A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO 2 concentration (original) (raw)
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Geophysical Research Letters, 2001
We discuss aspects of the Atlantic thermohaline circulation (THC) and its response to increased greenhouse gas concentration, using a coupled atmosphere-ocean general circulation model (AOGCM) whose oceanic component is a new hybrid-isopycnal model. Two 200-year model integrations are carried out-a control run assuming fixed atmospheric composition and a perturbation run assuming gradual doubling of CO2. We employ no flux corrections at the air-sea interface, nor do we spin up the ocean prior to coupling. The surface conditions in the control run stabilize after several decades. When doubling CO2 at the rate of 1% per year, the model responds with a 2 • C increase in global mean surface air temperature (SAT) after 200 years and a virtually unchanged Atlantic meridional overturning circulation. The latter is maintained by a salinity increase that counteracts the effect of global warming on the surface buoyancy.
Journal of Climate, 2001
Models of the North Atlantic thermohaline circulation (THC) show a range of responses to the high-latitude warming and freshening characteristic of global warming scenarios. Most simulate a weakening of the THC, with some suggesting possible interruption of the circulation, but others exhibit little change. The mechanisms of the THC response to climate change using the HadCM3 coupled ocean-atmosphere general circulation model, which gives a good simulation of the present-day THC and does not require flux adjustment, were studied. In a range of climate change simulations, the strength of the THC in HadCM3 is proportional to the meridional gradient of steric height (equivalent to column-integrated density) between 30ЊS and 60ЊN. During an integration in which CO 2 increases at 2% per year for 70 yr, the THC weakens by about 20%, and it stabilizes at this level if the CO 2 is subsequently held constant. Changes in surface heat and water fluxes are the cause of the reduction in the steric height gradient that derives the THC weakening, 60% being due to temperature change (greater warming at high latitudes) and 40% to salinity change (decreasing at high latitude, increasing at low latitude). The level at which the THC stabilizes is determined by advective feedbacks. As the circulation slows down, less heat is advected northward, which counteracts the in situ warming. At the same time, northward salinity advection increases because of a strong increase in salinity in the subtropical Atlantic, due to a greater atmospheric export of freshwater from the Atlantic to the Pacific. This change in interbasin transport means that salinity effects stabilize the circulation, in contrast to a single basin model of the THC, where salinity effects are destabilizing. These results suggest that the response of the Atlantic THC to anthropogenic forcing may be partly determined by events occurring outside the Atlantic basin.
1] Most climate models predict a weakening of the North Atlantic thermohaline circulation for the 21st century when forced by increasing levels of greenhouse gas concentrations. The model spread, however, is rather large, even when the forcing scenario is identical, indicating a large uncertainty in the response to forcing. In order to reduce the model uncertainties a weighting procedure is applied considering the skill of each model in simulating hydrographic properties and observation-based circulation estimates. This procedure yields a ''best estimate'' for the evolution of the North Atlantic THC during the 21st century by taking into account a measure of model quality. Using 28 projections from 9 different coupled global climate models of a scenario of future CO 2 increase (SRESA1B) performed for the upcoming fourth assessment report of the Intergovernmental Panel on Climate Change, the analysis predicts a gradual weakening of the North Atlantic THC by 25(±25)% until 2100. Citation: Schmittner, A., M. Latif, and B. Schneider (2005), Model projections of the North Atlantic thermohaline circulation for the 21st century assessed by observations, Geophys. Res. Lett., 32, L23710,
Journal of Climate, 2007
Two coupled atmosphere-ocean general circulation models developed at GFDL show differing stability properties of the Atlantic thermohaline circulation (THC) in the Coupled Model Intercomparison Project/ Paleoclimate Modeling Intercomparison Project (CMIP/PMIP) coordinated "water-hosing" experiment. In contrast to the R30 model in which the "off" state of the THC is stable, it is unstable in the CM2.1. This discrepancy has also been found among other climate models. Here a comprehensive analysis is performed to investigate the causes for the differing behaviors of the THC. In agreement with previous work, it is found that the different stability of the THC is closely related to the simulation of a reversed thermohaline circulation (RTHC) and the atmospheric feedback. After the shutdown of the THC, the RTHC is well developed and stable in R30. It transports freshwater into the subtropical North Atlantic, preventing the recovery of the salinity and stabilizing the off mode of the THC. The flux adjustment is a large term in the water budget of the Atlantic Ocean. In contrast, the RTHC is weak and unstable in CM2.1. The atmospheric feedback associated with the southward shift of the Atlantic ITCZ is much more significant. The oceanic freshwater convergence into the subtropical North Atlantic cannot completely compensate for the evaporation, leading to the recovery of the THC in CM2.1. The rapid salinity recovery in the subtropical North Atlantic excites large-scale baroclinic eddies, which propagate northward into the Nordic seas and Irminger Sea. As the large-scale eddies reach the high latitudes of the North Atlantic, the oceanic deep convection restarts. The differences in the southward propagation of the salinity and temperature anomalies from the hosing perturbation region in R30 and CM2.1, and associated different development of a reversed meridional density gradient in the upper South Atlantic, are the cause of the differences in the behavior of the RTHC. The present study sheds light on important physical and dynamical processes in simulating the dynamical behavior of the THC.
