Systematic Adjustments of Hydrographic Sections for Internal Consistency

Abstract

A significant legacy of the World Ocean Circulation Experiment (WOCE) is the large number of high quality, high-resolution, full-depth, transoceanic hydrographic sections occupied starting in the mid-1980s. The section data provide an unprecedented survey of World Ocean water properties. Most stations sampled pressure, temperature, salinity, dissolved oxygen, and nutrients (nitrate, phosphate, and silicic acid) at up to 36 depths. While the WOCE Hydrographic Program (WHP) strenuously advocated employing standardized measurement techniques on all sections, small but significant systematic differences among cruise legs are found. A simple method for adjusting measurements to maximize internal consistency is presented and applied to available WOCE data in the Pacific Basin. First, the sections are broken into distinct cruise legs between port stops. Then, crossovers are identified where two different cruise legs cross or approach each other. Using hydrographic data from each cruise leg near each crossover, linear fits are made of properties on potential temperature surfaces against distance along cruise track. These fits are then used to evaluate property differences and their uncertainties at crossovers. A set of least squares models are used to generate sets of adjustments, with related uncertainties, for all the properties of each cruise leg. These adjustments minimize differences of water properties among cruise legs at the crossovers in a least squares sense. The adjustments can be weighted by difference uncertainties, and damped by a priori estimates of the expected differences. Initial standard deviations of crossover differences are 0.0028 for salinity, 2.1% for oxygen, 2.8% for nitrate, 1.6% for phosphate, and 2.1% for silicic acid. The adjustments roughly halve these values, bringing cruise legs into agreement within WHP target accuracies.