Stability Of Thermohaline Circulation Research Articles

Rates of thermohaline recovery from freshwater pulses in modern, Last Glacial Maximum, and greenhouse warming climates

Recovery rates of the thermohaline circulation after a freshwater pulse in the North Atlantic vary considerably depending on the background climate, as demonstrated in the Community Climate System Model. The recovery is slowest in a Last Glacial Maximum (LGM) climate, fastest in a modern climate, and intermediate between the two in a greenhouse warming (4XCO2) climate. Previously proposed mechanisms to explain thermohaline circulation stability involving altered horizontal freshwater transport in the North Atlantic are consistent with relative recovery rates in the modern and 4XCO2 climates, but fail to explain the slow LGM recovery. Instead, sea ice expansion inhibits deep‐water formation after freshening in the LGM climate by reducing heat loss to the atmosphere and providing additional surface freshwater. In addition, anomalous vertical freshwater transport across ∼1 km depth after freshening is most effective at weakening the stratification in the modern case but is negligible in the LGM case.

Stability of the thermohaline circulation under millennial CO2 forcing and two alternative controls on Atlantic salinity

A large ensemble of experiments with an efficient climate model is carried out to examine stability of the oceanic thermohaline circulation (THC) as a function of two key processes that maintain high Atlantic salinities: the “Atmospheric Bridge” by which moisture is exported from the Atlantic to the Pacific; and “Agulhas Leakage” of salty Indian Ocean waters into the Atlantic. We find that irreversible THC collapse during the next millennium is five times more likely if Agulhas Leakage dominates over the Atmospheric Bridge. This finding is consistent with freshwater import to the Atlantic sector associated with the overturning circulation, when the Atmospheric Bridge dominates. In contrast, slight freshwater export is associated with the overturning circulation under strong Agulhas Leakage, helping to maintain higher Atlantic salinity. Predictions of future climate change therefore critically depend on better understanding of the relative importance of the Atmospheric Bridge and Agulhas Leakage.

Passive mechanism of decadal variation of thermohaline circulation

The decadal variation of thermohaline circulation (THC) is investigated in a simple coupled ocean‐atmospheric two‐box model with the new approach of conditional nonlinear optimal perturbation (CNOP). First, the nonlinear optimal initial perturbations with different constrains are found by this approach. There are two different types of perturbations in the nonlinear regime. One is the freshwater flux perturbation, which is the CNOP of THC and has stronger amplification. The other is salinity flux perturbation, whose amplification is weaker. Freshwater (salt) perturbations weaken (enhance) the mean circulation and hence weaken (enhance) the stability of THC. Second, the passive variabilities of THC are investigated by superposing the initial perturbations to the thermohaline circulation. The passive variabilities found in this model are due to nonnormal and nonlinear growth of initial perturbations. These variabilities, measured as recovering time of perturbations, can cause decadal variability of THC. The results of this paper suggest that CNOP approach is applicable to the investigation of the dependence of the THC sensitivity on the background climate state and is of potential application to the interpretation of past climate change linked to THC variations in nonlinear regime.

Thermohaline Circulation Stability: A Box Model Study. Part II: Coupled Atmosphere–Ocean Model

Abstract A thorough analysis of the stability of a coupled version of an interhemispheric three-box model of thermohaline circulation (THC) is presented. This study follows a similarly structured analysis of an uncoupled version of the same model presented in Part I of this paper. The model consists of a northern high-latitude box, a tropical box, and a southern high-latitude box, which can be thought of as corresponding to the northern, tropical, and southern Atlantic Ocean, respectively. This paper examines how the strength of THC changes when the system undergoes forcings representing global warming conditions. Since a coupled model is used, a direct representation of the radiative forcing is possible because the main atmospheric physical processes responsible for freshwater and heat fluxes are formulated separately. Each perturbation to the initial equilibrium is characterized by the total radiative forcing realized, by the rate of increase, and by the north–south asymmetry. Although only weakly asymmetric or symmetric radiative forcings are representative of physically reasonable conditions, general asymmetric forcings are considered in order to get a more complete picture of the mathematical properties of the system. The choice of suitably defined metrics makes it possible to determine the boundary dividing the set of radiative forcing scenarios that lead the system to equilibria characterized by a THC pattern similar to the present one, from those that drive the system to equilibria where the THC is reversed. This paper also considers different choices for the atmospheric transport parameterizations and for the ratio between the high-latitude and tropical radiative forcing. It is generally found that fast forcings are more effective than slow forcings in disrupting the present THC pattern, forcings that are stronger in the northern box are also more effective in destabilizing the system, and very slow forcings do not destabilize the system whatever their asymmetry, unless the radiative forcings are very asymmetric and the atmospheric transport is a relatively weak function of the meridional temperature gradient. In this latter case some relevant hysteresis graphs of the system are presented. The changes in the strength of the THC are primarily forced by changes in the latent heat transport in the hemisphere because of its sensitivity to temperature, which arises from the Clausius–Clapeyron relation.

