SI units in oceanography: Some of the problems

Abstract

Communication between different scientific disciplines can be facilitated by avoiding specialized jargon and, in particular, by using SI (International System) units. The SI base units are metre, kilogram, second, ampere, kelvin, mole, and candela. Oceanographers are urged to change their habits in reporting certain common quantities. Some of the recommendations are illustrated by the following examples: volume flux as 30 x 10⁶ m³/s (not 30 Sv); geopotential difference as 9.2 m²s⁻² (m²s⁻² = J/kg = 0.1 ‘dynamic metre'); density of seawater as 1,027.436 kg m⁻³; shearing stress as 0.102 Pa (Pa = N m⁻² = 10 dyn cm⁻²).

For other oceanographic quantities the appropriate SI units are not so obvious. The SI unit of pressure is the pascal (Pa), while the common pressure unit in oceanography is the decibar (dbar =10⁴ Pa). The megapascal (MPa =100 dbar) has been proposed as a new unit for use in oceanography, but oceanographers may resist a change from decibar to megapascal. For specific volume, the simple SI unit is the cubic metre per kilogram, which is far too large to use with seawater. The present common unit for steric anomaly (used in routine reporting of specific volume of seawater) is the centilitre per ton (cl/t = 10⁻⁵ cm³/g = 10⁻⁸ m³/kg). Perhaps oceanographers should adopt as new unit the cubic centimetre per kilogram (cm³/kg = 10⁻⁶ m³/kg = 10² cl/t), or the cubic millimetre per kilogram (mm³/kg = 10⁻⁹ m³/kg = 0.1 cl/t), but the former seems inconveniently large (most seawater has steric anomaly less than 1.000 cm³/kg), and the latter may be inconveniently small.