CSIRO Marine and Atmospheric Research
Past Seminars

Friday 27 February, 11.30am (Tas time)

CSIRO Auditorium, Hobart and via videoconference to Floreat and Cleveland

Trevor McDougall
CSIRO Marine, Hobart

The thinness of the ocean in salinity – temperature – pressure
space, and the limitations of orthobaric density

The hydrography of the global ocean chooses to occupy very little volume in the three-dimensional parameter space of salinity, temperature and pressure (S, T, p). Were this not so, seawater could rise vertically in helical spiraling motions, so crossing mean density surfaces without requiring diapycnal mixing and the dissipation of mechanical energy. This remarkable thinness (the ocean seems to be hollow or anorexic) will be illustrated, but a satisfactory explanation is not yet known. A philosophical explanation will be attempted, but perhaps the ocean’s thinness is simply happenstance. Were the ocean not so thin, then there would be little meaning to any kind of “density” surface in the ocean as fluid could move through any such average surface with impunity.

Orthobaric density has recently been suggested as a new density variable for displaying ocean data and as a coordinate for ocean modelling. Here we quantify the extent to which orthobaric density surfaces are neutral, finding that orthobaric density surfaces are no closer to being neutral in the world ocean than are potential density surfaces that are referenced to the sea surface: technology that oceanographers have used for almost a century. It is shown that this rather serious limitation to the accuracy of orthobaric density is due to the different water mass structure of the waters of the northern and southern hemispheres and is not due to the small extent of helical spiral motions.

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