Upon entering the sinking basin, the northward flow of the upper branch of the MOC forms a western boundary current, with a velocity that is independent of the sinking location. This western boundary current is superimposed on the western boundary current associated with the wind-driven gyres.
In the simplified geometry presented here, the MOC northward velocity is larger than the southward SPG western boundary current in the narrow basin with width similar to the Atlantic but not in the wide basin with width similar to the Pacific.
The net result is that salty water from the subtropical gyre is carried into the western portion of the SPG when sinking occurs in the narrow basin but not when sinking occurs in the wide basin.
Instead, in the wide basin, the southward western boundary current in the SPG brings freshwater from the far north, where the freshwater flux is maximum.
The resulting fresh pool suppresses local deep-water formation, and the faster zonal velocity efficiently spreads the freshwater eastward, causing the water in the SPG of the wide basin to become fresher than the water in the SPG of the narrow basin. This process, diagnosed in the 3D model experiments, is documented in section 2. In section 3 , a 2D advection—diffusion model of the upper branch of the MOC is used to explore the salinity distribution for various flow fields and zonal arrangements of deep-water formation.
Further idealized experiments with the same 2D model show that neither the MOC western boundary current alone nor the wind-driven gyres alone can produce different salinity fields based on the basin width.
However, salt advection by the combined velocity fields, and the associated feedback on deep-water formation, selects the narrow basin as the preferred deep-water mass formation site. Section 4 provides a summary and draws conclusions. Citation: Journal of Physical Oceanography 47, 11; The Redi tensor is tapered to horizontal diffusion in regions of weak stratification, as described by Danabasoglu and McWilliams Each simulation was run for at least years, until equilibrium was reached.
Additional details of the model configuration are given in Jones and Cessi Two configurations are considered in the 3D model: one in which the wide basin is twice as wide as the narrow basin W2N and one in which the wide basin is 3 times as wide as the narrow basin W3N. These geometries are shown in Fig.
Under zonally uniform forcing, deep-water formation occurs in the narrow basin only, regardless of the initial condition, and the surface salinity in the SPG of the narrow basin is higher for W3N than for W2N. Sinking in the wide basin can be coerced by reducing the freshwater flux at the northern end of the wide-basin sector, while compensating this reduction by a uniform freshwater flux increase everywhere else.
A larger asymmetry in freshwater flux is needed to force wide sinking in the W3N geometry than in the W2N geometry see the bottom panel of Fig. For both W2N and W3N, the wide sinking state reverts to narrow sinking when the forcing is slowly over 20 years returned to zonal symmetry.
In summary, wide sinking is unstable under zonally uniform freshwater forcing. In practice, this isopycnal contour is chosen to pass as close as possible through the maxima of both the deep overturning cell in the sinking basin and the shallow overturning cell in the nonsinking basin.
The upwelling across this isopycnal contour is approximately fixed by wind stress in the Southern Ocean plus eddy transport of buoyancy and global diapycnal diffusion Gnanadesikan ; Allison ; Jones and Cessi , setting the cross-equatorial northward transport of the upper branch of the MOC in the sinking basin to approximately 11 Sv regardless of the location of sinking Fig.
This cross-equatorial transport, augmented by the diapycnal upwelling across b m in the Northern Hemisphere of the sinking basin, determines the maximum transport of the MOC. In the nonsinking basin, diffusive upwelling feeds a shallow cell in the Northern Hemisphere and an abyssal cell mostly in the Southern Hemisphere. The upwelled water flows northward, sinking to about m at high northern latitudes, and then returns southward. The meridional transport in the nonsinking basin integrated zonally and above b m is shown in Fig.
The numerical simulations indicate why wide sinking is unstable when the surface freshwater flux is symmetric: Fig. The values in the sinking region of the wide basin red dashed lines in the gray box of Figs.
In other words, it appears that it is the zonal asymmetry in freshwater forcing that keeps the wide basin slightly saltier than the narrow basin.
Without this asymmetry, the salinities of the basins reverse and narrow sinking occurs. To quantify how the salinity might be distributed under zonally uniform forcing for wide sinking, we advect and diffuse a passive tracer with the velocity, diffusivity, and convective adjustment time series from the wide sinking state.
Unlike salt, the tracer is forced with the zonally uniform surface flux given by the solid line in Fig. The resulting tracer field vertically averaged above b m is shown in the bottom panel of Fig.
Compared to the salinity anomaly middle panel of Fig. To make a more quantitative comparison between the three cases, it is useful to examine the salinity and tracer concentrations averaged above b m and then zonally averaged.
Figure 6 shows that, for wide sinking, the tracer concentration anomaly at the latitudes of sinking is larger in the narrow basin than in the wide basin cf. At high latitudes, the temperature is approximately independent of the location of sinking and thus does not contribute directly to the preference for narrow over wide sinking.
However, at the sinking latitudes, the temperature of the sinking basin is slightly higher than the temperature of the nonsinking basin, so it partially counteracts the effects of salinity on the buoyancy. Consequently, when the salinity in the sinking region is only marginally larger than the salinity in the nonsinking basin, the negative temperature advection feedback destabilizes the wide sinking state.