Thermohaline Circulation Stability: A Box Model Study. Part I: Uncoupled Model

Abstract A thorough analysis of the stability of the uncoupled Rooth interhemispheric three-box model of thermohaline circulation (THC) is presented. The model consists of a northern high-latitude box, a tropical box, and a southern high-latitude box, which correspond to the northern, tropical, and southern Atlantic Ocean, respectively. Restoring boundary conditions are adopted for the temperature variables, and flux boundary conditions are adopted for the salinity variables. This paper examines how the strength of THC changes when the system undergoes forcings that are analogous to those of global warming conditions by applying the equilibrium state perturbations to the moisture and heat fluxes into the three boxes. In each class of experiments, using suitably defined metrics, the authors determine the boundary dividing the set of forcing scenarios that lead the system to equilibria characterized by a THC pattern similar to the present one from those that drive the system to equilibria with a reversed THC. Fast increases in the moisture flux into the northern high-latitude box are more effective than slow increases in leading the THC to a breakdown, while the increases of moisture flux into the southern high-latitude box strongly inhibit the breakdown and can prevent it, as in the case of slow increases in the Northern Hemisphere. High rates of heat flux increase in the Northern Hemisphere destabilize the system more effectively than low ones; increases in the heat fluxes in the Southern Hemisphere tend to stabilize the system.

The Sensitivity and Stability of the Ocean's Thermohaline Circulation to Finite-Amplitude Perturbations

Abstract Within a simple model context, the sensitivity and stability of the thermohaline circulation to finite-amplitude perturbations are studied. A new approach is used to tackle this nonlinear problem. The method is based on the computation of the so-called conditional nonlinear optimal perturbation (CNOP), which is a nonlinear generalization of the linear singular vector approach (LSV). It is shown that linearly stable thermohaline circulation states can become nonlinearly unstable, and the properties of the perturbations with optimal nonlinear growth are determined. An asymmetric nonlinear response to perturbations exists with respect to the sign of finite-amplitude freshwater perturbations, on both thermally dominated and salinity-dominated thermohaline flows. This asymmetry is due to the nonlinear interaction of the perturbations through advective processes.

Global warming and thermohaline circulation stability.

The Atlantic thermohaline circulation (THC) plays an important role in global climate. Theoretical and palaeoclimatic evidence points to the possibility of rapid changes in the strength of the THC, including a possible quasi-permanent shutdown. The climatic impacts of such a shutdown would be severe, including a cooling throughout the Northern Hemisphere, which in some regions is greater in magnitude than the changes expected from global warming in the next 50 years. Other climatic impacts would likely include a severe alteration of rainfall patterns in the tropics, the Indian subcontinent and Europe. Modelling the future behaviour of the THC focuses on two key questions. (i) Is a gradual weakening of the THC likely in response to global warming, and if so by how much? (ii) Are there thresholds beyond which rapid or irreversible changes in the THC are likely? Most projections of the response of the THC to increasing concentrations of greenhouse gases suggest a gradual weakening over the twenty-first century. However, there is a wide variation between different models over the size of the weakening. Rapid or irreversible THC shutdown is considered a low-probability (but high-impact) outcome; however, some climate models of intermediate complexity do show the possibility of such events. The question of the future of the THC is beset with conceptual, modelling and observational uncertainties, but some current and planned projects show promise to make substantial progress in tackling these uncertainties in future.

