A theoretical framework for the description of the thermal state and circulation in the ocean is presented. Relations between differential heating, diffusive heat flux and surface drift are derived and discussed in the light of the basic mechanical property of the system, i.e. the tendency for light water to spread on top of heavier water. We find that for low and medium temperatures, the poleward surface drift can be determined directly from a knowledge of the heat flux through the sea surface. Preliminary quantitative estimates indicate that the "Hadley circulation" for the entire ocean involves a volume flux of order 70 f 30 Sverdrup. For the North Atlantic we find a poleward drift of order 10 Sverdrup which compares well with previous estimates of the southward deep flow. Mean values for the diffusive flux in the ocean are found to be of order 20 W m-2 at 15 "C and 60 W m-' at 25 OC.Tellus 34 (1982), 2 0040-2826/82/020187-09$02.50/0 0 1982 Munksgaard, Copenhagen

A theoretical framework for the description of the thermal state and circulation in the ocean is presented. Relations between differential heating, diffusive heat flux and surface drift are derived and discussed in the light of the basic mechanical property of the system, i.e. the tendency for light water to spread on top of heavier water. We find that for low and medium temperatures, the poleward surface drift can be determined directly from a knowledge of the heat flux through the sea surface. Preliminary quantitative estimates indicate that the “Hadley circulation” for the entire ocean involves a volume flux of order 70 ± 30 Sverdrup. For the North Atlantic we find a poleward drift of order 10 Sverdrup which compares well with previous estimates of the southward deep flow. Mean values for the diffusive flux in the ocean are found to be of order 20 W m−2 at 15 °C and 60 W m−2 at 25 °C.

The possibility that a decreased equator-to-Pole surface density difference could imply stronger rather than weaker thermohaline circulation (THC) is explored theoretically as well as with the aid of numerical simulations. The idea builds on the classical thermocline scaling, stating that the THC should increase with density difference as well as with vertical diffusivity. To explore possible changes in vertical diffusivity that would follow a change in the oceanic density difference, simple models of internal wave mixing are considered. For reasonable assumptions concerning the energy supply to vertical mixing, the overall diffusivity tends to increase with decreasing density difference. This enhancement of the vertical diffusivity acts to deepen the thermocline, an effect that can cause the THC to amplify despite that the surface density difference is reduced. This remarkable state of affairs is illustrated with simulations from a one-hemisphere ocean circulation model. In the simulations, two stratification-dependent diffusivity representations are investigated, which both imply that a weaker density difference will be associated with a stronger THC. The more common mixing representation, where the diffusivity is taken to be fixed, yields the opposite and well-known result: a weaker density difference will be associated with a weaker THC.

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Making use of a simple two-layer model, we analyze the impact of freshwater forcing on the thermohaline circulation. We consider the forward-type circulation dominated by thermal forcing, implying that the freshwater forcing acts to reduce the density contrast associated with the equator-to-pole temperature contrast (prescribed in the model). The system is described by two variables: the depth of the upper layer (H ) and the density contrast between the upper and lower layer (Dr), which decreases with salinity contrast. The rate of poleward flow of light surface water and the diapycnal flow (i.e., upwelling) driven by widespread small-scale mixing are both modeled in terms of H and Dr. Steady states of thermohaline circulation are found when these two flows are equal. The representation of the diapycnal flow (M D ) is instrumental for the dynamics of the system. We present equally plausible examples of a physically based representation of M D for which the thermohaline circulation either decreases or increases with density contrast. In the latter case, contrary to the traditional wisdom, the freshwater forcing amplifies the circulation and there exists a thermally dominated equilibrium for arbitrary intensity of freshwater forcing. Here, Stommel's famous feedback between circulation and salinity contrast is changed from a positive to a negative feedback. The interaction of such a freshwater boosted thermohaline circulation with the climate system is fundamentally different from what is commonly assumed, an issue which is briefly addressed.