The Role of Salt in the Ocean's Adiabatic Overturning Circulation

The adiabatic overturning circulation is that part of the oceanic meridional overturning circulation (MOC) that is closed by wind-driven upwelling in the Southern Ocean. In contrast to the diffusive MOC, the adiabatic component of the MOC is independent of the strength of diapycnal mixing in the ocean interior, but instead is controlled by the strength of Southern Ocean winds and the size of the window of isopycnals shared between the Southern Ocean and the North Atlantic Ocean.

Most recent models of the adiabatic overturning circulation have assumed that the density of seawater depends only on temperature. Since the sea surface temperature is tightly coupled to the atmospheric temperature just above the surface, this allows the surface density distribution---and hence, the size of the shared isopycnal window---to be specified by specifying the atmospheric state. In contrast, precipitation is not strongly influenced by ocean salinity; specifying the atmospheric state specifies the surface freshwater flux rather than the surface salinity. Thus, in the presence of salt, the surface buoyancy distribution cannot be specified simply by specifying the atmospheric state. Therefore, existing models of the adiabatic overturning circulation must be modified to remain predictive when salinity forcing is present.

The role of salt in the adiabatic overturning circulation is examined through a series of idealized process studies using a oceanic general circulation model. In the presence of symmetric salinity forcing, a salt-advection feedback forms, which increases the size of the shared isopycnal window and strengthens the MOC relative to the case without salinity forcing. For small pole-to-pole temperature gradients, the window expands to its maximum extent and fills the entire channel; the window's extent is limited for larger pole-to-pole temperature gradients. The processes limiting the size of the isopycnal window are discussed.

Isopycnals shared between the channel and the northern subpolar region are subjected to a net negative salinity flux in both hemispheres. Since the circulation is adiabatic beneath the mixed layer, the salinity flux must be conserved along isopycnals and cannot be negative at both outcrops. This conflict is resolved by horizontal salt transport in the mixed layer, which helps set the surface salinity distribution. This phenomenon will be illustrated and quantified by examining heat and salt budgets on isopycnals.