Mechanisms Determining the Atlantic Thermohaline Circulation Response to Greenhouse Gas Forcing in a Non-Flux-Adjusted Coupled Climate Model (original) (raw)
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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.
Geophysical Research Letters, 2005
In an experiment coordinated as part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic thermohaline circulation (THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the circulation strength declines gradually in all models, by between 10 and 50%. This weakening is consistent with the expected effect of reduced heat loss and increased net freshwater input in the north Atlantic. No model shows a rapid or complete collapse. The models having the strongest overturning in the control climate tend to show the largest THC reductions. Despite the reduced ocean heat transport, no model shows a cooling anywhere, because the greenhouse warming is dominant. In all the models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux.
Climate Dynamics, 2006
On the time scale of a century, the Atlantic thermohaline circulation (THC) is sensitive to the global surface salinity distribution. The advection of salinity toward the deep convection sites of the North Atlantic is one of the driving mechanisms for the THC. There is both a northward and a southward contributions. The northward salinity advection (Nsa) is related to the evaporation in the subtropics, and contributes to increased salinity in the convection sites. The southward salinity advection (Ssa) is related to the Arctic freshwater forcing and tends on the contrary to diminish salinity in the convection sites. The THC changes results from a delicate balance between these opposing mechanisms. In this study we evaluate these two effects using the IPSL-CM4 ocean-atmospheresea-ice coupled model (used for IPCC AR4). Perturbation experiments have been integrated for 100 years under modern insolation and trace gases. River runoff and evaporation minus precipitation are successively set to zero for the ocean during the coupling procedure. This allows the effect of processes Nsa and Ssa to be estimated with their specific time scales. It is shown that the convection sites in the North Atlantic exhibit various sensitivities to these processes. The Labrador Sea exhibits a dominant sensitivity to local forcing and Ssa with a typical time scale of 10 years, whereas the Irminger Sea is mostly sensitive to Nsa with a 15 year time scale. The GIN Seas respond to both effects with a time scale of 10 years for Ssa and 20 years for Nsa. It is concluded that, in the IPSL-CM4, the global freshwater forcing damps the THC on centennial time scales.
A Model of Atlantic Heat Content and Sea Level Change in Response to Thermohaline Forcing
Journal of Climate, 2011
The response of ocean heat content in the Atlantic to variability in the meridional overturning circulation (MOC) at high latitudes is investigated using a reduced-gravity model and the Massachusetts Institute of Technology (MIT) general circulation model (MITgcm). Consistent with theoretical predictions, the zonal-mean heat content anomalies are confined to low latitudes when the high-latitude MOC changes rapidly, but extends to mid- and high latitudes when the high-latitude MOC varies on decadal or multidecadal time scales. This low-pass-filtering effect of the mid- and high latitudes on zonal-mean heat content anomalies, termed here the “Rossby buffer,” is shown to be associated with the ratio of Rossby wave basin-crossing time to the forcing period at high northern latitudes. Experiments using the MITgcm also reveal the importance of advective spreading of cold water in the deep ocean, which is absent in the reduced-gravity model. Implications for monitoring ocean heat content a...
A Theory for the Surface Atlantic Response to Thermohaline Variability
Journal of Physical Oceanography, 2002
The response of the upper, warm limb of the thermohaline circulation in the North Atlantic to a rapid change in deep-water formation at high latitudes is investigated using a reduced-gravity ocean model. Changes in deepwater formation rate initiate Kelvin waves that propagate along the western boundary to the equator on a timescale of months. The response in the North Atlantic is therefore rapid. The Southern Hemisphere response is much slower, limited by a mechanism here termed the ''equatorial buffer.'' Since to leading order the flow is in geostrophic balance, the pressure anomaly decreases in magnitude as the Kelvin wave moves equatorward, where the Coriolis parameter is lower. Together with the lack of sustained pressure gradients along the eastern boundary, this limits the size of the pressure field response in the Southern Hemisphere. Interior adjustment is by the westward propagation of Rossby waves, but only a small fraction of the change in thermohaline circulation strength is communicated across the equator to the South Atlantic at any one time, introducing a much longer timescale into the system. A new quantitative theory is developed to explain this long-timescale adjustment. The theory relates the westward propagation of thermocline depth anomalies to the net meridional transport and leads to a ''delay equation'' in a single parameter-the thermocline depth on the eastern boundary-from which the time-varying circulation in the entire basin can be calculated. The theory agrees favorably with the numerical results. Implications for predictability, abrupt climate change, and the monitoring of thermohaline variability are discussed.
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.
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.