Correcting Biases in Historical Bathythermograph Data Using Artificial Neural Networks

Historical estimates of ocean heat content (OHC) are important for understanding the climate sensitivity of the Earth system, and for tracking changes in the Earth’s energy balance over time. Prior to 2004, these estimates rely primarily on temperature measurements from mechanical and expendable bathythermograph (BT) instruments that were deployed on large scales by naval vessels and ships of opportunity. These BT temperature measurements are subject to well-documented biases, but even the best calibration methods still exhibit residual biases when compared to high-quality temperature datasets. Here, we use a new approach to reduce biases in historical BT data after binning them to a regular grid such as would be used for estimating OHC. Our method consists of an ensemble of artificial neural networks that corrects biases with respect to depth, year, and water temperature in the top 10 meters. A global correction, as well as corrections optimized to specific BT probe types are presented for the top 1800 m. Our approach differs from most prior studies by accounting for multiple sources of error in a single correction, instead of separating the bias into several independent components. These new global and probe-specific corrections perform on par with widely-used calibration methods on a series of metrics that examine the residual temperature biases with respect to a high-quality reference dataset. However, distinct patterns emerge across these various calibration methods when they are extrapolated to BT data not included in our cross-instrument comparison, contributing to uncertainty that will ultimately impact estimates of OHC. <br>