The buoyancy is displayed in the top panels of Fig. For wide sinking, the buoyancy in the sinking region is lower than at the same latitudes of the narrow basin cf. In the bottom panels of Fig. For wide sinking, the tracer buoyancy at high latitudes is indeed lower in the narrow basin than in the wide basin cf. This confirms that the wide sinking solution is unstable under zonally uniform surface salt flux forcing.
Salinity in turn affects the distribution of convective adjustment, 2 which is rather different for narrow and wide sinking. Convective adjustment is the main process determining the diapycnal velocity across the buoyancy b m. Thus, we use the diapycnal velocity, denoted with following the notation of Young , henceforth referred to as WRY12 , as a measure of deep-water formation. As illustrated in Fig. For wide sinking, is confined to the eastern two-thirds of the domain, whereas for narrow sinking, it is spread throughout the whole width of the basin.
This pattern reflects the zonal distribution of salinity cf. In the western third of the wide sinking basin, the surface is especially fresh, while it is relatively salty almost everywhere in the narrow sinking basin. We now demonstrate that the contrast in the zonal distribution of salinity results from differences in the velocities near the western boundary of the sinking basin. As shown in Fig. Therefore, salinity is transported farther north into the western portion of the SPG for narrow sinking.
The resulting high salinity in the narrow basin enables deep convection Fig. In the north of the wide basin, deep-water formation is suppressed on the western side of the SPG by the low salinities carried southward by the western boundary current, and the isopycnal b m does not outcrop there Fig. The western boundary velocity in the sinking basin is the sum of the northward flow of the MOC plus the western boundary current associated with the locally wind-driven Sverdrup gyre, which is northward in the subtropical gyre and southward in the SPG.
This interior transport produces a southward western boundary current in the SPG, which is twice as fast in the wide basin than in the narrow basin. The zonal velocity associated with the Sverdrup gyre is also faster in the wide basin. However, the MOC transport, largely confined to a western boundary current within each basin, is about 15 Sv, independent of the width of the sinking basin. The total flow above b m is approximately a linear combination of the gyres and the MOC western boundary current Stommel The MOC velocity is of similar amplitude to the SPG western boundary current in the narrow basin and in the opposite direction, but it is half of the velocity of the SPG western boundary current in the wide basin.
As a result, the western boundary current is strong and southward in all the SPG of the wide basin but not in the narrow basin as shown in Fig.
The difference in velocities on the western side of the basins leads to different salinity distributions in the SPG region: fresher in the wide sinking basin than in the narrow sinking basin.
The halocline that forms on the western side of the wide basin suppresses deep-water formation, localizing the diapycnal velocity to the eastern side of the wide basin. This localization, given a zonally uniform freshwater flux, reduces the efficiency of the salt feedback on water mass formation in the wide basin, giving a preference to narrow-basin sinking. To further demonstrate this process, in the following section we examine the salinity distribution obtained with zonally uniform freshwater fluxes and velocities that are simplified relative to the full 3D field.
Unlike the 3D salinity computations, here the freshwater flux is zonally uniform for both narrow and wide sinking, just as in the passive tracer experiments described in the previous section. The goal of this 2D experiment is to diagnose which components of the velocities are essential to determine the differences in the salinity distribution between narrow and wide sinking.
Despite ignoring the baroclinic terms, the 2D salinity obtained with 8 reproduces fairly well some aspects of the 3D computations and in particular the salinity vertically averaged above b m cf.
The agreement is less satisfactory in the subtropical regions, where the depth of b m is largest: here, the vertical correlation of the baroclinic components dominates the salinity transport diagnostic not shown.
The resulting salinities are higher at the northern end of the SPG in the narrow basin for both narrow and wide sinking velocities red solid and blue dashed lines in the gray box of Fig. Because the 2D advection—diffusion model [ 8 ] treats as a passive scalar, it does not incorporate the salt feedback controlling the onset of convective adjustment in the sinking region and the associated large diapycnal velocity. To demonstrate that the spatial distribution of is important, we conduct an experiment where this spatial distribution is varied.
In the 2D experiment reported in the left panels of Fig. The changes in are accommodated by changing the zonal velocity U so that 10 is satisfied. The zonal integral of is conserved at every latitude in this process, and the velocities outside the SPG of the sinking basin remain unchanged.
With advection by this modified velocity field the sinking wide basin gets saltier cf. The salinity remains essentially unchanged for narrow sinking cf. Therefore, the preference for narrow sinking is reduced when the suppression of convective adjustment by freshening, that is, the salt advection feedback, is removed. In summary, the salinity in the upper branch of the MOC is controlled by the velocity vertically averaged above the isopycnal b m and by the pattern of strong diapycnal velocity because of convective adjustment.
The associated deep-water mass formation is suppressed by a halocline on the western side of the wide basin. This latter feedback is not captured by 8 , which treats salinity as a passive scalar, but the contrast between the two experiments with the localized versus homogenized illustrates this effect. Both experiments reveal that the velocity field in the SPG is responsible for the halocline formation on the western side of the wide sinking region.
