Persistence of the Large‐Scale Interior Deep Ocean Circulation in Global Repeat Hydrographic Sections

An assumption of steady‐state is a common basis for deep ocean circulation theory and observational strategies. We use GO‐SHIP's Easy Ocean uniformly gridded CTD data from repeat hydrographic sections to test this assumption. In particular, we ask: for what regions of the world ocean is there evidence that the planetary scale deep geostrophic shear and potential vorticity fields, related to potential density gradients, are in quasi‐steady‐state over the modern observational period? We find that away from boundary currents, planetary‐scale potential density gradients in most parts of the deep ocean are stable from occupation to occupation, with higher variability in a few expected regions and for shorter sections. Median standard errors from all sections are 12%–17% in the Pacific, 10%–23% in the Atlantic, and 11%–36% in the Indian Ocean, with the highest values at 2,000 dbar and lowest at 4,000 dbar. Plain Language Summary In analyzing ocean circulation, it is common practice to use all available density data, so as to average out shorter variations (hours to years) and provide values that represent longer, more persistent underlying circulation patterns. In doing so, a “steady‐state” assumption has been 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. Our study uses observations from a global collection of long sampling lines, repeated every 10 years or so, to test the steady‐state assumption for density in the deep ocean, and finds that it 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. Key Points Large‐scale density gradients in most parts of the deep ocean away from boundaries are stable over the modern observational period The structure of large‐scale (planetary) potential vorticity in the deep interior ocean is also persistent over recent decades These results support the assumption of quasi‐steady state that underlies many analyses, including hydrographic inverse models