Formation of Low Potential Vorticity over the Deep Pacific

Low potential vorticity extends over the deep waters of the North Pacific and, possibly, the bottom waters of the North Atlantic. Isopycnic model integrations are conducted to investigate how these potential vorticity distributions are controlled, first, for an idealized double-hemisphere and, second, for the Pacific with realistic topography. Dense water is released from a southern, high-latitude source and circulates over the domain with diapycnic mixing gradually reducing its stratification. The potential vorticity contrast is large over the Southern Hemisphere, but weak over the Northern Hemisphere where the meridional changes in planetary vorticity and layer thickness oppose each other. Including an active eddy field inhibits the grounding of dense water, which increases the potential vorticity contrast in the overlying layer. Incorporating realistic topography leads to the dense fluid spreading via deep channels with tight recirculations and jets bifurcating. The experiments suggest that extensive regions of low potential vorticity are formed whenever there is both enhanced bottom mixing and a basin is filled by a single water mass entering from across the equator.