Description: Freshening of high latitude surface water in the North Atlantic can change the poleward oceanic transport of heat and salt with drastic effects on the global climate. The sensitivity of the thermohaline circulation is analyzed with respect to these perturbations. The study is based on coupled ocean-atmosphere (-sea ice) models with different levels of complexity in idealized geometries of the Atlantic ocean.An atmospheric energy balance model (EBM) is constructed which can predict the heat and fresh water fluxes at the surface. The response of the EBM to sea surface temperature anomalies and radiative forcing is consistent with complex atmospheric models.For a range of coupled models it is shown that the atmospheric transports affect the stability of the thermohaline circulation (THC). Coupled atmosphere EBM-ocean circulation model experiments show that the atmospheric heat transport is an important destabilizing effect while changes in fresh water flux are of minor importance for the THC. To understand the sensitivity of the THC for a range of atmospheric boundary conditions, a box model is designed, as it is considered the most simple atmosphere-ocean system. The analytical investigation shows how the stability of the THC is affected by the representation of the atmospheric transport of heat and moisture and the basic state.Depending on the meridional gradient in salinity, self-sustained oscillations do appear in a coupled atmosphere EBM-ocean circulation model caused by strong horizontal salinity gradients. It was found the the oscillatory state is more sensitive to perturbations than basic states with moderate meridional salinity gradients which is consistent with the analytical model.The sensitivity and feedback mechanisms affecting the THC are examined in a coupled ocean-atmosphere-sea ice system. The EBM is coupled with an ocean circulation model which includes a thermodynamic sea ice model. Due to a perturbation in high latitude salinity, the THC evolves into an other steady state with decreased atmospheric temperature, more sea ice, enhanced atmospheric heat transport, and decreased oceanic heat transport. The formation of intermediate water and cessation of deep convection in the northern North Atlantic is consistent with paleoclimatic findings of well documented climate shifts caused by a fresh water release.

Description: We investigate the sensitivity of the thermohaline circulation (THC) with respect to a subpolar salinity perturbation. Such perturbation simulates a fresh water release caused by retreating glaciers or anomalous sea ice. The feedback mechanisms amplifying or damping the initial anomaly are analyzed in the coupled ocean-atmosphere-sea ice model. Their understanding is essential for modelling climate variability on decadal and longer time scales.A 3-D ocean circulation model is coupled to an atmospheric energy balance and a thermodynamic sea ice model. The perturbation in the North Atlantic's subpolar salinity causes a cessation of deep convection and a climate state with decreased oceanic heat transport, decreased high latitude atmospheric temperature, and larger sea ice extent. The sea ice isolates the atmosphere from the warmer ocean reducing the heat flux and thus the vertical mixing in the ocean. This change in the local buoyancy flux is responsible for a reduced large-scale circulation. This change in the local buoyancy flux weakens the large scale circulation. High latitude cooling can not compensate for the freshening since the ocean temperature can not fall below the freezing point. Because deep convection is suppressed where sea ice is present, North Atlantic deep water formation is rather sensitive to the formation of sea ice. The insulating effect of sea ice is more important than its impact on salinity in our experiments. Different types of boundary conditions are used to isolate relevant feedback processes. The stability of the THC depends crucially on the atmospheric model component. Active atmospheric heat transport allows continued deep water formation because the sea ice margin is shifted poleward.You can find the model code for the EBM (Get the FORTRAN code of the model) . You may find also information in read.me.Reference StateMinimal overturning after 14 years .Development after the perturbation in the coupled model .Feedback mechanisms .

Description: In an analytical study the stability of the thermohaline circulation with respect to freshwater perturbations in high latitudes is investigated. The study is based on a coupled ocean and atmosphere box model in an idealized North Atlantic geometry. The box model provides a qualitative understanding of how the thermohaline circulation is affected by feedback mechanisms associated with changes in atmospheric transports of heat and moisture. Within a linear analysis we examine the stability of the thermohaline circulation for a range of different atmospheric boundary conditions. The stability of the coupled system depends on the imposed transport parameterizations and the basic state. For the underlying non-linear system we examine the sensitivity with respect to the strength of salinity perturbation.

Description: Changes in high latitude surface salinity have a strong effect on the North Atlantic Deep Water Formation (NADWF) which appears to be very important in driving the global thermohaline conveyor belt. Natural variations of sea surface salinity and sea ice have been observed in the North Atlantic, namely the Great Salt Anomaly (GSA) of the late sixties and seventies. When dealing with climate variability one must consider the sensitivity of the climate system to perturbations.In order to include the atmospheric heat transport mechanisms we coupled an atmosphere energy balance model with a 3-D ocean general circulation model which includes a thermodynamic sea ice model. We explore the feedback mechanisms in the ocean-atmosphere-sea ice system affecting the thermohaline circulation (THC) under perturbations in sea surface salinity at high latitudes.

