A quality-control procedure for surface temperature and surface layer inversion in the XBT data archive from the Indian Ocean

Abstract

A quality control procedure has been developed and validated for Expendable Bathythermograph (XBT) data from Indian Ocean archieved at Indian Oceanographic Data Centre (IODC). Two possible sources of errors are considered surface temperature and surface layer temperature inversion. XBT surface temperatrues (XST) are compared with the surface temperature from simultaneous CTD observations from four cruises and the former were found to be erroneous in a number of stations. XSTs are usually corrected by bucket temperature, observed prior to XBT launch. However, in the archive, nearly 50% of XBT stations were reported without bucket temperature. In the cases where bucket temperature are reported, it is found that in number of stations the error is too large to consider for XST correction. To minimize the error in XSTs it is suggested to ascertain the quality of those XSTs for which the difference between XST and bucket is less than 0.2 degrees C. When it exceeds 0.2 degrees C threshould and also when bucket temperature is not reported, it is suggested to take the 5m XBT-temperature(5-XT) as XST. This extrapolation procedure of 5-XT as XST using the XBT-CTD controlled data set of Thadathil et al. 1998 has been found valid. Surface layer temperature inversions observed in XBT data need not always be genuine. Inversion-like feature may occur in the XBT data due to wire stretch problem or pinhole leak in the cable. The characteristics of surface layer temperature inversions occurring in Indian Ocean have been analyzed using the IODC-CTD archive. There exists a significant relation between sea surface temperature and layer-thickness of inversions. This empirical, linear relation is used as an effective filter for flagging 'strange' inversions in XBT-archive