XBT Fall-rate study in the Southern Ocean

Several studies have observed a more prominent warming of the Southern Ocean when compared to other ocean regions of the world in response to global climate change. However, the vast majority of available temperature data for that region is composed by eXpendable BathyTermographers profiles (XBTs). These probes are not equipped with a pressure sensor and thus do not measure depth directly. Depth is inferred by a fall-rate equation offered by the manufacturer that does not seem to adequately represent the extremely cold and high viscosity conditions of the region. Probes fall slower than expected and thus lead to an overestimation in heat content for those areas. In this study, a set of 850 collocated XBT (DB/T7 type) and CTD stations obtained from World Ocean Database (2013) and separated by a maximum distance of 12.5 nm and 10 hours is used. Those pairs are used to identify and quantify the depth errors in the XBT’s temperature profiles, proposing a regional equation correction able to represent the peculiarities of the region. Hanawa et al. (1995) and Cheng et al. (2014) correction methods were applied to the dataset, with the latter producing better results. For the pairs in Drake Passage (151), South of Africa (244) and South of Tasmania (455), we found the ideal A coefficients to change by -0.88%, -1.4% and -2.2% from the original values, respectively. When a temperature profile is more homogeneous, there is no significant change if different values of B are used, which was proven to be true since they were .0021 for Drake Passage and .0020 for the other chokings. The ideal fall-rate equation for the Southern Ocean was determined using all pairs, being defined as instead of the proposed by the manufacturer. Overall, the results further support the hypothesis of a regional dependence of the XBT fall-rate on water temperature, and suggest the need of developing a bias correction scheme specific for the polar regions.