Evaluation of interior circulation in a high resolution global ocean model; Part II: Southern Hemisphere Intermediate, Mode and Thermocline Waters

A high-resolution, off-line, ocean general circulation model, incorporating a realistic parameterisation of mixed layer convection is used to diagnose pathways and timescales of Southern Hemisphere Intermediate, Mode and thermocline water ventilation. Simulated and observed 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 form the other basins via a pathway associated with the ACC that forms a circumpolar conduit of Intermediate and Mode water between all the basins. Intermediate water is injected into, and branches off this pathway in all basins but most vigorously in the south-eastern Pacific.