Diagnostics of the oceanic thermohaline circulation in a coupled climate model
Global and Planetary Change, 2004
Two century-scale integrations of a global coupled model consisting of the GISS atmospheric model and the HYCOM ocean model are carried out: a control run assuming fixed atmospheric composition, and a perturbation run assuming gradual doubling of CO 2. The model does not use flux corrections at the air-sea interface, nor is the ocean ''spun up'' prior to coupling. When increasing CO 2 at the rate of 1% per year to twice its original level and keeping it constant thereafter, the model responds with a 2 jC increase in 200 years in the global mean surface air temperature and a virtually unchanged Atlantic meridional overturning circulation (MOC). Due to the predominantly isopycnic character of HYCOM, geographic details of the 3-D thermohaline circulation in potential density space can be extracted from the model output with relative ease. The analysis confirms that even regional details of the MOC in this experiment are rather insensitive to the climate change brought on by CO 2 doubling. Furthermore, the analysis reveals strong similarities between the simulated and the observed MOC.
A low-order model for the response of the Atlantic thermohaline circulation to climate change
Ocean Dynamics, 2004
Concerns have been raised that anthropogenic climate change may lead to a slowdown or even collapse of the Atlantic thermohaline circulation (THC). Because of the possibly severe consequences that such an event could have on the northern North Atlantic and northwestern Europe, Integrated Assessment M o dels (IAMs) are needed to explore the associated political and socioeconomic implications. State-of-the-art climate models representing the THC are, however, often too complex to be incorporated into integrated assessment frameworks. In this paper we present a l o w-order model of the Atlantic THC which meets the main requirements of IAMs: it a) is physically based, b) is computationally highly e cient, c) allows for comprehensive uncertainty analysis, and d) can be linked to globally aggregated climate models that are mostly used in IAMs. The model is an interhemispheric extension of the seminal Stommel model. Its parameters are determined by a least square t to the output of a coupled climate model of intermediate complexity. Results of a number of transient global warming simulations indicate that the model is able to reproduce many features of the behavior of coupled ocean-atmosphere circulation models such as the sensitivity of the THC to the amount, regional distribution and rate of climate change. Key words Thermohaline Circulation { Box Model { Climate Change { Sensitivity Analysis 1 I n troduction The Atlantic thermohaline circulation (THC) transports large amounts of heat northward (up to 10 15 W), contributing a major part to the heat budget of the North Atlantic region (Ganachaud and Wunsch 2000). Paleoreconstructions (Dansgaard et al. 1993), theoretical con
Climate Dynamics, 2004
Different climate models simulate different behavior of the Atlantic meridional overturning circulation (MOC) under the same global warming scenario. We propose a plausible explanation for this and argue that a proper simulation of the present-day climate in the subpolar North Atlantic is important. This is illustrated using results from idealized global warming experiments, in which both the radiative forcing scenario and the model employed are the same, with the only major difference being the initial subpolar North Atlantic climate. The initial conditions are made progressively colder, with more extensive sea-ice cover in the northern North Atlantic.The key result is that starting from conditions which are too cold in the North Atlantic and with sea-ice that is too extensive leads to an MOC that is more stable to the radiative forcing. Furthermore, under considerably underestimated sea surface temperatures in subpolar regions, the MOC can even intensify. A reduction of freshwater flux associated with the reduction of sea-ice melt is shown to be important for such unusual behavior of the MOC. Other mechanisms are also considered, but not deemed as important in explaining published inter-model differences.
