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
17 citations
Cites background or methods from "Behavior of Double-Hemisphere Therm..."
...It has been suggested, using idealized double-hemisphere models, that the overturning circulation might respond differently to changes in the equator-to-pole or pole-to-pole meridional density gradient, with the equator-to-pole density gradient influencing the strength of local overturning in the dominant hemisphere and the pole-to-pole density gradient determining the degree of overturning asymmetry and cross-equatorial flow (Klinger and Marotzke 1999; Mohammad and Nilsson 2006)....
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...…gradient, with the equator-to-pole density gradient influencing the strength of local overturning in the dominant hemisphere and the pole-to-pole density gradient determining the degree of overturning asymmetry and cross-equatorial flow (Klinger and Marotzke 1999; Mohammad and Nilsson 2006)....
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17 citations
Cites background or result from "Behavior of Double-Hemisphere Therm..."
...It is well established that in a two-hemisphere basin the thermohaline circulation tends to attain an equatorially asymmetric state, rather than a state of reversed circulation, as the freshwater forcing is increased (Marotzke et al. 1988; Thual and McWilliams 1992; Klinger and Marotzke 1999 )....
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...However, this is probably misleading, as there tends to be an element of pole-to-pole circulation in the asymmetric regime, rather than two independent cells with different directions of flow (e.g., Klinger and Marotzke 1999 )....
[...]
...Focusing on thermally forced flows, it can be noted that the studies by Klinger and Marotzke (1999) and Marotzke and Klinger (2000) demonstrate that even a weak pole-to-pole temperature (i.e., density) difference yields a flow that is strongly asymmetric with respect to the equator....
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15 citations
Cites background from "Behavior of Double-Hemisphere Therm..."
...Thus, the scaling of the shallow meridional overturning streamfunction, Ψup, is given by Ψup ∼ wELxe , (46) independent of the diapycnal diffusivity....
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...In doublehemisphere geometries the thermally indirect cell has been attributed to the transequatorial intrusion of the anticlockwise MOC from the opposite hemisphere (Klinger and Marotzke, 1999)....
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15 citations
Cites background or result from "Behavior of Double-Hemisphere Therm..."
...The results here show that the subcritical bifurcation on the northern sinking branch is the origin of the hysteresis bifurcation that occurs for equatorially asymmetric conditions; this is consistent with the results in Klinger and Marotzke (1999)....
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...In ocean GCMs with idealized geometry, this type of symmetry breaking is responsible for the existence of northern and southern sinking solutions (Bryan 1986; Marotzke and Willebrand 1991; Klinger and Marotzke 1999)....
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15 citations
Cites background from "Behavior of Double-Hemisphere Therm..."
...The anomalous upwelling at these latitudes opposes the background flow which downwells and produces southern ocean intermediate water (Klinger and Marotzke, 1999)....
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References
3,107 citations
"Behavior of Double-Hemisphere Therm..." refers background or methods in this paper
...Some runs with restoring boundary conditions are also repeated with the ‘‘Gent– McWilliams’’ parameterization, which supplements isopycnal diffusion of T and S with additional advection by a tracer velocity representing the untilting of isopycnals by baroclinic instability (Gent and McWilliams 1990; Gent et al. 1995)....
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...…boundary conditions are also repeated with the ‘‘Gent– McWilliams’’ parameterization, which supplements isopycnal diffusion of T and S with additional advection by a tracer velocity representing the untilting of isopycnals by baroclinic instability (Gent and McWilliams 1990; Gent et al. 1995)....
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2,358 citations
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2,030 citations
"Behavior of Double-Hemisphere Therm..." refers background in this paper
...8.8 and 8.10 in Peixoto and Oort (1992)], with winter northern North Atlantic water roughly 0.5 psu saltier than austral winter Weddell Sea water (Levitus and Boyer 1994)....
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1,741 citations
"Behavior of Double-Hemisphere Therm..." refers methods in this paper
...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....
[...]