INSTRUMENTATION

An eXpendable BathyThermograph (XBT) is a probe that is dropped from a ship and measures the temperature as it falls through the water. A resistance in the head of the probe and a very thin twin-wire, connecting the probe to the equipment on the ship, compose the electronic circuit for measuring the water temperature. The probe is designed to fall at a known rate, so that the depth of the temperature profile can be inferred from the time since it enters the water.
XBTs are valuable because they represent the largest fraction of the temperature profile observations since 1970s and until the fully implementation of Argo profiling floats in approximately 2007, measure the seasonal and inter-annual fluctuations in the transport of mass, heat, and freshwater across transects, which define large enclosed ocean areas, determine the long-term mean, seasonal cycle, and inter-annual fluctuations of temperature, geostrophic velocity and large-scale ocean circulation in the top 800 m of the ocean, provide long time-series of temperature profiles at approximately repeated locations in order to unambiguously separate temporal from spatial variability, determine the space-time variability of temperature and geostrophic shear fields and provide appropriate in situ data for testing ocean and ocean-atmosphere models.