The Stabilization of the Thermohaline Circulation by the Temperature–Precipitation Feedback

Abstract The meridional freshwater flux in the atmosphere, and hence the strength of the hydrological cycle, undergoes variations on glacial–interglacial as well as on some shorter timescales. A significant portion of these changes to the hydrological cycle are due to the temperature–precipitation feedback according to which there is more precipitation over the higher latitudes during warm periods when the moisture holding capacity of the atmosphere is higher. It is proposed here that this feedback may play an important role in determining the stability of the thermohaline circulation (THC). The THC stability to different parameterizations of the meridional atmospheric freshwater flux is therefore investigated using a simple box model of the ocean, atmosphere, and sea ice. It is demonstrated that parameterizations that are consistent with the temperature–precipitation feedback, and hence with the observed variations of the hydrological cycle during glacial–interglacial cycles, stabilize the THC for a wide range of forcing parameters.

The Stability of Thermohaline Circulation in a Two-Box Model*

Abstract In Stommel’s simple two-box model, which has provided an insight on the thermohaline circulation and climate instability mechanisms, a linear mass transport was used. However, a scaling law based on geostrophy and advective–diffusive heat balance suggests a nonlinear mass transport relation for the oceans. By including this nonlinear mass transport relation to Stommel’s box model, it is possible to study the effects of the thermocline, which was not considered before, on the stability of the thermohaline circulation while keeping the simplicity of Stommel’s box model. The results were compared with those obtained with the traditional model using a linear mass transport relation. The thermal mode circulation of the nonlinear model is significantly more stable than that of the linear model, suggesting the thermohaline catastrophe is less likely to occur in the present North Atlantic if the thermocline is considered. In the nonlinear model, the circulation removes density anomalies rapidly so that s...

Stability of the interhemispheric thermohaline circulation in a coupled box model

Abstract The coupled atmosphere–ocean box model of the interhemispheric thermohaline circulation (THC) formulated by Scott et al. [Scott, J.R., Marotzke, J., Stone, P.H., 1999. Interhemispheric THC in a coupled box model. J. Phys. Oceanogr., 29, 351–365.] is solved analytically, by introducing the approximation that the time variations of salinity in the ocean are much slower than the time variations in the temperature. The analytic solution shows that there is an unstable limit cycle near the bifurcation where the flow becomes unstable, as suggested by Scott et al.'s numerical solutions. The solution also leads to an analytic expression for the conditions under which the instability discovered by Scott et al. sets in, which is more general than that found by Scott et al. In particular, it includes the effect of coupling the THC to the atmospheric meridional transports of heat and moisture. It shows that the stability of THC is much more sensitive to the representation of the atmospheric heat transport, i.e., to how it depends on the meridional temperature gradient, than it is in hemispheric models. In particular, it shows that interhemispheric ocean models that use mixed boundary conditions, or couple the ocean to a diffusive representation of the atmospheric heat transport, are less susceptible to this kind of instability than when the ocean is coupled to a representation of the atmospheric meridional heat transport which is more sensitive to the meridional temperature gradient, as is implied by observations and theory.

The stability of the North Atlantic thermohaline circulation in a coupled ocean-atmosphere general circulation model

The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input of 0.625 Sv and then reduced again to 0 (both instantaneously and linearly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient, the intertropical convergence zone over the Atlantic is displaced southward and the westerlies in the Northern Hemisphere gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence times of the surface water in high-northern latitudes, which allows them to accumulate more precipitation and runoff from the continents. As a consequence the stratification in the North Atlantic becomes more stable. This effect is further amplified by an enhanced northward atmospheric water vapour transport, which increases the freshwater input into the North Atlantic. The reduced northward oceanic heat transport leads to colder sea-surface temperatures and an intensification of the atmospheric cyclonic circulation over the Norwegian Sea. The associated Ekman transports cause increased upwelling and increased freshwater export with the East Greenland Current. Both the cooling and the wind-driven circulation changes largely compensate for the effects of the first two feedbacks. The wind-stress feedback destabilizes modes without deep water formation in the North Atlantic, but has been neglected in almost all studies so far. After the meltwater input stops, the North Atlantic deepwater formation resumed in all experiments and the meridional overturning returned within 200 years to a conveyor belt pattern. This happened although the formation of North Atlantic deep water was suppressed in one experiment for more than 300 years and the Atlantic overturning had settled into a circulation pattern with Antarctic bottom water as the only source of deep water. It is a clear indication that cooling and wind-stress feedback are more effective, at least in our model, than advection feedback and increased atmospheric water vapour transport. We conclude that the conveyor belt-type thermohaline circulation seems to be much more stable than hitherto assumed from experiments with simpler models.