Behavior of Double-Hemisphere Thermohaline Flows in a Single Basin
Summary (2 min read)
1. Introduction
- Though the earth’s global thermohaline circulation is dominated by temperature (the deepest water is generally the coldest), salinity variations play a crucial role in determining the location of deep-water formation.
- In the asymmetric state, one hemisphere will possess the densest surface water in the basin.
- In contrast to these studies, the authors want to relate the strength of the thermohaline circulation directly to external parameters; to clarify the relation they use simpler geometry and forcing.
- For simplicity the authors use a relatively idealized system here.
2. Numerical model
- All experiments are conducted with MOM-2, the Modular Ocean Model version of the GFDL Model (Pacanowski 1996; Cox 1984), a B-grid (Arakawa and Lamb 1977) finite-difference discretization of the primitive equations that computes solutions by stepping forward in time.
- The values chosen correspond to a linearization of the equation of state at surface pressure and a temperature of about 138C (see Table 1).
- The salinity is driven by setting a zonally uniform surface salinity flux to represent the effects of freshwater fluxes produced by evaporation, precipitation, and runoff.
- This parameterization has a stronger dynamical justification than horizontal diffusion, and allows numerical models to better represent the relatively thin thermocline and small deep-water formation regions of the real ocean and to eliminate spurious diapycnal diffusion in regions of strong horizontal gradients such as western boundary currents (Veronis 1975; Böning et al. 1995; Danabasoglu et al. 1994).
- The Gent–McWilliams runs are conducted with a flux-corrected transport scheme added to MOM-2 by Weaver and Eby (1997).
3. Restoring boundary conditions
- The authors conduct two-hemisphere experiments, which are forced only by restoring to a temperature profile that is asymmetric about the equator.
- The degree of asymmetry between the hemispheres is small, yet the circulation must be qualitatively different from a symmetric experiment because deep water is required to spread from the dominant hemisphere to fill the deepest region of the other ‘‘subordinate’’ hemisphere.
- The maximum buoyancy difference between the eastern and western boundaries is approximately bE 2 bW 5 0.25Db for all 1H and 2H runs, confirming the theoretical result of Marotzke (1997) and the hypothesis that the zonal buoyancy difference scales like Db.
- For smaller DbP, the peak northward heat transport is roughly proportional to the subordinate cell volume transport.
4. Mixed boundary conditions
- When the slightly asymmetric run with QS 5 1 is used as an initial condition for a high vertical resolution run with the same QS, the system falls into the intermediate asymmetric state.
- Therefore, most of the variation in DS, which spans nearly two orders of magnitude in the experiments, can be explained by (14).
- The low salinity tongue appears to be governed by a balance between southward advection of deep, low salinity from the northern boundary and downward diffusion of high salinity from the surface.
5. Conclusions
- One hemisphere, which the authors call the ‘‘dominant’’ hemisphere, has the strongest meridional circulation, minimizing the exposure of surface water to the surface fluxes of salinity (or, more realistically, of freshwater) at any particular latitude.
- Even a modest DbP forces the relatively small subpycnocline range of buoyancy to be filled by the dominant cell.
- The location of the regime boundaries must be viewed with some caution because, in reality, the ocean is coupled to an atmosphere in which meridional transports of heat and moisture can affect the stability of a state (Nakamura et al.
- A two-dimensional model shows that such a configuration is more unstable than a coupled model, in which moisture feedbacks are also included (Capotondi and Saravanan 1996).
- None of their attempts produced the multiple states seen here, nor did they display the vanishing of the asymmetric state at low QS described above, so the authors do not describe the model details in this paper.
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Citations
622 citations
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...[116] Concerning the relative formation rates of NADW and AABW, it appears that they are anticorrelated in model experiments with variations in the buoyancy fluxes [Klinger and Marotzke, 1999]....
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Cites background from "Behavior of Double-Hemisphere Therm..."
...In the picture of Bryan (1987), Klinger and Marotzke (1999), and Klinger et al. (2003) it is low-latitude diffusion that closes and drives the circulation....
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...On the other hand, a slight asymmetry between the density forcings in the two poles can result in a dramatically asymmetric deep THC circulation [Klinger and Marotzke, 1999]....
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Cites background from "Behavior of Double-Hemisphere Therm..."
...Theoretical considerations (Marotzke 1997) and GCM experiments for idealized geometry (Klinger and Marotzke 1999) have shown that the maximum east–west density difference scales as the north–south surface density difference....
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233 citations
References
191 citations
"Behavior of Double-Hemisphere Therm..." refers background in this paper
...Another weakness of the classical scaling employed here (and indeed of the vertical mixing parameterization in the model) is that vertical or diapycnal mixing in the ocean is known not to be uniform but concentrated near the margins (e.g., Munk 1966; Wunsch 1970; Armi 1978; Ledwell and Bratkovich 1995; Toole et al. 1997)....
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...…classical scaling employed here (and indeed of the vertical mixing parameterization in the model) is that vertical or diapycnal mixing in the ocean is known not to be uniform but concentrated near the margins (e.g., Munk 1966; Wunsch 1970; Armi 1978; Ledwell and Bratkovich 1995; Toole et al. 1997)....
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187 citations
177 citations
"Behavior of Double-Hemisphere Therm..." refers background or result in this paper
...The existence of such states would imply a much richer range of responses for the thermohaline circulation, which is usually portrayed as choosing between convection or no-convection states for various oceans (Marotzke and Willebrand 1991) or ocean regions (Hughes and Weaver 1994)....
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...The existence of such states would imply a much richer range of responses for the thermohaline circulation, which is usually portrayed as choosing between convection or no-convection states for various oceans (Marotzke and Willebrand 1991) or ocean regions (Hughes and Weaver 1994)....
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...The power law for overturning F found here appears to be more complicated than the linear relationships between flow strength and meridional density differences found in global GCMs (Hughes and Weaver 1994; Rahmstorf 1996)....
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...Hughes and Weaver (1994) showed in an idealized global GCM under mixed boundary conditions that Atlantic overturning strength is linearly related to the basinwide meridional gradient in zonally and vertically averaged steric height (i.e., a double vertical integral of zonally averaged density)....
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...Hughes and Weaver (1994) found a linear relationship between overturning strength F and the pole-to-pole difference in vertically integrated steric height, C....
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175 citations
175 citations
"Behavior of Double-Hemisphere Therm..." refers background or methods or result in this paper
...…subordinate cell is clearly related to the mixed layer depth but, since the meridional overturning is proportional to a double vertical integral of the density difference between eastern and western walls, one cannot expect a simple correspondence [see Marotzke (1997) for a detailed discussion]....
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...One might ask whether the zonal buoyancy difference in (5) must scale like Db, but Marotzke (1997) demonstrates that this is actually a reasonable assumption, and we show below that it holds fairly well in our numerical experiments....
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...Again, Marotzke (1997) has shown that some scaling and numerical results are reasonably insensitive to assumptions about how localized the mixing is....
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...How overturning strength depends on external parameters has received relatively thorough scrutiny in single-basin systems driven by temperature-only boundary conditions (Bryan 1986; Colin de Verdiere 1988; Winton 1996; Marotzke 1997) or freshwater-only boundary conditions (Huang and Chou 1994)....
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...In order to compare 2H flows to 1H, it is therefore more appropriate to use the continuity equation (see Marotzke 1997; Winton 1996)....
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