Is the deep ocean in steady state?

In making observational estimates of deep ocean ocean currents a “steady-state” assumption is often made—that the underlying ocean circulation does not have a long-term trend over the four decades of the modern observational period. This assumption may be problematic given the known warming of the deepest parts of the ocean. School of Oceanography associate professor Susan Hautala and Garrett Finucane, who graduated with a B. Sc. in 2021, recently published a study in the journal Geophysical Research Letters to test this assumption. They made use of a global collection of long sampling lines using full depth CTD (Conductivity-Temperature-Depth) casts that are repeated every 10 years or so. They find that the assumption generally holds true, except near intense currents that occur at boundaries (coasts and ridges) and in parts of the ocean close to the few locations where dense surface waters sink to great depth (aka, deep water formation). This finding supports the use of methods that combine all available density data, so as to average out shorter variations (hours to years) and provide values that represent longer, more persistent underlying deep circulation patterns.