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Willem P. Sijp and Matthew H. England
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Willem P. Sijp and Matthew H. England

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In Part I of this paper an evolution equation for the Atlantic salinity and the reverse cell strength in the North Atlantic Deep Water (NADW) OFF state was formulated. Here, an analytical solution to this equation is used to test its validity in the context of transient solutions. In this study several transient scenarios in the general circulation model are examined to determine the accuracy of the predictions made with the material in Part I. The authors also determine how well the basic premises of Part I hold throughout these transient behaviors. The unstable equilibria that mark the upper boundary of the OFF state attraction basin are elucidated by the time-dependent behavior shown here. Transient equilibration from one stable NADW OFF state to another in response to changes in the anomalous salt flux H is accurately described by the evolution equation. The theory also explains the distribution of decay times for the NADW OFF state around the maximum critical Atlantic surface flux. Exceedingly long collapse times in excess of 50 000 years are found for surface flux values slightly in excess of the critical value.

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Willem P. Sijp and Matthew H. England

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This study shows that a reduction in vertical mixing applied inside the Atlantic basin can drastically increase North Atlantic Deep Water (NADW) stability with respect to freshwater perturbations applied to the North Atlantic. This is contrary to the notion that the stability of the ocean’s thermohaline circulation simply scales with vertical mixing rates. An Antarctic Intermediate Water (AAIW) reverse cell, reliant upon upwelling of cold AAIW into the Atlantic thermocline, is found to be associated with stable states where NADW is collapsed. Transitions between NADW “on” and “off” states are characterized by interhemispheric competition between this AAIW cell and the NADW cell. In contrast to the AAIW reverse cell, NADW eventually upwells outside the Atlantic basin and is thus not as sensitive to changes in vertical mixing within the Atlantic. A reduction in vertical mixing in the Atlantic weakens the AAIW reverse cell, resulting in an enhanced stability of NADW formation. The results also suggest that the AAIW reverse cell is responsible for the stability of NADW collapsed states, and thereby plays a key role in maintaining multiple equilibria in the climate system. A global increase of vertical mixing in the model results in significantly enhanced NADW stability, as found in previous studies. However, an enhancement of vertical mixing applied only inside the Atlantic Ocean results in a reduction of NADW stability. It is concluded that the stability of NADW formation to freshwater perturbations depends critically on the basin-scale distribution of vertical mixing in the world’s oceans.

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Willem P. Sijp and Matthew H. England

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The role of a Southern Ocean gateway in permitting multiple equilibria of the global ocean thermohaline circulation is examined. In particular, necessary conditions for the existence of multiple equilibria are studied with a coupled climate model, wherein stable solutions are obtained for a range of bathymetries with varying Drake Passage (DP) depths. No transitions to a Northern Hemisphere (NH) overturning state are found when the Drake Passage sill is shallower than a critical depth (1100 m in the model described herein). This preference for Southern Hemisphere sinking is a result of the particularly cold conditions of the Antarctic Bottom Water (AABW) formation regions compared to the NH deep-water formation zones. In a shallow or closed DP configuration, this forces an exclusive production of deep/bottom water in the Southern Hemisphere. Increasing the depth of the Drake Passage sill causes a gradual vertical decoupling in Atlantic circulation, removing the influence of AABW from the upper 2000 m of the Atlantic Ocean. When the DP is sufficiently deep, this shifts the interaction between a North Atlantic Deep Water (NADW) cell and an AABW cell to an interaction between an (shallower) Antarctic Intermediate Water cell and an NADW cell. This latter situation allows transitions to a Northern Hemisphere overturning state.

