The nonlinear equation of state of sea water and the global water mass distribution

A vertical section through the South Atlantic shows a salinity maximum at mid-depth consisting of North Atlantic Deep Water (NADW), and fresher water both below (Antarctic Bottom Water, AABW) and above (Antarctic Intermediate Water, AAIW). What causes this structure? It is here shown that the nonlinearities of the equation of state (EOS) of seawater are crucial for this. The role of these nonlinearities for the distribution of water masses in the global ocean is examined through simulations with an ocean general circulation model with various manipulated versions of the EOS.

A simulation with a strongly simplified EOS, which contains only two nonlinear terms, still produces a realistic water-mass distribution, demonstrating that these two nonlinearities are indeed the essential ones. Further simulations show that each of these two nonlinear terms affects a specific aspect of the water-mass distribution: the thermobaric term is crucial for for the layering of NADW and AABW, and the cabbeling term for the formation of AAIW. With a realistic equation of state, there is a narrow region of downwelling near the Antarctic Circumpolar Current (ACC) that densifies because of cabbeling and sinks through the density surfaces, forming AAIW. Cabbeling is strong at the ACC because of strong lateral mixing along the sloping density surfaces. When the cabbeling term in the EOS is omitted, there is no such downwelling water, and essentially no AAIW.