4.14 Finding the True Temperature of the Ocean Surface

The characteristics of the SST data will depend on how the measurements were made. Most of the SST measurements in the period 1970 to 1997 were made using either a bucket and thermometer or by reporting the temperature of the engine intake cooling water. A smaller number of reports were made using hull sensors which are thought to be more accurate. ICOADS contains a metadata flag giving the method of measurements for some of the reports. Information from the SST Indicator “SI” flag is plotted in Figure 1a which shows that prior to 1981 only a subset of SST reports from buckets can be positively identified. However we can also appeal to external metadata contained in the World Meteorological Organisation (WMO) Report No. 47 the “List of Selected, Supplementary and Auxiliary Ships” which gives methods of measurements for many of the ships. WMO Report No. 47 is available in annual files in digital form since 1973 and was published most years in paper form since 1954 (e.g. WMO 1994). The method of measurement can be associated with individual reports using the ship callsign. Figure 1b shows the combined information from both sources, the SI flag and WMO Report No. 47. Using all the available metadata gives a much larger subset of data for analysis and allows the comparison of bucket and engine intake SST extending further back in time.