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- Author or Editor: James C. Mc Williams x
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Abstract
An idealized coupled ocean–atmosphere is constructed to study climatic equilibria and variability. The model focuses on the role of large-scale fluid motions in the climate system. The atmospheric component is an eddy-resolving two-level global primitive equation model with simplified physical parameterizations. The oceanic component is a zonally averaged sector model of the thermohaline circulation. The two components exchange heat and freshwater fluxes synchronously. Coupled integrations are carried out over periods of several centuries to identify the equilibrium states of the ocean–atmosphere system. It is shown that there exist at least three types of equilibria, which are distinguished by whether they have upwelling or downwelling in the polar regions. Each of the coupled equilibria has a close analog in the ocean-only model with mixed boundary conditions. The oceanic circulation in the coupled model exhibits natural variability on interdecadal and longer timescales. The dominant interdecadal mode of variability is associated with the advection of oceanic temperature anomalies in the sinking regions. The sensitivity of the coupled model to climatic perturbations is studied. A rapid increase in the greenhouse gas concentrations leads to a collapse of the meridional overturning in the ocean. Introduction of a large positive surface freshwater anomaly in the high latitudes leads to a temporary suppression of the sinking motion, followed by a rapid recovery, due primarily to the high latitude cooling associated with the reduction of oceanic heat transport. In this evolution, the secondary roles played by the atmospheric heat transport and moisture transport in destabilizing the thermohaline circulation are compared, and the former is found to be dominant.
Abstract
An idealized coupled ocean–atmosphere is constructed to study climatic equilibria and variability. The model focuses on the role of large-scale fluid motions in the climate system. The atmospheric component is an eddy-resolving two-level global primitive equation model with simplified physical parameterizations. The oceanic component is a zonally averaged sector model of the thermohaline circulation. The two components exchange heat and freshwater fluxes synchronously. Coupled integrations are carried out over periods of several centuries to identify the equilibrium states of the ocean–atmosphere system. It is shown that there exist at least three types of equilibria, which are distinguished by whether they have upwelling or downwelling in the polar regions. Each of the coupled equilibria has a close analog in the ocean-only model with mixed boundary conditions. The oceanic circulation in the coupled model exhibits natural variability on interdecadal and longer timescales. The dominant interdecadal mode of variability is associated with the advection of oceanic temperature anomalies in the sinking regions. The sensitivity of the coupled model to climatic perturbations is studied. A rapid increase in the greenhouse gas concentrations leads to a collapse of the meridional overturning in the ocean. Introduction of a large positive surface freshwater anomaly in the high latitudes leads to a temporary suppression of the sinking motion, followed by a rapid recovery, due primarily to the high latitude cooling associated with the reduction of oceanic heat transport. In this evolution, the secondary roles played by the atmospheric heat transport and moisture transport in destabilizing the thermohaline circulation are compared, and the former is found to be dominant.
Abstract
The isopycnal transport parameterization of Gent and Mc Williams has been implemented in the GFDL ocean general circulation model, replacing the physically unjustifiable horizontal mixing of tracers. The effects of this parameterization are investigated in a global domain. A comparison of its results with those of the conventional horizontal diffusion shows substantial and significant improvements in several climatically important aspects of the ocean circulation. These improvements include a sharper main thermocline, cooler abyssal ocean, elimination of the Deacon cell as a tracer transport agent, zonally integrated meridional heat transport and surface heat fluxes in better agreement with observations, and better confinement of the locations where deep convection occurs. The sensitivity of the model to the magnitude of the horizontal and isopycnal diffusion coefficients is also studied, showing that the domain averages of potential temperature and salinity, the mass transport of the Antarctic Circumpolar Current, and the meridional mass transport in the Northern Hemisphere all increase with decreasing diffusivity. The northward heat and freshwater transports also reveal significant sensitivities to these coefficients. In addition, the effects of spatially and temporally varying transport and of unequal values of the eddy-induced transport and isopycnal diffusion coefficients are examined.
Abstract
The isopycnal transport parameterization of Gent and Mc Williams has been implemented in the GFDL ocean general circulation model, replacing the physically unjustifiable horizontal mixing of tracers. The effects of this parameterization are investigated in a global domain. A comparison of its results with those of the conventional horizontal diffusion shows substantial and significant improvements in several climatically important aspects of the ocean circulation. These improvements include a sharper main thermocline, cooler abyssal ocean, elimination of the Deacon cell as a tracer transport agent, zonally integrated meridional heat transport and surface heat fluxes in better agreement with observations, and better confinement of the locations where deep convection occurs. The sensitivity of the model to the magnitude of the horizontal and isopycnal diffusion coefficients is also studied, showing that the domain averages of potential temperature and salinity, the mass transport of the Antarctic Circumpolar Current, and the meridional mass transport in the Northern Hemisphere all increase with decreasing diffusivity. The northward heat and freshwater transports also reveal significant sensitivities to these coefficients. In addition, the effects of spatially and temporally varying transport and of unequal values of the eddy-induced transport and isopycnal diffusion coefficients are examined.