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Willem P. Sijp and Matthew H. England

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The role of the Southern Ocean in global climate is examined using three simulations with a coupled model employing geometries different only at the location of Drake Passage (DP). The results of three main experiments are examined: 1) a simulation with DP closed, 2) an experiment with DP at a shallow (690 m) depth, and 3) a realistic DP experiment. The climate with DP closed is characterized by warmer Southern Hemisphere surface air temperature (SAT), little Antarctic ice, and no North Atlantic Deep Water (NADW) overturn. On opening the DP to a shallow depth of 690 m there is an increase in Antarctic sea ice and a cooling of the Southern Hemisphere but still no North Atlantic overturn. On fully opening the DP, the climate is mostly similar in the Southern Hemisphere to DP at 690 m, but the model now simulates NADW formation and a warming in the Northern Hemisphere. This suggests the North Atlantic thermohaline circulation depends not only on the existence of a DP throughflow, but also on the depth of the sills in the Southern Ocean. The closed DP experiment exhibits a large amount of deep-water formation [57 Sv (Sv ≡ 106 m3 s−1)] in the Southern Hemisphere; this reduces to 39 Sv for the shallow DP case and 14 Sv when DP is at 2316 m, its modern-day depth. NADW formation is shut down in both DP closed and shallow experiments, which accounts for the warming in the Northern Hemisphere observed when the DP is opened. SAT differences between the DP open and closed climate are seasonal. The largest SAT changes occur during winter in areas of large sea ice change. However, summer conditions are still significantly warmer when DP is closed (regionally up to 4°C). Summer SAT is the most important factor determining whether an Antarctic ice sheet can build up. Therefore our study does not exclude the possibility that changes in ocean gateways may have contributed to the glaciation of Antarctica. Overall, these experimental results support paleoclimatic evidence of rapid cooling of the Southern Ocean region soon after the isolation of Antarctica.

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Willem P. Sijp and Matthew H. England

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The absence of the Drake Passage (DP) gateway in coupled models generally leads to vigorous Antarctic bottom water (AABW) formation, Antarctic warming, and the absence of North Atlantic deep-water (NADW) formation. Here the authors show that this result depends critically on atmospheric moisture transport by midlatitude storms. The authors use coupled model simulations employing geometries different only at the location of DP to show that oceanic circulation similar to that of the present day is possible when DP is closed and atmospheric moisture transport values enhanced by Southern Ocean storm activity are used. In this case, no Antarctic warming occurs in conjunction with DP closure. The authors also find that the changes in poleward heat transport in response to the establishment of the Antarctic Circumpolar Current (ACC) are small. This result arises from enhanced atmospheric moisture transport at the midlatitudes of the Southern Hemisphere (SH), although the values used remain within a range appropriate to the present day. In contrast, homogeneous or (near) symmetric moisture diffusivity leads to strong SH sinking and the absence of a stable Northern Hemisphere (NH) overturning state, a feature familiar from previous studies. The authors’ results show that the formation of NADW, or its precursor, may have been possible before the opening of the DP at the Eocene/Oligocene boundary, and that its presence depends on an interplay between the existence of the DP gap and the hydrological cycle across the midlatitude storm tracks.

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Willem P. Sijp and Matthew H. England

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The effect of the position of the Southern Hemisphere subpolar westerly winds (SWWs) on the thermohaline circulation (THC) of the World Ocean is examined. The latitudes of zero wind stress curl position exert a strong control on the distribution of overturning between basins in the Northern Hemisphere. A southward wind shift results in a stronger Atlantic THC and enhanced stratification in the North Pacific, whereas a northward wind shift leads to a significantly reduced Atlantic THC and the development of vigorous sinking (up to 1500-m depth) in the North Pacific. In other words, the Atlantic dominance of the meridional overturning circulation depends on the position of the zero wind stress curl over the Southern Ocean in the experiments. This position has a direct influence on the surface salinity contrast between the Pacific and the Atlantic, which is then further amplified by changes in the distribution of Northern Hemisphere sinking between these basins. The results show that the northward location of the SWW stress maximum inferred for the last glacial period may have contributed to significantly reduced North Atlantic Deep Water formation during this period, and perhaps an enhanced and deeper North Pacific THC. Also, a more poleward location of the SWW stress maximum in the current warming climate may entail stronger salinity stratification of the North Pacific.

