Direct Evidence of a Changing Fall-Rate Bias in XBTs Manufactured during 1986-2008

This paper presents direct evidence of systematic depth errors consistent with a fall-rate bias in 52 temperature profiles collected using expendable bathythermographs (XBTs). The profiles were collected using the same recording system and under the same ocean conditions, but with XBTs manufactured during years 1986, 1990, 1991, 1995, and 2008. The depth errors are estimated by comparing each XBT profile with a collocated profile obtained from conductivity-temperature-depth (CTD) casts using a methodology that unambiguously separates depth errors from temperature errors. According to the manufacture date of the probes, the XBT fall-rate error has changed from (-3.77 +/- 0.57)% of depth in 1986 to (-1.05 +/- 1.34)% of depth in 2008. The year dependence of the fall-rate bias can be identified with statistical significance (1 sigma) below 500 m, where the effect of the fall-rate bias is larger. This result is the first direct evidence of changes in the XBT fall-rate characteristics. Therefore, for the 1986-2008 period, the hypothesis that the XBT errors are due to a time-varying fall-rate bias, as hypothesized by Wijffels et al., cannot be rejected. Additional implications for current efforts to correct the historical temperature profile database are discussed.