XBT Fall Rate in Waters of Extreme Temperature: A Case Study in the Antarctic Ocean

Abstract 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 th...