Depth Biases in XBT Data Diagnosed Using Bathymetry Data

Expendable bathythermograph (XBT) profiles are an important component of the historical record of subsurface ocean temperature. To correct for time-varying biases in these data, adjustments to XBT depths and/or temperatures have been proposed by a number of groups based on comparisons between XBT data and profiles recorded using other types of instruments. In this study, an alternative method for diagnosing biases has been developed that uses ocean depth information from the XBT profiles and from the General Bathymetry Chart of the Oceans (GEBCO) 30-arc-sec gridded bathymetry. This isolates any depth biases from additional, unrelated temperature biases. Corrections to depths obtained with this method for the Sippican T4 XBT follow a time evolution similar to that found in other studies that derived time-varying adjustments, but are relatively large during the 1980s. Similarities in the evolution of Sippican T7 XBT biases were also observed, but with differences in recent years. Corrections from a study that proposed non-time-varying adjustments were found to broadly remove the biases in the T4 depths but overcorrected the T7 data, with temporal variations in the biases remaining. For the Sippican T10 XBT a more detailed time evolution of depth biases has been obtained than was previously possible. Although corrections have also been derived for approximately 50% of the XBTs for which type and manufacturer are unknown, these should only be used with caution as this study necessarily focuses on shallow water and proportions of different XBT types in use there are not typical of the wider ocean.