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Willem P. Sijp and Matthew H. England

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Increasing the value of along-isopycnal diffusivity in a coupled model is shown to lead to enhanced stability of North Atlantic Deep Water (NADW) formation with respect to freshwater (FW) perturbations. This is because the North Atlantic (NA) surface salinity budget is dominated by upward salt fluxes resulting from winter convection for low values of along-isopycnal diffusivity, whereas along-isopycnal diffusion exerts a strong control on NA surface salinity at higher diffusivity values. Shutdown of wintertime convection in response to a FW pulse allows the development of a halocline responsible for the suppression of deep sinking. In contrast to convection, isopycnal salt diffusion proves a more robust mechanism for preventing the formation of a halocline, as surface freshening leads only to a flattening of isopycnals, leaving at least some diffusive removal of anomalous surface FW in place. As a result, multiple equilibria are altogether absent for sufficiently high values of isopycnal diffusivity. Furthermore, the surface salinity budget of the North Pacific is also dominated by along-isopycnal diffusion when diffusivity values are sufficiently high, leading to a breakdown of the permanent halocline there and the associated onset of deep-water formation.

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Willem P. Sijp, Matthew H. England, and Jonathan M. Gregory

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This study examines criteria for the existence of two stable states of the Atlantic Meridional Overturning Circulation (AMOC) using a combination of theory and simulations from a numerical coupled atmosphere–ocean climate model. By formulating a simple collection of state parameters and their relationships, the authors reconstruct the North Atlantic Deep Water (NADW) OFF state behavior under a varying external salt-flux forcing. This part (Part I) of the paper examines the steady-state solution, which gives insight into the mechanisms that sustain the NADW OFF state in this coupled model; Part II deals with the transient behavior predicted by the evolution equation. The nonlinear behavior of the Antarctic Intermediate Water (AAIW) reverse cell is critical to the OFF state. Higher Atlantic salinity leads both to a reduced AAIW reverse cell and to a greater vertical salinity gradient in the South Atlantic. The former tends to reduce Atlantic salt export to the Southern Ocean, while the latter tends to increases it. These competing effects produce a nonlinear response of Atlantic salinity and salt export to salt forcing, and the existence of maxima in these quantities. Thus the authors obtain a natural and accurate analytical saddle-node condition for the maximal surface salt flux for which a NADW OFF state exists. By contrast, the bistability indicator proposed by De Vries and Weber does not generally work in this model. It is applicable only when the effect of the AAIW reverse cell on the Atlantic salt budget is weak.

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Jan D. Zika, Willem P. Sijp, and Matthew H. England

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Vertical transport of heat by ocean circulation is investigated using a coupled climate model and novel thermodynamic methods. Using a streamfunction in temperature–depth coordinates, cells are identified by whether they are thermally direct (flux heat upward) or indirect (flux heat downward). These cells are then projected into geographical and other thermodynamic coordinates. Three cells are identified in the model: a thermally direct cell coincident with Antarctic Bottom Water, a thermally indirect deep cell coincident with the upper limb of the meridional overturning circulation, and a thermally direct shallow cell coincident with the subtropical gyres at the surface. The mechanisms maintaining the thermally indirect deep cell are investigated. Sinking water within the deep cell is more saline than that which upwells, because of the coupling between the upper limb and the subtropical gyres in a broader thermohaline circulation. Despite the higher salinity of its sinking water, the deep cell transports buoyancy downward, requiring a source of mechanical energy. Experiments run to steady state with increasing Southern Hemisphere westerlies show an increasing thermally indirect circulation. These results suggest that heat can be pumped downward by the upper limb of the meridional overturning circulation through a combination of salinity gain in the subtropics and the mechanical forcing provided by Southern Hemisphere westerly winds.

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