Neither of these patterns in isolation lead to the observed differences in SPG salinities. In the following, we show that, in isolation, neither the gyral velocities alone nor the MOC velocity alone can lead to a halocline in the wide-basin SPG.
We first show the salinity distribution with advection by idealized vertically integrated velocities U gyre and V gyre representing the gyres in the 2D model.
The resulting horizontally nondivergent transport is described by a single streamfunction shown in Fig. Figure 13 shows the distribution of the resulting tracer solution of 8 , forced by the zonally uniform freshwater flux of Fig. In the along-streamlines direction, the dominant balance is between the isopycnal advection terms and the surface salinity flux.
In the across-streamlines direction, the dominant balance is between the isopycnal diffusion terms and the surface salinity flux. These differences are too small to explain the preference for narrow sinking seen in the 3D model, and a scale analysis in appendix B confirms that the salinity in this configuration is independent of basin width.
We now examine the salinity distribution with a velocity field characterized by sinking and a western boundary current associated with the MOC without wind-driven gyres. The velocities U , V and in 8 are defined analytically.
They are confined to a western boundary current in a single basin fed by upwelling in the periodic channel. In the western boundary current and most of the channel, the dominant balance in 8 is between isopycnal salinity advection and the surface salinity flux.
Elsewhere, velocities are very small, so the dominant balance is between isopycnal diffusion and the surface salinity flux. The idealized velocity fields for both narrow and wide sinking are shown in Figs. The resulting zonally averaged salinities are shown in Fig. In a configuration with two basins of different widths connected by a reentrant channel, sinking occurs in the narrow basin under zonally uniform forcing.
Deep-water formation in the wide basin can be coerced by reducing the freshwater flux over the north of the wide basin and then we find that a stronger reduction is needed for larger ratios of basin widths. Despite the reduction in freshwater flux for wide sinking, the salinity difference between the sinking basin and the nonsinking basin is smaller when sinking occurs in the wide basin.
High salinity in the north of the sinking basin is always reinforced by a large cross-equatorial overturning cell, which transports salt northward across the equator: this is the salt advection positive feedback Stommel However, this feedback is less effective for wide sinking.
In particular, for zonally uniform salinity flux, we show that the wide sinking state is unstable. This is because higher salinity and therefore lower buoyancy is found at the surface in the north of the narrow basin, and freshwater is more efficiently advected southward by the subpolar wind-driven gyre in the wide basin. The temperature advection negative feedback also plays a small role. When the width of the narrow basin is reduced further, keeping the width of the wide basin constant, the salinity difference between the basins increases, as does the preference for narrow sinking.
A 2D advection—diffusion model shows that the advection of salinity in the upper branch of the 3D overturning is well represented by the velocity vertically integrated above the isopycnal that divides the upper and lower branch of the MOC. The vertically integrated velocities show that there is a crucial difference in the sense of circulation on the western side of the SPG between the wide and narrow sinking basins. For narrow sinking, the western boundary current in the SPG is very weak, and for wide sinking it is strong and southward, advecting freshwater from the north, forming a halocline that is absent in the narrow sinking basin.
This halocline is advected eastward by the southern branch of the SPG, suppressing deep-water mass formation. We rationalize the difference between narrow and wide sinking by invoking the linear superposition of the western boundary velocities associated with the wind-driven SPG and with the MOC.
The latter is independent of the basin size, while the former is larger for a wide basin, and it prevails over the MOC in the wide sinking basin. We further emphasize the interaction of the gyral velocities with the MOC by contrasting the effect of the two velocity components in isolation. Advection by gyres-only velocities or MOC-only velocities leads to no preference for narrow sinking.
Our arguments work well in the idealized context of our model, with simple coastlines, flat bottom, and zonally uniform steady forcing. Thus, it is not clear how robust the narrow sinking preference is in more complex settings. It is difficult to compare observations of the SPG transport between the Atlantic and Pacific because most long-term observations are limited to m depth and hence include the upper branch of the MOC.
Deep-water formation largely occurs in marginal seas rather than in the open ocean, as it does in our model. The W2N experiment described here was repeated with sills at the end of both of the continents and this led to no qualitative changes to the overturning. In this section, we use Cartesian coordinates, but the actual computations are in the spherical coordinates appropriate for the sector shown in Fig. Sign in Sign up. Advanced Search Help. Journal of Physical Oceanography.
Sections Abstract 1. Introduction 2. Because of this, the Atlantic constantly loses fresh water to other oceans and becomes saltier. The tallest ocean waves have been measured at feet 30 meters high! Tidal waves are not caused by tides.
The correct name is tsunami. The amount of fresh water this mechanism creates is significant, Schmittner said, about , cubic meters per second. The mountains of East Africa keep water transport originating in the Indian Ocean from reaching the Atlantic. Meanwhile, the massive Antarctic ice sheet also plays a major role, the researchers report in their study.
This ice sheet intensifies the winds and shifts the Antarctic Circumpolar Current to the south. Without the sheet, the temperature contrasts between the land mass and the atmosphere at lower latitudes would lessen, decreasing winds, Schmittner said. Climate model simulations by the researchers found that removing the mountain ranges creates a fresh North Atlantic and a salty North Pacific. Andreas Schmittner ,
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