Glacial thermohaline circulation and climate: Forcing from the north or south?
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Forcing (mathematics)
Atlantic Equatorial mode
Observational analyses suggest that the northern North Atlantic is fresher in the late 20th century in comparison to the 1950–60s. Here we examine possible contributors to these observed changes by analyzing a four‐member ensemble of 20th century simulations from a coupled climate model. Results show that a weaker meridional freshwater divergence related to a weaker thermohaline circulation in the North Atlantic is the primary cause for the freshening in the northern North Atlantic in the model. The increased P‐E tends to enforce it, but the reduced sea ice flux into this region tends to weaken it.
Atlantic Equatorial mode
Tropical Atlantic
Gulf Stream
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The connection between water vapor transport and the thermohaline circulation is examined with a simple global coupled ocean‐atmosphere energy‐salinity balance model (ESBM). Both latitudinal intrabasin and interbasin water vapor fluxes are considered. It is demonstrated that interbasin and intrabasin water vapor fluxes play interdependent and competitive roles in affecting the state of the thermohaline circulation. Increasing intrabasin water vapor flux in the North Atlantic, by decreasing water density in the high latitudes, decreases the North Atlantic deep water production and hence the thermohaline circulation, while increasing interbasin water vapor flux from the Atlantic to the Pacific, by increasing the mean density of the Atlantic and decreasing that of the Pacific, increases the strength of the global thermohaline circulation. The global thermohaline circulation and its asymmetry are sensitive to the latitudinal hydrological cycle in the North Atlantic because of the large water vapor flux from the Atlantic to Pacific Ocean. Global thermohaline circulation exhibits bimodal equilibria as a consequence of imbalances in rates of change of advective and eddy freshwater fluxes in the high‐latitude North Atlantic. One equilibrium mode resembles the modern ocean circulation with a strong global asymmetric thermohaline circulation associated with dominant deep water production in the North Atlantic and an effective “heat pump” operating in the Atlantic Ocean. In the other equilibrium, deep water is produced primarily in the Southern Ocean; in particular, North Atlantic deep water is replaced by Southern Ocean deep water. Deep water is produced in the North Pacific in this mode, but is, for reasonably large interbasin water vapor transport, insufficient to reverse the direction of deep water flow into the South Pacific. Based on estimated water vapor fluxes for the present climate, our study suggests that the present thermohaline circulation is dynamically stable, i.e., far from the critical regions of rapid transition between two modes.
Deep ocean water
Atlantic Equatorial mode
Circumpolar deep water
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Atlantic Equatorial mode
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Atlantic Equatorial mode
Tropical Atlantic
Intertropical Convergence Zone
Westerlies
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Atlantic Equatorial mode
Tropical Atlantic
Atlantic hurricane
Gulf Stream
Abrupt climate change
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Atlantic Equatorial mode
Gulf Stream
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Recent climate model experiments, as well as paleoclimate records, suggest that the meridional overturning circulation or “thermohaline circulation” in the Atlantic Ocean could change abruptly as a result of global warming, and that this could have a significant impact on European climate. We use a reduced‐gravity model to investigate the response of the Atlantic overturning circulation to changes in forcing. We find that variability at decadal and higher frequencies is confined to a single hemisphere. This implies that (a) overturning variability resulting from high frequency changes in buoyancy forcing in the Labrador and Greenland Seas will be limited to the North Atlantic, and (b) any observed decadal and higher frequency fluctuations in North Atlantic overturning can only result from changes in the surface fluxes within the North Atlantic basin itself. These results suggest that Southern Ocean wind forcing is not important for North Atlantic overturning on decadal and shorter timescales.
Atlantic Equatorial mode
Atlantic hurricane
Forcing (mathematics)
Abrupt climate change
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Circulation (fluid dynamics)
Forcing (mathematics)
Atlantic Equatorial mode
Mode (computer interface)
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Sea surface temperature (SST) observations in the North Atlantic indicate the existence of strong multidecadal variability with a unique spatial structure. It is shown by means of a new global climate model, which does not employ flux adjustments, that the multidecadal SST variability is closely related to variations in the North Atlantic thermohaline circulation (THC). The close correspondence between the North Atlantic SST and THC variabilities allows, in conjunction with the dynamical inertia of the THC, for the prediction of the slowly varying component of the North Atlantic climate system. It is shown additionally that past variations of the North Atlantic THC can be reconstructed from a simple North Atlantic SST index and that future, anthropogenically forced changes in the THC can be easily monitored by observing SSTs. The latter is confirmed by another state-of-the-art global climate model. Finally, the strong multidecadal variability may mask an anthropogenic signal in the North Atlantic for some decades.
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Gulf Stream
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