XBT fall rate in waters of extreme temperature: A case study in the Antarctic Ocean

XBT fall-rate variation in waters of extreme temperature and the resulting depth error has been addressed using controlled XBT-CTD datasets collected from two cruises in the Southern Ocean. Mean depth errors deduced from both the datasets are significantly different from those reported earlier for tropical and subtropical regions. The comprehensive study of Hanawa et al. (making use of controlled XBT-CTD data, mostly from tropical and subtropical waters) showed that the manufacturer's equation underestimates the probe's fall rate. This is manifested by the mean negative depth error reported from this region. However, results from the present study show that the manufacturer's equation slightly overestimates the fall rate in this region, as indicated by the small positive error (5-10 m). In order to provide theoretical support to the observed depth error, an analytical approach is adopted based on the viscosity effect on the probe's fall rate. Observed as well as analytical results suggest that the probe has a decelerating tendency due to the viscosity effect in high-latitude waters, and the existing correction scheme is not appropriate for XBT data from regions of such extreme low temperature. The existing correction scheme is valid for tropical and subtropical waters of negative depth error zones. However, for XBT data from high-latitude waters it is reasonable not to correct XBT data based on the existing scheme until the exact nature of depth error from this region is known. Though the mean depth errors from both the datasets show nearly identical values, it is necessary to conduct more controlled XBT-CTD experiments in this region in order to substantiate the exact nature of error for this region and then develop an appropriate depth-correction scheme.