Description: We analyze the sensitivity of the oceanic thermohaline circulation (THC) regarding perturbations in fresh water flux for a range of coupled oceanic general circulation - atmospheric energy balance models. The energy balance model (EBM) predicts surface air temperature and fresh water flux and contains the feedbacks due to meridional transports of sensible and latent heat. In the coupled system we examine a negative perturbation in run-off into the southern ocean and analyze the role of changed atmospheric heat transports and fresh water flux. With mixed boundary conditions (fixed air temperature and fixed surface fresh water fluxes) the response is characterised by a completely different oceanic heat transport than in the reference case. On the other hand, the surface heat flux remains roughly constant when the air temperature can adjust in a model where no anomalous atmospheric transports are allowed. This gives an artificially stable system with nearly unchanged oceanic heat transport. However, if meridional heat transports in the atmosphere are included, the sensitivity of the system lies between the two extreme cases. We find that changes in fresh water flux are unimportant for the THC in the coupled system.

Description: We present a simple, deterministic energy balance model in this report. The model is designed to represent the atmospheric component of the coupled atmosphere-ocean system. It is a one dimensional, global model with time and space resolutions of one year and 10^0 of latitude respectively. The model predicts the surface air temperature and estimates the surface freshwater flux diagnostically.The coupling between the atmospheric model and an ocean model is accomplished by heat and freshwater fluxes at their interface. The heat flux is calculated according to the difference in the surface air temperature and ocean surface temperature, while the freshwater flux is estimated from the latent heat transport in the atmosphere by a diagnostic equation since no explicit hydrologic cycle or water vapour budget is kept in the model. Two parameterizations for the latent heat transport are proposed, which distinguishes the two versions of the model. The assumptions made in the first version are that the total heat transport in the combined system is invariant and that the atmospheric sensible heat transport can be approximated by a diffusion process, whereas in the second version both atmospheric sensible heat and water vapour transport are treated as a diffusion process.Before proceeding with interactive runs, we study the behaviour of the model in a decoupled mode. Some experiments with initial conditions altered and external forcings changed are carried out to investigate the sensitivity and stability of the model. In particular, the influence of the ice-albedo feedback on model solutions is examined. The results of these experiments may be helpful both in understanding the characteristics of the model and in interpreting results when the model is coupled to an OGCM.2 x CO2 Experiment

Description: Palaeoclimatic records indicate several abrupt changes in North Atlantic climate which are assumed to be caused by disturbances of the thermohaline circulation (THC). By means of an idealized ocean box model we investigate the sensitivity of the THC with respect to a high-latitudinal salinity reduction, simulating a sudden meltwater release of glaciers or icebergs. We study the influence of various surface heat and freshwater flux parameterization schemes. By coupling an atmospheric energy balance model to the ocean model the importance of atmospheric heat and moisture transports in destabilizing the THC is demonstrated.Furthermore, the THC and its sensitivity under different climatic conditions is investigated. Due to the temperature-dependence of the thermal expansion coefficient the oceanic circulation weakens and becomes more vulnerable with decreasing global temperatures. The results indicate that during Ice Ages even relatively weak freshwater invasions might have caused considerable variations in the intensity of the THC, accompanied by severe cold snaps in high latitudes due to the weakened oceanic heat transport.Greenland ice cores and other climate records clearly indicate that various abrupt changes in the North Atlantic climate occured in the past. A well- known example is the Younger Dryas cold event, around 11,000 years BP. It is presumed that these changes are connected with freshwater invasions into the deep water formation areas of the northern North Atlantic, resulting in a weakening of the thermohaline-driven ocean circulation (THC) and consequently in a decreased oceanic poleward heat transport (Broecker, 1991). Paleoclimatic studys actually show relations between meltwater events, owing to retreating glaciers (Keigwin et al., 1991) or massive discharges of icebergs launched from Canada (Bond, 1995; Bond et al., 1992) and cold snaps in the North Atlantic region. Obviously, the sensitivity of the THC plays a key role for the climate variability and is the subject matter of the present paper.Several feedback mechanisms influence the sensitivity of the THC and either amplify (positive feedbacks) or weaken (negative feedbacks) an initial perturbation. Some of these feedback mechanisms, which are associated with heat and salt/freshwater transports in ocean and atmosphere and with outgoing longwave radiation from atmosphere to space, will be discussed in detail in this paper.