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J. R. Toggweiler and B. Samuels

Abstract

Brine rejection during the formation of Antarctic sea ice is known to enhance the salinity of dense shelf waters in the Weddell and Ross Seas. As these shelf waters flow off the shelves and descend to the bottom, they entrain ambient deep water to create new bottom water. It is not uncommon for ocean modelers to modify salinity boundary conditions around Antarctica in an attempt to include a “sea ice effect” in their models. However, the degree to which Antarctic salinities are enhanced is usually not quantified or defended.

In this paper, studies of shelf hydrography and δ 18O are reviewed to assess the level of salinity enhancement appropriate for ocean general circulation models. The relevant quantities are 1) the salinity difference between the water masses modified on the shelves and the final offshelf flow and 2) the flux of salt (or freshwater) that gives rise to this salinity difference. Onshelf/offshelf salinity changes in the Weddell and Ross Seas appear to be fairly small, 0.15–0.20 salinity units. The quantity of brine needed to produce this salinification is equivalent to the salt drained from ⩽0.50 m of new sea ice every year.

Salt fluxes and salinity distributions from three GCM simulations are then compared. The first model has its surface salinities simply restored to the Levitus observations. Levitus restoring produces a slight freshening in the area of the Weddell and Ross Sea shelves. The global-mean bottom-water salinity in this model is 34.57 psu, which is 0.16 units less than observed. The second model includes a very modest salinity enhancement in the area of the Weddell and Ross Sea shelves This produces a salt flux equivalent to the formation of ∼0.50 m yr−1 of new sea ice. Even though this amount of salt input is close to the amount observed, global-average deep salinities in the second model are only 0.02 units greater than the deep salinities in the fist model. The third model includes a large salinity enrichment, which is applied throughout the Weddell and Row embayments without regard to water depth. Its deep saliinities are 0.18 units higher than the deep salinities in the first model, but the amount of salt pumped into the model greatly exceeds the salt flux in nature.

The authors conclude that salt from sea ice is probably not a major influence on the salinity of Antarctic bottom waters. Predicted salinities in ocean GCMs are too fresh because of circulation deficiencies, not because of inadequate boundary conditions. Models that employ large salinity modifications near Antarctica run the risk of grossly distorting the processes of deep-water formation.

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J. R. Toggweiler and B. Samuels

Abstract

By convention, the ocean’s large-scale circulation is assumed to be a thermohaline overturning driven by the addition and extraction of buoyancy at the surface and vertical mixing in the interior. Previous work suggests that the overturning should die out as vertical mixing rates are reduced to zero. In this paper, a formal energy analysis is applied to a series of ocean general circulation models to evaluate changes in the large-scale circulation over a range of vertical mixing rates. Two different model configurations are used. One has an open zonal channel and an Antarctic Circumpolar Current (ACC). The other configuration does not. The authors find that a vigorous large-scale circulation persists at the limit of no mixing in the model with a wind-driven ACC. A wind-powered overturning circulation linked to the ACC can exist without vertical mixing and without much energy input from surface buoyancy forces.

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Eli Tziperman, J. R. Toggweiler, Kirk Bryan, and Yizhak Feliks

Abstract

A global primitive equations oceanic GCM and a simple four-box model of the meridional circulation are used to examine and analyze the instability of the thermohaline circulation in an ocean model with realistic geometry and forcing conditions under mixed boundary conditions. The purpose is to determine whether this instability should occur in such realistic GCMs.

It is found that the realistic GCM solution is near the stability transition point with respect to mixed boundary conditions. This proximity to the transition point allows the model to make a transition between the unstable and stable regimes induced by a relatively minor change in the surface freshwater flux and in the interior solution. Such a change in the surface flux may be induced, for example, by changing the salinity restoring time used to obtain the steady model solution under restoring conditions. Thus, the steady solution of the global GCM under restoring conditions may be either stable or unstable upon transition to mixed boundary conditions, depending on the magnitude of the salinity restoring time used to obtain this steady solution. The mechanism by which the salinity restoring time affects the model stability is further confirmed by carefully analyzing the stability regimes of a simple four-box model. The proximity of the realistic ocean model solution to the stability transition point is used to deduce that the real ocean may also be near the stability transition point with respect to the strength of the freshwater forcing.

Finally, it is argued that the use of too short restoring times in realistic models is inconsistent with the level of errors in the data and in the model dynamics, and that this inconsistency is a possible reason for the existence of the thermohaline instability in GCMs of realistic geometry and forcing. A consistency criterion for the magnitude of the restoring times in realistic models is formulated, that should result in steady states that are also stable under mixed boundary conditions. The results presented here may be relevant to climate studies that run an ocean model under restoring conditions in order to initialize a coupled ocean–atmosphere model.

