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

M. Hamon, P. Y. Le Traon, G. Reverdin
2011 Ocean Science Discussions  
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 sec-5 ond order correction on the depth as well as a depth offset representing measurement errors during XBT deployment. We had to separate data in several
more » ... ta 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 10 content signal is derived from corrected XBT. From this analysis, we confirm that the maximum heat content in the top 700 meters found during the 70's in early papers can be explained by the XBT biases. In addition, a trend of 0.32.10 22 J/year is observed between the period 1970 and 2008. upper ocean's temperature. The XBT system does not directly measure depth. The accuracy of the depth associated with each temperature depends on an equation which converts the time elapsed since the probe entered the water to depth. Gouretski and Koltermann (2007) used an ocean climatology based on high quality data (Conductivity Temperature Depth (CTD) and Nansen casts) to identify biases in XBT observations. They found a positive bias by 0.2-0.4 • C on average with some variations from year OSD Abstract 25
doi:10.5194/osd-8-291-2011 fatcat:siwjyrinpjbzpb4aqcszbz5qay