Signs of disequilibrium in the deep and abyssal circulations of seawater and tracers

Modern-day tracer observations provide a wealth of information about surface-to-deep circulation pathways. Inverting these observations for pathway information without the use of dynamical information is useful to provide an independent check of dynamical models and to avoid transferring model biases into our estimates. Here I review results regarding seawater pathways in the Nordic Seas and the Southern Ocean. Nordic Seas tracer data, for example, allows quantification of the detailed pathways of dense overflow water, but also decadal changes in ocean composition. In the Southern Ocean, transient tracers primarily reflect the large-scale statistically-steady circulation, but CFC values near the bottom water formation sites appear to be affected by a changing circulation. Taken together, pathway information and rate constraints permit the construction of an empirical ocean model that produces global metrics such an ocean heat and carbon uptake that are consistent with other estimates. Going forward, however, regional errors in such an empirical model are destined to become larger as circulation changes become larger.