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A high-resolution, offline ocean general circulation model, incorporating a realistic parameterization of mixed layer convection, is used to diagnose pathways and time scales of Southern Hemisphere intermediate, mode, and lower thermocline water ventilation. The use of such an offline methodology represents the only feasible way of simulating the long time scales required to validate the internal pathways of a high-resolution ocean model. Simulated and observed chlorofluorocarbon-11 (CFC-11) are in reasonably good agreement, demonstrating the model’s skill in representing realistic ventilation. Regional passive dye and age tracer experiments aid in the identification of pathways originating from different Southern Hemisphere locations. Northern Hemisphere penetration of intermediate, mode, and thermocline waters is most extensive and rapid into the North Atlantic Ocean because these waters are involved in closing the Atlantic meridional overturning cell. However, less than 8% of this ventilation is derived from subduction within the South Atlantic in the simulation. Instead, this water enters the Atlantic just to the south of South Africa, having originally subducted primarily in the east Indian Ocean, but also in the west Indian Ocean and the west Pacific region where a pathway advects water westward to the south of Australia. This pathway also plays a large part, together with water overturned in the east Indian Ocean, in ventilating the northern reaches of the Indian basin. Northward propagation in the Pacific Ocean is limited to the low latitudes of the Northern Hemisphere and is almost exclusively accomplished by water subducted in the South Pacific. A small contribution is made from the other basins from water that spreads northward, fed by a circumpolar pathway associated with the Antarctic Circumpolar Current that forms a conduit for intermediate and mode water exchange between all three basins. Intermediate water is injected into and branches off this pathway in all basins, but most vigorously in the southeastern Pacific. [ABSTRACT FROM AUTHOR]

Global watermass ventilation pathways and time scales are investigated using an “eddy permitting” (¼°) offline tracer model. Unlike previous Lagrangian trajectory studies, here an offline model based on a complete tracer equation that includes three-dimensional advection and mixing is employed. In doing so, the authors are able to meaningfully simulate chlorofluorocarbon (CFC) uptake and assess model skill against observation. This is the first time an eddy-permitting model has been subjected to such an assessment of interior ocean ventilation. The offline model is forced by seasonally varying prescribed velocity, temperature, and salinity fields of a state-of-the-art ocean general circulation model. A seasonally varying mixed layer parameterization is incorporated to account for the degradation of surface convection processes resulting from the temporal averaging. A series of CFC simulations are assessed against observations to investigate interdecadal-time-scale ventilation using a variety of mixed layer criteria. Simulated tracer inventories and penetration depths are in good agreement with observations, especially for thermocline, mode, and surface waters. Deep water from the Labrador Sea is well represented, forming a distinct deep western boundary current that branches at the equator, although concentrations are lower than observed. The formation of bottom water, which occurs around the Antarctic margin, is also generally too weak, although there is excellent qualitative agreement with observations in the region of the Ross and Weddell Seas. Multicentury ventilation of the outflow of North Atlantic Deep Water and bottom water from the Antarctic Margin are investigated using 1000-yr passive tracer experiments with specified interior source regions. The model captures many of the detailed pathways evident from observations, with much of the discrepancy accounted for by differences between actual and modeled topography. A comparison between model-derived “tracer age” and Δ14C “advection age” provides a semiquantitative assessment of model skill at these longer time scales. [ABSTRACT FROM AUTHOR]