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Important note about XBT and MBT bias corrections applied to standard level data:
As of November, 2, 2009, the WOD09 standard depth level data for the expendable bathythermograph (XBT) and mechanical bathythermograph (MBT) datasets will include adjustment for documented time-dependent temperature biases as described by Levitus et al. (2009). This is in addition to the correction of depths to the Hanawa et al. (1995) fall-rate for all XBT data which can be corrected. OBSERVED LEVEL DATA DO NOT HAVE ANY ADJUSTMENTS. The corrected standard level data are the data as they were used to calculate the World Ocean Atlas 2009 temperature climatologies. Those XBT data with a second-header 33 (depth_correction) value of -1 (not enough information on fall-rate used) do not have any fall-rate correction applied, they do have the temperature bias correction applied. These XBT data without sufficient fall-rate information were not used in the World Ocean Atlas temperature climatology. This subset of data consists of XBT only for years 1996-present, the period when the Sippican and TSK XBT software included two choices for fall-rate for T4, T6, T7, and Deep Blue XBTs. Prior to this period, when a probe type and fall-rate were not identified in the data, if the maximum depth of measurement was <= 840 meters, the depths were adjusted to the Hanawa fall-rate equation. Until the Deep Blue was manufactured to reach depths deeper than 900 meters, the only probe type which measured below 840 meters was the T5. Data for T5s have either no adjustment (Sippican) or a small adjustment (for TSK models) (Kizu et al., 2005). Information on the decision made on depth adjustment is included for each XBT in second-header 54 (depth_fix).
The bias adjustment (and fall-rate equation) is an area of current study, and may change in the future. The addition of more XBT and CTD data to the World Ocean Database may also cause adjustments in the temperature bias calculation. The temperature bias data provided by Levitus et al. 2009 along with other temperature bias/ fall-rate recalculations are available on XBT bias page. The XBT temperature bias and MBT temperature bias adjustments for World Ocean Database 2009 are slightly different from Levitus et al., 2009, especially for recent years, due to the addition of more XBT (MBT)/CTD data.
Hanawa, K., P. Raul, R. Bailey, A. Sy, and M. Szabados, 1995: A new depth-time equation for Sippican or TSK T-7, T-6 and T-4 expendable bathythermographs (XBT). Deep-Sea Res. I, 42, 1423-1451.
Kizu, S., H. Yoritaka, and K. Hanawa, 2005: A new fall-rate equation for T-5 Expendable Bathythermograph (XBT) by TSK. J. Oceanogr., 61, 115-121.
Levitus, S., J. I. Antonov, T. P. Boyer, R. A. Locarnini, H. E. Garcia, and A. V. Mishonov, 2009: Global ocean heat content 1955-2008 in light of recently revealed instrumentation problems. Geophys. Res. Lett., 36, L07608, DOI: 10.1029/2008GL037155.
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