Examining the Influence of Recording System on the Pure Temperature Error in XBT Data

Expendable bathythermographs (XBTs) have been widely deployed for ocean monitoring since the late 1960s. Improving the quality of XBT data is a vital task in climatology. Many factors (e.g., temperature, probe type, and manufacturing time) have been identified as major influences of XBT systematic bias. In addition, the recording system (RS) has long been suspected as another factor. However, this factor has not been taken into account in any global XBT correction schemes, partly because its impact is poorly understood. Here, based on analysis of an XBT–CTD side-by-side dataset and a global collocated reference dataset, the influence of RSs on the pure temperature error (PTE) is examined. Results show a clear time dependency of PTE on the RS, with maximum values occurring in the 1970s. In addition, the method used to convert thermistor resistance into temperature in the RS (using a resistance–temperature equation) has changed over time. These changes, together with the decadal changes in RSs, might contribute a small error (10% on average) to the RS dependency. Here, an improvement of global XBT bias correction that accounts for the RS dependency is proposed. However, more than 70% of historical global XBT data are missing RS-type information. We investigate several assumptions about the temporal distribution of RS types, and all scenarios lead to at least a ;50% reduction in the time variation of PTE compared with the uncorrected data. Therefore, the RS dependency should be taken into account in updated XBT correction schemes, which would have further implications for climatology studies.

**Citation: **

Tan Z., Reseghetti F, Abraham J, Cowley R, Chen K, Zhu J, Zhang B, Cheng L, 2021: Examining the Influence of Recording System on the Pure Temperature Error in XBT Data. Journal of Atmospheric and Oceanic Technology, 38, 759-776.

[Chinese Version]

很多海洋现场观测数据中均存在系统性偏差，需要对偏差进行订正，针对XBT的温度观测，子课题近期发现了新的影响因子：记录系统。研究表明，依赖于记录系统的XBT纯温度偏差（PTE）与年份之间存在明显的依赖关系：Sippican Strip Chart记录系统在1970s的纯温度偏差达到最大，随后逐渐减少，在2000s之后不同的记录系统的温度偏差已经几乎接近于0。这可能是由于不同的记录系统中使用了不同的热敏电阻阻值与温度之间的转换关系方程（R-T方程）而导致了不同的温度偏差。此外，课题还提出了新的全球XBT温度偏差订正方案：如果XBT订正方案考虑了记录系统的这一新的影响因子，将明显改善XBT温度偏差与时间的依赖性。

Recommended citation: Tan Z., Reseghetti F*, Abraham J, Cowley R, Chen K, Zhu J, Zhang B, Cheng L, 2021: Examining the Influence of Recording System on the Pure Temperature Error in XBT Data. Journal of Atmospheric and Oceanic Technology, 38, 759-776.
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