Empirical correction of XBT fall rate and its impact on heat content analysis

Abstract. We used a collocation method between XBT and CTD/OSD (Ocean Station Data including bottle cast and low resolution CTD) from WOD05 (1°×2°×15 days) to statistically correct the XBT fall rate. An analysis of the annual median bias on depth showed that it is necessary to apply a thermal correction linked to probe calibration error, a second order correction on the depth as well as a depth offset representing measurement errors during XBT deployment. We had to separate data in several categories: shallow and deep XBT and deployment sea temperatures (below or above 10 °C). We also processed separately XBT measurements close to Japan between 1968 and 1985 due to large regional biases. Once the corrections have been applied, the analysis of heat content signal is derived from corrected XBT. From this analysis, we confirm that the maximum heat content in the top 700 m found during the 70's in early papers can be explained by the XBT biases. In addition, a trend of 0.32.10²² J/year is observed between the period 1970 and 2008.