Time, Probe Type, and Temperature Variable Bias Corrections to Historical Expendable Bathythermograph Observations

Systematic biases in historical expendable bathythermograph (XBT) data are examined using two datasets: 4151 XBT CTD side-by-side pairs from 1967 to 2011 and 218 653 global-scale XBT CTD pairs (within one month and 1) extracted from the World Ocean Database 2009 (WOD09) from 1966 to 2010. Using the side-by-side dataset, it was found that both the pure thermal bias and the XBT fall rate (from which the depth of observation is calculated) increase with water temperature. Correlations between the terminal velocity A and deceleration B terms of the fall-rate equation (FRE) and between A and the offset from the surface terms are obtained, with A as the dominant term in XBT fall-rate behavior. To quantify the time variation of the XBT fall-rate and pure temperature biases, global-scale XBT CTD pairs are used. Based on the results from the two datasets, a new correction scheme for historical XBT data is proposed for nine independent probe-type groups. The scheme includes corrections for both temperature and depth records, which are all variable with calendar year, water temperature, and probe type. The results confirm those found in previous studies: a slowing in fall rate during the 1970s and 2000s and the large pure thermal biases during 1970-85. The performance of nine different correction schemes is compared. After the proposed corrections are applied to the XBT data in the WODO9 dataset, global ocean heat content from 1967 to 2010 is reestimated.