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A variety of observational and modeling studies show that changes in the Atlantic meridional overturning circulation (AMOC) can induce rapid global-scale climate change. In particular, a substantially weakened AMOC leads to a southward shift of the intertropical convergence zone (ITCZ) in both the Atlantic and the Pacific Oceans. However, the simulated amplitudes of the AMOC-induced tropical climate change differ substantially among different models. In this paper, the sensitivity to cloud feedback of the climate response to a change in the AMOC is studied using a coupled ocean–atmosphere model [the GFDL Coupled Model, version 2.1 (CM2.1)]. Without cloud feedback, the simulated AMOC-induced climate change in this model is weakened substantially. Low-cloud feedback has a strong amplifying impact on the tropical ITCZ shift in this model, whereas the effects of high-cloud feedback are weaker. It is concluded that cloud feedback is an important contributor to the uncertainty in the global response to AMOC changes. [ABSTRACT FROM AUTHOR]

This paper focuses on the abrupt increase in the oceanic and atmospheric temperature in the Northern Hemisphere at the beginning of the Holocene, approximately 11 000 yr before the present. De Boer and Nof hypothesized that, at that time, the Bering Strait (BS) opened up abruptly because of the breakup of an ice dam (by rising sea levels). It is proposed further here that this sudden opening caused an abrupt increase in the mean temperature of the Northern Hemisphere. An analytical, coupled ocean–atmosphere model is applied to the North Atlantic in an attempt to quantify the temperature change resulting from the opening of the BS. Heat, salt, and mass are all conserved within a box in the North Atlantic. A convection condition allows water to enter the deep layer and the island rule relates the wind field to the mass fluxes. The conventional approach that the meridional overturning cell (MOC) was not operating during the Younger Dryas because of an overwhelming freshwater flux is adopted here. Opening the BS in the early Holocene allowed these freshwater anomalies to be flushed out into the Pacific, reviving convection and the transport of heat northward. Restarting convection with an open BS increases mean oceanic and atmospheric temperature by 3° and 23°C, respectively. These values are comparable to those found in both the Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE) alkenone and Greenland Ice Sheet Project 2 (GISP 2) ice core records. Of course, restarting convection increases the temperature even with a closed BS, but in the closed BS case the oceanic increase is slightly higher (4°C instead of 3°C), whereas the atmospheric is much lower (17.5°C instead of 23°C). This is because, by requiring a continuous sea level around the Americas, an open BS allows the wind field to limit the amount of Southern Ocean water that enters the South Atlantic. This controlled volume flux (8 Sv) is considerably smaller than that allowed into the Atlantic in the no-wind control closed BS case (17 Sv). [ABSTRACT FROM AUTHOR]

This review paper discusses the physical basis and the potential for decadal climate predictability over the Atlantic and its adjacent land areas. Many observational and modeling studies describe pronounced decadal and multidecadal variability in the Atlantic Ocean. However, it still needs to be quantified to which extent the variations in the ocean drive variations in the atmosphere and over land. In particular, although a clear impact of the Tropics on the midlatitudes has been demonstrated, it is unclear if and how the extratropical atmosphere responds to midlatitudinal sea surface temperature anomalies. Although the mechanisms behind the decadal to multidecadal variability in the Atlantic sector are still controversial, there is some consensus that some of the longer-term multidecadal variability is driven by variations in the thermohaline circulation. The variations in the North Atlantic thermohaline circulation appear to be predictable one to two decades ahead, as shown by a number of perfect model predictability experiments. The next few decades will be dominated by these multidecadal variations, although the effects of anthropogenic climate change are likely to introduce trends. Some impact of the variations of the thermohaline circulation on the atmosphere has been demonstrated in some studies so that useful decadal predictions with economic benefit may be possible. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]