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Where Does Dense Water Sink? a Subpolar Gyre Example*

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Where Does Dense Water Sink? a Subpolar Gyre Example* 810 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 Where Does Dense Water Sink? A Subpolar Gyre Example* MICHAEL A. SPALL AND ROBERT S. PICKART Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts (Manuscript received 21 July 1999, in ®nal form 1 June 2000) ABSTRACT It is proposed that a dominant component of the downwelling limb of the thermohaline circulation takes place in regions where convective mixing is found adjacent to steep topography. A simple theoretical estimate of the overturning forced by such boundary convection is derived that depends only on the properties of the oceanic mixed layer along the boundary. Scaling estimates indicate that sinking forced by boundary convection is an order of magnitude greater than sinking in the open ocean resulting from large-scale dynamics or baroclinic instability of deep convective sites. Recent hydrographic observations in the Labrador Sea are used to estimate the downwelling due to these different mechanisms and support the notion that boundary sinking dominates. The theory compares well with the overturning rates diagnosed in a noneddy-resolving general circulation model over a wide range of parameters. As a direct consequence of these dynamics, the high-latitude hydrography and overturning circulation in the model are very sensitive to the presence of cyclonic rim currents. Lateral density advection by the rim currents in the subpolar gyre increases the strati®cation and limits the mixing near the boundaries, thus reducing the maximum downwelling. As a result, most of the high-latitude meridional heat transport is carried by the horizontal circulation instead of the overturning circulation. Such rim currents are found in different con®gurations of the model, including 1) a continental slope and standard diffusion parameters and 2) zero horizontal diffusion and a ¯at bottom. 1. Introduction ocean (and where they are not). This paper attempts to provide a dynamical framework for understanding The meridional overturning circulation is of funda- where the high-latitude downwelling takes place and mental importance to the global climate system. Water what constrains its magnitude. in the upper ocean ¯ows from low latitudes toward the There has been a great deal of recent interest in the poles, where it is cooled and heat is released into the meridional overturning circulation and its subsequent atmosphere. The modi®ed (more dense) water is then heat transport. Many low-resolution modeling studies returned to middle and low latitudes as intermediate and have focused on the relationship between the overturn- deep water masses. The mass ¯ux in the meridional ing rate and the model paramerizations of unresolved plane is thought to be closed by upwelling, primarily at processes such as vertical mixing of density, air±sea middle and low latitudes. While this scenario is no doubt exchange, and exchange with other basins (e.g., Bryan correct in the broadest sense, a better understanding of where these vertical mass ¯uxes take place, and of what 1987; Marotzke 1997; DoÈscher et al. 1994). Attempts physical processes control the location and amplitude to scale the overturning strength with model and at- of the vertical exchange, is necessary if we are to un- mospheric parameters have achieved some level of suc- derstand the ocean's circulation and its role in climate. cess (Bryan 1987; Colin de Verdiere 1988; Marotzke Recent results from high-resolution microstructure pro- 1997; Zhang et al. 1999, Park and Bryan 2000). Low- ®lers (Polzin et al. 1997) and tracer release experiments resolution models have also shown that the very nature (Ledwell et al. 1993) are beginning to shed light on of the thermohaline circulation can be strongly time where these deep waters are upwelled into the upper dependentÐexperiencing total collapse, periodic oscil- lations, or chaotic oscillations (Stommel 1961; Bryan 1987; Weaver et al. 1993). The variability of the over- turning circulation and associated meridional heat trans- * Woods Hole Oceanographic Institution Contribution Number port in these models is associated with the shutting down 10103. of deep convection and sinking at high latitudes, al- though the dynamics that control these sinking rates and regions are not well understood. A recent study by Mar- Corresponding author address: Michael A. Spall, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. otzke and Scott (1999) has begun to explore the dy- E-mail: [email protected], [email protected] namics of sinking regions and to distinguish down- q 2001 American Meteorological Society MARCH 2001 SPALL AND PICKART 811 welling from convective mixing. Many of these models urx 1 yry 5 Q. (1) are con®gured in the most simple way possible by mak- Within the mixed layer rz 5 0 and there is no contri- ing use of a ¯at bottom and forcing the circulation with bution from vertical advection. The cooling of parcels zonally averaged properties. The resulting circulations, requires that the ¯ow cross isopycnals