Discussion

When comparing Release 1a with interim data, the large amount of delayed ship reports (ref. Fig. 1B) provides the largest increase in global 2-degree box coverage (illustrated for SST in Fig. 2). Globally, the number of year-month-2-degree boxes during 1980-91 containing data in the standard statistics increases by about 4% in comparison to the interim data. However, the more heterogeneous platform mixture used for both the enhanced and interim statistics complicates comparison with the ship-based standard statistics (see Table 2). Variables commonly measured by drifting buoys, such as SST and sea level pressure, gain up to an additional 3% coverage in the enhanced statistics (e.g., Fig. 2). This increase combines the effects of inclusion of more delayed data from automated platforms and of wider trimming limits.

By itself, relaxation of the trimming limits to 4.5 sigma appears to have had a relatively small effect on overall data density and coverage (on the order of 1%) in the enhanced set. However, testing of the 4.5 sigma limits on subsets of Release 1a data, which did not consider effects of changes in platform mixture, showed important smaller-scale impacts:

  • For particularly large climate extremes such as the 1982-83 ENSO, local increases over 10% in the number of observations were possible for varying time intervals.

  • The number of 2-degree boxes with any observation was increased in areas with low observational density. The net effect was to reduce large gaps and spotty coverage (polar ice-edges, extratropical southern hemisphere) while well-observed regions (northern midlatitudes) did not gain additional coverage.

  • The size of recorded climate anomalies is less restricted by the 4.5 sigma versus the 3.5 sigma limits, thus overall climate variability is enlarged. For instance, during the 1982-83 ENSO, peak SST anomalies in the eastern equatorial Pacific are up to 1 degree C higher.

Preliminary comparisons between mean fields from the standard and enhanced statistics show compelling evidence that the combination of ship and buoy data is subject to systematic differences in boxes dominated by observations from one platform type or another (see, e.g., Wilkerson and Earle, 1990; Woodruff et al., 1991). Figure 3 illustates this for wind speed, and also shows frequent increases in wind speed in high latitudes stemming from the relaxed trimming limits.

Introduction | Data sources and corrections | Improvements in processing and products | Discussion | Future plans | Acknowledgements | References

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