XBT Temperature Errors during French Research Cruises (1999-2007)

Data from French cruises in 1999-2007, a period during which Deep Blue (DB) or T7 expendable bathythermographs (XBTs) were deployed, and for which ancillary temperature data are available in the northeast Atlantic and equatorial Atlantic regions, are examined. There was a total of 16 cruises with XBTs launched between conductivity-temperature-depth (CTD) stations; during most of these, as well as during three additional cruises that were also considered, intake temperature was measured. XBT data from two voluntary observing ships in the North Atlantic subpolar gyre for which intake temperature was measured were also investigated. There is an XBT cold bias due to stirring of a stratified upper layer by the ship, resulting in differences between XBT temperatures at 3-5 m and intake measurements. This is most pronounced for midlatitude spring or summer cruises, when it averages about 0.10 degrees C. When these situations are removed, the comparisons clearly indicate positive biases in XBT temperature measurements in 1999-2006, with individual cruise averages generally between 0 degrees and 0.1 degrees C, and a tendency to have larger biases when surface temperature is high. In addition, a positive depth-estimate bias of the XBTs in the upper thermocline (on the order of 4 m) is identified, as well as a depth overestimation through the profile, averaging 1.7% (1.2%) for the equatorial (midlatitude) cruises (with respect to a previously published depth estimate).