Behavior of double-hemisphere thermohaline flows in a single basin

A coarse resolution, three-dimensional numerical model is used to study how external parameters control the existence and strength of equatorially asy mmetric thermohaline overturning in a large-scale, rotating ocean basin. In itially, the meridional surface density gradient is directly set to be larg er in a "dominant" hemisphere than in a "subordinate" hemisphere. The two-h emisphere system has a broader thermocline and weaker upwelling than the sa me model with the dominant hemisphere only. This behavior is in accord with classical scaling arguments, providing that the continuity equation is emp loyed, rather than the linear vorticity equation. The dominant overturning cell, analogous to North Atlantic Deep Water forma tion, is primarily controlled by the surface density contrast in the domina nt hemisphere, which in turn is largely set by temperature. Consequently, i n experiments with mixed boundary conditions, the dominant cell strength is relatively insensitive to the magnitude Q(s) of the salinity forcing. Howe ver, a, strongly influences subordinate hemisphere properties, including th e volume transport of a shallow overturning cell and the meridional extent of a tongue of low-salinity intermediate water reminiscent of Antarctic Int ermediate Water. The minimum Q(s) is identified for which the steady, asymmetric how is stab le; below this value, a steady, equatorially symmetric, temperature-dominat ed overturning occurs. For high salt flux, the asymmetric circulation becom es oscillatory and eventually gives way to an unsteady, symmetric, salt-dom inated overturning. For given boundary conditions, it is possible to have a t least three different asymmetric states, with significantly different lar ge-scale properties. An expression for the meridional salt transport allows one to roughly predict the surface salinity and density profile and stabil ity of the asymmetric state as a function of Q(s) and other external parame ters.