|Met Office Hadley Centre observations datasets|
|> Home > HadSST3 >|
The following is a brief description of the data set construction process. For more detail please read the papers.
Observations were taken from version 2.5 of ICOADS. Observations were subject to a number of quality-control checks. First the observation had to have a valid time and location. The location had to be over the ocean and have a measured sea-surface temperature. The locations and times of the reports from each ship were then checked to ensure that the ship was not travelling unrealistically fast. This check removes observations where the location or time has been misreported.
SST observations were compared to the climatological average and rejected if they were more than 8 degrees from it. Observations below the freezing point of sea water, -1.8°C, were also rejected. Observations were then buddy-checked using other nearby observations.
ICOADS is composed of a number of 'decks' which refer back to the decks of punched cards on which many of the data were originally stored. A number of observations in deck 732 were rejected before other QC checks were applied because they were obviously erroneous. It appears as if a number of 5-degree latitude and longitude areas in the deck were somehow swapped around during the digitization process so that the observations were either much warmer or cooler than the climatological average and very different to observations in neighbouring grid cells.
The gridding process proceeded as in HadSST2.
For a more thorough description of the process see Part 2 of the paper (1Mb).
Measurements of sea surface temperature made by Voluntary Observing Ships have been taken typically either using a bucket to collect a water sample or by measuring the temperature of the water pumped in to the ship to cool the engine (known as Engine Room Intake or ERI measurements). The material used to make these buckets has changed from wood to canvas to rubber. The material used to make the bucket affects the measurement taken, through the ability of the material to effectively insulate the water sample: canvas buckets led to measurements being generally biased cool, wooden and rubber buckets are better insulated so the biases are smaller. Measurements of engine room intake water tend to be biased relatively warm.
Drifting and moored buoys, on the whole, tend to provide fairly accurate measurements of sea surface temperature, although an individual buoy's instrumentation can be biased. Near-coincident measurements from ships and drifting buoys show that ship measurements are, on average, biased warm by between 0.1 and 0.2K relative to drifting buoy measurements.
We have attributed a known or likely measurement method to nearly all observations of sea-surface temperature made in situ, contained within the International Comprehensive Ocean-Atmosphere Data Set (ICOADS, version 2.5), except for a small fraction. Where we have no direct information about measurement method, we infer a likely method from the country of origin of the ship. This attribution of measurement method, coupled with our understanding of the relative biases between measurement methods, has allowed us to develop adjustments for each monthly, grid-box average sea-surface temperature anomaly value. However, we do not know perfectly the method used to make all measurements because some of the information is uncertain, e.g. the dates individual countries switched from using one type of bucket to another. We account for this by creating many sets of bias adjustments, varying the assumptions we make about these uncertain aspects within their likely ranges each time. The following parameters were varied in each of 100 realisations:
Fields of the numbers of observations associated with each measurement method and the biases estimated for each measurement method were combined to calculate the bias in the gridded temperature fields. The spread of the 100 realisations gives an estimate of the uncertainty of the bias adjustment process.
For a more thorough description of the process see Part 1 of the paper (1Mb).
The uncertainties inherent in the bias adjustment process (described above) are most important at longer time scales and larger space scales and are most clearly expressed in long-term trends in the global average. At smaller scales, two other sources of uncertainty are also important.
When calculating area average sea-surface temperatures, such as the global average, using the gridded fields there will often be large areas for which no observations are available. Therefore, any area average will accrue an additional uncertainty from the imperfect sampling. This uncertainty can be estimated by using SST analyses that are globally complete and subsampling them at the locations where observations were available historically. Comparison between the complete fields and the subsampled fields gives an idea of the likely uncertainty accruing from imperfect sampling.
It should be noted that this component of the uncertainty depends, to a certain extent, on the statistical and physical assumptions made to produce the globally complete SST data sets used to estimate the coverage uncertainty. This possibility was explored somewhat by calculating the coverage uncertainty using three different interpolated analyses. For the regions considered in the paper, the three different analyses give similar results despite making quite different assumptions about how interpolation should be done.
Commercial and media enquiries
You can access the Met Office Customer Centre, any time of the day or night by phone, fax or e-mail. Trained staff will help you find the information or products that are right for you.
© Crown Copyright