The North Atlantic thermohaline circulation simulated by the GISS climate model during 1970–99
Atmosphere-Ocean, 2007
Evidence based on numerical simulations is presented for a strong correlation between the North Atlantic Oscillation (NAO) and the North Atlantic overturning circulation. Using an ensemble of numerical experiments with a coupled ocean-atmosphere model including both natural and anthropogenic forcings, it is shown that the weakening of the thermohaline circulation (THC) could be delayed in response to a sustained upward trend in the NAO, which was observed over the last three decades of the twentieth century, 1970-99. Overall warming and enhanced horizontal transports of heat from the tropics to the subpolar North Atlantic overwhelm the NAO-induced cooling of the upper ocean layers due to enhanced fluxes of latent and sensible heat, so that the net effect of warmed surface ocean temperatures acts to increase the vertical stability of the ocean column. However, the strong westerly winds cause increased evaporation from the ocean surface, which leads to a reduced fresh water flux over the western part of the North Atlantic. Horizontal poleward transport of salinity anomalies from the tropical Atlantic is the major contributor to the increasing salinities in the sinking regions of the North Atlantic. The effect of positive salinity anomalies on surface ocean density overrides the opposing effect of enhanced warming of the ocean surface, which causes an increase in surface density in the Labrador Sea and in the ocean area south of Greenland. The increased density of the upper ocean layer leads to deeper convection in the Labrador Sea and in the western North Atlantic. With a lag of four years, the meridional overturning circulation of the North Atlantic shows strengthening as it adjusts to positive density anomalies and enhanced vertical mixing. During the positive NAO trend, the salinity-driven density instability in the upper ocean, due to both increased northward ocean transports of salinity and decreased atmospheric freshwater fluxes, results in a strengthening overturning circulation in the North Atlantic when the surface atmospheric temperature increases by 0.3°C and the ocean surface temperature warms by 0.5°to 1°C. RÉSUMÉ [Traduit par la rédaction] Nous présentons des preuves basées sur des simulations numériques d'une forte corrélation entre l'oscillation nord-atlantique (ONA) et la circulation de renversement dans l'Atlantique Nord. Au moyen d'un ensemble d'expériences numériques avec un modèle couplé océan-atmosphère qui inclut à la fois les forçages naturel et anthropique, nous montrons que l'affaiblissement de la circulation thermohaline pourrait être retardé par suite d'une tendance à la hausse soutenue dans l'ONA, tendance qui a été observée au cours des trois dernières décennies du vingtième siècle, soit la période 1970-1999. Le réchauffement général et l'augmentation des transports horizontaux de chaleur des tropiques vers l'Atlantique Nord subpolaire font plus que compenser le refroidissement causé par l'ONA des couches supérieures de l'océan à cause de l'augmentation des flux de chaleur latente et sensible, de sorte que l'effet net des températures plus élevées de la surface de l'océan fait augmenter la stabilité verticale de la colonne océanique. Cependant, les forts vents d'ouest entraînent une évaporation accrue à la surface de l'océan, ce qui mène à un flux d'eau douce réduit dans la partie ouest de l'Atlantique Nord. Le transport horizontal vers le pôle des anomalies de salinité à partir de l'Atlantique tropical est le principal contributeur de l'accroissement de salinité dans les régions de plongée d'eau de l'Atlantique Nord. L'effet des anomalies positives de salinité sur la densité de la surface océanique annule l'effet opposé du réchauffement accru de la surface océanique, ce qui entraîne un accroissement de la densité à la surface dans la mer du Labrador et dans la région de l'océan au sud du Groenland. La densité accrue des couches supérieures de l'océan occasionne une convection plus profonde dans la mer du Labrador et dans l'ouest de l'Atlantique Nord. Avec un retard de quatre ans, la circulation de renversement méridienne dans l'Atlantique Nord se renforce en réponse aux anomalies de densité positives et au mélange vertical accru.