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A. M. de Boer, J. R. Toggweiler, and D. M. Sigman

Abstract

North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents. Here, instead, the overturning is viewed as a global energy–driven system and the robustness of NA dominance is investigated within this framework. Using an idealized geometry ocean general circulation model coupled to an energy moisture balance model, various climatic forcings are tested for their effect on the strength and structure of the overturning circulation. Without winds or a high vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through winds or mixing (purposefully included or due to numerical diffusion) starts the deep-water formation. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e., negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP), and Southern Ocean (SO), while near the haline limit, convection is restricted to the NA. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An increase in SO winds or global winds tends to weaken the salinity gradients, also pushing the ocean to the thermal state. Paleoobservations of more distributed sinking in warmer climates in the past suggest that mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning circulation over long time scales.

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Willem P. Sijp, Matthew H. England, and J. R. Toggweiler

Abstract

The role of tectonic Southern Ocean gateway changes in driving Antarctic climate change at the Eocene–Oligocene boundary remains a topic of debate. One approach taken in previous idealized modeling studies of gateway effects has been to alter modern boundary conditions, whereby the Drake Passage becomes closed. Here, the authors follow this approach but vary atmospheric pCO2 over a range of values when comparing gateway configurations. They find a significantly greater sensitivity of Antarctic temperatures to Southern Ocean gateway changes when atmospheric pCO2 is high than when concentrations are low and the ambient climate is cool. In particular, the closure of the Drake Passage (DP) gap is a necessary condition for the existence of ice-free Antarctic conditions at high CO2 concentrations in this coupled climate model. The absence of the Antarctic Circumpolar Current (ACC) is particularly conducive to warm Antarctic conditions at higher CO2 concentrations, which is markedly different from previous simulations conducted under present-day CO2 conditions. The reason for this is the reduction of sea ice associated with higher CO2. Antarctic sea surface temperature and surface air temperature warming due to a closed DP gap reach values around ∼5° and ∼7°C, respectively, for high concentrations of CO2 (above 1250 ppm). In other words, the authors find a significantly greater sensitivity of Antarctic temperatures to atmospheric CO2 concentration when the DP is closed compared to when it is open. The presence of a DP gap inhibits a return to warmer and more Eocene-like Antarctic and deep ocean conditions, even under enhanced atmospheric greenhouse gas concentrations.

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Joellen L. Russell, Keith W. Dixon, Anand Gnanadesikan, Ronald J. Stouffer, and J. R. Toggweiler

Abstract

A coupled climate model with poleward-intensified westerly winds simulates significantly higher storage of heat and anthropogenic carbon dioxide by the Southern Ocean in the future when compared with the storage in a model with initially weaker, equatorward-biased westerlies. This difference results from the larger outcrop area of the dense waters around Antarctica and more vigorous divergence, which remains robust even as rising atmospheric greenhouse gas levels induce warming that reduces the density of surface waters in the Southern Ocean. These results imply that the impact of warming on the stratification of the global ocean may be reduced by the poleward intensification of the westerlies, allowing the ocean to remove additional heat and anthropogenic carbon dioxide from the atmosphere.

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J. R. Toggweiler, B. Samuels, Eli Tziperman, Yizhak Feliks, Kirk Bryan, and Stephen M. Griffies

Abstract

The comment by Rahmstorf suggests that a numerical problem in Tziperman et al. (1994, TTFB) leads to a noisy EP field that invalidates TTFB's conclusions. The authors eliminate the noise, caused by the Fourier filtering used in the model, and show that TTFB's conclusions are still valid. Rahmstorf questions whether a critical value in the freshwater forcing separates TTFB's stable and unstable runs. By TTFB's original definition, the unstable runs in both TTFB and in Rahmstorf's comment have most definitely crossed a stability transition point upon switching to mixed boundary conditions. Rahmstorf finally suggests that the instability mechanism active in TTFB is a fast convective mechanism, not the slow advective mechanism proposed in TTFB. The authors show that the timescale of the instability is, in fact, consistent with the advective mechanism.

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Woo Geun Cheon, Young-Gyu Park, J. R. Toggweiler, and Sang-Ki Lee

Abstract

The Weddell Polynya of the mid-1970s is simulated in an energy balance model (EBM) sea ice–ocean coupled general circulation model (GCM) with an abrupt 20% increase in the intensity of Southern Hemisphere (SH) westerlies. This small upshift of applied wind stress is viewed as a stand in for the stronger zonal winds that developed in the mid-1970s following a long interval of relatively weak zonal winds between 1954 and 1972. Following the strengthening of the westerlies in this model, the cyclonic Weddell gyre intensifies, raising relatively warm Weddell Sea Deep Water to the surface. The raised warm water then melts sea ice or prevents it from forming to produce the Weddell Polynya. Within the polynya, large heat loss to the air causes surface water to become cold and sink to the bottom via open-ocean deep convection. Thus, the underlying layers cool down, the warm water supply to the surface eventually stops, and the polynya cannot be maintained anymore. During the 100-yr-long model simulation, two Weddell Polynya events are observed. The second one occurs a few years after the first one disappears; it is much weaker and persists for less time than the first one because the underlying layer is cooler. Based on these model simulations, the authors hypothesize that the Weddell Polynya and open-ocean deep convection were responses to the stronger SH westerlies that followed a prolonged weak phase of the southern annular mode.

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