The accuracy of measurement of salinity, oxygen and temperature in the deep ocean

Abstract

At depths greater than about 3000 db and between latitudes 20° and 50° the northeast Atlantic has a remarkably uniform θ-S relationship. Measurements of salinity made by different laboratories can be compared without resorting to simultaneous observations. Systematic differences between "high quality" measurements are about 0.003, considerably exceeding in situ variations. The origin of the differences is principally, we believe, due to variations in standard sea water. The concentration of dissolved oxygen is also remarkably uniform throughout the same region and has a value of 247 μmol kg-1 (5.67 ml l-1). The rms deviation of the best observations is only 1 μmol kg-1 (0.02 ml l-1). The above figures are for very careful researchers: most do considerably less well and are urged to reconcile their data against values put forward in the text of this paper. Recognizing temperature change in the deep northeastern Atlantic also requires the highest quality data. Institute of Oceanographic Sciences (IOS) experience comparing temperatures measured with reversing thermometers, CTD and current meters is described. With considerable care these laboratory calibrated instruments are shown to be capable of an absolute accuracy of a few millidegrees. It is proposed that carefully calibrated IOS moored instruments be used as comparison sites for other workers.