from light to particularly in the subpolar gyre, can be very unrealistic dense. The advection of density is achieved by the when compared to observations. depth-averaged ¯ow within the mixed layer because the There have also been numerous studies using both nonlinear terms cancel for the vertically sheared geo- low- and high-resolution models that are con®gured in strophic ¯ow. If the horizontal ¯ow with magnitude V realistic domains and are forced with realistic air±sea and direction u (relative to east) is assumed to be in ¯uxes. The exchanges with other basins are often pa- geostrophic balance, then (1) may be manipulated to rameterized in these calculations by restoring the tem- represent the rotation of the velocity vector with depth perature and salinity ®elds toward climatological values as near the meridional boundaries (Bryan and Holland gQ 1989; BoÈning et al. 1996). The circulation and density u 52 , (2) z 2 structure in these models are generally much more re- f r 0V alistic than are found in the ¯at bottom, idealized cal- where f is the Coriolis parameter, g is the gravitational culations. Recent advances in the parameterization of acceleration, and r 0 is a reference density of seawater mesoscale processes have resulted in meridional heat [similar derivations may be found in Schott and Stom- transports and overturning circulations that are in gen- mel (1978), Stommel (1979), and Spall (1992)]. For eral agreement with observational estimates (BoÈning et regions in which convective mixing is increasing the al. 1995). However, many aspects of the subpolar cir- density of the ocean, the velocity vector will spiral coun- culation and meridional heat transport in basin-scale terclockwise with depth. Such rotation with depth was models are sensitive to how the exchanges with high inferred from hydrographic data in the subpolar gyre by latitude basins are parameterized within the buffer zones Luyten et al. (1985) and Stommel (1979). The direction (DoÈscher et al. 1994; BoÈning et al. 1996). In part because of ¯ow as a function of depth may be found by inte- of this sensitivity to boundary conditions, little attention grating (2), has been paid in the analysis of these models to the gQz dynamics that control the downwelling limb of the ther- u(z) 5 u 2 . (3) 0 2 mohaline circulation. f r 0V In the present study, we seek to develop a basic un- For simplicity, we have assumed here that the mag- derstanding of what determines the regions and rate of nitude of the velocity vector is constant in depth, in sinking at high latitudes. While the discussion is for- general agreement with the observations in the interior mulated in the context of a subpolar gyre circulation, of the subpolar gyre (Stommel 1979) and near the the results are relevant to the general problem of dense boundaries (Pickart and Torres 1998). This expression water-sinking, such as occurs in the subarctic seas or is valid throughout the basin where the ¯ow is in geo- semienclosed seas (i.e., the Nordic seas, the Mediter- strophic balance. In the open ocean, the ¯ow can satisfy ranean Sea, or Red Sea) or near the west coast of Aus- geostrophic and thermodynamic balances by remaining tralia (Godfrey and Ridgway 1985). Some aspects of essentially horizontal and spiraling with depth. This is this anaysis will involve convective mixing, but our consistent with the velocity structure and buoyancy forc- primary focus is on the downward mass ¯ux, not on the ing inferred by Luyten et al. (1985) in the eastern North processes of water mass transformation. The sinking Atlantic subpolar gyre. mechanisms that are considered in this study include: The physical interpretation of (3) is straightforward. diapycnal mixing near steep topography, large-scale Flow perpendicular to the isopycnals is required in order open ocean sinking, baroclinic instability of convective to balance the heat loss to the atmosphere; however, this sites, Ekman convergence, and the subduction at col- ¯ow has no vertical shear. The ¯ow component parallel liding western boundary currents. to the isopycnals within the mixed layer must have a vertical shear in order to satisfy thermal wind balance. 2. Cooling spirals and downwelling Thus, the direction of the ¯ow must rotate counter- clockwise with depth. The assumption that V is constant Because we anticipate that sinking at high latitudes is reasonable provided that the ¯ow perpendicular to the may result when heat is lost to the atmosphere and the isopycnals is larger that the thermal wind associated water is made more dense, our analysis begins by con- with the horizontal density gradient within the mixed sidering the thermodynamic balances within a mixed layer. layer subject to cooling. In a steady state with no hor- izontal diffusion, the horizontal advection of density is balanced by buoyancy loss to the atmosphere and con- a. Cooling near a boundary vective mixing, represented here as Q (positive values Large vertical velocities can result from the cooling represent cooling): spiral if the cooling takes place adjacent to a boundary 812 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 or steep topography.
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