern Hemisphere for he 5 0 [this case was ing surface geostrophic current above a motionless low- originally considered by Anderson and Moore (1979)]. er layer and against a western vertical wall can only For a ®nite value of he a jet can penetrate into the North- exist in the Northern Hemisphere if it has no outcrop ern Hemisphere, whereby hw decreases until hw 5 0 on the open ocean side. Nof considered a shallow water when the jet must then separate from the wall. Nof and layer of thickness h in the vicinity of the equator, which Olson (1993) extended the work of Nof (1990) by in- is geostrophic in the cross-stream direction; that is, cluding bathymetry in the form of a parabolic channel straddling the equator in a meridional direction. In this ]h fy 5 g9 , (1) case bathymetry relaxes the outcropping constraint, and ]x inertial effects allow their abyssal current to cross iso- Unauthenticated | Downloaded 09/24/21 01:29 PM UTC 624 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 FIG. 2. Locations of basins and topographic features referred to in the text. Shading indicates depths shallower than 4000 m (bathymetry from Row et al. 1995). baths and ¯ip sides in the channel as it crosses the closed geostrophic contours, regardless of the sign of the equator, strongly resemblant of the AABW ¯ow. topography, by a vortex stretching effect due to the con- In a related study to explain the transposition of vergence of the ageostrophic frictional ¯ow, which bal- AABW, Speer and McCartney (1992) introduced vari- ances upwelling over the closed contour region. Johnson able layer thickness into the Stommel±Arons model (1998) ®nds support for this mechanism in relation to (thereby distorting the potential vorticity contours from abyssal ¯ows over ocean trenches; alternatively Dewar zonal contours of the Coriolis parameter) and showed (1998) argues for a balance between bottom friction and that, for a prescribed eastern boundary thickness, sep- potential vorticity mixing over the bathymetry determin- aration of the abyssal layer also occured from the west- ing the strength of the anticyclonic circulation. ern boundary. The separation mechanism is analagous to Parsons (1969) and Nof (1990), although the north- ward ¯ow in this case is driven by upwelling. b. A generalized Parsons mechanism Further studies investigating the effect of bathymetry on the abyssal circulation include Rhines (1989), who In the presence of bathymetry, such as in Nof and considered planetary-scale zonal ¯ows over topography Olson (1993), eastern and western boundary currents and demonstrated blocking and resonance structures. can occur quite naturally as ¯ows that are geostrophic Straub and Rhines (1990) investigated isolated regions in the cross-stream direction, and the strong damping of closed geostrophic contours due to bathymetric hills limit of Kawase (1987) may thus be avoided. We can and bowls submerged in the abyssal layer and found generalize the result of Nof and Olson described above internal jets linking the closed contour region to the west- to an isopycnal layer that has two outcrops (as abyssal ern boundary. Kawase and Straub (1991) examined the water masses do) against arbitrary bathymetry. We rep- spinup of a similar scenario and demonstrated that a vig- resent the AABW as a shallow homogeneous layer over- orous cyclonic circulation is generated over regions of lain by a motionless upper layer of slightly lesser den- Unauthenticated | Downloaded 09/24/21 01:29 PM UTC MARCH 2000 STEPHENS AND MARSHALL 625 to achieve a cross-equatorial ¯ow, although Edwards and Pedlosky (1998a) and Edwards and Pedlosky (1998b) have recently pointed out the importance of both inertia and dissipation in the vicinity of the equator.
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